NZ720073B2 - Grid frequency response - Google Patents
Grid frequency response Download PDFInfo
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- NZ720073B2 NZ720073B2 NZ720073A NZ72007314A NZ720073B2 NZ 720073 B2 NZ720073 B2 NZ 720073B2 NZ 720073 A NZ720073 A NZ 720073A NZ 72007314 A NZ72007314 A NZ 72007314A NZ 720073 B2 NZ720073 B2 NZ 720073B2
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R19/00—Arrangements for measuring currents or voltages or for indicating presence or sign thereof
- G01R19/25—Arrangements for measuring currents or voltages or for indicating presence or sign thereof using digital measurement techniques
- G01R19/2513—Arrangements for monitoring electric power systems, e.g. power lines or loads; Logging
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
- G01R27/28—Measuring attenuation, gain, phase shift or derived characteristics of electric four pole networks, i.e. two-port networks; Measuring transient response
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J13/00—Circuit arrangements for providing remote indication of network conditions, e.g. an instantaneous record of the open or closed condition of each circuitbreaker in the network; Circuit arrangements for providing remote control of switching means in a power distribution network, e.g. switching in and out of current consumers by using a pulse code signal carried by the network
- H02J13/00002—Circuit arrangements for providing remote indication of network conditions, e.g. an instantaneous record of the open or closed condition of each circuitbreaker in the network; Circuit arrangements for providing remote control of switching means in a power distribution network, e.g. switching in and out of current consumers by using a pulse code signal carried by the network characterised by monitoring
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J13/00—Circuit arrangements for providing remote indication of network conditions, e.g. an instantaneous record of the open or closed condition of each circuitbreaker in the network; Circuit arrangements for providing remote control of switching means in a power distribution network, e.g. switching in and out of current consumers by using a pulse code signal carried by the network
- H02J13/00004—Circuit arrangements for providing remote indication of network conditions, e.g. an instantaneous record of the open or closed condition of each circuitbreaker in the network; Circuit arrangements for providing remote control of switching means in a power distribution network, e.g. switching in and out of current consumers by using a pulse code signal carried by the network characterised by the power network being locally controlled
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J13/00—Circuit arrangements for providing remote indication of network conditions, e.g. an instantaneous record of the open or closed condition of each circuitbreaker in the network; Circuit arrangements for providing remote control of switching means in a power distribution network, e.g. switching in and out of current consumers by using a pulse code signal carried by the network
- H02J13/00032—Systems characterised by the controlled or operated power network elements or equipment, the power network elements or equipment not otherwise provided for
- H02J13/00034—Systems characterised by the controlled or operated power network elements or equipment, the power network elements or equipment not otherwise provided for the elements or equipment being or involving an electric power substation
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- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2310/00—The network for supplying or distributing electric power characterised by its spatial reach or by the load
- H02J2310/10—The network having a local or delimited stationary reach
- H02J2310/12—The local stationary network supplying a household or a building
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- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
- H02J3/14—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load by switching loads on to, or off from, network, e.g. progressively balanced loading
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- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
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- H04B2203/54—Aspects of powerline communications not already covered by H04B3/54 and its subgroups
- H04B2203/5404—Methods of transmitting or receiving signals via power distribution lines
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/56—Power conversion systems, e.g. maximum power point trackers
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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Abstract
Method and apparatus for determining inertia within a synchronous area of an electric power grid are described. Power flow to and/or from a group of one or more power units is modulated on the basis of a sequence of control signals (figure 4a) and a frequency characteristic relating to a frequency of electricity flowing in the electric power grid measured (solid lines of figures 4b-4d). A frequency response characteristic associated with at least one area of the electric power grid is determined on the basis of the measured frequency characteristic and the determined magnitude characteristic. From this it is determined whether the inertia in the area of the grid in which the power unit is located is low (figure 4b), medium (figure 4c) or high (figure 4d), and it enables frequency response characteristics within a synchronous area of the electric power grid to be easily determined. f electricity flowing in the electric power grid measured (solid lines of figures 4b-4d). A frequency response characteristic associated with at least one area of the electric power grid is determined on the basis of the measured frequency characteristic and the determined magnitude characteristic. From this it is determined whether the inertia in the area of the grid in which the power unit is located is low (figure 4b), medium (figure 4c) or high (figure 4d), and it enables frequency response characteristics within a synchronous area of the electric power grid to be easily determined.
Description
GRID FREQUENCY RESPONSE
Technical Field
The present invention relates to methods and apparatus for determining
frequency response characteristics within an electric power grid.
Background
The exchange of electrical power between providers and consumers takes
place via an electricity distribution k or electric power grid. In such an electric
power grid, electrical power is typically supplied by a combination of relatively large
capacity power stations and relatively small capacity renewable energy sources.
Generators in large power stations, such as fossil fuel-burning or r
power stations, typically se rotating parts that have relatively high mass that
are rotating at relatively high speeds, and accordingly are ed to as spinning
tion. In the course of their normal ion, the spinning generators store
relatively large amounts of kinetic energy. Smaller renewable energy sources, such as
wind turbines and solar power generators store a much smaller amount of energy, or
even no energy at all.
Typically, an ic power grid operates at a nominal grid frequency that is
uniform throughout a synchronous area of the grid. For example, the UK mains
supply nominally es at 50 Hz. Grid operators are usually obliged to maintain
the grid frequency to within predefined limits, for example the UK mains supply
should be kept within 0.4% of the nominal 50 Hz grid frequency. If the balance
between generation and consumption of electrical energy is not maintained (for
example, if the total amount of generation cannot meet consumption during high
demand periods, or if the output from a power generator changes, s due to a
fault in the generator) the net amount of energy stored in the generators of the grid can
vary. This results in a change of the rotational speed of the spinning generators and a
corresponding change in the ing frequency of the grid. Grid operators therefore
use the system operating frequency as a measure of the balance between consumption
and generation of electrical power in the grid.
A frequency response characteristic describes the response of the grid
frequency to a change in balance n generation and consumption of electrical
power in the grid. Examples of such frequency response characteristics include grid
“stiffness” and grid “inertia”.
Grid stiffness is a property of the grid describing the extent (i.e. magnitude) of
grid frequency response for a given power balance change. A onous electric
power grid with a relatively high ess, for example, exhibits a relatively small
change in grid frequency for a given power balance change. A stiff or strong grid
typically has a low grid impedance and is typical of grids where the system generation
capacity is large. Whilst stiffness is in general a static property of a given grid, it
should be noted that in practice, for example in large grids, the generation and
consumption capacity s ntly, for example, when new providers are added
to or removed from the grid and/or from intermittent sources of generation such as
wind and solar. This means that in practice, grid stiffness can be a ntially
dynamic property of a grid.
Grid inertia is a measure of the amount of energy stored in the electric power
grid and influences the rate at which the operating frequency of the grid changes in
response to a change in grid balance. Regions of a synchronous electric power grid
that have a high proportion of spinning generation typically have a large amount of
energy stored as rotational kinetic energy in the generators (that is they have high
inertia) and therefore have a larger capacity to maintain the ing frequency of the
grid at the nominal grid frequency. In contrast, regions of a synchronous electric
power grid that have a low proportion of spinning generation have a relatively low
amount of stored energy (that is they have low inertia) and therefore have less
capacity to maintain the operating frequency of the grid at the nominal grid frequency.
Consequently, the rate of change of frequency in high inertia regions of the grid is less
than it is in low inertia regions of the grid, and the “inertia” may refer to this rate of
change of frequency.
Since frequency response characteristics in the grid can provide an tion
of how the grid will d to sudden changes in consumption or generation, it is
useful for grid ors to understand how grid frequency response teristics
vary across the electric power grid. Conventionally, grid frequency response
teristics are determined by using phasor instrumentation to make precise and
high-resolution measurements of the grid operating ncy. Since such
instrumentation is expensive, it is not practical to bute widely; typically,
measurements are made at a limited number of central nodes in a transmission grid.
This means that the measurement is relatively insensitive to local variation within the
grid.
Further, due to the large amounts of data that the measurements produce, the
measurements are often analysed off-line. This means that there is a delay in the
ination of frequency response characteristics; this makes it difficult for network
operators and the like to react in a timely manner to s in frequency response
characteristics.
It is an object of the present invention to at least mitigate some of the problems
of the prior art.
Summary
According to a first aspect of the present invention, there is provided a method
of determining, in a measurement system, a frequency response characteristic within a
synchronous area of an electric power grid, electricity flowing in the grid in
accordance with a grid ncy, wherein the electric power grid is connected to a
first group of one or more power units each arranged to consume electric power from
and/or provide electric power to the electric power grid such that a change in power
provision and/or ption by said first group of one or more power units results in
a change in power flow in the electric power grid, wherein power flow to and/or from
each of the power units is modulated on the basis of a sequence of control signals,
thereby modulating the grid frequency to provide a frequency modulated signal
ing to the sequence of control signals, the method comprising:
measuring, in the measurement system, a frequency characteristic ng to a
frequency of icity flowing in the electric power grid;
accessing a database storing data relating to power characteristics of said one
or more power units and determining, on the basis thereof, a characteristic relating to
said power flow modulation; and
determining a frequency response characteristic associated with at least one
area of said electric power grid on the basis of the measured frequency characteristic
and the said determined power flow tion characteristic.
Measuring a characteristic of a frequency modulated signal produced by
existing power units with known power characteristics within a synchronous area of
an electric power grid enables real-time, or near real-time, determination of frequency
response characteristics at many points within the grid, with a relatively low cost, due
to the relative simplicity of the required frequency characteristic measurement device.
In some ments, determination of the frequency response characteristic
comprises ating the measured frequency teristic with said power flow
modulation characteristic.
In some embodiments, determining the frequency response characteristic
comprises correlating the measured frequency characteristic with said power flow
modulation characteristic.
In some embodiments, determining the frequency response characteristic
comprises determining a ratio of said power flow tion characteristic and the
measured frequency characteristic.
In some embodiments, said power flow modulation characteristic ses a
magnitude characteristic relating to said power flow modulation.
In some embodiments, said magnitude characteristic comprises an ude of the
power flow.
In some embodiments, the said measured frequency characteristic is measured
on the basis of one or more of: a frequency of alternating voltage, a ncy of
alternating current, a measured frequency of power flowing in the electric power grid;
a rate of change of frequency; a period of alternating current or voltage.
In some embodiments, said measured frequency characteristic comprises a
time variation in ncy ated with said ted signal.
In some embodiments, said frequency response characteristic comprises an
inertia characteristic.
In some embodiments, said inertia characteristic comprises at least one of a
rise time and a fall time associated with said frequency modulated signal.
In some embodiments, said frequency response characteristic comprises a
characteristic relating to a magnitude of variation in grid frequency per unit change in
power e.
In some embodiments, the first group of power units is a distributed group of
power units, and there is a method comprising:
modulating power flow to and/or from each of the first group of power units in
accordance with a l pattern, such that the consumption and/or provision of
power by the plurality of power units is coordinated to provide a tive frequency
modulated signal, having a collective frequency characteristic, that is detectable by
the measurement system.
Modulating power flow to and/or from each of the plurality of power units in
accordance with a control pattern enables a method of collective communication to
the measurement system by one up to all of the power units capable of power flow
tion connected to the electric power grid. This may be advantageous when
using power units which draw small amounts of power, which individually may not
be able to produce a frequency modulated signal strong enough to be detectable by the
measurement system over other signals or grid noise, but which when coordinated can
collectively produce a frequency modulated signal strong enough for detection at a
desired point in the electric power grid. In such a way the number of localities in
which ncy response characteristics may be determined may be increased.
In some ments, a signal specifying said control n to each power
unit of the first group of power units is sent. .
In some embodiments, the control pattern comprises a ing pattern, and
the power to and/or from the first group of one or more power units continuously
according to the repeating pattern.
The control pattern comprising a repeating pattern may be advantageous for
the provision of multiple opportunities for the measurement system to measure
characteristics of the power flow pattern, and for averaging of successive identical
power flow patterns by the measurement system to allow, for example, a higher
precision in the determination of a in the electric power grid.
In some embodiments, power to and/or from the first group of one or more
power units is controlled intermittently according to the l pattern.
The intermittent control may be advantageous for the purposes of power
, allowing the power units to, for example, remain off when not required. This
may also be ageous for the coding of data into the power flow control pattern,
and hence for communicating data to the measurement system by means of the
resulting power flow pattern in the electrical power grid.
In some ments, there is provided a method in which said collective
modulated signal includes an identifier identifying said group of power units, the
method comprising:
accessing a database storing one or more identifiers each associated with said
first group of one or more power units; and
determining a correspondence n the identifier included in the collective
ted signal and one or more of the identifiers stored in the database, thereby
identifying said first group of one or more power units.
In some embodiments, each identifier stored in the database is associated with
at least one area of the electric power grid and the method comprises determining an
area with which the determined frequency response characteristic is associated on the
basis of the determined identifier correspondence.
In some embodiments, there is provided a method in which the electric power
grid is ted to a second group of one or more power units arranged to consume
power from and/or provide power to the electric power grid, the method comprising:
determining, on the basis of the determined frequency response characteristic,
one or more parameters for use in triggering a change in consumption and/or
provision of power by the second group of one or more power units; and
transmitting said one or more parameters for receipt at said second group of
power units.
In some embodiments, there is provided a method comprising:
receiving, at the second group of power units, said one or more parameters;
ng, on the basis of the ed parameters, a trigger condition;
determining, based on a measured ncy characteristic of electric power
flowing in the grid locally to a second group of power units, whether the trigger
condition is satisfied; and
in response to a determination that the trigger condition is satisfied, changing a
power flow to and/or from the second group of power units.
ring a change in consumption and/or ion of power by power units
can be useful in limiting the impact of a change in power flow elsewhere in the grid
on grid frequency. Incorporating frequency response characteristics into the derivation
of triggering parameters is advantageous since the frequency response teristics
provide information relating to, for example, the rate of change of grid frequency in
response to a change in power flow. The triggering parameters for the power units can
ore be tailored to provide, for example, a relatively early response in the case
where the local grid inertia for example is relatively low, and a relatively late response
in cases where the local grid inertia for example is relatively high. This is an
advantageous arrangement, since it avoids both ses that are too late in for
example low inertia environments, which can lead to an unacceptable grid ncy
shift, as well as responses that are too early for high inertia environments for example,
which would cause unnecessary disruption to the use of the power units. Similarly,
incorporating stiffness characteristics for example into the derivation of r
ters is advantageous since, for example, it provides an indication of the likely
magnitude of the measured ncy characteristic change for a given likely power
balance change.
In some embodiments, there is provided a method comprising:
defining, at the measurement system, a first series of values associated with
the frequency characteristic during a first time period and a second series of values
associated with the frequency teristic during a second, later, time period;
determining, at the measurement system, a first polynomial function having a
first set of coefficients on the basis of said first series of values and a second
polynomial function having a second set of coefficients on the basis of said second
series of values; and
determining, at the measurement system, r the trigger condition is
satisfied on the basis of a difference between the first set of coefficients and the
second set of coefficients.
In some embodiments, there is provided a method in which the electric power
grid is connected to a second group of one or more power units arranged to consume
power from and/or provide power to the electric power grid, the method comprising:
determining, based on the determined frequency response characteristic
associated with an area ated with the second group of power units, one or more
parameters for use in triggering a change in consumption and/or provision of power
by the second group of one or more power units;
deriving a trigger condition on the basis of the measured frequency response
characteristic;
measuring, in an area associated with the second group of power units, a
ncy characteristic ng to a frequency of electricity flowing in the electric
power grid;
communicating the ed ncy characteristic measured in the area
associated with the second group of power units to the measurement system;
determining, based on the communicated measured frequency characteristic,
r the r condition is satisfied; and
in response to a determination that the trigger condition is satisfied, sending a
request to the second group of power units to change a power flow to and/or from the
second group of power units.
The above embodiment would enable the determination of r the
triggering condition is satisfied or not, for example, to be carried out at a central
control centre serving some or all of the electric power grid. This may be
advantageous since it would enable centralised control of triggering, a centralised
override of triggering, a means to centralise control of power flow at the power units,
and may also enable a more cost effective method of trigger condition determination
as compared to prior art methods requiring apparatus for these functions at every
device.
In some ments, the second group of power units is the same as the first
group of power units.
In some embodiments, the power modulation comprises modulation of at least
one of real power and reactive power.
According to a second aspect of the t invention, there is provided a
measurement system for determining a ncy response characteristic within a
synchronous area of an electric power grid, wherein electricity flows in the grid in
ance with a grid ncy and the ic power grid is connected to a group
of one or more power units each arranged to consume electric power from and/or
provide electric power to the electric power grid such that a change in power
provision and/or consumption by said power unit results in a change in power flow in
the grid, wherein power flow to and/or from each of the power units is modulated on
the basis of a sequence of control signals, thereby modulating the grid frequency to
provide a frequency ted signal, the measurement system being arranged to:
measure a frequency characteristic relating to a frequency of electricity
flowing in the electric power grid;
access a database storing data relating to power characteristics of said one or
more power units and determine, on the basis thereof, a characteristic relating to said
power flow modulation; and
determine a ncy response characteristic associated with at least one area
of said electric power grid on the basis of the measured frequency characteristic and
the said determined power flow tion characteristic.
In some embodiments, the second aspect of the invention es features
corresponding to all of the features associated with the various embodiments listed
above in relation to the first aspect of the invention.
According to a third aspect of the present invention, there is provided a power
control device for use with one or more associated power units to provide a response
to s in a frequency of electricity flowing in a synchronous area of an electric
power grid, wherein the electric power grid is connected to a measurement system
arranged to determine a ncy response characteristic of the grid in said area and
to ine one or more trigger parameters on the basis of the measured frequency
response characteristic, the power control device being arranged to:
intermittently receive one or more parameters from the measurement system,
the parameters being derived from a said determined frequency response
characteristic;
derive, on the basis of the received one or more parameters, a trigger
condition;
determine, based on a measured frequency characteristic of electric power
flowing in the grid, whether the trigger condition is satisfied; and
in response to a determination that that the trigger condition is satisfied,
change a power flow to and/or from the power unit.
In some embodiments, one of the received one or more parameters comprise
said r condition.
In some embodiments, said frequency response teristic comprises an
inertia characteristic.
In some embodiments, said frequency response characteristic comprises a
teristic relating to a magnitude of variation in grid frequency per unit change in
power balance.
In some embodiments, the power control device is arranged to:
define a first series of values associated with the frequency characteristic
during a first time period and a second series of values associated with the frequency
teristic during a second, later, time period;
determine a first polynomial function having a first set of coefficients on the
basis of said first series of values and a second polynomial function having a second
set of cients on the basis of said second series of values; and
determine whether the trigger condition is satisfied on the basis of a difference
between the first set of coefficients and the second set of coefficients.
The above embodiment allows for the trigger condition to be based on the way
in which the frequency is changing as a function of time. This may be ageous
since it can provide a relatively early response to a vely fast change in frequency
characteristic, which may allow a rapid restoration of the ical power grid
frequency, and alternatively e a relatively late response to a relatively slow
change in frequency teristic, preventing unnecessary disruption to power units.
It is also advantageous since it provides a means to filter out frequency characteristic
fluctuations occurring on a given time scale, which may represent noise or other
ations which are not of interest in determining whether the triggering condition
is satisfied.
In some ments, the first and second polynomial functions are second
order polynomial functions.
In some embodiments, the frequency change event is identified on the basis of
a value of at least one coefficient of the second set of coefficients differing from a
corresponding coefficient in the first set of coefficients by more than a predetermined
amount.
In some embodiments, the power control device is arranged to e said
series of values according to a polynomial extrapolation technique and/or conic
extrapolation technique.
In some embodiments, the power control device comprises phasor
measurement instrumentation arranged to measure said measured ncy
characteristic on the basis of a phasor measurement.
In some ments the phasor measurement instrumentation is arranged to
measure a phase ated with a vector of voltage measured in the electric power
grid with reference to an absolute time reference.
In some embodiments, the measured frequency characteristic includes one or
more of: a frequency of alternating voltage, a frequency of alternating current, a
frequency of power flowing in the electric power grid; a rate of change of frequency;
and a period of alternating t.
In some embodiments the power control device is arranged to receive a signal,
said signal indicating a time period during which power flow may be controlled.
In some embodiments the power modulation comprises modulation of at least
one of real power and reactive power.
According to a fourth aspect of the present invention, there is provided a
system for responding to changes of frequency in an electric power grid, the system
sing:
a distributed plurality of power control devices, each controlling a respective
power unit connected to the electric power grid; and
a measurement system for transmitting one or more trigger parameters to each
of the plurality of distributed power l devices. In some ments the
measurement system is arranged to:
define, a ity of groups of power control devices from said distributed
plurality of power control devices;
assign different respective trigger ions to each of the plurality of groups;
transmit, to each of power control devices a trigger ion assigned to the
group to which it is assigned.
In some embodiments, the measurement system is arranged to:
access a power unit database storing profile information relating to the
consumption and/or provision of power by the power units associated with the power
l devices; and
define said plurality of groups on the basis of the accessed profile information.
In some embodiments, the measurement system is arranged to: receive data
indicative of a polynomial function representative of a measured frequency
characteristic;
extrapolate, based on said mial function, future expected values
associated with the measured frequency characteristic; and
determine, on the basis of the extrapolated future expected values, an expected
power flow requirement for responding to the frequency change event.
The forecasting of a ncy teristic provided for in the above
embodiment is advantageous as it allows more time to organise an efficient response
to a change in frequency characteristic which may result in a trigger condition being
ied. This forecasting is also advantageous since it provides a means compensate
for a frequency characteristic change which is likely to happen in the near future,
rather than compensating for changes which have happened already, which allows for
a tighter control of the frequency characteristic.
In some embodiments, the measurement system is arranged to:
access a power unit database comprising profile ation relating to the
consumption and/or provision of power by the power units;
define, on the basis of the ed power flow requirement and said profile
information, one or more groups of one of more power units for ding to the
frequency change event.
In some embodiments, the measurement system is arranged to transmit one or
more requests, for receipt at the power l devices of the defined groups, to
control consumption and/or provision of electrical power by the power units
associated with power control devices, thereby varying a net consumption of electrical
energy in said area.
According to a fifth aspect of the t invention, there is provided a power
control device for use with one or more associated power units to provide a response
to changes in a frequency of electricity flowing in an ic power grid, wherein
electricity flows in the electric power grid in accordance with a grid frequency, the
power control device comprising a frequency measurement device and being ed
monitor, using said frequency measurement device, changes in said grid
frequency at the power control device;
based at least partly on said monitoring, determine a trigger condition;
determine, based on a measured frequency characteristic of ic power
flowing in the grid, whether the trigger condition is satisfied; and
in response to a determination that that the trigger condition is ied,
change a power flow to and/or from the power unit.
In some embodiments, the power control device is arranged, responsive to the
measured frequency teristic crossing a threshold value, to:
perform an analysis of the measured frequency characteristic at times
preceding said threshold value being crossed; and
determine said trigger condition at least partly on the basis of said analysis.
In some embodiments the power control device is arranged to:
monitor, using said frequency ement , changes in the grid
frequency at the power control device subsequent to the derivation of said r
condition, and
based on the subsequent monitoring, derive an updated r condition.
In some embodiments, the derivation of the updated trigger condition is based
in part on said first r condition.
In the above embodiments, the device is able to control power flow based on a
trigger condition which it itself has derived. This device would be advantageous, for
example, in areas of the electric power grid which may not have access to
communication networks, or in cases where d via these networks is not cost
effective.
Further features and advantages of the ion will become apparent from
the following description of preferred embodiments of the invention, given by way of
e only, which is made with reference to the accompanying drawings.
Brief Description of the Drawings
Figure 1 is a schematic m rating a synchronous electric power grid
in which the invention may be implemented;
Figure 2a is a schematic diagram illustrating a frequency modulation device;
Figure 2b is a diagram rating the relationship between modulated power
generation/consumption balance and the resulting grid frequency modulation in an
electric power grid;
Figure 3 is a schematic diagram illustrating a measurement device;
Figure 4a is a graph g an exemplary square-wave power modulated
signal;
Figure 4b is a graph showing a frequency modulated signal in a low inertia
region of an electric power grid;
Figure 4c is a graph showing a frequency modulated signal in a medium
inertia region of an electric power grid;
Figure 4d is a graph showing a frequency modulated signal in a high inertia
region of an electric power grid;
Figure 5 is a schematic diagram illustrating an example method and apparatus
for ining frequency response characteristics in an area of a grid.
Figure 6a is a diagram illustrating a series of intervals defined for a measured
frequency characteristic;
Figure 6b is a m illustrating a measured frequency characteristic can be
fitted with a polynomial function;
Figure 6c is a diagram illustrating a measured frequency teristic can be
fitted with a polynomial function;
Figure 6d is a diagram illustrating a measured frequency characteristic can be
fitted with a polynomial function;
Figure 6e is a diagram illustrating a measured frequency characteristic can be
fitted with a polynomial function;
Figure 6f is a diagram illustrating a measured frequency characteristic can be
fitted with a polynomial function;
Figure 7 is a graph showing a change in frequency in three exemplary regions
in response to a sudden change in grid balance; and
Figure 8 is a schematic diagram illustrating a power control .
Detailed ption
Supply of electricity from ers such as power stations, to consumers,
such as ic households and businesses, typically takes place via an electricity
distribution k or electric power grid. Figure 1 shows an ary electric
power grid 100, in which embodiments of the present invention may be implemented,
comprising a transmission grid 102 and a distribution grid 104.
The transmission grid 102 is connected to power generators 106, which may
be nuclear plants or gas-fired plants, for example, from which it transmits large
quantities of electrical energy at very high voltages (typically of the order of hundreds
of kV), over power lines such as overhead power lines, to the distribution grid 104.
The transmission grid 102 is linked to the distribution grid 104 via a
transformer 108, which converts the ic supply to a lower voltage (typically of
the order of 50kV) for distribution in the distribution grid 104.
The distribution grid 104 is connected via substations 110 comprising further
ormers for converting to still lower voltages to local networks which provide
electric power to power consuming devices ted to the electric power grid 100.
The local networks may include networks of domestic consumers, such as a city
network 112, that supplies power to domestic appliances within private residences
113 that draw a relatively small amount of power in the order of a few kW. Private
residences 113 may also use photovoltaic s 117 to provide relatively small
amounts of power for ption either by appliances at the residence or for
provision of power to the grid. The local networks may also include industrial
premises such as a factory 114, in which larger appliances ing in the rial
premises draw larger amounts of power in the order of several kW to MW. The local
networks may also include networks of smaller power generators such as wind farms
116 that provide power to the electric power grid.
Although, for conciseness, only one transmission grid 102 and one distribution
grid 104 are shown in figure 1, in practice a typical transmission grid 102 supplies
power to le distribution grids 104 and one transmission grid 102 may also be
onnected to one or more other transmission grids 102.
Electric power flows in the electric power grid 100 as alternating current (AC),
which flows at a system frequency, which may be referred to as a grid frequency
(typically 50 or 60 Hz, ing on country). The electric power grid 100 operates
at a synchronized frequency so that the ncy is substantially the same at each
point of the grid.
The electric power grid 100 may include one or more direct current (DC)
interconnects 117 that provide a DC connection between the electric power grid 100
and other electric power grids. Typically, the DC interconnects 117 connect to the
typically high voltage transmission grid 102 of the electrical power grid 100. The DC
interconnects 117 provide a DC link between the various electric power grids, such
that the electric power grid 100 s an area which operates at a given,
synchronised, grid frequency that is not affected by s in the grid frequency of
other electric power grids. For example, the UK transmission grid is connected to the
Synchronous Grid of Continental Europe via DC interconnects.
The electric power grid 100 also includes one or more devices for use in
modulating an operating frequency of the ic power grid 100 (herein referred to
as “frequency modulation devices” 118) and a measurement system in the form of a
measurement device 120 arranged to measure a characteristic relating to the operating
frequency of the grid (hereinafter referred to as the grid ncy).
Each ncy modulation device 118 is associated with a power unit 119
(which may consume power from or provide power to the electric power grid 100) or
a group of power units 119 and is arranged to modulate power flow to and/or from the
power unit 119 or group of power units 119 as described below with reference to
figure 2a. The frequency modulation devices 118 may be provided separately to,
and/or installed on, the power units 119. The power units 119 may include power
generators 106, appliances in residential premises 113 or industrial premises 114
and/or a scale power generators such as wind turbines 116 or solar panels 117.
It may be advantageous in this t for the power units 119 to have low inertia to
enable effective modulation of the power flow.
The one or more frequency modulation devices 118 may be located at power units
119 in the distribution grid 104 or in the transmission grid 102, or at any other
location of the electric power grid 100. The frequency modulation devices 118 operate
with the power units 119 to transmit code sequences within the electric power grid
100. Although, for the sake of simplicity, only seven frequency modulation devices
118 are shown in Figure 1, it will be understood that, in practice, the electric power
grid 100 may comprise ds or nds of such devices, ing upon the
ty of power units 119 with which the frequency modulation devices 118 are
associated. Furthermore, it will be tood that gh, for the sake of simplicity,
only one measurement device 120 is shown in Figure 1, in practice multiple
measurement devices 120 may operate in the same synchronous electric power grid
100. Where ncy modulation devices 118 are associated with large capacity
power units 119 (such as a power unit in an rial premises) there may only be a
small number of frequency modulation devices 118 in the electric power grid 100. In
some embodiments, there may only be one frequency modulation device 118 in the
electric power grid 100.
The frequency modulation devices 118 may be distributed among a relatively
large number of smaller capacity power units 119 (each providing, for example, a few
W to tens of kW, such that the contribution to the frequency modulation by each
power unit 119 is smaller but so that a combined frequency modulation signal has the
same strength as a single larger power unit 119. bution of the frequency
modulation devices 118 has the advantage that the switching of smaller loads can be
med without the need for expensive power switching apparatus (switching can
instead be performed with semiconductor-based switches, for example, which may be
mass produced), and the switching of smaller loads only introduces a relatively small
amount of voltage noise into the local grid environment so that, for example, the
supply voltage stays within limits.
Typically, the total modulated load required to it a frequency modulated
signal across the electric power grid 100 is dependent on the particular coding scheme
used for transmitting information, as described below. Different coding schemes
result in different amounts of gain at the measurement device 120 and hence the
required power for tion may range significantly, for example from W to MW.
The frequency modulation devices 118 each modulate the power flow to
and/or from respective ated power units 119. Where there is more than one
frequency tion device, each of the one or more frequency modulation devices
118 may be synchronised with each of the other ncy modulation devices 118
and arranged to te power flow according to a control pattern such that the
frequency modulation devices 118 cause a collective tion of the power flow in
the electric power grid 100. That is, the frequency modulation devices 118
collectively cause a ted change in power balance in the electric power grid
100, the change in power balance being the combined effect of the modulated power
flow to/from each of the power units 119 that have an associated frequency
modulation device 118.
The frequency modulation devices 118 may be arranged to modulate a reactive
power flow to and/or from their associated power units 119. For e, the
frequency modulation devices 118 may include inverters for modifying a reactive
power contribution of their associated power units 119. Modulating the reactive
power contribution of the power units causes a local modulation of the efficiency of
the electric power grid 100 with a corresponding modulation of the available real
power. In turn, these cause a modulation of the grid balance which as described above
causes a modulation of the grid frequency.
In certain embodiments the frequency modulation devices 118 may be
ed to modulate just real power, just reactive power, or both real and reactive
power.
Figure 2a shows an exemplary arrangement of a frequency modulation device
118. The frequency modulation device 118 forms an interface between the electric
power grid 100 and one or more power units 119 and operates with the one or more
power units 119 to propagate a frequency modulated signal within the electric power
grid 100. The frequency tion device 118 comprises an output (I/O)
interface 202, a data store 204, a processor 206, a modulator 208, and a clock 210.
The ncy modulation device 118 is ed to e data from a
controller via the I/O interface 202. The controller may be part of the measurement
device 120. Alternatively, the controller may not be directly connected to the electric
power grid 100 but instead the data may be received via the I/O interface 202. The
I/O ace 202 is arranged to receive information via a fixed or wireless
communications network, which may include one or more of Global System for
Mobile Communications (GSM), Universal Mobile Telecommunications System
(UMTS), Long Term Evolution (LTE), fixed wireless access (such as IEEE 802.16
WiMax), and wireless networking (such as IEEE 802.11 WiFi).
Information received via the I/O interface 202 may be stored in the data store
204. Information stored in the data store 204 may e representations of a control
sequence in accordance with which the grid frequency is to be modulated by the
frequency tion device 118 (referred to herein as “codes”). The codes may
represent control signals for controlling the modulator 208 according to a predefined
control pattern.
The processor 206 is arranged to retrieve the codes from the data store 204 and
to generate control signals for controlling the modulator 208. The processor 206
accesses the data store 204, retrieves a code and, based on the code, generates control
s and sends those control s to the modulator 208 to control power flow
to/from a power unit 119. The control signals may be in the form of a bit pattern of a
signal that is to be propagated in the electric power grid 100. The code typically
defines a time-varying pattern of control signals provided with reference to the clock
210. The clock 210 may be synchronised with the clocks of other frequency
tion devices 118 in order that each of the frequency modulation devices 118
connected to the electric power grid 100 is synchronised with each other frequency
modulation device 118. This enables propagation of frequency modulated signals to
be ted at each frequency modulation device 118 at the same time.
Synchronisation of the clock 210 may be performed on the basis of a synchronisation
signal received via the (I/O) interface 206, or by any other means.
The modulator 208 is arranged to modulate power flow to/from a power unit
119 in se to the control signals generated by the processor 206. The modulator
208 may comprise a switch for connecting/disconnecting the power unit 119 to/from
the ic power grid 100 and/or any electrical or onic means allowing power
flow to/from the power unit 119 to be modulated. For example, the power unit 119
may not necessarily be completely turned off during modulation but may instead be
modulated between set points of power consumption and/or provision. The modulator
208 may be an attenuator or some other means for ng the power
consumption/provision by the power unit 119 (for example, inverter-based chargers
for electric es and/or other electric devices, grid-tie inverters for photovoltaic
generators, Combined Heat and Power (CHP) generators, or wind generators.
Modulating the grid power balance s a modulation in the grid
ncy that in a synchronous electric power grid is the same throughout the entire
electric power grid.
For example, considering figure 2b, which depicts a theoretical inertia-less
electric power grid, at point A the electric power grid 100 is balanced (that is, the total
demand for electric power is approximately equal to the total amount of power being
generated in or provided to the electric power grid 100) and the grid frequency is
stable at, for example, 50Hz. At point B, the grid power balance is shifted such that
there is excess consumption from point B to point C. This s in a corresponding
fall in the grid frequency at point B′, which is maintained until point C′. At point C,
the grid power balance is shifted such that there is excess generation at point D, which
is maintained until point E. This results in a corresponding rise in the grid frequency
between points C′ and D′, which is maintained from point D′ to point E′. The extent to
which the grid frequency changes in response to a given change in grid power balance
is characterised by the grid stiffness, and is represented as the gradient of the straight
line in the graph of figure 2b. That is, for example, in figure 2b, a relatively stiff grid
would have a line with a vely small gradient, such that a relatively large grid
power balance change would only result in a relatively small grid ncy change. It
should be noted that although the onship between grid frequency change and grid
power balance change is generally monotonic, it may depart from the linearity as
depicted in figure 2b, and may have, for example, some curvature, at some times.
Typically, the maintenance of the increased grid frequency between, for
example, points D′ and E′ depends on a modulation frequency (i.e. the ncy at
which power flow is modulated). In particular, the increased grid frequency may be
maintained where the modulation period (the inverse of the modulation frequency) is
less than a characteristic reaction time for automatic correction and/or for the grid
operator to react to changes in grid power balance. In embodiments where the power
flow is modulated relatively y, power balance compensation mechanisms
employed automatically and/or by the grid operator cannot react quickly enough to
counteract the modulation, whereas where the power flow is modulated relatively
slowly, the power balance compensation mechanisms may begin to degrade the effect
of the intended tion by the compensation mechanism counteraction.
Typically, the amplitude of the grid frequency modulation is in the range of
µHz up to several mHz and so does not exceed the agreed limits within which grid
operators must maintain the grid frequency (the nominal system frequency) and does
not cause the grid or to te any manual or automatic grid ing
measures in response to the modulation. Furthermore, modulating the grid frequency
at a rate that is less than the grid frequency avoids attenuation of the frequency
modulated signal by transformers 108, 110 in the electric power grid 100.
The modulator 208 is typically ed to modulate power flow to/from the
power unit 119 at a modulation frequency typically up to 10 Hz h, again, this
s on the nature of each electric power grid). In some embodiments, power
flow to and/or from a power unit 119 is modulated at a modulation frequency of less
than half of the predefined grid frequency. In some embodiments, power flow is
modulated at a modulation frequency less than a quarter of the predefined grid
frequency. In some embodiments, power flow is modulated at a modulation frequency
less than a tenth of the ined grid frequency.
At this frequency range, switching of moderately high loads is possible.
Because the modulator 208 modulates power flow to/from the power unit 119 at a
modulation frequency less than the grid frequency, the modulated signal is not
ted by the infrastructure of the electric power grid 100 any more than an unmodulated
AC electrical power would be. This removes the need to e an
additional communications route around devices such as transformers 108, 110 as is
required by Powerline ications systems which overlay a High Frequency
(100’s Hz up to MHz) signal onto the base system (e.g. 50Hz) frequency.
It should be noted that it may be advantageous to modulate the power flow
to/from a power unit at a zero-crossing of the AC waveform. For example, in the case
where the modulation comprises turning the power flow m the device on and
off, the transitions between on and off may be made at the zero-crossing point. This
minimises the generation of unwanted harmonics uently distributed into the
electric power grid, and hence ses unnecessary grid noise due to the
modulation.
Although the frequency modulation device 118 is shown in Figure 2a as being
separate to the power unit 119, it will be understood that in some embodiments the
frequency modulation device 118 may be integral to power unit 119.
It should be noted that, although the codes are described above as being stored
in the data store 204 of the frequency modulation device 118, in some embodiments
they may be stored remotely (for e at the controller) and ed by the
frequency modulation device 118 when required. For example, the codes may be
transmitted to the frequency modulation device 118, in which case they may not be
stored at the ncy modulation device 118, or stored only in a temporary data
store.
Modulation of power flow by the ncy modulation device 118 causes a
corresponding modulation of the grid ncy, which is the same throughout a given
synchronous electric power grid 100.
As explained below with reference to figure 4, the frequency response for a
given power flow modulation is dependent on the frequency response characteristics
local to the frequency modulation device or devices 118 providing the power
modulation.
Since the grid frequency is the same throughout the electric power grid 100,
the modulated frequency is also the same throughout the electric power grid 100 and
so a measurement device 120 able to detect the modulated grid frequency is able to
measure the modulated frequency signal at any point at which it can be connected to
the grid 100.
Figure 3 is a diagram illustrating an exemplary measurement device 120
configured to measure a modulated frequency signal propagated within an electric
power grid 100. The measurement device 120 comprises a or 302, a data store
304, a processor 306, an input-output (I/O) interface 308, and a clock 310.
The or 302 may be any device capable of detecting or measuring a
characteristic relating to the grid ncy with sufficient precision.
In some embodiments, a time period relating to the grid frequency is used as a
characteristic measure of the grid frequency. For example, a ement of the halfcycle
, which is the period between times at which the voltage crosses 0V, may be
used as a characteristic relating to the grid frequency.
In some embodiments, the taneous grid frequency, corresponding to the
e of the time it takes to complete a half-cycle (or a full-cycle) may be
ined. The frequency data may be equalised and digitally filtered to remove
frequency components outside a known and desired range of signal frequencies. For
example, frequency components corresponding to the grid frequency and/or frequency
components relating to noise may be removed.
In an embodiment, the or 302 may comprise a voltage detector arranged
to sample the voltage at a frequency higher than the grid frequency and an analogue to
digital converter arranged to convert the sampled voltage to a digital voltage signal.
For example, the voltage detector may be arranged to sample the voltage 1000 times
per cycle. The digital voltage signal may then be processed to determine with a high
degree of precision (within the range µs to ms) the times at which the voltage s
In another embodiment, the detector 302 may comprise a current or
arranged to sample the current at a ncy higher than the grid frequency, and an
analogue to digital converter arranged to convert the sampled current to a digital
current signal, which may then be processed to determine with a high degree of
precision (within the range of, for example, µs to ms) the times at which the current
s 0V, or other characteristics associated with the t waveform.
In still r embodiment, the detector 302 may comprise both a voltage
detector and a current detector. Measuring the times at which both the voltage and
current crosses 0V enables the measurement device 120 to determine a change in the
relative phase of the voltage and current, thereby enabling the measurement device
120 to compensate for changes in reactive power in the grid. This in turn enables a
more accurate measurement of frequency (or a characteristic relating to frequency).
In addition to, or as an alternative to, measuring the grid ncy, the
or 302 may measure a rate of change of frequency of power flowing in the grid
based on measurements of voltage and/or current, as bed above.
The data store 304 may store data indicating the power modulation pattern
used by the one or more frequency modulation devices 118; for example it may
indicate a square wave pattern, as described below in relation to s 4a to 4d. The
processor 306 may use the stored data pattern format to aid extraction of the
frequency modulated signal from the measured frequency signal by correlating the
stored data n format with the measured frequency signal. The measurement
device 120 may include a ator arranged to correlate the measured frequency
signal with a known modulation pattern to identify transitions between high and low
states in a digital frequency modulated signal. This enables the modulated signal to
be extracted from the measured frequency even when the amplitude of the modulated
signal (which, as described above, could be up to l mHz) is less than the level
of noise in the grid frequency (which is typically in the range of 10 to 200 mHz).
These typical values vary significantly from one synchronous grid to another and in a
given synchronous grid over time.
The data store 304 may also store identification data relating to one or more
power units 119, or one or more groups of power units 119 (or their associated
frequency modulation s 118). Such identification data, hereinafter referred to
as identifiers, may be used to identify the source of a frequency ted signal as
described below. The identifiers may correspond to the “codes” mentioned above; in
other words, the groups of power units 119 (or their associated frequency modulation
devices) may be identified by the frequency modulation pattern that they produce.
The data store 304 also stores data relating to one or more power
characteristics of the power units whose electric power consumption and/or provision
is ted by the ncy modulation devices 118, and ations between this
data and the identifiers mentioned above. The data store 304 may also store data
indicating an area of the grid at which frequency modulation devices 118 are located
(such as a y, , city or postcode), and associations between this data and
the identifiers mentioned above.
The data store 304 may be used to store measured and extracted frequency
modulated signal data that has been propagated within the electric power grid.
The processor 306 may be any processor capable of processing data relating to
a frequency characteristic of electricity flowing in the electric power grid 100. The
processor may include, but not be limited to, one or more of an application specific
integrated circuit (ASIC), a field programmable gate array (FPGA), a l signal
processor (DSP), and a general-purpose programmable processor.
The processor 306 is configured to process the data relating to the measured
frequency characteristic to determine frequency response characteristics within a
synchronous area of the electric power grid 100. In particular, the processor 306 is
configured to access the data store 304 to retrieve the data relating to power
characteristics of one or more power units 119, whose electric power consumption
and/or provision is modulated by the frequency modulation devices 118, and to
determine a characteristic relating to power flow modulation of the one or more power
units 119. Based on the measured frequency characteristic and the determined power
flow characteristic, the processor ines frequency response characteristics
associated with at least one area of the electric power grid 100.
Figure 4a shows a plot of an exemplary power modulated signal provided by a
frequency modulation device 118, and s 4b to 4d show corresponding plots of
frequency modulated signals for regions of a onous electric power grid that
have low, medium and high amounts of inertia. In particular, Figure 4a shows a
square wave power modulated signal that has a constant period and a duty cycle of
50%.
In each of Figures 4b to 4d, the dotted line represents the waveform of the
ion in power flow, with the solid line showing the corresponding frequency
variation. Note that the electric power grid for each of s 4b to 4d has the same
stiffness, meaning that given sufficient time, the extent of ncy change in
response to a given change in power balance would be the same. The electric power
grids represented in these graphs do, however, have g a, which is the cause
of their g form of frequency response for the given power flow variation.
Figure 4b shows the ncy variation resulting from power modulation in
regions of the electric power grid that have relatively low inertia. The frequency
modulated signal ponds closely to the power modulated signal.
Figure 4c shows the frequency variation resulting from power tion in
regions of the electric power grid that have a medium amount of inertia. The
frequency modulated signal is modified compared to the power modulated signal and
therefore corresponds less closely to the power modulated signal than that of Figure
4b. In particular, the rate at which the frequency changes is slower than that shown in
Figure 4b and therefore the frequency modulated signal appears delayed and
ed with respect to the input power modulated signal of Figure 4a.
Figure 4d shows the ncy variation resulting from power modulation in
regions of the electric power grid that have relatively high inertia. The frequency
modulated signal is heavily modified compared to the power modulated signal. In the
particular example shown, the degree of delay and ing is such that in the time
frame of the power modulated signal, the ncy modulated signal does not reach
its maximum value before the power modulation next switches.
Thus, by ining parameters relating to the form of the frequency
modulated signal resulting from an identified power unit 119 (or group thereof), and
comparing this with the known characteristics of the power flow modulation
producing the signal (as fied by accessing the data store 304), the measurement
device 120 is able to determine an inertia in the area of the grid in which the power
device 119 (or group thereof) is located. Further, an analysis of the extent of
frequency change in response to a known power modulated signal also enables the
measurement device to determine the grid stiffness.
Using the measurement method described above, a grid system operator (or
other party) is able to ine frequency response characteristics representative of
the entire synchronous electric power grid 100 and/or of local, regional areas of the
synchronous electric power grid. Thus, the grid system operator can assess the
ncy response characteristics in the various s of the electric power grid
100 and, on the basis of those characteristics, plan for future changes in grid balance.
Furthermore, the determined frequency response characteristics can be used by the
measurement device 120 for automatic or semi-automatic control of power units in
the electric power grid 100 to e corrective measures to changes in grid
frequency, as described below.
The frequency response characteristic measurements described above may be
used to predict future s in frequency response characteristics in geographically
adjacent areas utilizing measured ncy response characteristics and available
additional information including, for example, current and predicted weather
information and/or characteristics of electricity generation in the relevant area, such as
the mix of renewable to non-renewable generation sources (based on the fact that
renewable sources tend to have, for example, relatively low inertia).
The frequency response characteristics referred to above may be a relative
indication of, for e, the inertia or stiffness of the grid, based on a relative
frequency se with respect to a power flow modulation. Such an tion may
provide qualitative information regarding the inertia or stiffness of the grid without
necessarily determining their absolute tative values. An indication of a
frequency response characteristic, may, for example, be determined by correlating the
measured frequency characteristic with a magnitude characteristic of the power flow
modulation using a cross-correlation, or other correlation process. Alternatively, an
indication of stiffness may be determined by determining a ratio of the characteristic
of a magnitude and the measured ncy characteristic.
Alternatively or additionally the frequency response characteristics may
comprise an absolute numerical value of inertia and/or stiffness.
As an e, an inertia characteristic may be ined by analysing a
time ion of a frequency characteristic, for example, by analysing the impulse
response of grid frequency resulting from an e resulting from a known
modulation of power flow in the electric power grid. The se in grid frequency
as a function of time may, for example, be fitted by an exponential function with a
characteristic time constant. If the power flow impulse ponds to an increase in
power flow, the time constant corresponds to a “rise time”, and if the power flow
impulse corresponds to a decrease in power flow, the time constant corresponds to a
“fall time”. A characteristic relating to inertia may then be defined, for example, to be
proportional to this rise time or fall time, and hence longer rise and/or fall times
indicate regions of the electric power grid with a larger a.
As a further example, a stiffness characteristic may be determined by
analysing the magnitude of a frequency characteristic change in response to a known
magnitude of power balance change. This could se, for example, the
determination of the ratio of the measured magnitude of the impulse response of grid
frequency (for example in units of Hz) to the known magnitude of impulse of power
flow (for example in units of W) which caused it, giving rise, in this example, to a
stiffness teristic with units of Hz W-1.
The characteristic of magnitude relating to the power flow tion
provided by the one or more power units 119 referred to above may, for example, be
an amplitude of the power modulation provided by one power unit 119 and its
corresponding frequency modulation device 118 or, as described in more detail below,
where more than one power unit 119 and its associated one or more frequency
modulation devices 118 are used to provide a frequency modulated signal, the
magnitude characteristic may be a summed amplitude of modulation of all the
modulated power units 119.
In some embodiments, the power flow of a group, such as a distributed group,
of power units 119 may be ted in a coordinated way by their corresponding
ncy modulating devices 118 in accordance with a single control pattern. In this
way, the consumption and/or provision of power by the group of power units 119 is
coordinated to provide a collective frequency modulated signal having a collective
ed frequency characteristic that is measurable/detectable by the measurement
device 120. As described above, this enables the switching of smaller loads to be
performed by removing the need for expensive power ing tus hing
can instead be performed with semiconductor-based switches, for example, which
may be mass produced), and also only introducing a relatively small amount of
voltage noise into the local grid environment.
In order that the resulting ncy ted signal from each member of
the group constructively combines with that from the other members, the use of the
codes may be synchronised i.e. the processor 206 of each group member should
activate the code in coordination (e.g. substantially aneously) with the other
members of the group. This could be achieved in a number of ways; for example, the
clocks 210 of each frequency modulating devices 118 could be onised, and the
devices 118 configured to activate the code at a predetermined time.
By coordinating the frequency modulated signal from a group of s, it is
le to produce a magnitude of signal that is more readily detectable by the
measurement device 120.
The group of power units 119 (and/or their corresponding frequency
modulation devices 118) may be grouped on the basis of their location such that
power units 119 in a particular area or region, or at a particular location in the electric
power grid 100 are collectively modulated to propagate a collective frequency
modulated signal at the same. Grouping power units 119 (or their associated
frequency modulation devices 118) in this way enables the measurement device 120
to determine local frequency response characteristics for that particular area or region,
or location in the electric power grid 100.
In order to enable a frequency modulated signal propagated by a given group
to be distinguished from the background variation in grid frequency and/or from
frequency modulated signals from other groups, as mentioned above, the
measurement device 120 may store, in a data store 304, codes (hereinafter referred to
as identifiers) each ated with one or more frequency modulation devices 118
and the frequency modulation devices 118 may include information relating to their
respective identifier in frequency modulated signals propagated in the electric power
grid 100.
Each frequency tion device 118 in a given group may be provided with
data indicative of the identifier ated with that frequency modulation device 118,
and may generate control signals according to which each of the power units 119 in
the group is modulated that correspond with or include the identifier assigned to that
group. In this way the frequency modulation devices 118 include the identifier in
ncy modulated signals that are propagated in the electric power grid 100. To
identify the group of power units, the measurement device 120 may access a database
(for example, in a data store 304 of a measurement device 120) storing one or more of
the identifiers associated with the group of power units 119 (or their associated
frequency modulation devices 118) and determine a pondence between the
identifier included in a measured collective ncy ted signal and one or
more of the identifiers stored in the database. For example, the measurement system
device 120 may correlate the frequency modulated signal (including the fier)
with its stored identifiers to perform this identification.
In order to enable the measurement device 120 to distinguish n
frequency modulated signals from different groups, or from different specific areas, it
is useful for the identifiers to be orthogonal or quasi-orthogonal, and result in
onal or quasi-orthogonal frequency modulated signals; that is, a respective
pattern associated with a given group is not correlated with patterns associated with
other groups, or is only very weakly correlated therewith. Moreover, the codes
associated with different groups or ent areas of the grid may be orthogonal or
quasi-orthogonal.
The control patterns may be programmed into each ncy modulation
device 118 at the time of manufacture or installation. Alternatively or in on,
control patterns may be provided to the power units’ frequency modulation devices
118 via a communication network by the measurement device 120. For example, the
control patterns may be provided as an update to an existing control pattern stored in
the data store 204 of the frequency modulation device 118.
The control patterns may include a repeating n, according to which the
frequency modulation device 118 continuously controls power flowing to and/or from
the power unit 119. Alternatively, the frequency modulation device 118 may control
power flowing to and/or from the power unit 119, according to the l pattern,
intermittently.
Frequency modulated signals propagated by each of the frequency modulation
devices 118 or groups of frequency modulation devices 118 may be separated (to be
orthogonal or quasi-orthogonal in time) based on a time difference. Each of the
groups of ncy modulation devices 118 may start propagating a frequency
modulated signal in the electric power grid that is measurable by the measurement
device 120 at a random start time. For example, frequency modulation s 118
located in different phical areas may be arranged to, or requested to, ate
a frequency modulated signal within the electric power grid 100 at a particular time or
within a particular time-frame. In order to prevent each of the frequency modulation
devices 118 propagating a frequency modulated signal at the same time, the
measurement device 120 may be arranged such that distributed frequency modulation
devices 118 each have sufficiently different modulation start times to each of the other
frequency modulation devices 118. For example, the frequency modulation devices
118 may be arranged to add a random time delay to the time at which they receive a
t to propagate a frequency modulated signal. This increases the likelihood that
the frequency modulated signals by each of the frequency modulation devices 118 is
measured by the measurement device 120 at sufficiently separated times (that is, times
separated by more than the length of time it takes to propagate the signal) so that the
measurement device 120 can distinguish n signals propagated by different
ncy modulation devices 118.
Each frequency modulation device 118 may determine the random time delay
based on information that is unique to that frequency modulation device 118. For
example, the random time delay may be determined based on a serial number of the
frequency modulation device 118. This s the likelihood of two or more
frequency modulation devices 118 using the same time delay, and therefore tates
separation at the measurement device 120 of signals propagated by different
frequency modulation devices 118.
An e method to determine frequency response characteristics, for
example, inertia or stiffness, of given area of a grid at a given time is presented with
reference to figure 5.
Figure 5 shows a power unit 119 connected to part of a synchronous electric
power grid 504 via a modulation device 118. The tion device 118 may be the
same as that shown in figure 2a, but in figure 5, only the data store 204 and modulator
208 components are shown for clarity. The data store 204 of the modulation device
118 has stored within it the code 502 assigned for use with the group of power unit(s)
119 and this code 502 is transmitted to the modulator 208 to produce a power
modulated signal, which is added to the power flow of the rest of the grid at the point
506 at which the modulation device connects to the grid.
As bed above, the power modulation gives rise to a corresponding
frequency modulation. These modulation signals propagate h the measurement
area of the grid 508. hstanding the attenuation of the s with distance, and
any filtering, these signals propagate, in principle, over the entire synchronous grid.
The frequency response characteristics associated with the measurement area
508 give rise to a modification of the frequency modulated signal as illustrated by 510
and, for example, figure 4c.
The modified frequency modulated signal 510 is measured and processed by
measurement device 120. The measurement device 120, which may be the same as
that ted in figure 3, is shown in figure 5 to be comprised of a detector 302, an
analogue to digital converter 512, a data store 304, and a correlator 514. (Note that the
correlator 514 need not be a dedicated device and may be implemented as part of the
processor 306 shown in figure 3, for e as software or firmware.)
The detector 302 detects the frequency modulated signal and feeds it into the
correlator 514 via the analogue to digital converter.
As described above, the modulated signal includes an fier identifying the
power unit(s). The measurement device 120 es the data store 304 storing
identifiers and determines a correspondence between the identifier included in the
modulated signal 510 and one or more of the identifiers stored in the data store 304,
thereby identifying the power units. On identification of the power unit(s) the
measurement device 120 accesses the data store 304 to establish the associated power
flow characteristics used to generate the detected frequency modulated signal 510.
The aforementioned power flow characteristics may include, for example, any
of: the consumption and/or provision capacity of the unit(s), the magnitude of the
power flow change for an individual unit, the total magnitude of power flow change
for a group of units, the power modulation magnitude as a on of consumption
and/or provision capacity of the unit(s), details of al, electrical, unit class and/or
geographic attributes, an indication of whether the unit(s) are providers or consumers
of electrical power, the time duration of each power flow level used in modulation,
the rise and/or fall time associated with switching between different levels of power
consumption and/or provision during modulation, the precise form of the tion
used for power flow tion of the unit(s), the reactive power contribution to the
ted power (e.g. the capability to vary the power ).
The identifiers and the ated power flow characteristics stored in the data
store 304 may be set and updated, for example, by control pattern, user interface,
ications interface, factory setting or any other means. Alternative means of
communicating the type, grouping and or power flow characteristics of the unit(s) to
the measurement device 120 may also be, for e, by user interface,
communications interface, or y setting.
As a result of aforementioned identification of power unit(s) and
establishment of power flow characteristics, the ement device 120 constructs a
copy of the transmitted power modulated signal.
The digitised detected frequency modulated signal and the copy of the
transmitted power modulated signal are fed into the correlator 514.
The correlator 514 then correlates the two signals and fits parameters of a
function h(t) that describes the effect of the measurement area of the grid 508 on the
detected frequency modulated signal for the given power flow modulation, that is, h(t)
describes the impulse response of the measurement area of the grid.
The fitted parameters which parameterise h(t) may include, for example, hmax,
the maximum value of h(t), which characterises the magnitude of grid frequency
change for a given power balance change, and as such characterises the stiffness of
the measured area of the grid. The parameter hmax may be an absolute magnitude of
grid stiffness, and, for e, be ascribed the units Hz W-1.
The fitted ters of h(t) may also include, for example, tfall, a
characteristic time constant of an exponential decay function fitted to h(t) at values of
t occurring after hmax. This tfall characterises the lag of the grid frequency change
behind an impulse of power flow balance, and as such is characteristic of inertia of the
measured area of the grid. This tfall may algebraically manipulated and normalised as
necessary to provide an absolute magnitude of local grid a, for example with
units of kg m2.
The fitting of the parameters of the impulse response function h(t) by the
measurement device 120 in the illustrative e above therefore provides for the
aneous determination of various frequency response characteristics, including
for example inertia and stiffness, for a given area of a grid at a given time. It should
be noted that the above rative example is a non-limiting e of an
ment of an aspect of the current invention. For example, certain steps in the
above example need not necessarily take place in the order presented, and may also
occur aneously.
The electrical power referred to above can be real and/or reactive power. The
modulation of real power flow may result from modulation using a purely resistive
load such that the phase difference between (alternating) voltage and current remains
close to zero. The magnitude of modulation in this case relates to a variation of real
power flow. In the case of a variation in reactive power, the magnitude of modulation
may relate to a magnitude of a reactive contribution, which can be varied by varying,
for example, the phase difference between voltage and current, or the power factor.
Alternatively, or in addition, modulation using a combination of real and reactive
power modulation may be used.
A frequently updated grid stiffness characteristic may be useful, for e,
for grid management, both since a significant change in the grid stiffness indicates
that there has been a significant change in the capacity of the grid in te terms,
relative terms and/or national or regional (e.g. distribution networks) terms, which
may alert the grid operator to take steps towards identification and correction of the
change, and also to enable efficient monitoring of the actual results of ed
changes to grid stiffness the grid operator may wish to make.
Conventionally, actions to d to changes in grid balance (to control the
grid frequency to within agreed limits) are triggered by the grid ncy reaching a
threshold value, as measured by e.g. locally buted measurement devices. For
example, in the UK the conventional trigger point for responding to a reduction in
grid frequency is 49.8 Hz. As described below with nce to figure 7, the time
taken to reach such a conventional trigger point, and therefore to begin ding to
a change in grid frequency, is typically of the order of several seconds.
In ance with an aspect of the invention, in order to reduce the time taken
to respond to such changes in grid balance, in some embodiments, locally ed
frequency characteristics may be analysed to enable early identification of significant
changes in the grid frequency. This analysis may be performed at the measurement
device, for example, based on data collected from meters at local devices, as
described below. The analysis may be performed by fitting a mathematical function,
such as a mial extrapolation function and/or conic extrapolation function, to a
series of values of the measured frequency characteristic (for example, measured at a
series of times over a time interval). This may involve using a “sliding interval”
approach to fit the function to a first series of values of the measured frequency
characteristic covering a first interval. The interval is then moved to fit the function
to a second series of values of measured frequency characteristic covering a second,
later, time interval. The mathematical function has associated coefficients or
parameters that define the form of the function. Therefore, for each time interval, the
associated cients can be determined, and the coefficients for one time interval
can be compared with the coefficients for another time interval to determine r
there is a change in the form of the fitted mathematical function. Such a method of
identifying changes in the measured frequency characteristic is advantageous in that it
typically enables changes to be detected earlier than they would otherwise be detected
using a simple old value comparison.
Further, different ing can be given for measured frequency
characteristics in ent time intervals, such that, for example, the largest weighting
is given to measurements in the most recent time interval. This weighting procedure
can act as a filter in that it can decrease the influence of spurious ents of the
measured frequency characteristic on the determination of the coefficients of the fitted
function.
Fitting of the values of measured frequency characteristic also enables
extrapolation of future values of the frequency characteristic which in turn enables
forecasting of the amount of resources in the electric power grid 100 that will need to
be utilised to react to a detected or predicted change.
Furthermore, by analysing the cients of a fitted mathematical function,
rather than making a comparison with a fixed threshold, it is possible to pate
significant changes to the measured frequency teristic in areas that have
different local frequency response characteristics based on relatively small frequency
level changes.
In a particular example, as shown in figures 6a to 6f, which show variations of
frequency with time t as measured locally, for example at a power unit 119, values of
the ed frequency characteristic are fitted with a second order polynomial
function. The functional form of the second order polynomial function is at2+bt+c
and the parameters defining the form of the function are the coefficients a, b, and c.
The polynomial function is fitted to the frequency characteristic measurements for
each time interval successively, where “t = 0” for the purposes of fitting is
successively redefined to a consistent point within each successive time interval. In
this exemplary implementation, changes in the grid frequency can be identified by
determining changes in the values of a, b, and c.
Figure 6a shows a measured frequency characteristic over a period of ten time
intervals, labelled 1 to 10. It can be seen that over the course of the 10 time intervals
there is a change in the frequency teristic. In particular, the frequency
teristic is stable during time intervals 1, 2, and 3 and then begins to reduce in
value in time al 4. The rate of change of the frequency teristic increases
slightly to a maximum rate of change at interval 6 and then the rate of change
decreases to al 10.
Figures 6b to 6f show the fitting of a second order polynomial function to the
measured frequency characteristic shown in Figure 6a.
During time interval 2 (Figure 6b), the ncy characteristic is stable such
that the fitted polynomial function reduces to a linear function whose gradient close to
zero.
During time interval 4 (Figure 6c), the ncy teristic begins to
decrease. In this interval, the frequency characteristic measurements may be best
fitted with a polynomial function which describes an inverted parabola, as indicated
by the dashed curve. This inverted parabola maybe characterised, for example, by a
negative value of cient “a”. During time interval 5 (Figure 6d), the rate of
change (rate of decrease) of the frequency characteristic increases. Therefore, for
example, the ncy characteristic ements of interval 5 (Figure 6d) may be
best fitted with a polynomial describing a sharper inverted parabola with a steeper
nt over the interval. This steeper gradient might be characterised, for example,
by an increase in the magnitude of cient “b”.
During time interval 6 (Figure 6e), the ncy characteristic reduces
further, but in a substantially nic manner, and therefore may be best fitted with
a linear function. Linear functions have coefficient a = 0, which also marks a point of
inflection in the notional onal form of the ncy characteristic.
During time interval 8 (Figure 6f), the frequency characteristic is passed a
point of inflection and the rate of change of the frequency characteristic is decreasing.
Accordingly, the frequency characteristic measurements may be best fitted with a
non-inverted parabola. This non-inverted parabola may be characterised, for example,
by a positive value of coefficient “a”. It can be seen from the above-described
example that by comparing the coefficients of a polynomial function fitted to
measured frequency characteristic values for one time interval with the coefficients
for a subsequent time interval, for example, it is possible to detect significant changes
in the form of the fitted function such as the onset of a decrease (or indeed increase)
of the ncy characteristic (by detecting that the coefficients have a non-zero
value), a change in the rate of change of the frequency characteristic (by detecting a
change in the magnitude of the coefficients) and a turning point or point of inflection
in the frequency characteristic (by detecting a change in sign of one or more of the
coefficients).
Furthermore, by determining how the cients of the polynomial function
change between time als it is possible to extrapolate the amount by which the
frequency teristic is likely to change. lly, an accurate estimate of the
total decrease (or increase) of the frequency characteristic can be made as the
frequency characteristic approaches the turning point (Figure 6e); this typically
corresponds with a time following the onset of the decrease of the frequency
characteristic of about 500 ms, which is a icantly shorter time frame than the
time taken to reach a threshold value (for example, on the order of a few seconds).
The determined regional inertia teristics may be used to determine
parameters that may in turn be used to trigger some response to the change in grid
frequency. For e, in response to a change in the grid frequency it may be
desirable to send a signal, from the ement device 120 to control a group of
power units 119 to change their power consumption and/or provision behaviour.
If there is a sudden drop in grid frequency (due to a sudden loss of generation
capability or sudden increase in power consumption, for example), it may be desirable
to send a signal to control power consuming power units 119 in the group to cease
operating and/or to control power providing power units to start operating, to reduce a
net power ption in the electric power grid 100 and therefore restore the
balance of power consumption and generation and consequently restore the grid
frequency to its nominal level.
If however there is a sudden rise in grid frequency (due to a reconnection of a
generation resource or a sudden reduction in power consumption, for example), it may
be desirable to send a signal to control power consuming power units 119 in the group
to begin ing and/or to control power providing power units to cease operating to
increase a net power consumption in the electric power grid 100 and therefore restore
the balance of power consumption and generation and consequently restore the grid
frequency to its nominal level.
It should be noted that these power units 119 whose power consumption
and/or ion changes as a response to a change in grid frequency need not be the
same power units whose power consumption and/or provision is modulated as part of
the frequency response characteristic measurement system. Indeed, it may be that
power units whose power consumption and/or provision changes as a response to a
change in grid frequency generally are selected to have more aggregated power
available than those whose consumption and/or provision is to be modulated as part of
the measurement system, in order that a change in their power consumption and/or
provision behaviour should have a more substantial impact on the restoration of grid
frequency to its nominal level.
The ement device may derive, based on measured frequency response
characteristics associated with an area of the electric power grid 100, a triggering
condition relating to a state of the grid frequency when restorative action should be
taken. The triggering condition may be a level of the measured frequency
characteristic itself or may, for example, be based one or more parameters, such as
parameters relating to a fitting function applied to the measured frequency
teristic, or s to those parameters, as described above with reference to
Figure 6.
Figure 7 is a plot showing an exemplary frequency response of an electric
power grid 100 to a sudden shift in power balance. The particular data shown in
Figure 7 s to a sudden disconnection at an interconnector neighbouring electric
power grids. The graph of Figure 7 shows the ncy se ed in 3
regions of one of the grids; namely Area A, Area B and Area C. Each of these
regions has a different mix of power generation, and therefore different s of
grid inertia.
A hypothetical trigger point of 49.8 Hz, representing the frequency at which
action is taken to respond to a frequency change in the grid, is shown in Figure 7. It
can be seen that following a sudden change in the grid balance, the time taken to reach
the trigger point is of the order of several seconds. In the case of the particular event
depicted the time taken to reach the conventional trigger point is 3s.
In Area A, which has a relatively high inertia due to a relatively high
proportion of spinning tion, the frequency responds to the same change in grid
balance but at a much d rate of change.
In Area B there is a larger proportion of spinning generation than there is in
Area A and therefore the response to the sudden change in grid balance, while
immediate, is much smoother.
In Area C there is a relatively low proportion of so-called “spinning”
generation; that is generation by conventional large-scale power stations that store
relatively large quantities of mechanical energy in their associated turbines.
Consequently, the response to the sudden change in grid balance is immediate and
rapid.
It can therefore be seen that the nature of the initial frequency se of a
synchronous electric power grid to sudden s in grid balance varies regionally
depending on the local grid inertia. ingly, the triggering conditions d in
different regions needs to be different in order to enable consistent triggering of a
response. For example, in areas with relatively high inertia, a relatively small l
change in frequency may indicate a vely large forthcoming change in frequency,
whereas the same forthcoming change in frequency may be indicated by a relatively
large change in frequency in an area with relatively low inertia. Accordingly, based
on determined inertia values for ent areas of the grid as described above, and/or
other frequency se teristics such as, for example, grid stiffness, the
ement device 120 may determine ent conditions relating to the
coefficients of the polynomial described above in relation to Figures 6a to 6f, on the
basis of which a restorative change in power consumption and/or provision of the
corresponding group of one or more power units is to be performed, as is described
below. It should be noted that an inertia characteristic is an especially useful
frequency response characteristic, since it s prediction of the time dependence
of a frequency characteristic response, and hence useful trigger conditions may also
be derived solely on the basis of an inertia characteristic. It should be noted that a
stiffness characteristic is also a useful ncy response characteristic for ing
trigger conditions, since it can inform a prediction of the extent to which the measured
frequency characteristic may change given a likely or common power balance change.
As a specific example with reference to Figure 7 and the specific case of the
response of the grid frequency in Area C, the trigger condition could, for example, be
satisfied if the “a” coefficient of the fitted polynomial was still negative after the
initial reduction in frequency of 0.08 Hz in 500 ms. The negative “a” coefficient
indicates the reduction in frequency has not reached a point of inflection, and hence is
likely to continue to reduce at a faster rate, which, when coupled with the high inertia
of the grid in Area C, indicates a frequency change event significant enough to satisfy
the trigger condition.
In some embodiments, a local measurement of a frequency characteristic may
be made by a ement instrument (such as a phasor) associated with one or more
power units 119 in a group of power units 119; the group of power units referred to
here may be the same group as those referred to above as generating the modulated
frequency signal, or it may be a ent group of power units 119. In this case, each
ement instrument may include a communications interface for icating
the measured frequency teristic to the measurement device 120. The
measurement device 120 may then ine whether a measured frequency
characteristic, received from the measurement instrument, satisfies a determined
triggering condition and, in response to determining that the triggering condition is
ied, the ement device 120may send a request to one or more of the power
units 119 in the group to change their consumption or provision of power.
Alternatively, the measurement device 120 may, transmit parameters for
receipt at the power units 119 of the group, the parameters relating to a g
function applied to the measured frequency characteristic and determined based on the
ined frequency response characteristics. The parameters may then be received
at the power units 119 of the group and used to derive a trigger condition. Whether
the trigger condition is satisfied can be determined, at the power units 119 of the
group, based on a frequency characteristic measured locally to the group. In response
to a determination that the trigger condition is satisfied, power flow to and/or from
each of the power units 119 in the group may be changed.
To change the flow to and/or from the power unit 119, the power unit 119 may
have an associated power control device. An exemplary arrangement of a power
control device 800 is shown in Figure 8. The power flow control device 800
cooperates with an associated power unit 119 in the same, or a similar way as the
frequency modulation device 118 described above with reference to Figure 2. As
with the frequency modulation device 118, the power control device 800 may be
external to the power unit 119 or may be integrated with the power unit 119. In some
es, the functions of the frequency modulation device 118 and the power
control device may be performed by a single device.
The power control device 800 forms an interface between the electric power
grid 100 and one or more power units 119 and operates with the one or more power
units 119 to change power flow to and/or from the electric power grid 100. The
power control device 800 comprises a detector 802, a data store 804, an input/output
(I/O) interface 806, a processor 808, and a switch 810.
The detector 802 may be any device capable of detecting or measuring a
characteristic relating to the grid frequency with sufficient ion.
In some embodiments, a time period relating to the grid frequency is used as a
characteristic measure of the grid frequency. For example, a measurement of the half-
cycle, which is the period between times at which the voltage crosses 0V, may be
used as a teristic ng to the grid frequency.
In some embodiments, the actual instantaneous grid frequency, corresponding
to the inverse of the time it takes to complete a half-cycle (or a ycle) may be
determined. The frequency data may be equalised and digitally filtered to remove
frequency components outside a known and desired range of signal frequencies. For
example, frequency components corresponding to the grid frequency and/or ncy
ents relating to noise may be removed.
In one embodiment, the detector 802 may comprise a voltage detector
arranged to sample the e at a frequency higher than the grid frequency and an
analogue to digital converter arranged to convert the sampled voltage to a digital
voltage signal. For example, the voltage detector may be arranged to sample the
voltage 1000 times per cycle. The digital voltage signal may then be processed to
determine with a high degree of precision (within the range µs to ms) the times at
which the voltage crosses 0V.
In another embodiment, the detector 802 may comprise a current detector
ed to sample the current at a frequency higher than the grid frequency, and an
analogue to digital converter arranged to convert the sampled current to a digital
t signal, which may then be processed to ine with a high degree of
precision (within the range µs to ms) the times at which the current crosses 0V.
In still another embodiment, the detector 802 may comprise both a voltage
detector and a current detector. Measuring the times at which both the voltage and
current crosses 0V enables the detector 802 to determine a change in the relative
phase of the voltage and current, thereby enabling the detector 802 to compensate for
s in reactive power in the grid. This in turn enables a more accurate
measurement of frequency (or a characteristic relating to frequency).
In addition to, or as an alternative to, measuring the grid frequency, the
detector 802 may measure a rate of change of frequency of power flowing in the grid
based on measurements of voltage and/or t, as described above.
The detector 802 may include phasor measurement instrumentation arranged
to measure said frequency characteristic on the basis of a phasor ement, in
which a phase associated with a vector of voltage measured in the electric power grid
is measured with reference to an te time reference.
The I/O interface 806 of the power control device 800 enables communication
between the power l device and the measurement device 120. The power
control device 800 intermittently receives one or more parameters derived, as
described above, from ncy response characteristics of the electric power grid
100. In particular, the power control device 800 receives ters that are derived
based on frequency response characteristics specific to the area or on in which it
is located so that the power control device 800 can derive, on the basis of a received
parameter, a trigger condition that it specific to the location or area in which it is
operating. The trigger condition may be the received one or more parameters
themselves or may be some other condition derived from the received one or more
parameters.
Data relating to the trigger condition and/or the received parameters may be
stored in the data store 804. Similarly, measured frequency characteristic of electric
power flowing in the ic power grid 100 may be stored in the data store 804.
The data store 804 may also store identification data relating to the power
control device 800, the power unit or units 119 with which the power control device is
associated, or groups to which the power unit 119 or units 119 belong. It should be
noted that, although the identification data is described above as being stored in the
data store 804 of the power control device 800, in some embodiments, the codes may
be itted to the power control device 800, for example from the ement
device, in which case they may not be stored at the power control device 800, or
stored only in a temporary data store. The identifier stored in the power control device
800 may be prescribed at the point of manufacture or installation of the power control
device 800, or it may be communicated to the power control device 800 via the I/O
interface 806.
The processor 808 accesses the data store 804, to access the data relating to
trigger ion and, based on a measured frequency characteristic (which may also
be accessed from the data store 804), determines whether the trigger condition is
satisfied.
In response to a determination that the trigger condition is satisfied, the
processor 808 sends a control signal to the switch 810 for controlling the flow of
power to the power unit 119.
The switch 810 may be a simple relay device which turns power supply onand
off in response to a l signal from the processor 808. Alternatively or
additionally, the switch 810 may comprise an ator or a phase inverter, etc. used
to attenuate real or reactive power flowing to or from the power unit 119. The action
of the switch 810 thus provides a change to the power flowing to or from the power
unit 119 which has a corresponding effect on the grid balance and accordingly has a
corresponding effect on the grid frequency. The amplitude of the effect depends on
the power consumption of the power unit 119. In order to coordinate the power
control devices such that a combined change in the net grid e is sufficient to
maintain the grid frequency within (or restore the grid frequency to within) agreed
limits, groups of one or more power control devices may each respond to a
determination that the trigger condition is satisfied and therefore each change the
power flow to or from their tive power unit 119 or power units 119.
There may be different rules stored in the data store defining the extent,
on and scheduling of attenuation of the power flow to the unit 119 following a
determination that the trigger condition is satisfied. These rules may include
conditions on a measured frequency characteristic, for example that the attenuation of
power flow is maintained for as long as the frequency characteristic is outside a
predefined range centred on the nominal grid frequency. The duration and extent of
ation may also be based, for example, on characteristics indicating the severity
of the frequency characteristic change. There may be rules ng ling, for
example, relating to times when an attenuation is permitted to take place. These rules
may be stored in the data store 804 for the processor 808 to access when the trigger
condition is satisfied, or at other times. These rules held in the data store 804 may be
updated from time to time via the I/O ace 806 and communications network.
Additionally, or alternatively, these rules may be prescribed at the point of
manufacture or installation of the device.
Although the power control device 800 is shown in Figure 8 as a separate
device to the power unit 119, in some cases the power control device may be
ated in the power unit 119, Further, the switch 810 is not necessarily d
exterior to the power unit 119, but may instead be installed in the unit, and be
arranged to control power supply from the interior of the device; this latter case is
advantageous where the power unit 119 may move from location to location, as is the
case, for example if the power unit is a Personal Electric Vehicle or other device.
The power control device 800 may also be arranged to process measured data.
For example, the processor may execute a computer program stored in the data store
804 that it configured to fit the measured frequency teristics, as described above
with reference to figures 6a to 6f.
The measurement device 120 described above with reference to Figure 3, may
be employed in combination with a distributed plurality of power control devices 800
to form a system for responding to s in the grid frequency. In such a system,
the measurement device 120 determines a frequency response teristic
associated with each area of the electric power grid 100, determines a trigger
condition for each area and transmits area specific trigger condition to each of the
power control devices.
The power l devices 800 may each measure a local frequency
characteristic and it data indicative of a polynomial function representative of
the local frequency characteristic to the measurement device 120. The ement
device 120 may then extrapolate, based on the polynomial function, future expected
values associated with the frequency characteristic to determine, for example, an
expected power flow requirement for responding to an expected change in grid
frequency.
It will be tood that in some implementations, the measurement device
120 may receive the frequency characteristic itself and ine a polynomial
function fitting the measured ncy characteristic. Furthermore, it will be
understood that in some implementations the power control devices 119 may perform
the extrapolation of the future expected values and/or the determination of the
ed power flow requirement, and may transmit this information to the
measurement device 120.
The measurement device 120 may access a database storing profile
information ng to power ption and/or provision characteristics of power
units 119 connected to the electric power grid 100. The measurement device 120 may
use the profile information to define one or more groups of power units 119 based on
the profile information and the expected power flow requirement, such that the groups
of power unit 119 have a net power consumption and/or provision capacity capable of
satisfying the power flow requirement and transmit signals to the defined one or more
groups ingly.
The measurement device 120 may, for example, transmit requests or
commands to the power control devices 800 of the defined groups to control
consumption and/or provision of electrical power by the power units 119 associated
with the power l devices 800. In this way the system can increase or decrease a
net consumption of electrical energy in each of the areas of the ic power grid
100 in response to changes of ncy in those areas.
In a further example embodiment, a system for providing a dynamic response
to a change in grid frequency is ed. The system comprises a measurement
device, such as the measurement device 120 described above, arranged to access the a
database, such as the power unit data store 304 described above, comprising, for
example, profile information relating to the consumption and/or provision of power
by the power units 119 and/or their location or grid on.
The measurement device 120 is then arranged to define, on the basis of the
profile information, one or more groups of power units 119 associated with power
control devices 118. For example, there could be three groups defined for a given grid
area, each with a similar number of power units of a similar class. The system is then
arranged to assign ent trigger conditions to each of the different groups and
transmit the assigned trigger conditions to the groups.
The trigger conditions could be derived from measurements of frequency
response characteristics such as, for example, related to local grid inertia and stiffness,
as described above.
The trigger conditions may be set such that one or more groups may have their
r conditions satisfied simultaneously, or near simultaneously, in order to provide
a response commensurate to a given frequency characteristic change event. In an
example, the trigger conditions for the three groups of power units could be set to
correspond to a low, medium, and high trigger sensitivity. For only a minor frequency
characteristic change event therefore, only the group with high sensitivity trigger
condition may have a satisfied trigger condition and therefore o a change in
power flow to/from the associated unit. For a medium severity ncy
characteristic change event, both the group with the high ivity trigger ion,
and the group with the medium sensitivity trigger condition may have a ied
trigger ion and therefore change the power flow to/from their associated power
units. For a severe frequency characteristic frequency change event, all three groups,
including the group with the low sensitivity trigger condition, may have a satisfied
trigger condition and therefore change the power flow to/from their associated power
units. The satisfaction of the trigger conditions of the different groups may be
simultaneous, near simultaneous, or offset in time from one another.
Alternatively, the triggering conditions of the different groups may be set such
that during a frequency characteristic change event, groups may be sequentially
red to provide a commensurate se to the frequency characteristic change
event. For example, the triggering conditions may be set such that if a frequency
characteristic change event is not iently corrected by an initial response provided
by the triggering of a first group, a further group may be triggered to enable an
enhanced se towards correction. Further groups may be further triggered up
until a predefined limit of response, or until the frequency characteristic has been
corrected to its nominal value, for example.
The groups of power units need not necessarily be of similar number or class.
The power class of each group may be d, for example, to achieve a desired
dynamic response. For example, a group defined including units of a high power flow
class may be assigned a r condition satisfied by a high severity event. Also, for
example, the number of power units in each group could be changed, or the number of
groups may be changed to provide for different forms of dynamic response.
In such a way, groups of power units in a given area connected to power
control devices may provide a ed dynamic correction response to a frequency
change event, and after an initial communication of ed trigger conditions, may
do so autonomously. This is advantageous since it provides for a corrective response
to a frequency change event that is commensurate to the event, and hence avoids
changing power flow to/from units unnecessarily.
The above embodiments are to be understood as illustrative es of the
invention. Further embodiments of the invention are envisaged. For example,
gh in the above description, trigger conditions are d or defined in the
measurement device 120, it will be understood that in some implementations the
power control device itself may be capable of measuring or determining a local
frequency response characteristics and may, on the basis of those measured
characteristics, derive appropriate trigger conditions. The power control device 800
may measure a frequency characteristic and determine whether the frequency
characteristic satisfies the trigger condition and, in the event that it does, the power
l device 800 may change the real and/or reactive power flowing to and/or from
its associated power unit or units 119 without reference to the measurement device
120.
In a further embodiment, an autonomous power control device 800, which
“learns” an appropriate trigger ion for the grid area in which it is placed, is
arranged to measure, at detector 802, a frequency characteristic of electric power
flowing in the electric power grid 100, ine whether the measured frequency
teristic has satisfied a ring condition, and in response to a determination
that the trigger condition is satisfied, change, using switch 810, a power flow to and/or
from the one or more associated power units 119 for a n amount of time.
The amount of time for which the power flow should be changed, as discussed
above in other embodiments, can be, for example, factory set, set by a user ace,
or dependent on the grid frequency returning to within a predefined limit of its
nominal value.
In order to determine whether the trigger ion has been satisfied, any of
the s described in the above ments may be used, such as, for example,
monitoring of parameters of polynomial functions fit to successive measurements of
frequency teristic.
More simply, a trigger condition may set to be satisfied if the measured
frequency response characteristic is measured to have changed to a certain extent in a
certain amount of time.
The ring condition in an l “out of the box” instance may, for
example, be factory set or set by user interface. There may also be no set trigger
condition in the first instance.
The autonomous power control device 118 is arranged to analyse the measured
frequency characteristic measured at times around the point of satisfaction of a
threshold condition of measured frequency characteristic. This threshold condition
may be set such that it is satisfied when the measured frequency characteristic is
outside a range set about a nominal value. The analysis times may be set to, for
example, a few seconds either side of the time when the threshold condition is
satisfied. The analysis may include, for example, fitting of polynomial or exponential
functions to the measured frequency characteristic for the set time window around the
satisfaction of the threshold condition. Other analysis could determine, for example,
teristics relating to the total extent of measured frequency characteristic change,
the total time for the measured frequency teristic to change between two
substantially stable values, and/or the average rate of change of the measured
frequency teristic between the two substantially stable values.
In any case, the autonomous power control device 800 is arranged to derive
one or more parameters from said analysis characterising the change in frequency
characteristic around the point of satisfaction of the threshold condition.
The autonomous power control device 800 is ed to then derive a r
condition based on the parameters derived from said analysis. It may be, for example,
that the analysis ined that the measured ncy characteristic changed a
relatively large amount in a relatively short amount of time. (This may be the case in
an area of the grid characterised by a relatively low inertia and a relatively low
stiffness, for example.) The r condition may therefore, for example, be set to be
satisfied by changes in ed frequency characteristic corresponding to a
relatively small change over a given time on.
The autonomous power control device 800 is arranged to update any existing
trigger condition used previously in the power control device 800, with the trigger
condition d from the analysis. If no trigger condition was set previously in the
device, the derived r condition may define an initial trigger condition. The
updated trigger ion may be partly based on previously determined trigger
conditions, and/or on statistics associated with these previously determined trigger
conditions. This may help to reduce the effect of spurious determinations of trigger
conditions. For example, a new trigger condition may be formed of an average of all
trigger ions determined by the device. This average, for example, may include
only a set of the latest determined trigger conditions, for example, the last ten, so that
the trigger condition may also easily adapt to changes in the grid nature of the area of
the grid to which it is local.
The learning algorithms described above should be taken as examples
only. Relatively simple algorithms such as those described above may be
advantageous in that they require relatively little computing resources. However,
other, for example, more complex, algorithms are envisaged. ically state of the
art Self-Organizing Maps (e.g. Kohonen SOM) and other neural network and/or
artificial intelligence algorithms may be used.
The autonomous power control device 800 is arranged to repeat the above
ing, deriving and updating for every sive change in measured frequency
characteristic that results in the threshold condition being satisfied. The autonomous
power control device 800 therefore “learns”, by its own analysis of the nature of the
frequency characteristic change events, the trigger condition that is most riate
and effective for the area of the grid in which it is ed.
The threshold condition used in each learning repetition may be linked to the
trigger condition, for example if an updated trigger condition is more sensitive than
the preceding trigger condition, then the threshold condition may also be updated to
correspond to a narrower range of frequency characteristic about the nominal value.
The autonomous power control device 800 may be ed to analyse the
measured frequency characteristic at times about a satisfaction of the threshold
condition a certain number times t defining a trigger condition. This would
allow the device to learn parameters for use in derivation of a trigger condition
appropriate for the area in which the device is located before implementing the trigger
condition in the device. This may avoid erroneous trigger condition satisfaction in the
early stages of the deployment of the mous power control device in a given
grid area.
The above bed embodiment of a power control device 800 is
advantageous since it is an autonomous, standalone device which requires no
communication means, and therefore may be relatively cost effective in its operation
and easy to implement.
In various embodiments detailed in the above description, reference is made to
a measurement system in the form of a single measurement device 120. It should be
noted, however, that in some embodiments, a distributed ement system may be
used. This distributed measurement system may comprise, for example, a combination
of the components of the measurement device referred to in fig. 3, namely a data store
304, a clock 310, an I/O interface 308, a processor 306, and a detector 302 arranged in
a distributed way. The measurement system may also include one or more centralised
l units. These lised control units may be used, for example, for centralised
processing of measurements taken by the measurement devices, or any other device
described herein, , for e, for performing the receiving and transmitting
of characteristics, parameters, and/or conditions described herein from and/or to any
of the devices described herein. The centralised control unit may also perform
ons of data storage otherwise implemented in the various devices described
herein. The measurement system may also take the form of a non-distributed device
similar to the exemplary embodiment described with reference to fig 3.
It is to be understood that any feature described in relation to any one
embodiment may be used alone, or in combination with other features described, and
may also be used in combination with one or more features of any other of the
embodiments, or any combination of any other of the embodiments. Furthermore,
lents and modifications not described above may also be employed without
departing from the scope of the invention, which is defined in the anying
claims.
Claims (43)
1. A method of determining, in a measurement system, a ncy response characteristic within a onous area of an electric power grid, electricity flowing 5 in the grid in ance with a grid frequency, wherein the electric power grid is connected to a first group of one or more power units each arranged to consume electric power from and/or provide electric power to the electric power grid such that a change in power provision and/or consumption by said first group of one or more power units results in a change in power flow in the electric power grid, wherein power flow to 10 and/or from each of the power units is modulated on the basis of a sequence of control signals, thereby ting the grid frequency to provide a frequency modulated signal according to the sequence of control signals, the method comprising: measuring, in the measurement system, a frequency characteristic relating to a frequency of electricity flowing in the electric power grid; 15 accessing a database storing data relating to power characteristics of said one or more power units and determining, on the basis f, a characteristic relating to said power flow modulation; and determining a frequency se characteristic associated with at least one area of said electric power grid on the basis of the ed frequency characteristic and the 20 determined power flow modulation characteristic, wherein the frequency response characteristic s to an inertia characteristic and/or a stiffness characteristic of the electric power grid.
2. A method according to claim 1, in which determining the frequency response 25 characteristic ses correlating the measured frequency characteristic with said power flow modulation characteristic.
3. A method according to claim 1, in which determining the frequency se characteristic comprises determining a ratio of said power flow modulation 30 characteristic and the measured frequency characteristic.
4. A method according to any preceding claim in which said power flow modulation characteristic comprises a magnitude characteristic relating to said power flow tion.
5 5. A method according to claim 4, in which said magnitude characteristic comprises an amplitude of the power flow.
6. A method according to any preceding claim, in which said ed frequency characteristic is measured on the basis of one or more of: a frequency of alternating 10 voltage, a frequency of alternating current, a measured frequency of power flowing in the electric power grid; a rate of change of ncy; a period of alternating current or voltage.
7. A method ing to any preceding claim, in which said measured frequency 15 characteristic comprises a time variation in frequency associated with said modulated signal.
8. A method according to any preceding claim, in which said frequency se characteristic comprises an inertia characteristic.
9. A method according to claim 8, in which said inertia characteristic comprises at least one of a rise time and a fall time associated with said ncy modulated signal.
10. A method according to any of claims 1 to 7, in which said frequency response 25 characteristic comprises a characteristic relating to a magnitude of ion in grid frequency per unit change in power balance.
11. A method according to any preceding claim, in which of the first group of power units is a distributed group of power units, the method comprising: 30 modulating power flow to and/or from each of the first group of power units in accordance with a control pattern, such that the ption and/or provision of power by the plurality of power units is coordinated to provide a collective frequency modulated signal, having a collective frequency characteristic, that is able by the measurement system.
12. A method according to claim 11, comprising sending a signal ying said 5 control pattern to each power unit of the first group of power units.
13. A method according to claim 11 or claim 12, in which the control pattern comprises a ing pattern, and the method comprises controlling power to and/or from the first group of one or more power units continuously according to the repeating 10 n.
14. A method according to any of claim 11 to claim 13, comprising controlling power to and/or from the first group of one or more power units intermittently according to the control pattern.
15. A method according to any of claim 11 to claim 14, in which said collective modulated signal includes an identifier identifying said group of power units, the method comprising: accessing a database storing one or more identifiers each associated with said 20 first group of one or more power units; and determining a correspondence between the identifier included in the collective modulated signal and one or more of the identifiers stored in the database, thereby identifying said first group of one or more power units. 25
16. A method according to claim 15, in which each identifier stored in the database is associated with at least one area of the electric power grid and the method ses determining an area with which the determined frequency response characteristic is associated on the basis of the determined identifier correspondence. 30
17. A method according to claim 1, the method comprising: determining, on the basis of the determined frequency response characteristic, one or more ters for use in triggering a change in consumption and/or provision of power by the first group of one or more power units or by a second group of one or more power units, the second group of one or more power units being connected to the electrical power grid and arranged to consume power from and/or provide power to the electric power grid; and 5 transmitting said one or more parameters for receipt at the first group of one or more power units or said second group of power units.
18. A method according to claim 17, comprising: receiving, at the first group of power units or the second group of power units, 10 said one or more parameters; deriving, on the basis of the received parameters, a trigger condition; determining, based on a measured frequency characteristic of electric power flowing in the grid locally to the first group of power units or the second group of power units, whether the trigger condition is satisfied; and 15 in se to a determination that the trigger ion is satisfied, changing a power flow to and/or from the first group of power units or the second group of power units.
19. A method according to claim 17 or 18, comprising: 20 defining, at the measurement system, a first series of values associated with the frequency characteristic during a first time period and a second series of values ated with the frequency teristic during a second, later, time period; determining, at the measurement system, a first polynomial function having a first set of coefficients on the basis of said first series of values and a second mial 25 function having a second set of coefficients on the basis of said second series of ; determining, at the measurement system, whether the trigger condition is satisfied on the basis of a difference between the first set of coefficients and the second set of coefficients.
20. A method according to any of claim 1 to claim 16, in which the electric power grid is connected to a second group of one or more power units arranged to consume power from and/or provide power to the electric power grid, the method comprising: determining, based on the ined frequency response characteristic 5 associated with an area associated with the second group of power units, one or more parameters for use in triggering a change in consumption and/or provision of power by the second group of one or more power units; deriving a trigger ion on the basis of the ed frequency response characteristic; 10 measuring, in the area associated with the second group of power units, a frequency characteristic relating to a frequency of electricity flowing in the electric power grid; communicating the measured frequency characteristic measured in the area associated with the second group of power units to the measurement system; 15 determining, based on the communicated measured frequency characteristic, whether the trigger condition is satisfied; and in response to a determination that the trigger condition is satisfied, g a request to the second group of power units to change a power flow to and/or from the second group of power units.
21. A method according to any preceding claim, in which the power modulation comprises modulation of at least one of real power and reactive power.
22. A measurement system for determining a frequency response characteristic within a synchronous area of an ic power grid, wherein electricity flows in the grid in accordance with a grid ncy and the electric power grid is ted to a group of one or more power units each arranged to consume electric power from and/or 30 provide electric power to the electric power grid such that a change in power provision and/or consumption by said power unit results in a change in power flow in the grid, wherein power flow to and/or from each of the power units is modulated on the basis of a sequence of control signals, thereby modulating the grid frequency to provide a frequency modulated , the measurement system being arranged to: measure a frequency characteristic relating to a frequency of electricity flowing in the electric power grid; 5 access a database storing data relating to power characteristics of said one or more power units and determine, on the basis thereof, a characteristic relating to said power flow modulation; and determine a ncy response characteristic associated with at least one area of said electric power grid on the basis of the measured frequency characteristic and the 10 determined power flow modulation characteristic, wherein the frequency se characteristic relates to an a characteristic and/or a stiffness characteristic of the electric power grid.
23. A power l device for use with one or more associated power units to 15 e a response to changes in a frequency of icity flowing in a synchronous area of an electric power grid, n the electric power grid is connected to a measurement system arranged to determine a frequency response teristic of the grid in said area and to determine one or more trigger parameters on the basis of the measured frequency response characteristic, the power control device being arranged 20 to: intermittently receive one or more parameters from the measurement system, the one or more parameters being derived from a said determined frequency response teristic; derive, on the basis of the received one or more parameters, a trigger condition; 25 determine, based on a measured frequency characteristic of electric power flowing in the grid, whether the trigger condition is satisfied; and in response to a determination that that the trigger condition is satisfied, change a power flow to and/or from the power unit, wherein when the trigger condition being satisfied indicates a rise in grid 30 frequency, changing the power flow comprises increasing a power flow to and/or reducing a power flow from the one or more power units, and when the trigger ion being satisfied indicates a drop in grid frequency, changing the power flow comprises reducing a power to and/or increasing a power flow from the one or more power units. 5
24. A power control device according to claim 23, in which one of the received one or more parameters se said trigger condition.
25. A power control device according to claim 23 or claim 24, in which said frequency response characteristic comprises an inertia characteristic.
26. A power l device ing to claim 23 or claim 24, in which said frequency response characteristic comprises a characteristic relating to a magnitude of variation in grid ncy per unit change in power balance. 15
27. A power control device according to any of claims 23 to 26, arranged to: define a first series of values associated with the frequency characteristic during a first time period and a second series of values associated with the frequency characteristic during a second, later, time period; determine a first polynomial on having a first set of coefficients on the 20 basis of said first series of values and a second polynomial function having a second set of coefficients on the basis of said second series of values; and determine whether the trigger condition is satisfied on the basis of a difference between the first set of coefficients and the second set of coefficients. 25
28. A power control device according to claim 27, wherein the first and second polynomial functions are second order polynomial ons.
29. A power control device according to claim 27 or claim 28, in which the frequency change event is identified on the basis of a value of at least one coefficient 30 of the second set of coefficients differing from a corresponding coefficient in the first set of cients by more than a predetermined amount.
30. A power control device according to any of claim 27 to claim 29, arranged to measure said series of values according to a polynomial extrapolation technique and/or conic extrapolation technique. 5
31. A power control device ing to any of claim 1 to 30 comprising a detector for measuring a frequency characteristic.
32. A power control device according to any of claim 23 to claim 31, comprising phasor measurement mentation arranged to measure said measured frequency 10 characteristic on the basis of a phasor measurement.
33. A power control device according to claim 32, in which the phasor measurement instrumentation is arranged to measure a phase ated with a vector of voltage measured in the electric power grid with reference to an absolute time reference.
34. A power control device according to any of claim 23 to claim 33, wherein the measured frequency characteristic includes one or more of: a frequency of alternating voltage, a frequency of alternating t, a frequency of power flowing in the electric power grid; a rate of change of frequency; and a period of alternating current.
35. A power control device according to any of claim 23 to claim 34, ed to e a signal, said signal indicating a time period during which power flow may be controlled.
36. A power control device according to any of claims 23 to 35, in which the power 25 modulation comprises modulation of at least one of real power and reactive power.
37. A power l device according to any of claims 1 to 36, in which said trigger ion is a level of the measured frequency characteristic or is based on one or more parameters ng to a fitting function applied to the measured frequency 30 characteristic, or changes to the one or more parameters.
38. A system for responding to changes of frequency in an electric power grid, the system comprising: a distributed plurality of power control devices according to any of claim 23 to claim 37, each controlling a respective power unit connected to the electric power grid; 5 and a measurement system for transmitting one or more trigger ters to each of the plurality of distributed power control devices.
39. A system according to claim 38, wherein the ement system is arranged 10 to: define, a plurality of groups of power l devices from said buted plurality of power control devices; assign different respective trigger conditions to each of the plurality of groups; 15 transmit, to each of the power control devices a trigger condition assigned to the group to which it is assigned.
40. A system according to claim 39, wherein the measurement system is arranged 20 access a power unit database storing profile information relating to the consumption and/or provision of power by the power units associated with the power control devices; and define said plurality of groups on the basis of the accessed profile ation. 25
41. A system according to claim 38, wherein the ement system is arranged e data indicative of a polynomial function representative of a measured frequency characteristic; extrapolate, based on said polynomial function, future expected values 30 associated with the measured ncy teristic; and determine, on the basis of the extrapolated future expected values, an expected power flow requirement for responding to the frequency change event.
42. A system according to claim 41, wherein the measurement system is arranged access a power unit se comprising profile ation relating to the 5 consumption and/or provision of power by the power units; define, on the basis of the expected power flow requirement and said profile information, one or more groups of one or more power units for responding to the frequency change event. 10
43. A system according to claim 42, wherein the measurement system is arranged to transmit one or more requests, for receipt at the power control devices of the defined groups, to control ption and/or provision of electrical power by the power units associated with power control devices, thereby g a net consumption of electrical energy in said area. ission Grid 102 Distribution Grid 104
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB1319624.1 | 2013-11-06 | ||
GB1319624.1A GB2515358B (en) | 2013-11-06 | 2013-11-06 | Grid Frequency Response |
PCT/EP2014/073694 WO2015067602A2 (en) | 2013-11-06 | 2014-11-04 | Grid frequency response |
Publications (2)
Publication Number | Publication Date |
---|---|
NZ720073A NZ720073A (en) | 2020-11-27 |
NZ720073B2 true NZ720073B2 (en) | 2021-03-02 |
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