AU2018206230A1

AU2018206230A1 - Power grid stabilising system

Info

Publication number: AU2018206230A1
Application number: AU2018206230A
Authority: AU
Inventors: Guy Nicholson
Original assignee: Statkraft Ireland Ltd
Current assignee: Statkraft Ireland Ltd
Priority date: 2017-01-09
Filing date: 2018-01-09
Publication date: 2019-08-15
Anticipated expiration: 2038-01-09
Also published as: EP3566277A1; AU2018206230B2; GB201700316D0; GB2559949A; GB2559949B; WO2018127609A1

Abstract

The present invention provides a method and apparatus for providing response to frequency fluctuations in a power grid system. The apparatus comprises an energy storage means (1) that is configured to be connected to a power grid, and a bidirectional power converter (4) connected between the energy storage means (1) and the power grid. The power converter (4) is configured to convert current at the grid frequency to current specific to the energy storage means (1).

Description

POWER GRID STABILISING SYSTEM

Technical Field

The present invention is concerned with power grids and apparatus for managing grid stability and providing response to fluctuations in grid frequency.

Background

Electricity networks or power grids have to safely and reliably distribute electricity from supply/generators to consumers/loads/demands. Power grids need to have control systems that keep supply and demand in balance.

The grid frequency, usually 50Hz, 60Hz or400Hz, synchronises all generation and synchronous load on the system and varies due to supply/load imbalance.

As load on the system is constantly varying, the grid frequency fluctuates, although mechanical inertia in the system limits the rate at which the frequency can change.

In conventional power grids, the generators and synchronous loads provide inertia so that the generators only respond relatively slowly to changes in grid frequency rather than stopping suddenly.

In power grids using renewable power sources, such as wind turbines, frequency fluctuations affect the grid even more, because, in contrast to conventional generators, wind turbines, solar panels etc. do not provide any inertia. One solution would be to hold conventional plant on at minimum generation levels, to provide inertia, but this is inefficient and expensive.

There is a need, therefore, to provide apparatus that can provide inertia and/or response to grid frequency fluctuations in an efficient, reliable and cost effective manner.

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-2 Statement of Invention

In one aspect, the present invention provides apparatus for providing response to frequency fluctuations in a power grid system; the apparatus comprising: energy storage means configured to be connected to a power grid; and a bi-directional power converter connected between the energy storage means and the power grid, the power converter configured to convert current at the grid frequency to current specific to the energy storage means.

In another aspect, there is provided a method of providing response to frequency fluctuations in a power grid; the method comprising: connecting energy storage means to a power grid; and converting current at the grid frequency to current specific to the energy storage means.

The energy storage means may be a rotating machine, and the current specific thereto is current appropriate for the speed at which the rotating machine is rotating.

Alternatively, the energy storage system may be e.g. battery, capacitor, super-capacitor, inductor, super-conducting magnetic energy storage system, rotating machine which is not synchronised and including a high speed flywheel, etc. configured to be connected to a power grid. The power converter would then be configured to convert current at the grid frequency to direct current for the examples above with the exception of the rotating machine which is not synchronised including high speed flywheel.

Preferred embodiments of the invention will now be described by way of example only, with reference to the drawings.

Brief Description of the Drawings

Fig. 1 is a schematic diagram of one apparatus that could be used to implement the invention using a rotating machine.

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-3Fig. 2 is a graph showing the response of the apparatus when there is a sudden drop in grid frequency.

Fig. 3 shows a possible steady-state response provided by the invention.

Fig. 4 is a schematic diagram of one apparatus that could be used to implement the invention using an energy storage system which requires direct current.

Fig. 5 is a schematic diagram of one apparatus that could be used to implement the invention using a rotating machine as the energy storage systems.

Fig. 6 is a schematic diagram of one apparatus that could be used to implement the invention using two separate energy storage systems, one of which is a rotating machine which is not synchronised and one of which requires direct current.

Detailed Description

The invention will now be described in relation to a stand-alone apparatus that can be connected to the grid as and when required, to provide inertia and/or response to grid frequency fluctuations. In theory, the apparatus could, however, be permanently connected to, or form part of, the grid.

The apparatus of Fig. 1 provides a synchronous mode of operation and a decoupled mode of operation, described further below. In other embodiments, also described further below, the apparatus operates only in a decoupled mode - i.e. to only provide response without the need to operate in a synchronous mode to provide inertia - in which case the circuit breaker 3 is not required.

The apparatus is made available to be connected to the grid when inertia needs to be added and/or response is required to large frequency fluctuations on the grid, such as due to a sudden increase or drop in demand for power from the grid, a sudden increase or drop in power generation, component failure etc. The apparatus should be such that it can be connected and started up in a short time, e.g. in less than 10 minutes from being called up.

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-4 In normal operation, the apparatus will be connected to the grid whilst the grid frequency is around the nominal operating frequency, and preferably within a so-called deadband where frequency deviations are not too great. At this point, in one embodiment, as shown in Figs. 1 and 2, the apparatus will operate in synchronous mode, adding inertia to the grid.

As can be seen in Fig. 1, in this embodiment, a synchronous machine (motor/generator) 1 is connected to the grid via a circuit breaker 3, or similar e.g. disconnector. In synchronous operation, the circuit breaker 3 is closed so that the synchronous machine 1 is connected directly to the grid.

The additional inertia provided to the system by the synchronous machine 1 means that in the event of a sudden loss of generation, the grid frequency will fall more slowly than would otherwise be the case. This gives more time for other generation to respond and/or for loads to be disconnected to reduce demand on the grid. The added inertia also reduces the risk of cascade tripping of further generation.

Similarly, in the event of a sudden loss of demand or increase in generation then the added inertia means the grid frequency will rise more slowly than would otherwise be the case.

In synchronous operation, the synchronous machine operates as a motor, taking power from the grid to feed the machine losses (mechanical and electrical). As a synchronous machine, the rotational speed matches the grid frequency. While the grid frequency oscillates around the nominal value, there is a continuous exchange of active power between the rotating machine and the grid. As the grid frequency falls, the (absolute value of the) machine load angle will transiently decrease so that the synchronous machine reduces its power demand from the grid, releasing stored energy by slowing down. If the grid frequency fall is severe, the machine will deliver power into the grid. Similarly, if the grid frequency increases, the machine will accelerate drawing more power from the grid. This dynamic response (inertial response) of taking in more or less power in order for the machine to remain synchronised to the system is a characteristic feature of synchronous machines.

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-5The amount of power and energy which is fed into or drawn out of the machine in the event of a change in grid frequency is a function of the inertia of the machine.

If there is a sudden, large frequency deviation and the apparatus determines a need for a controlled response, the circuit breaker 3 is opened and the equipment operates in decoupled mode, as shown in Fig. 2, so that the machine rotational speed and the grid frequency are no longer synchronous.

In decoupled mode, power can be imported to or exported from the machine in a controlled way through the bi-directional power converter 4. The converter is required to allow the equipment to draw current in from the network at the grid frequency and to drive current into the machine at a frequency related to the machine speed, or vice versa. The controlled response will allow the machine speed to remain constant in a set of given conditions (e.g. when the frequency remains within a deadband around the nominal frequency, as shown in Fig. 3) and will provide a controlled power output (e.g. proportional to the frequency deviation when outside of the deadband up to the maximum power capability of the apparatus, as shown in Fig. 3), with power being extracted from the machine and injected into the system when the frequency is low and power injected into the machine, and thus drawn from the network, when the frequency is high.

Importing power into the machine will accelerate the rotating machine and extracting power from the machine will decelerate the rotating machine. The maximum rotational speed, and thus the maximum amount of energy stored in the rotating machine, is limited by the design of the machine, specifically the ability of the design and materials to withstand the mechanical stress on the rotating parts of the machine. The minimum speed is limited by the amount of current which can be drawn out of the machine, and through the converter, when the machine electro-motive force (EMF) is low; typically machine EMF falls with the machine speed to prevent flux saturation.

Where synchronous mode is utilised, and when the control system determines it is appropriate (e.g. when the grid frequency returns to the deadband), the circuit breaker 3 can be re-closed and the machine resynchronised (i.e. the speed, voltage and phase angle of the rotating machine matches the grid frequency, voltage and

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-6phase angle of the grid). In order to resynchronise the machine, the control system manages the active power exchange with the grid so that power is gradually imported (machine speed low) or exported (machine speed high) to match the machine speed to the grid frequency. When the circuit breaker 3 is closed then the operation returns to the synchronous mode described above.

The synchronous machine 1 can be any known or future machine for use in ac power systems. The machine could, for example, be a wound field synchronous machine - this could be a two pole (3000RPM at 50Hz I 3600 RPM at 60Hz) or a higher pole number machine rotating more slowly.

The machine could alternatively be a permanent magnet synchronous machine, an induction machine, a doubly fed induction generator (DFIG) or any other type. Additional reactive power equipment may be required if the machine is not a wound field synchronous machine.

The machine can be either a horizontal or vertical axis machine.

If there is a requirement to absorb a large amount of energy from the grid such that the design of the machine becomes difficult and expensive from the high maximum speed, it would be possible to import the energy (possibly but not necessarily through the power converter) and dissipate it without storing the energy in increasing the rotational speed of the machine: e.g. the power could be dissipated as heat in resistors. The heat developed could be used if there were a suitable heat demand.

If there is a requirement to deliver a large amount of energy to the grid such that the design of the machine becomes difficult and expensive from the high current limit in the machine and converter (for a fixed inertia), it would be possible to use additional generating plant to export power into the grid by directly connecting the generating plant to the grid. The advantage this scheme would have over just connecting the generating plant to the grid is the reduced number of start cycles the generating plant would have, the reduced requirement for rapid starting, and the reduced capital and operation costs of the generating plant as it would be operational for less time and would have a lower power rating.

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-7It would also be possible to use additional energy storage (e.g. batteries) to allow the apparatus to export power for longer into the system through the power converter. The advantage that this scheme would have over just connecting the additional energy storage through the converter would be the reduced number of cycles the additional energy storage would experience (high cycle numbers tend to reduce the lifetime of energy storage systems).

In situations where additional inertia is not needed, and only response is needed, the machine 1 can be connected to the grid through the power converter 4 the whole time it is connected, without the need for the circuit breaker 3. This is shown in Fig. 5. This also means that the machine does not have to be a synchronous machine, and other machines can be used if preferred, e.g. on cost or size grounds. In such an embodiment the machine speed can be operated at a different steadystate speed (i.e. different amount of stored energy) from the synchronous speed. If the steady-state speed were lower than the synchronous speed then this would result in lower windage losses compared with operation at synchronous speed. For a given maximum machine speed, steady-state operation at less than synchronous speed would allow the machine to import (absorb) more energy from the network; steady-state operation at above synchronous speed would allow the machine to export (deliver) more energy into the network. Designing a machine for a higher maximum speed will increase the machine cost. The optimum steady-state machine speed depends on the relative requirements (or values) in holding down the grid frequency in the event of frequency going high through importing power and to support the grid frequency in the event of frequency going low through exporting power, while considering losses and capital cost.

In addition, as discussed above and shown in Fig. 6, additional energy storage (e.g. batteries) can be used to extend the time for which the machine can operate. It is possible for the apparatus to not include a machine and to rely only on the additional energy storage, as shown in Fig. 4. The steady-state amount of energy stored in the additional energy storage can be controlled depending on the relative requirements in holding down or supporting the grid frequency.

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-8The bi-directional power converter 4 can be any power converter suitable for controlling motor speed, whether the motor is synchronous, induction or another type. Bi-directional power converters are used to export regenerated power from a motor or machine, temporarily acting as a dynamic brake and feeding power back into the grid.

Bi-directional power converters are sometimes referred to as Active Front-End (AFE) converters to differentiate them from passive rectifiers using diodes or thyristors which cannot export power onto the grid.

In the arrangement of Fig. 1, it is not envisaged that there is any significant energy storage within the converter and so power out of the system is the same as power into the machine, neglecting losses, and vice versa. However, another arrangement could include energy storage on the HVDC bus e.g. with capacitors or batteries, similar to the situation shown in Fig. 6 for the rotating machine which is not synchronised.

Depending on the system voltage, the bi-directional power converter grid side converter voltage, the bi-directional power converter machine side converter voltage, and the machine stator voltage, a transformer or transformers (not shown) may be necessary to match the voltages at different points of the design.

In Figure 1, the inertia of the machine is increased by the addition of a flywheel 2 to the rotor of the machine through a stiff shaft 5, and, possibly, including a gearbox. Alternatively, inertia can be increased through the design of the synchronous machine.

During operation in the decoupled mode, the amount of power which is fed into or drawn out of the system is controlled by the converter. Fig. 3 shows a suggested steady-state controlled response of power imported or exported against measured grid frequency including an optional deadband. This diagram shows the machine having capability to both import and export power.

If the energy storage system is a rotating machine, the duration that the machine can import power, i.e. the maximum amount of energy which can be imported,

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-9depends upon the maximum speed which the machine is designed to withstand, the machine inertia and the speed prior to importing energy. Similarly, the maximum amount of energy which can be exported depends upon the speed prior to starting to export, the machine inertia and the minimum speed at which the power output can be maintained (subject to the current limit in the machine and power converter). Other energy storage technologies will also have finite energy import and export constraints, depending on the technology chosen.

In the region close to the nominal frequency (deadband), the apparatus neither imports nor exports active power, irrespective of small variations in the grid frequency. It is also possible for the apparatus to import or export energy when the frequency is close to the nominal value (which may correspond to the deaband but may be wider or narrower in frequency), in order to bring the energy storage system to the preferred steady-state state of charge. In the example of a rotating machine being used as the energy storage system this means bringing the rotating machine to the preferred steady-state speed.

In some grid systems, users are paid when they export energy and pay when they import energy if the frequency is close to the nominal value (e.g. within the deadband). However, apparatus which imports or exports energy to control the grid frequency, when the frequency is away from the nominal value (e.g. outside of the deadband) does not generally have to pay or get paid for the energy exchange between the apparatus and the grid, or pays I gets paid for energy at a different rate. A control system is envisaged in which energy is not imported when the grid frequency is in a small region close to the nominal frequency to avoid paying energy import costs. This small region close to the nominal frequency may or may not be the same as the deadband in response. A control system is also envisaged in which energy is neither imported nor exported in a small region close to the nominal frequency to avoid the fixed costs of trading energy imports and exports.

The addition of inertia and the response described above enable the apparatus to respond and stabilise the grid in the event of a rapid drop in frequency.

An additional development of the apparatus of the invention provides response to a rapid increase in grid frequency. In the event of such a high frequency event, the

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-10power converter controls the rotating machine to speed up to a super-synchronous speed.

If the grid frequency goes high, e.g. as a result of a loss of load or exporting inter5 connector, the power converter can import power from the system to hold the frequency down. This imported power will cause the machine to speed up, running faster than the synchronous speed - the maximum speed depending upon the amount of power imported, the duration and the machine inertia. The machine needs to be designed to withstand the shear stresses resulting from the higher speeds. This could require higher grade materials and I or additional bracing/fixing of machine subsections to prevent the machine coming apart at high speeds. This additional bracing/fixing could be achieved through use of stronger adhesives or through mechanical means such as pinning of the rotor pole sections and/or windings. Machines are designed for a wide range of overspeed requirements depending on applications; typically some generators for hydro generation require high overspeed capabilities.

Claims

Claims

1. An apparatus for providing response to frequency fluctuations in a power grid system; the apparatus comprising:

energy storage means configured to be connected to a power grid; and a bi-directional power converter connected between the energy storage means and the power grid, the power converter configured to convert current at the grid frequency to current specific to the energy storage means.
2. The apparatus for providing response to frequency fluctuations in a power grid system of claim 1, wherein the energy storage means is connected to the grid via a circuit breaker.
3. The apparatus for providing response to frequency fluctuations in a power grid system of claim 1 or 2, wherein the energy storage means is a rotating machine.
4. The apparatus for providing response to frequency fluctuations in a power grid system of claim 3, wherein the current specific to the energy storage means is a current appropriate for the speed at which the rotating machine is rotating.
5. The apparatus for providing response to frequency fluctuations in a power grid system of claim 3, wherein the rotating machine is not synchronised with the current specific thereto and includes a high speed flywheel.
6. The apparatus for providing response to frequency fluctuations in a power grid system of claim 1 or 2, wherein the energy storage means is at least one of a battery, a capacitor, a super-capacitor, inductor, super-conducing magnetic energy storage system.
7. The apparatus for providing response to frequency fluctuations in a power grid system of claim 6, wherein the power converter is configured to convert current at the grid frequency to direct current for the energy storage means.

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8. The apparatus for providing response to frequency fluctuations in a power grid system of any preceding claim further comprising an additional generating plant to export power to the grid.
9. The apparatus for providing response to frequency fluctuations in a power grid system of any preceding claim further comprising an additional energy storage means.
10. A method of providing a response to frequency fluctuations in a power grid; the method comprising:

connecting energy storage means to the power grid; and converting current at the grid frequency to current specific to the energy storage means.
11. The method of claim 10, further comprising providing power from the energy storage means to the grid in response to a decrease in grid frequency.
12. The method of claim 10 or 11, further comprising providing power to the energy storage means from the grid in response to an increase in grid frequency.
13. The method of any of claims 10 to 12, further comprising dissipating power from the grid without storing energy in the energy storage means.
14. The method of any of claims 10 to 13, wherein the energy storage means is at least one of a battery, a capacitor, a super-capacitor, inductor, superconducing magnetic energy storage system, and wherein the method further comprises converting current at the grid frequency to direct current for the energy storage means.