CN101123349A - Island effect detection method based on active frequency deviation and its device - Google Patents
Island effect detection method based on active frequency deviation and its device Download PDFInfo
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Abstract
A detached island effect test method and a device based on active frequency deviation relate to a test method and a device to test distributive new energy grid integration power supply, in particular to a detached island effect test method and a device based on active frequency deviation. Regarding the utility model, 25 cyclic waves form a major cycle. Chopped wave rate cf of the first 15 cyclic waves is 0, and variance trend of the frequency is tested. Under the circumstance that frequency tends to rise, frequency disturbance with the chopped wave rate of cf is added in the grid frequency in the last ten cyclic waves, and initial value cfmax of the chopped wave rate cf is positive. Under the circumstance that the frequency tends to reduce, frequency disturbance with the chopped wave rate of cf is added in the grid frequency in the last ten cyclic waves, and initial value cfmin of the chopped wave rate cf is negative. By adopting the invention, not only the operation probability of the detached island tested is improved; but also testing time is reduced, and current harmonics injected into the grid is reduced.
Description
Technical Field
The invention relates to a measurement and control method and a measurement and control device for distributed new energy grid-connected power supply, in particular to an island effect detection method and an island effect detection device based on active frequency deviation.
Background
Distributed power generation systems composed of new energy resources are receiving more and more attention, and wind power and solar grid-connected power generation are particularly taken as representatives. This is because: (1) The demand of electric power is increasing day by day, and the non-renewable energy sources such as coal, petroleum and the like are gradually exhausted, so that the alternative energy sources are urgently sought; (2) Environmental pollution caused by traditional centralized large-scale fuel power generation seriously influences social production and life, and green new energy is beneficial to relieving environmental pressure; (3) Because new energy such as wind power and photovoltaic power generation has the characteristics of wide distribution, low density and the like, local-source small-capacity and multi-distributed grid-connected power generation has more advantages than independent power generation. The distributed power supply emerging in recent years and a traditional large power grid simultaneously supply power to distribution network users, so that the obvious economic effect is generated, and the application prospect is bright.
Because the electric energy generated by the new energy source is unstable, a grid-connected mode that the electric energy is converted into stable direct current through a power electronic technology and then synchronous alternating current is obtained through an inverter is mostly adopted at present. Compared with the traditional power supply mode, the distributed grid-connected power generation system reversely transmits electric energy, and the quality of the electric energy and the safety of a power grid become very important and outstanding problems. Among them, the most important topic is the detection and prevention of islanding.
The existing of a decentralized power generation system is not considered in the design and construction of the current power system, and if a certain line in a distribution network breaks down, the fault can be cleared by means of a nearest relay protection device. Once a grid-connected distributed power generation system exists in the distribution network, the distributed power supply can continuously supply power to the distribution network disconnected with the large power grid, and an independent small power generation system is formed under some special conditions. This phenomenon is called "islanding". The island operation can bring very big harm to electric wire netting, user, personal safety: (1) Originally, because the circuit which is not electrified after the breaker is disconnected still continues to supply power, a line maintenance worker is likely to be unwittingly contacted with the circuit to cause personal injury; (2) The distributed power supply is not supported by a large power grid, the power quality of the distributed power supply cannot be guaranteed, and the equipment of a user is directly damaged; (3) The wide use of reclosure causes different-phase closing caused by inconsistent voltage and phase during reclosure, larger accidents are caused, and the power grid is prevented from supplying power again.
Fig. 1 is a general block diagram of a power grid including distributed power generation. The original bus 1 which is connected to the load after the switch S3 leads the distributed power generation system into the traditional power grid through the grid-connected transformer and the two grid-connected switches to transmit reverse electric energy. Taking a fault between the switch S3 and the bus 2 as an example, if the relay protection trips the switch S3, the distributed power supply and the load form an isolated island operation, which is very likely to cause great harm to equipment and personal safety in the small system. Therefore, in order to detect the islanding operation in the shortest time, a tripping signal for tripping the grid-connected switch S1 or the grid-connected switch S2 is sent out, and the safe operation of the system is maintained.
Firstly, a grid-connected inverter system is introduced, and a good hardware foundation and a good platform are provided for the introduction of an anti-islanding scheme.
Fig. 2 is a working block diagram of a grid-connected distributed power generation system including an island detection and control system. The whole system is roughly composed of five parts. The block diagram 1 represents a power conversion main loop consisting of a power supply energy source, a direct current stabilization control system, a stabilized direct current voltage output, an inverter and a transformer grid-connected switch; block 2 represents a local load; block 3 represents the utility grid to be connected; block 4 represents a trip control relay; a block diagram 5 represents an anti-islanding operation detection and control module, which is a control core of the whole system; a block diagram 6 represents a driving module for converting the PWM signal sent by the control system into a signal capable of driving the switching device in the inverter bridge of the block diagram 1 to be turned off and on;
in this document, photovoltaic power generation is taken as a specific example of the application of the anti-islanding algorithm, which is convenient for explanation. Block 1 represents the inversion of the dc power generated by the photovoltaic into ac, in parallel with the grid in block 3, to supply the load RLC circuit with electric energy. The block diagram 5 contains various control circuits, of which the anti-islanding device is an important part.
The block diagram 5 is the whole system core, including: a hall sensor 53 to collect a voltage value at the common node in real time; the AD conversion module 52 is configured to convert the analog signal acquired by the hall sensor 53 into a digital signal that can be processed by the Digital Signal Processor (DSP); the DSP control module 51 is a core in the core, and on one hand, receives the digital signal of the voltage at the common node converted by the AD conversion module 52 in real time, and on the other hand, processes the acquired signal, and finally sends a suitable control signal to the relay and a PWM signal for controlling the on/off of the switching device in the inverter to the switch driving module 6.
Application of traditional voltage and frequency parameter measurement in island passive detection method
Fig. 5 is a simplified diagram of the entire system, with RLC parallel circuits representing local loads connected to the inverted photovoltaic power generation system through a transformer via a common node a, while connecting circuit breakers operating in parallel with the large grid. When the whole system works normally, the large power grid can be regarded as an ideal voltage source with infinite capacity and stable voltage. The voltage clamp at the common node a is at the grid voltage normal and the power output by the photovoltaic array is delivered to the grid as current. The local load RLC parallel circuit can obtain sufficient electric energy from the photovoltaic power generation system and the large power grid, namely:
P load ＝P inv +ΔP(1)
Q load ＝Q inv +ΔQ(2)
in the formula P load 、Q load The active power and the reactive power required by the normal operation of the load are represented; p is inv 、Q inv Representative of photovoltaic power generation systemsActive and reactive power output by the inverter; Δ P, Δ Q represent the electrical energy required by the grid to "replenish" the load RLC shunt circuit (Δ P, Δ Q may be positive or negative, i.e. the actual flow of grid power may be in phase or in anti-phase with that shown in fig. 5). Meanwhile, the voltage and the frequency of the two ends of the RLC parallel circuit have the following relations with the active power and the reactive power of the load:
in the formula V a Representing the value of the voltage at the common node a and ω represents the angular frequency of the voltage at node a. Simultaneous equations (1), (2), (3) and (4) can be obtained:
from the equations (5) and (6), if the power grid is broken due to a fault (e.g. the switch 2 in fig. 5 is opened), the load obtains the electric energy P load ′、Q load ' is provided entirely by the photovoltaic power generation system, as shown in formulas (7) (8):
in the formula V a ', ω' represent the voltage at point a and its frequency, respectively. In most cases, the photovoltaic power generation system outputs power P inv 、 Q inv The power demand P corresponding to the load normal work group load 、Q load The phase difference is very large (so-called mismatch), resulting in common nodesAt a voltage V a The frequency omega fluctuates to V a ', ω ' to P ' load ＝P inv ，Q′ load ＝Q inv . Since the operation of the photovoltaic power generation system is controlled at unity power factor (the current input to the grid is synchronized with the grid voltage), i.e. Q' load ＝Q inv =0. The following are obtained by (8):
arg{R -1 +(jωL) -1 +jωC} -1 ＝0(9)
i.e. the RLC parallel circuit operating frequency drifts to the load resonance frequency
Whether V or not a 'also drift to ω' of the load resonant frequency, this apparent change in voltage and frequency can be detected using passive methods of over/under voltage (OVR/UVR) and over/under frequency (OFR/UFR).
Inspired by the most basic principle, the basic principle of anti-islanding operation is to detect whether the voltage or frequency at the common node a is within a normal range. Although the traditional passive method for directly detecting the voltage and the frequency has high speed and cannot influence a power grid, with the development of the technology, the capacity of a grid-connected photovoltaic power generation system is continuously enlarged, and the situation that the inverter output power is completely failed when matched with the load requirement is very likely to occur. Meanwhile, due to the input and the removal of the load, voltage and frequency jitter may occur in the power grid, and the change may cause a false trip. Therefore, the active method of anti-islanding operation has received more attention.
Conventional active frequency offset method (AFD)
The Active Frequency shift Method, i.e. the AFD Method, changes the Frequency of the current injected into the grid so that the Frequency of the grid voltage continuously rises or falls away from the resonant Frequency of equation (9) during islanding operation until the allowable range is exceeded.
1. Introduction to AFD Process
FIGS. 6a and 6b show the current i injected into the grid with reference to the grid voltage sine wave at 50Hz PV And (6) wave-form. T is Vutil Is the period of the grid voltage; t is iPV Is the period of the sinusoidal part of the output current. Zero output time t z Half period of mains voltage
The ratio is called the chopping rate cf (choppingfraction). The grid-tie current is synchronized with the grid voltage at its zero crossings, but then cf advances (or lags) the output current by the voltage. When the whole large power grid has no fault, namely, no breaker trips, the voltage of the end connected with the power grid is always synchronous with the power grid under the clamping action of the large power grid. Although the current injected into the power grid is slightly distorted, the synchronization of the current and the power grid voltage is recovered each time when the power grid voltage crosses zero, and meanwhile, the distortion rate of the output current is controlled within an allowable range, and the influence on the power grid voltage is avoided. Once islanding occurs, the voltage is determined by the output current and the load impedance, and reaches the zero point in advance or in a delay mode. And repeating the above steps, continuously increasing or decreasing the voltage frequency of the distribution network side, and finally detecting by the OFR/UFR to cut off the operation of the photovoltaic power generation system.
According to Fourier analysis, the higher harmonics of the injected grid current are negligible, and V is considered a Is i PV A fundamental function. Zero output results in a fundamental component to i in the current input to the grid PV With a phase shift, i PV Fundamental component ratio i of PV T ahead of time z /2. Therefore, when the system is in islanding operation, with i PV Fundamental component synchronous V a Will advance
. If V a And i PV And synchronizing, the system frequency is definitely satisfied:
arg{R -1 +(jωL) -1 +jωC} -1 ＝0.5ωt z ＝0.5πcf(10)
i.e. when the system load satisfies equation (10)，V a And i PV And then, no phase difference exists, the system frequency is kept unchanged, and the system enters a detection blind area.
2. Disadvantages of the AFD method
When the grid-connected power generation inverter system runs in an isolated island, the theoretical Detection blind area of the AFD method is much smaller than the Detection blind area (Non-Detection Zone) of the basic voltage and frequency Detection method. However, the detection blind area of the method still needs to be further reduced to meet the increasingly severe conditions of the power grid.
The traditional AFD detection method can reduce the detection blind area by increasing the value of cf (because cf can be positive or negative, the reference of increasing the value of cf in the method refers to the absolute value of cf). However, cf is too large, current distortion injected into a power grid can seriously affect the quality of electric energy; and cf is too small, so that the superiority of the method cannot be highlighted compared with the traditional voltage frequency detection method. Therefore, the method is key to reasonably select the value and the size of the cf.
When the AFD method is applied, if the system load does not satisfy the formula (10), theoretically, the system frequency can always shift to be out of the normal working range, and the abnormal operation condition can be detected. However, the islanding detection is time-defined, especially when high-speed reclosing is used at present, if the anti-islanding speed is too low, the grid-connected power generation inverter system is still not disconnected before reclosing, and severe oscillation is possibly generated on a power grid to cause an accident. This is likely to occur in the AFD method. Taking the local load being inductive, the voltage phase has a tendency of leading current, and after the island operation occurs, if AFD is not applied, the frequency of the voltage will continuously rise. However, once the AFD method is applied, cf takes a fixed negative value, which counteracts the voltage frequency variation trend caused by the load property, and slows down the voltage frequency fluctuation, but delays the trip time. And vice versa.
The above disadvantages indicate that the AFD method provides a good idea for islanding detection, but further improvements are still needed.
Disclosure of Invention
The invention provides an island effect detection method and device based on active frequency deviation, aiming at the defect that the detection blind area is larger when the traditional passive method of voltage and frequency detection is matched with a source and a load.
The invention adopts the following technical scheme:
step 1: taking 25 cycles as a large cycle, simultaneously detecting the change trend of the frequency in 15 cycles with the chopping rate cf =0, adding frequency disturbance with the chopping rate cf to the power grid frequency in the last 10 cycles when the change trend of the frequency belongs to the situation that the frequency has an ascending trend, and setting an initial value cf of the chopping rate cf max The frequency change trend is positive, and for the situation that the frequency change trend is a descending trend, frequency disturbance with the chopping rate cf is added to the power grid frequency in the last 10 cycles, and the initial value cf of the chopping rate cf min The value of the negative value is the negative value,
the detection method of the frequency variation trend comprises the following steps: in the first 15 cycles in each major cycle, detecting the frequency value of the cycle by using a phase-locked circuit of a grid-connected inverter system, setting the initial frequency of the voltage cycle as power frequency 50Hz, comparing the detected first cycle frequency value with the initial frequency value, if the first cycle frequency value is greater than the initial frequency value, recording the first cycle frequency value as an ascending trend for 1 time, otherwise, recording the first cycle frequency value as a descending trend for 1 time, then comparing the detected current cycle frequency value with the previous cycle frequency value, if the current cycle frequency value is greater than the previous cycle frequency value, recording the ascending trend for 1 time, otherwise, recording the descending trend for 1 time, finally, comparing the ascending trend with the descending trend for the number of times in the 15 cycles, if the ascending trend number of times is greater than the descending trend, judging that the change trend of the major cycle belongs to the situation that the voltage frequency has the ascending trend, otherwise, judging that the change trend of the major cycle belongs to the situation that the voltage frequency has the descending trend;
and 2, step: monitoring the voltage frequency of the power grid, wherein the monitoring method comprises the following steps: the phase-locked PLL circuit acquires a power grid voltage frequency value f when the power grid voltage crosses zero, the condition that the power grid voltage frequency value f when the voltage crosses zero is less than or equal to 49.5Hz or the condition that the power grid voltage frequency value f when the voltage crosses zero is more than or equal to 50.5Hz is taken as the abnormal phenomenon of the power grid voltage frequency, if the abnormal phenomenon of the power grid voltage frequency occurs in continuous 5 cycles, the island effect is judged to occur, and a tripping signal is sent out.
The invention comprises a control module (51), a voltage digital input signal of the control module (51) is collected by a Hall sensor 53 in real time, then an AD conversion module (52) converts an analog signal collected by the Hall sensor 53 into a voltage digital signal, and the control module (51) sends a grid-connected switch instruction according to a calculation result, wherein the control module (51) comprises a grid voltage, frequency calculation module (511) and an anti-islanding operation detection and control module (512), the grid voltage and frequency calculation module (511) is used for processing a collected voltage digital signal at a common node to obtain information of a voltage amplitude and a voltage frequency at the common node, the grid voltage and anti-islanding operation detection and control module (512) comprises a frequency monitoring module 5123, a frequency change trend detection module 5121 and a chopping rate application module 5122, the frequency calculation module (511) transmits the obtained voltage frequency information at the common node after the voltage signal is processed to the frequency change trend detection module 5121 and the chopping rate application module 5122, and the frequency change detection module 5121 is used for determining the frequency change of front frequency cycle waves of 15 cycle waves of large-wave change frequency waves through the frequency change trend detection module 5125; when the frequency change trend made by the chopping rate applying module 5122 according to the frequency change trend detecting module 5121 belongs to an ascending trend, the frequency disturbance with the chopping rate cf and the initial value cf of the chopping rate cf are added to the power grid frequency in the last 10 cycles max The frequency is a positive value, otherwise, the frequency disturbance with the chopping rate cf is added to the power grid frequency in the last 10 cycles, and the initial value cf of the chopping rate cf is min Is a negative value; the frequency monitoring module 5123 is usedWhen the power grid is monitored, and the frequency value f of the power grid voltage when continuous 5 cycles have zero crossing is less than or equal to 49.5Hz or the frequency value f of the power grid voltage when the continuous 5 cycles have zero crossing is greater than or equal to 50.5Hz, the power grid is judged to have an island effect, and a tripping signal is sent.
Compared with the prior art, the invention has the following advantages:
according to the scheme, on the basis of a traditional active frequency migration method, an improved method that the voltage frequency change trend of the power grid is detected firstly and then the reasonable chopping rate is applied is adopted, so that the detection blind area is greatly reduced, the detection speed is accelerated, the current harmonic waves injected into the power grid are reduced to the greatest extent, and the influence of the active method on the power grid is reduced.
(1) The island effect detection method can effectively change the frequency of the voltage at two ends of the load when an island occurs in operation by changing the frequency of the injected current, can force the voltage frequency to deviate from a normal range even under the condition of source-load matching, detects the abnormal operation condition and sends a trip signal, thereby avoiding the defect that the traditional method for detecting whether the island effect occurs or not by detecting the voltage frequency range fails because the voltage frequency does not change under the condition of source-load matching.
(2) The island effect detection method overcomes the defect that the detection time is prolonged due to the fact that the traditional active frequency change rate method possibly exists in the contradiction between the load property and the chopping rate cf property by the improved method of firstly detecting the voltage frequency change trend of a power grid and then applying a reasonable chopping rate, and can detect island operation and disconnect a switch before reclosing is conducted so as to ensure the operation safety of a system.
(3) The active frequency deviation method inevitably has interference on a power grid, combines the international standard, and takes a cf value as large as possible on the premise of meeting the national conditions of China, thereby reducing detection blind areas, accelerating the detection speed, reducing harmonic waves injected into the power grid and reducing the influence on the power grid.
Drawings
FIG. 1 is a block diagram of a distributed grid-connected power generation system;
FIG. 2 is a block diagram of a distributed grid-connected power generation system including a novel island detection and control system according to the present invention;
fig. 3 is a detailed internal structure diagram of the DSP control module 51 in fig. 2;
FIG. 4 is a detailed diagram of the internal structure of the anti-islanding operation detection module 512 in FIG. 3;
FIG. 5 is a simplified diagram of the system shown in FIG. 2;
FIG. 6a is a graph showing the current i injected into the grid when a positive chopping rate is applied using active frequency offset with the grid voltage frequency at 50Hz PV The waveform of (2).
FIG. 6b shows the current i injected into the grid when applying a negative chopping rate using active frequency offset with a grid voltage frequency of 50Hz PV The waveform of (2).
FIG. 7 is a flow chart of a method of islanding detection and control according to the present invention;
FIG. 8 is a detailed flow diagram of the chop rate application 5122 module of FIG. 7;
FIG. 9 is a detailed flow diagram of the frequency monitoring module 5123 of FIG. 7;
fig. 10 is a graph showing a relationship between a chopping rate cf value and a harmonic content THD.
Detailed Description
Example 1
Step 1: taking 25 cycles as a large cycle, simultaneously detecting the change trend of frequency in the first 15 cycles with the chopping rate cf =0, and adding frequency disturbance with the chopping rate cf to the power grid frequency in the last 10 cycles when the change trend of frequency belongs to the situation that the frequency has an ascending trend, wherein the initial value cf of the chopping rate cf max Is positiveThe value is that for the situation that the frequency change trend is that the frequency has a descending trend, frequency disturbance with the chopping rate cf is added to the power grid frequency in the last 10 cycles, and the initial value c of the chopping rate cf fmin The value of the negative value is the negative value,
the detection method of the frequency variation trend comprises the following steps: in the first 15 cycles in each major cycle, detecting the frequency value of the cycle by using a phase-locked circuit of a grid-connected inverter system, setting the initial frequency of the voltage cycle as power frequency 50Hz, comparing the detected first cycle frequency value with the initial frequency value, if the first cycle frequency value is greater than the initial frequency value, recording the first cycle frequency value as an ascending trend for 1 time, otherwise, recording the first cycle frequency value as a descending trend for 1 time, then comparing the detected current cycle frequency value with the previous cycle frequency value, if the current cycle frequency value is greater than the previous cycle frequency value, recording the ascending trend for 1 time, otherwise, recording the descending trend for 1 time, finally, comparing the ascending trend with the descending trend for the number of times in the 15 cycles, if the ascending trend number of times is greater than the descending trend, judging that the change trend of the major cycle belongs to the situation that the voltage frequency has the ascending trend, otherwise, judging that the change trend of the major cycle belongs to the situation that the voltage frequency has the descending trend;
step 2: monitoring the voltage frequency of the power grid, wherein the monitoring method comprises the following steps: the method comprises the steps that a phase-locked PLL circuit obtains a power grid voltage frequency value f when the power grid voltage crosses zero, the situation that the power grid voltage frequency value f when the power grid voltage crosses zero is smaller than or equal to 49.5Hz or the situation that the power grid voltage frequency value f when the power grid voltage crosses zero is larger than or equal to 50.5Hz is taken as the power grid voltage frequency abnormality phenomenon, if the power grid voltage frequency abnormality phenomenon occurs in continuous 5 cycles, the situation that an island effect occurs is judged, and a tripping signal is sent.
The frequency disturbance implementation method comprises the following steps:
step 1: the phase signal θ = ω t of the grid voltage is detected using a phase-locked PLL circuit,
step 2: when the power grid voltage is in a positive half cycle (theta is more than or equal to 0 and less than or equal to pi), the power grid voltage phase signal theta is used for calculating the application cf
Corresponding to the phase theta '= (1 + cf) × theta of grid-connected current later, if theta' < pi, the phase of the synchronous grid-connected current signal is
Theta'; if theta' is more than or equal to pi, the phase of the synchronous grid-connected current signal is 0,
and step 3: when the grid voltage is detected to be in a negative half cycle (theta is not less than pi and not more than 2 pi), the phase theta '= (1 + cf) × (theta-pi) of the corresponding grid-connected current after cf is applied is calculated by the grid voltage phase signal theta, if theta' < pi, the phase of the synchronous grid-connected current signal is 2 pi-theta ', and if theta' > pi, the phase of the synchronous grid-connected current signal is 0.
Example 2
The voltage digital input signal of the control module 51 is acquired by a Hall sensor 53 in real time, and then an analog signal acquired by the Hall sensor 53 is converted into a voltage digital signal by an AD conversion module 52, the control module 51 sends a grid-connected switch instruction according to a calculation result, the control module 51 is characterized in that the control module 51 comprises a grid voltage and frequency calculation module 511 and an anti-islanding operation detection and control module 512, the grid voltage and frequency calculation module 511 is used for processing the acquired voltage digital signal at the common node to acquire information of a voltage amplitude and a voltage frequency at the common node, the grid voltage and anti-islanding operation detection and control module 512 comprises a frequency monitoring module 5123, a frequency change trend detection module 5121 and a chopping rate application module 5122, the frequency calculation module 511 transmits the voltage frequency information at the common node obtained after the voltage signal processing to the frequency change trend detection module 5121 and the chopping rate application module 5122, and the frequency change trend detection module 5121 is used for determining the frequency change trend of the first 15 cycles of the 25 cycles through frequency detection; when the frequency change trend made by the chopping rate applying module 5122 according to the frequency change trend detection module 5121 belongs to a rising trend, the frequency disturbance with the chopping rate cf and the initial value cf of the chopping rate cf are added to the power grid frequency in the last 10 cycles max Is positive, otherwise, inAdding frequency disturbance with a chopping rate cf to the power grid frequency in the last 10 cycles, and adding an initial value cf of the chopping rate cf min Is a negative value; the frequency monitoring module 5123 is used for monitoring the power grid, and when the power grid voltage frequency value f at zero crossing is less than or equal to 49.5Hz or the power grid voltage frequency value f at zero crossing is greater than or equal to 50.5Hz in the case of continuous 5 cycles, it is determined that an islanding effect occurs, and a trip signal is sent. . The control module 51 can be implemented by a digital signal processing chip DSP, and a power grid voltage and frequency calculation module (511) in the control module 51 is used for processing the acquired voltage digital signal at the common node to obtain information of the voltage amplitude and the voltage frequency at the common node; an anti-islanding operation detection and control module (512) in the control module 51 comprises a frequency monitoring module 5123, a frequency change trend detection module 5121 and a chopping rate application module 5122, wherein the frequency monitoring module 5123 is used for monitoring the frequency at a common node, judging whether the frequency is in a normal range, and when the grid voltage frequency value f when continuous 5 cycles have zero crossing is less than or equal to 49.5Hz or the grid voltage frequency value f when the zero crossing is greater than or equal to 50.5Hz, determining that an islanding effect occurs, and sending a trip signal; the frequency variation trend detection module 5121 is used for determining the frequency variation trend in the first 15 cycles of the 25 cycles of the major cycle through frequency detection; the choppingThe wave rate applying module 5122 determines to apply a corresponding chopping rate according to the frequency variation trend made by the frequency variation trend detecting module 5121: when the frequency belongs to the rising trend, frequency disturbance with the chopping rate cf is added to the power grid frequency in the last 10 cycles, and the initial value cf of the chopping rate cf is added max The frequency is a positive value, otherwise, the frequency disturbance with the chopping rate cf is added to the power grid frequency in the last 10 cycles, and the initial value cf of the chopping rate cf is min Is negative.
The invention is explained in more detail below with reference to the drawings:
1.1. islanding detection scheme
1.1.1. Detecting frequency variation trend to determine chopping rate value
When the system in the solution of the present invention operates, the hall sensor module 53 samples the voltage at the common node, the analog signal obtained by sampling is subjected to digital conversion by the AD conversion module 52, and the DSP control module receives the converted voltage signal and processes the digital signal, as shown in fig. 2. The central content of the island detection is as follows: in order to overcome the defects of the AFD method, the detection method firstly judges the change trend of the frequency to determine the sign of the cf strengthening the same trend, then selects the strongest cf value according to the maximum value of the THD of the current injected into the power grid allowed by the national relevant standards, and periodically applies the cf. The specific flow is as shown in fig. 7 to 9 and explained below.
1. Frequency detection
The frequency detection mainly monitors whether the frequency of the inverter grid-connected voltage tends to rise or fall. The frequency detection takes 25 cycles of the grid voltage (one cycle is basically maintained at 0.02s when the grid normally operates) as a cycle period. The first 15 cycles are voltage frequency change trend detection periods, and the last 10 cycles are chopping rate cf application periods. In the first 15 cycles (voltage frequency change trend detection period), the anti-islanding system detects whether the voltage frequency of the power grid has an ascending trend or a descending trend, and the detection is stopped in the last 10 cycles (chopping rate cf application period).
In the first 15 detection cycles (voltage frequency variation trend detection periods), the anti-islanding scheme does not change the waveform of the output current of the inverter, and the current and the grid voltage detected on two sides of the load are synchronously output in the same phase. At the same time, the voltage frequency variation trend is determined during this period. Starting from the 16 th cycle, the detection of the frequency change is stopped. By judging the rising or falling trend of the first 15 cycle voltage frequencies, the anti-islanding system applies a proper cf value to current in the last 10 cycle (a chopping rate cf application period), so that the trend of frequency change is strengthened under the condition of islanding operation, and the islanding operation is detected at the fastest speed.
The specific frequency variation trend detection steps are as follows:
step 1: counters min and max for accumulating the number of times the grid frequency rises and falls within these 15 cycles. max represents
The number of times of the rise of the grid voltage frequency compared with the last cycle is min, and the number of times of the fall of the grid voltage frequency compared with the last cycle is accumulated.
Whether the grid voltage is in a normal operating state or the inverter is in an island operation, the voltage frequency always fluctuates.
The two counters can express the approximate change trend of the system frequency in the period of time.
Step 2: at the beginning of the voltage frequency variation trend detection period (i.e., at the beginning of the 1 st cycle), the min and max counters are reset to zero.
A frequency register PreF is preset, and the frequency of the last voltage cycle is recorded (if the program is just started, the power frequency is set to be 50 Hz).
And step 3: when one cycle is finished, the frequency value of the cycle is detected by using a phase locking circuit of a grid-connected inverter system and is compared with the frequency PreF of the previous cycle. If the frequency is increased or unchanged, defaulting that the frequency has an ascending trend, and adding 1 to a max counter; similarly, if the frequency decreases, indicating that it is trending downward, the min counter is incremented by 1. And replacing the original value in the PreF by the voltage frequency value of the cycle, and recording a numerical value for frequency comparison of the next period.
And 4, step 4: in the first 15 cycles, step 3 is repeated.
And 5: during the 16 th to 25 th cycles, the detection of the frequency rise or fall is stopped, and the obtained min and max counter values are kept unchanged. But the voltage frequency update of PreF cannot be stopped.
After 15 cycles (voltage frequency variation trend detection period) are finished, the numerical updating of max and min is stopped in the last 10 cycles (chopping rate cf application period). At this point, max is compared to the min counter. If max > = min, the voltage frequency is mostly increased in 15 cycles, and it is considered that the voltage frequency tends to increase; if max < min, on the contrary, it is considered that the voltage frequency tends to decrease.
The flow of this part is shown in FIG. 7.
As can be seen from the flowchart of fig. 7, the 5121 module function is indicated by the dashed line box, and the operation in the first 15 cycles (voltage frequency variation trend detection period) corresponds to the above detection steps. It is noted that the block 5122 shows the application of the chopping rate as described below, which operates in the last 10 cycles (the chopping rate cf application period), and is also depicted here to make the whole process more clear. Meanwhile, the frequency monitoring system in the block diagram 5123 is mainly used for constantly observing whether the voltage frequency exceeds the normal working range, and if abnormal operation is found, the islanding operation is considered to occur, a trip signal is sent out, and the grid-connected inverter is disconnected from the power grid. This function will also be explained in detail below. Blocks 51211 and 511 represent the assignment of the variable PreF and the acquisition of the grid frequency by means of digital phase-lock processing, which are performed for each grid voltage cycle of the entire frequency detection cycle.
2. Chopping rate cf application
The application of the chopping rate cf is essentially an application of the AFD method. To overcome the traditional AFD method, fixation is always appliedThe scheme only adds the proper chopping rate in the last 10 cycles of chopping rate cf application periods. When the power grid works normally, the voltage frequency may also fluctuate. However, no matter what the max and min in the first 15 cycles are, the plus or minus of the cf applied by the grid-connected current in the last 10 cycles does not have great influence on the voltage of the power grid, and the distortion generated to the grid-connected current in a periodic manner rather than a continuous manner greatly reduces the average harmonic injection to the power grid. If islanding occurs, the grid-connected inverter has load properties (inductive or capacitive) which cause the voltage frequency to rise or fall, and the trend is obviously detected by a max and min counter. If max = min, the voltage frequency tends to rise, and the islanding operation load is inductive, and a positive cf value (cf) is applied to the grid-connected current max ) Electricity injected into the grid in each cycleAs shown in fig. 6a, the flow reaches the zero point in advance, so that the frequency increase trend is strengthened, and the islanding detection speed is increased. If max is less than min, the voltage frequency tends to decrease, the load carried by the island operation is considered to be capacitive, and a negative cf value (cf) is applied to the grid-connected current min ) The current injected into the grid in each cycle lags to zero as shown in fig. 6b, so that the frequency slowing trend is strengthened, and the speed of frequency deviation out of the normal range is increased.
The detection of the frequency variation trend determines the sign of the applied cf max And cf min The determination of the specific value needs to be limited by means of the total distortion rate (THD) of the grid-connected current. Although the larger the absolute value of cf is, the smaller the detection blind area is, the more the intensity of cf is increased, the distortion rate of the injected power grid current is increased rapidly, and harmonic pollution to the power grid cannot be ignored. IEEEStd929-2000 states that THD of injected power grid current cannot be larger than 5%, which means that cf value has certain limit.
Fig. 10 shows the relationship between cf value and THD. In the preceding stage grid-connected inverter circuit in the scheme, current ripples exist in grid connection actually, and even if cf =0, harmonic waves with THD of about 2% are injected into a power grid. cf max 4.9%, corresponding to THD4.97%; cf min Take-4.81% to correspond to THD4.9%. Under different circuit conditions, the proper cf value is slightly different, and the normal cf max It is feasible to take 4.5 percent and cfmin about-4.5 percent. Of course, a relation curve between different grid-connected inverter circuits similar to fig. 10cf and THD can be obtained through experiments, and it is determined that cf is not more than 5% of THD max And cf min Selection of (2).
The specific cf application steps are as follows:
step 1: and comparing the values of max and min. If max > = min, take cf = cf max =4.9%; if max is less than min, taking cf = cf min ＝-4.81。
Step 2: when the power grid voltage is detected to be in a positive half cycle, a phase-locked PLL circuit is used for detecting a phase signal theta = omegat (wherein 0 is larger than or equal to theta and smaller than or equal to pi) of the power grid voltage.
And step 3: and calculating the phase theta' = (1 + cf) × theta of the corresponding grid-connected current after cf is applied according to the power grid voltage phase signal theta. If theta '< pi, outputting i = sin (theta') as a grid-connected current synchronization signal; if theta' is equal to pi, 0 is output.
And 4, step 4: when the grid voltage is detected to be in a negative half cycle, a phase-locked PLL circuit is used for detecting a phase signal theta = omegat (wherein pi is less than or equal to theta and less than or equal to 2 pi) of the grid voltage.
And 5: and calculating the phase theta' = (1 + cf) × (theta-pi) of the corresponding grid-connected current after cf is applied according to the power grid voltage phase signal theta. If theta '< pi, outputting i = -sin (theta') as a synchronization current synchronization signal; if theta' is equal to pi, 0 is output.
A flow chart of the cf applying step is shown in fig. 8.
3. Frequency monitoring system
The anti-islanding control described above causes the voltage frequency to shift out of the normal range at the fastest rate after islanding by periodically applying disturbances to the grid. As shown in fig. 7, a frequency monitoring system (block 5123 in fig. 7) is present for each cycle throughout a large cycle of 25 cycles, and this function primarily detects whether the frequency of the grid voltage is within a normal range of values.
IEEEStd929-2000 stipulates technical indexes required to be achieved by a grid-connected power generation inverter system, wherein the technical indexes comprise the requirements of power grid quality, detection time for abnormal operation and island effectThe time of detection should be, etc. However, this international standard is directed to a 60Hz system, and in order to adapt to the chinese power system, according to the national power standard "power quality power system frequency tolerance", the present document improves the detection requirements: when the frequency of the system voltage continuously drifts out of the range of 49.5-50.5 Hz by 5 cycles, the control system sends a tripping signal to disconnect a breaker between the inverter and the power grid, and the inverter stops supplying power to the power grid. Due to the improvement of the working performance of the reclosing at present, the reclosing time is greatly shortened, and the requirement of detecting island detection in the traditional 2 seconds can not be met. Therefore, the abnormal operation state is detected by the control algorithm of the method within 0.5 second after the islanding, which is why 25 cycles are selected as a large cycle, and enough time is provided for judging the frequency change trend (15 cycles) and applying cf max And cf min (10 cycles).
The frequency monitoring steps are as follows:
step 1: a counter frequency counter is set to an initial value of zero. This counter is used to record the number of cycles that the grid voltage frequency is abnormally maintained.
Step 2: and detecting whether the power grid voltage crosses zero. If a zero crossing (i.e., rising, falling edge) is detected, a grid voltage frequency value f is obtained by the phase-locked PLL circuit.
And 3, step 3: if f is less than or equal to 49.5Hz or f is more than or equal to 50.5Hz, the power grid voltage frequency is abnormal, and the frequency counter is increased by 1: if the voltage frequency f is restored to the normal state, the frequency counter is cleared.
And 4, step 4: if the frequency counter does not satisfy 10, repeating the steps 2 and 3. Once the frequency counter is full of 10, the power grid is considered to be abnormal for 5 continuous cycles, islanding operation occurs, and a tripping signal is sent out, as shown in fig. 1. This is because the frequency counter is on both rising and falling edges of Sinewave, so the frequency counter reaches 10 instead of 5 to initiate a trip.
A flow chart of the frequency monitoring system steps is shown in fig. 9:
2. implementation examples
After the device is electrified, firstly phase information of power grid voltage is obtained through a phase-locked loop (PLL) technology to determine synchronous phase of grid-connected current, meanwhile, amplitude of the grid-connected current is obtained according to a preceding-stage Maximum Power Point Tracking (MPPT) function, and after proper output current is determined according to the phase and the amplitude, synchronous grid connection with a power grid is achieved. When the inverter system and the power grid are in parallel normal operation, the influence of the island detection method on the power grid is small as can be seen from a oscillogram, even in a period that cf is not zero, the detected harmonic waves injected into the power grid are within an allowable range, and if the system works for a long time, the average harmonic waves injected into the power grid are smaller. When the system runs for 5 seconds, the connection between the power grid and the inverter system is intentionally disconnected, and island operation is manufactured. At this time, a plurality of loads are also connected to the inverter. The abnormal condition can be detected within 0.2 seconds usually because the frequency of the output current is changed continuously by the island detection method, so that the inverter stops working, the connection between the inverter and the load is disconnected, and the island operation is effectively prevented.
Claims (3)
1. An island effect detection method based on active frequency deviation is characterized in that:
step 1: taking 25 cycles as a large cycle, simultaneously detecting the change trend of the frequency in 15 cycles with the chopping rate cf =0, adding frequency disturbance with the chopping rate cf to the power grid frequency in the last 10 cycles when the change trend of the frequency belongs to the situation that the frequency has an ascending trend, and setting an initial value cf of the chopping rate cf max For the situation that the frequency variation trend is a descending trend, adding frequency disturbance with the chopping rate cf to the power grid frequency in the last 10 cycles, wherein the initial value cf of the chopping rate cf is min The value of the negative of the number of the positive,
the detection method of the frequency variation trend comprises the following steps: in the first 15 cycles in each major cycle, detecting the frequency value of the cycle by using a phase-locked circuit of a grid-connected inverter system, setting the initial frequency of the voltage cycle as power frequency 50Hz, comparing the detected first cycle frequency value with the initial frequency value, if the first cycle frequency value is greater than the initial frequency value, recording the first cycle frequency value as an ascending trend for 1 time, otherwise, recording the first cycle frequency value as a descending trend for 1 time, then comparing the detected current cycle frequency value with the previous cycle frequency value, if the current cycle frequency value is greater than the previous cycle frequency value, recording the ascending trend for 1 time, otherwise, recording the descending trend for 1 time, finally, comparing the ascending trend with the descending trend for the number of times in the 15 cycles, if the ascending trend number of times is greater than the descending trend, judging that the change trend of the major cycle belongs to the situation that the voltage frequency has the ascending trend, otherwise, judging that the change trend of the major cycle belongs to the situation that the voltage frequency has the descending trend;
step 2: monitoring the voltage frequency of the power grid, wherein the monitoring method comprises the following steps: the method comprises the steps that a phase-locked PLL circuit obtains a power grid voltage frequency value f when the power grid voltage crosses zero, the situation that the power grid voltage frequency value f when the power grid voltage crosses zero is smaller than or equal to 49.5Hz or the situation that the power grid voltage frequency value f when the power grid voltage crosses zero is larger than or equal to 50.5Hz is taken as the power grid voltage frequency abnormality phenomenon, if the power grid voltage frequency abnormality phenomenon occurs in continuous 5 cycles, the situation that an island effect occurs is judged, and a tripping signal is sent.
2. The islanding detection method based on active frequency deviation according to claim 1, wherein the frequency disturbance is implemented by:
step 1: the phase signal θ = ω t of the grid voltage is detected using a phase-locked PLL circuit,
step 2: when the power grid voltage is in a positive half cycle (theta is more than or equal to 0 and less than or equal to pi), calculating the phase theta ' = (1 + cf) × theta of the corresponding grid-connected current after cf is applied according to the power grid voltage phase signal theta, and if theta ' < pi, the phase of the synchronous grid-connected current signal is theta '; if theta' is more than or equal to pi, the phase of the synchronous grid-connected current signal is 0,
and step 3: when the grid voltage is detected to be in a negative half cycle (theta is not less than pi and not more than 2 pi), the phase theta '= (1 + cf) × (theta-pi) of the corresponding grid-connected current after cf is applied is calculated by the grid voltage phase signal theta, if theta' < pi, the phase of the synchronous grid-connected current signal is 2 pi-theta ', and if theta' > pi, the phase of the synchronous grid-connected current signal is 0.
3. An apparatus for implementing the active frequency offset-based islanding detection method according to claim 1, including a control module (51), wherein a voltage digital input signal of the control module (51) is obtained by a hall sensor 53 in real time, and then an AD conversion module (52) converts an analog signal obtained by the hall sensor 53 into a voltage digital signal, and the control module (51) sends a grid-connected switching command according to a calculation result, the apparatus is characterized in that the control module (51) includes a grid voltage and frequency calculation module (511) and an anti-islanding operation detection and control module (512), the grid voltage and frequency calculation module (511) is configured to process the collected voltage digital signal at the common node to obtain information of amplitude and voltage frequency at the common node, the grid voltage and anti-islanding operation detection and control module (512) includes a frequency monitoring module 5123, a frequency change trend detection module 5121 and a frequency application module 5122, the frequency calculation module (511) transmits the voltage frequency information at the common node obtained after the voltage signal processing to the frequency change detection module 5121 and the frequency change trend detection module 5122, and the frequency change detection module is configured to determine a frequency change of the frequency change through a cycle detection module 5115 that detects a cycle frequency change of the frequency change; when the frequency variation trend made by the chopping rate applying module 5122 according to the frequency variation trend detecting module 5121 is in an ascending trend, the frequency disturbance with the chopping rate cf and the initial value cf of the chopping rate cf are added to the power grid frequency in the last 10 cycles max Positive, whereas chopping is added to the grid frequency during the last 10 cyclesFrequency disturbance with a rate cf and an initial value cf of the chopping rate cf min Is a negative value; the frequency monitoring module 5123 is used for monitoring the power grid, and when the power grid voltage frequency value f at zero crossing is less than or equal to 49.5Hz or the power grid voltage frequency value f at zero crossing is greater than or equal to 50.5Hz in the case of continuous 5 cycles, it is determined that an islanding effect occurs, and a trip signal is sent.
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