CN112260307B - Coordination operation method for island protection and low voltage ride through - Google Patents
Coordination operation method for island protection and low voltage ride through Download PDFInfo
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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
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
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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
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/388—Islanding, i.e. disconnection of local power supply from the network
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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
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/56—Power conversion systems, e.g. maximum power point trackers
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Abstract
The invention provides a coordinated operation method for island protection and low voltage ride through, aiming at the problem of operation conflict between the island protection and the low voltage ride through of a photovoltaic grid-connected system. When the fact that the frequency of the point of common coupling is normal and the voltage is abnormal is detected, the strategy coordinates two functions of isolated island protection and low-voltage ride through of the photovoltaic grid-connected system according to the difference of harmonic characteristics under different faults. The invention has the advantages that no current is injected into the system when two operation modes are coordinated, so that the island protection and the low voltage ride through are not interfered with each other on the premise of not influencing the quality of electric energy, the operation is simple and easy, and the invention can be matched with the frequency protection and the voltage protection of the island.
Description
Technical Field
The invention relates to a strategy for coordinating island protection and low voltage ride through, and belongs to a fault detection technology in the field of new energy.
Background
In order to ensure safe and stable operation of a power grid, a photovoltaic grid-connected system needs to have island protection and low-voltage ride through capability at the same time. The island protection means that when a network side circuit breaker is disconnected and a system is in an island state, a photovoltaic power station is timely disconnected to ensure the safety of personnel and equipment; the low voltage ride through means that when the voltage on the grid side drops for a short time, the photovoltaic power station is required to keep running for a period of time in a grid-connected state so as to maintain the stability of the voltage and the frequency. This presents a problem, namely how the pv grid-connected system should select low voltage ride through or islanding protection action when a Point of Common Coupling (PCC) voltage drop is detected. When the photovoltaic grid-connected system is in an island state, the low-voltage ride-through operation is executed, so that not only is the island detection effect influenced, but also equipment and personnel are damaged; when transient disturbance occurs to the grid-side voltage, the low voltage ride through fails due to the shutdown action of performing the island protection.
In order to avoid the contradiction, the low voltage ride through and the island detection of the power grid in China are not performed synchronously at present, but the island detection is performed after the grid-connected state is kept running for the low voltage ride through time specified by the national standard when voltage drops. However, according to the regulations of domestic and foreign standards (references [1-9 ]) such as GB/T19964-2012 and IEEE std.1547.1-2020, after the PCC voltage drops, the islanding protection requires the system to be disconnected within 2s, and the low voltage ride through requires the system to keep 2s from being disconnected. Obviously, the execution mode preferentially ensures the low voltage ride through capability of the photovoltaic grid-connected system, and is contrary to each grid-connected criterion in the aspect of island protection action time.
In order to enable both the island protection action time and the low voltage ride through duration to meet grid connection standards, a photovoltaic grid connection system needs to perform low voltage ride through and island detection operations at the same time. Reference [10] proposes a low voltage ride through and islanding synchronous detection algorithm based on reactive power disturbance, but the algorithm is based on the premise that the frequency is not changed when the voltage on the grid side is in transient disturbance, and the frequency deviation of the main grid in fault is not considered. Reference [11] utilizes negative sequence current injection to achieve islanding protection and unbalanced fault ride-through, but fails to achieve low voltage ride-through in three-phase symmetric faults. The reference documents [12] and [13] divide an upper area of a low voltage ride through curve specified in a grid-connection rule into a low voltage ride through area, and a lower area of the low voltage ride through curve into an island protection area, so that the problem of conflict between the two areas cannot be solved fundamentally, and island misjudgment still possibly occurs.
Reference documents
[1] The technical regulation of the photovoltaic power station access power system of the national standardization Committee of China (GB/T19964-2012), beijing, china Standard Press, 2012.
[2]IEEE standard for interconnecting distributed resources with electric power systems:IEEE Std 1547.1－2020[S].2020.
[3]IEEE recommended practice for utility interface of photovoltaic(PV)systems:IEEE Std 929－2000[S].2000.
[4] The requirement of the China national standardization Committee for grid connection of distributed power supplies is GB/T33593-2017S, beijing, china Standard Press, 2017.
[5] The technical regulation of the photovoltaic power generation system accessing to a power distribution network is GB/T29319-2012S, beijing, china Standard Press, 2012.
[6] The technical specification of the distributed power supply access power distribution network is NB/T32015-2013 [ 2] S ]. Beijing: 2013.
[7] The technical specification of the national energy agency and the photovoltaic grid-connected inverter is NB/T32004-2018 [ S ]. Beijing: 2018.
[8] Q/GDW 1617-2015 photovoltaic power station access grid technical Specification [ S ]. Beijing: department of science and technology of national grid company, 2016.
[9] Q/GDW 1480-2015 specification for distributed power Access to grid technology [ S ]. Beijing, department of science and technology of national grid corporation, 2015.
[10] Maclen, rush, liruisheng, etc. the reactive power disturbance algorithm for synchronous detection of low voltage ride through and islanding of a new energy grid-connected power generation system [ J ] grid technology, 2016,40 (05): 1406-1414.
[11]TUYEN N D,FUJITA G.Negative-sequence current injection of dispersed generation for islanding detection and unbalanced fault ride-through[C].46th International Universities’ Power Engineering Conference(UPEC),Sep 5-8,2011,Soest,Germany:6p.
[12]DAS P P,CHATTOPADHYAY S.A Voltage-Independent Islanding Detection Method and Low-Voltage Ride Through of a Two-Stage PV Inverter[J].IEEE Transactions on Industry Applications,2018,54(3):2773-2783.
[13]DIETMANNSBERGER M,SCHULZ D.Compatibility of fault-ride-through capability and anti-islanding-detection in inverters connected to low voltage distribution grids[C].42nd Annual Conference of the IEEE Industrial Electronics Society,Oct 23-26,2016,Florence,Italy:7010-7015.
[14] The national Committee for standardization of China [ electric energy quality ] -public grid harmonic wave ]. GB/T14549-1993S ]. Beijing, china Standard Press, 1993.
Disclosure of Invention
The invention aims to provide a coordinated operation strategy aiming at the problem of operation conflict between isolated island protection and low voltage ride through of a photovoltaic grid-connected system.
The purpose of the invention is realized by the following technical scheme: a coordinated operation method for island protection and low voltage ride through comprises the following steps:
the method comprises the following steps: collecting PCC voltage, current and frequency information, performing fast Fourier transform on the voltage, and extracting h-order harmonic voltage (effective value) Uh。
Step two: frequency f to be detected in real timePCCWith a normal frequency f0And (6) comparing. If fPCCIf the current time is within the normal range, executing a step three; if fPCCOut of grid connection criteria (references 1-9)]) And if the specified allowable frequency range (48 Hz-50.5 Hz) is maintained for more than 0.1s, the system is judged to be in an island state and an island protection action is executed.
Step three: effective value U of voltage to be detected in real timePCCAnd normal voltage U0And (6) comparing. If U isPCCIf the value is obviously lower than the normal value, the system is judged to be in an abnormal operation state, and the step four is executed; if U is detectedPCCWithin the normal range, normal operation continues.
Step four: according to any one extracted UhThe change of the voltage is coordinated with two functions of island protection and low voltage ride through. If U is detectedh>Uh，setIf the voltage on the network side is discontinuous, judging that transient disturbance occurs to the voltage on the network side, and executing a fifth step; if U is detectedh>Uh，setAnd if the situation is continuous, the system is judged to have the island phenomenon and immediately executes the island protection action. Note that the reason why continuity is considered here is that the waveform of the fault transient voltage has a large distortion, and the amplitude of the harmonic inevitably reaches a high peak value.
Step five: and judging whether the low voltage ride through condition is met or not according to the voltage and frequency state information, if so, executing a sixth step, otherwise, immediately splitting the photovoltaic power station.
Step six: into a low voltage ride through operating state, according to the relevant standards (references [1-9]]) Transient reactive support is provided for the PCC voltage during transient operation. Passing through the maximum crossing time limit TmaxAnd then, judging whether the voltage reaches a normal value, if so, recovering normal operation, otherwise, failing in low voltage ride through, and immediately disconnecting the photovoltaic power station.
Further, in the second step, the allowable range of the frequency is 48Hz to 50.5Hz.
Furthermore, in the third step, the normal value range is 0.85-1.1 per unit value.
In the fourth step, the harmonic voltage threshold is less than U ″)h，detAnd is greater than U'h，detWherein U ″)h，detIs the harmonic voltage at PCC in island operation, U'h，detHarmonic voltages at the PCC during transient disturbance of the grid side voltage can be obtained through simulation.
The invention has the advantages that no current is injected into the system when two operation modes are coordinated, so that the island protection and the low voltage ride through are not interfered with each other on the premise of not influencing the quality of electric energy, the operation is simple and easy, and the invention can be matched with the frequency protection and the voltage protection of the island.
Drawings
FIG. 1 is a coordination strategy algorithm flow;
FIG. 2 is a simulation model of a photovoltaic grid-connected system;
FIG. 3 (a) shows effective values of voltages at PCC before and after an island;
FIG. 3 (b) is the effective value of the voltage at PCC before and after the voltage transient disturbance;
FIG. 3 (c) shows the effective value of the voltage at the PCC before and after two faults occur simultaneously;
FIG. 4 (a) frequencies at PCC before and after islanding;
FIG. 4 (b) is the frequency at PCC before and after a voltage transient disturbance;
FIG. 4 (c) is the frequency at PCC before and after two faults occur simultaneously;
FIG. 5 (a) is the effective value of the harmonic voltage at PCC before and after islanding;
FIG. 5 (b) is the harmonic voltage effective value at PCC before and after the voltage transient disturbance;
fig. 5 (c) shows the effective value of the harmonic voltage at PCC before and after two faults occur simultaneously.
Detailed Description
The specific implementation flow of the coordination strategy provided by the invention is shown in fig. 1, and the strategy is based on the PCC harmonic voltage U during the abnormal period of the system running statehThe difference of the variation characteristics distinguishes the islanding phenomenon from the voltage transient disturbance phenomenon. It can be seen from the figure that in order to comprehensively consider the effects of various electrical quantities such as voltage, frequency, harmonic waves and the like, the strategy provided by the invention also considers the frequency protection of the island. Before judging whether the voltage is abnormal, firstly judging whether the frequency is abnormal, and if the frequency is abnormal, directly judging that the system is in an island state. The reason for this is that: on one hand, the frequency detection method separately takes the frequency as a criterion, and a larger detection blind area exists, so that the fault is judged by combining a voltage detection method; on the other hand, the voltage detection method may confuse the islanding phenomenon and the voltage transient disturbance phenomenon, therebyLeading to island misjudgment and low voltage ride through failure, further combining with a harmonic detection method to analyze the system operation state is needed.
In the present specification, a photovoltaic grid-connected system model as shown in fig. 2 in the appendix is built based on the MATLAB/Simulink simulation platform, and a specific embodiment of the present invention is described below in conjunction with the simulation model.
The simulation time length of the model is 1.8S, the fault occurrence time is 1S, and the simulation parameter setting is shown in table 1, wherein SK1And δ I2PIs referred to national standard GB/T14549-1993 (reference document [14]]) And (4) obtaining the product. The simulation mainly studies the change of harmonic wave before and after the fault, and controls the frequency at the boundary of a normal range (49.5-50.2 Hz) in order to amplify the influence of frequency spectrum leakage on the harmonic wave after the fault as much as possible and avoid the interference of a frequency protection system on the strategy provided by the text. The frequency at PCC stabilizes around 49.5Hz after islanding by adjusting the local load.
Table 1 simulation parameter settings
The corresponding simulation results are as follows.
Firstly, the islanding phenomenon and the voltage transient disturbance phenomenon are respectively simulated, and the voltage and the frequency before and after the fault are respectively shown in the attached appendix in fig. 3 and fig. 4. To control the variables for the control experiment, the voltage and frequency at the PCC during the voltage transient disturbance were set to be the same as during islanding by adjusting the load parameters and setting the main grid parameters.
Then, whether the strategy can make an accurate judgment when the islanding phenomenon and the voltage transient disturbance occur simultaneously is considered, and corresponding simulation results are respectively shown in fig. 3 (c), fig. 4 (c) and fig. 5 (c). It can be seen that the simulated image curve when the two faults occur simultaneously and the image curve when the island occurs are basically coincident, which indicates that the proposed strategy can effectively identify the island without being influenced by the voltage on the network side under the condition that the island phenomenon and the voltage transient disturbance occur simultaneously.
The simulation results in fig. 5 were calculated, recorded, and collated to obtain table 2. Wherein U'h，detIs the harmonic voltage, U', at PCC in normal operationh，detIs the harmonic voltage at PCC in island operation, U'h，detIs the harmonic voltage at the PCC at the time of the grid side voltage transient disturbance. U 'in the experiment due to very small background harmonic'h，det≈U′h，KU′h≈KU′h，det。KhThe ratio of harmonic voltage before and after the occurrence of the island is theoretically calculated according to the data in table 1 as follows:
wherein R, L and C are parallel load parameters, omega0＝2πf0Angular frequency, L, corresponding to power frequencySIs the equivalent inductance of the power grid side.
Table 2 simulation results data analysis
Note: the voltage data in the table are all effective values.
Analyzing the data in table 2 can lead to the following conclusions:
1) Comparison Uh，detAnd U'h，detIt can be seen from the two sets of data that, for the same number of harmonics, the harmonic voltage amplitude during islanding operation is greater than the harmonic voltage amplitude during voltage transient disturbance, so that one of the harmonic voltage amplitudes may be selected to be less than U ″h，detAnd is greater than U'h，detHarmonic voltage threshold value U ofh，setTo distinguish between two types of failures. And the higher the harmonic frequency is, the smaller the difference of harmonic voltages under two faults is. This shows that the harmonic voltage is used for distinguishing the island state and the voltage transient disturbance state only in a certain rangeEffectively, when the harmonic frequency in the experiment exceeds 5, the harmonic voltage is no longer suitable as the criterion for distinguishing two faults.
2) Comparison Uh，detAnd KhU′hTwo groups of data show that the theoretical value of the harmonic voltage in the island state is smaller than the actual value through calculation, because the actual value is influenced by frequency spectrum leakage, namely the actual value is influenced by the original harmonic U of the island systemhaAnd harmonics U due to spectral leakagehbAre superimposed and have KhU′h＜Uha+Uhb＜Uhc+KhU′h. According to Uh，det/(KhU′h) The data of x 100% can also be seen that the difference between the theoretical value and the actual value increases with the increase of the harmonic times, that is, the higher the harmonic times, the more obviously the harmonic voltage in the island state is affected by the frequency spectrum leakage.
3) For the 5 th harmonic, U ″)5，detAnd U'5，detSo close that an accurate voltage threshold cannot be used to distinguish between two faults, and K5U′5And U ″)5，detThe difference between them is large. This is because U5aThe numerical value is small, so that the influence of frequency spectrum leakage on the 5 th harmonic is very obvious, and the size of the 5 th harmonic is basically determined by U when the island operates5bAnd (6) determining.
4) For the 2 nd harmonic, U ″)2，det＝5.649V，U″′2，det=1.518V. Apparently U2,set=3.527V satisfying the requirement as harmonic threshold, i.e. U ″'2，det＜U2，set＜U″2，detThus U is2，setWhen the coordination strategy provided by the invention is applied (as shown in fig. 5 (a) and fig. 5 (b)), the island protection and low voltage ride through functions of the photovoltaic grid-connected system can be effectively coordinated.
It should be understood that the above examples are only for clarity of illustration and are not intended to limit the embodiments. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. This need not be, nor should all embodiments be exhaustive. And obvious variations or modifications of the invention may be made without departing from the scope of the invention.
Claims (3)
1. A coordinated operation method for island protection and low voltage ride through is characterized by comprising the following steps:
the method comprises the following steps: collecting voltage, current and frequency information of a Point of Common Coupling (PCC), and extracting h-th harmonic voltage Uh；
Step two: frequency f to be detected in real timePCCCompared to the normal frequency: if the current time is within the allowable range, executing a step three, and if the current time exceeds the allowable range and lasts for more than 0.1s, executing an island protection action;
step three: effective value U of voltage to be detected in real timePCCComparison with the normal voltage: if the value is obviously lower than the normal value, executing a step four, and if the value is within the normal value range, continuing normal operation;
step four: according to any one time extracted UhThe change of the voltage is coordinated with two functions of island protection and low voltage ride through: if U is detectedhIf the voltage is larger than the harmonic voltage threshold but not continuous, executing step five, and if U is detectedhIf the persistence is larger than the harmonic voltage threshold value, executing an island protection action; the harmonic voltage threshold value is less than Uh，detAnd is greater than U'h，detWherein U ″)h，detIs the harmonic voltage at PCC in island operation, U'h，detIs the harmonic voltage at the PCC when the network side voltage is in transient disturbance, h is less than 5;
step five: judging whether a low voltage ride through condition is met, if so, executing a sixth step, otherwise, immediately splitting the photovoltaic power station;
step six: and entering a low voltage ride through operation state, after the maximum ride through time limit, if the voltage reaches a normal value, recovering the normal operation, and if not, immediately splitting.
2. The method according to claim 1, wherein in the second step, the allowable range of the frequency is 48Hz to 50.5Hz.
3. The method according to claim 1, wherein the normal value in the third step is in the range of 0.85-1.1 per unit.
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