CN112260307B - A coordinated operation method of islanding protection and low voltage ride through - Google Patents

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

A Coordinated Operation Method of Island Protection and Low Voltage Ride Through

technical field

The invention relates to a strategy for coordinating island protection and low-voltage ride-through, which belongs to the fault detection technology in the field of new energy.

Background technique

In order to ensure the safe and stable operation of the power grid, the photovoltaic grid-connected system needs to have both island protection and low voltage ride-through capabilities. Island protection means that when the grid-side circuit breaker is disconnected and the system is in an island state, the photovoltaic power station should be disconnected in time to ensure the safety of personnel and equipment; while low-voltage ride-through means that when the grid-side voltage drops for a short time, the photovoltaic power station should maintain The network state runs for a period of time to maintain the stability of voltage and frequency. This brings about a difficult problem, that is, when a point of common coupling (point of common coupling, PCC) voltage drop is detected, how should the photovoltaic grid-connected system choose low voltage ride through or island protection action. When the photovoltaic grid-connected system is in an island state, performing low-voltage ride-through operation will not only affect the islanding detection effect, but also cause damage to equipment and personnel; Make low voltage ride through fail.

In order to avoid this contradiction, at present, the low voltage ride through and island detection of my country's power grid are not carried out synchronously. Instead, when a voltage drop occurs, the island detection is performed after the low voltage ride through time specified by the national standard is maintained in the grid-connected state. However, according to GB/T 19964-2012, IEEE Std.1547.1-2020 and other domestic and foreign standards (references [1-9]), when the PCC voltage drops, the islanding protection requires the system to be disconnected within 2s. For traversal, the system is required to remain on the network for 2 seconds. Obviously, the above-mentioned execution method gives priority to ensuring the low-voltage ride-through capability of the photovoltaic grid-connected system, but it is contrary to each grid-connected criterion in terms of islanding protection action time.

In order to make the operating time of the island protection and the duration of low voltage ride-through meet the grid-connection criteria, the photovoltaic grid-connected system needs to perform low-voltage ride-through and island detection operations at the same time. Reference [10] proposed a low-voltage ride-through and islanding synchronous detection algorithm based on reactive power disturbances, but this algorithm is based on the premise that the frequency remains unchanged when the grid-side voltage is transiently frequency offset. Reference [11] used negative-sequence current injection to achieve island protection and unbalanced fault ride-through, but failed to achieve low-voltage ride-through during three-phase symmetrical faults. Reference [12] and Reference [13] divide the upper area of the low-voltage ride-through curve specified in the grid-connected code into the low-voltage ride-through area, and the lower area into the island protection area, which fails to fundamentally solve the two problems. There is still the possibility of misjudgment of isolated islands.

references

[1] National Standardization Committee of China. Technical Regulations for Connecting Photovoltaic Power Stations to Power Systems: GB/T 19964－2012[S]. Beijing: China Standards 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] National Standardization Committee of China. Requirements for Grid-connected Distributed Power: GB/T 33593-2017[S]. Beijing: China Standards Press, 2017.

[5] China National Standardization Committee. Technical Regulations for Connecting Photovoltaic Power Generation System to Distribution Network: GB/T 29319-2012[S]. Beijing: China Standard Press, 2012.

[6] National Energy Administration. Technical Regulations for Distributed Power Generation Access to Distribution Network: NB/T 32015-2013[S]. Beijing: 2013.

[7] National Energy Administration. Technical Specifications for Photovoltaic Grid-connected Inverters: NB/T 32004-2018[S]. Beijing: 2018.

[8] State Grid Corporation of China. Q/GDW 1617-2015 Technical Regulations for Connecting Photovoltaic Power Stations to the Grid [S]. Beijing: Ministry of Science and Technology of State Grid Corporation of China, 2016.

[9] State Grid Corporation of China. Q/GDW 1480-2015 Technical Regulations on Distributed Power Grid Access [S]. Beijing: Ministry of Science and Technology of State Grid Corporation of China, 2015.

[10] Cong Ma, Gao Feng, Ruisheng Li, et al. Reactive Power Disturbance Algorithm for Synchronous Detection of Low Voltage Ride-through and Islanding in New Energy Grid-connected Power Generation System[J]. Power 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-throughcapability 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] National Standardization Committee of China. Power Quality-Public Power Grid Harmonics: GB/T 14549-1993[S]. Beijing: China Standards Press, 1993.

Contents of the invention

The purpose of the present invention is to propose a coordinated operation strategy for the operational conflict between island protection and low voltage ride-through of photovoltaic grid-connected systems.

The purpose of the present invention is achieved through the following technical solutions: a coordinated operation method for island protection and low voltage ride through, comprising the following steps:

Step 1: Collect information such as PCC voltage, current, and frequency, perform fast Fourier transform on the voltage, and extract h-order harmonic voltage (effective value) U _h .

Step 2: Compare the frequency f _PCC detected in real time with the normal frequency f ₀ . If f _PCC is within the normal range, go to step 3; if f _PCC exceeds the allowable frequency range (48Hz-50.5Hz) stipulated in grid-connected guidelines (references [1-9]) and lasts for more than 0.1s, then determine the system It is in an island state and performs island protection actions.

Step 3: Compare the real-time detected voltage effective value U _PCC with the normal voltage U ₀ . If the U _PCC is significantly lower than the normal value, it is determined that the system is in an abnormal operating state, and step 4 is performed; if it is detected that the U _PCC is within the normal value range, the normal operation is continued.

Step 4: Coordinating the two functions of island protection and low voltage ride-through according to any extracted U _h change. If it is detected that U _h > U _{h, set} but not continuous, it is determined that the grid-side voltage has a transient disturbance, and perform step 5; if it is detected that U _h > U _{h, set} and continuous, it is determined that the system has an islanding phenomenon and execute immediately Island Protection Action. Note that continuity is considered here because the voltage waveform at the moment of fault will be greatly distorted, and the harmonic amplitude will inevitably reach a very high peak value.

Step 5: Determine whether the low voltage ride-through condition is met according to the voltage and frequency status information, and if so, perform step 6, otherwise, the photovoltaic power station will be disconnected immediately.

Step 6: Enter the low-voltage ride-through operation state, and provide transient reactive power support for the PCC voltage during transient operation according to relevant standards (references [1-9]). After the maximum ride-through time limit T _max , it is judged whether the voltage has reached the normal value, and if it reaches the normal operation, it will resume normal operation, otherwise the low-voltage ride-through fails, and the photovoltaic power station will be disconnected immediately.

Further, in the second step, the allowable range of the frequency is 48Hz-50.5Hz.

Further, in the third step, the normal value range is 0.85-1.1 per unit.

In the step 4, the harmonic voltage threshold is less than U″ _{h, det} and greater than U″′ _{h, det} , wherein U″ _{h, det} is the harmonic voltage at the PCC during island operation, U″′ _{h, det} is the harmonic voltage at the PCC when the grid-side voltage is transiently disturbed, which can be obtained through simulation.

The beneficial effect of the present invention is that the present invention does not inject any current into the system when coordinating the two operating modes, and on the premise of not affecting the power quality, the island protection and low voltage ride-through do not interfere with each other. In fact, the operation is simple and easy. Compatible with islanding frequency protection and voltage protection.

Description of drawings

Figure 1 is the coordination strategy algorithm flow;

Figure 2 is the simulation model of photovoltaic grid-connected system;

Figure 3(a) shows the effective value of the voltage at the PCC before and after the island;

Figure 3(b) shows the effective value of voltage at PCC before and after voltage transient disturbance;

Figure 3(c) shows the effective value of voltage at PCC before and after two kinds of faults occur simultaneously;

Figure 4(a) shows the frequency at the PCC before and after the island;

Figure 4(b) shows the frequency at PCC before and after the voltage transient disturbance;

Figure 4(c) shows the frequency at PCC before and after two kinds of faults occur at the same time;

Figure 5(a) shows the effective value of the harmonic voltage at the PCC before and after the island;

Figure 5(b) shows the effective value of the harmonic voltage at the PCC before and after the voltage transient disturbance;

Figure 5(c) shows the RMS harmonic voltage at PCC before and after two kinds of faults occur simultaneously.

Detailed ways

The specific implementation process of the coordination strategy proposed by the present invention is shown in Fig. 1. This strategy distinguishes the islanding phenomenon from the voltage transient disturbance phenomenon according to the variation characteristics of the PCC harmonic voltage U _h during abnormal system operation. It can be seen from the figure that in order to comprehensively consider the effects of various electrical quantities such as voltage, frequency, and harmonics, the strategy proposed in the present invention also considers island frequency protection. Before judging whether the voltage is abnormal, first judge whether the frequency is abnormal. If the frequency is abnormal, it can be directly determined that the system is in an island state. The reason for this setting is: on the one hand, the frequency detection method uses frequency alone as the criterion, and there is a large detection blind spot, so it is necessary to combine the voltage detection method to determine the fault; on the other hand, the voltage detection method may confuse the islanding phenomenon with the voltage Transient disturbance phenomenon, resulting in island misjudgment and low voltage ride-through failure, so it is necessary to further combine the harmonic detection method to analyze the operating state of the system.

This manual is based on the MATLAB/Simulink simulation platform to build a photovoltaic grid-connected system model as shown in Figure 2 in the appendix. The specific implementation of the present invention will be described below in conjunction with the simulation model.

The simulation time of this model is 1.8s, and the fault occurrence time is 1s. The simulation parameter settings are shown in Table 1, where S _K1 and δI _2P are obtained from the national standard GB/T 14549-1993 (reference [14]). The simulation mainly studies the changes of harmonics before and after the fault. In order to amplify the influence of spectrum leakage on the harmonics after the fault as much as possible, and to avoid the interference of the frequency protection system on the strategy proposed in this paper, the frequency is controlled within the normal range (49.5 ~ 50.2 Hz) borders. After the occurrence of islanding, the frequency at the PCC is stabilized at about 49.5 Hz by adjusting the local load.

Table 1 Simulation parameter settings

The corresponding simulation results are described below.

Firstly, the islanding phenomenon and the voltage transient disturbance phenomenon are simulated separately, and the voltage and frequency before and after the fault are shown in Figure 3 and Figure 4 in the appendix, respectively. In order to control the variables for a control experiment, the voltage and frequency at the PCC during the voltage transient disturbance period are set to be the same as those during the islanding period by adjusting the load parameters and setting the main grid parameters.

Then consider whether the strategy can make accurate judgments when the islanding phenomenon and voltage transient disturbances occur simultaneously. The corresponding simulation results are shown in Figure 3(c), Figure 4(c) and Figure 5(c). It is not difficult to see that the simulation image curves when the two faults occur at the same time basically coincide with the image curves when the islanding occurs, which shows that the proposed strategy can effectively identify the islanding when the islanding phenomenon and the voltage transient Grid-side voltage influence.

Calculate, record and organize the simulation results in Figure 5, and get Table 2. Among them, U′ _{h, det} is the harmonic voltage at PCC during normal operation, U″ _{h, det} is the harmonic voltage at PCC during island operation, U″′ _{h, det} is the harmonic voltage at PCC when the grid side voltage is transiently disturbed harmonic voltage. Since the background harmonic is very small, in this experiment U′ _{h, det} ≈ U′ _h , KU′ _h ≈ KU′ _{h, det} . K _h is the theoretical ratio of harmonic voltage before and after islanding, which can be calculated by the following formula according to the data in Table 1:

Among them, R, L, and C are parallel load parameters, ω ₀ =2πf ₀ is the angular frequency corresponding to the power frequency, and L _S is the equivalent inductance of the grid side.

Table 2 Simulation results data analysis

Note: The voltage data in the table are valid values.

Analyzing the data in Table 2, the following conclusions can be drawn:

1) Comparing the two sets of data of U″ _{h, det} and U″′ _{h, det} , it can be seen that for the same order of harmonics, the harmonic voltage amplitude during island operation is greater than that during voltage transient disturbance value, so a harmonic voltage threshold U _{h, set} smaller than U″ _{h, det} and larger than U″′ _{h, det} can be selected to distinguish two kinds of faults. Moreover, the higher the harmonic order, the smaller the difference of the harmonic voltage under the two kinds of faults. This shows that the use of harmonic voltage to distinguish between the island state and the voltage transient disturbance state is only effective within a certain range. When the harmonic order in this experiment exceeds 5 times, the harmonic voltage is no longer suitable as a criterion for distinguishing the two faults .

2) Comparing the two sets of data of U″ _{h, det} and K _h U′ _h , it can be seen that the theoretical value of the harmonic voltage in the island state calculated by calculation is smaller than the actual value, because the actual value is affected by the spectrum leakage The influence of , that is, the actual value is formed by the superposition of the original harmonic U _ha of the island system and the harmonic U _hb generated by the spectrum leakage, and there is K _h U′ _h <U _ha +U _hb <U _hc +K _h U′ _h . According to the data of U″ _{h, det} /(K _h U′ _h )×100%, it can also be seen that the gap between the theoretical value and the actual value increases with the increase of the harmonic order , that is, the higher the harmonic order, the more obvious the harmonic voltage in the islanding state is affected by the spectrum leakage.

3) For the 5th harmonic, U″ _5, det is so close to U″′ _{5, det} that an accurate voltage threshold cannot be used to distinguish the two faults, and K ₅ U′ ₅ and U″ _{5, det} There is a large gap between them. This shows that the 5th harmonic is significantly affected by spectrum leakage because the value of U _5a is small, and the magnitude of the 5th harmonic is basically determined by U _5b during island operation.

4) For the 2nd harmonic, U″ _{2, det} = 5.649V, U″′ _{2, det} = 1.518V. Obviously U _2,set =3.527V satisfies the requirement as the harmonic threshold value i.e. U″' _2,det <U _2,set <U" _2,det , so U _2,set is applied to the coordination strategy proposed by the present invention ( As shown in Figure 5(a) and Figure 5(b), it can effectively coordinate the island protection and low voltage ride-through functions of the photovoltaic grid-connected system.

Apparently, the above-mentioned embodiments are only examples for clear description, rather than limiting the implementation. For those of ordinary skill in the art, other changes or changes in different forms can also be made on the basis of the above description. It is not necessary and impossible to exhaustively list all implementation modes here. However, the obvious changes or changes derived therefrom still fall within the protection scope of the present invention.

Claims (3)

1. A coordinated operation method for island protection and low voltage ride through, characterized in that it comprises the following steps: Step 1: collecting point of common coupling (point of common coupling, PCC) voltage, current and frequency information, and extracting the h-order harmonic voltage U _h ; Step 2: Compare the real-time detected frequency f _PCC with the normal frequency: if it is within the allowable range, execute step 3, and if it exceeds the allowable range and lasts for more than 0.1s, execute the island protection action; Step 3: Compare the real-time detected voltage effective value U _PCC with the normal voltage: if it is obviously lower than the normal value, perform step 4, and if it is within the normal value range, continue to operate normally; Step 4: Coordinate the two functions of islanding protection and low voltage ride-through according to the change of any extracted U _h : if it is detected that U _h is greater than the harmonic voltage threshold but not continuous, then perform step 5, if it is detected that U _h is continuously greater than The harmonic voltage threshold then executes the island protection action; the harmonic voltage threshold is less than U″ _{h, det} and greater than U″’ _{h, det} , where U″ _{h, det} is the harmonic voltage at the PCC during island operation, U″ ′ _{h, det} is the harmonic voltage at PCC when the grid side voltage is transiently disturbed, h is less than 5; Step 5: Determine whether the low-voltage ride-through condition is met, and if so, execute step 6, otherwise, the photovoltaic power station will be disconnected immediately; Step 6: Enter the low-voltage ride-through operation state. After the maximum ride-through time limit, if the voltage reaches the normal value, it will resume normal operation, otherwise it will be disconnected immediately. 2. The method according to claim 1, characterized in that, in the second step, the allowable range of the frequency is 48Hz-50.5Hz. 3. The method according to claim 1, characterized in that, in said step 3, the normal value range is 0.85-1.1 per unit.

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