A photovoltaic grid-connected line protection method, system, computing device and storage medium

Abstract

The application provides a photovoltaic grid-connected line current differential protection method and system based on phase correction, a computing device and a storage medium, and the method comprises the following steps: when a short-circuit fault occurs in the photovoltaic grid-connected line, on the photovoltaic side, whether it is a two-phase interphase short-circuit fault is judged by using a split-phase starting criterion and a zero-sequence current starting criterion; if it is a two-phase interphase short-circuit fault, which phase is the current lagging phase is further judged; then the current of the current lagging phase is processed by lagging 60 degrees; and the processed current is further used for protection discrimination by using traditional differential protection. By processing the current of the lagging phase on the photovoltaic side, when a two-phase interphase short-circuit fault occurs in the photovoltaic grid-connected line, the phase difference of the currents at both ends of the fault phase is greatly reduced, which is beneficial to improving the sensitivity of the ratio braking current differential protection, and the situation that the current differential protection of one phase is refused to act when a two-phase interphase short-circuit fault occurs in the photovoltaic grid-connected line is avoided.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of power system and its automation, and particularly relates to a current differential protection method and system based on phase correction suitable for photovoltaic grid-connected lines. BACKGROUND

[0002] The statements in this section merely provide background information related to the present application and do not necessarily constitute the prior art.

[0003] In order to reduce the carbon emissions caused in the process of economic development, at present, the scale of new energy development is increasing, making the power grid increasingly showing the characteristics of "double high", i.e. high proportion of renewable energy and high proportion of power electronic devices. The direct current generated by photovoltaic panels needs to be converted into alternating current by an inverter before being connected to the power system. When the grid voltage is unbalanced, the direct current side of the inverter will generate secondary ripple current and ripple voltage, which seriously affects the control performance of the three-phase inverter, so the photovoltaic inverter adopts the control mode of positive and negative sequence decoupling and suppresses the negative sequence.

[0004] The low inertia characteristics, low overcurrent capacity and unique control mode of the photovoltaic inverter make its short-circuit transient characteristics significantly different from those of the traditional synchronous machine. At present, experts and scholars have made a lot of research on the applicability of traditional protection methods applied to full-power grid-connected lines, involving ratio braking current differential protection, distance protection, directional element, zero sequence overcurrent protection, etc., and pointed out the problems of reliability, sensitivity, etc. when the traditional protection methods are applied to photovoltaic grid-connected lines.

[0005] The document "Adaptability Analysis of Current Differential Protection in Inverter New Energy Station Sending-out Line" analyzes the performance of current differential protection in the strong and weak conditions of the grid-connected system when the sending-out line is subjected to asymmetric short circuit. The results show that when the sending-out line is subjected to two-phase short circuit, the differential protection in the weak system has the risk of refusal to act. In view of the problem of reduced reliability of differential protection for inverter new energy station sending-out line, the document does not give an effective solution.

[0006] The document "Research on Sensitivity of Differential Protection for Asymmetric Faults in Photovoltaic Grid-connected System Sending-out Line" analyzes the sensitivity of the ratio braking characteristic differential protection under the conditions of with and without reactive power compensation when the grid-connected sending-out line is subjected to asymmetric faults. The results show that in the reactive power compensation state, the sensitivity of the traditional ratio braking characteristic differential protection of the photovoltaic grid-connected power generation system sending-out line decreases. The document verifies that the differential protection based on zero sequence current still has high reliability for ground faults, but does not give a specific solution for non-ground faults.

[0007] The document "Influence of weak power characteristics of photovoltaic power station on relay protection of outgoing line" studies the fault current characteristics of photovoltaic power generation system, analyzes the influence of weak power characteristics on the action performance of outgoing line current differential protection and distance protection, and points out that the weak power characteristics of photovoltaic power station will cause the sensitivity of photovoltaic side current differential protection of outgoing line to decrease. The solution strategy proposed in the document is to ignore the photovoltaic side when setting the fixed value, and to set it according to single-sided power supply. However, the scenario where the short-circuit current of the photovoltaic side is larger than that of the system side is not considered.

[0008] At present, experts and scholars have made a lot of research on the applicability of traditional protection methods applied to full-power grid-connected lines, and pointed out that for two-phase interphase short-circuit faults, the reliability and sensitivity will decrease, however, the existing research has not proposed a current differential protection action reliability improvement strategy in the high proportion of photovoltaic access scenarios. SUMMARY

[0009] In order to overcome the shortcomings of the prior art, the present application provides a photovoltaic grid-connected line current differential protection method based on phase correction, which can ensure the reliability of the protection.

[0010] In order to solve the technical problem, the technical scheme adopted by the present application is as follows: a photovoltaic grid-connected line protection method, comprising the following steps:

[0011] S01), when a short-circuit fault occurs in the photovoltaic grid-connected line, the photovoltaic side sampling data is identified by using the split-phase starting criterion and the zero-sequence starting criterion to identify the fault type;

[0012] S02), whether it is a two-phase interphase short-circuit is determined by the above fault type identification method, if it is a two-phase interphase short-circuit, then enter the phase correction module, and the lagging phase current in the two-phase short-circuit current is processed by lagging N degrees, N is greater than 0, so as to reduce the phase difference of the current at both ends of the fault phase;

[0013] S03), the photovoltaic side sampling data after phase correction enters the traditional current differential protection program to perform corresponding protection discrimination on the photovoltaic grid-connected line.

[0014] Further, if it is determined in step S01) that it is a ground fault or a three-phase short-circuit fault, then the fault is discriminated according to the traditional current differential protection.

[0015] Further, N=60.

[0016] The present application also discloses a photovoltaic grid-connected line protection system, comprising:

[0017] The fault type identification module is configured to identify the fault type by using the split-phase starting criterion and the zero-sequence starting criterion when a short-circuit fault occurs in the photovoltaic grid-connected line.

[0018] The phase lag module is configured to: if the fault type is identified by the fault type identification module as a two-phase-to-phase short circuit, lagging the phase of the fault current by N degrees, N being greater than 0, so as to reduce the phase difference of the current between the two fault phases.

[0019] The differential protection module is configured to: perform differential protection identification on the phase-corrected photovoltaic side sampling data.

[0020] Further, if the fault type identification module identifies the fault type as a ground fault or a three-phase short circuit fault, the traditional current differential protection is performed for fault identification.

[0021] Further, N=60.

[0022] The application further discloses a computing device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to implement the steps of the method in any one of claims 1-3.

[0023] The application further discloses a computer readable storage medium, which stores a computer program, wherein the program is executed by a processor to implement the steps of the method in any one of claims 1-3.

[0024] The application has the following beneficial effects: the photovoltaic side lagging phase current is lagged by 60 degrees, so that when a two-phase-to-phase short circuit occurs in a photovoltaic grid-connected line, the phase difference of the current between the two fault phases is greatly reduced, the sensitivity of the ratio braking current differential protection is improved, and the current differential protection of one phase is prevented from being refused when a two-phase-to-phase short circuit fault occurs in the photovoltaic grid-connected line. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 Fig. 1 is a diagram of fault current phases of a photovoltaic grid-connected line according to an embodiment of the application;

[0026] Figure 2 Fig. 2 is a diagram of fault current phases after phase correction according to an embodiment of the application;

[0027] Figure 3 Fig. 3 is a flowchart of the method according to embodiment 1. DETAILED DESCRIPTION

[0028] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the application belongs.

[0029] It should be noted that the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the exemplary embodiments according to the application.

[0030] In the case of no conflict, the embodiments in the application and the features in the embodiments can be combined with each other.

[0031] Embodiment one

[0032] The embodiment discloses a photovoltaic grid-connected line current differential protection method based on phase correction, comprising:

[0033] When a short-circuit fault occurs in the photovoltaic grid-connected line, the fault phase protection is started by using the split-phase starting criterion and the zero-sequence starting criterion;

[0034] If it is a ground fault or a three-phase short-circuit fault, the traditional current differential protection is used for fault discrimination;

[0035] If it is an inter-phase short-circuit fault, the phase correction is performed on the fault phase current, the corrected current is used for ratio restraint type differential current protection discrimination, and then the intra-zone and extra-zone faults are identified.

[0036] In order to better illustrate the technical scheme of the application, first, the fault characteristic analysis is carried out, and then a current differential protection method based on phase correction is introduced.

[0037] First, the fault characteristic analysis: for the new energy side, when a short-circuit fault occurs and the positive-sequence voltage drops to 0.9 p.u. or below, the photovoltaic inverter controller enters the low-voltage ride-through control stage. Since the controller adopts the positive and negative sequence decoupling control strategy, the given reference value of the negative sequence is 0, so even if an asymmetric short-circuit fault occurs, the three-phase current output by the photovoltaic side is still symmetrical.

[0038] For the system side, when an inter-phase short-circuit fault occurs, the phase difference of the fault phase current is almost 180 degrees, but due to the suppression of the negative sequence current by the photovoltaic side, the negative sequence impedance of the photovoltaic side is infinite. Therefore, the negative sequence current all flows to the system side, resulting in that the negative sequence current of the system side is greater than the positive sequence current, and further resulting in that the phase sequence of the three-phase current of the system side is reversed. Therefore, when an AB inter-phase short-circuit fault occurs, the B-phase current leads the A-phase current, but the phase difference is less than 180 degrees.

[0039] According to Kirchhoff's current law, when an AB inter-phase short-circuit fault occurs, ignoring the line capacitance current, the following equation can be obtained

[0040] (1),

[0041] Where IA1, IB1, IA2, IB2 are the A, B phase currents of the photovoltaic side and the system side respectively. Since the three-phase current of the photovoltaic side is symmetrical after the fault, it can be obtained that

[0042] (2),

[0043] Since the three-phase currents IA1, IB1, and IC1 on the photovoltaic side are symmetrical, only IA2 and IB2 need to be determined, and the other current quantities can be determined.

[0044] The above analysis shows that the B-phase current in the system always leads the A-phase current, and the phase angle difference is close to 180 degrees. Using IA2 as the reference vector, the phase diagrams of each current can be drawn as shown in the attached figure. Figure 1 As shown. By Figure 1 It is known that the phase difference between the A-phase current on the new energy side and the system side is less than 90 degrees, and the phase difference between the B-phase current is greater than 90 degrees but less than 120 degrees.

[0045] Therefore, as the scale of new energy access increases (or the strength of the connected system decreases), the ratio of short-circuit current between new energy and system side increases, and the phase difference between phase B current on the new energy side and system side is close to 120 degrees. At this time, the difference between phase B braking quantity and differential quantity decreases, that is, the reliability of phase B current differential protection decreases.

[0046] As can be seen from the above analysis, due to the suppression effect of the photovoltaic controller on negative sequence current, the three-phase current is symmetrical when an asymmetrical fault occurs, but the phases of the currents of the two faulty phases on the system side are almost opposite, which leads to a larger phase angle difference between the currents on both sides of one phase line, thereby reducing the reliability of the differential protection of that phase.

[0047] like Figure 3 As shown, the current differential protection method based on phase correction includes:

[0048] Fault type identification: When a short-circuit fault occurs in a photovoltaic grid-connected line, the fault type is identified using phase-by-phase start-up criteria and zero-sequence start-up criteria. If it is a ground fault or a three-phase short-circuit fault, the existing differential protection method is used to distinguish between faults inside and outside the protection zone. If it is a two-phase short circuit, the phase correction stage is entered to correct the phase of the photovoltaic side current entering the protection device.

[0049] Photovoltaic-side fault current phase correction: Select the lagging phase of the two fault phases on the photovoltaic side and lag its current phase by 60 degrees. (See attached...) Figure 2 As shown, the faulty phases are phases A and B, and the phase of phase B current is delayed by 60 degrees. After phase correction, the phase angle between the photovoltaic side and the system side phase B current is greatly reduced, which is beneficial to the operation of the ratio-controlled differential protection. Phase correction is then performed before the phase-by-phase current differential protection is judged.

[0050] Example 2

[0051] The purpose of this embodiment is to provide a computing device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to implement the steps of the above-described method.

[0052] Example 3

[0053] The purpose of this embodiment is to provide a computer-readable storage medium.

[0054] A computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, performs the steps of the above method.

[0055] Example 4

[0056] The purpose of this embodiment is to provide a phase-corrected photovoltaic grid-connected line current differential protection system, including:

[0057] The fault phase discrimination module is configured to: when a short circuit fault occurs in a photovoltaic grid-connected line, use the phase-by-phase start-up criterion and the zero-sequence start-up criterion to determine the fault type;

[0058] The phase correction module is configured to: determine the type of fault by using the aforementioned fault phase discrimination module; if it is a two-phase short circuit, then lag the current phase of the lagging phase by 60 degrees and proceed to the subsequent phase-differential protection procedure.

[0059] The steps and methods involved in the apparatuses of Embodiments 2, 3, and 4 above correspond to those in Embodiment 1. For specific implementation details, please refer to the relevant description section of Embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or multiple media including one or more instruction sets; it should also be understood as including any medium capable of storing, encoding, or carrying an instruction set for execution by a processor and enabling the processor to perform any of the methods in this invention.

[0060] Those skilled in the art will understand that the modules or steps of the present invention described above can be implemented using general-purpose computer devices. Optionally, they can be implemented using computer-executable program code, thereby allowing them to be stored in a storage device for execution by a computer device, or they can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. The present invention is not limited to any particular combination of hardware and software.

[0061] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A photovoltaic grid-tie line protection method, characterized by: The method comprises the following steps: S01), when a short-circuit fault occurs in the photovoltaic grid-connected line, the photovoltaic side sampling data is used to identify the fault type by using a split-phase starting criterion and a zero-sequence starting criterion; S02), whether it is a two-phase interphase short-circuit is determined by the above fault type identification method, if it is a two-phase interphase short-circuit, a phase correction module is entered, a lagging phase current in the two-phase short-circuit current is processed by lagging N degrees, N is greater than 0, so as to reduce the current phase difference between both ends of the fault phase; S03), the photovoltaic side sampling data after phase correction enters a traditional current differential protection program to perform corresponding protection identification on the photovoltaic grid-connected line.

2. The photovoltaic grid-tie line protection method of claim 1, wherein: If it is a ground fault or a three-phase short-circuit fault determined in step S01), the fault is identified according to the traditional current differential protection.

3. The photovoltaic grid-tie line protection method of claim 1, wherein: N=60。 4. A photovoltaic grid-tie line protection system, characterized by: It comprises: a fault type identification module configured to identify the fault type by using a split-phase starting criterion and a zero-sequence starting criterion when a short-circuit fault occurs in the photovoltaic grid-connected line; a phase lag module configured to, if it is a two-phase interphase short-circuit determined by the above fault type identification module, process the lagging phase of the fault current by lagging N degrees, N is greater than 0, so as to reduce the current phase difference between both ends of the fault phase; a differential protection module configured to perform differential protection identification on the photovoltaic side sampling data after phase correction.

5. The photovoltaic grid-tie line protection system of claim 4, wherein: If it is a ground fault or a three-phase short-circuit fault determined by the fault type identification module, the fault is identified according to the traditional current differential protection.

6. The photovoltaic grid-tie line protection system of claim 4, wherein: N=60。 7. A computing device comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, wherein: The processor executes the program to realize the steps of the method of any one of claims 1-3.

8. A computer-readable storage medium having stored thereon a computer program, characterized in that, The program is executed by the processor to execute the steps of the method of any one of claims 1-3.

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