Method for testing and evaluating continuous commutation failure resistance of multi-infeed direct current system

Abstract

The invention relates to a method for testing and evaluating continuous commutation failure resistance of a multi-infeed direct current system, and belongs to the field of high-voltage direct current transmission. The method comprises the steps of performing a multi-infeed direct current system commutation failure simulation test; and constructing a multi-infeed direct current system commutation failure simulation test event set, wherein the multi-infeed direct current system commutation failure simulation test event set construction method considering multiple factors is adopted. And aiming at the condition of lack of actual system fault event occurrence probability statistics, considering equal-proportion uniform distribution of event occurrence probability, and designing multi-infeed direct current system commutation failure resistance capability evaluation based on probability analysis. A commutation failure resisting effect evaluation method considering actual occurrence probabilities of different fault events is provided, the comprehensive commutation failure probability considering multiple fault events is calculated by adopting a conditional probability mode, and the commutation failure resisting effect of an actual direct current system is described more accurately.

Description

Technical Field

[0001] The invention relates to the field of high voltage direct current power transmission, and in particular to a test and evaluation method for a multi-feed direct current system to resist continuous commutation failure. Background Art

[0002] Line Commutate Converter Based High Voltage Direct Current (LCC-HVDC) has significant advantages in the connection of renewable energy power generation to the grid, long-distance large-capacity power transmission and large-area networking, and has now become an important supporting technology for my country's implementation of the West-to-East Power Transmission Strategy.

[0003] Commutation failure is one of the common faults of LCC-HVDC. In a multi-infeed DC transmission system, there is interaction between multiple DCs, and there is a coupling relationship between multiple types of DC equipment, and the fault characteristics are complex. At present, the East China Power Grid and Guangdong Power Grid have 11 and 9 DC feeds respectively, which are the largest multi-infeed DC transmission systems in my country. In addition, the Central China Power Grid and the North China Power Grid also have multiple DC feeds. The multi-infeed DC system can effectively improve the system's transmission capacity and the flexibility of the operating mode, but it also brings huge challenges to the safe and stable operation of the power grid.

[0004] At present, a large number of research results have been achieved in the commutation failure test method, but there are still the following limitations: (1) The event that leads to commutation failure is related to the electrical location of the fault, the time in the AC electrical cycle, the fault type, and the fault duration. In order to comprehensively analyze the commutation failure characteristics of the multi-input DC system, the traversal simulation method is often used, which has the problems of long time consumption and low efficiency; (2) The commutation failure immunity factor is limited to the fault of the inverter side commutation bus, and does not consider the situation of faults at other locations. Although the commutation failure probability index CFPI can make up for the shortcomings of CFII, it is generally based on a certain fault severity, fault type, and fault location. It uses multiple simulations to analyze the system's commutation failure probability, which cannot comprehensively and intuitively reflect the commutation failure resistance capability. Summary of the invention

[0005] The purpose of the present invention is to provide a method for testing and evaluating a multi-infeed DC system against continuous commutation failures, which solves the problems of long time consumption and low efficiency in the commutation failure characteristic test of the multi-infeed DC system and fails to comprehensively and intuitively reflect the commutation failure resistance capability.

[0006] The above-mentioned object of the present invention is achieved by the following technical solutions: A method for testing and evaluating a multi-infeed DC system against continuous commutation failures comprises the following steps: Step 1: Simulation test of commutation failure in multi-infeed DC system Step 1.1: Extraction of characteristic quantities and judgment criteria for commutation failure simulation test; Step 1.2: Consider the multi-factor influence of multi-infeed DC system commutation failure simulation test method, the multi-factors specifically include fault location, fault type, fault occurrence time, fault duration, DC system operation mode; for the construction of the fault area of ​​commutation failure test, the test idea is: preliminarily identify the commutation failure risk area by conducting fault tests on the AC bus from near to far from the commutation bus, and then conduct detailed tests on the fault line, preliminarily identify the commutation failure test area, and conduct detailed commutation failure simulation tests for areas with commutation failure risks; for the commutation failure test fault type setting, during the test, the fault type is set in the system in equal proportion and the fault is set according to the proportion of actual power grid statistical data; for the setting of the fault occurrence time of the commutation failure test, the fault occurrence time is evenly distributed within an AC cycle, and a total of 50 faults are set at different fault times to carry out commutation failure tests; for the fault duration of the commutation failure test, it is set according to the protection action time limit; for the DC system operation mode of the commutation failure test, the DC transmission power, DC system strength and the interaction between multiple DCs are considered.

[0007] Step 2: Construct a simulation test event set for commutation failure in a multi-infeed DC system According to the priority of the DC system operation mode, fault duration, fault type, fault location, and fault occurrence time from high to low, they are arranged and combined in sequence to form a commutation failure test event set, and a multi-infeed DC system commutation failure simulation test event set construction method considering multiple factors is adopted; according to the priority of the DC system operation mode A, fault duration B, fault type C, fault location D, and fault occurrence time E from high to low, they are arranged and combined in sequence to form a commutation failure test event set. The specific steps are as follows: Step 2.1: When the operation mode A=1 and the fault duration B=1, for the fault type C=1, the commutation failure test is carried out according to the commutation failure area test method. This process already includes the fault location D and the fault occurrence time E; Step 2.2: Repeat step 2.1 for other fault types such as fault type C=2, 3, ...; Step 2.3: When the operating mode A=1 and the fault duration B=2, 3, ..., repeat steps 2.1 and 2.2; Step 2.4: In the case of operation mode A=2,3,…, repeat steps 2.1 to 2.3; The symbols are explained as follows: A represents the operation mode of the DC system, A=1,2,…, iRepresent different DC system operation modes respectively, and the event set consists of i Types of operation; B represents the fault duration, B=1,2,…, h They represent different fault durations, B=1 represents a fault duration of 0.1s, B=2 represents a fault duration of 0.5s, and the event set consists of h The duration of the fault; C represents the fault type, C=1,2,…, k They represent different fault types, C=1 represents three-phase short circuit, C=2 represents single-phase ground short circuit, and the event set consists of k Types of failures; D represents the fault location, D=1,2,…, m Respectively represent AC bus 1, 2, ..., m The fault location of the event set consists of m Fault location; D=x—y—z% represents the fault location on line x—y with a distance of z% from the line head end; E represents the time when the fault occurs.

[0008] Step 3: Evaluation of commutation failure resilience of multi-infeed DC systems based on probability analysis Step 3.1: In view of the lack of actual system fault event probability statistics, the event occurrence probability is considered to be evenly distributed in proportion. The event set is composed of the event set in the commutation failure risk area and the set composed of commutation failure events. A commutation failure resistance evaluation method for multi-infeed DC systems based on probability analysis is designed. When the event j The probability of continuous commutation failure under P j ,but N Comprehensive continuous commutation failure probability under events P 综合 As shown in formula (1):

[0009] Step 3.2: In the case of having actual fault event probability statistics, a commutation failure resistance effect evaluation method considering the actual occurrence probabilities of different fault events is used. When the probability of occurrence of each event is different, the comprehensive commutation failure probability considering multiple fault events is calculated based on the actual occurrence probabilities of different fault events using the conditional probability method; When the event j The probability of continuous commutation failure under P j , and based on historical data events j The probability of occurrence in the power grid is Pwj ,but N Comprehensive continuous commutation failure probability under events P 综合 As shown in formula (2): .

[0010] The beneficial effects of the present invention are: providing a reference for testing the commutation failure characteristics of an actual power grid system, providing a basis for solving the difficult problem of how to evaluate the commutation failure resistance capability of a multi-input DC system, and having important significance for the planning and operation of commutation failure prevention technology and an actual power grid system. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The drawings described herein are used to provide further understanding of the present invention and constitute a part of this application. The illustrative examples of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0012] Figure 1 A schematic diagram of commutation failure classification of the present invention; Figure 2 This is a schematic diagram of classification of multiple DC commutation failures of the present invention; Figure 3 Constructing a flow chart for the commutation failure region of the present invention; Figure 4 A schematic diagram of setting different fault time points of the present invention; Figure 5 This is a flow chart of the commutation failure probability calculation of the present invention.

[0013] Figure 6 Schematic diagram of the test system of the present invention. DETAILED DESCRIPTION

[0014] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0015] See also Figures 1 to 6As shown, the test and evaluation method for resisting continuous commutation failure of a multi-infeed DC system of the present invention extracts the characteristic quantity of commutation failure simulation test, analyzes the judgment criteria of commutation failure under different situations, and proposes a commutation failure simulation test method for a multi-infeed DC system considering the influence of multiple factors. The influence of different types of events in a complex AC / DC system on commutation failure is studied, and the commutation failure test event set is formed according to the priority of the DC system operation mode, fault duration, fault type, fault location, and fault occurrence time from high to low, and a commutation failure test event set is arranged and combined in sequence, and a method for constructing a commutation failure simulation test event set for a multi-infeed DC system considering multiple factors (region + fault type + fault time + fault duration + operation mode) is proposed. In view of the lack of actual system fault event probability statistics, considering the uniform distribution of event occurrence probability in equal proportion, a commutation failure resistance evaluation method for a multi-infeed DC system based on probability analysis is designed. In view of the situation where there is an actual fault event probability statistics, a commutation failure resistance effect evaluation method considering the actual occurrence probability of different fault events is proposed. Assuming that the probability of occurrence of each event is different, based on the actual occurrence probability of different fault events, the conditional probability method is used to calculate the comprehensive commutation failure probability considering multiple fault events. It more accurately describes the commutation failure resistance effect of the actual DC system. The specific process is as follows: 1. Commutation failure simulation test of multi-input DC system: Commutation failure refers to the phenomenon that the valve that has exited conduction during the commutation process fails to restore its blocking capacity in time under the action of reverse voltage, or the commutation process fails to end during the action of reverse voltage, causing the valve that should have been closed to reopen under the action of forward voltage.

[0016] Commutation failure is analyzed at two levels: valve level and system level. The specific classification of commutation failure is as follows: Figure 1 As shown in the figure, the commutation failure judgment criteria and the extraction of the commutation failure simulation test feature quantities are finally proposed, and a commutation failure simulation test method for a multi-input DC system considering the influence of multiple factors is proposed, including fault location, fault type, fault occurrence time, fault duration, DC system operation mode and other factors.

[0017] 1.1 Definition and classification of commutation failure According to the different commutation situations of the converter valve in one power frequency cycle, the commutation failure can be divided into the first commutation failure of the valve stage, the continuous commutation failure of the valve stage and the second discontinuous commutation failure of the valve stage.

[0018] The first commutation failure of the valve stage is defined as follows: the commutation from VT1 to VT3 is unsuccessful, and VT1 may not be able to resume blocking until the next cycle.

[0019] The definitions of valve-level continuous commutation failure are as follows: Definition method 1: If the commutation from VT1 to VT3 is unsuccessful, in the next cycle, if VT3 with commutation failure still cannot conduct normally, it is called "valve-level continuous commutation failure"; Definition method 2: If there are two commutation failures in the same cycle, such as the commutation from VT1 to VT3 is unsuccessful, and then the commutation from VT2 to VT4 is also unsuccessful, it is also called "valve-level continuous commutation failure".

[0020] The definition of the valve-level two discontinuous commutation failures is as follows: after the first commutation failure occurs in the converter valve, it is successfully turned on in the next cycle, but because the commutation conditions are not met, it fails to turn on again in the adjacent cycle.

[0021] In engineering practice, system-level commutation failure is divided into system-level first commutation failure and system-level continuous commutation failure according to the power recovery characteristics of high-voltage DC after the first commutation failure.

[0022] The definition of the first commutation failure at the system level is as follows: the commutation failure of the valve group occurs within 160~200ms after the first commutation failure caused by the system fault.

[0023] The definition of continuous commutation failure at the system level is as follows: after the first commutation failure, the next commutation failure occurs after a time interval of 160~200ms.

[0024] For multi-circuit DC systems, commutation failures can be divided into "simultaneous commutation failures" and "sequential commutation failures" according to the order in which they occur. Figure 2 As shown: Simultaneous commutation failure: If the voltage drop caused by the fault directly leads to simultaneous commutation failure in two or more converter stations, this phenomenon is called "simultaneous commutation failure". "Simultaneous commutation failure" is the most serious commutation failure in a multi-input DC system because multiple DC lines fail to commutate in the same time period. The resulting DC power fluctuation will have a greater impact on the system. The fundamental reason is that the electrical distance between the converter stations of the receiving power grid is relatively close. A single AC system fault may cause a sharp drop in the voltage of multiple commutation buses at the same time, thereby causing simultaneous commutation failure.

[0025] Successive commutation failure: If the voltage drop caused by the fault only causes commutation failure at the converter station near the fault, but causes a secondary voltage drop during the recovery process of the commutation failure at the converter station, and then causes commutation failure at the converter station far from the fault, this commutation failure with a transmission nature should be called "successive commutation failure". The fundamental reason is that due to the complex structure of the multi-input DC system, once the receiving AC power grid fails, although the bus voltage of each converter station is affected, it only causes commutation failure in one DC system. During the recovery process of the commutation failure, the imbalance of reactive power will cause a secondary drop in the voltage of the converter bus. Due to the mutual influence of the voltages between the converter stations, other converter stations will successively experience commutation failure.

[0026] 1.2 Feature extraction and judgment of commutation failure simulation test The feature extraction of commutation failure test is mainly carried out from system operating state quantities such as system turn-off angle and AC voltage.

[0027] During the system operation, the time required for the valve to restore its blocking ability is about 400μs, and the corresponding electrical angle is about 7°, which is defined as the limit shutoff angle. γ min In the commutation failure simulation test, the characteristic value of the turn-off angle is extracted γ Minimum cut-off angle γ min For comparison, if γ < γ min It is considered that commutation failure occurs.

[0028] As one of the characteristic quantities of commutation failure test, AC voltage is easy to grasp and control in real time. By comparing the commutation voltage drop value at the time of fault with the critical voltage drop value, it is judged whether the system has commutation failure. If the commutation voltage drop value exceeds the critical value, it can be judged that the system has commutation failure. This is used as the criterion for commutation failure, but it ignores the influence of waveform distortion in the actual operation of the system.

[0029] In actual engineering, when commutation failure occurs in the system, waveforms such as power pits and voltage pits will be generated. When conducting simulation tests, the degree of drop in system DC power and voltage can be used as a criterion for commutation failure.

[0030] 1.3 Commutation failure simulation test method for multi-infeed DC system considering multiple factors Based on the above commutation failure judgment criteria, a commutation failure simulation test method for a multi-infeed DC system is proposed considering the influence of multiple factors, including fault location, fault type, fault occurrence time, fault duration, DC system operation mode and other factors.

[0031] 1.3.1 Commutation failure test area construction For the simulation area of ​​the commutation failure test, the commutation failure risk area is preliminarily identified. For the area with the commutation failure risk, a detailed commutation failure simulation test is carried out to avoid the full network traversal simulation of the multi-input DC system, effectively improving the simulation test efficiency.

[0032] For a certain system operation mode, fault type, and fault duration, consider 50 fault moments evenly distributed in an AC cycle for testing. For a fault location, if the probability of commutation failure under the above conditions is not 0, it is considered that there is a commutation failure risk. The specific test method is as follows: Test idea: Perform fault tests on the AC busbars from near to far from the commutation busbars, preliminarily identify the commutation failure test area, and then conduct detailed tests on the faulty line. The specific flow chart of the test steps is as follows: Figure 3 shown.

[0033] The specific steps are as follows: (1) According to the network topology, perform fault tests on different AC buses in sequence along the path with electrical distance from the commutation bus from near to far, and observe the probability of commutation failure. If there is no commutation failure risk for three consecutive AC buses on a certain path, stop the commutation failure test on that path. The first commutation bus on that path that does not have a commutation failure risk and all buses with a commutation failure risk together form a commutation failure test area. (2) For the fault positions where the probability of phase commutation failure of the AC busbar at both ends of the line is 100%, no more testing of different fault positions of the line is performed, and it is assumed that the probability of phase commutation failure of each fault position of the entire line is 100%; (3) For the fault locations where the probability of commutation failure of the AC busbar at both ends of the line is not 100%, 20%, 50%, and 80% of the line length are selected as the fault locations. Based on the actual data distribution of the actual 500 kV line of the actual power grid provided, the fault probabilities at different locations are considered, and the commutation failure test is performed on the assumption that the fault probabilities at the near end, middle end, and far end of the line are 15%, 70%, and 15%, respectively; (4) For the fault locations where the probability of AC bus commutation failure at both ends of the line is not all 0%, 20%, 50%, and 80% of the line length are selected as fault locations. Based on the actual data distribution of the actual 500 kV line of the actual power grid provided, the fault probabilities at different locations are considered. It is assumed that the fault probabilities at the near end, middle end, and far end of the line are 15%, 70%, and 15%, respectively, for commutation failure testing.

[0034] 1.3.2 Phase-change failure test fault type setting For the fault type setting of the commutation failure test, different fault types such as single-phase short circuit to ground, two-phase short circuit, two-phase short circuit to ground, and three-phase short circuit are considered, and detailed commutation failure simulation tests are carried out.

[0035] For a certain system operation mode, fault location, and fault duration, 50 fault moments evenly distributed in an AC cycle are tested. The specific test methods for different fault types are as follows: Tests are conducted for each fault type. In each fault type commutation failure risk area, single-phase short circuit to ground, two-phase short circuit, two-phase short circuit to ground, and three-phase short circuit are set to conduct commutation failure tests. The steps are as follows: (1) The probability of occurrence of any fault type in the entire network is evenly distributed. During the test, the fault types are set in the system in equal proportions; (2) There are significant differences in the probability of occurrence of different fault types. Based on the proportion distribution of historical data of single-phase short circuit and grounding, two-phase short circuit, two-phase short circuit and grounding, and three-phase short circuit in the actual power grid, the typical fault set of AC single-phase short circuit and grounding, two-phase short circuit, two-phase short circuit and grounding, and three-phase short circuit at 500 kV and above AC three-phase nodes in the standard model is determined. According to the proportion of the probability of occurrence of different types of events, single-phase short circuit and grounding: two-phase short circuit: two-phase short circuit and grounding: three-phase short circuit 93:4:2:1, the fault events are set.

[0036] Taking the standard model proposed by China Southern Power Grid as an example, the comparison between the DC commutation failure area caused by three-phase fault and the DC commutation failure area caused by single-phase fault obtained based on PSD-BPA simulation scanning shows that there are significant differences in the commutation failure of multi-infeed DC systems under different fault types such as single-phase fault and three-phase fault.

[0037] 1.3.3 Phase-change failure test fault occurrence time setting For the fault occurrence time setting of the commutation failure test, different fault times have an impact on the occurrence of commutation failure. For a certain system operation mode, fault location, fault type and fault duration, such as Figure 4 As shown, the fault occurrence time is evenly distributed within one AC cycle, and a total of 50 faults are set at different fault times to carry out commutation failure tests.

[0038] 1.3.4 Setting the duration of the commutation failure test The fault duration of the commutation failure test is mainly set according to the protection action time limit.

[0039] For a certain system operation mode, fault location, and fault type, consider 50 fault moments evenly distributed in an AC cycle for testing. For the duration of commutation failure, the specific test method is as follows: (1) Considering the action time limit of AC main protection to clear the fault, the fault duration is set to 0.1s and the commutation failure test is carried out; (2) Considering the action time limit of the main protection refusal to operate and the backup protection action to clear the fault, the fault duration is set to 0.5s and the commutation failure test is carried out; (3) Considering the occurrence of a permanent fault, after the protection action clears the fault, the reclosing action is delayed for 1 second and the reclosing fails, and a phase change failure test is carried out.

[0040] 1.3.5 Commutation failure test Multi-infeed DC system operation mode settings For the DC system operation mode of commutation failure test, the DC transmission power, DC system strength and the interaction between multiple DCs are mainly considered.

[0041] For a certain fault type, fault location, and fault duration, 50 fault moments evenly distributed in an AC cycle are considered for testing, and different operating modes of the DC system are changed to carry out commutation failure tests.

[0042] (1) Change the DC transmission power P dc In a multi-infeed DC system, a commutation failure test is performed by adjusting the transmission power of one or several DC systems to 90% and 80% of the rated power, or by changing the reference value of the DC current.

[0043] (2) Change the DC system strength SCR In a multi-infeed DC system, the AC system strength is changed by adjusting the AC system impedance near one or several DC lines, and a commutation failure test is performed.

[0044] (3) Change the multi-input interaction factor MIIF In a multi-infeed DC system, the multi-infeed interaction factor MIIF is changed by adjusting the mutual impedance between the two DCs, and a commutation failure test is performed.

[0045] 2. Construct a simulation test event set for commutation failure in multi-infeed DC systems The events that lead to commutation failure are related to the electrical location of the fault, the time in the AC electrical cycle, the fault type, and the fault duration. The above-mentioned fault area, fault type, fault occurrence time, fault duration and the operation mode of the DC system are all permutations and combinations, forming a multi-input DC system commutation failure simulation test event set of "area + fault type + fault time + fault duration + DC system operation mode".

[0046] The symbols are explained as follows: A represents the operation mode of the DC system, A=1,2,…, i Represent different DC system operation modes respectively, and the event set consists of i Types of operation; B represents the fault duration, B=1,2,…, h They represent different fault durations, B=1 represents a fault duration of 0.1s, B=2 represents a fault duration of 0.5s, and the event set consists of h The duration of the fault; C represents the fault type, C=1,2,…, k They represent different fault types, C=1 represents three-phase short circuit, C=2 represents single-phase ground short circuit, and the event set consists of k Types of failures; D represents the fault location, D=1,2,…, m Respectively represent AC bus 1, 2, ..., m The fault location of the event set consists of m Fault location; D=x—y—z% represents the fault location on line x—y with a distance of z% from the line head end; E represents the time when the fault occurs.

[0047] According to the priority of DC system operation mode A, fault duration B, fault type C, fault location D, and fault occurrence time E from high to low, they are arranged and combined in sequence to form a commutation failure test event set. The specific steps are as follows: 2.1 When the operation mode A=1 and the fault duration B=1, for the fault type C=1, the commutation failure test is carried out according to the commutation failure area test method (this process already includes the fault location D and the fault occurrence time E); 2.2 For other fault types such as fault type C=2, 3, ..., repeat step 2.1; 2.3 When the operation mode A=1 and the fault duration B=2, 3, ..., repeat steps 2.1 and 2.2; 2.4 In the case of operating mode A=2,3,…, repeat steps 2.1 to 2.3.

[0048] 3. Evaluation of Commutation Failure Resistance Capability of Multi-infeed DC System Based on Probabilistic Analysis Based on the above commutation failure classification, a commutation failure resistance evaluation method can be constructed under different conditions. Taking into account the operation mode of the multi-infeed DC system and the fault type, location, duration and occurrence time, two commutation failure probability calculation methods are designed to evaluate the commutation failure resistance capability of the multi-infeed DC system. The specific block diagram is as follows: Figure 5 shown.

[0049] 3.1 Probabilistic analysis of single-circuit DC system For a single-circuit DC system, the following two calculation methods are carried out: Method 1: In view of the lack of actual system failure event probability statistics, it is assumed that the events in the event set are evenly distributed in equal proportions, and the total number of events is N , where the number of commutation failure events is n , the probability of commutation failure at this time is as follows (3-1):

[0050] The composition of the event set can be considered in the following two ways: (1) The selected commutation failure test event is an event set consisting of fault points belonging to the commutation failure risk area. The total number of events is N For some fault points with commutation failure risk, the probability of commutation failure is not 100%. In this case, the number of events that may cause commutation failure is N A1 Less than N After adopting the comprehensive method to resist commutation failure, the number of events that can cause commutation failure becomes N B1 , under the typical fault set, the probability of commutation failure of the original system is A1, and the probability of continuous commutation failure of the system after adopting the comprehensive resistance method of commutation failure is B1. At this time, the value of the continuous commutation failure probability of the system is calculated as shown in formula (3-2):

[0051] (2) The selected commutation failure test event is a set of events that cause commutation failure. The total number of events is N , for all events, commutation failure occurs. In this case, the number of events that can cause commutation failure is N A2 After adopting the commutation failure suppression method, the number of events that can cause commutation failure becomes N B2 , under the typical fault set, the probability of commutation failure of the original system is A2, and the probability of continuous commutation failure of the system after adopting the comprehensive resistance method of commutation failure is B2. At this time, the probability of continuous commutation failure of the system is reduced as shown in formula (3-3):

[0052] Method 2: For the situation where we have actual probability statistics of fault events, we assume that the probability of each event is different, C represents the fault type, C=1,2,…, kThey represent different types of fault events, such as: C=1 represents three-phase short circuit, C=2 represents single-phase ground short circuit, ...; CF represents commutation failure, CF=1 represents commutation failure, CF=0 represents no commutation failure; E represents the time of fault occurrence, which is uniformly distributed within an AC cycle. During the test, multiple short-circuit fault tests are set at equal time intervals. P(C=1) represents the probability of a three-phase short circuit event, and P[(CF=1)|(C=1)] represents the probability of commutation failure (CF=1) under the condition of a three-phase short circuit (C=1) event at different fault times E. Therefore, for a certain system operation mode, fault location, and fault duration, only the fault time and fault type are considered. The probability of commutation failure is as shown in formula (3-4):

[0053] Similarly, if A, B, and D represent the DC system operation mode, duration, and fault location, respectively, A=1,2,…, i , B=1,2,…, h， D = 1,2,…, m Represent different DC system operation modes, different fault duration events and fault location events respectively. The comprehensive commutation failure probability considering multiple fault events is obtained as shown in formula (3-5):

[0054] In this case, it is also necessary to consider the composition of the two event sets in method 1, and the specific probability calculation results will also be different. Under the typical fault set, the probability of phase switching failure of the original system is A3, and the probability of continuous phase switching failure of the system after adopting the comprehensive protection method of phase switching failure is B3. P (CF=1)=B3, at this time, the probability of continuous commutation failure of the calculation system is reduced as shown in formula (3-6):

[0055] 3.2 Probabilistic analysis of multi-circuit DC systems For multi-circuit DC systems, the following two calculation methods are specifically carried out: Method 1: In view of the lack of actual system failure event probability statistics, it is assumed that the events in the event set are evenly distributed in equal proportions, and the total number of events is N , the composition of the event set can be considered in the following two ways: (1) The selected commutation failure test event is an event set consisting of fault points belonging to the commutation failure risk area. The total number of events is N , resulting in the number of DC commutation failure events being N The number of DC commutation failure events in 1 and 2 isN 2， The number of 5 DC commutation failure events is N 5. i The number of DC commutation failure events is N i , the probability of commutation failure at this time is as follows (3-7):

[0056] Under the typical fault set, the probability of commutation failure of the original system is A4, and the probability of continuous commutation failure of the system after adopting the comprehensive resistance method of commutation failure is B4. At this time, the value of the continuous commutation failure probability of the system is calculated as shown in formula (3-8):

[0057] (2) The selected commutation failure test event is a set of events that cause commutation failure. The total number of events is N , for all events, commutation failure occurs. The calculation method of commutation failure probability of this configuration is consistent with (1). The original system commutation failure probability is A5. The continuous commutation failure probability of the system after adopting the comprehensive commutation failure prevention method is B5. At this time, the calculation of the continuous commutation failure probability of the system is reduced as shown in formula (3-9):

[0058] Method 2: For the situation with actual fault event probability statistics, assuming that the probability of each event is different, the probability of one DC commutation failure is P The probability of failure of DC commutation in circuits 1 and 2 is P The probability of failure of 2…5 DC commutation is P 5. i The probability of DC commutation failure is P i , the probability of commutation failure at this time is as follows (3-10):

[0059] In this kind of fault situation, it is also necessary to consider the composition of the two event sets in method 1, and the specific probability calculation results will also be different. Under the typical fault set, the probability of phase change failure of the original system is A6, and the probability of continuous phase change failure of the system after adopting the comprehensive protection method of phase change failure is B6. The value of the continuous phase change failure probability of the system is calculated as shown in formula (3-11):

[0060] In summary, the probability of continuous commutation failure in the calculation system is reduced by Δ P%, and the evaluation of the commutation failure resistance capability of multi-infeed DC systems can be realized based on this value, which is of great significance for the planning and operation of power grids.

[0061] Example: by Figure 6 Taking the test system with 2 DC lines and 4 nodes as shown in the figure as an example, a commutation failure simulation test event set is constructed. Based on the simulation test event set, the test results of the number of commutation failure converter stations for each fault event are obtained respectively, and the continuous commutation failure probability under a single fault event is calculated. Then, the comprehensive continuous commutation failure probability under multiple fault events is calculated. The specific process is as follows: (1) For the test results of a single fault event, the probability of continuous commutation failure of the system is calculated as follows: Total N If a converter station n The probability of continuous commutation failure in the system is P= n / N .

[0062] Taking a DC system with two lines as an example, the results of commutation failure are divided into three cases: Case 1: 0 converter stations have continuous commutation failures, and the probability of continuous commutation failures in the system is 0%; Case 2: Continuous commutation failure occurs in one converter station, and the probability of continuous commutation failure in the system is 50%; Case 3: Continuous commutation failure occurs in two converter stations, and the probability of continuous commutation failure of the system is 100%.

[0063] (2) Based on the continuous commutation failure probability of a single fault event, the comprehensive continuous commutation failure probability under all fault events is calculated using the conditional probability method as follows: If the event j The probability of continuous commutation failure under P j , and assuming that based on historical data events j The probability of occurrence in the power grid is P wj , N The comprehensive continuous commutation failure probability of each event P 综合 As shown in formula (3-12):

[0064] by Figure 6 Taking a DC system with two lines as an example, the comprehensive continuous commutation failure probability is calculated: If busbars 1, 2, 3, and 4 fail, and the number of commutation failures in the converter station is 2, 2, 1, and 0, respectively, then the probability of continuous commutation failure under the four events is 100%, 100%, 50%, and 0%, respectively. Assuming that the probability of busbars 1, 2, 3, and 4 failing is 30%, 30%, 25%, and 15%, respectively, then the comprehensive probability of continuous commutation failure under the four events is P 综合 =(30%*100%)+(30%*100%)+(25%*50%) +(15%*0%)=72.5%.

[0065] (3) Evaluation method for the effect of resisting continuous commutation failure: The defense method adopts the continuous commutation failure probability of the front and rear systems as A and B respectively, and the continuous commutation failure probability reduction value is as shown in formula (3-13): .

[0066] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made to the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for testing and evaluating a multi-infeed DC system against continuous commutation failure, characterized in that: The steps include: Step 1: Simulation test of commutation failure of multi-infeed DC system; Step 2: Construct a commutation failure simulation test event set for a multi-infeed DC system; Step 3: Evaluation of commutation failure resistance capability of multi-infeed DC system based on probability analysis; Step 3.1: In view of the lack of actual system fault event probability statistics, the event occurrence probability is considered to be evenly distributed in proportion. The event set is composed of the event set in the commutation failure risk area and the set composed of commutation failure events. A commutation failure resistance evaluation method for multi-infeed DC systems based on probability analysis is designed. When the event j The probability of continuous commutation failure under P j ,but N Comprehensive continuous commutation failure probability under events P 综合 for: ； Step 3.2: In the case of having actual fault event probability statistics, a commutation failure resistance effect evaluation method considering the actual occurrence probabilities of different fault events is used. When the probability of occurrence of each event is different, the comprehensive commutation failure probability considering multiple fault events is calculated based on the actual occurrence probabilities of different fault events using the conditional probability method; When the event j The probability of continuous commutation failure under P j , and based on historical data events j The probability of occurrence in the power grid is P wj ,but N Comprehensive continuous commutation failure probability under events P 综合 for: 。 2. The method for testing and evaluating the protection of a multi-infeed DC system against continuous commutation failure according to claim 1, characterized in that: The multi-infeed DC system commutation failure simulation test described in step 1 includes: Step 1.1: Extraction of characteristic quantities and judgment criteria for commutation failure simulation test; Step 1.2: Consider the multi-factor influence of multi-infeed DC system commutation failure simulation test method, the multi-factors specifically include fault location, fault type, fault occurrence time, fault duration, DC system operation mode; for the construction of the fault area of ​​commutation failure test, the test idea is: preliminarily identify the commutation failure risk area by conducting fault tests on the AC bus from near to far from the commutation bus, and then conduct detailed tests on the fault line, preliminarily identify the commutation failure test area, and conduct detailed commutation failure simulation tests for areas with commutation failure risks; for the commutation failure test fault type setting, during the test, the fault type is set in the system in equal proportion and the fault is set according to the proportion of actual power grid statistical data; for the setting of the fault occurrence time of the commutation failure test, the fault occurrence time is evenly distributed within an AC cycle, and a total of 50 faults are set at different fault times to carry out commutation failure tests; for the fault duration of the commutation failure test, it is set according to the protection action time limit; for the DC system operation mode of the commutation failure test, the DC transmission power, DC system strength and the interaction between multiple DCs are considered.

3. The method for testing and evaluating the protection of a multi-infeed DC system against continuous commutation failure according to claim 1, characterized in that: The construction of the multi-infeed DC system commutation failure simulation test event set described in step 2 is specifically: according to the priority of the DC system operation mode, fault duration, fault type, fault location, and fault occurrence time from high to low, they are arranged and combined in sequence to form a commutation failure test event set, and a multi-factor multi-infeed DC system commutation failure simulation test event set construction method is adopted; according to the priority of the DC system operation mode A, fault duration B, fault type C, fault location D, and fault occurrence time E from high to low, they are arranged and combined in sequence to form a commutation failure test event set, and the specific steps are as follows: Step 2.1: When the operation mode A=1 and the fault duration B=1, for the fault type C=1, the commutation failure test is carried out according to the commutation failure area test method. This process already includes the fault location D and the fault occurrence time E; Step 2.2: For the fault type C=2,3,…, repeat step 2.1; Step 2.3: When the operating mode A=1 and the fault duration B=2, 3, ..., repeat steps 2.1 and 2.2; Step 2.4: In the case of operation mode A=2,3,…, repeat steps 2.1 to 2.3; The symbols are explained as follows: A represents the operation mode of the DC system, A=1,2,…, i Represent different DC system operation modes respectively, and the event set consists of i Types of operation; B represents the fault duration, B=1,2,…, h They represent different fault durations, B=1 represents a fault duration of 0.1s, B=2 represents a fault duration of 0.5s, and the event set consists of h The duration of the fault; C represents the fault type, C=1,2,…, k They represent different fault types, C=1 represents three-phase short circuit, C=2 represents single-phase ground short circuit, and the event set consists of k Types of faults; D represents the fault location, D=1,2,…, m Respectively represent AC bus 1, 2, ..., m The fault location of the event set consists of m Fault location; D=x—y—z% represents the fault location on line x—y with a distance of z% from the line head end; E represents the time when the fault occurs.

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