A method and system for diagnosing open circuit fault of submodule of MMC under NLM strategy

Abstract

The present invention proposes a method and system for diagnosing submodule open circuit faults of MMC under the NLM strategy. The method selects the absolute error between the actual value and the predicted value of the SM capacitor voltage as a diagnostic mark, and can accurately and quickly diagnose two types of MMC submodule open circuit faults at different power points. At the same time, it can cope with the situation where multiple SMs in the bridge arm fail. By utilizing the sorting results of the voltage balancing control, the amount of calculation is greatly reduced, and the diagnosis process can be completed within one power frequency cycle, thereby improving the reliability of the modular multilevel converter and ensuring the stable operation of the modular multilevel converter.

Description

Technical Field

[0001] The invention relates to the field of power electronics, and in particular to a method and system for diagnosing submodule open circuit faults of an MMC under an NLM strategy. Background Art

[0002] Modular multilevel converter (MMC) has the characteristics of modular structure, redundant configuration and scalable power. These advantages make MMC widely used in high-voltage and high-power scenarios, such as high voltage direct current (HVDC) transmission systems, static synchronous compensators and power electronic transformers.

[0003] The MMC topology consists of a large number of sub-modules (SMs) connected in series. Reliability is one of its important issues. Any IGBT failure in an SM will affect the normal operation of the MMC, and may even further damage other components, eventually leading to the collapse of the entire system. IGBTs are divided into short-circuit faults and open-circuit faults. Short-circuit faults usually cause overcurrent, which is very harmful, but there are mature solutions in engineering, that is, short-circuit protection is integrated in the gate driver, and the switch will be quickly closed in the event of a short-circuit fault. Compared with short-circuit faults, open-circuit faults will not immediately damage the SM and may not be detected for a long time. Recently, level approximation modulation has been widely used in MMCs with high level counts due to its good dynamic performance and simple implementation.

[0004] Traditional fault diagnosis methods usually require additional sensors, and have problems such as high computational complexity and long diagnostic time. More importantly, the diagnostic process relies on the accurate setting of empirical thresholds. When the operating power of the MMC changes, the setting and promotion of empirical thresholds are more difficult. Therefore, it is necessary to provide a new submodule open circuit fault diagnosis method for NLM modulated MMCs under different operating powers. Summary of the invention

[0005] The purpose of the present invention is to propose a submodule open circuit fault diagnosis method and system for MMC under NLM strategy for NLM modulation MMC under different operating powers. The method can accurately and quickly diagnose two MMC submodule open circuit faults at different power points by selecting the absolute error between the actual value and the predicted value of the SM capacitor voltage as a diagnostic mark, thereby ensuring the stable operation of the modular multilevel converter.

[0006] The present invention is achieved through the following technical solutions:

[0007] A method for diagnosing an open circuit fault of a submodule of an MMC under an NLM strategy comprises the following steps:

[0008] Step 1, obtaining the actual value of the capacitor voltage of the setting submodule in the current control cycle in the MMC, and estimating the estimated value of the capacitor voltage of the setting submodule in the current control according to the bridge arm current of the previous control cycle of the MMC;

[0009] Step 2: determine the absolute error between the estimated value of the capacitor voltage of the setting submodule and the actual value of the capacitor voltage, compare the absolute error with a setting threshold, and determine whether the setting submodule fails;

[0010] Step 3: When a submodule fails, the actual value of the capacitor voltage of the submodule is used as the maximum voltage value, and all submodules are sorted in descending order according to the capacitor voltage value to obtain a submodule sorting sequence;

[0011] Step 4: Obtain the actual value of the capacitor voltage of the lower-level submodule adjacent to the set submodule in the submodule sorting sequence in the current control cycle; estimate the capacitor voltage estimate value of the lower-level submodule in the current control cycle based on the bridge arm current of the MMC in the previous cycle and the number of submodules put into operation in the previous cycle;

[0012] Step 5: determine the absolute error between the estimated value of the capacitor voltage of the lower-level submodule and the actual value of the capacitor voltage, compare the absolute error with a set threshold, and determine whether the lower-level submodule fails;

[0013] Step 6: If a lower-level submodule fails, step 4 is repeated to perform fault diagnosis on the lower-level submodule of the lower-level submodule until the diagnosed submodule is free of faults and all faulty submodules are obtained.

[0014] Preferably, in step 1, the estimated value of the capacitor voltage currently controlled by the setting submodule is estimated according to the positive or negative value of the bridge arm current in the previous control cycle of the MMC.

[0015] Preferably, in step 1, if the bridge arm current is positive, the estimated value of the capacitor voltage currently controlled by the setting submodule is equal to the actual value of the capacitor voltage of the setting submodule in the previous control cycle.

[0016] Preferably, in step 1, if the bridge arm current i ua (k-1) is non-positive, then the capacitor voltage estimation value of the submodule in the current control cycle is set as follows:

[0017]

[0018] Where C is the capacitance of the submodule, T c is the duration of the control cycle, U c_number_rea (k-1) is the actual value of the capacitor voltage of the submodule in the previous control cycle.

[0019] Preferably, in step 2, when the absolute error is greater than a set threshold, the setting submodule fails, otherwise the setting submodule does not fail.

[0020] Preferably, the expression for setting the threshold in step 2 is as follows:

[0021]

[0022] Where C is the capacitance of the submodule, T c is the duration of the control cycle, i ua (k) is the bridge arm current of the current control cycle, i ua (k-1) is the bridge arm current of the previous control cycle, err normal is the absolute error, err _th To set the threshold.

[0023] Preferably, the method for determining the capacitor voltage estimation value of the lower-level submodule in the current control cycle in step 4 is as follows:

[0024] If the bridge arm current is positive and the number of submodules N put into operation in the previous control cycle on (k-1) <N-m+1,N on The number of submodules put into operation in the previous control cycle, N is the total number of submodules on the bridge arm, and the estimated capacitor voltage value U of the lower submodule under the current control c_pos_m It is equal to the actual value of the capacitor voltage of the submodule in the previous control cycle.

[0025] Preferably, the method for determining the capacitor voltage estimation value of the lower-level submodule in the current control cycle in step 4 is as follows:

[0026] If the bridge arm current is non-positive and the number of sub-modules N put into operation in the previous control cycle on (k-1)>m-1, the estimated value of the capacitor voltage of the lower submodule is U c_neg_m .

[0027]

[0028] Where C is the capacitance of the submodule, T c is the duration of the control cycle, i ua (k) is the bridge arm current of the current control cycle, i ua (k-1) is the bridge arm current of the previous control cycle, U c_number_m (k-1) is the actual value of the capacitor voltage of the submodule in the previous cycle.

[0029] The present invention also provides a system for diagnosing a submodule open circuit fault of an MMC under an NLM strategy, comprising:

[0030] The first capacitor voltage acquisition module is used to obtain the actual value of the capacitor voltage of the setting submodule in the MMC in the current control cycle, and estimate the estimated value of the capacitor voltage of the setting submodule in the current control according to the bridge arm current of the previous control cycle of the MMC;

[0031] The MMC diagnostic module is used to determine the absolute error between the estimated value of the capacitor voltage of the setting submodule and the actual value of the capacitor voltage, and compare the absolute error with a setting threshold to determine whether the setting submodule fails;

[0032] A sorting module is used to, when a submodule fails, use the actual value of the capacitor voltage of the submodule as the maximum voltage value, sort all submodules in descending order according to the capacitor voltage value, and obtain a submodule sorting sequence;

[0033] The second capacitor voltage acquisition module is used to obtain the actual value of the capacitor voltage of the lower-level submodule adjacent to the set submodule in the submodule sorting sequence in the current control cycle; based on the bridge arm current of the MMC in the previous cycle and the number of submodules put into operation in the previous cycle, estimate the capacitor voltage estimation value of the lower-level submodule in the current control cycle;

[0034] A submodule diagnosis module, used to determine the absolute error between the estimated value of the capacitor voltage of the lower submodule and the actual value of the capacitor voltage, compare the absolute error with a set threshold value, and determine whether the set submodule fails;

[0035] The output module is used to diagnose the fault of the next-level submodule of the next-level submodule when a fault occurs in the next-level submodule of the next-level submodule, until the diagnosed submodule is free of faults and all faulty submodules are obtained.

[0036] Preferably, the method for determining the capacitor voltage estimation value of the lower-level submodule in the current control cycle is as follows:

[0037] If the bridge arm current is non-positive and the number of sub-modules N put into operation in the previous control cycle on (k-1)>m-1, the estimated value of the capacitor voltage of the lower submodule is U c_neg_m ;

[0038]

[0039] Where C is the capacitance of the submodule, T c is the duration of the control cycle, i ua (k) is the bridge arm current of the current control cycle, i ua (k-1) is the bridge arm current of the previous control cycle, U c_number_m (k-1) is the actual value of the capacitor voltage of the submodule in the previous cycle.

[0040] Compared with the prior art, the present invention has the following beneficial technical effects:

[0041] The present invention discloses a method for diagnosing an open-circuit fault of a submodule of an MMC under an NLM strategy. By selecting the absolute error between the actual value and the predicted value of the SM capacitor voltage as a diagnostic mark, two types of MMC submodule open-circuit faults can be accurately and quickly diagnosed at different power points. At the same time, the method can cope with the situation where multiple SMs in the bridge arm fail. The method also greatly reduces the amount of calculation by utilizing the sorting result of the voltage-sharing control. The method can complete the diagnosis process within one power frequency cycle, thereby improving the reliability of the modular multilevel converter and ensuring the stable operation of the modular multilevel converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The MMC main circuit and half-bridge submodule topology of the present invention;

[0043] Figure 2 Schematic diagrams of two types of submodule open circuit failures of the present invention;

[0044] Figure 3 It is a flow chart of the submodule open circuit fault diagnosis method of the present invention;

[0045] Figure 4 This is the simulation waveform of a type I fault occurring at the rated load of the MMC in the inverter mode of the present invention;

[0046] Figure 5 This is the simulation waveform of the inverter mode MMC light load type I fault of the present invention;

[0047] Figure 6 This is the simulation waveform of the inverter mode MMC rated load with type II fault;

[0048] Figure 7 This is the simulation waveform of type II fault occurring in the inverter mode MMC light load of the present invention;

[0049] Figure 8 This is the simulation waveform of a type II fault occurring at the rated load of the MMC in the rectification mode of the present invention;

[0050] Fig. 9 This is the simulation waveform of a type II fault occurring in the light-load MMC rectifier mode of the present invention.

[0051] Figure 1 L is the bridge arm reactor, U dc is the DC bus voltage, I dc is the total DC current, i x is the AC output phase current. DETAILED DESCRIPTION

[0052] The present invention will be further described in detail below in conjunction with the accompanying drawings, which are intended to explain the present invention rather than to limit it.

[0053] See also Figure 1-3 , a method for diagnosing an open circuit fault of a submodule of an MMC under an NLM strategy, comprising the following steps:

[0054] Step 1: Get the bridge arm current i of the current control cycle of the MMC ua (k), and the actual value U of the capacitor voltage of the submodule SM with serial number number c_number_rea (k);

[0055] Figure 1 The topology of a typical MMC (modular multilevel converter) is shown; a three-phase six-bridge structure is adopted, each phase includes two upper and lower bridge arms, and each bridge arm is composed of N sub-modules (SM) and a bridge arm reactor L. The sub-module adopts a half-bridge structure, including 2 IGBTs, 2 anti-parallel diodes and 1 DC capacitor.

[0056] Step 2: According to the bridge arm current i of the previous control cycle of MMC ua The positive or negative of (k-1) estimates the estimated capacitor voltage U of the submodule SM with the serial number number at the current control c_number_est (k).

[0057] If the bridge arm current i ua (k-1) is positive, based on which the estimated capacitance voltage value U of the submodule SM with serial number number can be estimated c_pos The submodule SM with the serial number number is at the current controlled capacitor voltage estimated value U c_pos It is equal to the actual value of the capacitor voltage of the submodule in the previous control cycle. The expression of the estimated value of the capacitor voltage is as follows:

[0058] U c_pos =U c_number_rea (k-1)

[0059] Wherein, k is the current control cycle.

[0060] If the bridge arm current i ua (k-1) is non-positive, based on which the estimated capacitance voltage value U of the submodule SM with serial number number can be estimated c_neg ; Then the estimated value of the capacitor voltage of the submodule SM with serial number number in the current control cycle is as follows:

[0061]

[0062] Where C is the capacitance of the submodule, T c is the duration of the control cycle, Uc _number_rea (k-1) is the actual value of the capacitor voltage of the submodule in the previous control cycle.

[0063] Step 3: Calculate the absolute error err between the estimated value of the capacitor voltage of the submodule SM with the serial number number and its actual value of the capacitor voltage in the current control cycle, and compare it with the set threshold err _th A comparison is performed, and whether the MMC has an open circuit fault is determined based on the comparison result.

[0064] Specifically, if the absolute error err is less than the set threshold err _th , it means that there is no fault in the MMC, and the submodule SM with the serial number number is updated to the submodule with the largest capacitor voltage at the current moment and its capacitor voltage is recorded as U c_number (k), complete the detection of the current control cycle, return to step 1, and wait for the next control cycle detection process.

[0065] If the absolute error err is greater than the set threshold err _th , indicating that the submodule fails, that is, the MMC fails, and the submodule SM with the serial number number is updated to the submodule with the largest capacitor voltage at the current moment, and all submodules are arranged in descending order according to their actual values ​​of capacitor voltage to obtain a submodule sorting sequence;

[0066] Set threshold err _th The specific setting method is:

[0067]

[0068] Among them, err normal It is the absolute error between the actual value of the capacitor voltage of the submodule SM without fault and the estimated value of the capacitor voltage, which is generally close to zero. Considering the possible sensor error, submodule capacitance tolerance and other factors in practice, when setting the threshold err th It can be appropriately made close to the right side of the inequality.

[0069] After completing this step, the diagnosis of whether the MMC has a fault is achieved. It should be noted that in the MMC fault diagnosis process, not all faulty sub-modules can be detected, so in subsequent steps, all faulty sub-modules in the MMC need to be diagnosed and located.

[0070] Step 4: When an open circuit fault is detected in the MMC, the bridge arm current i of the current control cycle of the MMC is obtained. ua (k), and the actual value U of the capacitor voltage of the submodule with serial number number_m c_number_m (k);

[0071] The submodule numbered number_m is a submodule adjacent to the submodule SM with the largest capacitor voltage in the submodule sorting sequence, and m is the sorting number of the submodule in the submodule sorting sequence.

[0072] Figure 2 Schematic diagrams of two types of submodule open circuit faults studied in the present invention are shown: Type I fault and Type II fault.

[0073] Step 5: Obtain the bridge arm current i of the MMC a phase in the previous cycle of the current control cycle described in step 4. ua The positive or negative of (k-1) is combined with the input amount of the submodule in the previous cycle to estimate the capacitor voltage estimated value of the submodule with the serial number of number_m.

[0074] Specifically, if the bridge arm current is positive and the number of submodules N put into operation in the previous control cycle on (k-1) <N-m+1,N on The number of submodules put into operation in the previous control cycle, N is the total number of submodules on the bridge arm, based on which the estimated capacitor voltage value U of the submodule SM with the serial number number_m can be estimated c_pos_m , the submodule SM with serial number number_m is at the current controlled capacitor voltage estimated value U c_pos_m It is equal to the actual value of the capacitor voltage of the submodule in the previous control cycle. The expression of the estimated value of the capacitor voltage is as follows:

[0075] U c_pos_m =U c_number_m (k-1)

[0076] If the bridge arm current is positive but N on (k-1)≥N-m+1, then return to step 4 and wait for the positioning process of the next control cycle;

[0077] If the bridge arm current is non-positive and the number of sub-modules N put into operation in the previous control cycle on (k-1)>m-1, based on which the estimated capacitance voltage value U of the submodule SM with serial number number_m can be estimated c_neg_m for:

[0078]

[0079] If it is non-positive but N on (k-1)≤m-1, then return to step 4 and wait for the positioning process of the next control cycle.

[0080] Step 6: Calculate the absolute error err between the estimated value and the actual value of the capacitor voltage of the submodule SM with serial number number_m _m and compare it with the set threshold err _thAfter comparison, if it is greater than the set threshold, it means that this submodule is a faulty submodule.

[0081] Step 7: Add 1 to m and repeat steps 4-6 to diagnose the submodule with the sequence number of number_m+1 in the submodule sorting sequence until the absolute error err corresponding to the submodule with the sequence number of number_m+i is _m If it is less than the set threshold, it means that this submodule is a normal submodule. At this point, all faulty submodules in the MMC open circuit fault are obtained, the diagnosis process is completed, and the number of faulty submodules and the corresponding serial numbers are output.

[0082] The present invention also provides a system for MMC submodule open circuit fault diagnosis method under NLM strategy, including a first capacitor voltage acquisition module, an MMC diagnosis module, a sorting module, a second capacitor voltage acquisition module, a submodule diagnosis module and an output module.

[0083] The first capacitor voltage acquisition module is used to obtain the actual value of the capacitor voltage of the setting submodule in the MMC in the current control cycle, and estimate the estimated value of the capacitor voltage of the setting submodule in the current control according to the bridge arm current of the previous control cycle of the MMC;

[0084] The MMC diagnostic module is used to determine the absolute error between the estimated value of the capacitor voltage of the setting submodule and the actual value of the capacitor voltage, and compare the absolute error with a setting threshold to determine whether the setting submodule fails;

[0085] A sorting module is used to, when a submodule fails, use the actual value of the capacitor voltage of the submodule as the maximum voltage value, sort all submodules in descending order according to the capacitor voltage value, and obtain a submodule sorting sequence;

[0086] The second capacitor voltage acquisition module is used to obtain the actual value of the capacitor voltage of the lower-level submodule adjacent to the set submodule in the submodule sorting sequence in the current control cycle; based on the bridge arm current of the MMC in the previous cycle and the number of submodules put into operation in the previous cycle, estimate the capacitor voltage estimation value of the lower-level submodule in the current control cycle;

[0087] If the bridge arm current is positive and the number of submodules N put into operation in the previous control cycle on (k-1) <N-m+1,N on The number of submodules put into operation in the previous control cycle, N is the total number of submodules on the bridge arm, and the estimated capacitor voltage value U of the lower submodule under the current control c_pos_m It is equal to the actual value of the capacitor voltage of the submodule in the previous control cycle.

[0088] If the bridge arm current is non-positive and the number of sub-modules N put into operation in the previous control cycle on (k-1)>m-1, the estimated value of the capacitor voltage of the lower submodule is Uc_neg_m ;

[0089]

[0090] Where C is the capacitance of the submodule, T c is the duration of the control cycle, i ua (k) is the bridge arm current of the current control cycle, i ua (k-1) is the bridge arm current of the previous control cycle, U c_number_m (k-1) is the actual value of the capacitor voltage of the submodule in the previous cycle.

[0091] A submodule diagnosis module, used to determine the absolute error between the estimated value of the capacitor voltage of the lower submodule and the actual value of the capacitor voltage, compare the absolute error with a set threshold value, and determine whether the set submodule fails;

[0092] The output module is used to repeat step 4 when an adjacent submodule fails, and perform fault diagnosis on the subordinate submodule of the subordinate submodule until the diagnosed submodule is free of faults, thereby obtaining all faulty submodules.

[0093] Figure 3 The flowchart of the diagnosis method in the present invention is shown: the diagnosis process includes a detection process and a positioning process.

[0094] Example 1

[0095] Taking a single-ended MMC system as an example, the effectiveness of the NLM modulation strategy MMC submodule open circuit fault diagnosis method based on capacitor voltage imbalance proposed in the present invention is verified. The modulation mode adopts NLM modulation, and the voltage balancing strategy adopts the traditional voltage balancing strategy. This embodiment analyzes the following six situations:

[0096] (1) A type I fault occurs in the inverter mode rated load MMC;

[0097] In case (1), if Figure 4 As shown in the figure, at 1.0s, type I faults occurred simultaneously on bridge arms 1 and 3 of the inverter mode rated load MMC a phase. After the fault occurred, the absolute error of the faulty SM exceeded the set threshold. The system could quickly detect the fault. Then, through the positioning process, it was determined that the number of faulty SMs in the current bridge arm was 2, with serial numbers 1 and 3 respectively. The diagnosis time was 12.14ms, which verified the applicability of the proposed strategy in this case.

[0098] (2) Type I fault occurs in the light-load MMC in inverter mode;

[0099] In case (2), if Figure 5As shown in the figure, at 1.0s, type I faults occurred simultaneously on bridge arms 1 and 3 of the inverter mode rated load MMC a phase. After the fault occurred, the absolute error of the faulty SM exceeded the set threshold. The system could quickly detect the fault. Then, through the positioning process, it was determined that the number of faulty SMs in the current bridge arm was 2, with serial numbers 1 and 3 respectively. The diagnosis time was 12.20ms, which verified the applicability of the proposed strategy in this case.

[0100] (3) Type I fault occurs in the rated load MMC in inverter mode;

[0101] In case (3), if Figure 6 As shown in the figure, at 1.0s, type II faults occurred simultaneously on bridge arms 1 and 3 of the inverter mode rated load MMC a phase. After the fault occurred, the absolute error of the faulty SM exceeded the set threshold. The system could quickly detect the fault. Then, through the positioning process, it was determined that the number of faulty SMs in the current bridge arm was 2, with serial numbers 1 and 3 respectively. The diagnosis time was 0.46ms, which verified the applicability of the proposed strategy in this case.

[0102] (4) Type II fault occurs in the light-load MMC in inverter mode;

[0103] In case (4), if Figure 7 As shown in the figure, at 1.0s, type II faults occurred simultaneously in the upper bridge arms 1 and 3 of the light-load MMC a phase in the inverter mode. After the fault occurred, the absolute error of the faulty SM exceeded the set threshold. The system could quickly detect the fault. Then, through the positioning process, it was determined that the number of faulty SMs in the current bridge arm was 2, with serial numbers 1 and 3 respectively. The diagnosis time was 0.47ms, which verified the applicability of the proposed strategy in this case.

[0104] (5) A type I fault occurs in the rated load MMC in rectification mode;

[0105] In case (5), if Figure 8 As shown in the figure, at 1.0s, type II faults occurred simultaneously on bridge arms 1 and 3 of the MMC a phase in the rectifier mode with rated load. After the fault occurred, the absolute error of the faulty SM exceeded the set threshold. The system could quickly detect the fault. Then, through the positioning process, it was determined that the number of faulty SMs in the current bridge arm was 2, with serial numbers 1 and 3 respectively. The diagnosis time was 11.93ms, which verified the applicability of the proposed strategy in this case.

[0106] (6) A Type II fault occurs in the light-load MMC in rectification mode.

[0107] In case (6), if Fig. 9As shown in the figure, at 1.0s, type II faults occurred simultaneously in bridge arms 1 and 3 of phase a of the light-load MMC in rectification mode. After the fault occurred, the absolute error of the faulty SM exceeded the set threshold. The system could quickly detect the fault. Then, through the positioning process, it was determined that the number of faulty SMs in the current bridge arm was 2, with serial numbers 1 and 3 respectively. The diagnosis time was 11.95ms, which verified the applicability of the proposed strategy in this case.

[0108] Table 1 Main circuit parameters of the embodiment

[0109] parameter Parameter Value Rated voltage on AC side 290kV AC side voltage frequency 50Hz DC side rated voltage 500kV Rated active power 750MW Number of submodules per bridge arm 244 Bridge arm reactor 0.1H Submodule capacitance value 8mF

[0110] The present invention proposes a method for diagnosing open-circuit faults of submodules of a modular multilevel converter under an NLM strategy. The method selects the absolute error between the actual value and the predicted value of the SM capacitor voltage as a diagnostic mark, and can accurately and quickly diagnose two MMC submodule open-circuit faults at different power points. At the same time, it can cope with the situation where multiple SMs in the bridge arm fail. By utilizing the sorting results of the voltage balancing control, the amount of calculation is greatly reduced, and the diagnosis process can be completed within one power frequency cycle, thereby improving the reliability of the modular multilevel converter and ensuring the stable operation of the modular multilevel converter.

[0111] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of complete hardware embodiments, complete software embodiments, or embodiments in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code. The scheme in the embodiments of the present application can be implemented in various computer languages, for example, object-oriented programming language Java and literal scripting language JavaScript, etc.

[0112] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0113] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0114] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0115] Although the preferred embodiments of the present application have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0116] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.

Claims

1. A method for diagnosing open circuit faults of submodules of MMC under NLM strategy, characterized in that: The steps include: Step 1, obtaining the actual value of the capacitor voltage of the setting submodule in the current control cycle in the MMC, and estimating the estimated value of the capacitor voltage of the setting submodule in the current control according to the bridge arm current of the previous control cycle of the MMC; Step 2: determine the absolute error between the estimated value of the capacitor voltage of the setting submodule and the actual value of the capacitor voltage, compare the absolute error with a setting threshold, and determine whether the setting submodule fails; Step 3: When a submodule fails, the actual value of the capacitor voltage of the submodule is used as the maximum voltage value, and all submodules are sorted in descending order according to the capacitor voltage value to obtain a submodule sorting sequence; Step 4: Obtain the actual value of the capacitor voltage of the lower-level submodule adjacent to the set submodule in the submodule sorting sequence in the current control cycle; estimate the capacitor voltage estimate value of the lower-level submodule in the current control cycle based on the bridge arm current of the MMC in the previous cycle and the number of submodules put into operation in the previous cycle; Step 5: determine the absolute error between the estimated value of the capacitor voltage of the lower-level submodule and the actual value of the capacitor voltage, compare the absolute error with a set threshold, and determine whether the lower-level submodule fails; Step 6: If a lower-level submodule fails, step 4 is repeated to perform fault diagnosis on the lower-level submodule of the lower-level submodule until the diagnosed submodule is free of faults and all faulty submodules are obtained.

2. The method for diagnosing an open circuit fault of a submodule of an MMC under an NLM strategy according to claim 1, characterized in that: In step 1, the estimated value of the capacitor voltage currently controlled by the setting submodule is estimated according to the positive or negative value of the bridge arm current in the previous control cycle of the MMC.

3. The method for diagnosing an open circuit fault of a submodule of an MMC under an NLM strategy according to claim 2, characterized in that: In step 1, if the bridge arm current is positive, the estimated value of the capacitor voltage currently controlled by the setting submodule is equal to the actual value of the capacitor voltage of the setting submodule in the previous control cycle.

4. The method for diagnosing an open circuit fault of a submodule of an MMC under an NLM strategy according to claim 2, characterized in that: In step 1, if the bridge arm current i ua (k-1) is non-positive, then the capacitor voltage estimation value of the submodule in the current control cycle is set as follows: Where C is the capacitance of the submodule, T c is the duration of the control cycle, U c_number_rea (k-1) is the actual value of the capacitor voltage of the submodule in the previous control cycle.

5. The method for diagnosing open circuit faults of submodules of MMC under the NLM strategy according to claim 1, characterized in that: In step 2, when the absolute error is greater than the set threshold, the setting submodule fails, otherwise the setting submodule does not fail.

6. The method for diagnosing an open circuit fault of a submodule of an MMC under an NLM strategy according to claim 1, characterized in that: The expression for setting the threshold in step 2 is as follows: Where C is the capacitance of the submodule, T c is the duration of the control cycle, i ua (k) is the bridge arm current of the current control cycle, i ua (k-1) is the bridge arm current of the previous control cycle, err normal is the absolute error, err _th To set the threshold.

7. The method for diagnosing an open circuit fault of a submodule of an MMC under an NLM strategy according to claim 1, characterized in that: The method for determining the capacitor voltage estimation value of the lower-level submodule in the current control cycle in step 4 is as follows: If the bridge arm current is positive and the number of submodules N put into operation in the previous control cycle on (k-1) <N-m+1,N on The number of submodules put into operation in the previous control cycle, N is the total number of submodules on the bridge arm, and the estimated capacitor voltage value U of the lower submodule under the current control c_pos_m It is equal to the actual value of the capacitor voltage of the submodule in the previous control cycle.

8. The method for diagnosing an open circuit fault of a submodule of an MMC under an NLM strategy according to claim 7, characterized in that: The method for determining the capacitor voltage estimation value of the lower-level submodule in the current control cycle in step 4 is as follows: If the bridge arm current is non-positive and the number of sub-modules N put into operation in the previous control cycle on (k-1)>m-1, the estimated value of the capacitor voltage of the lower submodule is U c_neg_m ; Where C is the capacitance of the submodule, T c is the duration of the control cycle, i ua (k) is the bridge arm current of the current control cycle, i ua (k-1) is the bridge arm current of the previous control cycle, U c_number_m (k-1) is the actual value of the capacitor voltage of the submodule in the previous cycle.

9. A system for diagnosing open circuit faults of submodules of MMC under NLM strategy, characterized in that: include, The first capacitor voltage acquisition module is used to obtain the actual value of the capacitor voltage of the setting submodule in the MMC in the current control cycle, and estimate the estimated value of the capacitor voltage of the setting submodule in the current control according to the bridge arm current of the previous control cycle of the MMC; The MMC diagnostic module is used to determine the absolute error between the estimated value of the capacitor voltage of the setting submodule and the actual value of the capacitor voltage, and compare the absolute error with a setting threshold to determine whether the setting submodule fails; A sorting module is used to, when a submodule fails, use the actual value of the capacitor voltage of the submodule as the maximum voltage value, sort all submodules in descending order according to the capacitor voltage value, and obtain a submodule sorting sequence; The second capacitor voltage acquisition module is used to obtain the actual value of the capacitor voltage of the lower-level submodule adjacent to the set submodule in the submodule sorting sequence in the current control cycle; based on the bridge arm current of the MMC in the previous cycle and the number of submodules put into operation in the previous cycle, estimate the capacitor voltage estimation value of the lower-level submodule in the current control cycle; A submodule diagnosis module, used to determine the absolute error between the estimated value of the capacitor voltage of the lower submodule and the actual value of the capacitor voltage, compare the absolute error with a set threshold value, and determine whether the set submodule fails; The output module is used to diagnose the fault of the next-level submodule of the next-level submodule when a fault occurs in the next-level submodule of the next-level submodule, until the diagnosed submodule is free of faults and all faulty submodules are obtained.

10. The system of the method for diagnosing submodule open circuit fault of MMC under NLM strategy according to claim 9, characterized in that: The method for determining the capacitor voltage estimation value of the lower-level submodule in the current control cycle is as follows: If the bridge arm current is non-positive and the number of sub-modules N put into operation in the previous control cycle on (k-1)>m-1, the estimated value of the capacitor voltage of the lower submodule is U c_neg_m ; Where C is the capacitance of the submodule, T c is the duration of the control cycle, i ua (k) is the bridge arm current of the current control cycle, i ua (k-1) is the bridge arm current of the previous control cycle, U c_number_m (k-1) is the actual value of the capacitor voltage of the submodule in the previous cycle.

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