MMC sub-module IGBT open-circuit fault diagnosis method based on capacitor energy

Abstract

The embodiment of the invention discloses an MMC sub-module IGBT open-circuit fault diagnosis method based on capacitor energy, and the method comprises the steps: carrying out the integration of capacitor voltages of all SM sub-modules of an MMC, and accumulating the integral value of each SM sub-module; selecting a maximum value and a minimum value from the accumulated integral values of all the SM sub-modules, calculating a difference value as a capacitance energy imbalance value, and comparing the capacitance energy imbalance value with a preset fault threshold value to determine a fault SM sub-module; and giving a fault SM sub-module detection signal, sampling a capacitor voltage value, carrying out differential calculation on the capacitor voltage value sampled twice to obtain a fault positioning value, and determining the fault type of the fault SM sub-module. Sampling errors and random fluctuation can be smoothed through integral accumulation, the false alarm and rejection probability is reduced, and meanwhile the fault diagnosis performance and reliability of the MMC are guaranteed. By giving a specific detection signal, the fault type of the SM sub-module is judged according to the change of the capacitor voltage of the fault SM sub-module.

Description

Technical Field

[0001] The invention relates to the technical field of MMC fault diagnosis, and in particular to a method for diagnosing an open-circuit fault of an MMC submodule IGBT based on capacitor energy. Background Art

[0002] Modular Multilevel Converter (MMC) has a modular design structure and can be used as an important converter topology in high voltage power fields such as high voltage direct current (HVDC) transmission, flexible alternating current transmission, medium and high voltage motor drives, and energy storage systems. MMC has convenient assembly, scalability, low switching frequency, and excellent harmonic performance, making its application more flexible. Semiconductor switching devices in MMC, such as insulated gate bipolar transistors (IGBT), will lose performance and cause failures during use due to improper use or natural aging. Since MMC is composed of a large number of sub-modules (SM), each sub-module contains an upper tube IGBT and a lower tube IGBT. When the IGBT of the sub-module fails in an open circuit, the capacitor voltage of the affected sub-module in the MMC will continue to increase, causing the output voltage and current of the MMC to be distorted, and then causing the MMC to shut down or be damaged.

[0003] Since MMC contains a large number of submodules, each of which contains two IGBTs, the fault diagnosis of IGBTs in MMC usually requires a detection process and a positioning process. The detection process samples the capacitor voltage of the submodule and compares it with the preset detection threshold to determine the submodule with a fault, and then determines the fault type of the submodule, whether it is an upper tube fault or a lower tube fault, based on the working status and output voltage of the faulty submodule. However, in an environment with low switching frequency and low sampling frequency, it is easy to be affected by instantaneous errors caused by grid fluctuations, resulting in false alarms or rejections, affecting the efficiency of fault diagnosis. Summary of the invention

[0004] The embodiment of the present invention provides a method for diagnosing an open-circuit fault of an MMC submodule IGBT based on capacitor energy, so as to solve the technical problem of false alarm or rejection caused by sampling error or random fluctuation during MMC fault diagnosis.

[0005] The embodiment of the present invention provides a method for diagnosing an open circuit fault of an MMC submodule IGBT based on capacitor energy, comprising:

[0006] Integrate and calculate the capacitor voltages collected by all SM submodules of the MMC within a preset sampling period to obtain a sampling period integral value of each SM submodule;

[0007] Accumulate the integral values ​​of all sampling periods of each SM submodule within a preset integral period to obtain the integral period capacitance energy value of each SM submodule;

[0008] The maximum and minimum values ​​are selected from the integrated period capacitor energy values ​​of all SM submodules, and the difference between the maximum and minimum values ​​is calculated to obtain the capacitor energy unbalance value;

[0009] Compare the capacitor energy imbalance value with a preset fault threshold to determine the faulty SM submodule;

[0010] A detection signal is given to the faulty SM submodule, and the capacitor voltage value of the faulty submodule is sampled at preset time intervals to obtain a voltage value at a first moment and a voltage value at a second moment;

[0011] The voltage value at the first moment and the voltage value at the second moment are used to perform differential calculation to obtain a fault location value, and the fault type of the faulty SM submodule is determined according to the fault location value.

[0012] Furthermore, the capacitor voltages collected by all SM submodules of the MMC within a preset sampling period are integrated and calculated to obtain the sampling period integral value of each SM submodule, including:

[0013] The capacitor voltage of each SM submodule of the MMC is sampled in a preset sampling period, wherein the preset sampling period includes a plurality of continuous sampling cycles, and the time interval between each two adjacent sampling points is a sampling cycle, so as to obtain the capacitor voltage value of each sampling cycle;

[0014] According to the time sequence of the sampling periods, the capacitor voltage value of each sampling period in the preset sampling period is integrated and calculated to obtain the sampling period integral value of each SM submodule.

[0015] Furthermore, the preset integration period is a fundamental frequency period of the alternating current, and each fundamental frequency period includes a plurality of continuous sampling time periods.

[0016] Further, the comparing the capacitor energy imbalance value with a preset fault threshold to determine the faulty SM submodule includes:

[0017] The capacitor energy imbalance value is compared with a preset fault threshold, and when the capacitor energy imbalance value is greater than the fault threshold, the SM submodule corresponding to the maximum value of the integral period capacitor energy value is determined to be a faulty submodule.

[0018] Furthermore, the method further comprises:

[0019] The capacitor energy imbalance value is compared with a preset fault threshold. When the capacitor energy imbalance value is less than the fault threshold, the capacitor energy imbalance value is then compared with a preset warning threshold. When the capacitor energy imbalance value is less than the fault threshold and greater than the warning threshold, a warning message is issued to the SM sub-module corresponding to the maximum value of the capacitor energy value in the integration period.

[0020] Further, the detection signal is given to the faulty SM submodule, and the capacitor voltage value of the faulty submodule is sampled at preset time intervals to obtain a voltage value at a first moment and a voltage value at a second moment, including:

[0021] Given a faulty SM submodule removal signal, the switch function of the faulty SM submodule is set to 0;

[0022] The capacitor voltage value of the faulty SM submodule is sampled at a first sampling moment and a second sampling moment respectively to obtain a voltage value at a first moment and a voltage value at a second moment.

[0023] Furthermore, the preset time interval is 1 ms.

[0024] Further, determining the fault type of the faulty SM submodule according to the fault location value includes:

[0025] When the fault location value is greater than 0, it is determined that the fault type of the faulty SM submodule is a lower tube fault;

[0026] When the fault location value is equal to 0, it is determined that the fault type of the faulty SM submodule is an upper tube fault.

[0027] Furthermore, the fault threshold is 1.5% of the MMC DC side voltage.

[0028] Furthermore, the warning threshold is 1.0% of the MMC DC side voltage.

[0029] The embodiment of the present invention provides a MMC submodule IGBT open circuit fault diagnosis method based on capacitor energy, which integrates the capacitor voltage of each SM submodule in the sampling period, and then accumulates the integral value to obtain the capacitor energy value of each SM submodule, and uses the difference between the maximum and minimum values ​​of the capacitor energy values ​​between the SM submodules to determine the capacitor imbalance of the SM submodule, and uses the capacitor imbalance and the preset fault threshold to diagnose whether the SM submodule corresponding to the maximum value has a fault. By integrating and accumulating, the sampling error and random fluctuation can be smoothed, the probability of false alarm and rejection can be reduced, and the fault diagnosis performance and reliability of the MMC can be guaranteed. By giving a specific detection signal of the faulty SM submodule, the capacitor voltage change sampled by the faulty SM submodule at a certain time interval is differentiated and calculated, and the fault type of the SM submodule is determined by whether the faulty SM submodule is in a special working state. Only the situation of the lower tube fault is judged by the differential result. Since the SM submodule that has been determined to be faulty is detected, if it is not a lower tube fault, it is an upper tube fault, which reduces the data and calculation required for fault location and improves the efficiency of fault location. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0031] Figure 1 This is a flow chart of a method for diagnosing an open circuit fault of an MMC submodule IGBT based on capacitor energy according to Embodiment 1 of the present invention;

[0032] Figure 2 Schematic diagram of four operating states of the SM submodule according to the first embodiment of the present invention;

[0033] Figure 3 A circuit diagram of four operating states of the SM submodule according to the first embodiment of the present invention;

[0034] Figure 4 This is a schematic diagram of the state of an upper tube failure of the SM submodule according to the first embodiment of the present invention;

[0035] Figure 5 This is a schematic diagram of the state of a lower tube failure of the SM submodule according to the first embodiment of the present invention;

[0036] Figure 6 This is a flow chart of a method for diagnosing an open-circuit fault of an MMC submodule IGBT based on capacitor energy according to Embodiment 2 of the present invention;

[0037] Figure 7This is a flow chart of a method for diagnosing an open circuit fault of an MMC sub-module IGBT based on capacitor energy as described in Embodiment 4 of the present invention. DETAILED DESCRIPTION

[0038] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for ease of description, only parts related to the present invention, rather than all structures, are shown in the accompanying drawings.

[0039] Embodiment 1

[0040] Figure 1 This is a flow chart of a method for diagnosing an open-circuit fault of an MMC submodule IGBT based on capacitor energy according to Embodiment 1 of the present invention. This embodiment accumulates the integral of the capacitor voltage sampling values ​​within a period of time, detects the faulty SM submodule by using the capacitor energy difference between the accumulated SM submodules, and locates the fault type of the SM submodule by giving a removal signal. The method specifically includes the following steps:

[0041] The SM submodule of MMC has four operating states under normal working conditions, such as Figure 2 , 3 As shown:

[0042] State I: The SM submodule is in the on state and the current direction is positive, the switch function is 1, and the upper tube (S 1 ) is turned on, the lower tube (S 2 ) is turned off, and the current flows through the anti-parallel diode D 1 and the SM submodule capacitor C, the capacitor is in a charged state;

[0043] State II: When the current direction is positive and the switching function is 0, the upper tube (S 1 ) is turned off, the lower tube (S 2 ) is turned on, and the current only flows through S 2 , the capacitor voltage remains unchanged;

[0044] State III: When the current direction is negative, the switch function is 1. At this time, the upper tube (S 1 ) is turned on, the lower tube (S 2 ) is turned off, and the current flows through S 1 , the capacitor is in a discharged state;

[0045] State IV: When the current direction is negative and the switching function is 0, the upper tube (S 1 ) is turned off, the lower tube (S 2 ) is turned on, and the current only flows through D 2 , the capacitor voltage remains unchanged.

[0046] When an upper tube failure (Type I failure) occurs, the upper tube IGBT (S 1 ) open circuit, such as Figure 4 As shown in the figure, the red part is the fault location, which will only change in the above state III. 1 The open circuit fault cannot be turned on, and the current can only flow through the anti-parallel diode D 2 , the actual SM submodule is in the cut-off state, and the capacitor voltage is 0, such as Figure 4 (c); Other operating states are the same as the normal operating state, such as Figure 4 (a)(b)(d).

[0047] When a lower tube fault (Type II fault) occurs, the lower tube IGBT (S 2 ) open circuit, such as Figure 5 As shown in the figure, the red part is the fault location, which will only change in the above state II. 2 The open circuit fault cannot be turned on, and the current can only flow through the anti-parallel diode D1 and the SM capacitor C. The actual SM submodule is in the input state, and the capacitor is charged. Figure 5 (b); Other operating states are the same as normal operating states, such as Figure 5 (a)(c)(d).

[0048] Therefore, by sampling the capacitor voltage of the SM submodule, the fault of the SM submodule can be diagnosed through the voltage change obtained by sampling. When analyzing the capacitor voltage of the SM submodule for fault diagnosis, if the capacitor voltage of a single sampling cycle is compared, it is easy to be affected by the error of the sampling device, resulting in a large change in the sampling value of a single sampling cycle; if a detection group is formed by several consecutive sampling cycles, it is also difficult to eliminate the large error influence of a single sampling cycle when comparing between detection groups, which leads to the sampling value exceeding the threshold used for fault detection, resulting in false alarm or rejection.

[0049] S101, performing integral calculation on the capacitor voltages collected by all SM submodules of the MMC within a preset sampling period to obtain an integral value of each SM submodule within the sampling period.

[0050] During a period of time including multiple continuous sampling cycles, the capacitor voltage of the SM submodule is sampled, and the capacitor voltage obtained by sampling is used to perform area integration calculation in time sequence. The integral value reflects the change of capacitor energy accumulated over a period of time, which can smooth random fluctuations in a single sampling cycle or a short period of time and reduce the impact of sampling errors, instantaneous fluctuations in the power grid, etc.

[0051] S102, accumulating the integral values ​​of all sampling periods of each SM submodule within a preset integral period to obtain the integral period capacitance energy value of each SM submodule.

[0052] By accumulating the integral value, the energy change of the capacitor within an integration period is determined. By combining the change of the capacitor energy with the working state of the SM submodule, it is possible to analyze whether the SM submodule is in a normal working state. The capacitor energy accumulated through integration can further smooth the impact of random fluctuations in a short period of time and more comprehensively reflect the change trend of the capacitor voltage. The calculation formula is as follows:

[0053]

[0054] in, t i The integral value of the capacitor voltage at time .

[0055] At the same time, due to the initial stage of the SM submodule failure, the change trend of the capacitor voltage is not obvious enough, and the capacitor voltage will be closer to the normally operating SM submodule with a higher capacitor voltage. Therefore, by accumulating the integral value, the difference between the faulty SM submodule and the normally operating SM submodule can be more clearly reflected.

[0056] S103, selecting a maximum value and a minimum value from the integrated period capacitor energy values ​​of all SM sub-modules, and calculating a difference between the maximum value and the minimum value to obtain a capacitor energy unbalance value.

[0057] When a type I fault occurs, if the current in the bridge arm where the SM submodule is located is positive when the fault occurs, the working state of the faulty SM submodule is the same as that of the normal SM submodule; when the bridge arm current turns negative, the capacitor of the faulty SM submodule is bypassed, and the current does not flow through the capacitor, so the capacitor voltage remains unchanged; but at this time, the capacitors of other normally working SM submodules will enter a discharge state, and the capacitor voltage will gradually decrease, causing the capacitor voltage of the faulty SM submodule to become the maximum value in the bridge arm. If the current in the bridge arm where the SM submodule is located is negative when the fault occurs, the same change will occur when the bridge arm current turns positive.

[0058] When a type II fault occurs, if the current in the bridge arm where the SM submodule is located is positive when the fault occurs, the capacitor of the faulty SM submodule is always in a charging state, and the capacitor voltage continues to increase. At this time, other SM submodules in normal working state will be charged in sequence according to the capacitor voltage sorting, and the increase in capacitor energy will be significantly smaller than that of the faulty SM submodule. Therefore, the capacitor voltage of the faulty SM submodule is the maximum value in the bridge arm; if the bridge arm current is negative when the fault occurs, the state of the faulty SM submodule is the same as that of the SM submodule in a normal state, but when the bridge arm current turns positive, the capacitor of the faulty SM submodule will also become always in a charging state.

[0059] According to the variation characteristics of the capacitor voltage of the faulty SM submodule, the integrated period capacitor energy value accumulated by all SM submodules can be selected. The maximum value reflects the capacitor energy of the faulty SM submodule. The capacitor energy between the normally working SM submodules is relatively close to each other and will be significantly lower than the capacitor energy of the faulty SM submodule. The difference between the maximum value and the minimum value, that is, the capacitor energy imbalance value, can reflect the state difference between the SM submodules. It can be judged whether the SM submodule is faulty according to the degree of difference. The calculation formula is as follows:

[0060]

[0061] in, Indicates the maximum capacitance energy of the SM submodule. It represents the minimum capacitance energy of the SM submodule, and σ represents the capacitance energy imbalance.

[0062] S104, comparing the capacitor energy imbalance value with a preset fault threshold to determine the faulty SM submodule.

[0063] When all SM submodules are operating normally, in theory, the capacitor voltages of all SM submodules are the same, and the capacitor energy imbalance value should be 0. However, in actual work, due to sampling errors, random fluctuations and other reasons, there will be a certain degree of capacitor energy difference, and the capacitor energy imbalance value will be at a low level; when an SM submodule fails, its capacitor energy will be significantly different from other SM submodules that are operating normally, and the faulty SM submodule can be identified according to the degree of difference. At the same time, in order to avoid the continuous accumulation of factors such as errors and random fluctuations in the sampling process during the integration and accumulation process, by setting the alarm threshold to reserve judgment space for the error, the capacitor energy imbalance value is compared with the alarm threshold, which greatly reduces the false alarm rate and ensures the fault detection performance. The alarm threshold is determined according to the sampling accuracy and sampling frequency. The larger the sampling accuracy and sampling frequency, the smaller the impact of the error, and the alarm threshold can be determined lower.

[0064] S105 , providing a detection signal to the faulty SM submodule, sampling the capacitor voltage value of the faulty submodule at preset time intervals, and obtaining a voltage value at a first moment and a voltage value at a second moment.

[0065] By giving a specific switch signal to the SM submodule that detects a fault, the switch state of the SM submodule is changed, and the capacitor voltage of the faulty SM submodule is sampled twice. It is analyzed whether the change pattern of the capacitor voltage between the two sampling times conforms to the change pattern of the capacitor voltage in the normal operating state.

[0066] S106, performing differential calculation using the voltage value at the first moment and the voltage value at the second moment to obtain a fault location value, and determining the fault type of the faulty SM submodule according to the fault location value.

[0067] After the faulty SM submodule receives a specific switching signal, if an upper tube fault or a lower tube fault occurs, the faulty SM submodule will be in a special working state. By using the differential calculation based on the change law of the capacitor voltage between two samplings, this special working state can be distinguished. When it is determined that the faulty SM submodule is in this special working state, the faulty SM submodule can be determined as one of the fault types according to the fault location value. Since there are only two fault types of the faulty SM submodule, and the submodule has been determined to be in a fault state, if the submodule is determined not to be one of the fault types, it is determined to be another fault type.

[0068] This embodiment integrates the capacitor voltage within the sampling period, and then accumulates and sums the integral values ​​of all sampling periods within the integration period to determine the capacitor energy value of each SM submodule within the integration period. According to the difference degree of the capacitor energy values ​​between the SM submodules, when the difference degree exceeds the preset fault threshold, it can be diagnosed that the SM submodule corresponding to the maximum value has a fault. By integrating and accumulating, the sampling error and random fluctuation can be smoothed, the probability of false alarm and rejection can be reduced, and the fault diagnosis performance and reliability of the MMC can be guaranteed at the same time. After that, a specific detection signal is given to the SM submodule with a fault, so that the faulty SM submodule is switched to a special working state, and the capacitor voltage of the faulty SM submodule is sampled before and after a specific time interval, and the change of the capacitor voltage sampled before and after the time interval is calculated by differential calculation, and then the fault type of the SM submodule is determined. Only the situation of whether it is a lower tube fault is judged by the differential result. Since the SM submodule that has been determined to be a fault is detected, if it is not a lower tube fault, it is an upper tube fault, which reduces the data and calculation required for fault location and improves the efficiency of fault location.

[0069] Embodiment 2

[0070] Figure 6 This is a flow chart of a method for diagnosing an open-circuit fault of an MMC submodule IGBT based on capacitor energy according to Embodiment 2 of the present invention. This embodiment is optimized based on the above embodiment. In this embodiment, the capacitor voltage collected by all SM submodules of the MMC within a preset sampling period is integrated and calculated to obtain the sampling period integral value of each SM submodule. The specific optimization is as follows:

[0071] The capacitor voltage of each SM submodule of the MMC is sampled in a preset sampling period, wherein the preset sampling period includes a plurality of continuous sampling cycles, and the time interval between each two adjacent sampling points is a sampling cycle, so as to obtain the capacitor voltage value of each sampling cycle;

[0072] According to the time sequence of the sampling periods, the capacitor voltage value of each sampling period in the preset sampling period is integrated and calculated to obtain the sampling period integral value of each SM submodule.

[0073] Accordingly, the MMC submodule IGBT open circuit fault diagnosis method based on capacitor energy provided in this embodiment specifically includes:

[0074] S201, sampling the capacitor voltage of each SM submodule of the MMC in a preset sampling period, wherein the preset sampling period includes a plurality of continuous sampling cycles, and the time interval between each two adjacent sampling points is a sampling cycle, to obtain the capacitor voltage value of each sampling cycle.

[0075] Sampling multiple consecutive sampling cycles can reflect the changes in the capacitor voltage of the SM sub-module during the sampling period. The capacitor voltage values ​​of multiple consecutive sampling cycles can avoid false alarms caused by instantaneous changes caused by sampling errors, random fluctuations, etc. that occur in a single sampling cycle to a certain extent, and also reduce the dependence on high sampling accuracy and high sampling frequency, which is beneficial to data collection.

[0076] S202 , performing integral calculation on the capacitor voltage value of each sampling period in a preset sampling period according to the time sequence of the sampling periods, to obtain the sampling period integral value of each SM submodule.

[0077] After integrating all the capacitor voltages obtained by sampling in the order of sampling time, the integral value can reflect the change in capacitor energy during the sampling period. The calculation formula is as follows:

[0078]

[0079] in, is the change in capacitor energy, t 1 The energy value of the capacitor at the moment, t 2 The energy value of the capacitor at the moment, V sm is the submodule capacitor voltage.

[0080] The change in capacitor energy can more comprehensively reflect the fault development trend during the sampling period, smooth the error of a single sampling cycle, and reduce the false alarm and rejection rates.

[0081] S203, accumulating the integral values ​​of all sampling periods of each SM submodule within a preset integral period to obtain the integral period capacitance energy value of each SM submodule.

[0082] S204, selecting a maximum value and a minimum value from the integrated period capacitor energy values ​​of all SM sub-modules, and calculating a difference between the maximum value and the minimum value to obtain a capacitor energy unbalance value.

[0083] S205, comparing the capacitor energy imbalance value with a preset fault threshold to determine the faulty SM submodule.

[0084] S206 , providing a detection signal to the faulty SM submodule, sampling the capacitor voltage value of the faulty submodule at preset time intervals, and obtaining a voltage value at a first moment and a voltage value at a second moment.

[0085] S207, performing differential calculation using the voltage value at the first moment and the voltage value at the second moment to obtain a fault location value, and determining the fault type of the faulty SM submodule according to the fault location value.

[0086] This embodiment calculates the change of the capacitor voltage through area integration and accumulates the change, which can smooth the sampling error of a single sampling period and reduce the false alarm rate caused by sampling error or random fluctuation.

[0087] Exemplarily, the preset integration period is the base frequency period of the alternating current, and each base frequency period includes multiple continuous sampling periods. For example, the alternating current in my country is 50Hz, the base frequency period is 0.02 seconds, and the integration period is also preset to 0.02 seconds. The integral values ​​of all sampling periods within 0.02 seconds are accumulated according to the time sequence of the sampling periods, and the capacitance energy change within a preset integration period can be obtained.

[0088] Embodiment 3

[0089] This embodiment is optimized based on the above embodiment. In this embodiment, the capacitor energy imbalance value is compared with a preset fault threshold to determine the faulty SM submodule. The specific optimization is as follows:

[0090] The capacitor energy imbalance value is compared with a preset fault threshold, and when the capacitor energy imbalance value is greater than the fault threshold, the SM submodule corresponding to the maximum value of the integral period capacitor energy value is determined to be a faulty submodule.

[0091] In order to avoid false alarms caused by the accumulation of errors during integration and accumulation, a buffer zone needs to be reserved when setting the fault threshold to filter sampling errors and random fluctuations, while ensuring that real faults can be discovered, reducing the false alarm rate to avoid overreaction in fault detection, and ensuring the reliability and practicality of fault detection. Exemplarily, the fault threshold is 1.5% of the MMC DC side voltage. When the capacitor energy imbalance value is greater than 1.5% of the MMC DC side voltage value, it is determined that the capacitor energy imbalance value exceeds the normal range of sampling errors and random fluctuations, and the SM submodule corresponding to the maximum value is determined as a faulty submodule.

[0092] An optional implementation of the present embodiment is to compare the capacitor energy imbalance value with a preset fault threshold; when the capacitor energy imbalance value is less than the fault threshold, then compare the capacitor energy imbalance value with a preset warning threshold; when the capacitor energy imbalance value is less than the fault threshold and greater than the warning threshold, issue a warning message to the SM sub-module corresponding to the maximum value of the capacitor energy value in the integration period.

[0093] In order to prejudge the failure of the SM submodule in advance, by setting a warning threshold value less than the fault threshold, the SM submodule with a large capacitor energy imbalance value but not reaching the fault threshold is warned, indicating that the SM submodule corresponding to the current maximum value has a risk of failure, and the attention to the submodule needs to be increased. The submodule failure can be identified as early as possible by increasing the sampling of the submodule. Exemplarily, the warning threshold is 1.0% of the MMC DC side voltage. The situation where the capacitor energy imbalance value is less than 1.0% of the MMC DC side voltage value is filtered to avoid the accumulation of errors or random fluctuations in the integration and accumulation process to cause false alarms and reduce the false alarm rate.

[0094] Embodiment 4

[0095] Figure 7 This is a flow chart of a method for diagnosing an open-circuit fault of an MMC submodule IGBT based on capacitor energy according to Embodiment 4 of the present invention. This embodiment is optimized based on the above embodiment. In this embodiment, a detection signal is given to the faulty SM submodule, and the capacitor voltage value of the faulty submodule is sampled at preset time intervals to obtain a voltage value at a first moment and a voltage value at a second moment. The specific optimization is as follows:

[0096] Given a faulty SM submodule removal signal, the switch function of the faulty SM submodule is set to 0;

[0097] The capacitor voltage value of the faulty SM submodule is sampled at a first sampling moment and a second sampling moment respectively to obtain a voltage value at a first moment and a voltage value at a second moment.

[0098] Accordingly, the MMC submodule IGBT open circuit fault diagnosis method based on capacitor energy provided in this embodiment specifically includes:

[0099] S401, performing integral calculation on the capacitor voltages collected by all SM submodules of the MMC within a preset sampling period to obtain an integral value of each SM submodule within the sampling period.

[0100] S402 , accumulating the integral values ​​of all sampling periods of each SM submodule within a preset integral period to obtain the integral period capacitance energy value of each SM submodule.

[0101] S403 , selecting a maximum value and a minimum value from the integrated period capacitor energy values ​​of all SM submodules, and calculating a difference between the maximum value and the minimum value to obtain a capacitor energy unbalance value.

[0102] S404, comparing the capacitor energy imbalance value with a preset fault threshold to determine the faulty SM submodule.

[0103] S405 , a faulty SM submodule removal signal is given, and a switch function of the faulty SM submodule is set to 0.

[0104] A cut-off signal is given to the SM submodule determined to have a fault, so that the upper tube IGBT of the faulty SM submodule is in a turned-off state and the lower tube IGBT is in a turned-on state.

[0105] S406 , sampling the capacitor voltage value of the faulty SM submodule at a first sampling time and a second sampling time respectively to obtain a voltage value at a first time and a voltage value at a second time.

[0106] The capacitor voltage is sampled, and the capacitor voltage is sampled again at a second sampling time interval t from the first sampling time, and the capacitor voltage values ​​sampled twice are used as the first time voltage value and the second time voltage value, respectively. Exemplarily, the preset time interval t is 1 ms.

[0107] S407, using the voltage value at the first moment and the voltage value at the second moment to perform differential calculation to obtain a fault location value, and determining the fault type of the faulty SM submodule according to the fault location value. The calculation formula of the fault location value is as follows:

[0108]

[0109] Where, μ represents the fault location value, which is the differential of the submodule capacitor voltage with respect to time, ΔV sm is the change in the submodule capacitor voltage, Δt is the change in time, that is, t 1 Time and t 2 The time interval between moments, t1 The submodule capacitor voltage at time t 2 Submodule capacitor voltage at time.

[0110] In this embodiment, a removal signal is given to the faulty SM submodule, and the removal signal is used to actually change the SM submodule with the lower tube fault into a locked state. The capacitor voltage sampled at different times is used for differential calculation, and it can be known from the change of the capacitor voltage whether the faulty SM submodule is in a locked state at this time, and then it can be determined whether it is a lower tube fault.

[0111] Specifically, determining the fault type of the faulty SM submodule according to the fault location value includes:

[0112] When the fault location value is greater than 0, it is determined that the fault type of the faulty SM submodule is a lower tube fault; when the fault location value is equal to 0, it is determined that the fault type of the faulty SM submodule is an upper tube fault.

[0113] When the fault type is an upper tube fault, since the switching function is 0 when the cut-off signal is given, the state of the SM submodule is that the upper tube is turned off and the lower tube is turned on. The faulty SM is always in the cut-off state from the first sampling moment to the second sampling moment, which is no different from the normal working state. Therefore, the capacitor voltage will not change, and the fault location value obtained by differential calculation is 0.

[0114] When the fault type is a lower tube fault, since the switch function is 0 when the cut-off signal is given, the state of the SM submodule is that the upper tube is turned off and the lower tube is turned on. The lower tube fault is that the lower tube IGBT is open. At this time, the actual state of the SM submodule is a locked state, that is, both the upper tube and the lower tube are open. When the bridge arm current is in the positive phase, the capacitor will be in a charging state, and the capacitor voltage obtained by sampling will show a significant increase trend. At this time, the fault detection value is greater than 0. Otherwise, the fault detection value can be used to determine that there is a fault in the lower tube of the SM submodule. Through the differential calculation of two samples at a specific interval, the change of the capacitor voltage of the faulty SM submodule is judged by the differential result, and then the actual working state of the SM submodule is judged, and it can be known whether the fault type is a lower tube fault. If it is not a lower tube fault, it is an upper tube fault. It is no longer necessary to calculate the two IGBTs separately, which reduces the data and calculation amount required for the calculation and improves the efficiency of fault diagnosis.

[0115] Note that the above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A method for diagnosing open-circuit faults of MMC submodule IGBTs based on capacitor energy, characterized in that: include: Integrate and calculate the capacitor voltages collected by all SM submodules of the MMC within a preset sampling period to obtain a sampling period integral value of each SM submodule; Accumulate the integral values ​​of all sampling periods of each SM submodule within a preset integral period to obtain the integral period capacitance energy value of each SM submodule; The maximum and minimum values ​​are selected from the integrated period capacitor energy values ​​of all SM submodules, and the difference between the maximum and minimum values ​​is calculated to obtain the capacitor energy unbalance value; Compare the capacitor energy imbalance value with a preset fault threshold to determine the faulty SM submodule; A detection signal is given to the faulty SM submodule, and the capacitor voltage value of the faulty submodule is sampled at preset time intervals to obtain a voltage value at a first moment and a voltage value at a second moment; The voltage value at the first moment and the voltage value at the second moment are used to perform differential calculation to obtain a fault location value, and the fault type of the faulty SM submodule is determined according to the fault location value.

2. The method according to claim 1, characterized in that The step of integrating and calculating the capacitor voltages collected by all SM submodules of the MMC within a preset sampling period to obtain the sampling period integral value of each SM submodule includes: The capacitor voltage of each SM submodule of the MMC is sampled in a preset sampling period, wherein the preset sampling period includes a plurality of continuous sampling cycles, and the time interval between each two adjacent sampling points is a sampling cycle, so as to obtain the capacitor voltage value of each sampling cycle; According to the time sequence of the sampling periods, the capacitor voltage value of each sampling period in the preset sampling period is integrated and calculated to obtain the sampling period integral value of each SM submodule.

3. The method according to claim 1, characterized in that: The preset integration period is the fundamental frequency period of the alternating current, and each fundamental frequency period includes a plurality of continuous sampling time periods.

4. The method according to claim 1, characterized in that: The step of comparing the capacitor energy imbalance value with a preset fault threshold to determine the faulty SM submodule includes: The capacitor energy imbalance value is compared with a preset fault threshold, and when the capacitor energy imbalance value is greater than the fault threshold, the SM submodule corresponding to the maximum value of the integral period capacitor energy value is determined to be a faulty submodule.

5. The method according to claim 1, characterized in that The method further comprises: The capacitor energy imbalance value is compared with a preset fault threshold. When the capacitor energy imbalance value is less than the fault threshold, the capacitor energy imbalance value is then compared with a preset warning threshold. When the capacitor energy imbalance value is less than the fault threshold and greater than the warning threshold, a warning message is issued to the SM sub-module corresponding to the maximum value of the capacitor energy value in the integration period.

6. The method according to claim 1, characterized in that The method of providing a detection signal to the faulty SM submodule and sampling the capacitor voltage value of the faulty submodule at preset time intervals to obtain a voltage value at a first moment and a voltage value at a second moment includes: Given a faulty SM submodule removal signal, the switch function of the faulty SM submodule is set to 0; The capacitor voltage value of the faulty SM submodule is sampled at a first sampling moment and a second sampling moment respectively to obtain a voltage value at a first moment and a voltage value at a second moment.

7. The method according to claim 1, characterized in that: The preset time interval is 1 ms.

8. The method according to claim 1, characterized in that Determining the fault type of the faulty SM submodule according to the fault location value includes: When the fault location value is greater than 0, it is determined that the fault type of the faulty SM submodule is a lower tube fault; when the fault location value is equal to 0, it is determined that the fault type of the faulty SM submodule is an upper tube fault.

9. The method according to claim 4, characterized in that: The fault threshold is 1.5% of the MMC DC side voltage.

10. The method according to claim 5, characterized in that: The warning threshold is 1.0% of the MMC DC side voltage.

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