Method for open-circuit fault diagnosis of a sub-module of a modular multilevel converter

Abstract

The application discloses an open-circuit fault diagnosis method of a sub-module based on a modular multilevel converter, and the method comprises the following steps: determining a sub-module switch state value by using carrier phase shift pulse width modulation; calculating group voltage residuals of all sub-modules according to the sub-module switch state value; and performing open-circuit fault diagnosis of the sub-modules according to the group voltage residuals of all the sub-modules. The open-circuit fault diagnosis method of the sub-module provided by the application is used for diagnosing single or multiple sub-modules in which switch tube open-circuit faults occur in a voltage sensor multiplexing group, the method does not need additional sensors and hardware circuits, and the diagnosis can be completed in a single triangular carrier cycle, the diagnosis speed is much faster than that of a traditional fault diagnosis method, and the reliability of the system is effectively improved.

Description

Technical Field

[0001] This invention relates to the field of open-circuit fault diagnosis, and more specifically to an open-circuit fault diagnosis method based on a submodule of a modular multilevel converter. Background Technology

[0002] Modular multilevel converters have become one of the most promising topologies for medium and high voltage industrial applications in recent years due to their excellent harmonic performance, inherent scalability, and modularity.

[0003] Modular multilevel converters require knowledge of the capacitor voltages of their submodules for safe and reliable operation. Because they often consist of hundreds of submodules, single-phase modular multilevel converters require a large number of voltage sensors, such as… Figure 1 As shown, the Individual voltage sensor is an independent voltage sensor, C SM S is the capacitor of the submodule. u For the switching transistor S u S l For the switching transistor S l SM1 is the first submodule, SM2 is the second submodule, SM N For the Nth submodule, L arm For the bridge arm inductance, V dc For external DC voltage, I dc For external DC current, u u The first voltage of the single-phase modular multilevel converter, u o For the second voltage of a single-phase modular multilevel converter, u l This is the third voltage in a single-phase modular multilevel converter. A modular multilevel converter with voltage sensor multiplexing replaces a single voltage sensor with a group of voltage sensors, such as... Figure 2 As shown, S1 is switch S1, S2 is switch S2, Group 1 is the first voltage sensor multiplexing group, Group k is the kth voltage sensor multiplexing group, and k is the total number of voltage sensor multiplexing groups. N-M+1 For the (N-M+1)th submodule, SM N-M+2 For the (N-M+2)th submodule, VoltageSensor 1 is the first multiplexed voltage sensor, and Voltage Sensor k is the kth multiplexed voltage sensor. The modular multilevel converter with multiplexed voltage sensors employs a series of software-based techniques, using the measured group voltage to estimate the capacitor voltage of each submodule in the group. This configuration helps reduce hardware costs and communication overhead, but it presents challenges for submodule status monitoring and fault diagnosis.

[0004] Existing technologies offer a model-based fault detection method that utilizes a Kalman filter to detect faults, achieving a diagnosis process in just tens of milliseconds. However, for modular multilevel converter systems with numerous submodules, this method demands stringent accuracy from the mathematical model, drastically increasing its complexity. Other existing technologies propose sensor-based fault diagnosis methods, monitoring faults by comparing the output of monitored sensors with expected voltages. Sensor-based fault diagnosis methods are as fast as microseconds, but they increase equipment costs, require consideration of redundant sensor reliability, and are generally difficult to port to modular multilevel converters that reuse voltage sensors. Summary of the Invention

[0005] To address the aforementioned shortcomings in the prior art, the present invention provides a method for diagnosing open-circuit faults in sub-modules of modular multilevel converters, which can diagnose open-circuit faults in sub-modules of modular multilevel converter groups with voltage sensor multiplexing without the need for additional sensors and hardware circuits.

[0006] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0007] The open-circuit fault diagnosis method for submodules based on modular multilevel converters includes the following steps:

[0008] S1. Determine the switching state value of the submodule using carrier phase-shift pulse width modulation;

[0009] S2. Calculate the group voltage residual of all sub-modules based on the sub-module switch status values ​​in step S1.

[0010] S3. Diagnose open-circuit faults of submodules based on the group voltage residuals of all submodules in step S2.

[0011] Further, step S1 includes the following sub-steps:

[0012] S11. The sampling time is determined by using carrier phase-shift pulse width modulation to determine the time when the triangular carrier of the corresponding sub-module reaches its peak value.

[0013] S12. Obtain the submodule switch state value at the sampling time in step S11 using carrier phase-shift pulse width modulation.

[0014] Furthermore, in step S12, the specific process of obtaining the submodule switch state value at the sampling time is as follows: when the modulation wave is greater than the triangular carrier wave, the submodule switch state value is 1; when the modulation wave is less than the triangular carrier wave, the submodule switch state value is 0.

[0015] Furthermore, step S2 includes the following sub-steps:

[0016] S21. Calculate the capacitor voltage of the submodule based on the submodule switch status value in step S1;

[0017] S22. Calculate the estimated group voltage of all sub-modules based on the capacitor voltage of the sub-module in step S21.

[0018] S23. Subtract the measured group voltage values ​​of all sub-modules from the estimated group voltage values ​​of all sub-modules in step S22 to obtain the group voltage residuals of all sub-modules.

[0019] Further, in step S21, the capacitor voltage of the submodule is calculated, expressed as:

[0020]

[0021] Where: u mg (t1) represents the measured group voltage value of all submodules at sampling time t1, s1(t1) represents the switching state value of the first submodule at sampling time t1, and u c1 s1 is the capacitor voltage of the first submodule, s2(t1) is the switching state value of the second submodule at sampling time t1, and u c2 The capacitor voltage of the second submodule, s M (t1) represents the switch state value of the Mth sub-module at sampling time t1, u cM Let u be the capacitor voltage of the Mth submodule. mg (t2) represents the measured group voltage value of all sub-modules at sampling time t2, s1(t2) represents the switching state value of the first sub-module at sampling time t2, and s2(t2) represents the switching state value of the second sub-module at sampling time t2. M (t2) represents the switch state value of the Mth sub-module at sampling time t2, u mg (t M (t) represents the sampling time. M The measured group voltage values ​​of all submodules at that time, s1(t) M (t) represents the sampling time. M The switch state value of the first submodule at time s2(t) M (t) represents the sampling time. M The switch state value of the second submodule at time, s M (t M (t) represents the sampling time. M The switch status value of the Mth submodule at that time.

[0022] Further, in step S22, the estimated group voltage values ​​for all submodules are calculated, expressed as:

[0023]

[0024] Where: u eg (t) represents the estimated group voltage of all submodules at sampling time t, where i is the submodule number, M is the total number of submodules in the sensor multiplexing group, and s i (t) represents the switch state value of the i-th submodule at sampling time t, u ci Let be the capacitor voltage of the i-th submodule.

[0025] Furthermore, step S3 includes the following sub-steps:

[0026] S31. Determine whether the group voltage residual of all sub-modules in step S2 is equal to zero; if so, diagnose the sub-module as normal; otherwise, diagnose the sub-module as open circuit fault and proceed to sub-step S32.

[0027] S32. Determine whether the group voltage residual of all sub-modules in step S2 is equal to a positive integer multiple of the rated voltage of the sub-module capacitor; if so, diagnose the open circuit fault as the first open circuit fault and perform fault location; otherwise, diagnose the open circuit fault as the second open circuit fault and perform fault location.

[0028] Furthermore, in step S32, diagnosing the open-circuit fault as the first open-circuit fault and locating the fault includes the following steps:

[0029] A1. Based on the output characteristics of the submodule, the bridge arm current i arm <0 and the switching transistor S u On and switch S l When disconnected, the switching state value of the submodule is determined to be 1, and the switching transistor S when the switching state value of the submodule is 1 is set. u The open-circuit fault was identified as the first open-circuit fault.

[0030] A2. Based on the fact that the group voltage residual of all sub-modules in step S2 is equal to a positive integer multiple of the rated voltage of the sub-module capacitor, the open circuit fault is determined to be the first open circuit fault in step A1.

[0031] A3. Based on the open circuit fault in A2, which is the first open circuit fault, mark the submodule with a switch status value of 1 as the suspected fault submodule.

[0032] A4. When the group voltage residual of all submodules falls back to zero, the suspected faulty submodule whose switch status value in A3 changes from 1 to 0 is identified as the faulty submodule.

[0033] Furthermore, in step S32, diagnosing the open-circuit fault as a second open-circuit fault and locating the fault includes the following steps:

[0034] B1. Based on the output characteristics of the submodule, the bridge arm current i arm >0 and the switching transistor S uDisconnect and switch S l When the submodule is turned on, its switching state value is set to 0, and the switching transistor S when the submodule's switching state value is 0 is set to 0. l The open circuit fault was identified as the second open circuit fault.

[0035] B2. Based on the fact that the group voltage residual of all sub-modules in step S2 is not equal to zero and is not equal to a positive integer multiple of the rated voltage of the sub-module capacitor, the open circuit fault is determined to be the second open circuit fault in step B1.

[0036] B3. Based on the open circuit fault in B2, which is the second open circuit fault, mark the submodule with a switch status value of 0 as the suspected fault submodule.

[0037] B4. When the group voltage residual of all submodules is increased to zero, the suspected faulty submodule whose switch state value in B3 changes from 0 to 1 is identified as the faulty submodule.

[0038] The beneficial effects of this invention are as follows:

[0039] (1) The open circuit fault diagnosis method of the sub-module provided by the present invention is used to diagnose one or more sub-modules in the voltage sensor multiplexing group that have open circuit faults of switching transistors. The method diagnoses the sub-modules that have open circuit faults of switching transistors by judging the group voltage residual value of all sub-modules, without the need for additional sensors and hardware circuits.

[0040] (2) The present invention can complete the diagnosis of faulty sub-modules within a single triangular carrier cycle by carrier phase shift pulse width modulation. Its diagnosis speed is much faster than traditional fault diagnosis methods, which effectively improves the reliability of the system. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of a single-phase modular multilevel converter.

[0042] Figure 2 A schematic diagram of a modular multilevel converter for multiplexing voltage sensors;

[0043] Figure 3 This is a schematic diagram of the open-circuit fault diagnosis method for submodules based on a modular multilevel converter.

[0044] Figure 4 This is a schematic diagram illustrating the periodic variation of the modulating wave and the triangular carrier wave in carrier phase-shift pulse width modulation.

[0045] Figure 5 This is a schematic diagram illustrating the positional changes of the triangular carrier wave in carrier phase-shift pulse width modulation.

[0046] Figure 6 The diagram shows the current path of the faulty submodule. A is a diagram of the first open-circuit fault, and b is a diagram of the second open-circuit fault.

[0047] Figure 7 This is a schematic diagram of fault location according to the present invention. Detailed Implementation

[0048] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0049] like Figure 3 As shown, the open-circuit fault diagnosis method for submodules based on a modular multilevel converter includes steps S1-S3, as detailed below:

[0050] The open-circuit fault diagnosis method for submodules based on modular multilevel converters includes the following steps:

[0051] S1. Determine the switching state value of the submodule using carrier phase-shift pulse width modulation.

[0052] In an optional embodiment of the present invention, the present invention utilizes carrier phase-shift pulse width modulation to determine the sampling time of the switching state value of the submodule, and determines the switching state value of the submodule within the sampling time according to the magnitude relationship between the modulating wave and the triangular carrier in the carrier phase-shift pulse width modulation.

[0053] Step S1 includes the following sub-steps:

[0054] S11. The sampling time is determined by using carrier phase-shift pulse width modulation to determine the time when the triangular carrier of the corresponding submodule reaches its peak value.

[0055] like Figure 4 As shown, Reference represents the modulated wave. In carrier phase-shift pulse width modulation, the phase difference between adjacent triangular carriers is 2π / N, where N is the total number of sub-modules in the modular multilevel converter. Each sub-module has a corresponding triangular carrier. This invention determines the sampling time as the moment when the triangular carrier of the corresponding sub-module reaches its peak value. During consecutive sampling times, all triangular carriers will periodically shift sequentially.

[0056] S12. Obtain the submodule switch state value at the sampling time in step S11 using carrier phase-shift pulse width modulation.

[0057] The specific process of obtaining the submodule switch state value at the sampling time in this invention is as follows: when the modulation wave is greater than the triangular carrier wave, the submodule switch state value is 1; when the modulation wave is less than the triangular carrier wave, the submodule switch state value is 0.

[0058] like Figure 4 As shown, the modulating wave and the triangular carrier wave exhibit periodic variations. The modulating wave is not a straight line, but a 50 Hz sinusoidal signal, while the frequency of the triangular carrier wave is on the order of kHz. Therefore, this invention envisions the modulating wave as a straight line.

[0059] like Figure 5 As shown, C1 is the triangular carrier wave of phase 1, C2 is the triangular carrier wave of phase 2, C3 is the triangular carrier wave of phase 3, C4 is the triangular carrier wave of phase 4, and C5 is the triangular carrier wave of phase 5. N-1 For the triangular carrier wave of phase (N-1), C N The triangular carrier wave represents the Nth phase, and the black arrow represents the modulation wave. At adjacent sampling times, one submodule's triangular carrier wave will cross the modulation wave downwards, and another submodule's triangular carrier wave will cross the modulation wave upwards. When one submodule's triangular carrier wave crosses the modulation wave downwards, the submodule's switch state value changes from 1 to 0; when one submodule's triangular carrier wave crosses the modulation wave upwards, the submodule's switch state value changes from 0 to 1. The switch state of all submodules in the bridge arm changes twice within one triangular carrier wave cycle, making fault diagnosis possible within one carrier wave cycle.

[0060] S2. Calculate the group voltage residual of all sub-modules based on the sub-module switch status values ​​in step S1.

[0061] In an optional embodiment of the present invention, the present invention can obtain the measured group voltage values ​​of all sub-modules, calculate the capacitor voltage of the sub-modules based on the sub-module switch state values ​​in step S1, and then calculate the estimated group voltage values ​​of all sub-modules. Furthermore, the group voltage residuals of all sub-modules can be calculated by using the obtained measured group voltage values ​​of all sub-modules and the calculated estimated group voltage values ​​of all sub-modules.

[0062] Step S2 includes the following sub-steps:

[0063] S21. Calculate the capacitor voltage of the submodule based on the submodule switch state value in step S1.

[0064] The capacitor voltage of the calculation submodule in this invention is expressed as:

[0065]

[0066] Where: u mg (t1) represents the measured group voltage value of all submodules at sampling time t1, s1(t1) represents the switching state value of the first submodule at sampling time t1, and u c1 s1 is the capacitor voltage of the first submodule, s2(t1) is the switching state value of the second submodule at sampling time t1, and u c2The capacitor voltage of the second submodule, s M (t1) represents the switch state value of the Mth sub-module at sampling time t1, u cM Let u be the capacitor voltage of the Mth submodule. mg (t2) represents the measured group voltage value of all sub-modules at sampling time t2, s1(t2) represents the switching state value of the first sub-module at sampling time t2, and s2(t2) represents the switching state value of the second sub-module at sampling time t2. M (t2) represents the switch state value of the Mth sub-module at sampling time t2, u mg (t M (t) represents the sampling time. M The measured group voltage values ​​of all submodules at that time, s1(t) M (t) represents the sampling time. M The switch state value of the first submodule at time s2(t) M (t) represents the sampling time. M The switch state value of the second submodule at time, s M (t M (t) represents the sampling time. M The switch status value of the Mth submodule at that time.

[0067] S22. Calculate the estimated group voltage of all sub-modules based on the capacitor voltage of the sub-module in step S21.

[0068] This invention calculates the group voltage estimate for all submodules, expressed as:

[0069]

[0070] Where: u eg (t) represents the estimated group voltage of all submodules at sampling time t, where i is the submodule number, M is the total number of submodules in the sensor multiplexing group, and s i (t) represents the switch state value of the i-th submodule at sampling time t, u ci Let be the capacitor voltage of the i-th submodule.

[0071] S23. Subtract the measured group voltage values ​​of all sub-modules from the estimated group voltage values ​​of all sub-modules in step S22 to obtain the group voltage residuals of all sub-modules.

[0072] S3. Diagnose open-circuit faults of submodules based on the group voltage residuals of all submodules in step S2.

[0073] In an optional embodiment of the present invention, the present invention can diagnose whether all submodules are normal based on the group voltage residual of all submodules. If all submodules are normal, the group voltage residual of all submodules will be zero; if one or more submodules have an open circuit fault, the group voltage residual of all submodules will not be zero. When the present invention further determines that one or more submodules have an open circuit fault, it can further locate the specific submodule with the open circuit fault by monitoring the change of the group voltage residual.

[0074] Step S3 includes the following sub-steps:

[0075] S31. Determine whether the group voltage residual of all sub-modules in step S2 is equal to zero; if so, diagnose the sub-module as normal, otherwise diagnose the sub-module as open circuit fault and proceed to sub-step S32.

[0076] If the group voltage residual of all submodules is equal to zero, that is, the estimated group voltage of all submodules is equal to the measured group voltage of all submodules, it means that all submodules are in normal working condition; if the group voltage residual of all submodules is not equal to zero, that is, the estimated group voltage of all submodules is not equal to the measured group voltage of all submodules, the output voltage of one or more submodules in the voltage sensor multiplexing group is different from the output voltage during normal operation, therefore the submodule has an open circuit fault.

[0077] S32. Determine whether the group voltage residual of all sub-modules in step S2 is equal to a positive integer multiple of the rated voltage of the sub-module capacitor; if so, diagnose the open circuit fault as the first open circuit fault and perform fault location; otherwise, diagnose the open circuit fault as the second open circuit fault and perform fault location.

[0078] The group voltage residual of all submodules is not equal to zero, that is, the estimated group voltage of all submodules is not equal to the measured group voltage of all submodules. The output voltage of one or more submodules in the voltage sensor multiplexing group is different from the output voltage during normal operation, as shown in Table 1. When the bridge arm current i arm <0 and the switching transistor S u On and switch S l When disconnected, the switching transistor S u An open-circuit fault will cause the submodule's output voltage to be 0, which is different from the normal operating voltage u. C Different, such as Figure 6 As shown in Figure a, the normal current path is the path of normal current flow, represented by a light gray dashed arrow, and the fault current path is the path of fault current flow, represented by a dark gray dashed arrow; when the bridge arm current i arm >0 and the switching transistor S u Disconnect and switch S l When turned on, the switching transistor S lAn open-circuit fault occurs when the submodule's output voltage is u. C This is different from the voltage that outputs 0 during normal operation, such as... Figure 6 As shown in b. Therefore, the open-circuit faults existing in the submodule include the first open-circuit fault and the second open-circuit fault.

[0079] Table 1 Submodule Output Characteristics

[0080]

[0081] Where: 1 indicates conduction, 0 indicates deactivation, u C This is the rated voltage of the submodule capacitor.

[0082] This invention diagnoses open-circuit faults as first open-circuit faults and performs fault location, including steps A1-A4:

[0083] A1. Based on the output characteristics of the submodule, the bridge arm current i arm <0 and the switching transistor S u On and switch S l When disconnected, the switching state value of the submodule is determined to be 1, and the switching transistor S when the switching state value of the submodule is 1 is set. u The open circuit fault was identified as the first open circuit fault.

[0084] A2. Based on the fact that the group voltage residual of all sub-modules in step S2 is equal to a positive integer multiple of the rated voltage of the sub-module capacitor, the open circuit fault is determined to be the first open circuit fault in step A1.

[0085] A3. Based on the open circuit fault in A2, which is the first open circuit fault, mark the submodule with the switch status value of 1 as the suspected fault submodule.

[0086] like Figure 7 As shown, Case is a case study, S u The open-circuit fault is the switching transistor S. u Open circuit fault, SFSM is the suspected fault submodule, FSM is the faulty submodule, the switching transistor S of the second submodule in the sensor multiplexing group. u An open-circuit fault occurs when the bridge arm current is less than zero. The residual voltage across all submodules is equal to (or nearly equal to) the rated voltage of the submodule capacitors, indicating the presence of a switching transistor S within the group. u The submodule with an open-circuit fault. At the sampling time t1 when the open-circuit fault is detected by this invention, other submodules within the sensor multiplexing group may also have a switch state value of 1 at this sampling time. Therefore, this invention cannot temporarily identify which specific submodule has an open-circuit fault. Therefore, at the sampling time t1 when the open-circuit fault is detected, this invention marks the submodule with a switch state value of 1 within the group at this time as a suspected faulty submodule. Figure 7The gray boxes in the text represent suspected faulty submodules.

[0087] A4. When the group voltage residual of all submodules falls back to zero, the suspected faulty submodule whose switch status value in A3 changes from 1 to 0 is identified as the faulty submodule.

[0088] Within one carrier cycle, the submodule will switch between different states (Table 1 includes the four possible states of the submodule), and the switching transistor S will occur. u The submodule with an open-circuit fault will operate normally except under specific switching states where it will exhibit fault characteristics (the actual output voltage differs from the normal output voltage). Therefore, when it switches from a state exhibiting fault characteristics to a state that can mimic a normal submodule (i.e., the corresponding switching state value changes from 1 to 0), this invention can locate the open-circuit fault submodule by measuring the group voltage residual of all submodules returning to zero. Figure 7 The black box in the diagram represents a faulty submodule. For a suspected faulty submodule where the switch state value changes from 1 to 0, but the group voltage residual of all submodules still equals the rated voltage of the submodule capacitor, this invention removes the suspicion flag for that submodule. After locating a submodule with an open-circuit fault, this invention controls... Figure 6 The switch Q in the module can disconnect the submodule.

[0089] This invention can complete fault diagnosis within one triangular carrier cycle. Specifically, if the carrier frequency is 1kHz and the number of sub-modules in the bridge arm is 10, the fault diagnosis time is as fast as 0.1ms and as slow as 1ms. Furthermore, the fault diagnosis method proposed in this invention is not limited to a single sub-module open-circuit fault. When multiple sub-modules within a sensor multiplexing group have a first open-circuit fault, the group voltage residual of all sub-modules is equal to a positive integer multiple of the rated voltage of the sub-module capacitors, and the fault diagnosis is performed with reference to the above steps.

[0090] This invention diagnoses an open-circuit fault as a second open-circuit fault and performs fault location, including steps B1-B4:

[0091] B1. Based on the output characteristics of the submodule, the bridge arm current i arm >0 and the switching transistor S u Disconnect and switch S l When the submodule is turned on, its switching state value is set to 0, and the switching transistor S when the submodule's switching state value is 0 is set to 0. l The open circuit fault was identified as the second open circuit fault.

[0092] B2. Based on the fact that the group voltage residual of all sub-modules in step S2 is not equal to zero and is not equal to a positive integer multiple of the rated voltage of the sub-module capacitor, the open circuit fault is determined to be the second open circuit fault in step B1.

[0093] B3. Based on the open circuit fault in B2, which is the second open circuit fault, mark the submodule with the switch status value of 0 as the suspected fault submodule.

[0094] The switching transistor S of the second submodule within the sensor multiplexing group l An open-circuit fault occurs when the bridge arm current is greater than zero, and the group voltage residual of all submodules is equal to (or nearly equal to) the rated voltage of the negative submodule capacitor. This indicates the presence of a switching transistor S within the group. l The submodule with an open-circuit fault. At sampling time t1, when the invention detects the existence of an open-circuit fault, other submodules within the sensor multiplexing group may also have a switch state value of 0 at this sampling time. Therefore, the invention cannot temporarily identify which specific submodule has an open-circuit fault. Therefore, at sampling time t1, when the existence of an open-circuit fault is detected, the invention marks the submodule with a switch state value of 0 within the group as a suspected faulty submodule.

[0095] B4. When the group voltage residual of all submodules is increased to zero, the suspected faulty submodule whose switch state value in B3 changes from 0 to 1 is identified as the faulty submodule.

[0096] Within a carrier cycle, the submodule switches between different states (Table 1 includes the four possible states of the submodule), and the switching transistor S occurs. l A submodule with an open-circuit fault exhibits fault characteristics (the actual output voltage differs from the normal output voltage) only under specific switching states; otherwise, it operates normally. Therefore, when it switches from a state exhibiting fault characteristics to a state that can mimic a normal submodule (i.e., the switching state value changes from 0 to 1), this invention can locate the open-circuit fault submodule by increasing the group voltage residual of all submodules to zero. For suspected fault submodules where the switching state value changes from 0 to 1, but the group voltage residual of all submodules still equals the negative rated voltage of the submodule capacitor, this invention eliminates the suspicion flag for that submodule. After locating the submodule with an open-circuit fault, this invention controls... Figure 6 The switch Q in the module can disconnect the submodule.

[0097] This invention can complete fault diagnosis within one triangular carrier cycle. Specifically, if the carrier frequency is 1kHz and the number of sub-modules in the bridge arm is 10, the fault diagnosis time is as fast as 0.1ms and as slow as 1ms. Furthermore, the fault diagnosis method proposed in this invention is not limited to a single sub-module open-circuit fault. When multiple sub-modules within a sensor multiplexing group have a second open-circuit fault, the group voltage residual of all sub-modules is equal to an integer multiple of the negative rated voltage of the sub-module capacitor, and the fault diagnosis is performed with reference to the above steps.

[0098] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of this invention.

Claims

1. A method for diagnosing open-circuit faults in submodules of a modular multilevel converter, characterized in that, Includes the following steps: S1. Determine the switching state value of the submodule using carrier phase-shift pulse width modulation; S2. Calculate the group voltage residual of all sub-modules based on the sub-module switch status values ​​in step S1. S3. Diagnose open-circuit faults of sub-modules based on the group voltage residuals of all sub-modules in step S2. Step S3 includes the following sub-steps: S31. Determine whether the group voltage residual of all sub-modules in step S2 is equal to zero; if so, diagnose the sub-module as normal; otherwise, diagnose the sub-module as open circuit fault and proceed to sub-step S32. S32. Determine whether the group voltage residual of all sub-modules in step S2 is equal to a positive integer multiple of the rated voltage of the sub-module capacitor; if so, diagnose the open circuit fault as the first open circuit fault and perform fault location; otherwise, diagnose the open circuit fault as the second open circuit fault and perform fault location. In step S32, diagnosing the open circuit fault as the first open circuit fault and locating the fault includes the following steps: A1. Based on the output characteristics of the submodule, the bridge arm current... i arm <0 and switching transistor S u On and switching transistor S l When disconnected, the switch state value of the submodule is determined to be 1, and the switch transistor when the switch state value of the submodule is 1 is set to 1. S u The open-circuit fault was identified as the first open-circuit fault. A2. Based on the fact that the group voltage residual of all sub-modules in step S2 is equal to a positive integer multiple of the rated voltage of the sub-module capacitor, the open circuit fault is determined to be the first open circuit fault in step A1. A3. Based on the open circuit fault in A2, which is the first open circuit fault, mark the submodule with a switch status value of 1 as the suspected fault submodule. A4. When the group voltage residual of all submodules falls back to zero, the suspected faulty submodule whose switch status value in A3 changes from 1 to 0 is identified as the faulty submodule. In step S32, diagnosing the open circuit fault as a second open circuit fault and locating the fault includes the following steps: B1. Based on the output characteristics of the submodule, the bridge arm current... i arm >0 and switching transistor S u Disconnect and switch transistor S l When the submodule is turned on, its switching state value is set to 0, and the switching transistor with a switching state value of 0 is set to... S l The open circuit fault was identified as the second open circuit fault. B2. Based on the fact that the group voltage residual of all sub-modules in step S2 is not equal to zero and is not equal to a positive integer multiple of the rated voltage of the sub-module capacitor, the open circuit fault is determined to be the second open circuit fault in step B1. B3. Based on the open circuit fault in B2, which is the second open circuit fault, mark the submodule with a switch status value of 0 as the suspected fault submodule. B4. When the group voltage residual of all submodules is increased to zero, the suspected faulty submodule whose switch state value in B3 changes from 0 to 1 is identified as the faulty submodule.

2. The open-circuit fault diagnosis method for submodules based on a modular multilevel converter according to claim 1, characterized in that, Step S1 includes the following sub-steps: S11. The sampling time is determined by using carrier phase-shift pulse width modulation to determine the time when the triangular carrier of the corresponding sub-module reaches its peak value. S12. Obtain the submodule switch state value at the sampling time in step S11 using carrier phase-shift pulse width modulation.

3. The open-circuit fault diagnosis method for submodules based on a modular multilevel converter according to claim 2, characterized in that, In step S12, the specific process of obtaining the submodule switch state value at the sampling time is as follows: when the modulation wave is greater than the triangular carrier wave, the submodule switch state value is 1; when the modulation wave is less than the triangular carrier wave, the submodule switch state value is 0.

4. The open-circuit fault diagnosis method for submodules based on a modular multilevel converter according to claim 1, characterized in that, Step S2 includes the following sub-steps: S21. Calculate the capacitor voltage of the submodule based on the submodule switch status value in step S1; S22. Calculate the estimated group voltage of all sub-modules based on the capacitor voltage of the sub-module in step S21. S23. Subtract the measured group voltage values ​​of all sub-modules from the estimated group voltage values ​​of all sub-modules in step S22 to obtain the group voltage residuals of all sub-modules.

5. The open-circuit fault diagnosis method for submodules based on a modular multilevel converter according to claim 4, characterized in that, In step S21, the capacitor voltage of the submodule is calculated and expressed as: in: Sampling time The measured group voltage values ​​of all submodules at that time. Sampling time The switch status value of the first submodule at that time. This is the capacitor voltage of the first submodule. Sampling time The switch status value of the second submodule at that time. The capacitor voltage of the second submodule. Sampling time The first time The on / off status values ​​of each submodule For the first The capacitor voltage of each submodule Sampling time The measured group voltage values ​​of all submodules at that time. Sampling time The switch status value of the first submodule at that time. Sampling time The switch status value of the second submodule at that time. Sampling time The first time The on / off status values ​​of each submodule Sampling time The measured group voltage values ​​of all submodules at that time. Sampling time The switch status value of the first submodule at that time. Sampling time The switch status value of the second submodule at that time. Sampling time The first time The on / off status values ​​of each submodule.

6. The open-circuit fault diagnosis method for submodules based on a modular multilevel converter according to claim 5, characterized in that, In step S22, the estimated group voltage values ​​for all sub-modules are calculated, as follows: in: Sampling time The estimated group voltage values ​​of all submodules at that time. This is the sequence number of the submodule. This represents the total number of submodules within the sensor reuse group. Sampling time Time The on / off status values ​​of each submodule For the first The capacitor voltage of each submodule.

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