CN113281678A - Method for positioning open-circuit fault of tubes on half-bridge submodule of modular multilevel converter - Google Patents

Method for positioning open-circuit fault of tubes on half-bridge submodule of modular multilevel converter Download PDF

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CN113281678A
CN113281678A CN202110390870.4A CN202110390870A CN113281678A CN 113281678 A CN113281678 A CN 113281678A CN 202110390870 A CN202110390870 A CN 202110390870A CN 113281678 A CN113281678 A CN 113281678A
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CN113281678B (en
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孙向东
张梦楠
安杨
任碧莹
陈桂涛
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Xian University of Technology
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Abstract

The invention discloses a method for positioning an open-circuit fault on a tube of a half-bridge submodule of a modular multilevel converter, which achieves the purpose of diagnosing and positioning a fault submodule by judging the positive and negative of a submodule capacitor current; the submodule capacitor current is used as a characteristic parameter for fault diagnosis, analysis shows that the submodule capacitor current has positive and negative values under the normal operation condition, namely the normal submodule capacitor can be charged and discharged normally, and after an upper tube open circuit fault occurs, the capacitor of the fault submodule cannot be discharged normally, so that the submodule capacitor current does not have the negative value, and the fault submodule can be diagnosed and positioned according to the characteristic; the sub-modules of the modular multilevel converter are of a half-bridge structure, and the upper bridge arm and the lower bridge arm of each phase are respectively formed by cascading N sub-modules. The method also well solves the problem that the capacitance voltage of the normal submodule is consistent with the capacitance voltage of the fault submodule under the condition that the open circuit fault of the submodule tubes occurs under the sorting voltage-sharing algorithm.

Description

Method for positioning open-circuit fault of tubes on half-bridge submodule of modular multilevel converter
Technical Field
The invention relates to the technical field of power electronics, in particular to a method for positioning an open-circuit fault of a tube on a half-bridge submodule of a modular multilevel converter.
Background
The modular multilevel Converter (modular multilevel Converter) is a novel voltage source Converter topology proposed by r.marquardt and a.leincica in 2002, and has the advantages of low device withstand voltage change rate, strong expansion capability, small switching loss, easiness in redundancy design and the like, so that the modular multilevel Converter has a wide application prospect in the fields of high-voltage direct-current power transmission, asynchronous interconnection of alternating-current power grids, train traction, wind power plant access, offshore platform power supply and the like. In a modular multilevel converter system, a large number of switching devices are used, and when the system has abnormal operation conditions or the operation mode is suddenly changed, the switching devices are likely to be damaged along with severe conditions such as overcurrent, overvoltage, and overhigh voltage change rate or current change rate, overhigh temperature and the like in the circuit. The reliability is an important standard for judging the operation performance of the modular multilevel converter system, if the switching devices of individual sub-modules fail during operation, the output voltage and the bridge arm current of the system are distorted, even linkage effect occurs, the range of fault influence is enlarged, and the normal operation of the modular multilevel converter is seriously damaged. Therefore, it is necessary to monitor the operating status of the sub-modules in real time, and to remove and replace the sub-modules with failures in time, so as to shorten the duration and influence range of failures. How to quickly and accurately detect and locate the open-circuit fault of the switching device is the key point of the problem research.
At present, the fault diagnosis and positioning method for the submodule of the modular multilevel converter system comprises a hardware method, a model analysis method, a data analysis method and other methods. For the hardware method, an additional hardware circuit is needed to perform fault diagnosis, the principle is simple, and the diagnosis speed is high, but the method can increase the hardware cost, and the existence of the additional hardware can also form a potential fault point. For the model analysis method, a Kalman filtering algorithm is taken as an example, and the deviation between the estimated value and the actual measured value is taken as the basis of fault diagnosis. Another disadvantage is that there may be problems with model mismatch and the time required for fault diagnosis and localization is relatively long. For a data analysis method, a machine learning method is taken as an example, when the method is used for sub-module fault diagnosis, data characteristics after normal operation and fault occurrence need to be analyzed, and whether the fault occurs can be judged only by evaluating the data characteristics of measured values of characteristic parameters. In summary, how to accurately diagnose and locate the open circuit fault of the single or multiple sub-module switch devices under the condition of controlling the cost and improve the diagnosis speed is a problem which needs to be solved urgently. The invention provides a method for positioning an open-circuit fault on a tube of a half-bridge submodule of a three-phase modular multilevel converter, which is based on the problems, so that accurate positioning and rapid diagnosis of the fault submodule are realized.
Disclosure of Invention
Technical problem to be solved
The invention monitors the capacitance current of each submodule in each sampling period by utilizing the characteristic that the submodule capacitor can not normally discharge after the open circuit fault occurs on the tube of the half-bridge submodule and accumulates the times of the negative current. And if the accumulated quantity of the negative current times is equal to zero within the specified detection time, determining that the half-bridge submodule has an upper tube open-circuit fault, thereby realizing the quick diagnosis and accurate positioning of the fault submodule. The method not only can effectively solve the problem that the fault sub-modules cannot be distinguished due to the fact that the capacitor voltages of all sub-modules of a bridge arm where the fault sub-modules are located rise synchronously under the control of a carrier stacking modulation strategy and a sorting voltage-sharing algorithm, but also can quickly diagnose and position the open-circuit faults of the upper tubes of the plurality of fault sub-modules.
(II) technical scheme
In order to achieve the purpose, the invention is realized by the following technical scheme: the method for positioning the open-circuit fault of the tube on the half-bridge submodule of the modular multilevel converter achieves the purpose of diagnosing and positioning the fault submodule by judging the positive and negative of the capacitance current of the submodule; the submodule capacitor current is used as a characteristic parameter for fault diagnosis, analysis shows that the submodule capacitor current has positive and negative values under the normal operation condition, namely the normal submodule capacitor can be charged and discharged normally, and after an upper tube open circuit fault occurs, the capacitor of the fault submodule cannot be discharged normally, so that the submodule capacitor current does not have the negative value, and the fault submodule can be diagnosed and positioned according to the characteristic; the sub-modules of the modular multilevel converter are of a half-bridge structure, and the upper bridge arm and the lower bridge arm of each phase are respectively formed by cascading N sub-modules; the method comprises the following specific steps:
step 1: sampling capacitor voltages of all half-bridge sub-modules; the capacitor voltage of the ith sub-module of the j-phase upper and lower bridge arms is ucrjiCurrent i flowing through the sub-module capacitorcrjiWith the voltage u across the sub-module capacitorcrjiThe following relationships exist:
Figure BDA0003016708280000031
in the formula, C is the capacitance value of the sub-module parallel capacitor;
sub-module capacitance voltage ucrjiThe differential value of (a) is estimated to obtain:
Figure BDA0003016708280000032
in the formula ucrji(k) Represents the capacitance voltage u of the ith sub-module of the upper and lower bridge arms at the time of kcrji(k-1) represents the capacitance voltage of the ith sub-module of the upper and lower bridge arms at j phase at the time of k-1, TsRepresents a sampling period;
the capacitance current i of the ith sub-module of the j-phase upper and lower bridge arms can be obtained according to the formula (1) and the formula (2)crjiThe specific calculation expression is:
Figure BDA0003016708280000041
step 2: calculating the capacitance current of all half-bridge sub-modules according to the formula (3); in the formula, C is the capacitance value of the sub-module parallel capacitor;
and step 3: all half-bridge sub-modules SM for a three-phase modular multilevel converterpa1～SMpaN、SMna1～SMnaN、SMpb1～SMpbN、SMnb1～SMnbN、SMpc1～SMpcN、SMnc1～SMncNSetting a cumulative amount num of negative current times of a corresponding capacitance currentpa1～numpaN、numna1～numnaN、numpb1～numpbN、numnb1～numnbN、numpc1～numpcN、numnc1～numncN；
And 4, step 4: initializing the accumulated quantity of negative current times of all sub-module capacitor currents to be zero;
and 5: calculating the accumulated quantity of negative current times of all the sub-module capacitor currents; calculating the capacitance current i of each submodule through the formula (3)crjiJudging the capacitance current of each submodule, if icrjiIf the current is less than 0, the sub-module capacitor current is judged to be negative, and then the accumulated quantity num _ neg of the negative current times of the sub-module capacitor is judgedrji Adding 1; on the contrary, the accumulated quantity num _ neg of negative current times of the sub-module capacitorrjiRemain unchanged.
Step 6: judging whether the diagnosis time is over;
and 7: and judging the accumulated quantity of the negative current times of the capacitor currents of all the sub-modules.
Further, the step 6 specifically includes: when the diagnosis time T is equal to TthWhen the time is 20ms, the accumulated quantity num _ neg of negative current times of all sub-module capacitances is terminatedrjiStep 7 is entered; otherwise, returning to the step 5, and continuing to accumulate the negative current times of all the sub-module capacitors.
Further, when the diagnosis time is over, the accumulated quantity num _ neg of the negative current times of the capacitor current of each submodule is judgedrjiIf num _ negrjiIf the sub-module is 0, the sub-module is judged to have the T1 pipe open-circuit fault, and the sub-module is diagnosed and positioned to the fault sub-module SMrji(ii) a If num _ negrjiIf the number of the sub-module is more than 0, the sub-module is in a normal operation state, the step 4 is returned, and the accumulated number num _ neg of the negative current times corresponding to the sub-module capacitor is countedrjiAnd (6) clearing.
(III) advantageous effects
The invention provides a method for positioning an open-circuit fault on a tube of a half-bridge submodule of a modular multilevel converter. The method has the following beneficial effects:
the method of the invention only needs to sample the capacitance voltage signal of each submodule without detecting the bridge arm current signal or the submodule capacitance current signal, thereby greatly reducing the hardware cost. The method can accurately diagnose and position the upper tube open-circuit fault of the single-phase modular multilevel converter, and can also accurately diagnose and position the upper tube open-circuit fault of the three-phase modular multilevel converter. Meanwhile, the diagnosis and the positioning of the fault sub-module when the open-circuit fault of the upper tube occurs to a single sub-module or a plurality of sub-modules can be carried out within one diagnosis period, namely 20ms, so that the speed of the diagnosis and the positioning of the fault is effectively improved. The method of the invention is to accumulate the discharge quantity times of the sub-module capacitance current for one diagnosis period, if the value at a certain moment has error, the diagnosis result will not be influenced, thereby effectively reducing the possibility of error diagnosis and improving the accuracy of fault diagnosis and positioning. In addition, the method of the invention also well solves the problem that the capacitance voltage of the normal submodule and the capacitance voltage of the fault submodule are consistent under the condition that the open circuit fault of the submodules occurs under the sorting voltage-sharing algorithm.
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Fig. 1 is a diagram of a main circuit topology of a three-phase modular multilevel converter suitable for use in the present invention;
FIG. 2 is a schematic diagram of the variables to be sampled according to the present invention;
FIG. 3 is a flow chart of the method for diagnosing and locating the open-circuit fault on the tube of the half-bridge submodule of the three-phase modular multilevel converter system according to the present invention;
FIG. 4 is a diagram showing the diagnosis result of an open-tube fault occurring in the first sub-module of the a-phase upper bridge arm of the three-phase modular multilevel converter of the present invention;
FIG. 5 is a diagram showing the diagnosis result of an upper tube open-circuit fault occurring in the first sub-module of the upper bridge arm and the first sub-module of the lower bridge arm of the three-phase modular multilevel converter c-phase of the invention;
fig. 6 is a diagram showing the diagnosis result of the open-circuit fault of the upper tube of the first sub-module of the a-phase lower bridge arm, the third sub-module of the b-phase upper bridge arm and the fourth sub-module of the c-phase upper bridge arm of the three-phase modular multilevel converter.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example (b):
FIG. 1 is a main circuit topology structure diagram of a three-phase modular multilevel converter, wherein three phases a, b and c are highly symmetrical, each phase consists of an upper bridge arm and a lower bridge arm, the three phases have six bridge arms, the upper bridge arm and the lower bridge arm of each phase are also symmetrical, and each bridge arm consists of N half-bridge sub-modules SMrji(r ═ p, N; j ═ a, b, c; i ═ 1,2,3, … N) and a bridge arm inductance Lrj(r ═ p, n; j ═ a, b, c) in series. Each submodule is of a half-bridge type structure, and each half-bridge submodule is composed of two switching devices T1 and T2 and a capacitor C connected in parallel, wherein the two switching devices T1 and T2 are respectively connected with a diode VD1 and a diode VD2 in an anti-parallel mode. Three-phase upper bridge arm first submodule SMpj1The emitting electrodes of the T1 tubes are connected together and connected with the positive end of the direct current bus; three-phase lower bridge arm last submodule SMnjNThe emitters of the T2 tubes are connected together and to the negative terminal of the dc bus. The support capacitor on the direct current side is formed by connecting two capacitors C1 and C2 in series, and the direct current side is connected with a load resistor R in parallel. Three-phase upper and lower bridgeBridge arm inductance L of armpj、LnjThe connection point and the filter inductor L on the AC power grid sidesjConnected, filter inductance LsjParasitic resistance R of j-phase linesj，RsjConnecting j alternating voltage usj。
The half-bridge sub-module has three working states: plunge, cut and latch states. When the upper tube is in an on state and the lower tube is in an off state, the submodule is in an on state, the output voltage of the submodule is the submodule capacitor voltage, and the capacitor is in a charge-discharge state; when the upper tube is in a turn-off state and the lower tube is in a turn-on state, the submodule is in a cutting-off state, the output voltage of the submodule is 0 at the moment, and the capacitor is in a bypass state; when the upper pipe is in an off state, and the lower pipe is also in an off state, the submodule is in a locking state, and the state can only occur under the abnormal operation condition.
When an open-circuit fault occurs to the upper tube T1, if the submodule is in a cutting-off state, the normal working condition is the same as the above normal working condition; if the sub-module is in the input state, if the bridge arm current is larger than zero, the normal working condition is the same; if the bridge arm current is smaller than zero, the bridge arm current flows through an anti-parallel diode VD2 of the T2 tube due to the fact that the T1 tube is open-circuited, the capacitance current of the submodule is 0, and the voltage output by the submodule at the moment is reduced to 0 instead of the normal capacitance voltage of the submodule.
Fig. 2 is a schematic diagram of variables to be sampled by one j-phase bridge arm of the three-phase modular multilevel converter, wherein each sub-module capacitor voltage of each phase needs to be sampled.
Fig. 3 is a flowchart of a diagnosis and location method for an open-tube fault of one or more sub-modules of a modular multilevel converter according to the present invention. The general idea of the invention is to achieve the purpose of diagnosing and positioning the fault submodule by judging the positive and negative of the submodule capacitor current. The submodule capacitor current is used as a characteristic parameter for fault diagnosis, analysis shows that the submodule capacitor current has positive and negative values under the normal operation condition, namely, the normal submodule capacitor can be charged and discharged normally, and after an upper tube open circuit fault occurs, the capacitor of the fault submodule can not discharge normally, so that the submodule capacitor current does not have the negative value, and the fault submodule can be diagnosed and positioned according to the characteristic. The sub-modules of the modular multilevel converter are of a half-bridge structure, and the upper bridge arm and the lower bridge arm of each phase are respectively formed by cascading N sub-modules. The method comprises the following specific steps:
step 1: the capacitor voltage of all half-bridge sub-modules is sampled. The capacitor voltage of the ith sub-module of the j-phase upper and lower bridge arms is ucrjiCurrent i flowing through the sub-module capacitorcrjiWith the voltage u across the sub-module capacitorcrjiThe following relationships exist:
Figure BDA0003016708280000081
in the formula, C is the capacitance value of the sub-module parallel capacitor.
Sub-module capacitance voltage ucrjiThe differential value of (a) is estimated to obtain:
Figure BDA0003016708280000082
in the formula ucrji(k) Represents the capacitance voltage u of the ith sub-module of the upper and lower bridge arms at the time of kcrji(k-1) represents the capacitance voltage of the ith sub-module of the upper and lower bridge arms at j phase at the time of k-1, TsRepresenting the sampling period.
The capacitance current i of the ith sub-module of the j-phase upper and lower bridge arms can be obtained according to the formula (1) and the formula (2)crjiThe specific calculation expression is:
Figure BDA0003016708280000083
step 2: the capacitance currents of all half-bridge sub-modules are calculated according to equation (3).
And step 3: all half-bridge sub-modules SM for a three-phase modular multilevel converterpa1～SMpaN、SMna1～SMnaN、SMpb1～SMpbN、SMnb1～SMnbN、SMpc1～SMpcN、SMnc1～SMncNSetting a cumulative amount num of negative current times of a corresponding capacitance currentpa1～numpaN、numna1～numnaN、numpb1～numpbN、numnb1～numnbN、numpc1～numpcN、numnc1～numncN。
And 4, step 4: and initializing the accumulated quantity of the negative current times of all the sub-module capacitor currents to be zero.
And 5: and calculating the accumulated quantity of the negative current times of the capacitor currents of all the sub-modules. The capacitance current i of each submodule can be calculated by the formula (3)crjiJudging the capacitance current of each submodule, if icrjiIf the current is less than 0, the sub-module capacitor current is judged to be negative, and then the accumulated quantity num _ neg of the negative current times of the sub-module capacitor is judgedrjiAdding 1; on the contrary, the accumulated quantity num _ neg of negative current times of the sub-module capacitorrjiRemain unchanged.
Step 6: it is determined whether the diagnosis time is over. When the diagnosis time T is equal to TthWhen the time is 20ms, the accumulated quantity num _ neg of negative current times of all sub-module capacitances is terminatedrjiStep 7 is entered; otherwise, returning to the step 5, and continuing to accumulate the negative current times of all the sub-module capacitors.
And 7: and judging the accumulated quantity of the negative current times of the capacitor currents of all the sub-modules. When the diagnosis time is over, the accumulated quantity num _ neg of the negative current times of the capacitance current of each submodule is judgedrjiIf num _ negrjiIf the sub-module is 0, the sub-module is judged to have the T1 pipe open-circuit fault, and the sub-module is diagnosed and positioned to the fault sub-module SMrji(ii) a If num _ negrjiIf the number of the sub-module is more than 0, the sub-module is in a normal operation state, the step 4 is returned, and the accumulated number num _ neg of the negative current times corresponding to the sub-module capacitor is countedrjiAnd (6) clearing.
Fig. 4 is a diagram showing the diagnosis result of the open-circuit fault of the upper tube of the first submodule of the upper bridge arm of the phase a of the three-phase modular multilevel converter. and (3) the first submodule of the upper bridge arm of the phase a has an upper tube open-circuit fault in 0.2s, the accumulated quantity of the negative current times of the capacitance current of the first submodule is 0 in 0.22s, the fault flag bit of the submodule is changed from 0 to 1, and the fault submodule is diagnosed and positioned.
Fig. 5 is a diagram showing the diagnosis result of the open-circuit fault of the upper tube of the first submodule of the upper bridge arm and the first submodule of the lower bridge arm of the c-phase modular multilevel converter. c, when the first submodule of the upper bridge arm of the phase is subjected to an upper tube open-circuit fault in 0.2s, and when the fault flag bit of the submodule is changed from 0 to 1 in 0.22s, diagnosing and positioning the fault submodule; and c, when the first submodule of the lower bridge arm of the phase is in an upper tube open-circuit fault state at 0.3s, changing the fault flag bit of the submodule from 0 to 1 at 0.32s, and diagnosing and positioning the fault submodule.
Fig. 6 is a diagram showing the diagnosis results of the open-circuit fault of the upper tubes of the first sub-module of the a-phase lower bridge arm, the third sub-module of the b-phase upper bridge arm and the fourth sub-module of the c-phase upper bridge arm of the three-phase modular multilevel converter. The first submodule of the a-phase lower bridge arm has an upper tube open-circuit fault in 0.3s, the fault flag bit of the submodule is changed from 0 to 1 in 0.32s, and the fault submodule is diagnosed and positioned; b, when the third submodule of the upper bridge arm of the phase b generates an upper tube open-circuit fault in 0.4s, and when the fault flag bit of the submodule is changed from 0 to 1 in 0.42s, the fault submodule is diagnosed and positioned; and c, when the fourth submodule of the upper bridge arm of the phase c generates an upper tube open-circuit fault in 0.6s, and when the fault flag bit of the submodule is changed from 0 to 1 in 0.62s, the fault submodule is diagnosed and positioned.
Description of the symbols: SMrji(r ═ p, N; j ═ a, b, c; i ═ 1,2,3, … N) is the i-th submodule of the j-phase upper and lower bridge arms; u. ofcrji(r ═ p, N; j ═ a, b, c; i ═ 1,2,3, … N) is the capacitance voltage of the ith submodule of the upper and lower bridge arms of j phases; i.e. icrji(r ═ p, N; j ═ a, b, c; i ═ 1,2,3, … N) is the capacitance current of the ith submodule of the upper and lower bridge arms of j phases; r is a DC side resistive load, C1、C2Supporting a capacitor for the DC side; l ispj、Lnj(j ═ a, b, c) are j-phase upper and lower bridge arm inductances, respectively; i.e. ipj、inj(j ═ a, b, c) are j-phase upper and lower arm currents, respectively; u. ofpj、unj(j ═ a, b, c) are j-phase upper and lower bridge arm output voltages respectively; rsj(j ═ a, b, c) is j phaseLine parasitic resistance on the ac side; l issj(j ═ a, b, c) is the filter inductance on the current crossing side of j; u. ofsj(j ═ a, b, c) is an ac input voltage of j phase; i.e. isj(j ═ a, b, c) is an ac input current for the j phase; t issIs a sampling period; num _ negrji(r ═ p, N; j ═ a, b, c; i ═ 1,2,3, … N) is the accumulated quantity of negative current times of the ith sub-module capacitance current of the j-phase upper and lower bridge arms;
flagrji(r ═ p, N; j ═ a, b, c; i ═ 1,2,3, … N) is the fault flag of the ith sub-module of the j-phase upper and lower bridge arms; t isthIs the duration of a diagnostic period.
It is noted that, herein, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation. The use of the phrase "comprising one of the elements does not exclude the presence of other like elements in the process, method, article, or apparatus that comprises the element.
Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims (3)

1. The method for positioning the open-circuit fault of the tube on the half-bridge submodule of the modular multilevel converter is characterized in that the aim of diagnosing and positioning the fault submodule is fulfilled by judging the positive and negative of the capacitance current of the submodule; the submodule capacitor current is used as a characteristic parameter for fault diagnosis, analysis shows that the submodule capacitor current has positive and negative values under the normal operation condition, namely the normal submodule capacitor can be charged and discharged normally, and after an upper tube open circuit fault occurs, the capacitor of the fault submodule cannot be discharged normally, so that the submodule capacitor current does not have the negative value, and the fault submodule can be diagnosed and positioned according to the characteristic; the sub-modules of the modular multilevel converter are of a half-bridge structure, and the upper bridge arm and the lower bridge arm of each phase are respectively formed by cascading N sub-modules; the method comprises the following specific steps:
step 1: sampling capacitor voltages of all half-bridge sub-modules; the capacitor voltage of the ith sub-module of the j-phase upper and lower bridge arms is ucrjiCurrent i flowing through the sub-module capacitorcrjiWith the voltage u across the sub-module capacitorcrjiThe following relationships exist:
Figure FDA0003016708270000011
in the formula, C is the capacitance value of the sub-module parallel capacitor;
sub-module capacitance voltage ucrjiThe differential value of (a) is estimated to obtain:
Figure FDA0003016708270000012
in the formula ucrji(k) Represents the capacitance voltage u of the ith sub-module of the upper and lower bridge arms at the time of kcrji(k-1) represents the capacitance voltage of the ith sub-module of the upper and lower bridge arms at j phase at the time of k-1, TsRepresents a sampling period;
the capacitance current i of the ith sub-module of the j-phase upper and lower bridge arms can be obtained according to the formula (1) and the formula (2)crjiThe specific calculation expression is:
Figure FDA0003016708270000021
step 2: calculating the capacitance current of all half-bridge sub-modules according to the formula (3); in the formula, C is the capacitance value of the sub-module parallel capacitor;
and step 3: all half-bridge sub-modules SM for a three-phase modular multilevel converterpa1～SMpaN、SMna1～SMnaN、SMpb1～SMpbN、SMnb1～SMnbN、SMpc1～SMpcN、SMnc1～SMncNSetting a cumulative amount num of negative current times of a corresponding capacitance currentpa1～numpaN、numna1～numnaN、numpb1～numpbN、numnb1～numnbN、numpc1～numpcN、numnc1～numncN；
And 4, step 4: initializing the accumulated quantity of negative current times of all sub-module capacitor currents to be zero;
and 5: calculating the accumulated quantity of negative current times of all the sub-module capacitor currents; calculating the capacitance current i of each submodule through the formula (3)crjiJudging the capacitance current of each submodule, if icrjiIf the current is less than 0, the sub-module capacitor current is judged to be negative, and then the accumulated number num _ negr of negative current times of the sub-module capacitorji plus 1; on the contrary, the accumulated quantity num _ negr of negative current times of the sub-module capacitorji remains unchanged.
Step 6: judging whether the diagnosis time is over;
and 7: and judging the accumulated quantity of the negative current times of the capacitor currents of all the sub-modules.
2. The method for locating an open-tube fault on a submodule of a modular multilevel converter half-bridge according to claim 1, wherein the step 6 is specifically: when the diagnosis time T is equal to TthWhen the time is 20ms, the accumulated quantity num _ neg of negative current times of all sub-module capacitances is terminatedrjiStep 7 is entered; otherwise, returning to the step 5, and continuing to accumulate the negative current times of all the sub-module capacitors.
3. According to the rightThe method of claim 1, wherein the cumulative amount of negative current times num _ neg of each sub-module capacitor current is determined when the diagnostic time is overrjiIf num _ negrjiIf the sub-module is 0, the sub-module is judged to have the T1 pipe open-circuit fault, and the sub-module is diagnosed and positioned to the fault sub-module SMrji(ii) a If num _ negrjiIf the number of the sub-module is more than 0, the sub-module is in a normal operation state, the step 4 is returned, and the accumulated number num _ neg of the negative current times corresponding to the sub-module capacitor is countedrjiAnd (6) clearing.
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