CN116165573A - Method and system for diagnosing open-circuit faults of submodule of MMC under NLM strategy - Google Patents

Abstract

The invention provides a sub-module open-circuit fault diagnosis method and system for an MMC under an NLM strategy, wherein the method can accurately and rapidly diagnose two kinds of MMC sub-module open-circuit faults at different power points by selecting absolute errors of an actual value and a predicted value of SM capacitor voltage as diagnosis marks, can cope with the condition that a plurality of SMs in a bridge arm have faults at the same time, greatly reduces the calculated amount by utilizing the sequencing result of voltage equalizing control, can complete the diagnosis process in one power frequency period, improves the reliability of a modularized multi-level converter and ensures the stable operation of the modularized multi-level converter.

Description

Method and system for diagnosing open-circuit faults of submodule of MMC under NLM strategy

Technical Field

The invention relates to the field of power electronics, in particular to a submodule open circuit fault diagnosis method and system of an MMC under an NLM strategy.

Background

The modularized multi-level converter (modular multilevel converter, MMC) has the characteristics of modularized structure, redundant configuration, expandable power and the like. These advantages have led to widespread use of MMCs in high voltage high power scenarios, such as high voltage direct current transmission systems (high voltage direct current, HVDC), static synchronous compensators and power electronic transformers.

The MMC topology is formed by serially connecting a plurality of sub-modules (SMs), reliability is one of the important problems, the normal operation of the MMC can be influenced by the failure of the IGBT in any one SM, even other elements can be further damaged, and finally the whole system is crashed. IGBTs are classified into short circuit faults and open circuit faults. Short-circuit faults usually lead to overcurrent phenomena and are serious in damage, but a mature solution exists in the current engineering, namely short-circuit protection is integrated in a gate driver, and a switch is rapidly closed when the short-circuit faults occur. In contrast to a short circuit fault, an open circuit fault does not immediately damage the SM and may not be detected for a long period of time. Recently, the level approximation modulation is widely applied to MMCs with high level numbers due to the advantages of good dynamic performance, simple implementation and the like.

The traditional fault diagnosis method generally needs to additionally add a sensor, and meanwhile has the problems of high calculation complexity, long diagnosis time and the like, and more importantly, the diagnosis process depends on accurate setting of an experience threshold, and when the MMC operation power is changed, the setting and popularization of the experience threshold are difficult. Therefore, it is necessary to provide a new sub-module open fault diagnosis method for NLM modulation MMC at different operating powers.

Disclosure of Invention

The invention aims at adopting NLM modulation MMC under different operating powers, and provides a submodule open-circuit fault diagnosis method and system of the MMC under an NLM strategy.

The invention is realized by the following technical scheme:

a sub-module open circuit fault diagnosis method of MMC under NLM strategy includes the following steps:

step 1, acquiring an actual value of capacitance voltage of a set submodule in an MMC in a current control period, and estimating an estimated value of capacitance voltage of the set submodule in the current control period according to bridge arm current of a previous control period of the MMC;

step 2, determining an absolute error between the capacitance voltage estimated value of the setting submodule and the capacitance voltage actual value, comparing the absolute error with a setting threshold value, and determining whether the setting submodule fails or not;

step 3, when the set submodule fails, the actual value of the capacitor voltage of the set submodule is used as the maximum voltage value, and all the submodules are ordered in descending order according to the capacitor voltage value, so that a submodule ordering sequence is obtained;

step 4, acquiring the actual value of the capacitance voltage of the next sub-module adjacent to the setting sub-module in the sub-module sequencing sequence in the current control period; according to bridge arm current of the MMC in the previous period and combining the input quantity of the submodules in the previous period, estimating a capacitance voltage estimated value of the lower submodule in the current control period;

step 5, determining an absolute error between the capacitance voltage estimated value and the capacitance voltage actual value of the lower-level sub-module, comparing the absolute error with a set threshold value, and determining whether the lower-level sub-module has a fault or not;

and step 6, if the subordinate sub-module fails, repeating the step 4, and performing fault diagnosis on the subordinate sub-module of the subordinate sub-module until the diagnosed sub-module does not fail, so as to obtain all the failure sub-modules.

Preferably, in step 1, the capacitance voltage estimated value of the current control of the stator module is estimated and set according to the positive and negative of the bridge arm current of the previous control period of the MMC.

Preferably, in step 1, if the bridge arm current is positive, the estimated value of the capacitor voltage of the current control sub-module is set to be equal to the actual value of the capacitor voltage of the current control period of the setting sub-module.

Preferably, in step 1, if the bridge arm current i _ua (k-1) is non-positive, then the capacitance voltage estimation value of the submodule at the current control period is set as follows:

wherein C is the capacitance value of the submodule capacitor, T _c To control the duration of the period U _{c_number_rea} (k-1) is the actual value of the capacitor voltage of the submodule in the last control period.

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

Preferably, the expression of the threshold setting in step 2 is as follows:

wherein C is the capacitance value of the submodule capacitor, T _c To control the duration of the period, i _ua (k) Bridge arm current i for the current control period _ua (k-1) bridge arm current, err, of the previous control period _normal Is the absolute error err _{_th} To set a threshold.

Preferably, in step 4, the method for determining the estimated value of the capacitance voltage of the lower sub-module in the current control period is as follows:

if the bridge arm current is positive and the last control period inputs the number N of submodules _on (k-1)<N-m+1，N _on The number of submodules is input in the last control period, N is the total number of submodules on the bridge arm, and the lower submodule is used for estimating the value U of the capacitor voltage under current control _{c_pos_m} Is equal to the actual value of the capacitor voltage of the submodule in the last control period.

Preferably, in step 4, the method for determining the estimated value of the capacitance voltage of the lower sub-module in the current control period is as follows:

if the bridge arm current is non-positive and the last control period inputs the number N of submodules _on (k-1)>m-1, capacitance-voltage estimation value U of lower-level sub-module _{c_neg_m} 。

Wherein C is the capacitance value of the submodule capacitor, T _c To control the duration of the period, i _ua (k) Bridge arm current i for the current control period _ua (k-1) bridge arm current of last control period, U _{c_number_m} (k-1) is a childThe module has the actual value of the capacitor voltage in the last period.

The invention also provides a system of the sub-module open circuit fault diagnosis method of the MMC under the NLM strategy, which comprises,

the first capacitor voltage acquisition module is used for acquiring the actual value of the capacitor voltage of the set submodule in the MMC in the current control period, and estimating the estimated value of the capacitor voltage of the set submodule in the current control period according to the bridge arm current of the last control period of the MMC;

the MMC diagnosis module is used for determining the absolute error between the capacitance voltage estimated value and the capacitance voltage actual value of the setting submodule, comparing the absolute error with a setting threshold value and determining whether the setting submodule fails or not;

the sequencing module is used for sequencing all the submodules in descending order according to the capacitance voltage value to obtain a submodule sequencing sequence when the set submodule fails and the actual capacitance voltage value of the set submodule is used as the maximum voltage value;

the second capacitor voltage acquisition module is used for acquiring the actual value of the capacitor voltage of the next-level submodule adjacent to the setting submodule in the submodule sequencing sequence in the current control period; according to bridge arm current of the MMC in the previous period and combining the input quantity of the submodules in the previous period, estimating a capacitance voltage estimated value of the lower submodule in the current control period;

the sub-module diagnosis module is used for determining the absolute error between the capacitance voltage estimated value and the capacitance voltage actual value of the lower sub-module, comparing the absolute error with a set threshold value and determining whether the set sub-module fails or not;

and the output module is used for carrying out fault diagnosis on the next-stage submodule of the next-stage submodule until the diagnosed submodule does not have faults to obtain all fault submodules.

Preferably, the method for determining the capacitance-voltage estimated value of the lower-level sub-module in the current control period is as follows:

if the bridge arm current is non-positive and the last control period inputs the number N of submodules _on (k-1)>m-1, capacitance-voltage estimation of a subordinate sub-moduleValue U _{c_neg_m} ；

Wherein C is the capacitance value of the submodule capacitor, T _c To control the duration of the period, i _ua (k) Bridge arm current i for the current control period _ua (k-1) bridge arm current of last control period, U _{c_number_m} (k-1) is the actual value of the capacitor voltage of the submodule in the previous period.

Compared with the prior art, the invention has the following beneficial technical effects:

according to the method for diagnosing the open-circuit faults of the submodules of the MMC under the NLM strategy, the absolute errors of the actual value and the predicted value of the SM capacitor voltage are selected as diagnosis marks, so that the open-circuit faults of the submodules of the two MMC can be accurately and rapidly diagnosed at different power points, meanwhile, the fault conditions of a plurality of SMs in a bridge arm can be dealt with, the calculation amount is greatly reduced by utilizing the sequencing result of voltage equalizing control, the diagnosis process can be completed within one power frequency period, the reliability of the modularized multi-level converter is improved, and the stable operation of the modularized multi-level converter is ensured.

Drawings

FIG. 1 is a schematic diagram of an MMC main circuit and half-bridge submodule topology according to the present invention;

FIG. 2 is a schematic diagram of two types of sub-module open faults according to the present invention;

FIG. 3 is a flow chart of a sub-module open circuit fault diagnostic method of the present invention;

FIG. 4 is a waveform of the simulation of the type I fault of the inversion mode MMC rated load of the present invention;

FIG. 5 is a waveform of the light load of the inversion mode MMC of the present invention with type I fault simulation;

FIG. 6 is a simulated waveform of the type II fault occurring at the rated load of the MMC in the inversion mode according to the invention;

FIG. 7 is a simulation waveform of type II light load faults of the inversion mode MMC of the present invention;

FIG. 8 is a simulated waveform of a type II fault occurring at the rated load of the MMC in the rectification mode according to the present invention;

fig. 9 is a waveform of the light load II fault simulation of the rectification mode MMC according to the present invention.

L in FIG. 1 is a bridge arm reactor, U _dc Is the voltage of a direct current bus, I _dc I is the total direct current _x Is an ac output phase current.

Detailed Description

The invention will now be described in further detail with reference to the accompanying drawings, which illustrate but do not limit the invention.

Referring to fig. 1-3, a method for diagnosing an open circuit fault of a submodule of an MMC under an NLM strategy includes the following steps:

step 1, obtaining bridge arm current i of the current control period of the MMC _ua (k) And the actual value U of the capacitance voltage of the submodule SM with the sequence number _{c_number_rea} (k)；

Fig. 1 shows the topology of a typical MMC (modular multilevel converter); each phase comprises an upper bridge arm and a lower bridge arm, and each bridge arm consists of N sub-modules (SM) and a bridge arm reactor L. The sub-module adopts a half-bridge structure and comprises 2 IGBTs, 2 anti-parallel diodes and 1 direct current capacitor.

Step 2, according to bridge arm current i of previous control period of MMC _ua (k-1) positive and negative, estimating a capacitance-voltage estimated value U of a sub-module SM with a sequence number currently controlled _{c_number_est} (k)。

If the bridge arm current i _ua (k-1) being positive, whereby the capacitance-voltage estimation value U of the number-number submodule SM can be estimated _{c_pos} The number sub-module SM is used for estimating the current control capacitance voltage estimation value U _{c_pos} The capacitance voltage actual value equal to the last control period of the submodule is expressed as follows:

U _{c_pos} ＝U _{c_number_rea} (k-1)

where k is the current control period.

If the bridge arm current i _ua (k-1) being non-positive, whereby the power of the number sub-module SM can be estimatedCapacitance-voltage estimation value U _{c_neg} The method comprises the steps of carrying out a first treatment on the surface of the The estimated value of the capacitance voltage of the current control period of the submodule SM with the sequence number is as follows:

wherein C is the capacitance value of the submodule capacitor, T _c To control the duration of the period U _{c_number_rea} (k-1) is the actual value of the capacitor voltage of the submodule in the last control period.

Step 3, calculating the absolute error err between the estimated value of the SM capacitor voltage of the number sub-module and the actual value of the capacitor voltage in the current control period, and comparing the absolute error err with the set threshold err _{_th} And comparing, and determining whether the MMC has an open circuit fault according to a comparison result.

Specifically, if the absolute error err is smaller than the set threshold err _{_th} If the MMC has no fault, updating the submodule SM with the sequence number of number to the submodule with the maximum capacitance voltage at the current moment, and recording the capacitance voltage as U _{c_number} (k) And (3) finishing the detection of the current control period, returning to the step (1), and waiting for the detection process of the next control period.

If the absolute error err is greater than the set threshold err _{_th} Indicating that the submodule fails, namely that the MMC fails, updating the submodule SM with the sequence number of number into the submodule with the maximum capacitance voltage at the current moment, and arranging all the submodules in descending order according to the actual capacitance voltage value of the submodule to obtain a submodule ordering sequence;

setting a threshold err _{_th} The specific method for setting is as follows:

wherein err is _normal Is the absolute error between the actual value of the capacitance voltage and the estimated value of the capacitance voltage of the non-faulty submodule SM, is generally approximate to zero, and takes into account the possible sensor errors, submodule capacitance tolerance and the like in practiceFactors, at the setting of the set threshold err _th It may be suitably brought close to the right side of the inequality.

After this step, it is to be noted that, in the process of diagnosing the MMC fault, all fault sub-modules cannot be detected, so that in the subsequent step, all fault sub-modules in the MMC need to be diagnosed and located.

Step 4, after detecting that the MMC has an open circuit fault, acquiring bridge arm current i of the current control period of the MMC _ua (k) And the actual value U of the capacitance voltage of the submodule with the sequence number m _{c_number_m} (k)；

The submodule with the sequence number of number_m is a submodule adjacent to the submodule SM with the largest capacitor voltage in the submodule sequencing sequence, and m is the sequencing sequence number of the submodule in the submodule sequencing sequence.

Fig. 2 shows a schematic diagram of open circuit faults of two sub-modules investigated by the present invention: type I fault, type II fault.

Step 5, obtaining the bridge arm current i of the MMC a phase in the previous period of the current control period in step 4 _ua And (k-1) and estimating the capacitance voltage estimated value of the sub-module with the sequence number of number_m by combining the input quantity of the sub-module in the last period.

Specifically, if the bridge arm current is positive and the last control period inputs the number N of submodules _on (k-1)<N-m+1，N _on The number of submodules is input in the last control period, N is the total number of submodules on the bridge arm, and accordingly the capacitance voltage estimated value U of the submodule SM with the sequence number of number_m can be estimated _{c_pos_m} The submodule SM with the sequence number of number_m is used for estimating the value U of the currently controlled capacitance voltage _{c_pos_m} The actual value of the capacitance voltage of the sub-module in the last control period is equal to the estimated value of the capacitance voltage expressed as follows:

U _{c_pos_m} ＝U _{c_number_m} (k-1)

if the bridge arm current is positive but N _on (k-1) is not less than N-m+1, returning to the step 4, and waiting for the positioning process of the next control period;

if the bridge arm current is not positiveAnd the number of the submodules is N when the last control period is input _on (k-1)>m-1, from which the capacitance voltage estimation value U of the submodule SM with the sequence number number_m can be estimated _{c_neg} _ _m The method comprises the following steps:

if it is not positive but N _on (k-1) is less than or equal to m-1, returning to the step 4, and waiting for the positioning process of the next control period.

Step 6, calculating an absolute error err between the estimated value and the actual value of the SM capacitance voltage of the submodule with the sequence number of number_m _{_m} And compares it with a set threshold err _{_th} And comparing, and if the sub-module is larger than the set threshold value, indicating that the sub-module is a fault sub-module.

Step 7, adding 1 to m, and repeatedly executing the steps 4-6 to diagnose the sub-module with the sequence number of number_m+1 in the sub-module sequencing sequence until the absolute error err corresponding to the sub-module with the sequence number of number_m+i _{_m} And when the number of the fault sub-modules is smaller than the set threshold value, indicating that the sub-module is a normal sub-module, so as to obtain all fault sub-modules in the MMC open circuit fault, ending the diagnosis process, and outputting the number of the fault sub-modules and the corresponding serial numbers.

The invention also provides a system of the submodule open-circuit fault diagnosis method of the MMC under the NLM strategy, which comprises a first capacitor voltage acquisition module, an MMC diagnosis module, a sequencing module, a second capacitor voltage acquisition module, a submodule diagnosis module and an output module.

The first capacitor voltage acquisition module is used for acquiring the actual value of the capacitor voltage of the set submodule in the MMC in the current control period, and estimating the estimated value of the capacitor voltage of the set submodule in the current control period according to the bridge arm current of the last control period of the MMC;

the MMC diagnosis module is used for determining the absolute error between the capacitance voltage estimated value and the capacitance voltage actual value of the setting submodule, comparing the absolute error with a setting threshold value and determining whether the setting submodule fails or not;

the sequencing module is used for sequencing all the submodules in descending order according to the capacitance voltage value to obtain a submodule sequencing sequence when the set submodule fails and the actual capacitance voltage value of the set submodule is used as the maximum voltage value;

the second capacitor voltage acquisition module is used for acquiring the actual value of the capacitor voltage of the next-level submodule adjacent to the setting submodule in the submodule sequencing sequence in the current control period; according to bridge arm current of the MMC in the previous period and combining the input quantity of the submodules in the previous period, estimating a capacitance voltage estimated value of the lower submodule in the current control period;

if the bridge arm current is positive and the last control period inputs the number N of submodules _on (k-1)<N-m+1，N _on The number of submodules is input in the last control period, N is the total number of submodules on the bridge arm, and the lower submodule is used for estimating the value U of the capacitor voltage under current control _{c_pos_m} Is equal to the actual value of the capacitor voltage of the submodule in the last control period.

If the bridge arm current is non-positive and the last control period inputs the number N of submodules _on (k-1)>m-1, capacitance-voltage estimation value U of lower-level sub-module _{c_neg_m} ；

Wherein C is the capacitance value of the submodule capacitor, T _c To control the duration of the period, i _ua (k) Bridge arm current i for the current control period _ua (k-1) bridge arm current of last control period, U _{c_number_m} (k-1) is the actual value of the capacitor voltage of the submodule in the previous period.

The sub-module diagnosis module is used for determining the absolute error between the capacitance voltage estimated value and the capacitance voltage actual value of the lower sub-module, comparing the absolute error with a set threshold value and determining whether the set sub-module fails or not;

and the output module is used for repeatedly executing the step 4 when the adjacent sub-module fails, and performing fault diagnosis on the lower sub-module of the lower sub-module until the diagnosed sub-module does not fail, so as to obtain all the failure sub-modules.

FIG. 3 shows a flow chart of a diagnostic method of the present invention: the diagnostic process includes a detection process and a localization process.

Example 1

Taking a single-ended MMC system as an example, verifying the effectiveness of the NLM modulation strategy MMC submodule open-circuit fault diagnosis method based on the capacitor voltage unbalance degree, wherein the modulation mode adopts NLM modulation, and the voltage-sharing strategy adopts a traditional voltage-sharing strategy, and the embodiment totally analyzes the following six conditions:

(1) The inversion mode rated load MMC has I-type faults;

in the case of (1), as shown in fig. 4, when the type I faults occur simultaneously in the upper bridge arm 1 and 3 of the phase of the rated load MMC a of the inversion mode at 1.0s, the absolute error of the fault SM exceeds a set threshold after the fault occurs, the system can rapidly detect the fault, then the number of faults SM in the current bridge arm is determined to be 2 through a positioning process, the serial numbers are respectively 1 and 3, the diagnostic time is 12.14ms, and the applicability of the proposed strategy under the condition is verified.

(2) The inversion mode light-load MMC has I-type faults;

in the case of (2), as shown in fig. 5, when the type I faults occur simultaneously in the upper bridge arm 1 and 3 of the phase of the MMC a with the rated load of the inversion mode at 1.0s, the absolute error of the fault SM exceeds a set threshold after the fault occurs, the system can rapidly detect the fault, then the number of faults SM in the current bridge arm is determined to be 2 through the positioning process, the serial numbers are respectively 1 and 3, the diagnostic time is 12.20ms, and the applicability of the proposed strategy under the condition is verified.

(3) The inversion mode rated load MMC has I-type faults;

in the case of (3), as shown in fig. 6, when the type II faults occur simultaneously in the upper bridge arm 1 and 3 of the phase of the MMC a with the rated load of the inversion mode at 1.0s, the absolute error of the fault SM exceeds a set threshold after the fault occurs, the system can rapidly detect the fault, then the number of faults SM in the current bridge arm is determined to be 2, the serial numbers are respectively 1 and 3, the diagnostic time is 0.46ms, and the applicability of the proposed strategy under the condition is verified.

(4) The inversion mode light-load MMC has II type faults;

in the case of (4), as shown in fig. 7, when the inversion mode light load MMC a phase upper bridge arm 1 and 3 simultaneously generate II-type faults, the absolute error of the fault SM exceeds a set threshold after the fault occurs, the system can rapidly detect the fault, then the number of faults SM in the current bridge arm is determined to be 2, the serial numbers are respectively 1 and 3, the diagnostic time is 0.47ms, and the applicability of the proposed strategy under the condition is verified.

(5) The rectifying mode rated load MMC has I-type faults;

in the case of (5), as shown in fig. 8, when the II type faults occur simultaneously in the upper bridge arm 1 and 3 of the phase of the rated load MMC a of the rectifying mode at 1.0s, the absolute error of the fault SM exceeds a set threshold after the fault occurs, the system can rapidly detect the fault, then the number of faults SM in the current bridge arm is determined to be 2, the serial numbers are respectively 1 and 3, the diagnostic time is 11.93ms, and the applicability of the proposed strategy under the condition is verified.

(6) And the rectifying mode light-load MMC has a type II fault.

In the case of (6), as shown in fig. 9, when the type II faults occur simultaneously in the upper bridge arm 1 and 3 of the light-load MMC a phase in the rectification mode during 1.0s, the absolute error of the fault SM exceeds a set threshold after the fault occurs, the system can rapidly detect the fault, then the number of faults SM in the current bridge arm is determined to be 2, the serial numbers are respectively 1 and 3, the diagnostic time is 11.95ms, and the applicability of the proposed strategy under the condition is verified.

Table 2 embodiment main circuit parameters

Parameters (parameters)	Parameter value
		Ac side rated voltage	290kV
Ac side voltage frequency	50Hz
		DC side rated voltage	500kV
Rated active power	750MW
		Shan Qiaobei number of submodules	244 pieces
Bridge arm reactor	0.1H
		Capacitance value of sub-module	8mF

The invention provides a modular multilevel converter submodule open-circuit fault diagnosis method under an NLM strategy, which can accurately and rapidly diagnose open-circuit faults of two MMC submodules at different power points by selecting absolute errors of actual values and predicted values of SM capacitor voltages as diagnosis marks, can cope with the condition that a plurality of SMs in bridge arms are in fault, greatly reduces calculated amount by utilizing sequencing results of voltage equalizing control, can complete diagnosis process in one power frequency period, improves reliability of the modular multilevel converter, and ensures stable operation of the modular multilevel converter.

It will be appreciated by those skilled in the art that embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein. The solutions in the embodiments of the present application may be implemented in various computer languages, for example, object-oriented programming language Java, and an transliterated scripting language JavaScript, etc.

The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.

These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.

While preferred embodiments of the present application have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. It is therefore intended that the following claims be interpreted as including the preferred embodiments and all such alterations and modifications as fall within the scope of the application.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present application without departing from the spirit or scope of the application. Thus, if such modifications and variations of the present application fall within the scope of the claims and the equivalents thereof, the present application is intended to cover such modifications and variations.

Claims (10)

1. The sub-module open circuit fault diagnosis method of the MMC under the NLM strategy is characterized by comprising the following steps of:

step 1, acquiring an actual value of capacitance voltage of a set submodule in an MMC in a current control period, and estimating an estimated value of capacitance voltage of the set submodule in the current control period according to bridge arm current of a previous control period of the MMC;

step 2, determining an absolute error between the capacitance voltage estimated value of the setting submodule and the capacitance voltage actual value, comparing the absolute error with a setting threshold value, and determining whether the setting submodule fails or not;

step 3, when the set submodule fails, the actual value of the capacitor voltage of the set submodule is used as the maximum voltage value, and all the submodules are ordered in descending order according to the capacitor voltage value, so that a submodule ordering sequence is obtained;

step 4, acquiring the actual value of the capacitance voltage of the next sub-module adjacent to the setting sub-module in the sub-module sequencing sequence in the current control period; according to bridge arm current of the MMC in the previous period and combining the input quantity of the submodules in the previous period, estimating a capacitance voltage estimated value of the lower submodule in the current control period;

step 5, determining an absolute error between the capacitance voltage estimated value and the capacitance voltage actual value of the lower-level sub-module, comparing the absolute error with a set threshold value, and determining whether the lower-level sub-module has a fault or not;

and step 6, if the subordinate sub-module fails, repeating the step 4, and performing fault diagnosis on the subordinate sub-module of the subordinate sub-module until the diagnosed sub-module does not fail, so as to obtain all the failure sub-modules.

2. The method for diagnosing an open circuit fault of a submodule of an MMC according to claim 1, wherein in step 1, the estimated value of the capacitor voltage of the submodule currently controlled is estimated and set according to the positive and negative of the bridge arm current of the previous control period of the MMC.

3. The method for diagnosing an open circuit fault of a submodule of an MMC according to claim 2, wherein in step 1, if the bridge arm current is positive, the estimated value of the capacitor voltage of the submodule currently controlled is set to be equal to the actual value of the capacitor voltage of the submodule set in the previous control period.

4. The method for diagnosing an open-circuit fault of a submodule of an MMC under an NLM strategy as claimed in claim 2, wherein in step 1, if the arm current i is _ua (k-1) is non-positive, then the capacitance voltage estimation value of the submodule at the current control period is set as follows:

wherein C is the capacitance value of the submodule capacitor, T _c To control the duration of the period U _{c_number_rea} (k-1) is the actual value of the capacitor voltage of the submodule in the last control period.

5. The method for diagnosing an open circuit fault of a submodule of an MMC according to claim 1, wherein in step 2, when the absolute error is greater than a set threshold, the set submodule fails, and otherwise the set submodule fails.

6. The method for diagnosing an open circuit fault of a submodule of an MMC under an NLM strategy according to claim 1, wherein the expression of the set threshold in step 2 is as follows:

wherein C is the capacitance value of the submodule capacitor, T _c To control the duration of the period, i _ua (k) Bridge arm current i for the current control period _ua (k-1) bridge arm current, err, of the previous control period _normal Is the absolute error err _{_th} To set a threshold.

7. The method for diagnosing an open circuit fault of a submodule of an MMC under an NLM strategy according to claim 1, wherein the method for determining the estimated value of the capacitor voltage of the lower submodule in the current control period in step 4 is as follows:

if the bridge arm current is positive and the last control period inputs the number N of submodules _on (k-1)<N-m+1，N _on The number of submodules is input in the last control period, N is the total number of submodules on the bridge arm, and the lower submodule is used for estimating the value U of the capacitor voltage under current control _{c_pos_m} Is equal to the actual value of the capacitor voltage of the submodule in the last control period.

8. The method for diagnosing an open circuit fault of a submodule of an MMC under an NLM strategy of claim 7, wherein the method for determining the estimated value of the capacitor voltage of the lower submodule in the current control period in step 4 is as follows:

if the bridge arm current is non-positive and the last control period inputs the number N of submodules _on (k-1)>m-1, capacitance-voltage estimation value U of lower-level sub-module _{c_neg_m} ；

Wherein C is the capacitance value of the submodule capacitor, T _c To control the duration of the period, i _ua (k) Bridge arm current i for the current control period _ua (k-1) bridge arm current of last control period, U _{c_number_m} (k-1) is the actual value of the capacitor voltage of the submodule in the previous period.

9. A system for diagnosing open-circuit faults of a submodule of an MMC under an NLM strategy is characterized by comprising,

the first capacitor voltage acquisition module is used for acquiring the actual value of the capacitor voltage of the set submodule in the MMC in the current control period, and estimating the estimated value of the capacitor voltage of the set submodule in the current control period according to the bridge arm current of the last control period of the MMC;

the MMC diagnosis module is used for determining the absolute error between the capacitance voltage estimated value and the capacitance voltage actual value of the setting submodule, comparing the absolute error with a setting threshold value and determining whether the setting submodule fails or not;

the sequencing module is used for sequencing all the submodules in descending order according to the capacitance voltage value to obtain a submodule sequencing sequence when the set submodule fails and the actual capacitance voltage value of the set submodule is used as the maximum voltage value;

the second capacitor voltage acquisition module is used for acquiring the actual value of the capacitor voltage of the next-level submodule adjacent to the setting submodule in the submodule sequencing sequence in the current control period; according to bridge arm current of the MMC in the previous period and combining the input quantity of the submodules in the previous period, estimating a capacitance voltage estimated value of the lower submodule in the current control period;

the sub-module diagnosis module is used for determining the absolute error between the capacitance voltage estimated value and the capacitance voltage actual value of the lower sub-module, comparing the absolute error with a set threshold value and determining whether the set sub-module fails or not;

and the output module is used for carrying out fault diagnosis on the next-stage submodule of the next-stage submodule until the diagnosed submodule does not have faults to obtain all fault submodules.

10. The system of a submodule open fault diagnosis method of an MMC under an NLM strategy according to claim 9, wherein the method for determining the capacitance-voltage estimation value of the lower submodule in the current control period is as follows:

if the bridge arm current is non-positive and the last control period inputs the number N of submodules _on (k-1)>m-1, capacitance-voltage estimation value U of lower-level sub-module _{c_neg_m} ；

Wherein C is the capacitance value of the submodule capacitor, T _c To control the duration of the period, i _ua (k) Bridge arm current i for the current control period _ua (k-1) bridge arm current of last control period, U _{c_number_m} (k-1) is the actual value of the capacitor voltage of the submodule in the previous period.

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