Method for collaborative optimization of key devices in sub-module of modular multilevel converter

Abstract

The invention discloses a method for collaborative optimization of key devices in sub-modules of a modular multilevel converter, which comprises the following steps of: monitoring the break-over voltage of each sub-module switch tube on a bridge arm and the capacitance value of a capacitor to obtain a standard value; acquiring bridge arm current and capacitor voltage in each sub-module; calculating aging quantization parameters of the switch tube and the capacitor; formulating a sub-module classification label design rule; obtaining an actual capacitance voltage normalization model; calculating an additional component function of the elastic capacitance voltage model of the power device and the capacitor; obtaining an elastic capacitor voltage model of the power device and the capacitor; and carrying out collaborative operation optimization on key devices in the sub-modules to reduce the aging rate of the key devices. According to the method, an MMC sub-module key device aging state evaluation system is established, a collaborative operation optimization method of multiple types of key devices is provided, the overall reliability of the sub-module is improved, MMC hardware cost does not need to be increased, and the influence on output electric energy quality is small.

Description

Technical Field

[0001] The present invention relates to the technical field of multilevel power electronic converters, and specifically to a method for collaborative optimization of key devices in a sub-module of a modular multilevel converter. Background Art

[0002] Each phase of a modular multilevel converter (MMC) is composed of two upper and lower arms, each arm contains multiple sub-modules, and each sub-module usually contains two switching devices, two anti-parallel diodes, and a capacitor. With the development of medium-voltage direct current (MVDC) power transmission technology, the modular multilevel converter (MMC) has gradually received extensive attention and application in power conversion of medium-voltage direct current distribution systems due to its high scalability, high operating efficiency, good harmonic characteristics, etc.

[0003] There are multiple sub-modules in the MMC, and there are multiple types of key devices in each sub-module. Therefore, the aging of MMC sub-modules involves multiple internal key devices, and there are significant differences in the physical parameters and performance indicators of different types of devices. The key devices in the sub-module operation bear different thermal stresses, which leads to intermittent fatigue of the devices, affecting the aging degree of the key devices. The aging and failure of some devices cause the entire sub-module to exit the system operation in advance, which accelerates the overall aging rate of the MMC, thus greatly reducing the economy and service life of the MMC.

[0004] To address the problem of reducing the aging rate of key devices in MMC sub-modules, the conventional method is to optimize the thermal stress of power devices or capacitors. It mainly includes optimizing the capacitor voltage balance strategy of each sub-module in the same arm and optimizing the modulation strategy of the modular multilevel converter to optimize the thermal stress of power devices or capacitors. However, the above methods fail to evaluate the aging states of multiple key devices and only consider the operation optimization of a single key device (switching device or capacitor), resulting in uneven aging degrees of key devices and difficulties in improving the overall reliability of the sub-module. Therefore, there are many defects in practical applications. Summary of the Invention

[0005] Aiming at the above-mentioned technical deficiencies, the purpose of the present invention is to provide a method for collaborative optimization of key devices in a sub-module of a modular multilevel converter. This method proposes a collaborative operation optimization strategy for switching devices and capacitors based on device aging state classification labels, actual capacitor voltages, arm current directions, and elastic capacitor voltages; by optimizing the flow path of the arm current in the key components of the SM, the average value of the current flowing through severely aged devices is reduced to improve the service life of the MMC.

[0006] To solve the above technical problems, the present invention adopts the following technical solutions: Method for collaborative optimization of key devices in modular multilevel converter sub-modules, comprising the following steps: S1. Monitor the conduction voltage V CE(now) [i] (i = 1, 2, …, N) of each sub-module switch tube on the arm and the capacitance value C now [i] (i = 1, 2, …, N) of the capacitor, collect the arm current and the capacitor voltage U C [i] (i = 1, 2, …, N) in each sub-module; S2. Calculate the aging quantization parameter ∆V CE_age [i] of the switch tube; S3. Calculate the aging quantization parameter ∆C age [i] of the capacitor; S4. Formulate a sub-module classification label design rule based on the aging quantization parameters of the switch tube and the capacitor; S5. Calculate the actual capacitor voltage normalization parameter ∆U C [i]; S6. Calculate the additional components T SM_T [i] and T SM_C [i] of the elastic capacitor voltage of the power device switch tube and the capacitor; S7. Calculate the elastic capacitor voltage models L T [i] and L C [i] of the power device switch tube and the capacitor in each sub-module; S8. Regulate the operation mode of the sub-module according to the elastic capacitor voltage model, the arm current, the number N on of sub-modules to be put into operation, and the classification label, so as to realize the collaborative operation optimization of multiple types of key devices in the sub-module.

[0007] Further, the aging quantization parameter ∆V CE_age [i] of the switch tube in each sub-module on the arm in S2 is expressed as: (1) Wherein, V CE(now) [i] is the current conduction voltage of the sub-module switch tube, V CE(std) is the standard value of the switch tube conduction voltage, and the reference value V CE(max) in the case of switch tube aging failure is obtained according to the formula .

[0008] Further, the aging quantization parameter ∆C age [i] of the capacitor in each sub-module on the arm in S3 is expressed as: (2) Wherein, C now [i] and C stdThey are the current capacitance value of the sub-module and the standard value of the capacitance capacity, and the reference value C in the case of capacitor aging failure min According to the formula Obtained

[0009] Further, the sub-module classification label design rule formulated in S4 is: 1>∆V CE_age >∆C age ≥0 or ∆V CE_age ≥1, 1>∆C age ≥0, write the I-class label "put into use" for SM; 0≤∆V CE_age <∆C age <1 or 1>∆V CE_age ≥0, ∆C age ≥1, write the II-class label "bypass" for SM; 1>∆V CE_age =∆C age >0, write the III-label "ready" for SM; Use N c1 , N c2 , and N c3 To represent the number of sub-modules in each classification label I, II, and III

[0010] Further, the calculation formula of ∆U C [i] in S5 is: (3) Among them, the range of ∆U C [i] is 0~1, U C_min =min{u c1 ,u c2 ,u c3 ,u c4 ,…u ci}, U C_max =max{ u c1 ,u c2 ,u c3 ,u c4 ,…u ci}

[0011] Further, T SM_C [i] in S6 is obtained according to the formula: The T SM_T [i] in S6 is obtained according to the formula: Obtained

[0012] Further, the calculation formula of the elastic capacitance voltage model L C [i] of the capacitor in S7 is: (4).

[0013] Further, the elastic capacitance voltage model L of the switching tube in S7 T [i] is calculated by the formula: (5).

[0014] Further, the operation mode of the sub-module is regulated in S8 to optimize the coordinated operation of key devices in multiple types of sub-modules. The specific method is as follows: when the arm current is negative, the sub-module enters the "insertion" operation mode. According to the modulation result, N on sub-modules are inserted; when N on ≤ N c1 , the N T sub-modules with the lowest elastic capacitance voltage L in classification label I are inserted; when N on < N c1 ≤ N on ≤ N c1 + N c3 , N c1 sub-modules in classification label I and the N on - N c1 sub-modules with the highest actual capacitance voltage in classification label III are inserted; when N c1 + N c3 < N on ≤ N c1 + N c2 + N c3 , N c1 + N c3 sub-modules in classification labels I and III and the N C sub-modules with the highest elastic capacitance voltage L in classification label II are inserted. on - N c1 - N c3 sub-modules.

[0015] When the arm current is positive, the sub-module enters the "bypass" operation mode. The modulation result is that N - N on sub-modules in the bypass arm, where N is the total number of sub-modules in the arm; when N - N on ≤ N c2 , the N - N C sub-modules with the lowest elastic capacitance voltage L in classification label II are bypassed; when N on < N - N c2 ≤ N on ≤ N c2 + N c3 , N c2 sub-modules in classification label II and the N - N on - N c2 sub-modules with the highest actual capacitance voltage in classification label III are bypassed; when N c2 + N c3<N-N on ≤N c1 +N c2 +N c3 When, in bypass classification tags II and III, N c2 +N c3 sub-modules, and in bypass classification tag I, N-N on -N c2 -N c3 elastic capacitor voltages L T highest sub-module.

[0016] The beneficial effects of the present invention are as follows: (1) The method of the present invention determines the flow path of the arm current in the key components of the SM through collaborative operation optimization strategy and modulation strategy, reduces the average value of the current flowing through severely aged devices, and improves the service life of the MMC; considering the complex coupling relationship between the operating modes of the key devices in the MMC sub-module, the present invention realizes the collaborative optimal operation of multiple types of key devices in the sub-module, significantly improves the service life of the sub-module, and enhances the overall reliability and economy of the MMC.

[0017] (2) The method of the present invention does not need to change the topological structure of the modular multilevel converter sub-module, does not increase the hardware cost of the modular multilevel converter, has little impact on the output voltage waveform quality of the modular multilevel converter, is easy to implement in the existing modular multilevel converter system, and has strong practicability. Description of the Drawings

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0019] Figure 1 Schematic diagram of the three-phase modular multilevel converter topology provided by the embodiment of the present invention; Figure 2 Schematic diagram of the sub-module topology provided by the embodiment of the present invention; Figure 3 Flow chart of the method for collaborative optimization of key devices in the modular multilevel converter sub-module provided by the embodiment of the present invention. Detailed Embodiments

[0020] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0021] Embodiment In view of the problem of reducing the aging rate of key devices in the MMC sub-module, this embodiment proposes a method for collaborative optimization of key devices in the sub-module of a modular multilevel converter. The topological structures of the three-phase MMC and the sub-module are as Figure 1 、 Figure 2 shown. The three-phase MMC is composed of six arms, and each arm includes i (i = 1, 2,..., N) sub-modules (Submodule, SM) with the same topological structure and an arm inductor L S ; The sub-module is a half-bridge structure, consisting of two diodes D 1 、D 2 , two power switches T 1 、T 2 and a DC capacitor C aui .

[0022] As Figure 3 shown, based on the above topological structures of the three-phase MMC and the sub-module, a method for collaborative optimization of multiple types of key devices in the sub-module of a modular multilevel converter includes: monitoring the conduction voltage of the switching tubes of each sub-module on the arm and the capacitance value of the capacitor, and obtaining the standard values from the data sheet; collecting the arm current and the capacitor voltage in each sub-module; calculating the aging quantization parameters of the switching tubes and the capacitor according to the monitored data and the standard values; formulating the design rule of the sub-module classification label according to the aging quantization parameters; obtaining the actual capacitor voltage normalization model according to the capacitor voltage value, the maximum capacitor voltage and the minimum capacitor voltage of the sub-module; calculating the additional component function of the elastic capacitor voltage model of the power device and the capacitor according to the aging quantization parameters, and determining the weight of the additional component function by combining the failure rate of the device and the proportion of the key influencing factors of failure; obtaining the elastic capacitor voltage models of the power device and the capacitor according to the normalization model of the actual capacitor voltage value and the additional component function of the elastic capacitor voltage model of the key devices (power device, capacitor); determining the number of sub-modules to be put into operation by modulating the reference signal, and then regulating the operation mode of the sub-module according to the elastic capacitor voltage model, the arm current and the classification label, so as to optimize the collaborative operation of the key devices in the sub-module to reduce the aging rate of the key devices.

[0023] The above method specifically includes the following steps: S1. Monitor the conduction voltage V of the switching tubes of each sub-module on the arm CE(now)[i] (i = 1, 2, …, N) and the capacitance value C of the capacitor now [i] (i = 1, 2, …, N), collect the arm current and the capacitor voltage U in each sub-module C [i] (i = 1, 2, …, N); S2. Calculate the aging quantization parameter ∆V of the switch tube CE_age [i]; S3. Calculate the aging quantization parameter ∆C of the capacitor age [i]; S4. Develop a design rule for sub-module classification labels based on the aging quantization parameters of the switch tube and the capacitor; S5. Calculate the actual capacitor voltage normalization parameter ∆U C [i]; S6. Calculate the additional components T of the elastic capacitor voltage of the power device and the capacitor SM_T [i] and T SM_C [i]; S7. Calculate the elastic capacitor voltage models L of the power devices and capacitors in each sub-module T [i] and L C [i]; S8. Regulate the operation mode of the sub-module according to the elastic capacitor voltage model, the arm current, the number N of sub-modules to be put into operation on and the classification label to achieve the coordinated operation optimization of multiple types of key devices in the sub-module.

[0024] The aging quantization parameter ∆V of the switch tube in the arm sub-module in S2 CE_age [i] can be expressed as: (1) where V CE(now) [i] is the current conduction voltage of the sub-module switch tube, V CE(std) is the standard value of the switch tube conduction voltage, and the reference value V CE(max) in the case of switch tube aging failure can be obtained according to the formula . The aging quantization parameter ∆C of the capacitor in the arm sub-module in S3 age [i] can be expressed as: (2) where C now [i] and C std are the current capacitance value of the sub-module and the standard value of the capacitance capacity respectively, and the reference value C min in the case of capacitor aging failure can be obtained according to the formula .

[0025] The design rule for sub-module classification labels developed in S4 is: 1> ∆VCE_age > ∆C age ≥ 0 or ∆V CE_age ≥ 1, 1 > ∆C age Under the condition of ≥ 0, SM writes the class I label "input". 0 ≤ ∆V CE_age < ∆C age < 1 or 1 > ∆V CE_age ≥ 0, ∆C age Under the condition of ≥ 1, SM writes the class II label "bypass". 1 > ∆V CE_age = ∆C age Under the condition of > 0, SM writes the class III label "ready". Use N c1 , N c2 , and N c3 to represent the number of sub - modules in each classification label I, II, and III respectively.

[0026] ∆U C [i] in the above - mentioned S5 has the calculation formula of , where the range of ∆U C [i] is 0 to 1, U C_min = min{u c1 , u c2 , u c3 , u c4 , … u ci}, U C_max = max{u c1 , u c2 , u c3 , u c4 , … u ci}.

[0027] T SM_C [i] in the above - mentioned S6 is obtained according to the formula: , and T SM_T [i] in the above - mentioned S6 is obtained according to the formula: obtained.

[0028] The elastic capacitance voltage L C [i] of the capacitor in the above - mentioned S7 has the calculation formula of: (3) The elastic capacitance voltage L T [i] of the switching tube in the above - mentioned S7 has the calculation formula of: (4) In the above - mentioned S8, the operation mode of the sub - module is regulated to realize the collaborative operation optimization of multiple types of key devices in the sub - module. The specific method is that when the arm current is negative, the sub - module enters the "input" operation mode, and N on sub - modules are input according to the modulation result. When N on ≤ Nc1 When, input the N sub-modules with the lowest elastic capacitance voltage L in classification label I. T The lowest N on sub-modules. When N c1 < N on ≤ N c1 + N c3 When, input the N sub-modules in classification label I, and input the N sub-modules with the highest actual capacitance voltage in classification label III. c1 sub-modules, and input the N sub-modules with the highest actual capacitance voltage in classification label III. on - N c1 sub-modules. When N c1 + N c3 < N on ≤ N c1 + N c2 + N c3 When, input the N sub-modules in classification labels I and III, and input the N sub-modules with the highest elastic capacitance voltage L in classification label II. c1 + N c3 sub-modules, and input the N sub-modules with the highest elastic capacitance voltage L in classification label II. C The highest N on - N c1 - N c3 sub-modules. When the arm current is positive, the sub-module enters the "bypass" operation mode, and the modulation result is N - N sub-modules in the bypass arm, where N is the total number of sub-modules in the arm. on sub-modules, where N is the total number of sub-modules in the arm. When N - N on ≤ N c2 When, bypass the N - N sub-modules with the lowest elastic capacitance voltage L in classification label II. C the lowest N - N on sub-modules. When N c2 < N - N on ≤ N c2 + N c3 When, bypass the N sub-modules in classification label II, and bypass the N - N sub-modules with the highest actual capacitance voltage in classification label III. c2 sub-modules, and bypass the N - N sub-modules with the highest actual capacitance voltage in classification label III. on - N c2 sub-modules. When N c2 + N c3 < N - N on ≤ N c1 + N c2 + N c3 When, bypass the N sub-modules in classification labels II and III, and bypass the N - N sub-modules with the highest elastic capacitance voltage L in classification label I. c2 + N c3 sub-modules, and bypass the N - N sub-modules with the highest elastic capacitance voltage L in classification label I. on - N c2 - N c3 sub-modules with the highest elastic capacitance voltage L T highest sub-modules.

[0029] The present invention optimizes the flow path of the arm current in the key components of the SM, reduces the average value of the current flowing through the severely aged devices, and improves the service life of the MMC. Compared with the conventional method, the present invention takes into account the complex coupling relationship existing between the operating modes of the key devices of the MMC sub-module, realizes the collaborative operation optimization of the key devices of multiple types of sub-modules, improves the overall reliability and economy of the MMC, does not require an increase in the hardware cost of the MMC, and has little impact on the output power quality.

[0030] In the description of the present specification, the description referring to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In the present specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0031] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art of this industry should understand that the present invention is not limited by the above embodiments, and the above embodiments and the descriptions in the specification only illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed.

Claims

1. A method for collaborative optimization of key components in a modular multilevel converter submodule, characterized in that: The steps include: S1, monitor the on-state voltage V of each sub-module switch tube on the bridge arm CE(now) [i] (i = 1, 2, …, N) and the capacitance C now [i] (i = 1, 2, …, N), collect the bridge arm current and the capacitor voltage U in each submodule C [i] (i = 1, 2,…, N); S2. Calculate the aging quantitative parameter ∆V of the switch tube CE_age [i]; S3. Calculate the capacitance aging quantitative parameter ∆C age [i]; S4. Formulate submodule classification label design rules based on the aging quantitative parameters of switches and capacitors; S5. Calculate the actual capacitor voltage normalization parameter ∆U C [i]; S6. Calculate the additional component T of the voltage of the power device switch tube and the capacitor elastic capacitor SM_T [i] and T SM_C [i] ; S7. Calculate the elastic capacitor voltage model L of the power device switch tube and capacitor of each submodule T [i] and L C [i]; S8. According to the elastic capacitor voltage model, bridge arm current, and the number of sub-modules N required on The sub-module operation mode is regulated by classification labels to achieve coordinated operation optimization of multiple types of key components in the sub-module.

2. The method for optimizing the coordinated operation of key components in modular multilevel converter submodules according to claim 1, characterized in that: The aging quantitative parameter ∆V of the upper submodule switch tube in the bridge arm of S2 CE_age [i] is represented by: （1） Among them, V CE(now) [i] is the current on-state voltage of the submodule switch, V CE(std) is the standard value of the switch tube conduction voltage, and the reference value V CE(max) According to the formula To obtain.

3. The method for optimizing the coordinated operation of key components in modular multilevel converter submodules according to claim 1 or 2, characterized in that: The aging quantitative parameter ∆C of each submodule capacitor on the bridge arm of S3 is age [i] is represented by: （2） Among them, C now [i] and C std They are respectively the current capacitance value of the submodule, the standard value of the capacitance, and the reference value C under the condition of capacitor aging failure. min According to the formula To obtain.

4. The method for optimizing the coordinated operation of key components in modular multilevel converter submodules according to claim 3, characterized in that: The submodule classification label design rule formulated in S4 is: 1>∆V CE_age >∆C age ≥0 or ∆V CE_age ≥1, 1>∆C age ≥0, SM writes "input" to Class I tag; 0≤∆V CE_age <∆C age <1 or 1>∆V CE_age ≥0, ∆C age ≥1 condition SM writes to Class II tag "bypass"; 1>∆V CE_age =∆C age >0 condition, SM writes III tag "ready"; respectively, with N c1 , N c2 , and N c3 Indicates the number of submodules in each classification label I, II, and III.

5. The method for optimizing the coordinated operation of multiple types of key components in modular multi-level converter submodules according to claim 4, characterized in that: The S5 ∆U C The calculation formula for [i] is: （3） Among them, ∆U C [i]Special value 0~1, U C_min =min{u c1 ,u c2 ,u c3 ,u c4 ,…u ci }, U C_max =max{ u c1 ,u c2 ,u c3 ,u c4 ,…u ci }.

6. The method for optimizing the coordinated operation of multiple types of key components in modular multi-level converter submodules according to claim 5, characterized in that: The S6 in T SM_C [i] According to the formula: It is found that T in S6 SM_T [i] According to the formula: To obtain.

7. The method for optimizing the coordinated operation of key components in modular multilevel converter submodules according to claim 6, characterized in that: The elastic capacitance voltage model L of the capacitor in S7 C The calculation formula for [i] is: （4）。 8. The method for optimizing the coordinated operation of key components in modular multilevel converter submodules according to claim 7, characterized in that: The elastic capacitance voltage model L of the switch tube in S7 T The calculation formula for [i] is: （5）。 9. The method for optimizing the coordinated operation of key components in modular multilevel converter submodules according to claim 8, characterized in that: In the S8, the submodule operation mode is regulated to achieve the coordinated operation optimization of key components in multiple types of submodules. The specific method is: when the bridge arm current is negative, the submodule enters the "on" operation mode, and according to the modulation result, the N on submodules; when N on ≤N c1 When the elastic capacitor voltage L in the classification label I is input T The lowest N on Submodule; when N c1 <N on ≤N c1 +N c3 When N is input into the classification label I c1 submodules, and input the N with the highest actual capacitor voltage in classification label III on -N c1 submodules; when N c1 +N c3 <N on ≤N c1 +N c2 +N c3 When N is input into the classification labels I and III c1 +N c3 submodules, input elastic capacitor voltage L in classification label II C Highest N on -N c1 -N c3 Submodules; The bridge arm current is positive, the submodule enters the "bypass" operation mode, and the modulation result is NN in the bypass bridge arm. on sub-modules, where N is the total number of sub-modules in the bridge arm; when NN on ≤N c2 When the bypass classification label II elastic capacitor voltage L C The lowest NN on Submodule; when N c2 <N-N on ≤N c2 +N c3 When N in the bypass classification label II c2 submodules, bypass the NN with the highest actual capacitor voltage in classification label III on -N c2 submodules; when N c2 +N c3 <N-N on ≤N c1 +N c2 +N c3 When N in bypass classification labels II and III c2 +N c3 submodules, bypassing the classification label I in NN on -N c2 -N c3 The elastic capacitance voltage L T The highest submodule.

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