CN112039322B - MMC common-mode voltage suppression modulation method and system suitable for even number of sub-modules - Google Patents

MMC common-mode voltage suppression modulation method and system suitable for even number of sub-modules Download PDF

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CN112039322B
CN112039322B CN202010991500.1A CN202010991500A CN112039322B CN 112039322 B CN112039322 B CN 112039322B CN 202010991500 A CN202010991500 A CN 202010991500A CN 112039322 B CN112039322 B CN 112039322B
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CN112039322A (en
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蒋栋
陈嘉楠
赵炫
李桥
孙伟
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Huazhong University of Science and Technology
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/12—Arrangements for reducing harmonics from ac input or output
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/08—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters
    • H02M1/088—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters for the simultaneous control of series or parallel connected semiconductor devices
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/44—Circuits or arrangements for compensating for electromagnetic interference in converters or inverters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
    • H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
    • H02M7/53871—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
    • H02M7/53873—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current with digital control
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02S—GENERATION OF ELECTRIC POWER BY CONVERSION OF INFRARED RADIATION, VISIBLE LIGHT OR ULTRAVIOLET LIGHT, e.g. USING PHOTOVOLTAIC [PV] MODULES
    • H02S40/00—Components or accessories in combination with PV modules, not provided for in groups H02S10/00 - H02S30/00
    • H02S40/30—Electrical components
    • H02S40/32—Electrical components comprising DC/AC inverter means associated with the PV module itself, e.g. AC modules
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0003—Details of control, feedback or regulation circuits
    • H02M1/0025—Arrangements for modifying reference values, feedback values or error values in the control loop of a converter
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
    • H02M1/0074—Plural converter units whose inputs are connected in series
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/12—Arrangements for reducing harmonics from ac input or output
    • H02M1/123—Suppression of common mode voltage or current
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy

Abstract

The invention discloses a modulation method and a modulation system for suppressing common-mode voltage by an MMC (modular multilevel converter) suitable for an even number of sub-modules, and belongs to the field of electric power. The method comprises the following steps: converting the three-phase reference voltage analog quantity linear operation into three reference line voltage analog quantities injected with zero-sequence components; for an MMC with 2N sub-modules, respectively carrying out carrier comparison on three reference line voltage analog quantities and N in-phase laminated carriers to obtain three driving signal digital quantities; linearly converting the three driving signal digital quantities into three-phase reference voltage digital quantities of the MMC; and distributing the three-phase reference voltage digital quantity of the MMC to the sub-module to control the MMC so as to obtain the driving signals of all the switching tubes in the MMC. The invention realizes the transmission of the zero common-mode voltage vector equivalently by an addition and subtraction mode of carrier comparison, greatly reduces the common-mode voltage, inhibits the common-mode current coupled by stray capacitance, is simple to realize, and avoids the complicated calculation of reference vector positioning and the like based on the traditional space vector synthesis method.

Description

MMC common-mode voltage suppression modulation method and system suitable for even number of sub-modules
Technical Field
The invention belongs to the technical field of power electronics, and particularly relates to a modulation method and a modulation system for suppressing common-mode voltage by an MMC (modular multilevel converter) suitable for an even number of sub-modules.
Background
The Modular Multilevel Converter (MMC) is considered as the interface with the most application potential in the medium-high voltage electric energy conversion field by virtue of excellent performance. Besides being used as a converter valve in high-voltage direct-current transmission, the converter valve also has application prospects in occasions with relatively low voltage levels, such as motor control systems and photovoltaic power generation systems. At this time, the number of sub-modules of the MMC is relatively small, and thus the MMC is controlled by a modulation method with a high switching frequency, such as carrier stacking or carrier shifting.
The conventional carrier wave laminating or phase shifting method generates a common-mode voltage, so that the induced common-mode current is further propagated and derived to generate electromagnetic interference, which threatens the safe operation of the converter. In addition, the common mode problem can also produce leakage current in the photovoltaic system and damage the photovoltaic board, and produce shaft voltage in the motor system and harm the motor bearing. The existing method for realizing common-mode voltage suppression of the multilevel converter mainly depends on zero common-mode voltage vector synthesis. This method is very complex to implement, and needs to select the three closest vectors from all the zero common mode vectors to implement the combination of the reference voltages. As the number of levels increases, the number of zero common mode vectors increases and the calculation process becomes more complicated. For example: three-level 7 zero common-mode vectors, namely 3 selected from 7; five levels of 19 zero common mode vectors; 7 level 37 zero common mode vectors; the 9-level 61 zero common-mode vectors … … have no universality for different level converters, and a common-mode voltage suppression method for a three-level converter is one implementation method, and a common-mode voltage suppression method for a five-level converter is another method. The existing common-mode voltage suppression technology of the multilevel converter is aimed at three levels, the report of five levels is very common, 7 levels are less, and the report of more than 7 levels is almost not reported, also because of the reason of increasing complexity.
Disclosure of Invention
Aiming at the defect that a motor bearing of a motor system adopting MMC for electric energy conversion and a photovoltaic panel of a photovoltaic power generation system are easily damaged by common-mode voltage and the improvement requirement in the prior art, the invention provides a modulation method and a modulation system for restraining the common-mode voltage by the MMC, which are suitable for even numbers of sub-modules, and aims to reduce the common-mode current and common-mode interference caused by the high-frequency common-mode voltage output by the MMC in the traditional modulation mode.
To achieve the above object, according to a first aspect of the present invention, there is provided a modulation method for suppressing a common mode voltage by an MMC adapted to an even number of sub-modules, the method comprising the steps of:
s1, giving three-phase reference voltage analog quantity U to a controllera，Ub，UcThree reference line voltage analog quantity U converted from linear operation into injection zero sequence componentref1，Uref2，Uref3；
S2, for MMC with 2N sub-modules, three reference line voltage analog quantities U are usedref1，Uref2，Uref3Respectively carrying out carrier comparison with N in-phase laminated carriers to obtain three drive signal digital values V1，V2，V3；
S3, three driving signal digital values V1，V2，V3Linear transformation into three-phase reference voltage digital quantity V of MMCa，Vb，VcAnd guarantee Va，Vb，VcThe sum is zero;
s4, carrying out three-phase reference voltage digital quantity V on the MMCa，Vb，VcAnd the sub-module is allocated to control the MMC to obtain the driving signals of all the switching tubes in the MMC.
Preferably, in step S1, three reference line voltage analog quantities U of zero sequence component are injectedref1，Uref2，Uref3Obtained by the following method:
Figure BDA0002690272120000021
wherein the intermediate variable
Figure BDA0002690272120000022
Injected zero sequence component
Figure BDA0002690272120000031
Preferably, in step S2, three driving signal digital values V1，V2，V3Obtained by the following method:
Figure BDA0002690272120000032
wherein, the ith of MMC with 2N sub-modulesjMultiple stacked carrier waves
Figure BDA0002690272120000033
Figure BDA0002690272120000034
Carrier number for carrier comparison
Figure BDA0002690272120000035
wcAnd
Figure BDA0002690272120000039
respectively representing the frequency and initial phase of the triangular carrier, the ordinal number i, j represents the j th reference wave to be compared with the i th carrier, i is 1, 2, …, N, j is 1, 2, 3, ceil () function represents rounding up.
Preferably, in step S3, the MMC three-phase reference voltage digital value Va，Vb，VcObtained by the following method:
Figure BDA0002690272120000036
preferably, in step S4, within a few carrier cycles of the reference wave compared to the same carrier,
the number of submodules which are constantly input into each phase of lower bridge arm of the MMC and the number of the removed submodules are respectively as follows:
Figure BDA0002690272120000037
the number of submodules which are constantly input by each phase of bridge arm of the MMC and the number of the removed submodules are respectively as follows:
Figure BDA0002690272120000038
wherein, the function floor () represents rounding down, p ═ a, b, c;
for sub-modules of each bridge arm of the MMC, which are not determined to be put into or cut off, according to three-phase reference voltage digital quantity Va，Vb，VcAnd switching chopping control is carried out.
Preferably, the switching-in or switching-off of the sub-modules is determined according to the sequencing result of the capacitor voltages of the sub-modules.
To achieve the above object, according to a second aspect of the present invention, there is provided a carrier stacked modulation system adapted for suppressing a common mode voltage by an MMC having an even number of sub-modules, comprising:
a computer-readable storage medium and a processor;
the computer-readable storage medium is used for storing executable instructions;
the processor is configured to read executable instructions stored in the computer-readable storage medium, and execute the modulation method for suppressing the common-mode voltage by the MMC adapted for an even number of sub-modules according to the first aspect.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
(1) compared with the traditional modulation method for the MMC, the modulation method equivalently realizes the transmission of the zero common-mode voltage vector through an addition and subtraction method of carrier comparison, can greatly reduce the common-mode voltage, thereby inhibiting the common-mode current coupled by stray capacitance, preventing a motor bearing of a motor system and a photovoltaic panel of a photovoltaic system from being damaged, simultaneously preventing the common-mode current from being further transmitted and derived, reducing the possibility of electromagnetic interference and ensuring the safe and reliable operation of the system. The method is simple to realize, and avoids the complex calculation of reference vector positioning, zero common mode voltage vector selection and synthesis based on the traditional space vector synthesis method.
(2) The invention is suitable for the multi-level converter with any odd level number (even number of sub-modules), and is particularly suitable for the condition with less level number (higher switching frequency), such as the level numbers of 3, 5, 7, 9, 11, 13, 15, and the like. For a multi-level converter with 2N sub-modules, N carriers are adopted for carrier comparison, so that common-mode voltage elimination can be realized, 2N +1 levels with uniform intervals can be output, and the common-mode voltage of the MMC can be reduced, thereby inhibiting the generation of common-mode current and common-mode interference.
Drawings
Fig. 1 is a schematic diagram of a suitable modular multilevel converter provided by the present invention;
FIG. 2 is a zero common mode vector diagram of a five-level converter provided by the present invention;
FIG. 3 is a schematic diagram of a carrier comparison implementation with zero common mode voltage output provided by the present invention;
FIG. 4 is a flowchart of a modulation method for suppressing a common-mode voltage by an MMC applicable to an even number of sub-modules according to the present invention;
FIG. 5 is a schematic illustration of the sequencing moments provided by the present invention;
FIG. 6 is a graph comparing five-level common mode voltages provided by the present invention;
FIG. 7 is a comparison graph of seven level common mode voltages provided by the present invention;
fig. 8 is a comparison graph of the nine-level common mode voltage provided by the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
First, terms related to the present invention are explained as follows:
MMC: modular Multilevel Converters, belonging to one of Multilevel Converters, is particularly suitable for application occasions with more levels because of convenient and simple Modular production and assembly. The more the number of submodules is, the more the number of levels is; the number of the level ratios is more than the number of the sub-modules by 1; an even number of sub-modules, i.e. an odd number of levels.
Multi-level: the number of levels is greater than 2.
The present invention is suitable for the modular multilevel converter shown in fig. 1, the number of the sub-modules can be any number and even number, so that the MMC with any odd number of levels (even number of sub-modules) can be generated, and the present invention is particularly suitable for the case of less levels (higher switching frequency), such as 3, 5, 7, 9, 11, 13, 15, etc. The topology has 3 phase units, each phase unit is formed by connecting 2 bridge arms through inductors, each bridge arm is formed by connecting 2N sub-modules in series, and the sub-modules are typical half-bridge structures.
Fig. 2 is a space vector diagram of a five-level converter, with a total of 125 space vectors. There are also 19 vectors that can make the output common mode voltage zero. The vector synthesis method has clear physical concept, high direct current voltage utilization rate and good output electric energy quality. However, as the number of levels increases, the space voltage vector increases geometrically, and vector synthesis becomes more complicated and difficult to realize.
As shown in fig. 3, the present invention adopts a carrier comparison implementation manner to avoid a complex vector synthesis process, and proposes a modulation method for suppressing the common mode voltage by an MMC with an even number of sub-modules with a goal of common mode voltage cancellation, and the specific implementation steps are as shown in fig. 4:
(1) under the condition that a controller gives three-phase reference voltage (analog quantity and normalization) of the MMC, the three-phase reference voltage is linearly calculated and converted into three reference line voltages (analog quantity), and zero sequence components are injected;
obtaining a three-phase reference voltage value u by a controller of the MMC according to a corresponding control methoda，ubAnd uc(analog quantity) three reference line voltages (analog quantity) u 'are calculated'ref1，u’ref2And u'ref3：
Figure BDA0002690272120000061
Reference voltage value (analog quantity) u of three phases in the above formulaa，ubAnd ucSum and three reference line voltages (analog quantities) u'ref1，u’ref2And u'ref3The sum is zero, i.e. three-phase equilibrium.
The injected zero sequence components are:
Figure BDA0002690272120000062
in the above formula, max { } function means taking the maximum value, and min { } function means taking the minimum value.
Three reference line voltage (analog quantity) u with zero sequence component injectionref1，uref2And uref3Comprises the following steps:
Figure BDA0002690272120000063
(2) for an MMC with 2N sub-modules, carrier comparison is carried out on three reference line voltages and N in-phase laminated carriers respectively to obtain three driving signals (digital quantity);
n stacked carriers of MMC with 2N sub-modules
Figure BDA0002690272120000071
In the above formula, omegacAnd
Figure BDA0002690272120000076
respectively the frequency and the initial phase of the triangular carrier.
Three reference line voltage (analog quantity) u with injected zero sequence componentref1，uref2And uref3The carrier serial number for carrying out the carrier comparison is as follows:
Figure BDA0002690272120000072
in the above formula, the ceil (x) function represents the smallest integer greater than or equal to x, and the ordinal number i, j represents the j th reference wave and the i th carrier wave.
Three driving signals V obtained by carrier wave comparison1，V2And V3(numerical quantity) of
Figure BDA0002690272120000073
(3) Carrying out linear transformation on the three driving signals to obtain three-phase reference voltage (digital quantity) of the MMC;
MMC three-phase reference voltage Va，VbAnd Vc(digital quantity) with three drive signals V1，V2And V3The relationship (numerical quantity) is:
Figure BDA0002690272120000074
(4) and distributing three-phase reference voltage (digital quantity) to a submodule to control the MMC, wherein the submodule is divided into three states of input, cut-off and chopping.
In a plurality of carrier periods when the reference wave is compared with the same carrier, the number of the submodules which are constantly input into the lower bridge arm of each phase of the MMC and the number of the removed submodules are as follows:
Figure BDA0002690272120000075
in the above formula, the floor (x) function represents the largest integer less than or equal to x.
The number of submodules which are constantly input by each phase of bridge arm of the MMC and the number of the removed submodules are as follows:
Figure BDA0002690272120000081
one submodule of each bridge arm of the MMC, which is not determined to be put into or cut off, is used for generating three-phase reference voltage Va，VbAnd VcSwitching chopping control (digital quantity). The switching-in or switching-off of the sub-modules is determined according to the sequencing result of the capacitor voltage of the sub-modules.
Fig. 5 is a schematic diagram of sequencing time instants. And sequencing the voltage of the submodules only when the number of the constant input submodules is changed, and redistributing the states of input, cut-off and chopping for the submodules.
Fig. 6, 7 and 8 are diagrams of comparing common mode voltages of five, seven and nine levels, respectively, using the modulation method proposed by the present invention. The common mode voltage is basically reduced to be close to 0V, so that the common mode current coupled by stray capacitance is restrained, a motor bearing of a motor system and a photovoltaic panel of a photovoltaic system are prevented from being damaged, and the possibility of electromagnetic interference is reduced due to the restraint of the common mode voltage, so that the safe and reliable operation of the system is ensured.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (6)

1. The MMC common-mode voltage suppression modulation method suitable for an even number of sub-modules is characterized by comprising the following steps of:
s1, giving three-phase reference voltage analog quantity U to a controllera，Ub，UcThree reference line voltage analog quantity U converted from linear operation into injection zero sequence componentref1，Uref2，Uref3；
S2, for MMC with 2N sub-modules, three reference line voltage analog quantities U are usedref1，Uref2，Uref3Respectively carrying out carrier comparison with N in-phase laminated carriers to obtain three drive signal digital values V1，V2，V3；
S3, three driving signal digital values V1，V2，V3Linear transformation into three-phase reference voltage digital quantity V of MMCa，Vb，VcAnd guarantee Va，Vb，VcThe sum is zero;
s4, carrying out three-phase reference voltage digital quantity V on the MMCa，Vb，VcThe sub-modules are allocated to control the MMC to obtain driving signals of all switching tubes in the MMC;
in step S2, three driving signal digital values V1，V2，V3Obtained by the following method:
Figure FDA0003105128090000011
wherein, the ith of MMC with 2N sub-modulesjMultiple stacked carrier waves
Figure FDA0003105128090000012
Figure FDA0003105128090000013
Carrier number for carrier comparison
Figure FDA0003105128090000014
wcAnd
Figure FDA0003105128090000015
respectively representing the frequency and initial phase of the triangular carrier wave, the ordinal number i, j represents that the jth reference wave is compared with the ith carrier wave, i is 1, 2, …, N, j is 1, 2, 3, and the ceil () function represents rounding up.
2. The method of claim 1, wherein in step S1, three reference line voltage analog quantities U of zero sequence component are injectedref1，Uref2，Uref3Obtained by the following method:
Figure FDA0003105128090000021
wherein, the middle is changedMeasurement of
Figure FDA0003105128090000022
Injected zero sequence component
Figure FDA0003105128090000023
3. Method according to claim 1 or 2, characterized in that in step S3 the three-phase reference voltage digital values V of MMCa，Vb，VcObtained by the following method:
Figure FDA0003105128090000024
4. the method according to claim 1 or 2, characterized in that in step S4, during several carrier periods in which the reference wave is compared with the same carrier,
the number of submodules which are constantly input into each phase of lower bridge arm of the MMC and the number of the removed submodules are respectively as follows:
Figure FDA0003105128090000025
the number of submodules which are constantly input by each phase of bridge arm of the MMC and the number of the removed submodules are respectively as follows:
Figure FDA0003105128090000026
wherein, the function floor () represents rounding down, p ═ a, b, c;
for sub-modules of each bridge arm of the MMC, which are not determined to be put into or cut off, according to three-phase reference voltage digital quantity Va，Vb，VcAnd switching chopping control is carried out.
5. The method of claim 4, wherein the switching in or switching out of the sub-modules is determined based on the result of the sequencing of the sub-module capacitor voltages.
6. Carrier stack modulation system suitable for MMC that contains an even number of submodule restraines common mode voltage, characterized by, includes:
a computer-readable storage medium and a processor;
the computer-readable storage medium is used for storing executable instructions;
the processor is used for reading executable instructions stored in the computer readable storage medium and executing the modulation method for suppressing the common-mode voltage of the MMC suitable for an even number of sub-modules in any one of claims 1 to 5.
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