CN111711376A - Selection method of modulation strategy of MMC converter valve - Google Patents

Selection method of modulation strategy of MMC converter valve Download PDF

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Publication number
CN111711376A
CN111711376A CN201710113874.1A CN201710113874A CN111711376A CN 111711376 A CN111711376 A CN 111711376A CN 201710113874 A CN201710113874 A CN 201710113874A CN 111711376 A CN111711376 A CN 111711376A
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loss
converter valve
modulation
current
igbt
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荣飞
尹章涛
饶宏
黄守道
周保荣
马河涛
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Hunan University
CSG Electric Power Research Institute
Research Institute of Southern Power Grid Co Ltd
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Hunan University
Research Institute of Southern Power Grid Co Ltd
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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
    • 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
    • H02M7/49—Combination of the output voltage waveforms of a plurality of converters
    • 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
    • H02M1/0048—Circuits or arrangements for reducing losses
    • 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
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B70/00—Technologies for an efficient end-user side electric power management and consumption
    • Y02B70/10—Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

The invention discloses a selection method of a modulation strategy of an MMC converter valve, which comprises the steps of firstly solving the total loss of the converter valve obtained by adopting carrier phase shift modulation in a power frequency period under the condition of certain capacity and direct-current voltage, then solving the total loss of the converter valve obtained by adopting nearest level approximation modulation in the power frequency period, and finally making the losses under the two modulation modes equal to each other and solving the number of bridge arm modules of the MMC converter valve. When the number of the sub-modules in practical application is larger than or equal to the N value obtained by solving, adopting a modulation mode of nearest level approximation; and when the number of the sub-modules in practical application is smaller than the solved N value, a modulation mode of carrier phase shift is adopted. The selection method of the modulation strategy is beneficial to reducing the loss of the MMC converter valve system and reducing the running cost.

Description

Selection method of modulation strategy of MMC converter valve
Technical Field
The invention belongs to the field of flexible direct current transmission, and particularly relates to the aspects of modulation strategies and loss calculation of a modular multilevel converter.
Background
With the development of power electronic technology, Modular Multilevel (MMC) converter valves have greatly facilitated the development of high-voltage direct-current transmission technology. But high loss is a major limiting factor in high power transmission.
Each bridge arm of the Modular Multilevel (MMC) converter valve is connected with a plurality of identical submodules in series, output multilevel can be realized by controlling the conduction and the closing of the submodules, the harmonic content of output waveforms is low, a filter does not need to be additionally arranged in a power system, and the cost is greatly reduced. Losses in the MMC converter valve system are mainly generated by IGBTs and diodes in submodules, a common modulation strategy comprises carrier phase shift modulation and nearest level approximation modulation, and losses of the MMC converter valve system under different modulation strategies are different. And the proper modulation strategy can greatly reduce the running cost of the system.
Although the carrier frequency of each sub-module is fixed in the carrier phase-shift modulation mode, a plurality of PI regulators are needed, so that the complexity of a control system is high, and PI setting is difficult, so that the recent level approximation modulation is often adopted in the situation that the number of bridge arm sub-modules is large at present. However, under recent level approaching modulation, sub-module voltage-sharing control enables the average switching frequency of the sub-modules to increase along with the increase of the number of bridge arm sub-modules, and therefore system loss rises sharply. The carrier phase shift modulation method does not have the problem. The two modulation modes have advantages and disadvantages, so that a selection method of the modulation strategy of the MMC converter valve needs to be designed, so that the system loss is reduced, and the operation cost of the system is reduced.
Disclosure of Invention
The invention aims to solve the problem that in order to overcome the defects of the prior art, the invention provides a method for selecting a modulation strategy of an MMC converter valve, and the modulation strategy with the minimum loss under a specific system parameter is selected.
In order to achieve the above object, the invention adopts the technical scheme that:
a method for selecting a modulation strategy of an MMC converter valve is characterized in that the MMC converter valve adopts a three-phase six-bridge-arm symmetrical inversion structure, each phase comprises an upper bridge arm and a lower bridge arm, each bridge arm comprises N identical SM submodules and a bridge arm inductor which are mutually connected in series, and a phase line is led out from a connecting line of the upper bridge arm inductor and the lower bridge arm inductor; each SM submodule comprises two IGBT tubes T1And T2Two diodes D1And D2And a capacitor C; wherein D is1And D2Are respectively connected in reverse parallel at T1And T2Upper, T1The collector of the capacitor C is connected with the anode of the capacitor C, the emitter is used as the positive port of the SM submodule, T2The collector electrode of the capacitor C is connected to the positive port, the emitter electrode is used as the negative port of the SM submodule, the negative electrode of the capacitor C is connected to the negative port, and T1And T2The grid of the grid receives a control signal; under the condition of certain capacity and direct-current voltage, the selection method of the modulation strategy comprises the following steps:
step 1, calculating the total loss P of the MMC converter valve obtained by adopting a carrier phase shift modulation mode in a power frequency periodcps；
Step 2, calculating the total loss P of the MMC converter valve obtained by adopting the nearest level approximation modulation mode in a power frequency periodnlm；
Step 3, comparing the losses of the two, and determining a proper modulation strategy;
make the total loss of the MMC converter valves under two modulation modes equal, namely Pcps-PnlmSolving the number N of SM submodules in each bridge arm of the MMC converter valve when the number is 0;
when the number of SM submodules in practical application is larger than the N value obtained by solving, adopting a modulation mode of nearest level approximation; when the number of SM submodules in practical application is smaller than the solved N value, a modulation mode of carrier phase shift is adopted; and when the number of the sub-modules in practical application is equal to the N value obtained by solving, considering the cost of the control system and adopting a modulation mode of nearest level approximation.
Further, the step 1 specifically includes the following steps:
(1.1) calculating the average current and the effective current flowing through each device in a power frequency period:
flow through T in one power frequency period1The average current and the effective current of (a) are respectively:
Figure BDA0001235133770000021
Figure BDA0001235133770000022
flow through T in one power frequency period2The average current and the effective current of (a) are respectively:
Figure BDA0001235133770000023
Figure BDA0001235133770000024
current through D in a power frequency cycle1The average current and the effective current of (a) are respectively:
Figure BDA0001235133770000031
Figure BDA0001235133770000032
current through D in a power frequency cycle2The average current and the effective current of (a) are respectively:
Figure BDA0001235133770000033
Figure BDA0001235133770000034
wherein T is the power frequency period, m is the modulation ratio, omega is the fundamental angular frequency, T represents time, and k represents the kthThe SM submodule takes the value of 1,2, …, N; f. ofcIs carrier frequency, S is rated power of the MMC converter valve,
Figure BDA0001235133770000035
is a power factor angle, U, of an MMC converter valve in a rated statedcFor the direct-current side voltage of the MMC converter valve, the integral time t1、t2And t3Can be calculated by the following formula:
Figure BDA0001235133770000036
Figure BDA0001235133770000037
Figure BDA0001235133770000038
(1.2) calculating the on-state loss and the dynamic loss generated by the kth SM submodule:
Figure BDA0001235133770000039
Figure BDA00012351337700000310
wherein, UCEFor bias of the turn-on voltage of the IGBT tube, rCEIs its corresponding on-resistance; u shapefIs the on-voltage bias of the diode, rfIs its corresponding on-resistance; eT_refThe sum of single turn-on loss and single turn-off loss of the IGBT is obtained; eD_refSingle reverse recovery losses for the diode; u shapeCE、rCE、Uf、rf、、ET_refAnd ED_refAll can be obtained from the specification of IGBT manufacturers; u shaperefAnd IrefMeasurement by IGBT manufacturers ET_refCollector-emitter voltage and collector current;
(1.3) calculating the total loss of the MMC converter valve:
Figure BDA0001235133770000041
further, the step 1 specifically includes the following steps:
(2.1) calculating the on-state loss:
Figure BDA0001235133770000042
wherein n isapNumber of submodules put into the bridge arm on phase a, iapRepresenting the upper arm current of phase a, PT_con(iap) And PT_con(iap) To relate to iapCan be obtained by the following equations:
Figure BDA0001235133770000043
Figure BDA0001235133770000044
Figure BDA0001235133770000045
in the above formula, round () is a rounding function;
(2.2) calculating the necessary switching losses:
Figure BDA0001235133770000046
wherein, a1、b1、c1A quadratic fitting coefficient of IGBT turn-off loss; a is2、b2、c2Quadratic fitting coefficients are obtained for the IGBT turn-on loss; a is3、b3、c3A second-order fitting coefficient of the reverse recovery loss of the diode; k is a radical of1、k2And k3Turn-off loss of IGBT, turn-on loss of IGBT and diode respectivelyThe cutoff voltage correction factor for reverse recovery loss can be calculated using the following equation,
Figure BDA0001235133770000047
(2.3) calculating additional switching losses:
Figure BDA0001235133770000051
wherein C represents the capacitance size of the submodule, Deltau is the maximum deviation allowed by the capacitor voltage under the voltage-sharing control and is determined by a user, f is the control frequency, ImRepresents the maximum value of the a-phase output current, ImaxThese two values are calculated by the following equation for the maximum value of the bridge arm current:
Figure BDA0001235133770000052
Figure BDA0001235133770000053
(2.4) calculating the total loss of the converter valve under the recent level approximation modulation strategy:
Pnlm＝6(Pcon+Psw1+Psw2)。
further, in the step (2.2), a1、b1、c1For the IGBT turn-off loss quadratic fitting coefficient, the curve of' typical collector current-turn-off loss at 125 ℃ in the specification of IGBT manufacturers is obtained by adopting a quadratic curve fitting mode, a1Is the constant term coefficient in the fitting method, b1Is the coefficient of a first order term, c1Is the coefficient of the quadratic term; a is2、b2、c2For the quadratic fitting coefficient of the IGBT opening loss, the quadratic fitting coefficient is obtained by adopting a quadratic curve fitting mode to a typical collector current-opening loss curve at 125 ℃ in the specification of IGBT manufacturers, a2Is the constant term coefficient in the fitting method, b2Is the coefficient of a first order term, c2Is the coefficient of the quadratic term; a is3、b3、c3The secondary fitting coefficient of the reverse recovery loss of the diode is obtained by adopting a secondary curve fitting mode to a typical on-state current-reverse recovery loss curve at 125 ℃ in the specification of IGBT manufacturers, and a3Is the constant term coefficient in the fitting method, b3Is the coefficient of a first order term, c3Is the coefficient of the quadratic term.
Further, a1Is 378.2, b1Is 4.025, c1Is 0.00006071, a2Is 684.4, b2Is 3.059, c20.0006558; u shapefIs 1.079V, rfIs 0.001109 omega, a3Is 644.2, b3Is 3.103, c3Is-0.0007948.
Further, S is 2MVA, Udc10kV, m is 0.95, omega is 50Hz, T is 0.02s,
Figure BDA0001235133770000054
is 0, fcIs 200Hz, C is 50mF, Δ U is 100V, UrefIs 2800V, Iref1200A, the type of IGBT tube is Infineon-FZ1200R45HL, ET_refIs 10600mJ, ED_ref3200 mJ; u shapeCEIs 1.342V, rCEIs 0.00126 Ω.
The invention has the beneficial effects that:
the invention takes low system loss as a target, and obtains an optimal modulation strategy of MMC under specific capacity and voltage level by calculating the number of SM submodules corresponding to the equal loss in two modulation modes of carrier phase shift modulation and nearest level approximation modulation; the invention provides basis for selecting a modulation mode with lower system loss in practical application, thereby reducing the loss of the MMC converter valve and reducing the operation cost of the MMC converter valve.
Drawings
FIG. 1 is a structure diagram of an MMC converter valve
FIG. 2 shows a waveform of the current of the upper bridge arm in phase a
FIG. 3 is a relationship between loss and the number of sub-modules in two modulation modes
Detailed Description
In order to make the technical problems, technical solutions and advantageous effects solved by the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Example 1:
the invention discloses a selection method of a modulation strategy of an MMC converter valve, as shown in figure 1, the MMC converter valve adopts a three-phase six-bridge-arm symmetrical inversion structure, each phase comprises an upper bridge arm and a lower bridge arm, each bridge arm comprises N identical SM submodules and a bridge arm inductor which are mutually connected in series, and a phase line is led out from a connecting line of the upper bridge arm inductor and the lower bridge arm inductor; each SM submodule comprises two IGBT tubes T1And T2Two diodes D1And D2And a capacitor C; wherein D is1And D2Are respectively connected in reverse parallel at T1And T2Upper, T1The collector of the capacitor C is connected with the anode of the capacitor C, the emitter is used as the positive port of the SM submodule, T2The collector electrode of the capacitor C is connected to the positive port, the emitter electrode is used as the negative port of the SM submodule, the negative electrode of the capacitor C is connected to the negative port, and T1And T2The grid of the grid receives a control signal; under the condition of certain capacity and direct-current voltage, the selection method of the modulation strategy comprises the following steps:
step 1, calculating the total loss P of the MMC converter valve obtained by adopting a carrier phase shift modulation mode in a power frequency periodcps；
Fig. 2 is a waveform of the bridge arm current on the phase a, and it can be seen that the bridge arm current is obtained by superposing a direct current component on the basis of the sinusoidal current, and the time of the current zero crossing point can be obtained, and thus the loss can be calculated in a segmented manner.
(1.1) calculating the average current and the effective current flowing through each device in a power frequency period:
flow through T in one power frequency period1The average current and the effective current of (a) are respectively:
Figure BDA0001235133770000071
Figure BDA0001235133770000072
flow through T in one power frequency period2The average current and the effective current of (a) are respectively:
Figure BDA0001235133770000073
Figure BDA0001235133770000074
current through D in a power frequency cycle1The average current and the effective current of (a) are respectively:
Figure BDA0001235133770000075
Figure BDA0001235133770000076
current through D in a power frequency cycle2The average current and the effective current of (a) are respectively:
Figure BDA0001235133770000077
Figure BDA0001235133770000078
wherein T is a power frequency period, m is a modulation ratio, omega is a fundamental wave angular frequency, T represents time, k represents the kth SM submodule and takes the values of 1,2, … and N; f. ofcIs carrier frequency, S is rated power of the MMC converter valve,
Figure BDA0001235133770000079
is a power factor angle, U, of an MMC converter valve in a rated statedcIs MMCDC side voltage of converter valve, integral time t1、t2And t3Can be calculated by the following formula:
Figure BDA00012351337700000710
Figure BDA00012351337700000711
Figure BDA0001235133770000081
(1.2) calculating the on-state loss and the dynamic loss generated by the kth SM submodule:
Figure BDA0001235133770000082
Figure BDA0001235133770000083
wherein, UCEFor bias of the turn-on voltage of the IGBT tube, rCEIs its corresponding on-resistance; u shapefIs the on-voltage bias of the diode, rfIs its corresponding on-resistance; eT_refThe sum of single turn-on loss and single turn-off loss of the IGBT is obtained; eD_refSingle reverse recovery losses for the diode; u shapeCE、rCE、Uf、rf、、ET_refAnd ED_refAll can be obtained from the specification of IGBT manufacturers; u shaperefAnd IrefMeasurement by IGBT manufacturers ET_refCollector-emitter voltage and collector current;
(1.3) calculating the total loss of the MMC converter valve:
Figure BDA0001235133770000084
step 2, calculating the nearest level approximation tone adopted in a power frequency periodMMC converter valve total loss P obtained by system modenlm；
(2.1) calculating the on-state loss:
Figure BDA0001235133770000085
wherein n isapNumber of submodules put into the bridge arm on phase a, iapRepresenting the upper arm current of phase a, PT_con(iap) And PT_con(iap) To relate to iapCan be obtained by the following equations:
Figure BDA0001235133770000086
Figure BDA0001235133770000087
Figure BDA0001235133770000088
in the above formula, round () is a rounding function;
(2.2) calculating the necessary switching losses:
Figure BDA0001235133770000091
wherein, a1、b1、c1A quadratic fitting coefficient of IGBT turn-off loss; a is2、b2、c2Quadratic fitting coefficients are obtained for the IGBT turn-on loss; a is3、b3、c3A second-order fitting coefficient of the reverse recovery loss of the diode; a is1Is 378.2, b1Is 4.025, c1Is 0.00006071, a2Is 684.4, b2Is 3.059, c20.0006558; u shapefIs 1.079V, rfIs 0.001109 omega, a3Is 644.2, b3Is 3.103, c3Is-0.0007948; k is a radical of1、k2And k3Respectively IGBT OFFThe cut-off voltage correction coefficients for the turn-off loss, the turn-on loss of the IGBT, and the reverse recovery loss of the diode can be calculated by the following formula,
Figure BDA0001235133770000092
(2.3) calculating additional switching losses:
Figure BDA0001235133770000093
wherein C represents the capacitance size of the submodule, Deltau is the maximum deviation allowed by the capacitor voltage under the voltage-sharing control and is determined by a user, f is the control frequency, ImRepresents the maximum value of the a-phase output current, ImaxThese two values are calculated by the following equation for the maximum value of the bridge arm current:
Figure BDA0001235133770000094
Figure BDA0001235133770000095
(2.4) calculating the total loss of the converter valve under the recent level approximation modulation strategy:
Pnlm＝6(Pcon+Psw1+Psw2)。
the values of the parameters are as follows: s is 2MVA, Udc10kV, m is 0.95, omega is 50Hz, T is 0.02s,
Figure BDA0001235133770000096
is 0, fcIs 200Hz, C is 50mF, Δ U is 100V, UrefIs 2800V, Iref1200A, the type of IGBT tube is Infineon-FZ1200R45HL, ET_refIs 10600mJ, ED_ref3200 mJ; u shapeCEIs 1.342V, rCEIs 0.00126 Ω.
Step 3, comparing the losses of the two, and determining a proper modulation strategy;
let two modulation methodsMMC converter valve under formula has equal total loss, i.e. Pcps-PnlmSolving the number N of SM submodules in each bridge arm of the MMC converter valve when the number is 0;
when the number of SM submodules in practical application is larger than the N value obtained by solving, adopting a modulation mode of nearest level approximation; when the number of SM submodules in practical application is smaller than the solved N value, a modulation mode of carrier phase shift is adopted; and when the number of the sub-modules in practical application is equal to the N value obtained by solving, considering the cost of the control system and adopting a modulation mode of nearest level approximation.
FIG. 3 is a relationship between total system loss and the number of sub-modules under carrier phase shift modulation and nearest level approximation modulation when the carrier frequency is 200Hz, and it can be seen from the figure that when the number of SM sub-modules reaches 10, the loss under the two modulation modes is equal, and the nearest level approximation modulation is selected in consideration of cost; when the number of SM sub-modules is less than 10, the loss is low under the nearest level approximation modulation mode, and at the moment, nearest level approximation modulation should be selected; when the number of SM sub-modules is larger than 10, the loss under the carrier phase-shift modulation mode is small, and the carrier phase-shift modulation is selected at the moment.

Claims (6)

1. A method for selecting a modulation strategy of an MMC converter valve is characterized in that the MMC converter valve adopts a three-phase six-bridge-arm symmetrical inversion structure, each phase comprises an upper bridge arm and a lower bridge arm, each bridge arm comprises N identical SM submodules and a bridge arm inductor which are mutually connected in series, and a phase line is led out from a connecting line of the upper bridge arm inductor and the lower bridge arm inductor; the method for selecting the modulation strategy is characterized by comprising the following steps of:
step 1, calculating total loss P of the MMC converter valve obtained by adopting a carrier phase shift modulation mode in a power frequency periodcps；
Step 2, calculating the total loss P of the MMC converter valve obtained by adopting the nearest level approximation modulation mode in a power frequency periodnlm；
Step 3, comparing the losses of the two, and determining a proper modulation strategy;
make the total system loss equal under two modulation modes, i.e. Pcps-PnlmSolving the value N when the value is 0;
when the number of SM submodules in practical application is larger than the N value obtained by solving, adopting a modulation mode of nearest level approximation; when the number of SM submodules in practical application is smaller than the solved N value, a modulation mode of carrier phase shift is adopted; and when the number of the sub-modules in practical application is equal to the N value obtained by solving, considering the cost of the control system and adopting a modulation mode of nearest level approximation.
2. The method for selecting the modulation strategy of the MMC converter valve according to claim 1, wherein the step 1 comprises the following steps:
(1.1) calculating the average current and the effective current flowing through each device in a power frequency period:
flow through T in one power frequency period1The average current and the effective current of (a) are respectively:
Figure FDA0001235133760000011
Figure FDA0001235133760000012
flow through T in one power frequency period2The average current and the effective current of (a) are respectively:
Figure FDA0001235133760000013
Figure FDA0001235133760000014
current through D in a power frequency cycle1The average current and the effective current of (a) are respectively:
Figure FDA0001235133760000021
Figure FDA0001235133760000022
current through D in a power frequency cycle2The average current and the effective current of (a) are respectively:
Figure FDA0001235133760000023
Figure FDA0001235133760000024
wherein T is a power frequency period, m is a modulation ratio, omega is a fundamental wave angular frequency, T represents time, k represents the kth SM submodule and takes the values of 1,2, … and N; f. ofcIs carrier frequency, S is rated power of the MMC converter valve,
Figure FDA00012351337600000210
is a power factor angle, U, of an MMC converter valve in a rated statedcFor the direct-current side voltage of the MMC converter valve, the integral time t1、t2And t3Calculated by the following formula:
Figure FDA0001235133760000025
Figure FDA0001235133760000026
Figure FDA0001235133760000027
(1.2) calculating the on-state loss and the dynamic loss generated by the kth SM submodule:
Figure FDA0001235133760000028
Figure FDA0001235133760000029
wherein, UCEFor bias of the turn-on voltage of the IGBT tube, rCEIs its corresponding on-resistance; u shapefIs the on-voltage bias of the diode, rfIs its corresponding on-resistance; eT_refThe sum of single turn-on loss and single turn-off loss of the IGBT is obtained; eD_refSingle reverse recovery losses for the diode; u shapeCE、rCE、Uf、rf、、ET_refAnd ED_refAll can be obtained from the specification of IGBT manufacturers; u shaperefAnd IrefMeasurement by IGBT manufacturers ET_refCollector-emitter voltage and collector current;
(1.3) calculating the total loss of the MMC converter valve:
Figure FDA0001235133760000031
3. the method for selecting the modulation strategy of the MMC converter valve according to claim 2, wherein the step 2 comprises the following steps:
(2.1) calculating the on-state loss:
Figure FDA0001235133760000032
wherein n isapNumber of submodules put into the bridge arm on phase a, iapRepresenting the upper arm current of phase a, PT_con(iap) And PT_con(iap) To relate to iapIs obtained by the following equation:
Figure FDA0001235133760000033
Figure FDA0001235133760000034
Figure FDA0001235133760000035
in the above formula, round () is a rounding function;
(2.2) calculating the necessary switching losses:
Figure FDA0001235133760000036
wherein, a1、b1And c1A quadratic fitting coefficient of IGBT turn-off loss; a is2、b2And c2Quadratic fitting coefficients are obtained for the IGBT turn-on loss; a is3、b3And c3A second-order fitting coefficient of the reverse recovery loss of the diode; a is1、b1、c1、a2、b2、c2、a3、b3And c3Obtained from the specification of IGBT manufacturers; k is a radical of1、k2And k3The cut-off voltage correction coefficients, which are the turn-off loss of the IGBT, the turn-on loss of the IGBT and the reverse recovery loss of the diode, are calculated by the following formula,
Figure FDA0001235133760000037
(2.3) calculating additional switching losses:
Figure FDA0001235133760000041
wherein C represents the capacitance size of the submodule, Deltau is the maximum deviation allowed by the capacitor voltage under the voltage-sharing control and is determined by a user, f is the control frequency, ImRepresents the maximum value of the a-phase output current, ImaxThese two values are calculated by the following equation for the maximum value of the bridge arm current:
Figure FDA0001235133760000042
Figure FDA0001235133760000043
(2.4) calculating the total loss of the converter valve under the recent level approximation modulation strategy:
Pnlm＝6(Pcon+Psw1+Psw2)。
4. the method for selecting the modulation strategy of an MMC converter valve according to claim 3, characterized in that in step (2.2), a1、b1、c1For the IGBT turn-off loss quadratic fitting coefficient, the curve of' typical collector current-turn-off loss at 125 ℃ in the specification of IGBT manufacturers is obtained by adopting a quadratic curve fitting mode, a1Is the constant term coefficient in the fitting method, b1Is the coefficient of a first order term, c1Is the coefficient of the quadratic term; a is2、b2、c2For the quadratic fitting coefficient of the IGBT opening loss, the quadratic fitting coefficient is obtained by adopting a quadratic curve fitting mode to a typical collector current-opening loss curve at 125 ℃ in the specification of IGBT manufacturers, a2Is the constant term coefficient in the fitting method, b2Is the coefficient of a first order term, c2Is the coefficient of the quadratic term; a is3、b3、c3The secondary fitting coefficient of the reverse recovery loss of the diode is obtained by adopting a secondary curve fitting mode to a typical on-state current-reverse recovery loss curve at 125 ℃ in the specification of IGBT manufacturers, and a3Is the constant term coefficient in the fitting method, b3Is the coefficient of a first order term, c3Is the coefficient of the quadratic term.
5. The MMC converter valve modulation strategy of claim 4, wherein a1Is 378.2, b1Is 4.025, c1Is 0.00006071, a2Is 684.4, b2Is 3.059, c20.0006558; u shapefIs 1.079V, rfIs 0.001109 omega, a3Is 644.2, b3Is 3.103, c3Is-0.0007948.
6. The MMC converter valve modulation strategy of any of claims 2-5, wherein S is 2MVA, Udc10kV, m is 0.95, omega is 50Hz, T is 0.02s,
Figure FDA0001235133760000044
is 0, fcIs 200Hz, C is 50mF, Δ U is 100V, UrefIs 2800V, Iref1200A, the type of IGBT tube is Infineon-FZ1200R45HL, ET_refIs 10600mJ, ED_ref3200 mJ; u shapeCEIs 1.342V, rCEIs 0.00126 Ω.
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