CN109149916B - Method for suppressing current pulsation of direct current side of modular multilevel converter - Google Patents

Abstract

The invention discloses a method for inhibiting direct current side current pulsation of a Modular Multilevel Converter (MMC). A three-phase MMC system adopts a carrier lamination pulse width modulation method and adds a method for inhibiting double times of a bridge armThe frequency circulation control algorithm provides that carrier waves are divided into two groups: group 1 comprisesn-1 carrier, and a sinusoidal reference wave ±)y _j Comparing; group 2 includes 1 carrier wave, whose phase is variable, and a compensated reference wave output by the double frequency circulation controllery _j2A comparison is made. The phases of the 2 nd group of carriers in the three-phase bridge arm are respectively moved in each carrier period, so that the sum of pulse voltages on the inductors of the three-phase bridge arm is zero, high-frequency currents caused by the pulse voltages in the three-phase bridge arm are mutually offset and cannot flow into the direct-current bus, and the high-frequency current pulsation on the direct-current bus is restrained. The method does not need to change any circuit topology, inhibits the high-frequency harmonic current brought to the direct current side of the MMC by the traditional control method through improving the traditional PD-PWM control method, and makes up for the defects of the traditional method.

Description

Method for suppressing current pulsation of direct current side of modular multilevel converter

Technical Field

The invention belongs to the technical field of multilevel power electronic converters, and relates to a method for inhibiting current pulsation on a direct current side of a Modular Multilevel Converter (MMC).

Background

Compared with the traditional two-level and three-level converters, the MMC has the characteristics of high efficiency, small harmonic wave of output alternating voltage, high modularization and the like, is more suitable for application occasions with high voltage and high power, and is widely applied to the fields of high-voltage direct-current transmission and the like.

A carrier-stacked pulse width modulation (PD-PWM) method is a relatively common MMC modulation method. The method is that n triangular carrier waves with the same frequency, phase and amplitude are laminated and compared with a modulation wave, so that trigger pulses of the switching device are obtained. Generally, the phases of the triangular carriers of the upper and lower arms of each phase are set to be opposite. If the sinusoidal modulation waves given by the upper and lower legs of each phase are symmetric about the x-axis, the sum of the number of submodules put in each phase at any time is n. Neglecting the voltage drop across the bridge arm impedances, the sum of the sub-module voltages dropped per phase is thus equal to the dc-side bus voltage.

When the converter normally operates, the current flowing through the MMC bridge arm contains a larger double frequency component, so that the loss of the converter is increased. To reduce losses in MMCs, it is common toAnd a controller is also required to be designed to inhibit the circular current of the bridge arm. At present, a common strategy for suppressing double frequency circulating current is to extract double frequency circulating current, obtain a reference voltage compensation signal through a circulating current controller, and add the reference voltage compensation signal to sinusoidal modulation waves of upper and lower bridge arms of each phase, respectively, thereby achieving the purpose of suppressing double frequency circulating current. However, the double frequency circulating current is suppressed while another adverse effect is brought about. Because the same voltage compensation component of double frequency is added in the sine modulation waves of the upper and lower bridge arms, the reference modulation waves of the upper and lower bridge arms are not symmetrical about the x axis any more. At this time, the PD-PWM modulation algorithm is adopted, and the number of submodules put into each phase at any time is not constant any more. When the voltage of frequency doubling compensates the component y_j2>When 0, the number of the sub-modules put into each phase unit is n or n-1; when the voltage of frequency doubling compensates the component y_j2<At 0, the number of submodules put in each phase unit is n or n + 1. Let the voltage of each sub-module capacitor equal to V_dcN, wherein V_dcThe direct current bus voltage results in the inconsistency between the sum of the voltages of the submodules input by each phase unit and the sum of the direct current side bus voltage, so that pulse voltage is generated on the bridge arm inductor, and the amplitude of the pulse voltage is equal to the voltage of 1 submodule capacitor. Thereby generating a high frequency current in the bridge arm. The phases of the pulse voltages on the three-phase bridge arm inductors are arbitrary, and the addition of the pulse voltages at each moment cannot be guaranteed to be zero, so that high-frequency currents generated on the three-phase bridge arms cannot be mutually offset and then flow into a direct-current side bus, and high-frequency pulsation of the current on the direct-current bus is caused.

Disclosure of Invention

In view of the above problems, an object of the present invention is to provide a method for suppressing a direct-current side current ripple of a Modular Multilevel Converter (MMC), which can suppress a high-frequency current ripple on a direct-current bus by dividing A, B, C carriers for each arm of three phases into two groups and shifting the phase of the 2 nd group of carriers for each carrier cycle so that the sum of pulse voltages applied to three-phase arm inductances is zero and the generated high-frequency current does not flow to the direct-current bus.

In order to achieve the above-mentioned objects,the technical scheme adopted by the invention is as follows: a Modular Multilevel Converter (MMC) direct current side current pulsation suppression method adopts a PD-PWM modulation method, and a control algorithm for suppressing double frequency circulation of a bridge arm is added. The method specifically comprises the following steps: considering no redundant sub-modules, the number of the sub-modules of A, B, C three-phase bridge arms is n, the number of the isosceles triangle carriers of each phase of upper bridge arm and lower bridge arm is n, and the isosceles triangle carriers are divided into two groups: the 1 st group comprises n-1 carriers, wherein the carriers of the upper and lower bridge arms are respectively marked as W_{ju_1}～W_{ju_n-1}And W_{jl_1}～W_{jl_n-1}(j ═ a, b, c, each representing A, B, C three phases) uniformly distributed from top to bottom in [ -1,1]Within the range of (1), the frequency and the phase are the same, the amplitude is the same and is equal to 2/(n-1), and the carrier phases of the j-phase upper bridge arm and the j-phase lower bridge arm are opposite; the 2 nd group comprises 1 carrier, wherein the carrier of the upper and lower bridge arms is respectively marked as W_{ju_n}And W_{jl_n}The amplitude range is [ -1,1 [)]The frequency is the same as that of the 1 st group, the phase is calculated by a control algorithm, and the carrier phases of the j-phase upper bridge arm and the j-phase lower bridge arm are opposite. Accordingly, the modulated wave of each phase of the upper bridge arm and the lower bridge arm is divided into two parts: part 1 is a given power frequency sinusoidal reference wave, wherein the power frequency sinusoidal reference waves given by the upper bridge arm and the lower bridge arm are respectively-y_jAnd y_jComparing with the 1 st group of carriers of the upper and lower bridge arms respectively to obtain the modulation level numbers n_{ju_1}And n_{jl_1}(ii) a Part 2 is a frequency-doubled sinusoidal reference wave y derived from the circulation controller_j2Comparing with the 2 nd group of carriers of the upper and lower bridge arms respectively to obtain the modulation level numbers n respectively_{ju_2}And n_{jl_2}. The number of the levels obtained by modulating the j-phase upper bridge arm and the j-phase lower bridge arm is n_juAnd n_jl. Wherein n is_ju＝n_{ju_1}+n_{ju_2}，n_jl＝n_{jl_1}+n_{jl_2}. Then through a traditional voltage-sharing strategy, PWM pulses for driving each submodule of the j-phase upper bridge arm and the j-phase lower bridge arm are generated finally, so that the j-phase upper bridge arm and the j-phase lower bridge arm are respectively fed with n_ju、n_jlAnd a sub-module.

As an improvement of the present invention, the phase of the 2 nd group of carriers of each of the A, B, C three-phase bridge arms is obtained by a control algorithm, which specifically includes the following steps:

(1) and in each carrier period, A, B, C three-phase compensation voltage output by the sampling circulation controller is respectively marked as y_a2、y_b2And y_c2；

(2) Calculating the widths of the high-frequency pulse voltages applied to the three-phase bridge arm inductors, and respectively recording the widths as theta_a、θ_bAnd theta_cThen calculate their absolute value | θ_a|、|θ_bAnd theta_c|；

(3) For | theta_a|、|θ_bAnd theta_cI sorting is carried out, and the obtained corresponding serial numbers are respectively marked as K_θa、K_θbAnd K_θcThe serial numbers are in the range of 0, 1 and 2. Wherein, | θ_a|、|θ_bAnd theta_cThe corresponding serial number of the largest in | is equal to 0; [ theta ]_a|、|θ_bAnd theta_cThe smallest of | has a corresponding serial number equal to 2; otherwise, its corresponding sequence number is equal to 1.

(4) Calculating A, B, C the position angle of the corresponding 2 nd group carrier when the three-phase sequencing serial numbers are respectively 0, 1 and 2, and obtaining K according to (3)_θa、K_θbAnd K_θcAnd respectively moving the 2 nd group of carriers of the upper and lower bridge arms of each phase to corresponding phases.

As an improvement of the present invention, in the step (2), in each triangular carrier period, 2 pulse voltages are applied to the bridge arm inductances of each phase, and the distance difference is pi. Width theta of each pulse voltage_a、θ_bAnd theta_cThe calculation formula of (2) is as follows: theta_j＝y_j2·π。y_j2>0 time theta_a>0，y_j2<0 time theta_a<0。

As an improvement of the present invention, in the step (4), for the positions of the 2 nd group carrier corresponding to each sequence number of the A, B, C three phases, it is defined that in 1 carrier cycle, the horizontal distances from the initial point of the carrier cycle to the negative peak point (or positive peak point) of the triangular carrier are θ_as、θ_bs、θ_csThe specific calculation method comprises the following steps:

for the phase A, the phase A is selected,

K_θa＝0，θ_as＝π

for the phase B, the phase B is selected,

K_θb＝0，θ_bs＝π

for the phase C, the phase C is selected,

K_θc＝0，θ_cs＝π

has the advantages that:

the MMC direct current side current ripple suppression method does not need to change a hardware circuit, and has the following advantages that:

1. can restrain the current pulsation on the direct current bus, practical value is high:

the carrier stacked pulse width modulation (PD-PWM) method is one of the most widely used MMC modulation methods at present. However, after the control system adds the double frequency circulation algorithm, it applies a pulse voltage to the bridge arm inductance, and injects a high frequency current into the dc bus, so that the dc bus current characteristic of the MMC is deteriorated. The method discovers the problem and provides a corresponding control algorithm. The method inhibits the high-frequency current flowing to the direct-current bus by moving the carrier phase, improves the performance of the MMC and has higher practical value.

2. The control is simple, and the understanding and the implementation are easy:

through analysis, the width theta of the pulse voltage applied to the three-phase bridge arm inductor in one carrier wave period is found_a、θ_bAnd theta_cThe algebraic sum is zero, but the phase of the pulses is random. Therefore, the invention proposes that the sum of the pulse voltages on the three-phase bridge arm inductance is equal to zero at any time by shifting the phase of the carrier. As shown in fig. 1, the carrier wave is divided into two groups, the 1 st group is compared with the sine modulation wave, so that the sum of the levels of the upper and lower bridge arms of each phase is fixed; group 2 has only one carrier, and the amplitude range is [ -1,1 [)]And comparing with the output reference voltage of the double frequency loop controller, each phase generates 2 pulse voltages in a carrier period, the distance is pi, and the position of the pulse is related to the position of the carrier. Therefore, the positions of the pulse voltages on the three-phase arm inductors can be controlled by shifting the phases of the 2 nd group of carriers of the three-phase arm respectively.

Drawings

FIG. 1 is a three-phase MMC and sub-module topology block diagram;

FIG. 2 is a schematic diagram of an implementation of the present method;

FIG. 3 is a schematic diagram of the modulation result when the double frequency circulation controller outputs the compensation reference wave greater than zero in the method of the present invention;

FIG. 4 is a schematic diagram of the modulation result when the double frequency circulation controller outputs the compensation reference wave less than zero in the method of the present invention;

fig. 5 is a schematic diagram of phase shift of the 2 nd group of carriers of the three-phase bridge arm and the bridge arm inductance voltage pulse in the method provided by the invention.

Detailed Description

The invention is further explained below with reference to the drawings.

The invention is suitable for a three-phase MMC direct current side current pulsation suppression method, wherein MMC topological junctionsAs shown in fig. 1, the three-phase MMC has 6 bridge arms, and each bridge arm is composed of n sub-module units (cells) and a series inductor. u. of_als、u_bls、u_clsWhich are the sum of the inductive voltages of A, B, C three-phase upper and lower bridge arms respectively. The sub-module unit is in a half-bridge topology, and can also be in other topologies such as a full-bridge topology, a clamping dual sub-module topology and the like.

As shown in fig. 2, a method for suppressing the direct-current side current ripple of the MMC adopts a carrier-stacked pulse width modulation (PD-PWM) method and adds a control algorithm for suppressing the double frequency circulation of the bridge arm. Considering no redundant sub-modules, the number of the sub-modules of A, B, C three-phase bridge arms is n, the number of the isosceles triangle carriers of each phase of upper bridge arm and lower bridge arm is n, and the isosceles triangle carriers are divided into two groups: the 1 st group comprises n-1 carriers, wherein the carriers of the upper and lower bridge arms are respectively marked as W_{ju_1}～W_{ju_n-1}And W_{jl_1}～W_{jl_n-1}Are uniformly distributed on [ -1,1 ] from top to bottom]Within the range of (1), the frequency and the phase are the same, the amplitude is the same and is equal to 2/(n-1), and the carrier phases of the j-phase upper bridge arm and the j-phase lower bridge arm are opposite; the 2 nd group comprises 1 carrier, wherein the carrier of the upper and lower bridge arms is respectively marked as W_{ju_n}And W_{jl_n}The amplitude range is [ -1,1 [)]The frequency is the same as that of the 1 st group, the phase is calculated by a control algorithm, and the carrier phases of the j-phase upper bridge arm and the j-phase lower bridge arm are opposite. Accordingly, the modulated wave of each phase of the upper bridge arm and the lower bridge arm is divided into two parts: part 1 is a given power frequency sinusoidal reference wave, wherein the power frequency sinusoidal reference waves given by the upper bridge arm and the lower bridge arm are respectively-y_jAnd y_jComparing with the 1 st group of carriers of the upper and lower bridge arms respectively to obtain the modulation level numbers n_{ju_1}And n_{jl_1}(ii) a Part 2 is a frequency-doubled sinusoidal reference wave y derived from the circulation controller_j2Comparing with the 2 nd group of carriers of the upper and lower bridge arms respectively to obtain the modulation level numbers n respectively_{ju_2}And n_{jl_2}. The number of the levels obtained by modulating the j-phase upper bridge arm and the j-phase lower bridge arm is n_juAnd n_jl. Wherein n is_ju＝n_{ju_1}+n_{ju_2}，n_jl＝n_{jl_1}+n_{jl_2}. Then through the traditional voltage-sharing strategy, finally, a driving j-phase upper and lower bridge is generatedPWM pulse of each sub-module of the arm makes the j-phase upper and lower bridge arms respectively input n_ju、n_jlAnd a sub-module.

A method for suppressing MMC direct current side current pulsation through phases of 2 groups of carriers of mobile bridge arms is disclosed, wherein the phase of each 2 groups of carriers of each bridge arm is obtained by a control algorithm, and the method specifically comprises the following steps:

(1) and in each carrier period, A, B, C three-phase compensation voltage output by the sampling circulation controller is respectively marked as y_a2、y_b2And y_c2. Where y is_a2、y_b2And y_c2Are per unit values. It should be noted here that since PD-PWM is a high-frequency modulation method, the carrier frequency is set to be much higher than the modulation wave frequency, and therefore, within one carrier period, y can be considered to be_a2、y_b2And y_c2Is constant.

(2) Calculating the widths of the high-frequency pulse voltages applied to the three-phase bridge arm inductors, and respectively recording the widths as theta_a、θ_bAnd theta_cAnd calculating a corresponding absolute value | θ_a|、|θ_bAnd theta_c|；；

(3) For | theta_a|、|θ_bAnd theta_cI sorting is carried out, and the obtained corresponding serial numbers are respectively marked as K_θa、K_θbAnd K_θcThe serial numbers have a value range of 0, 1 and 2. Wherein, | θ_a|、|θ_bAnd theta_cThe corresponding serial number of the largest in | is equal to 0; [ theta ]_a|、|θ_bAnd theta_cThe smallest of | has a corresponding serial number equal to 2; otherwise, its corresponding sequence number is equal to 1.

(4) Calculating A, B, C the position angle of the corresponding 2 nd group carrier when the three-phase sequencing serial numbers are respectively 0, 1 and 2, and obtaining K according to (3)_θa、K_θbAnd K_θcAnd respectively moving the 2 nd group of carriers of the upper and lower bridge arms of each phase to corresponding phases.

In the step (2), in each triangular carrier period, 2 pulse voltages are applied to the bridge arm reactance of each phase, and the distance difference is pi. Width theta of each pulse voltage_a、θ_bAnd theta_cThe calculation formula of (2) is as follows: theta_j＝y_j2·π。y_j2>0 time theta_a>0，y_j2<0 time theta_a<0。

In step (4), for the position of the 2 nd group of carriers corresponding to each sequence number of the A, B, C three phases, it is defined that in 1 triangular carrier cycle, the horizontal distances from the initial point of the carrier cycle to the negative peak point (or positive peak point) of the triangular carrier are respectively theta_as、θ_bs、θ_cs. For the 2 nd group of carriers of the upper and lower bridge arms of j phase, the phases are opposite. Therefore, if θ_jsAnd at the moment, the initial point of the carrier period of the upper bridge arm of the j phase is just on the positive peak value point, and the initial point of the carrier period of the lower bridge arm of the j phase is just on the negative peak value point. The initial point of each carrier period is varied in practice. Theta_as、θ_bs、θ_csAre all according to K_θa、K_θbAnd K_θcAnd the calculation method is obtained by taking pi as a reference, and comprises the following specific steps:

for the phase A, the phase A is selected,

K_θa＝0，θ_as＝π

for the phase B, the phase B is selected,

K_θb＝0，θ_bs＝π

for the phase C, the phase C is selected,

K_θc＝0，θ_cs＝π

y as shown in FIGS. 3 and 4, respectively_j2>0 and y_j2<And (5) a j-phase modulation result at 0. Taking fig. 3 as an example for explanation, in the control method provided by the present invention, the 1 st group of carriers includes n-1 carriers, a given sine wave is modulated, and the number of levels obtained by modulating the upper and lower bridge arms is n respectively_{ju_1}And n_{jl_1}Since the sinusoidal reference waves of the upper and lower arms are in anti-phase and the carrier wave is in anti-phase, the sum of the modulation results is constant, n_{ju_1}+n_{jl_1}N-1; the 2 nd group only has 1 carrier wave, modulates the output reference wave of the double frequency loop controller, and the level numbers obtained by modulating the upper bridge arm and the lower bridge arm are n respectively_{ju_2}And n_{jl_2}The modulation result is not constant, n_{ju_2}+ n _{jl_2}1 or 2. Let the capacitor voltage of each submodule equal to V_dcN, wherein V_dcIs the DC bus voltage, thus u_jsm＝V_dcOr (n +1) V_dcN, wherein u_jsmIs the sum of the sub-module voltages input by the upper bridge arm and the lower bridge arm of the j phase. Thus, u _jls0 or-V_dcN, wherein u_jlsIs the voltage on the j-phase upper and lower bridge arm inductors. Thereby generating high-frequency voltage pulses on the j-phase upper and lower bridge arm inductors. Each phase is modulated to generate 2 voltage pulses in each carrier period. It can be seen that the generated voltage pulse comprises 4 attributes: magnitude, sign, width, and position. The amplitude of the voltage pulse is the voltage of a sub-module capacitor, the positive and negative of the voltage pulse are related to the positive and negative of the output reference wave of the double-frequency loop controller, the width of the voltage pulse is related to the size of the output reference wave of the double-frequency loop controller, the position of the voltage pulse is related to the position of the 2 nd group of triangular carriers, and the distance of 2 voltage pulses in each carrier period is different from pi. For a three-phase MMC, θ is easily analytically obtained_a+θ_b+θ_c＝0。Therefore, theoretically, the phases of the 2 nd group of carriers can be moved in each carrier period, so that the three-phase pulses are added and offset, and the sum of the voltage pulses on the three-phase bridge arm inductance at any moment is equal to zero.

Fig. 5 shows a case where the carrier phase is shifted. A. B, C the relationship of the pulse width of three phases is theta_b|>|θ_a|>|θ_cL, thus K_θa＝1，K_θb＝0，K _θc2. Is kept stationary with reference to the B-phase triangular carrier wave, theta_bsPi. Moving the carrier of the A phase to the left to pi- (| theta)_b|-|θ_aI)/2, shifting the carrier of C phase to the right to pi + (| theta)_b|-|θ_cI))/2. At the moment, the pulse voltages on the three-phase bridge arm inductors are mutually offset, and the sum of the inductance voltages of the three-phase bridge arms is equal to zero in the whole period. Therefore, the sum of the generated high-frequency currents is equal to zero and does not flow into the direct current bus, so that high-frequency current pulsation on the direct current bus is suppressed.

The above description is only a preferred embodiment of the present invention, and the scope of the present invention is not limited to the above embodiment, but equivalent modifications or changes made by those skilled in the art according to the present disclosure should be included in the scope of the present invention as set forth in the appended claims.

Claims (4)

1. A method for suppressing current ripple at a direct current side of a modular multilevel converter is characterized by comprising the following steps:

considering no redundant sub-modules, the number of the sub-modules of A, B, C three-phase bridge arms is n, the number of the isosceles triangle carriers of each phase of upper bridge arm and lower bridge arm is n, and the isosceles triangle carriers are divided into two groups: the 1 st group comprises n-1 carriers, wherein the carriers of the upper and lower bridge arms are respectively marked as W_{ju_1}～W_{ju_n-1}And W_{jl_1}～W_{jl_n-1}J-a, b and c respectively represent A, B, C three phases which are uniformly distributed on [ -1,1 ] from top to bottom]Within the range of (1), the frequency and the phase are the same, the amplitude is the same and is equal to 2/(n-1), and the carrier phases of the j-phase upper bridge arm and the j-phase lower bridge arm are opposite; the 2 nd group comprises 1 carrier, wherein the carrier of the upper and lower bridge arms is respectively marked asW_{ju_n}And W_{jl_n}The amplitude range is [ -1,1 [)]The frequency is the same as that of the 1 st group, the phase is calculated by a control algorithm, and the carrier phases of the j-phase upper bridge arm and the j-phase lower bridge arm are opposite;

dividing the modulation wave of each phase of the upper bridge arm and the lower bridge arm into two parts: part 1 is a given power frequency sinusoidal reference wave,

wherein the power frequency sinusoidal reference waves given by the upper bridge arm and the lower bridge arm are respectively-y_jAnd y_jComparing with the 1 st group of carriers of the upper and lower bridge arms respectively to obtain the modulation level numbers n_{ju_1}And n_{jl_1}(ii) a Part 2 is a frequency-doubled sinusoidal reference wave y derived from the circulation controller_j2Comparing with the 2 nd group of carriers of the upper and lower bridge arms respectively to obtain the modulation level numbers n respectively_{ju_2}And n_{jl_2}(ii) a The number of the levels obtained by modulating the j-phase upper bridge arm and the j-phase lower bridge arm is n_juAnd n_jl(ii) a Wherein n is_ju＝n_{ju_1}+n_{ju_2}，n_jl＝n_{jl_1}+n_{jl_2}(ii) a And finally generating PWM (pulse-width modulation) pulses for driving each submodule of the j-phase upper bridge arm and the j-phase lower bridge arm through a voltage-sharing strategy, so that the j-phase upper bridge arm and the j-phase lower bridge arm are respectively fed with n_ju、n_jlAnd a sub-module.

2. The method for suppressing the direct current side current ripple of the modular multilevel converter according to claim 1, wherein: the phase of the 2 nd group of carriers of each A, B, C three-phase bridge arm is calculated by a control algorithm, and the method specifically comprises the following steps:

(1) and in each carrier period, A, B, C three-phase compensation voltage output by the sampling circulation controller is respectively marked as y_a2、y_b2And y_c2；

(2) Calculating the widths of the high-frequency pulse voltages applied to the three-phase bridge arm inductors, and respectively recording the widths as theta_a、θ_bAnd theta_cThen calculate their absolute value | θ_a|、|θ_bAnd theta_c|；

(3) For | theta_a|、|θ_bAnd theta_cI ordering to get correspondencesAre respectively marked as K_θa、K_θbAnd K_θcThe serial number ranges from 0, 1 and 2, wherein, | theta_a|、|θ_bAnd theta_cThe corresponding serial number of the largest in | is equal to 0; [ theta ]_a|、|θ_bAnd theta_cThe smallest of | has a corresponding serial number equal to 2; otherwise, the corresponding serial number is equal to 1;

(4) calculating the position angles of the corresponding 2 nd group carrier waves when the three-phase sequencing serial numbers of a, b and c are respectively 0, 1 and 2, and obtaining K according to the result in the step (3)_θa、K_θbAnd K_θcAnd respectively moving the 2 nd group of carriers of the upper and lower bridge arms of each phase to corresponding phases.

3. The method for suppressing the DC side current ripple of the modular multilevel converter according to claim 2, wherein in the step (2), in each triangular carrier period, 2 pulse voltages are applied to the bridge arm inductances of each phase, the distance between the pulse voltages is pi, and the width θ of each pulse voltage is_a、θ_bAnd theta_cThe calculation formula of (2) is as follows: theta_j＝y_j2·π。

4. The method for suppressing the current ripple on the dc side of the modular multilevel converter according to claim 2, wherein in the step (4), for the position angle of the 2 nd group carrier corresponding to each sequence number of A, B, C three phases, the horizontal distance from the initial point of the carrier period to the negative peak point or the positive peak point of the triangular carrier is defined as θ in 1 carrier period_as、θ_bs、θ_csThe specific calculation method comprises the following steps:

for the phase A, the phase A is selected,

K_θa＝0，θ_as＝π

K_θa＝1，

K_θa＝2，

for the phase B, the phase B is selected,

K_θb＝0，θ_bs＝π

K_θb＝1，

K_θb＝2，

for the phase C, the phase C is selected,

K_θc＝0，θ_cs＝π

K_θc＝1，

K_θc＝2，

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