CN112134477B

CN112134477B - Frequency reduction control method of modular multilevel converter with auxiliary sub-modules

Info

Publication number: CN112134477B
Application number: CN202010960359.9A
Authority: CN
Inventors: 荣飞; 孙宗卿; 徐爽; 潘烙; 陈志忠; 刘成
Original assignee: Hunan University
Current assignee: Hunan University
Priority date: 2020-09-14
Filing date: 2020-09-14
Publication date: 2021-07-27
Anticipated expiration: 2040-09-14
Also published as: CN112134477A

Abstract

The invention discloses a frequency reduction control method of a modular multilevel converter containing auxiliary sub-modules, wherein each bridge arm of the modular multilevel converter topology is formed by connecting N half-bridge sub-modules, 1 auxiliary sub-module and 1 bridge arm inductor in series, the auxiliary sub-modules adopt a full-bridge structure, and the capacitor voltage of the auxiliary sub-modules is controlled to be 1/2 of the half-bridge sub-modules; adopting a nearest level approximation modulation mode for each bridge arm half-bridge submodule, wherein the auxiliary submodule is only put into when the number of put-in bridge arm half-bridge submodules is less than or equal to a preset value; the forward input and the reverse input of the auxiliary sub-module are determined according to the deviation between the actual value and the reference value of the capacitance voltage of the auxiliary sub-module in the current state and the current direction of a bridge arm; the control method of the invention can ensure that the auxiliary sub-modules connected in series are only put into use in part of time periods, and effectively reduces the switching frequency of the auxiliary sub-modules and the loss of the system while increasing the output level number of the MMC and reducing the harmonic distortion rate of the output voltage of the modular multilevel converter.

Description

Frequency reduction control method of modular multilevel converter with auxiliary sub-modules

Technical Field

The invention belongs to the field of power electronic current transformation, and particularly relates to a frequency reduction control method of a modular multilevel converter with auxiliary sub-modules.

Background

In recent years, rapid development of industry and economy has led to rapid consumption of energy, and electric energy is favored by various countries due to its advantages of cleanness, convenience, sustainability, and the like, but the voltage level and the electric energy quality of the traditional power transmission mode can not meet the demand gradually. With the continuous progress of power electronic switching devices and control technologies, flexible direct current transmission technology has been developed rapidly. The Modular Multilevel Converter (MMC) becomes a preferred scheme of the current direct current transmission project due to the technical advantages of high power, high reliability, strong fault processing capability, convenience in expansion and the like.

Commonly used MMC modulation strategies mainly include the nearest level approximation modulation (NLM) and the carrier phase shift modulation (CPS-PWM) strategies. Compared with CPS-PWM modulation, NLM modulation does not need to increase a voltage-sharing controller, and has the remarkable advantages of simple control, low switching frequency and low switching loss. However, when the number of the sub-modules is small, the number of the output levels of the MMC is reduced, and the harmonic distortion rate of the output voltage is high, so that the power quality cannot meet the power transmission requirement.

Therefore, the method has great significance for researching the problem that the harmonic distortion rate of the MMC output voltage is high when the number of sub-modules of the NLM modulation is small.

Based on the defect of the traditional NLM under the condition of less sub-modules, the invention provides a frequency reduction control method of a modular multilevel converter with auxiliary sub-modules. Compared with the traditional MMC, the auxiliary submodule of a full-bridge structure is connected in each bridge arm in series, the capacitance voltage of the auxiliary submodule is controlled to be 1/2 of the half-bridge submodule, a frequency reduction control strategy of the auxiliary submodule is provided, the output level number of the MMC under NLM modulation is improved, the harmonic distortion of the output voltage is reduced, the switching frequency of the auxiliary submodule is effectively reduced, and the loss of an MMC system is reduced.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a frequency reduction control method of a modular multilevel converter with auxiliary sub-modules, which increases the output level number of an MMC system, reduces the harmonic distortion rate of output voltage and reduces the switching frequency of the sub-modules and the loss of the system.

The technical solution of the invention is as follows:

(1) on the basis of a traditional MMC three-phase six-bridge arm topology, each bridge arm is connected with an auxiliary submodule of a full-bridge structure in series; the half-bridge sub-modules are controlled by adopting an NLM (non-line-of-sight) modulation strategy with an integer function of floor (), the capacitance voltage of the auxiliary sub-modules is controlled to be 1/2 of the half-bridge sub-modules, and the auxiliary sub-modules are put into forward or reverse after the number of the half-bridge sub-modules is changed every time according to the deviation between the actual value and the reference value of the capacitance voltage and the direction of the bridge arm current.

(2) And the switching frequency of the auxiliary sub-modules is reduced by adopting a frequency reduction strategy of partial time investment, and the auxiliary sub-modules are invested again only when the number of the invested bridge arm half-bridge sub-modules is less than or equal to a preset value.

The invention has the beneficial effects that: 1) each bridge arm of the three phases of the MMC is connected with an auxiliary submodule in series, the capacitance voltage of the auxiliary submodule is controlled to be 1/2 of the half-bridge submodule, the auxiliary submodule can be used for increasing the output level, the harmonic distortion rate of the output voltage of the MMC is reduced, and the power quality is improved; 2) by adopting the frequency reduction strategy, the switching frequency of the auxiliary sub-module device can be reduced, the requirement and cost of the MMC system on the performance of the MMC sub-module switching device are reduced, and the loss of the system is reduced.

Drawings

FIG. 1 is a block diagram of a modular multilevel converter topology with auxiliary sub-modules;

FIG. 2 is a block diagram illustrating the frequency reduction control of the upper bridge arm of the modular multilevel converter with auxiliary sub-modules;

FIG. 3 is a voltage waveform of the A phase output of the modular multilevel converter with auxiliary sub-modules;

fig. 4 shows distortion rates of a phase a output voltage of the modular multilevel converter including the auxiliary sub-modules;

FIG. 5 is a waveform of a capacitor voltage of an upper bridge arm half bridge of phase A of the modular multilevel converter with auxiliary sub-modules and the auxiliary sub-modules;

FIG. 6 is a relationship between harmonic distortion of A-phase output voltage of the modular multilevel converter with the auxiliary sub-modules and switching frequency of the auxiliary sub-modules when kN takes different values;

fig. 7 shows pulse signals of 4 IGBTs of the a-phase upper arm auxiliary sub-module after the frequency reduction of the modular multilevel converter with the auxiliary sub-modules.

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects solved by the present invention more clear, 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.

Fig. 1 is a topological structure diagram of a modular multilevel converter with auxiliary sub-modules, wherein the modular multilevel converter is composed of 6 bridge arms; the 6 bridge arms are equally divided into 3 groups, and each group forms a phase unitEach phase unit comprises an upper bridge arm and a lower bridge arm; the upper bridge arm and the lower bridge arm are connected in series, the connection position is called as a phase unit midpoint, and the phase units are connected in parallel; the upper bridge arm of each phase unit is formed by sequentially connecting N half-bridge submodules, 1 full-bridge auxiliary submodule and 1 bridge arm inductor in series, and the lower bridge arm of each phase unit is formed by sequentially connecting 1 bridge arm inductor, 1 full-bridge auxiliary submodule and N half-bridge submodules in series; the N half-bridge submodules of the upper bridge arm of the x-th phase unit are sequentially recorded as SM_{x_p1}，SM_{x_p2}，…，SM_{x_pj}，…，SM_{x_pN}Auxiliary submodule denoted SM_{x_p(N+1)}(ii) a The N half-bridge submodules of the lower bridge arm of the x-th phase unit are sequentially recorded as SM_{x_n1}，SM_{x_n2}，…，SM_{x_nj}，…，SM_{x_nN}Auxiliary submodule denoted SM_{x_n(N+1)}The subscript x is 1, 2, 3, and represents three phase units of 1, 2, and 3, respectively, and the subscript j is 1, 2, …, n; the bridge arm inductance of the x-th phase upper bridge arm is recorded as L_{x_p}And the bridge arm inductance of the lower bridge arm is recorded as L_{x_n}(ii) a The positive poles of the first submodules of the three upper bridge arms are connected together and connected to the positive pole of a direct-current power supply, and the negative poles of the last submodules of the three lower bridge arms are connected together and connected to the negative pole of the direct-current power supply; the midpoints of the three phase units form three output ports of the modular multilevel converter respectively to form three phases A, B and C.

The half-bridge sub-module is composed of 2 IGBT tubes T₁₁、T₁₂And 1 capacitor C₁Forming; capacitor C₁Positive electrode and T₁₁Is connected to the collector of a capacitor C₁Negative electrode and T₁₂The emitting electrodes are connected; t is₁₁Emitter and T₁₂The collectors are connected to form the output positive electrode of the half-bridge submodule, T₁₂The emitter of the half-bridge submodule is used as the cathode of the output end of the half-bridge submodule; the auxiliary sub-module adopts a full-bridge structure and is composed of 4 IGBT tubes T₁、T₂、T₃、T₄And 1 capacitor C₂Forming; capacitor C₂Positive electrode and T₁、T₂Is connected to the collector of a capacitor C₂Negative electrode and T₃、T₄The emitting electrodes are connected; t is₁Emitter and T₃And the collectors of which are connected and form the output positive pole, T, of the auxiliary submodule₂Emitter and T₄The collector electrodes of the auxiliary submodules are connected and form the cathode of the output end of the auxiliary submodule.

In this embodiment, the number N of half-bridge submodules in each bridge arm is 8; half-bridge submodule capacitor C₁Rated value U of capacitor voltage of half-bridge submodule with size of 4700uF_{C_ref}Is 2.1 kV; auxiliary sub-module capacitor C₂4700uF, auxiliary submodule capacitor voltage rating U_{x_(N+1)_ref}Is 1.05 kV; bridge arm inductance L of upper and lower bridge arms_{x_p}And L_{x_n}The sizes are all 8 mH; the downconversion coefficient k is 0.4.

Fig. 2 is a block diagram of a frequency reduction control of an upper bridge arm of a modular multilevel converter including auxiliary sub-modules, and the frequency reduction control is performed on any one of three phase units of the modular multilevel converter by the following steps:

step 1: calculating the reference value U of the modulation voltage of the upper and lower bridge arms_{px_ref}And U_{nx_ref}Are respectively U_{px_ref}＝U_dc/2[1-m*sin(ωt+(x-1)*(2π/3))]And U_{nx_ref}＝U_dc/2[1+m*sin(ωt+(x-1)*(2π/3))]。

Wherein, U_dcThe voltage is the direct current power supply voltage and is obtained by measuring through a voltage transformer; m is a voltage modulation ratio and is selected according to the actual requirement of a user; omega is the angular frequency of the output voltage and is selected according to the actual requirement of a user; t represents the amount of time obtained by timing from the start of the operation of the modular multilevel converter; denotes the multiplication of the mathematical symbol.

Step 2: when the auxiliary submodules are not considered, the number n of the half-bridge submodules required to be put into the upper bridge arm and the lower bridge arm_{px_1}And n_{nx_1}Are each n_{px_1}＝floor(U_{px_ref}/U_{C_ref}) And n_{nx_1}＝floor(U_{nx_ref}/U_{C_ref})。

Wherein, the floor () function is a rounding-down function; u shape_{C_ref}And taking the reference value of the half-bridge submodule capacitor voltage as a rated value of the half-bridge submodule capacitor voltage.

Step 3: reference value U of upper and lower bridge arm modulation voltage_{px_ref}And U_{nx_ref}Reference value U of capacitor voltage of half-bridge submodule respectively_{C_ref}The quotient is then respectively compared with n_{px_1}And n_{nx_1}Taking difference to obtain difference values p and q, wherein p is equal to U_{px_ref}/U_{C_ref}-n_{px_1}And q ═ U_{nx_ref}/U_{C_ref}–n_{nx_1}。

And 4, step 4: the measured voltage value of the capacitor of the upper bridge arm auxiliary submodule is U_{x_p(N+1)}(ii) a Will U_{x_p(N+1)}And U_{x_(N+1)_ref}Respectively connecting to positive and negative input ends of a first hysteresis comparator; u shape_{x_(N+1)_ref}The reference value of the auxiliary sub-module capacitor voltage is taken as the rated value of the auxiliary sub-module capacitor voltage; when U is turned_{x_p(N+1)}≥U_{x_(N+1)_ref}When + delta, the output of the first hysteresis comparator is 1, wherein delta is the loop width of the hysteresis comparator and is a rated value of the half-bridge submodule capacitor voltage, and the value of the rated value is 0.5% times; when U is turned_{x_p(N+1)}<U_{x_(N+1)_ref}At + δ, the first hysteresis comparator output is-1.

And 5: the measured voltage value of the capacitor of the auxiliary sub-module of the lower bridge arm is U_{x_n(N+1)}(ii) a Will U_{x_n(N+1)}And U_{x_(N+1)_ref}Respectively connecting to positive and negative input ends of a second hysteresis comparator; when U is turned_{x_n(N+1)}≥U_{x_(N+1)_ref}When + delta, the output of the second hysteresis comparator is 1; when U is turned_{x_n(N+1)}<U_{x_(N+1)_ref}At + δ, the second hysteresis comparator output is-1.

Step 6: when n is_{px_1}kN or less, and upper bridge arm current i_pxNot less than 0 and not more than 0<At 0.25 hour, the switch tube T of the auxiliary submodule is controlled₁And T₂Conduction, T₃And T₄Turning off; take n_{px_2}＝0；n_{px_2}Representing the number of half-bridge submodules additionally input by an upper bridge arm; k represents a frequency reduction coefficient, the value of the frequency reduction coefficient is changed between (0, 1), and the value can be taken according to the actual frequency reduction requirement; the larger the k value is, the poorer the frequency reduction effect is, but the smaller the harmonic distortion rate of the output voltage of the modular multilevel converter system is; the lower the k value is, the better the frequency reduction effect is, but the modularized multi-level conversion isThe greater the output voltage harmonic distortion rate of the system.

When n is_{px_1}kN or less, and upper bridge arm current i_pxP is more than or equal to 0 and more than or equal to 0.25 and less than or equal to 0.75, and when the output of the first hysteresis comparator is-1, the switching tube T of the auxiliary submodule is controlled₁And T₄Conduction, T₂And T₃Turning off; take n_{px_2}＝0。

When n is_{px_1}kN or less, and upper bridge arm current i_pxP is more than or equal to 0 and more than or equal to 0.25 and less than or equal to 0.75, and when the output of the first hysteresis comparator is 1, the switching tube T of the auxiliary submodule is controlled₂And T₃Conduction, T₁And T₄Turning off; take n_{px_2}＝1。

When n is_{px_1}kN or less, and upper bridge arm current i_pxNot less than 0 and 0.75<p<1 hour, control the switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{px_2}＝1。

When n is_{px_1}kN or less, and upper bridge arm current i_px<0, and 0 is not more than p<At 0.25 hour, the switch tube T of the auxiliary submodule is controlled₁And T₂Conduction, T₃And T₄Turning off; take n_{px_2}＝0。

When n is_{px_1}kN or less, and upper bridge arm current i_px<P is more than or equal to 0.25 and less than or equal to 0.75, and the output of the first hysteresis comparator is-1, the switch tube T of the auxiliary submodule is controlled₂And T₃Conduction, T₁And T₄Turning off; take n_{px_2}＝1。

When n is_{px_1}kN or less, and upper bridge arm current i_px<P is more than or equal to 0.25 and less than or equal to 0.75, and the output of the first hysteresis comparator is 1, the switching tube T of the auxiliary submodule is controlled₁And T₄Conduction, T₂And T₃Turning off; take n_{px_2}＝0。

When n is_{px_1}kN or less, and upper bridge arm current i_px<0, and 0.75<p<1 hour, control the switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{px_2}＝1。

When n is_{px_1}>kN, and p is more than or equal to 0 and less than or equal to 0.5, controlling a switching tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{px_2}＝0。

When n is_{px_1}>kN, and 0.5<p<1 hour, control the switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{px_2}＝1。

And 7: when n is_{nx_1}kN or less, and lower bridge arm current i_nxNot less than 0 and not more than 0 and q<At 0.25 hour, the switch tube T of the auxiliary submodule is controlled₁And T₂Conduction, T₃And T₄Turning off; take n_{nx_2}＝0；n_{nx_2}And the number of half-bridge submodules which need to be additionally input by a lower bridge arm is shown.

When n is_{nx_1}kN or less, and lower bridge arm current i_nxQ is more than or equal to 0, q is more than or equal to 0.25 and less than or equal to 0.75, and the output of the second hysteresis comparator is-1, the switching tube T of the auxiliary submodule is controlled₁And T₄Conduction, T₂And T₃Turning off; take n_{nx_2}＝0。

When n is_{nx_1}kN or less, and lower bridge arm current i_nxQ is more than or equal to 0, q is more than or equal to 0.25 and less than or equal to 0.75, and the output of the second hysteresis comparator is 1, the switching tube T of the auxiliary submodule is controlled₂And T₃Conduction, T₁And T₄Turning off; take n_{nx_2}＝1。

When n is_{nx_1}kN or less, and lower bridge arm current i_nxNot less than 0 and 0.75<q<1 hour, control the switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{nx_2}＝1。

When n is_{nx_1}kN or less, and lower bridge arm current i_nx<0, and 0 is not less than q<At 0.25 hour, the switch tube T of the auxiliary submodule is controlled₁And T₂Conduction, T₃And T₄Turning off; take n_{nx_2}＝0。

When n is_{nx_1}kN or less, and lower bridge arm current i_nx<Q is 0, 0.25-0.75, and the second hysteresis comparator outputWhen the output is-1, the switch tube T of the auxiliary submodule is controlled₂And T₃Conduction, T₁And T₄Turning off; take n_{nx_2}＝1。

When n is_{nx_1}kN or less, and lower bridge arm current i_nx<Q is more than or equal to 0.25 and less than or equal to 0.75, and the output of the second hysteresis comparator is 1, the switching tube T of the auxiliary submodule is controlled₁And T₄Conduction, T₂And T₃Turning off; take n_{nx_2}＝0。

When n is_{nx_1}kN or less, and lower bridge arm current i_nx<0, and 0.75<q<1 hour, control the switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{nx_2}＝1。

When n is_{nx_1}>kN, and q is more than or equal to 0 and less than or equal to 0.5, controlling a switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{nx_2}＝0。

When n is_{nx_1}>kN, and 0.5<q<1 hour, control the switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{nx_2}＝1。

And 8: n is to be_{px_1}And n_{px_2}、n_{nx_1}And n_{nx_2}Respectively summing to obtain the number n of half-bridge submodules required to be put into by the upper and lower bridge arms when the auxiliary submodules are considered_pxAnd n_nxAre respectively n_px＝n_{px_1}+n_{px_2}And n_nx＝n_{nx_1}+n_{nx_2}。

And step 9: according to n_pxAnd n_nxAnd adopting nearest level approximation modulation for the half-bridge sub-modules of the upper and lower bridge arms.

Fig. 3 shows the waveform of the output voltage of the A phase of the modular multilevel converter with the auxiliary sub-modules, the fundamental amplitude of the output voltage is 8.268kV, and the error rate is 1.27% compared with the reference amplitude of the A phase output voltage of 8.164 kV.

Fig. 4 shows the distortion rate of the output voltage of the phase a of the modular multilevel converter with the auxiliary sub-modules, the total distortion rate is 3.61%, and the output voltage waveform smoothly meets the requirement of the power quality.

Fig. 5 shows capacitor voltage waveforms of an upper bridge arm half bridge and auxiliary submodules of a phase a of the modular multilevel converter including the auxiliary submodules, wherein the capacitor voltage of each half bridge submodule of the upper bridge arm fluctuates between 2.043 and 2.139, and the voltage ripple is about 4.57%; the capacitor voltage of the auxiliary sub-module of the upper bridge arm fluctuates between 1.045 and 1.055, the voltage ripple is about 0.95%, and the engineering requirement is met.

Fig. 6 is a relationship between a harmonic distortion rate of a phase a output voltage of the modular multilevel converter including the auxiliary sub-modules and a switching frequency of the auxiliary sub-modules when kN (0< k <1) takes different values; wherein, the dotted line represents the harmonic distortion rate curve of the A-phase output voltage, and the solid line represents the switching frequency curve of the auxiliary sub-module; and when kN is 0, the harmonic distortion rate of the A-phase output voltage and the switching frequency of the auxiliary sub-module are shown when only the half-bridge sub-module participates in switching.

Fig. 7 shows the pulse signals of 4 IGBTs of the a-phase upper bridge arm auxiliary sub-module after the frequency reduction of the modular multilevel converter with the auxiliary sub-module, and the average switching frequency of each IGBT is about 558 Hz.

Claims

1. A frequency reduction control method of a modular multilevel converter containing auxiliary sub-modules is characterized in that the modular multilevel converter is composed of 6 bridge arms; the 6 bridge arms are averagely divided into 3 groups, each group forms a phase unit, and each phase unit comprises an upper bridge arm and a lower bridge arm; the upper bridge arm and the lower bridge arm are connected in series, the connection position is called as a phase unit midpoint, and the phase units are connected in parallel; the upper bridge arm of each phase unit is formed by sequentially connecting N half-bridge submodules, 1 full-bridge auxiliary submodule and 1 bridge arm inductor in series, and the lower bridge arm of each phase unit is formed by sequentially connecting 1 bridge arm inductor, 1 full-bridge auxiliary submodule and N half-bridge submodules in series; the N half-bridge submodules of the upper bridge arm of the x-th phase unit are sequentially recorded as SM_{x_p1}，SM_{x_p2}，…，SM_{x_pj}，…，SM_{x_pN}Auxiliary submodule denoted SM_{x_p(N+1)}(ii) a The N half-bridge submodules of the lower bridge arm of the x-th phase unit are sequentially recorded as SM_{x_n1}，SM_{x_n2}，…，SM_{x_nj}，…，SM_{x_nN}Auxiliary submodule denoted SM_{x_n(N+1)}The subscript x is 1, 2, 3, and represents three phase units of 1, 2, and 3, respectively, and the subscript j is 1, 2, …, n; the bridge arm inductance of the x-th phase upper bridge arm is recorded as L_{x_p}And the bridge arm inductance of the lower bridge arm is recorded as L_{x_n}(ii) a The positive poles of the first submodules of the three upper bridge arms are connected together and connected to the positive pole of a direct-current power supply, and the negative poles of the last submodules of the three lower bridge arms are connected together and connected to the negative pole of the direct-current power supply; the midpoints of the three phase units respectively form three output ports of the modular multilevel converter to form three phases A, B and C;

the half-bridge sub-module is composed of 2 IGBT tubes T₁₁、T₁₂And 1 capacitor C₁Forming; capacitor C₁Positive electrode and T₁₁Is connected to the collector of a capacitor C₁Negative electrode and T₁₂The emitting electrodes are connected; t is₁₁Emitter and T₁₂The collectors are connected to form the output positive electrode of the half-bridge submodule, T₁₂The emitter of the half-bridge submodule is used as the cathode of the output end of the half-bridge submodule; the auxiliary sub-module adopts a full-bridge structure and is composed of 4 IGBT tubes T₁、T₂、T₃、T₄And 1 capacitor C₂Forming; capacitor C₂Positive electrode and T₁、T₂Is connected to the collector of a capacitor C₂Negative electrode and T₃、T₄The emitting electrodes are connected; t is₁Emitter and T₃And the collectors of which are connected and form the output positive pole, T, of the auxiliary submodule₂Emitter and T₄The collector electrodes are connected and form the output end cathode of the auxiliary submodule;

the frequency reduction control method of the modular multilevel converter with the auxiliary sub-modules is characterized by comprising the following steps of:

aiming at any one phase unit in three phase units of the modular multilevel converter, the following steps are respectively adopted for control:

step 1: calculating the reference value U of the modulation voltage of the upper and lower bridge arms_{px_ref}And U_{nx_ref}Respectively is as follows:

U_{px_ref}＝U_dc/2[1-m*sin(ωt+(x-1)*(2π/3))]

U_{nx_ref}＝U_dc/2[1+m*sin(ωt+(x-1)*(2π/3))]

wherein, U_dcThe voltage is the direct current power supply voltage and is obtained by measuring through a voltage transformer; m is a voltage modulation ratio and is selected according to the actual requirement of a user; omega is the angular frequency of the output voltage and is selected according to the actual requirement of a user; t represents the amount of time obtained by timing from the start of the operation of the modular multilevel converter; denotes the multiplication of the mathematical symbol;

step 2: when the auxiliary submodules are not considered, the number n of the half-bridge submodules required to be put into the upper bridge arm and the lower bridge arm_{px_1}And n_{nx_1}Respectively as follows:

n_{px_1}＝floor(U_{px_ref}/U_{C_ref})

n_{nx_1}＝floor(U_{nx_ref}/U_{C_ref})

wherein, the floor () function is a rounding-down function; u shape_{C_ref}Taking the reference value of the half-bridge submodule capacitor voltage as a rated value of the half-bridge submodule capacitor voltage;

and step 3: reference value U of upper and lower bridge arm modulation voltage_{px_ref}And U_{nx_ref}Reference value U of capacitor voltage of half-bridge submodule respectively_{C_ref}The quotient is then respectively compared with n_{px_1}And n_{nx_1}And (5) obtaining difference values p and q:

p＝U_{px_ref}/U_{C_ref}-n_{px_1}

q＝U_{nx_ref}/U_{C_ref}–n_{nx_1}

and 4, step 4: the measured voltage value of the capacitor of the upper bridge arm auxiliary submodule is U_{x_p(N+1)}(ii) a Will U_{x_p(N+1)}And U_{x_(N+1)_ref}Respectively connecting to positive and negative input ends of a first hysteresis comparator; u shape_{x_(N+1)_ref}The reference value of the auxiliary sub-module capacitor voltage is taken as the rated value of the auxiliary sub-module capacitor voltage; when U is turned_{x_p(N+1)}≥U_{x_(N+1)_ref}When + delta, the output of the first hysteresis comparator is 1, wherein delta is the loop width of the hysteresis comparator, and the value is 0.5 percent times of the half-bridge submoduleA nominal value of the capacitor voltage; when U is turned_{x_p(N+1)}<U_{x_(N+1)_ref}At + δ, the output of the first hysteresis comparator is-1;

and 5: the measured voltage value of the capacitor of the auxiliary sub-module of the lower bridge arm is U_{x_n(N+1)}(ii) a Will U_{x_n(N+1)}And U_{x_(N+1)_ref}Respectively connecting to positive and negative input ends of a second hysteresis comparator; when U is turned_{x_n(N+1)}≥U_{x_(N+1)_ref}When + delta, the output of the second hysteresis comparator is 1; when U is turned_{x_n(N+1)}<U_{x_(N+1)_ref}+ δ, the output of the second hysteresis comparator is-1;

step 6: when n is_{px_1}kN or less, and upper bridge arm current i_pxNot less than 0 and not more than 0<At 0.25 hour, the switch tube T of the auxiliary submodule is controlled₁And T₂Conduction, T₃And T₄Turning off; take n_{px_2}＝0；n_{px_2}Representing the number of half-bridge submodules additionally input by an upper bridge arm; k represents a frequency reduction coefficient, the value of the frequency reduction coefficient is changed between (0, 1), and the value can be taken according to the actual frequency reduction requirement; the larger the k value is, the poorer the frequency reduction effect is, but the smaller the harmonic distortion rate of the output voltage of the modular multilevel converter system is; the lower the k value is, the better the frequency reduction effect is, but the higher the harmonic distortion rate of the output voltage of the modular multilevel converter system is;

when n is_{px_1}kN or less, and upper bridge arm current i_pxP is more than or equal to 0 and more than or equal to 0.25 and less than or equal to 0.75, and when the output of the first hysteresis comparator is-1, the switching tube T of the auxiliary submodule is controlled₁And T₄Conduction, T₂And T₃Turning off; take n_{px_2}＝0；

When n is_{px_1}kN or less, and upper bridge arm current i_pxP is more than or equal to 0 and more than or equal to 0.25 and less than or equal to 0.75, and when the output of the first hysteresis comparator is 1, the switching tube T of the auxiliary submodule is controlled₂And T₃Conduction, T₁And T₄Turning off; take n_{px_2}＝1；

When n is_{px_1}kN or less, and upper bridge arm current i_pxNot less than 0 and 0.75<p<1 hour, control the switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{px_2}＝1；

When n is_{px_1}kN or less, and upper bridge arm current i_px<0, and 0 is not more than p<At 0.25 hour, the switch tube T of the auxiliary submodule is controlled₁And T₂Conduction, T₃And T₄Turning off; take n_{px_2}＝0；

When n is_{px_1}kN or less, and upper bridge arm current i_px<P is more than or equal to 0.25 and less than or equal to 0.75, and the output of the first hysteresis comparator is-1, the switch tube T of the auxiliary submodule is controlled₂And T₃Conduction, T₁And T₄Turning off; take n_{px_2}＝1；

When n is_{px_1}kN or less, and upper bridge arm current i_px<P is more than or equal to 0.25 and less than or equal to 0.75, and the output of the first hysteresis comparator is 1, the switching tube T of the auxiliary submodule is controlled₁And T₄Conduction, T₂And T₃Turning off; take n_{px_2}＝0；

When n is_{px_1}kN or less, and upper bridge arm current i_px<0, and 0.75<p<1 hour, control the switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{px_2}＝1；

When n is_{px_1}>kN, and p is more than or equal to 0 and less than or equal to 0.5, controlling a switching tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{px_2}＝0；

When n is_{px_1}>kN, and 0.5<p<1 hour, control the switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{px_2}＝1；

And 7: when n is_{nx_1}kN or less, and lower bridge arm current i_nxNot less than 0 and not more than 0 and q<At 0.25 hour, the switch tube T of the auxiliary submodule is controlled₁And T₂Conduction, T₃And T₄Turning off; take n_{nx_2}＝0；n_{nx_2}The number of half-bridge submodules which need to be additionally input by a lower bridge arm is represented;

when n is_{nx_1}kN or less, and lower bridge arm current i_nxQ is not less than 0 and not more than 0.25 and not more than 0.75, and a second hysteresis comparisonWhen the output of the device is-1, the switch tube T of the auxiliary submodule is controlled₁And T₄Conduction, T₂And T₃Turning off; take n_{nx_2}＝0；

When n is_{nx_1}kN or less, and lower bridge arm current i_nxQ is more than or equal to 0, q is more than or equal to 0.25 and less than or equal to 0.75, and the output of the second hysteresis comparator is 1, the switching tube T of the auxiliary submodule is controlled₂And T₃Conduction, T₁And T₄Turning off; take n_{nx_2}＝1；

When n is_{nx_1}kN or less, and lower bridge arm current i_nxNot less than 0 and 0.75<q<1 hour, control the switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{nx_2}＝1；

When n is_{nx_1}kN or less, and lower bridge arm current i_nx<0, and 0 is not less than q<At 0.25 hour, the switch tube T of the auxiliary submodule is controlled₁And T₂Conduction, T₃And T₄Turning off; take n_{nx_2}＝0；

When n is_{nx_1}kN or less, and lower bridge arm current i_nx<Q is more than or equal to 0.25 and less than or equal to 0.75, and the output of the second hysteresis comparator is-1, the switching tube T of the auxiliary submodule is controlled₂And T₃Conduction, T₁And T₄Turning off; take n_{nx_2}＝1；

When n is_{nx_1}kN or less, and lower bridge arm current i_nx<Q is more than or equal to 0.25 and less than or equal to 0.75, and the output of the second hysteresis comparator is 1, the switching tube T of the auxiliary submodule is controlled₁And T₄Conduction, T₂And T₃Turning off; take n_{nx_2}＝0；

When n is_{nx_1}kN or less, and lower bridge arm current i_nx<0, and 0.75<q<1 hour, control the switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{nx_2}＝1；

When n is_{nx_1}>kN, and q is more than or equal to 0 and less than or equal to 0.5, controlling a switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{nx_2}＝0；

When n is_{nx_1}>kN, and 0.5<q<1 hour, control the switch tube T of the auxiliary submodule₁And T₂Conduction, T₃And T₄Turning off; take n_{nx_2}＝1；

And 8: n is to be_{px_1}And n_{px_2}、n_{nx_1}And n_{nx_2}Respectively summing to obtain the number n of half-bridge submodules required to be put into by the upper and lower bridge arms when the auxiliary submodules are considered_pxAnd n_nx：

n_px＝n_{px_1}+n_{px_2}

n_nx＝n_{nx_1}+n_{nx_2}

2. The method for controlling the frequency reduction of the modular multilevel converter with the auxiliary sub-modules according to claim 1, wherein the number N of half-bridge sub-modules in each bridge arm is 8; half-bridge submodule capacitor C₁Rated value U of capacitor voltage of half-bridge submodule with size of 4700uF_{C_ref}Is 2.1 kV; auxiliary sub-module capacitor C₂4700uF, auxiliary submodule capacitor voltage rating U_{x_(N+1)_ref}Is 1.05 kV; bridge arm inductance L of upper and lower bridge arms_{x_p}And L_{x_n}The sizes are all 8 mH; the downconversion coefficient k is 0.4.