CN103337980B - Modular multilevel converter (MMC) circulating current suppression method - Google Patents

Abstract

A modular multilevel converter (MMC) circulating current suppression method is provided. The invention belongs to the technical field of electrical engineering, specifically relates to a suppression method for suppressing internal circulating current of an MMC, and solves problems of incompatibility with single phase and three phase systems, poor circulating current suppression capability and a complicated control method in conventional MMC circulating current suppression methods. According to the MMC circulating current suppression method, a wave trap is used for acquiring secondary harmonic signals in phase current; the harmonic signals in the phase current are subtracted from the average current of the phase current so that average current signals of an upper bridge arm and a lower bridge arm in which secondary harmonic waves are subtracted are acquired; the average current signals of an upper bridge arm and a lower bridge arm in which secondary harmonic waves are subtracted is taken to do subtraction with a reference current, and thus secondary harmonic signals of the circulating current is acquired; a proportional resonant controller is used for performing current tracing on the secondary harmonic waves of the circulating current, and thus voltage compensation signals are acquired; the compensation signals are processed and then are given back to sub-modules of the upper bridge arm of the MMC, and thus circulating current suppression is realized. The MMC circulating current suppression method is suitable for the technical field of electrical engineering.

Description

Modular multi-level converter circulation inhibition method

Technical field

The invention belongs to electrical technology field, be specifically related to a kind of suppressing method for the inner circulation of suppression module Multilevel Inverters.

Background technology

Increasing along with novel energy supply channel, the HVDC Transmission Technology (Voltage Sourced Converter Based HVDC, VSC-HVDC) based on voltage converter receives increasing concern.Two traditional level or the VSC topological structure of three level, the problems such as there is switching loss high, current harmonics is high, and reliability is low.And modular multi-level converter (the modular multilevel converter proposed by German scholar Rainer Marquardt, MMC), compared to other current transformers, have and such as can realize Redundant Control, export more level number, there is the advantages such as common DC bus, for the application of high-power energy transformation provides new solution.But modular multi-level converter, because number of modules is numerous, causes it to there is control strategy complexity, the technology barriers such as submodule capacitor voltage equalizing difficulty, switching loss are larger.And the problems referred to above, circulation (circulating current) inner with modular multi-level converter, especially all there is comparatively close relationship in its two times of fundamental components, therefore, launches research just seem particularly important to MMC inside circulation.

Existing at following several Restrain measurement for MMC inside circulation:

1) MMC brachium pontis inductance is increased.The method is effective to a certain extent, but the simple increase brachium pontis inductance that relies on suppresses inner circulation, limited efficiency, also can reduce the frequency response speed of system, and greatly can increase volume and the cost of MMC.

2) be 2 DC component by two frequency multiplication negative phase-sequence rotating coordinate transformations by the three-phase Circulation Decomposition of inverter inside, and design corresponding loop current suppression controller, the method need use two frequency multiplication negative phase-sequence coordinate transforms and alternate decoupling zero link, add the operand of MMC control system, the method is only applicable to three-phase system simultaneously.

3) by producing the loop current suppression device of the method design that loop pressure drops compensates respectively in upper and lower bridge arm voltage instruction on upper and lower brachium pontis inductance to circulation, the method principle is comparatively succinct, but owing to introducing differentiation element, working control difficulty increases.

Cannot compatible single-phase and three-phase system but adopt said method to exist, the problem of loop current suppression ability control method complexity.

Summary of the invention:

The present invention cannot compatible single-phase and three-phase system in order to solve that existing Multilevel Inverters circulation inhibition method exists, and the problem of loop current suppression ability control method complexity, proposes a kind of modular multi-level converter circulation inhibition method.

Modular multi-level converter circulation inhibition method of the present invention, it realizes based on modular multi-level converter, describedly to be made up of three-phase circuit based on modular multi-level converter, every circuitry phase is made up of upper and lower brachium pontis inductance and n sub-block coupled in series, wherein, 1≤n <+∞, each submodule is by a half-bridge converter and a storage capacitor composition, and described half-bridge converter comprises a diode, gate-controlled switch, No. two gate-controlled switches and No. two diodes; The negative electrode of a diode connects the collector electrode of a gate-controlled switch and one end of storage capacitor; The emitter of a gate-controlled switch is connected with the negative electrode of No. two diodes with the collector electrode of No. two gate-controlled switches, and the anode of No. two diodes connects the emitter of No. two gate-controlled switches and the other end of storage capacitor; The emitter of a gate-controlled switch is the signal input part of this submodule; The emitter of No. two gate-controlled switches is signal output parts of this submodule;

It is characterized in that: the method is used for the circulation of suppression module Multilevel Inverters, and the concrete steps of modular multi-level converter circulation inhibition method are:

Step one, employing phase-locked loop pll are modulated grid ac voltage, obtain reference current i _ref, reference current i _refadopt subtracter to be deducted by the phase current i of modular multi-level converter, obtain a current difference signal;

Step 2, adoption rate resonant controller carry out tracing compensation to the current difference signal obtained, and obtain compensating signal;

Step 3, grid alternating current stream is multiplied by proportionality coefficient k, adopts adder to be added with the compensating signal of acquisition by k grid alternating current stream signal doubly, acquisition reference voltage u _ref;

The reference voltage u that step 4, step 3 obtain _refbe added through adder with the direct voltage E of modular multi-level converter, obtain a voltage and signal;

Step 5, the reference voltage u that step 3 is obtained _refutilize subtracter to subtract the direct voltage E of modular multi-level converter, obtain No. two voltage differential signals;

The direct voltage E=U of described modular multi-level converter _dc/ 2, U _dcfor modular multi-level converter DC side total voltage;

Step 6, by modular multi-level converter export phase current be multiplied by proportionality coefficient 1/2, obtain the average current of upper and lower brachium pontis;

Step 7, the average current of employing loop current suppression device to the upper and lower brachium pontis obtained carry out suppression and compensate, and obtain compensating signal u _cc;

Adopt loop current suppression device to carry out suppression to the average current of upper and lower brachium pontis to compensate, obtain compensating signal u _ccconcrete grammar be:

Step July 1st, the average current signal of trapper to the upper and lower brachium pontis that step 6 obtains is adopted to carry out second harmonic extraction, acquisition second harmonic current signal;

Step 7 two, adopt the electric current of subtracter to the second harmonic that the average current signal of the upper and lower brachium pontis that step 6 obtains and step obtain the July 1st poor, acquisition removes the average current signal of the upper and lower brachium pontis of second harmonic;

Step 7 three, employing subtracter, by poor for the average current signal and reference current removing the upper and lower brachium pontis of second harmonic, obtain the second harmonic signal of circulation;

Step 7 four, the adoption rate resonant controller second harmonic signal to the circulation that step 7 three obtains carries out current tracking, obtains voltage compensation signal u _cc; And perform step 8;

The compensating signal u that step 8, No. two voltage differential signals utilizing subtracter step 5 to be obtained and step 7 obtain _ccdiffer from, obtain No. three voltage differential signals;

The compensating signal that the voltage utilizing subtracter step 4 to be obtained and signal and step 7 obtain is poor, obtains No. four voltage differential signals;

Step 9, No. three voltage differential signals step 8 obtained and No. four voltage differential signals are multiplied by proportionality coefficient 1/2E respectively; Obtain No. three voltage differential signals of 1/2E and No. four voltage differential signals of 1/2E;

Step 10, employing carrier phase modulator carry out carrier phase modulation to the 1/2E that step 9 obtains No. three voltage differential signals, and by the signal feedback that obtains after carrier modulation in the submodule of the upper brachium pontis of modular multi-level converter, realize the loop current suppression to upper bridge arm circuit;

Carrier phase modulator is adopted to carry out carrier phase modulation to the 1/2E that step 9 obtains No. four voltage differential signals, and by the signal feedback that obtains after carrier modulation in the submodule of the upper brachium pontis of modular multi-level converter, realize the loop current suppression to lower bridge arm circuit.

The present invention is directed to the arbitrary of modular multi-level converter three-phase output and carry out loop current suppression mutually, can compatible single-phase and three-phase system.And present invention reduces the circulation of inside circuit, reach the fluctuation of reduction MMC submodule capacitor voltage, reduce switching loss, loop current suppression ability improves more than 10% compared with existing circulation inhibition method.

Accompanying drawing explanation

Fig. 1 is the structure chart of modular multi-level converter, and in figure, the three-phase of a, b, c difference representation module Multilevel Inverters exports, SM1, SM2 ... SMn represents submodule;

Fig. 2 is modular multi-level converter Neutron module circuit structure diagram, and in figure, Uc is the capacitance voltage value of submodule;

Fig. 3 is the circulation control block diagram of loop current suppression device, i in figure _zfor the circulation that this phase of modular multi-level converter exports, i _z2for 2 component of degree n ns of this phase circulation;

Fig. 4 is the control block diagram comprising modular multi-level converter circulation inhibition method of the present invention;

Fig. 5 is loop current suppression design sketch; In figure, left-hand broken line is MMC start-up time, and right side dotted line is NCCS start-up time;

Fig. 6 is loop current suppression contrast effect figure; In figure, curve 1 is current curve before loop current suppression, and curve 2 is loop current suppression after-current curve;

Fig. 7 is submodule voltage inhibition comparison diagram; In figure, curve 3 is voltage curve before loop current suppression, and curve 4 is voltage curve after loop current suppression;

Fig. 8 is a phase power design sketch after loop current suppression.

Embodiment

Embodiment one: composition graphs 1, Fig. 2, Fig. 3, Fig. 4 illustrates present embodiment, modular multi-level converter circulation inhibition method described in present embodiment, it realizes based on modular multi-level converter, describedly to be made up of three-phase circuit based on modular multi-level converter, every circuitry phase is by upper, lower brachium pontis inductance and n sub-block coupled in series are formed, wherein, 1≤n <+∞, each submodule forms by a half-bridge converter and a storage capacitor 1-1, described half-bridge converter comprises a diode 1-2, a gate-controlled switch 1-3, No. two gate-controlled switch 1-4 and No. two diode 1-5, the negative electrode of a diode 1-2 connects the collector electrode of a gate-controlled switch 1-3 and one end of storage capacitor 1-1, the emitter of a gate-controlled switch 1-3 is connected with the negative electrode of No. two diode 1-5 with the collector electrode of No. two gate-controlled switch 1-4, and the anode of No. two diode 1-5 connects the emitter of No. two gate-controlled switch 1-4 and the other end of storage capacitor 1-1, the emitter of a gate-controlled switch 1-3 is the signal input part of this submodule, the emitter of No. two gate-controlled switch 1-4 is signal output parts of this submodule,

It is characterized in that: the method is used for the circulation of suppression module Multilevel Inverters, and the concrete steps of modular multi-level converter circulation inhibition method are:

Step one, employing phase-locked loop pll are modulated grid ac voltage, obtain reference current i _ref, reference current i _refadopt subtracter to be deducted by the phase current i of modular multi-level converter, obtain a current difference signal;

Step 2, adoption rate resonant controller carry out tracing compensation to the current difference signal obtained, and obtain compensating signal;

Step 3, grid alternating current stream is multiplied by proportionality coefficient k, adopts adder to be added with the compensating signal of acquisition by k grid alternating current stream signal doubly, acquisition reference voltage u _ref;

The reference voltage u that step 4, step 3 obtain _refbe added through adder with the direct voltage E of modular multi-level converter, obtain a voltage and signal;

Step 5, the reference voltage u that step 3 is obtained _refutilize subtracter to subtract the direct voltage E of modular multi-level converter, obtain No. two voltage differential signals;

The direct voltage E=U of described modular multi-level converter _dc/ 2, U _dcfor modular multi-level converter DC side total voltage;

Step 6, by modular multi-level converter export phase current be multiplied by proportionality coefficient 1/2, obtain the average current of upper and lower brachium pontis;

Step 7, the average current of employing loop current suppression device to the upper and lower brachium pontis obtained carry out suppression and compensate, and obtain compensating signal u _cc;

Adopt loop current suppression device to carry out suppression to the average current of upper and lower brachium pontis to compensate, obtain compensating signal u _ccconcrete grammar be:

Step July 1st, the average current signal of trapper to the upper and lower brachium pontis that step 6 obtains is adopted to carry out second harmonic extraction, acquisition second harmonic current signal;

Step 7 two, adopt the electric current of subtracter to the second harmonic that the average current signal of the upper and lower brachium pontis that step 6 obtains and step obtain the July 1st poor, acquisition removes the average current signal of the upper and lower brachium pontis of second harmonic;

Step 7 three, employing subtracter, by poor for the average current signal and reference current removing the upper and lower brachium pontis of second harmonic, obtain the second harmonic signal of circulation;

Step 7 four, the adoption rate resonant controller second harmonic signal to the circulation that step 7 three obtains carries out current tracking, obtains voltage compensation signal u _cc; And perform step 8;

The compensating signal u that step 8, No. two voltage differential signals utilizing subtracter step 5 to be obtained and step 7 obtain _ccdiffer from, obtain No. three voltage differential signals;

Utilize subtracter a to subtract voltage that step 4 obtains and the compensating signal that signal and step 7 obtain poor, obtain No. four voltage differential signals;

Step 9, No. three voltage differential signals step 8 obtained and No. four voltage differential signals are multiplied by proportionality coefficient 1/2E respectively; Obtain No. three voltage differential signals of 1/2E and No. four voltage differential signals of 1/2E;

Step 10, employing carrier phase modulator carry out carrier phase modulation to the 1/2E that step 9 obtains No. three voltage differential signals, and by the signal feedback that obtains after carrier modulation in the submodule of the upper brachium pontis of modular multi-level converter, realize the loop current suppression to upper bridge arm circuit;

Carrier phase modulator is adopted to carry out carrier phase modulation to the 1/2E that step 9 obtains No. four voltage differential signals, and by the signal feedback that obtains after carrier modulation in the submodule of the upper brachium pontis of modular multi-level converter, realize the loop current suppression to lower bridge arm circuit.

Embodiment two: present embodiment is further illustrating the modular multi-level converter circulation inhibition method described in embodiment one, trapper described in step July 1st adopts infinite impulse response digital filter, and the transfer function of this digital filter is:

$N\left(s\right)=\frac{s^2+{\mathrm{\&omega;}}_0^2}{s^2+2{\mathrm{\&tau;}}_0{\mathrm{\&omega;}}_0+{\mathrm{\&omega;}}_0^2}---\left(1\right)$

Wherein ω ₀for resonance frequency, s is Laplacian, τ ₀=1/2Q, Q are the quality factor of notch filter circuit;

And formula (1) is by formula

$H\left(z\right)=\frac{\underset{i=0}{\overset{M}{\mathrm{\&Sigma;}}}{a}_i{z}^{-i}}{\underset{i=0}{\overset{N}{\mathrm{\&Sigma;}}}{b}_i{z}^{-i}}=\frac{\underset{i=1}{\overset{M}{\mathrm{\&Pi;}}}(z-z_i)}{\underset{i=1}{\overset{N}{\mathrm{\&Pi;}}}(z-p_i)}---\left(2\right)$

In only containing the conjugation zero point being positioned at the imaginary axis for a pair, during a pair corresponding with zero point conjugate pole, be single-frequency trapper, in frequencies omega ₀place occurs that depression obtains;

In formula 2, a _iand b _ifilter function constant coefficient, z _ifor the zero point of transfer function, p _ifor the limit of transfer function, M is number at zero point, and N is limit number, and z is Z-transformation operator z ^-irepresent the i power of the inverse of z.

Embodiment three: present embodiment is further illustrating the modular multi-level converter circulation inhibition method described in embodiment one, and the transfer function of ratio resonant controller is: the transfer function of ratio resonant controller is:

$F_{\mathrm{PR}\_\mathrm{quasi}}\left(s\right)=K_p+\frac{2K_{r}s}{s^2+2{\mathrm{\&omega;}}_{c}s+{{\mathrm{\&omega;}}_0}^2}---\left(3\right)$

In formula, K _pfor proportional coefficient, K _rfor resonance term coefficient, ω ₀for resonance frequency, ω _cfor cut-off angular frequency, line voltage frequency allowable fluctuation range is ± 0.8Hz time, then have ω _c/ π=1.6Hz, thus ω _c=5Hz.

The transfer function of PR controller described in present embodiment overcomes transfer function

$F_{\mathrm{PR}\_\mathrm{ideal}}\left(s\right)=K_p+\frac{2K_{r}s}{s^2+{{\mathrm{\&omega;}}_0}^2}---\left(4\right)$

The application of desirable PR controller real system in two subject matters of depositing: (1), due to the restriction of analogue system component parameter precision and digital system precision, ratio resonant controller not easily realizes; (2) PR controller is very little in non-fundamental frequency place gain, when mains frequency produces skew, and the harmonic wave that just cannot effectively suppress electrical network to produce; Level off to infinity in the gain of resonance point, and almost do not decay outside this Frequency point.Although desirable PR controller can reach zero steady-state error, improve the ability of anti-line voltage interference selectively.

Meanwhile, below ratio resonant controller demand fulfillment some: quality factor are large as far as possible, to reduce steady-state error; The cut-off frequency of control system open-loop transfer function is less than 1/5 of switching frequency, to reduce switching harmonics; Phase stabilization allowance is large as far as possible, to reduce the overshoot of output current in each control cycle and the number of oscillation.

For a phase, based on the entire system control block diagram of above-mentioned suppression strategy, as Fig. 4: according to system block diagram, when qualitative analysis, getting PR is desired proportions resonance link, can obtain modular multi-level converter AC output current i _acirculation i _zaand secondary base frequency component i _{za_2}right transfer function is respectively:

$G_{z,o}\left(s\right)=\frac{I_a}{I_z}=\frac{s^2+{\mathrm{\&omega;}}_0^2}{s^2+{\mathrm{\&omega;}}_0^2+{\mathrm{sL}}_c[(R+\mathrm{sL})(s^2+{\mathrm{\&omega;}}_0^2)-(s^2+5s+{\mathrm{\&omega;}}_0^2)]}---\left(5\right)$

$G_{z2,o}\left(s\right)=\frac{I_a}{I_{z\_2}}=\frac{5s^2+50s+{5\mathrm{\&omega;}}_0^2}{(s^2+{\mathrm{\&omega;}}_0^2)(R+\mathrm{sL}+\frac{1}{2}{\mathrm{sL}}_c)-(s^2+5s+{\mathrm{\&omega;}}_0^2)]}---\left(6\right)$

L and R is net side equivalent inductance and resistance, L _cfor brachium pontis inductance, I _a, I _zand I _{z_2}be respectively i _a, i _zaand i _{za_2}image function, G _z,o(s) and G _{z2, o}s () is output current I _ato circulation I _zwith secondary base frequency component i _{za_2}transfer function.

Obtaining circulation secondary base frequency component by formula (1) and (2) to the transfer function of circulation is:

$G_{z2,z}\left(s\right)=\frac{G_{z2,o}\left(s\right)}{G_{z,o}\left(s\right)}=\frac{(s^2+{\mathrm{\&omega;}}_0^2)[(s^2+{\mathrm{\&omega;}}_0^2)(R+\mathrm{sL}+\frac{1}{2}s{L}_c)-s^2+5s+{\mathrm{\&omega;}}_0^2]}{(5s^2+50s+5{\mathrm{\&omega;}}_0^2)(s^2+{\mathrm{\&omega;}}_0^2+s{L}_c(R+\mathrm{sL})(s^2+{\mathrm{\&omega;}}_0^2)-s{L}_c(s^2+5s+{\mathrm{\&omega;}}_0^2)]}---\left(7\right)$

Get s=j ω ₀time, G _{z2, z}s ()=0, in circulation, secondary base frequency component obtains abundant suppression, and wherein j is imaginary unit.

Embodiment four, present embodiment are further illustrating the modular multi-level converter circulation inhibition method described in embodiment two, ω ₀=100 π.

ω described in present embodiment ₀because the target that will suppress is 2 times of fundamental components, therefore get 100 π.

Embodiment five, present embodiment are further illustrating the modular multi-level converter circulation inhibition method described in embodiment two, τ ₀=0.2.

By τ ₀=1/2Q, the known τ of the quality factor for notch filter circuit ₀less, limit is the closer to unit circle, then frequency response curve depression is darker, and the width of depression is also narrower.The present invention wants filtering secondary Fundamental-frequency Current, therefore gets ω ₀=100 π, and get τ ₀be 0.2 namely meet required precision.

The method of the invention is adopted to build the grid-connected emulation platform of single-ended MMC to verify the method proposed by the invention inhibition to circulation, system parameters is as shown in table 1, modulation strategy selects the phase-shifting carrier wave SPWM Technique of the double modulation ripple of phase place mutual deviation 180 degree to control upper and lower brachium pontis respectively, simulation result as Fig. 5,6, shown in 7 and 8.

Table 1 simulation parameter table

As shown in Figure 5, Figure 6, before adding NCCS, the circulation do not suppressed fluctuates loop current suppression Contrast on effect between-30A and 30A, and add CCS suppress stable after, circulation peak value is no more than 3.5A, is only original about 5%.The effect of suppressing method of the present invention clearly as can be seen here.

The contrast of submodule voltage waveform and a phase gross power before and after Fig. 7 and Fig. 8 gives and suppress, after adding CCS proposed by the invention, submodule capacitor voltage waveform obtains remarkable improvement, obviously reduces with phase power fluctuation, extend electric capacity useful life, improve MMC reliability.

Claims (5)

1. a modular multi-level converter circulation inhibition method, it realizes based on modular multi-level converter, describedly to be made up of three-phase circuit based on modular multi-level converter, every circuitry phase is by upper, lower brachium pontis inductance and n sub-block coupled in series are formed, wherein, 1≤n <+∞, each submodule is by a half-bridge converter and storage capacitor (1-1) composition, described half-bridge converter comprises a diode (1-2), a gate-controlled switch (1-3), No. two gate-controlled switches (1-4) and No. two diodes (1-5), the negative electrode of a diode (1-2) connects the collector electrode of a gate-controlled switch (1-3) and one end of storage capacitor (1-1), the emitter of a gate-controlled switch (1-3) is connected with the negative electrode of No. two diodes (1-5) with the collector electrode of No. two gate-controlled switches (1-4) simultaneously, and the anode of No. two diodes (1-5) connects the emitter of No. two gate-controlled switches (1-4) and the other end of storage capacitor (1-1) simultaneously, the emitter of a gate-controlled switch (1-3) is the signal input part of this submodule, the emitter of No. two gate-controlled switches (1-4) is the signal output part of this submodule,

It is characterized in that: the method is used for the circulation of suppression module Multilevel Inverters, and the concrete steps of modular multi-level converter circulation inhibition method are:

Step one, employing phase-locked loop pll are modulated grid ac voltage, obtain reference current i _ref, reference current i _refadopt subtracter to be deducted by the phase current i of modular multi-level converter, obtain a current difference signal;

Step 2, adoption rate resonant controller carry out tracing compensation to the current difference signal obtained, and obtain compensating signal;

Step 3, grid alternating current stream is multiplied by proportionality coefficient k, adopts adder to be added with the compensating signal of acquisition by k grid alternating current stream signal doubly, acquisition reference voltage u _ref;

The reference voltage u that step 4, step 3 obtain _refbe added through adder with the direct voltage E of modular multi-level converter, obtain a voltage and signal;

Step 5, the reference voltage u that step 3 is obtained _refutilize subtracter to subtract the direct voltage E of modular multi-level converter, obtain No. two voltage differential signals;

The direct voltage E=U of described modular multi-level converter _dc/ 2, U _dcfor modular multi-level converter DC side total voltage;

Step 6, by modular multi-level converter export phase current be multiplied by proportionality coefficient 1/2, obtain the average current of upper and lower brachium pontis;

Step 7, the average current of employing loop current suppression device to the upper and lower brachium pontis obtained carry out suppression and compensate, and obtain compensating signal u _cc;

Adopt loop current suppression device to carry out suppression to the average current of upper and lower brachium pontis to compensate, obtain compensating signal u _ccconcrete grammar be:

Step July 1st, the average current signal of trapper to the upper and lower brachium pontis that step 6 obtains is adopted to carry out second harmonic extraction, acquisition second harmonic current signal;

Step 7 two, the second harmonic current signal adopting subtracter to obtain the July 1st the average current signal of the upper and lower brachium pontis that step 6 obtains and step are poor, and acquisition removes the average current signal of the upper and lower brachium pontis of second harmonic;

Step 7 three, employing subtracter, by poor for the average current signal and reference current removing the upper and lower brachium pontis of second harmonic, obtain the second harmonic signal of circulation;

Step 7 four, the adoption rate resonant controller second harmonic signal to the circulation that step 7 three obtains carries out current tracking, obtains voltage compensation signal u _cc; And perform step 8;

The compensating signal u that step 8, No. two voltage differential signals utilizing subtracter step 5 to be obtained and step 7 obtain _ccdiffer from, obtain No. three voltage differential signals;

The compensating signal that the voltage utilizing subtracter step 4 to be obtained and signal and step 7 obtain is poor, obtains No. four voltage differential signals;

Step 9, No. three voltage differential signals step 8 obtained and No. four voltage differential signals are multiplied by proportionality coefficient 1/2E respectively; Obtain No. three voltage differential signals of 1/2E and No. four voltage differential signals of 1/2E;

Step 10, employing carrier phase modulator carry out carrier phase modulation to the 1/2E that step 9 obtains No. three voltage differential signals, and by the signal feedback that obtains after carrier modulation in the submodule of the upper brachium pontis of modular multi-level converter, realize the loop current suppression to upper bridge arm circuit;

Carrier phase modulator is adopted to carry out carrier phase modulation to the 1/2E that step 9 obtains No. four voltage differential signals, and by the signal feedback that obtains after carrier modulation in the submodule of the upper brachium pontis of modular multi-level converter, realize the loop current suppression to lower bridge arm circuit.

2. modular multi-level converter circulation inhibition method according to claim 1, is characterized in that, the trapper described in step July 1st adopts infinite impulse response digital filter, and the transfer function of this digital filter is:

$N\left(s\right)=\frac{s^2+{\mathrm{\&omega;}}_0^2}{s^2+2{\mathrm{\&tau;}}_0{\mathrm{\&omega;}}_0+{\mathrm{\&omega;}}_0^2}---\left(1\right)$

Wherein ω ₀for resonance frequency, s is Laplacian, τ ₀=1/2Q, Q are the quality factor of notch filter circuit;

And formula (1) is by formula

$H\left(z\right)=\frac{\underset{i=0}{\overset{M}{\mathrm{\&Sigma;}}}{a}_i{z}^{-i}}{\underset{i=0}{\overset{N}{\mathrm{\&Sigma;}}}{b}_i{z}^{-i}}=\frac{\underset{i=1}{\overset{M}{\mathrm{\&Pi;}}}(z-z_i)}{\underset{i=1}{\overset{N}{\mathrm{\&Pi;}}}(z-p_i)}---\left(2\right)$

In only containing the conjugation zero point being positioned at the imaginary axis for a pair, during a pair corresponding with zero point conjugate pole, be single-frequency trapper, in frequencies omega ₀place occurs that depression obtains;

In formula, a _iand b _ifilter function constant coefficient, z _ifor the zero point of transfer function, p _ifor the limit of transfer function, M is number at zero point, and N is limit number, and z is Z-transformation operator, z ^-irepresent the i power of the inverse of z.

3. modular multi-level converter circulation inhibition method according to claim 1, is characterized in that, the transfer function of ratio resonant controller is:

$F_{\mathrm{PR}\_\mathrm{quasi}}\left(s\right)=K_p+\frac{2K_{r}s}{s^2+2{\mathrm{\&omega;}}_{c}s+{{\mathrm{\&omega;}}_0}^2}---\left(3\right)$

In formula, K _pfor proportional coefficient, K _rfor resonance term coefficient, ω ₀for resonance frequency, ω _cfor cut-off angular frequency.

4. modular multi-level converter circulation inhibition method according to claim 2, is characterized in that, ω ₀=100 π.

5. modular multi-level converter circulation inhibition method according to claim 2, is characterized in that, τ ₀=0.2.