CN103337980A - 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 multilevel current transformer circulation inhibition method

Technical field

The invention belongs to electrical technology field, be specifically related to a kind of for the inhibition method that suppresses the inner circulation of modular multilevel current transformer.

Background technology

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

Existing in following several inhibition schemes at MMC inside circulation:

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

2) by two frequency multiplication negative phase-sequence rotating coordinate transformations three phase circulations of inverter inside are decomposed into 2 DC component, and design corresponding circulation and suppress controller, this method need be used two frequency multiplication negative phase-sequence coordinate transforms and alternate decoupling zero link, increased the operand of MMC control system, this method is only applicable to three-phase system simultaneously.

3) the circulation inhibitor that designs by the method that in upper and lower bridge arm voltage instruction, circulation is compensated respectively in the pressure drop of upper and lower brachium pontis inductance generation circulation, this method principle is comparatively succinct, but owing to introduced differentiation element, the working control difficulty increases.

Can't compatible single-phase and three-phase system but adopt said method to exist, circulation suppresses ability control method complicated problems.

Summary of the invention:

The present invention can't compatible single-phase and three-phase system in order to solve that existing many level current transformers circulation inhibition method exists, and circulation suppresses ability control method complicated problems, has proposed a kind of modular multilevel current transformer circulation inhibition method.

Modular multilevel current transformer circulation inhibition method of the present invention, it is based on the realization of modular multilevel current transformer, describedly formed by three-phase circuit based on the modular multilevel current transformer, every circuitry phase is made of upper and lower brachium pontis inductance and n submodule series connection, wherein, 1≤n＜+∞, each submodule is formed by a half-bridge converter and a storage capacitor, and described half-bridge converter comprises a diode, a 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 an end of storage capacitor; The emitter of a gate-controlled switch links to each other with the collector electrode of No. two gate-controlled switches and the negative electrode of No. two diodes, 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: this method is used for suppressing the circulation of modular multilevel current transformer, and the concrete steps of modular multilevel current transformer circulation inhibition method are:

Step 1, employing phase-locked loop pll are modulated grid ac voltage, obtain reference current i _Ref, reference current i _RefAdopt subtracter that the phase current i of modular multilevel current transformer is deducted, obtain No. one the difference between current signal;

Step 2, employing ratio resonant controller are followed the tracks of compensation to a difference between current signal that obtains, and obtain compensating signal;

Step 3, grid alternating current stream be multiply by proportionality coefficient k, adopt adder with k grid alternating current stream signal doubly and the compensating signal addition of acquisition, acquisition reference voltage u _Ref

The reference voltage u that step 4, step 3 obtain _RefWith the direct voltage E of modular multilevel current transformer through the adder addition, obtain voltage and signal;

Step 5, the reference voltage u that step 3 is obtained _RefUtilize subtracter to subtract the direct voltage E of modular multilevel current transformer, obtain No. two voltage differential signal;

The direct voltage E=U of described modular multilevel current transformer _Dc/ 2, U _DcBe modular multilevel current transformer DC side total voltage;

Step 6, the phase current that the modular multilevel current transformer is exported multiply by proportionality coefficient 1/2, obtain the average current of upper and lower brachium pontis;

Step 7, employing circulation inhibitor suppress compensation to the average current of the upper and lower brachium pontis of acquisition, obtain compensating signal u _Cc

Adopt the circulation inhibitor that the average current of upper and lower brachium pontis is suppressed compensation, obtain compensating signal u _CcConcrete grammar be:

Step July 1st, adopt the average current signal of the upper and lower brachium pontis that trapper obtains step 6 to carry out second harmonic to extract, obtain the second harmonic current signal;

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

Step 7 three, to adopt subtracter will remove average current signal and the reference current of upper and lower brachium pontis of second harmonic poor, the second harmonic signal of acquisition circulation;

Step 7 four, employing ratio resonant controller are carried out current tracking to the second harmonic signal of the circulation that step 7 three obtains, and obtain voltage compensation signal u _CcAnd execution in step eight;

Step 8, the compensating signal u of No. two voltage differential signals and step 7 acquisition that utilizes that subtracter obtains step 5 _CcDiffer from, obtain No. three voltage differential signal;

The voltage and the signal that utilize that subtracter obtains step 4 are poor with the compensating signal of step 7 acquisition, obtain No. four voltage differential signal;

Step 9, No. three voltage differential signals and No. four voltage differential signals that step 8 is obtained multiply 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 the carrier phase modulation to No. three voltage differential signals of the 1/2E that step 9 obtains, and with the signal feedback that obtains after the carrier modulation to the submodule of the last brachium pontis of modular multilevel current transformer, realize the circulation of last bridge arm circuit is suppressed;

Adopt the carrier phase modulator that No. four voltage differential signals of the 1/2E of step 9 acquisition are carried out the carrier phase modulation, and with the signal feedback that obtains after the carrier modulation to the submodule of the last brachium pontis of modular multilevel current transformer, realize the circulation of bridge arm circuit is down suppressed.

The arbitrary circulation that carries out mutually that the present invention is directed to the output of modular multilevel current transformer three-phase suppresses, can compatible single-phase and three-phase system.And the present invention has reduced the circulation of circuit inside, has reached to reduce MMC submodule capacitance voltage fluctuation, reduce switching loss, compare circulation inhibition ability with existing circulation inhibition method and improved more than 10%.

Description of drawings

Fig. 1 is the structure chart of modular multilevel current transformer, a, b, the c three-phase output of the many level current transformers of representation moduleization respectively among the figure, SM1, SM2 ... SMn represents submodule;

Fig. 2 is submodular circuits structure chart in the modular multilevel current transformer, and Uc is the capacitance voltage value of submodule among the figure;

Fig. 3 is the circulation control block diagram of circulation inhibitor, i among the figure _zBe this circulation of exporting mutually of modular multilevel current transformer, i _Z22 component of degree n ns for this phase circulation;

Fig. 4 is the control block diagram that comprises modular multilevel current transformer circulation inhibition method of the present invention;

Fig. 5 is circulation inhibition figure; Left-hand broken line is MMC start-up time among the figure, and the right side dotted line is NCCS start-up time;

Fig. 6 suppresses contrast effect figure for circulation; Curve 1 suppresses preceding current curve for circulation among the figure, and curve 2 is that circulation suppresses the after-current curve;

Fig. 7 is submodule voltage inhibition comparison diagram; Curve 3 suppresses preceding voltage curve for circulation among the figure, and curve 4 is that circulation suppresses the back voltage curve;

Fig. 8 is a phase power design sketch of circulation after suppressing.

Embodiment

Embodiment one: in conjunction with Fig. 1, Fig. 2, Fig. 3, Fig. 4 illustrates present embodiment, the described modular multilevel current transformer of present embodiment circulation inhibition method, it is based on the realization of modular multilevel current transformer, describedly formed by three-phase circuit based on the modular multilevel current transformer, every circuitry phase is by last, following brachium pontis inductance and n submodule series connection constitute, wherein, 1≤n＜+∞, each submodule is formed by a half-bridge converter and a storage capacitor 1-1, and described half-bridge converter comprises diode 1-2 No. one, 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 the end of storage capacitor 1-1; The emitter of a gate-controlled switch 1-3 links to each other with the collector electrode of No. two gate-controlled switch 1-4 and the negative electrode of No. two diode 1-5, 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 the signal output part of this submodule;

It is characterized in that: this method is used for suppressing the circulation of modular multilevel current transformer, and the concrete steps of modular multilevel current transformer circulation inhibition method are:

Step 1, employing phase-locked loop pll are modulated grid ac voltage, obtain reference current i _Ref, reference current i _RefAdopt subtracter that the phase current i of modular multilevel current transformer is deducted, obtain No. one the difference between current signal;

Step 2, employing ratio resonant controller are followed the tracks of compensation to a difference between current signal that obtains, and obtain compensating signal;

Step 3, grid alternating current stream be multiply by proportionality coefficient k, adopt adder with k grid alternating current stream signal doubly and the compensating signal addition of acquisition, acquisition reference voltage u _Ref

The reference voltage u that step 4, step 3 obtain _RefWith the direct voltage E of modular multilevel current transformer through the adder addition, obtain voltage and signal;

Step 5, the reference voltage u that step 3 is obtained _RefUtilize subtracter to subtract the direct voltage E of modular multilevel current transformer, obtain No. two voltage differential signal;

The direct voltage E=U of described modular multilevel current transformer _Dc/ 2, U _DcBe modular multilevel current transformer DC side total voltage;

Step 6, the phase current that the modular multilevel current transformer is exported multiply by proportionality coefficient 1/2, obtain the average current of upper and lower brachium pontis;

Step 7, employing circulation inhibitor suppress compensation to the average current of the upper and lower brachium pontis of acquisition, obtain compensating signal u _Cc

Adopt the circulation inhibitor that the average current of upper and lower brachium pontis is suppressed compensation, obtain compensating signal u _CcConcrete grammar be:

Step July 1st, adopt the average current signal of the upper and lower brachium pontis that trapper obtains step 6 to carry out second harmonic to extract, obtain the second harmonic current signal;

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

Step 7 three, to adopt subtracter will remove average current signal and the reference current of upper and lower brachium pontis of second harmonic poor, the second harmonic signal of acquisition circulation;

Step 7 four, employing ratio resonant controller are carried out current tracking to the second harmonic signal of the circulation that step 7 three obtains, and obtain voltage compensation signal u _CcAnd execution in step eight;

Step 8, the compensating signal u of No. two voltage differential signals and step 7 acquisition that utilizes that subtracter obtains step 5 _CcDiffer from, obtain No. three voltage differential signal;

The voltage and the signal that utilize that subtracter subtracts that step 4 obtains are poor with the compensating signal of step 7 acquisition, obtain No. four voltage differential signal;

Step 9, No. three voltage differential signals and No. four voltage differential signals that step 8 is obtained multiply 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 the carrier phase modulation to No. three voltage differential signals of the 1/2E that step 9 obtains, and with the signal feedback that obtains after the carrier modulation to the submodule of the last brachium pontis of modular multilevel current transformer, realize the circulation of last bridge arm circuit is suppressed;

Adopt the carrier phase modulator that No. four voltage differential signals of the 1/2E of step 9 acquisition are carried out the carrier phase modulation, and with the signal feedback that obtains after the carrier modulation to the submodule of the last brachium pontis of modular multilevel current transformer, realize the circulation of bridge arm circuit is down suppressed.

Embodiment two: present embodiment is further specifying embodiment one described modular multilevel current transformer circulation inhibition method, the step described trapper July 1st adopts infinite impulse response digital filter, and the transfer function of this digital filter is:

$N\left(s\right)=\frac{{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+{\mathrm{\&omega;}}_0^2}{{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+2{\mathrm{\&tau;}}_0{\mathrm{\&omega;}}_0+{\mathrm{\&omega;}}_0^2}---\left(1\right)$

ω wherein ₀Be 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 contain a pair of conjugation zero point that is positioned at the imaginary axis, during a pair of conjugate pole corresponding with zero point, for the single-frequency trapper, in frequencies omega ₀The place depression occurs and obtains;

In the formula 2, a _iAnd b _iBe the filter function constant coefficient, z _iBe zeros of transfer functions, p _iBe the limit of transfer function, M is the number at zero point, and N is the limit number, and z is Z-transformation operator z ^-iThe i power of the inverse of expression z.

Embodiment three: present embodiment is that the transfer function of ratio resonant controller is: the transfer function of ratio resonant controller is to the further specifying of embodiment one described modular multilevel current transformer circulation inhibition method:

$F_{\mathrm{PR}\_\mathrm{quasi}}\left(s\right)=K_p+\frac{2K_{r}s}{{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+2{\mathrm{\&omega;}}_{c}s+{{\mathrm{\&omega;}}_0}^2}---\left(3\right)$

In the formula, K _pBe proportional coefficient, K _rBe resonance item coefficient, ω ₀Be resonance frequency, ω _cBe cut-off angular frequency, the line voltage frequency allow fluctuation range be ± during 0.8Hz, ω is arranged then _c/ π=1.6Hz, thereby ω _c=5Hz.

The transfer function of the described PR controller of present embodiment has overcome transfer function

$F_{\mathrm{PR}\_\mathrm{ideal}}\left(s\right)=K_p+\frac{2K_{r}s}{{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+{{\mathrm{\&omega;}}_0}^2}---\left(4\right)$

Desirable PR controller real system two subject matters of depositing in using: because the restriction of analogue system component parameter precision and digital system precision, the ratio resonant controller is difficult for realizing (1); (2) the PR controller is very little in the gain of non-fundamental frequency place, when mains frequency produces skew, just can't effectively suppress the harmonic wave that electrical network produces; Level off to infinity in the gain of resonance point, and almost not decay outside this Frequency point.Though desirable PR controller can reach zero steady-state error, improves anti-line voltage interference capability selectively.

Simultaneously, the ratio resonant controller need satisfy following some: quality factor are big 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 the switch ripple; The phase stabilization allowance is big as far as possible, to reduce overshoot and the number of oscillation of output current in each control cycle.

Be example mutually with a, based on the entire system control block diagram of above-mentioned inhibition strategy, as Fig. 4: according to system block diagram, when qualitative analysis, getting PR is desired proportions resonance link, can get modular multilevel AC side of converter output current i _aCirculation i _ZaAnd secondary base frequency component i _{Za_2}Right transfer function is respectively:

$G_{z,o}\left(s\right)=\frac{I_a}{I_z}=\frac{s^2+{\mathrm{\&omega;}}_0^2}{{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+{\mathrm{\&omega;}}_0^2+{\mathrm{sL}}_c[(R+\mathrm{sL})({\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+{\mathrm{\&omega;}}_0^2)-({\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+5s+{\mathrm{\&omega;}}_0^2)]}---\left(5\right)$

${\mathrm{<figure-callout\; id="2"\; label="G\; z"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">G</figure-callout>}}_z\left(s\right)=\frac{I_a}{I_{z\_2}}=\frac{5{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+50s+{5\mathrm{\&omega;}}_0^2}{({\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+{\mathrm{\&omega;}}_0^2)(R+\mathrm{sL}+\frac{1}{2}{\mathrm{sL}}_c)-({\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+5s+{\mathrm{\&omega;}}_0^2)]}---\left(6\right)$

L and R are net side equivalent inductance and resistance, L _cBe 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) be output current I _aTo circulation I _zWith secondary base frequency component i _{Za_2}Transfer function.

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

${\mathrm{<figure-callout\; id="2"\; label="G\; z"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">G</figure-callout>}}_z\left(s\right)=\frac{{\mathrm{<figure-callout\; id="2"\; label="G\; z"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">G</figure-callout>}}_z\left(s\right)}{G_{z,o}\left(s\right)}=\frac{({\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+{\mathrm{\&omega;}}_0^2)[({\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+{\mathrm{\&omega;}}_0^2)(R+\mathrm{sL}+\frac{1}{2}s{L}_c)-{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+5s+{\mathrm{\&omega;}}_0^2]}{(5{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+50s+5{\mathrm{\&omega;}}_0^2)({\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+{\mathrm{\&omega;}}_0^2+s{L}_c(R+\mathrm{sL})({\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+{\mathrm{\&omega;}}_0^2)-s{L}_c({\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+5s+{\mathrm{\&omega;}}_0^2)]}---\left(7\right)$

Get s=j ω ₀The time, G _{Z2, z}(s)=0, in the circulation, the secondary base frequency component has obtained abundant inhibition, and wherein j is imaginary unit.

Embodiment four, present embodiment are that embodiment two described modular multilevel current transformer circulation inhibition methods are further specified ω ₀=100 π.

The described ω of present embodiment ₀Because the target that will suppress is 2 times of fundamental components, so get 100 π.

Embodiment five, present embodiment are that embodiment two described modular multilevel current transformer circulation inhibition methods are further specified τ ₀=0.2.

By τ ₀=1/2Q is the quality factor of notch filter circuit τ as can be known ₀More little, limit is the closer to unit circle, and then the frequency response curve depression is more dark, and the width of depression is also more narrow.The present invention wants filtering secondary fundamental frequency electric current, so get ω ₀=100 π, and get τ ₀Be 0.2 namely to satisfy required precision.

Adopt the method for the invention to build single-ended MMC and be incorporated into the power networks emulation platform to verify that method proposed by the invention is to the inhibition of circulation, system parameters is as shown in table 1, modulation strategy selects for use the phase-shifting carrier wave sinusoidal pulse width modulation technology of the double modulation ripple of phase place mutual deviation 180 degree to control upper and lower brachium pontis respectively, and simulation result is shown in Fig. 5,6,7 and 8.

Table 1 simulation parameter table

The circulation inhibition is to such as Fig. 5, shown in Figure 6, and before adding NCCS, the circulation that is not suppressed fluctuates between-30A and 30A, and adds after CCS suppresses to stablize, and the circulation peak value is no more than 3.5A, only is original about 5%.The effect that this shows inhibition method of the present invention is very obvious.

Fig. 7 has provided with Fig. 8 and has suppressed front and back submodule voltage waveform and a contrast of gross power mutually, after adding CCS proposed by the invention, submodule capacitance voltage waveform has obtained remarkable improvement, and the phase power fluctuation obviously reduces simultaneously, prolong electric capacity useful life, improved the MMC reliability.

Claims (5)

1. modular multilevel current transformer circulation inhibition method, it is based on the realization of modular multilevel current transformer, describedly formed by three-phase circuit based on the modular multilevel current transformer, every circuitry phase is made of upper and lower brachium pontis inductance and n submodule series connection, wherein, 1≤n＜+∞, each submodule is formed by a half-bridge converter and a storage capacitor (1-1), and 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 an end of storage capacitor (1-1); The emitter of a gate-controlled switch (1-3) links to each other with the collector electrode of No. two gate-controlled switches (1-4) and the negative electrode of No. two diodes (1-5) 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: this method is used for suppressing the circulation of modular multilevel current transformer, and the concrete steps of modular multilevel current transformer circulation inhibition method are:

Step 1, employing phase-locked loop pll are modulated grid ac voltage, obtain reference current i _Ref, reference current i _RefAdopt subtracter that the phase current i of modular multilevel current transformer is deducted, obtain No. one the difference between current signal;

Step 2, employing ratio resonant controller are followed the tracks of compensation to a difference between current signal that obtains, and obtain compensating signal;

Step 3, grid alternating current stream be multiply by proportionality coefficient k, adopt adder with k grid alternating current stream signal doubly and the compensating signal addition of acquisition, acquisition reference voltage u _Ref

The reference voltage u that step 4, step 3 obtain _RefWith the direct voltage E of modular multilevel current transformer through the adder addition, obtain voltage and signal;

Step 5, the reference voltage u that step 3 is obtained _RefUtilize subtracter to subtract the direct voltage E of modular multilevel current transformer, obtain No. two voltage differential signal;

The direct voltage E=U of described modular multilevel current transformer _Dc/ 2, U _DcBe modular multilevel current transformer DC side total voltage;

Step 6, the phase current that the modular multilevel current transformer is exported multiply by proportionality coefficient 1/2, obtain the average current of upper and lower brachium pontis;

Step 7, employing circulation inhibitor suppress compensation to the average current of the upper and lower brachium pontis of acquisition, obtain compensating signal u _Cc

Adopt the circulation inhibitor that the average current of upper and lower brachium pontis is suppressed compensation, obtain compensating signal u _CcConcrete grammar be:

Step July 1st, adopt the average current signal of the upper and lower brachium pontis that trapper obtains step 6 to carry out second harmonic to extract, obtain the second harmonic current signal;

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

Step 7 three, to adopt subtracter will remove average current signal and the reference current of upper and lower brachium pontis of second harmonic poor, the second harmonic signal of acquisition circulation;

Step 7 four, employing ratio resonant controller are carried out current tracking to the second harmonic signal of the circulation that step 7 three obtains, and obtain voltage compensation signal u _CcAnd execution in step eight;

Step 8, the compensating signal u of No. two voltage differential signals and step 7 acquisition that utilizes that subtracter obtains step 5 _CcDiffer from, obtain No. three voltage differential signal;

The voltage and the signal that utilize that subtracter obtains step 4 are poor with the compensating signal of step 7 acquisition, obtain No. four voltage differential signal;

Step 9, No. three voltage differential signals and No. four voltage differential signals that step 8 is obtained multiply 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 the carrier phase modulation to No. three voltage differential signals of the 1/2E that step 9 obtains, and with the signal feedback that obtains after the carrier modulation to the submodule of the last brachium pontis of modular multilevel current transformer, realize the circulation of last bridge arm circuit is suppressed;

Adopt the carrier phase modulator that No. four voltage differential signals of the 1/2E of step 9 acquisition are carried out the carrier phase modulation, and with the signal feedback that obtains after the carrier modulation to the submodule of the last brachium pontis of modular multilevel current transformer, realize the circulation of bridge arm circuit is down suppressed.

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

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

And formula (1) is by formula

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

In the formula, a _iAnd b _iBe the filter function constant coefficient, z _iBe zeros of transfer functions, p _iBe the limit of transfer function, M is the number at zero point, and N is the limit number, and z is Z-transformation operator z ^-iThe i power of the inverse of expression z.

3. modular multilevel current transformer circulation inhibition method according to claim 1, it is characterized in that the transfer function of ratio resonant controller is: the transfer function of ratio resonant controller is:

In the formula, K _pBe proportional coefficient, K _rBe resonance item coefficient, ω ₀Be resonance frequency, ω _cBe cut-off angular frequency.

4. modular multilevel current transformer circulation inhibition method according to claim 1 is characterized in that ω ₀=100 π.

5. modular multilevel current transformer circulation inhibition method according to claim 1 is characterized in that τ ₀=0.2.

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