CN105071403A - Reactive compensation device based on double H-bridge modular multilevel topology and control method - Google Patents

Abstract

The invention discloses a reactive compensation device based on a double H-bridge modular multilevel topology and a control method of the reactive compensation device, relates to a multi-level reactive compensation device with high voltage and high power and a control method of the multi-level reactive compensation device, and aims at solving the problems that a converter in the prior art is small in output level number and poor in circulating-current restraining effect. The reactive compensation device comprises three-phase alternating current power supply, load, a modular multilevel converter (MMC), a signal collection circuit, a control circuit and a drive circuit, wherein the MMC converter comprises three bridge arms which are the same in structure and are connected with one another in parallel; each bridge arm comprises an upper bridge arm and a lower bridge arm, which are symmetrical about the middle pint of the bridge arm and are connected with each other in series; each upper bridge arm comprises an electric reactor, a first H-bridge unit, a second H-bridge unit and a plurality of half-bridge units, which are connected with one another in series; the electric reactor of each upper bridge arm is connected with the electric reactor of each lower bridge arm in series; and the middle points of three bridge arms of the converter are connected between the three-phase alternating current power supply and the load through leads in parallel. According to the reactive compensation device, reactive compensation in the fields of high voltage and high power is realized.

Description

Based on reactive power compensator and the control method of dual H-bridge modular multilevel topology

Technical field

The present invention relates to a kind of novel reactive power compensator and control method thereof, be specifically related to the reactive power compensator based on dual H-bridge modular multilevel topology and control method thereof, belong to high-power, many level reactive power compensation technology field.

Background technology

Along with the development of electric power system scale, be proposed new requirement to the capacity of reactive power compensator and electric pressure, make it constantly to the development of high-power field, many level topological structure arises at the historic moment.

The basic thought of multilevel converter approaches Sine Modulated waveform with multiple output level number.More common many level topological structure has three kinds: diode clamp type, striding capacitance type and H bridge cascade connection type.Diode clamp type and striding capacitance type structure, along with the increase of level number, required switching device and clamp capacitor quantity increase greatly, be unfavorable for the translation circuit realizing more high level, and capacitance voltage are not easily balanced, applies and is restricted.H bridge cascade structure, when the current-unbalance that three-phase exports, can not transmit meritorious energy between brachium pontis, be difficult to the capacitor voltage balance realized between three-phase module.

Summary of the invention

The object of the invention is to solve the low and problem that loop current suppression effect is bad of prior art converter output level number.

Technical scheme of the present invention is: based on the reactive power compensator of dual H-bridge modular multilevel topology, comprise three-phase alternating-current supply, load, MMC converter, signal deteching circuit, DSP module and drive circuit, described converter comprises three identical brachium pontis be connected in parallel of structure, and each brachium pontis comprises about point symmetry in brachium pontis and the upper brachium pontis be connected in series and lower brachium pontis, and described upper brachium pontis comprises the resistance of series connection mutually, reactor, first H-bridge unit, second H-bridge unit and some half-bridge cells, the reactor of upper brachium pontis and the reactor of lower brachium pontis are connected in series, the mid point of converter three brachium pontis is connected between three-phase alternating-current supply and load by conductor in parallel, and the input of signal deteching circuit connects the output of three AC power respectively, the input of load, the output of converter, converter three brachium pontis, first H-bridge unit, second H-bridge unit and each half-bridge cells, the output of signal deteching circuit connects the input of DSP module, and the output of DSP module is connected by drive circuit and converter.

Described DSP module comprises the first control unit and the second control unit, and the output of the first control unit and the output of the second control unit connect with drive circuit respectively, and described first control unit comprises the first comparator, second comparator, 3rd comparator, 4th comparator, 5th comparator, one PI controller, 2nd PI controller, 3rd PI controller, first coordinate converter, second coordinate converter, first reactor, second reactor, first modulating unit and capacitance voltage sequencing unit, the first comparator, one PI controller, second comparator, 2nd PI controller and the 3rd comparator access the first coordinate converter after connecting successively, the 4th comparator, 3rd PI controller and the 5th comparator access the first coordinate converter after connecting successively, the output of the first coordinate converter connects the first modulating unit, the output of the first modulating unit connects capacitance voltage sequencing unit, the output of capacitance voltage sequencing unit connects drive circuit, the output of described converter connects the second coordinate converter, first output of the second coordinate converter connects the second comparator and the second reactor respectively, the output of the second reactor connects the 3rd comparator, another output of second coordinate converter connects the 4th comparator and the first reactor respectively, and the output of the first reactor connects the 5th comparator.

Second control unit comprises the 6th comparator, 7th comparator, 8th comparator, 9th comparator, first proportional controller, second proportional controller, 4th PI controller, 5th PI controller, function module and the second modulating unit, 6th comparator, first proportional controller, 7th comparator, 4th PI controller and the second proportional controller access the 9th comparator after connecting successively, 8th comparator, 5th PI controller sum functions module accesses the 9th comparator after connecting successively, the output of the 9th comparator connects the second modulating unit, the output of the second modulating unit connects drive circuit.

The described reactive power compensator based on dual H-bridge modular multilevel topology comprises voltage zero-crossing detection circuit, described voltage zero-crossing detection circuit comprises voltage sensor, comparison circuit and inverter, the input of voltage sensor connects the output of three-phase alternating-current supply, the output of voltage sensor connects the input of comparison circuit, and the output of comparison circuit accesses DSP module by inverter circuit.Although the voltage of general electrical network is power frequency 50Hz, the phenomenon fluctuated up and down also can be there is, so need to utilize voltage zero-crossing detection circuit to follow the tracks of mains frequency, the electric voltage frequency making SVG AC produce is consistent with mains frequency.

The described reactive power compensator based on dual H-bridge modular multilevel topology comprises current detecting and modulate circuit, described current detecting and modulate circuit comprise current sensor, optical isolation amplifier and biasing circuit, the output of current sensor connects optical isolation amplifier, the output of optical isolation amplifier connects biasing circuit, biasing circuit output is the output of current detecting and modulate circuit, can reduce the antijamming capability of the delayed and raising detection signal that sampling element causes as far as possible.

The described reactive power compensator based on dual H-bridge modular multilevel topology comprises current foldback circuit; current foldback circuit comprises comparator and catching diode circuit; the inverting input of comparator and the output of signal acquisition circuit connect; the output of comparator connects through the mid point of clamp circuit and DSP module, interrupts PDPINTA realize current protection process by the power drive protection of TMS320F2812.

The control method of the described reactive power compensator based on dual H-bridge modular multilevel topology, comprises the i adopted based on instantaneous reactive power theory _p-i _qelectric current testing detects electric current; Half-bridge cells, the first H-bridge unit and the second H-bridge unit are controlled respectively;

The described control to half-bridge cells comprises the compensation of System Reactive Power and the stable of maintenance half-bridge cells capacitance voltage, according to voltage, current double closed-loop control method generation modulation signal based on closed loop decoupling zero, adopt the capacitance voltage ranking method based on phase-shifting carrier wave that the capacitance voltage of half-bridge cells is stablized;

The described control to the first H-bridge unit comprises and utilizes the first H-bridge unit to increase level number, and maintains the stable of the first H-bridge unit capacitance voltage;

The described control to the second H-bridge unit comprises the suppression of the second H-bridge unit capacitance voltage equilibrium and whole reactive power compensator circulation.

Capacitance voltage ranking method based on phase-shifting carrier wave comprises:

(1) determination of level number need be dropped into

First, utilize phase-shifting carrier wave modulation technique to be compared by the phase shift triangular carrier of the modulating wave obtained through feed forward decoupling control and each brachium pontis, obtain the level number N needed, described phase shift triangular carrier is produced by the first modulating unit;

Then, judging obtaining level number N, obtaining new level number k; Corresponding k half-bridge cells is selected to drop into according to the flow direction of each brachium pontis half-bridge cells capacitance voltage height ordering scenario and bridge arm current again;

(2) step of half-bridge cells capacitance voltage ranking method

According to bridge arm current i _armto carry out judgement as follows in direction:

Work as i _armduring >0, then drop into k minimum half-bridge cells of capacitance voltage according to half-bridge cells capacitance voltage ranking results;

Work as i _armduring <0, then drop into k the highest half-bridge cells of capacitance voltage according to half-bridge cells capacitance voltage ranking results.

The method utilizing the first H-bridge unit to increase level number comprises:

Utilize phase-shifting carrier wave technology to approach modulation signal, obtain level number N, then judge;

If level number N is original 2n+1 level, then the first H-bridge unit does not need to put into operation, the level number k=N that half-bridge cells needs, and obtains the control signal of half-bridge cells according to level number k;

If level number is 2n new level, then the first H-bridge unit puts into operation, and the control signal of the first H-bridge unit is obtained by the first H-bridge unit control module, determine new level number k simultaneously, then obtained the control signal of half-bridge cells unit according to new level number k by half-bridge cells control module.

Because the input of the first H-bridge unit and excision will coordinate input and the excision of the half-bridge cells of series connection, the control procedure of the first H-bridge unit is comprised: the electric current of brachium pontis is crossed in first convection current and DC capacitor voltage detects, size according to the DC capacitor voltage of the sense of current and current first H-bridge unit obtains charging and discharging state needed for the first H-bridge unit, obtain output voltage state according to charging and discharging state and the sense of current again, and then obtain the drive singal of the first H-bridge unit.

Described second H-bridge unit capacitance voltage balancing procedure comprises: the set-point V of each mutually upper and lower brachium pontis second H-bridge unit capacitance voltage _{h, ref}compare with the actual capacitance voltage of the second H-bridge unit, through PI controller, after it exports and is multiplied by the sign function of this bridge arm current, by the second H-bridge unit voltage signal V generated _{r, ref}with obtain PWM ripple after triangular carrier, drive corresponding power switch pipe in the second H-bridge unit, charge and discharge control carried out to the electric capacity of the second H-bridge unit, realizes the equilibrium of the second H-bridge unit capacitance voltage, wherein, r=P, N.

Described loop current suppression process comprises: compared with the reference value of circulation respectively by each phase circulation, now the reference value i of circulation _{cir, ref}=i _dc/ 3, i _dcfor three-phase MMC DC bus current, the result obtained forms the voltage reference value of second H-bridge unit by a proportional controller, and this voltage reference value is divided into 2 parts, and the voltage signal of brachium pontis second H-bridge unit mutually upper and lower with this is added respectively.

The present invention compared with prior art has following effect: the invention provides a kind of reactive power compensator that can be applied to high-power field, advantage is that the topological structure of MMC converter has common DC bus, three alternate energy can flow mutually, also can normally run during system imbalance of three-phase voltage, therefore, the STATCOM based on MMC converter can realize reactive power, harmonic wave and unbalanced comprehensive compensation.The basis of former MMC system adds 2 H-bridge unit in every mutually upper and lower brachium pontis, wherein half-bridge cells is for controlling the first-harmonic load current in brachium pontis, first H-bridge unit can realize being multiplied to 4n+1 to output level number, the second H-bridge unit as voltage correction module for suppressing circulation.The loop current suppression device control program of novel MMC topological structure of the present invention is simple, the negative phase-sequence coordinate transform of two frequencys multiplication need not be carried out, the software resource taken is fewer comparatively speaking, and load current and circulation are controlled by half-bridge cells and H-bridge unit respectively, are independent of each other; Described a kind of capacitance voltage ranking method based on phase-shifting carrier wave, simple and be easy to realize, not only make half-bridge cells capacitance voltage keep stable.And the switching that half-bridge cells is unnecessary can be avoided, reduce the on-off times of power tube, reduce switching loss.

Accompanying drawing explanation

Fig. 1, entire system block diagram;

Fig. 2, traditional MMC topological structure schematic diagram;

Fig. 3, novel MMC topological structure schematic diagram;

Fig. 4, the determination flow chart of level number need be dropped into;

The control block diagram of Fig. 5, the first H-bridge unit;

The working state schematic representation of Fig. 6, half-bridge cells;

Fig. 7, H-bridge unit structural representation;

Fig. 8, i _p-i _qdetection method schematic diagram;

Fig. 9, meritorious, reactive current control block diagram;

Figure 10, Feedforward Decoupling equivalent control block diagram;

The control block diagram of Figure 11, STATCOM voltage and current double closed-loop;

Figure 12, STATCOM system main-control block diagram;

Figure 13, capacitance voltage ranking method flow chart;

The modulation strategy block diagram of Figure 14, novel topological structure;

The equivalent model schematic diagram of Figure 15, Three phase MMC topological structure;

The fundamental diagram of Figure 16, the second control unit;

Figure 17, voltage zero-crossing detection circuit schematic diagram;

Figure 18, current detecting and modulate circuit circuit diagram thereof;

Figure 19, current foldback circuit circuit diagram;

A phase voltage, current waveform figure before Figure 20, compensation;

A phase voltage, current waveform figure after Figure 21, compensation;

Figure 22, half-bridge cells capacitance voltage oscillogram;

Figure 23, conventional topologies a phase export phase voltage waveform figure;

Figure 24, New Topological a phase export phase voltage waveform figure;

Three phase circulation oscillograms before and after Figure 25, suppression;

A phase bridge arm current oscillogram before and after Figure 26, loop current suppression;

The capacitance voltage oscillogram of Figure 27, a phase second H-bridge unit.

Embodiment

Accompanying drawings the specific embodiment of the present invention, based on the reactive power compensator of dual H-bridge modular multilevel topology, comprise three-phase alternating-current supply, load, converter, signal acquisition circuit, DSP module and drive circuit, described converter comprises three identical brachium pontis be connected in parallel of structure, each brachium pontis comprises about point symmetry in brachium pontis and the upper brachium pontis be connected in series and lower brachium pontis, described upper brachium pontis comprises the resistance of series connection mutually, reactor, first H-bridge unit, second H-bridge unit and some half-bridge cells, resistance on described first brachium pontis is respectively R11 and R12, resistance on second brachium pontis is respectively R21 and R22, resistance on 3rd brachium pontis is respectively R31 and R32, the reactor of upper brachium pontis and the reactor of lower brachium pontis are connected in series, the mid point of converter three brachium pontis is connected between three-phase alternating-current supply and load by conductor in parallel, the input of signal acquisition circuit connects the output of three AC power respectively, the input of load, the output of converter, converter three brachium pontis, first H-bridge unit, second H-bridge unit and each half-bridge cells, the output of signal acquisition circuit connects the input of DSP module, the output of DSP module is connected by drive circuit and converter.Three phase feedback currents that signal acquisition circuit detection of grid three-phase voltage, load-side three-phase current, MMC export, each half-bridge cells and the capacitance voltage of H-bridge unit DC side and the bridge arm current of three-phase; Then, the feedback quantity of detection is carried out computing and adjustment in a control unit, obtains pwm control signal; Finally, control signal is carried out power amplifier to drive power switch pipe in MMC half-bridge cells and H-bridge unit, make converter export corresponding offset current, realize reactive power compensation.

The topological structure of the novel MMC modularization multi-level converter of present embodiment as shown in Figure 3, there is public DC bus, three alternate energy can flow mutually, when electrical network distorts, reactive power, harmonic wave and unbalanced comprehensive compensation can be realized, high modularization, be easy to Redundancy Design, and export as many level, close to sine wave, harmonic content is little.The every each n of the mutually upper and lower brachium pontis half-bridge cells of traditional MMC, as shown in Figure 2, it exports phase voltage level number is 2n+1 to its structure, and level number can be brought up to 4n+1 by novel topological structure that the present invention carries.

SM is half-bridge cells, and as shown in Figure 2, each half-bridge cells is made up of two IGBT with reverse fly-wheel diode and 1 storage capacitor C its structure, and bridge arm current is i _sm, the output voltage of each half-bridge cells is u _sm, each half-bridge cells capacitance voltage is V _d.

Each half-bridge cells can only export 0 and V _dtwo kinds of voltage statuss, use S ₁and S ₂represent T respectively ₁and T ₂on off state, DC capacitor voltage is V _d.The output voltage that the corresponding half-bridge cells of the different on off state of switching tube is different and capacitor charge and discharge state, as shown in table 1.

Table 1 half-bridge cells different on off state corresponding states table

As shown in Figure 6, in figure, arrow shows the flow direction of electric current to the half-bridge cells operating state of MMC.Half-bridge cells has three kinds of operating states:

1) T ₁(D ₁) open, T ₂(D ₂) turn off as input state, as Fig. 6 (a);

2) T ₁(D ₁) turn off, T ₂(D ₂) open as excision state, as Fig. 6 (b);

3) T ₁and T ₂all turn off as blocking, as Fig. 6 (c);

If s _ifor the switch function of MMC half-bridge cells, can be expressed as

The then equivalent output voltage u of each half-bridge cells _ocan be expressed as

u _o＝s _iV _d(2)

The HB1 unit of H-bridge unit is called the first H-bridge unit, and the HB2 unit of H-bridge unit is called the second H-bridge unit.First H-bridge unit and the second H-bridge unit have 3 kinds of on off states, s _j{ value of-1,0,1}, sj determines the polarity of H-bridge unit output voltage to ∈.First H-bridge unit and the second H-bridge unit are all H-bridge unit, its structure chart as shown in Figure 7, wherein u _hfor H-bridge unit output voltage; Use S ₁, S ₂, S ₃and S ₄represent 4 switch transistor T respectively ₁, T ₂, T ₃and T ₄on off state, DC capacitor voltage is V _h.The output voltage that the different corresponding H-bridge unit of on off state of 4 switching tubes is different and capacitor charge and discharge state, as shown in table 2.

Table 2H bridge unit different on off state corresponding states table

Described DSP module comprises the first control unit and the second control unit, and the output of the first control unit and the output of the second control unit connect with drive circuit respectively, and described first control unit comprises the first comparator 1, second comparator 3, 3rd comparator 5, 4th comparator 7, 5th comparator 9, one PI controller 2, 2nd PI controller 4, 3rd PI controller 8, first coordinate converter 6, second coordinate converter 12, first reactor 10, second reactor 11, first modulating unit 30 and capacitance voltage sequencing unit 32, first comparator 1, one PI controller 2, second comparator 3, 2nd PI controller 4 and the 3rd comparator 5 access the first coordinate converter, the 4th comparator 7 after connecting successively, 3rd PI controller 8 and the 5th comparator 9 access the first coordinate converter 6 after connecting successively, the output of the first coordinate converter 6 connects the first modulating unit 30, the output of the first modulating unit 30 connects capacitance voltage sequencing unit 32, the output of capacitance voltage sequencing unit 32 connects drive circuit, the output of described MMC converter connects the second coordinate converter, first output of the second coordinate converter connects the second comparator 3 and the second reactor 11 respectively, the output of the second reactor 11 connects the 3rd comparator 5, another output of second coordinate converter connects output connection the 5th comparator 9 of the 4th comparator 7 and the first reactor 10, first reactor 10 respectively.

Second control unit comprises the 6th comparator 13, 7th comparator 15, 8th comparator 18, 9th comparator 21, first proportional controller 14, second proportional controller 17, 4th PI controller 16, 5th PI controller 19, function module 20 and the second modulating unit 31, 6th comparator 13, first proportional controller 14, 7th comparator 15, 4th PI controller 16 and the second proportional controller 17 access the 9th comparator 21 after connecting successively, 8th comparator 18, 5th PI controller 19 sum functions module 20 accesses the 9th comparator 21 after connecting successively, the output of the 9th comparator 21 connects described second modulating unit 31, the output of the second modulating unit 31 connects drive circuit.

The DSP control module of present embodiment is with the TMS320F2812 of TI company for core, and realize the functions such as the collection of electric current and voltage, the generation of CPS-SPWM ripple, capacitance voltage sequence, auxiliary circuit is made up of the protective circuit etc. of Switching Power Supply and periphery.

The described reactive power compensator based on dual H-bridge modular multilevel topology comprises voltage zero-crossing detection circuit, as shown in figure 17, described voltage zero-crossing detection circuit comprises voltage sensor 22, comparison circuit 23 and inverter 24, the input of voltage sensor 22 connects the output of three-phase alternating-current supply, the output of voltage sensor 22 connects the input of comparison circuit 23, the output of comparison circuit 23 connects DSP module by inverter 24, the line voltage of sine wave is produced a rising edge by voltage zero-crossing detection circuit overlap with forward voltage zero-crossing point of power grid, and with the square-wave signal of electrical network same frequency, the cycle of line voltage can be obtained again by the time interval of measuring between adjacent two rising edges.

The described reactive power compensator based on dual H-bridge modular multilevel topology comprises current detecting and modulate circuit, as shown in figure 18, described current detecting and modulate circuit comprise current sensor 25, optical isolation amplifier 26 and biasing circuit 27, the output of current sensor 25 connects optical isolation amplifier 26, the output of optical isolation amplifier 26 connects biasing circuit 27, biasing circuit 27 output is the output of current detecting and modulate circuit, the input of current detecting and load export with converter and are connected, output is delivered to DSP and is carried out signal transacting, the present invention adopts current Hall module CHB-25NP at a high speed to realize three-phase current detection, and utilize optical isolation amplifier 26 to isolate, the model of the optical isolation amplifier that present embodiment adopts is HCPL7840.

The described reactive power compensator based on dual H-bridge modular multilevel topology comprises current foldback circuit; as shown in figure 19; described current foldback circuit comprises comparator 28 and clamp circuit 29; mid point and the DSP module of clamp circuit 29 connect; in the normal situation of electric current; the comparator LM393 of present embodiment exports high level; but when the electric current in circuit is excessive; the output level of comparator LM393 becomes low level; trigger fault protection is interrupted; thus make DSP block the output of all pwm pulse signals, to protect whole SVG system.

Based on the control method of the reactive power compensator of dual H-bridge modular multilevel topology, specifically comprise: adopt the i based on instantaneous reactive power theory _p-i _qelectric current testing detects electric current; Current detecting schematic diagram as shown in Figure 8, first needs to calculate instantaneous active current i _pwith instantaneous reactive current i _q:

$\left[\begin{array}{c}{i}_p\\ i_q\end{array}\right]=\left[\begin{array}{cc}sin\mathrm{\&omega;}t& -cos\mathrm{\&omega;}t\\ -cos\mathrm{\&omega;}t& -sin\mathrm{\&omega;}t\end{array}\right]\left[\begin{array}{c}{i}_{\mathrm{\&alpha;}}\\ i_{\mathrm{\&beta;}}\end{array}\right]=C\left[\begin{array}{c}{i}_{\mathrm{\&alpha;}}\\ i_{\mathrm{\&beta;}}\end{array}\right]={\mathrm{CC}}_{32}\left[\begin{array}{c}{i}_a\\ i_b\\ i_c\end{array}\right]---\left(3\right)$

Wherein: $C=\left[\begin{array}{cc}sin\mathrm{\&omega;}t& -cos\mathrm{\&omega;}t\\ -cos\mathrm{\&omega;}t& -sin\mathrm{\&omega;}t\end{array}\right],C_{32}=\sqrt{\frac{2}{3}}\left[\begin{array}{ccc}1& -\frac{1}{2}& -\frac{1}{2}\\ 0& \frac{\sqrt{3}}{2}& -\frac{\sqrt{3}}{2}\end{array}\right].$

By the i that formula (3) calculates _p, i _qafter low pass filter, obtain DC component three-phase fundamental current i is obtained again through inverse transformation _af, i _bf, and i _cf:

$\left[\begin{array}{c}{i}_{af}\\ i_{bf}\\ i_{cf}\end{array}\right]=C_{23}{C}^{-1}\left[\begin{array}{c}\stackrel{\&OverBar;}{i_p}\\ \stackrel{\&OverBar;}{i_q}\end{array}\right]---\left(4\right)$

Use three-phase current i _a, i _b, i _cdeduct corresponding three-phase first-harmonic component i _af, i _bf, i _cf, the harmonic wave of three-phase current and idle composition sum i can be obtained _ah, i _bh, i _ch.

Control method based on the reactive power compensator of dual H-bridge modular multilevel topology comprises and controlling respectively half-bridge cells, the first H-bridge unit and the second H-bridge unit;

The described control to half-bridge cells comprises the compensation of System Reactive Power and the stable of maintenance half-bridge cells capacitance voltage, according to voltage, current double closed-loop control method generation modulation signal based on Feedforward Decoupling, adopt the capacitance voltage ranking method based on phase-shifting carrier wave that the capacitance voltage of half-bridge cells is stablized;

Described Feedforward Decoupling process comprises:

The dq component v of line voltage and STATCOM output voltage is obtained by coordinate transform _sd, v _sq, v _cd, v _cqas follows:

$\left[\begin{array}{c}{v}_{sd}\\ v_{sq}\end{array}\right]=\sqrt{3}{U}_s\left[\begin{array}{c}1\\ 0\end{array}\right],\left[\begin{array}{c}{v}_{cd}\\ v_{cq}\end{array}\right]=\frac{\sqrt{3}}{2}{\mathrm{Mu}}_{dc}\left[\begin{array}{c}1\\ 0\end{array}\right]\left[\begin{array}{c}cos\mathrm{\&delta;}\\ sin\mathrm{\&delta;}\end{array}\right]---\left(5\right)$

Wherein: δ is the phase difference of STATCOM output voltage and line voltage, M is modulation ratio, and Us is line voltage, and udc is DC capacitor voltage.

By the component V of the output voltage of STATCOM under dq coordinate system _cdand V _cqas controlled quentity controlled variable, then by formula can find out, by control v _cdand v _cqsize just can regulate the exchange of STATCOM and electric network active and reactive power, thus reach the object of reactive power compensation.

Inverter output voltage v _cdand v _cqexpression formula be

$\left[\begin{array}{c}{v}_{cd}\\ v_{cq}\end{array}\right]=\left[\begin{array}{c}{v}_{sd}\\ v_{sq}\end{array}\right]+\left[\begin{array}{cc}-R& \mathrm{\&omega;}L\\ -\mathrm{\&omega;}L& -R\end{array}\right]\left[\begin{array}{c}{i}_d\\ i_q\end{array}\right]-L\frac{d}{dt}\left[\begin{array}{c}{i}_d\\ i_q\end{array}\right]---\left(6\right)$

Gained merit according to formula (6), reactive current control block diagram as shown in Figure 9.In order to realize dq decoupler shaft, the present invention takes feed forward decoupling control strategy.Wherein, the loss of whole current transformer is equivalent to fixed resistance R, linked reactor and line inductance are equivalent to inductance L.

Introduce x in the middle of variable ₁, x ₂:

$\left\{\begin{array}{c}{x}_1=v_{sd}-v_{cd}+{\mathrm{\&omega;Li}}_q\\ x_2=v_{sq}-v_{cq}-{\mathrm{\&omega;Li}}_d\end{array}\right.---\left(7\right)$

Obtained by formula (6), (7)

$\left\{\begin{array}{c}{x}_1=L\frac{{\mathrm{di}}_d}{dt}+{\mathrm{Ri}}_d\\ x_2=L\frac{{\mathrm{di}}_q}{dt}+{\mathrm{Ri}}_q\end{array}\right.---\left(8\right)$

By x ₁, x ₂be designed to pi controller, can obtain

$\left\{\begin{array}{c}{x}_1=k_1(i_d*-i_d)+\frac{k_1}{T_1}\&Integral;(i_d*-i_d)dt\\ x_2=k_2(i_q*-i_q)+\frac{k_2}{T_2}\&Integral;(i_q*-i_q)dt\end{array}\right.---\left(9\right)$

Then can obtain Feedforward Decoupling equivalent control block diagram as shown in Figure 10, by the given electric current under dq coordinate system and feedback current poor, export 2 intermediate variable x through 2 PI controllers ₁, x ₂, thus decoupling zero is realized under dq coordinate system, finally obtain the two close cycles STATCOM system architecture diagram based on Feedforward Decoupling as shown in Figure 10, the k in formula (9) ₁, k ₂for proportionality coefficient, T ₁, T ₂for the time of integration.

As seen from Figure 11, STATCOM system is made up of outer voltage and current inner loop 2 part, and Voltage loop is to regulate DC capacitor voltage, and its result is as given value of current value i _d*; Electric current loop process comprises the reactive current that need record load-side, is set to specified rate i _q*, inverter send electric current through dq conversion draw current feedback amount again with i _d*, i _q* compare, then carry out PI adjustment, finally obtain the voltage v that STATCOM wants to export _cd, v _cq;

The described control to the first H-bridge unit comprises and utilizes the first H-bridge unit to increase level number, and maintains the stable of the first H-bridge unit capacitance voltage;

The control method of the first H-bridge unit of the Double closed-loop of voltage and current strategy based on Feedforward Decoupling, the sequence control methods of the voltage based on phase-shifting carrier wave and increase level number is combined, forms STATCOM idle compensating control, be called the first control unit; The control of loop current suppression second H-bridge unit to selfcapacity voltage and suppression system circulation is called the second control unit.So can based on the STSTCOM system main-control strategy of New Topological as shown in figure 12.

For traditional MMC, when the half-bridge cells number of every mutually upper and lower brachium pontis is respectively n, coordinate phase-shifting carrier wave modulation technique, the output voltage maximum level number of its AC is 2n+1 simultaneously.

In novel MMC, respectively add first H-bridge unit in each brachium pontis, its DC capacitor voltage is the half of half-bridge cells DC capacitor voltage, is V _d/ 2, its output level number is just 3 like this, is respectively+V _d/ 2,0 and-V _d/ 2, the half-bridge cells coordinating each brachium pontis to connect just can make output level number be widened, and namely in the middle of original 2n+1 level number, inserts a new 2n level number, makes level number reach 4n+1.

The multiple half-bridge cells of each brachium pontis meeting cascade in modular multilevel topology, but the DC capacitor voltage of each half-bridge cells is separate, in practice due to capacitance fluctuations differential loss, switching loss difference and drive singal difference etc., imbalance between the DC capacitor voltage that can cause half-bridge cells, affects the output voltage of modular multilevel structure.In order to maintain the equilibrium of half-bridge cells capacitance voltage, the present invention adopts the half-bridge cells capacitance voltage sequence control strategy based on CPS-SPWM, thus makes half-bridge cells capacitance voltage keep stable.

Half-bridge cells capacitance voltage sequence control strategy based on CPS-SPWM is effectively combined CPS-SPWM modulation technique and half-bridge cells capacitance voltage ranking method, and its process comprises:

(1) need drop into the determination of level number, its process as shown in Figure 4

First, utilize phase-shifting carrier wave modulation technique to be compared by the phase shift triangular carrier of the modulating wave obtained through feed forward decoupling control and each brachium pontis, obtain the level number N needed, described phase shift triangular carrier is produced by capacitance voltage sequencing unit;

Then, judging obtaining level number N, obtaining new level number k; Corresponding k half-bridge cells is selected to drop into according to the flow direction of each brachium pontis half-bridge cells capacitance voltage height ordering scenario and bridge arm current again;

(2) half-bridge cells capacitance voltage ranking method, flow chart as shown in figure 13:

According to each bridge arm current i _armdirection judge, be specially:

Work as i _armduring >0, namely bridge arm current is to the charging of half-bridge cell capacitance, then drop into k minimum half-bridge cells of capacitance voltage according to half-bridge cells capacitance voltage ranking results, namely bridge arm current makes its voltage raise to this k half-bridge cells capacitor charging.

Work as i _armduring <0, namely bridge arm current is to the electric discharge of half-bridge cell capacitance, then drop into k the highest half-bridge cells of capacitance voltage according to half-bridge cells capacitance voltage ranking results, namely bridge arm current makes its voltage reduce to this k half-bridge cells capacitor discharge.So just achieve the balance of half-bridge cells capacitance voltage.

Add the control strategy block diagram of the novel MMC of the first H-bridge unit as shown in figure 14:

(1) first utilize phase-shifting carrier wave technology to approach modulation signal, obtain level number N, then level number is judged.

If level number N is original 2n+1 level, then the first H-bridge unit does not need to put into operation, the level number k=N that half-bridge cells needs, and obtains the control signal of half-bridge cells according to level number k;

If level number is 2n new level, then the first H-bridge unit puts into operation, and obtains the control signal of the first H-bridge unit by the first H-bridge unit control module, determines new level number k=N-sign (u simultaneously _h), then obtained the control signal of cascade half-bridge cells by half-bridge cells control module according to level number k.

(2) because the input of the first H-bridge unit and excision will coordinate input and the excision of the half-bridge cells of series connection, so the control method flow chart of the first H-bridge unit as shown in Figure 5: the electric current of brachium pontis is crossed in first convection current and DC capacitor voltage detects, size according to the DC capacitor voltage of the sense of current and current first H-bridge unit obtains charging and discharging state needed for module, obtain output voltage state according to charging and discharging state and the sense of current again, and then obtain the drive singal of the first H-bridge unit.Table 3 is the first H-bridge unit output voltage condition judgement table.

Table 3 first H-bridge unit output voltage condition judgement table

The described control to the second H-bridge unit comprises the inhibitory control of the circulation of the second H-bridge unit capacitance voltage equilibrium and whole reactive power compensator, is specially:

As shown in figure 15, wherein, DC bus current is i to the equivalent model of Three phase MMC power topology _dc, on three-phase, bridge arm current is respectively i _aP, i _bP, i _cP, under three-phase, bridge arm current is respectively i _aN, i _bN, i _cN, three-phase output current is respectively i _a, i _b, i _c.

Below for a phase, analyze the operation principle of novel MMC topological structure, according to KCL Circuit theory, a phase output current can be expressed as

i _a＝i _aP-i _aN(10)

If the circulation of a phase brachium pontis is i _{cir, a}, because the circuit structure of upper and lower brachium pontis is identical, then have

$i_{aP}=i_{cir,a}+\frac{i_a}{2}---\left(11\right)$

$i_{aN}=i_{cir,a}-\frac{i_a}{2}---\left(12\right)$

Formula (11) and (12) are added, obtain

$i_{cir,a}=\frac{1}{2}(i_{aP}+i_{aN})---\left(13\right)$

The current i of three-phase MMC DC bus _dcfor a, b, c tri-phase circulation sum, namely

i _dc＝i _cir,a+i _cir,b+i _cir,c(14)

Due to three-phase symmetrical, three phase circulations can be expressed as

$i_{cir,j}=\frac{i_{dc}}{3}+{i_{zj}}^{*}---\left(15\right)$

In formula, i _zj ^*two frequency multiplication negative phase-sequence of acs in circulation, wherein, j=a, b, c, convolution (11), (12) can obtain with (15):

$i_{aP}=\frac{i_{dc}}{3}+\frac{i_a}{2}+{i_{zj}}^{*}---\left(16\right)$

$i_{aN}=\frac{i_{dc}}{3}-\frac{i_a}{2}+{i_{zj}}^{*}---\left(17\right)$

Composite type (16) and (17), the two frequency multiplication negative phase-sequence alternating components that can obtain a phase circulation are

${i_{zj}}^{*}=\frac{1}{2}(i_{aP}+i_{aN})-\frac{i_{dc}}{3}---\left(18\right)$

The power model of MMC system can be equivalent to controlled voltage source V _jrwherein, j=a, b, c; R=P, N, then bridge output voltage V in a phase _aPwith the output voltage V of lower brachium pontis _aNcan be expressed as:

$V_{aP}=\underset{i=1}{\overset{N}{\&Sigma;}}{s}_j{V}_d---\left(19\right)$

$V_{aN}=\underset{i=N+1}{\overset{2N}{\&Sigma;}}{s}_j{V}_d---\left(20\right)$

In the equivalent model of Figure 15, the output voltage of the first H-bridge unit and the second H-bridge unit is for being respectively V _{h1, jr}and V _{h2, jr}, with the mid point of DC bus-bar voltage for reference, the three-phase voltage that MMC system exports is V _j, the resistance of each brachium pontis is Re, according to KVL Circuit theory, can obtain

$\frac{U_d}{2}-V_{aP}-V_{H1,aP}-V_{H2,aP}-V_a=R_e{i}_{aP}+L\frac{{\mathrm{di}}_{aP}}{dt}---\left(21\right)$

$\frac{U_d}{2}-V_{aN}-V_{H1,aN}-V_{H2,aN}+V_a=R_e{i}_{aN}+L\frac{{\mathrm{di}}_{aN}}{dt}---\left(22\right)$

Formula (21) and (22) are added, then convolution (11) is to (13), can obtain:

$2L\frac{{\mathrm{di}}_{cir,a}}{dt}+2R_e{i}_{cir,a}=(U_d-V_{aP}-V_{aN})-(V_{H1,aP}+V_{H1,aN})-(V_{H2,aP}+V_{H2,aN})---\left(23\right)$

As can be seen from formula (23), can by the output voltage sum (V of control two the second H-bridge unit _{h2, aP}+ V _{h2, aN}) size and voltage difference (U _d-V _aP-V _aN) equal thus reach and eliminate the object of circulation.

Formula (22) is deducted formula (21), can obtain

$V_a=\frac{1}{2}(V_{aN}-V_{aP})+\frac{1}{2}(V_{H1,aN}-V_{H1,aP})+\frac{1}{2}(V_{H2,aN}-V_{H2,aP})+\frac{R_e}{2}(i_{aN}-i_{aP})+\frac{L}{2}(\frac{{\mathrm{di}}_{aN}}{dt}-\frac{{\mathrm{di}}_{aP}}{dt})---\left(24\right)$

From formula (24), the output voltage of the second H-bridge unit on MMC does not almost affect, and reason has two: first, the output voltage grade of the second H-bridge unit relative to MMC system output voltage be very little; Secondly, it is equal for controlling the voltage that the second H-bridge unit inserts each mutually upper and lower brachium pontis, therefore (V in formula (24) _{h2, aN}-V _{h2, aP}) this can be similar to and regard 0 as, the not output voltage of influential system.From formula (24), by adopting rational control strategy, the output voltage controlling the second H-bridge unit just can suppress circulation.

Figure 16 is the schematic diagram of novel MMC system a phase second H-bridge unit controller, achieves the Balance route to H-bridge unit capacitance voltage while suppressing circulation.

As seen from Figure 16, by each phase circulation and circulation reference value i _{cir, ref}=i _dc/ 3 compare, the result passing ratio obtained regulates the voltage reference value of formation second H-bridge unit, this reference value is divided into 2 parts, be added in the Voltage Reference of this mutually upper and lower brachium pontis second H-bridge unit respectively, like this, also the impact of the second H-bridge unit on MMC output voltage is dropped to minimum while elimination circulation.

Second control unit also will maintain the equilibrium of the second H-bridge unit capacitance voltage while suppressing circulation.The set-point V of each mutually upper and lower brachium pontis second H-bridge unit capacitance voltage _{h, ref}compare with the actual capacitance voltage of the second H-bridge unit, through PI controller, it exports the sign function being multiplied by this bridge arm current, if bridge arm current is greater than 0, then symbol function is+1; If bridge arm current is less than 0, then symbol function is-1.Finally, the voltage signal V will generated _{r, ref}with obtain PWM ripple after triangular carrier, wherein r=P, N; Drive corresponding power switch pipe in the second H-bridge unit, charge and discharge control is carried out to the electric capacity of the second H-bridge unit, to realize the balance of capacitance voltage.

Control the switching frequency that the second H-bridge unit switching frequency of circulation is higher than power model, like this, the bandwidth of circulation controller namely the second control unit higher than the first control unit, thus restrained effectively the negative phase-sequence circulation being mainly two frequencys multiplication.Because in the second control unit, the capacitance voltage rated value of the second H-bridge unit is lower, its switching loss also can not be very large under higher switching frequency.

To System Reactive Power compensation effect, carried out simulating, verifying, Figure 20 be power network compensation before a phase voltage, current waveform.As can be seen from the figure, compensate before electric current obviously lag behind voltage, Figure 21 be compensate after a phase voltage and current waveform, the phase place of phase voltage, electric current is consistent.Visible the present invention has good compensation effect to idle.

Half-bridge cell capacitance electric voltage equalization effect is analyzed, Figure 22 is the capacitance voltage of half-bridge cells in the upper and lower brachium pontis of a phase, can see, half-bridge cells capacitance voltage is all stabilized in about 1000V, fluctuation, at about 10V, illustrates that the half-bridge cells capacitance voltage ranking method of CPS-SPWM has the equal pressure energy power of good half-bridge cells to STATCOM system.

Carry out simulation analysis to system output voltage, when the half-bridge cells number that each brachium pontis is connected is 2, the STATCOM based on conventional topologies structure MMC exports a phase voltage as shown in figure 24, is 5 level; And be 9 level based on a phase output voltage of the STATCOM of novel topological structure MMC, as shown in figure 24.Therefore, a capacitance voltage of connecting in conventional topologies structure MMC is the first H-bridge unit of half-bridge cells voltage half, and the novel MMC structure obtained achieves the increase of output-voltage levels number, the correctness of checking novel topological structure and feasibility.

Simulation analysis is carried out to the loop current suppression ability of system, in the MMC of New Topological, add the second H-bridge unit and carry out loop current suppression, Figure 25 gives and adds three phase circulation waveforms before and after the second H-bridge unit, wherein: Figure 25 (a) is three phase circulation waveforms before not adding the second H-bridge unit, can see that three phase circulations are about 15A.Figure 25 (b), for adding three phase circulation waveforms after the second H-bridge unit, can see that three phase circulations obtain effective suppression, within being reduced to 2A.Therefore propose to add the second H-bridge unit topology MMC herein and there is suppression circulation ability preferably.

Simulation analysis is carried out to bridge arm current, before and after loop current suppression, the contrast of a phase bridge arm current waveform as shown in figure 26, Figure 26 (a) is the electric current of the upper and lower brachium pontis of a phase before loop current suppression, before loop current suppression there is a large amount of harmonic componentss in the electric current of the upper and lower brachium pontis of a phase as we can see from the figure, and its amplitude reaches 30A; Figure 26 (b) is the current waveform of the upper and lower brachium pontis of a phase after loop current suppression, be the sinusoidal waveform that frequency is identical with first-harmonic, and amplitude is reduced to 10A.

To the second H-bridge unit capacitance voltage stabilizing ability carry out simulation analysis, Figure 27 is the capacitance voltage of a phase second H-bridge unit in New Topological, can see, the capacitance voltage value stabilization of the second H-bridge unit, at reference value 20V, proves that the second H-bridge unit also maintains the balance of self H-bridge unit capacitance voltage while suppressing circulation.

Claims (10)

1., based on the reactive power compensator of dual H-bridge modular multilevel topology, comprise three-phase alternating-current supply, load, MMC converter, signal deteching circuit, DSP module and drive circuit, described converter comprises three identical brachium pontis be connected in parallel of structure, and each brachium pontis comprises about point symmetry in brachium pontis and the upper brachium pontis be connected in series and lower brachium pontis, it is characterized in that: described upper brachium pontis comprises the resistance of series connection mutually, reactor, first H-bridge unit, second H-bridge unit and some half-bridge cells, the reactor of upper brachium pontis and the reactor of lower brachium pontis are connected in series, the mid point of converter three brachium pontis is connected between three-phase alternating-current supply and load by conductor in parallel, and the input of signal deteching circuit connects the output of three AC power respectively, the input of load, the output of converter, converter three brachium pontis, first H-bridge unit, second H-bridge unit and each half-bridge cells, the output of signal deteching circuit connects the input of DSP module, and the output of DSP module is connected by drive circuit and converter.

2. according to claim 1 based on the reactive power compensator of dual H-bridge modular multilevel topology, it is characterized in that: described DSP module comprises the first control unit and the second control unit, the output of the first control unit and the output of the second control unit connect with drive circuit respectively, described first control unit comprises the first comparator (1), second comparator (3), 3rd comparator (5), 4th comparator (7), 5th comparator (9), one PI controller (2), 2nd PI controller (4), 3rd PI controller (8), first coordinate converter (6), second coordinate converter (12), first reactor (10), second reactor (11), first modulating unit (30) and capacitance voltage sequencing unit (32), first comparator (1), one PI controller (2), second comparator (3), 2nd PI controller (4) and the 3rd comparator (5) access the first coordinate converter after connecting successively, 4th comparator (7), 3rd PI controller (8) and the 5th comparator (9) access the first coordinate converter (6) after connecting successively, the output of the first coordinate converter (6) connects the first modulating unit (30), the output of the first modulating unit (30) connects capacitance voltage sequencing unit (32), the output of capacitance voltage sequencing unit (32) connects drive circuit, the output of described MMC converter connects the second coordinate converter, first output of the second coordinate converter connects the second comparator (3) and the second reactor (11) respectively, the output of the second reactor (11) connects the 3rd comparator (5), another output of second coordinate converter connects the 4th comparator (7) and the first reactor (10) respectively, the output of the first reactor (10) connects the 5th comparator (9).

3. according to claim 2 based on the reactive power compensator of dual H-bridge modular multilevel topology, it is characterized in that: the second control unit comprises the 6th comparator (13), 7th comparator (15), 8th comparator (18), 9th comparator (21), first proportional controller (14), second proportional controller (17), 4th PI controller (16), 5th PI controller (19), function module (20) and the second modulating unit (31), the 6th comparator (13), first proportional controller (14), 7th comparator (15), 4th PI controller (16) and the second proportional controller (17) access the 9th comparator (21), the 8th comparator (18) after connecting successively, 5th PI controller (19) sum functions module (20) accesses the 9th comparator (21) after connecting successively, the output of the 9th comparator (21) connects described second modulating unit (31), and the output of the second modulating unit (31) connects drive circuit.

4. according to claim 1 based on the reactive power compensator of dual H-bridge modular multilevel topology, it is characterized in that: the described reactive power compensator based on dual H-bridge modular multilevel topology comprises voltage zero-crossing detection circuit, described voltage zero-crossing detection circuit comprises voltage sensor (22), comparison circuit (23) and inverter (24), the input of voltage sensor (22) connects the output of three-phase alternating-current supply, the output of voltage sensor (22) connects the input of comparison circuit (23), the output of comparison circuit (23) connects DSP module by inverter (24).

5. according to claim 1 based on the reactive power compensator of dual H-bridge modular multilevel topology, it is characterized in that: the described reactive power compensator based on dual H-bridge modular multilevel topology comprises current detecting and modulate circuit, described current detecting and modulate circuit comprise current sensor (25), optical isolation amplifier (26) and biasing circuit (27), the output of current sensor (25) connects optical isolation amplifier (26), the output of optical isolation amplifier (26) connects biasing circuit (27), biasing circuit (27) output is the output of current detecting and modulate circuit.

6. based on described in claim 1 based on the control method of the reactive power compensator of dual H-bridge modular multilevel topology, it is characterized in that: comprise and adopting based on the i of instantaneous reactive power theory _p-i _qelectric current testing detects electric current; Half-bridge cells, the first H-bridge unit and the second H-bridge unit are controlled respectively;

The described control to half-bridge cells comprises the compensation of System Reactive Power and the stable of maintenance half-bridge cells capacitance voltage, according to voltage, current double closed-loop control method generation modulation signal based on closed loop decoupling zero, adopt the capacitance voltage ranking method based on phase-shifting carrier wave that the capacitance voltage of half-bridge cells is stablized;

The described control to the first H-bridge unit comprises and utilizes the first H-bridge unit to increase level number, and maintains the stable of the first H-bridge unit capacitance voltage;

The described control to the second H-bridge unit comprises the suppression of the second H-bridge unit capacitance voltage equilibrium and whole reactive power compensator circulation.

7. according to claim 6 based on the control method of the reactive power compensator of double-H bridge structure multilevel converter, it is characterized in that: the capacitance voltage ranking method based on phase-shifting carrier wave comprises:

Determine the level number needing to drop into:

First, utilize phase-shifting carrier wave modulation technique to be compared by the phase shift triangular carrier of the modulating wave obtained through feed forward decoupling control and each brachium pontis, obtain the level number N needed, described phase shift triangular carrier is produced by the first modulating unit;

Then, judging obtaining level number N, obtaining new level number k; Corresponding k half-bridge cells is selected to drop into according to the flow direction of each brachium pontis half-bridge cells capacitance voltage height ordering scenario and bridge arm current again;

Described half-bridge cells voltage ranking method specifically comprises:

According to bridge arm current i _armto carry out judgement as follows in direction:

Work as i _armduring >0, then drop into K minimum half-bridge cells of capacitance voltage according to half-bridge cells capacitance voltage ranking results;

Work as i _armduring <0, then drop into K the highest half-bridge cells of capacitance voltage according to half-bridge cells capacitance voltage ranking results.

8., according to claim 7 based on the control method of the reactive power compensator of double-H bridge structure multilevel converter, it is characterized in that:

The method utilizing the first H-bridge unit to increase level number comprises:

Utilize phase-shifting carrier wave technology to approach modulation signal, obtain level number N, then judge;

If level number N is original 2n+1 level, then the first H-bridge unit does not need to put into operation, the level number k=N that half-bridge cells needs, and obtains the control signal of half-bridge cells according to level number k;

If level number is 2n new level, then the first H-bridge unit puts into operation, and the control signal of the first H-bridge unit is obtained by the first H-bridge unit control module, determine new level number k simultaneously, then obtained the control signal of half-bridge cells unit according to new level number k by half-bridge cells control module.

9. according to claim 8 based on the control method of the reactive power compensator of double-H bridge structure multilevel converter, it is characterized in that: the control procedure of the first H-bridge unit is specifically comprised: the electric current of brachium pontis is crossed in first convection current and DC capacitor voltage detects, size according to the DC capacitor voltage of the sense of current and current first H-bridge unit obtains charging and discharging state needed for the first H-bridge unit, obtain output voltage state according to charging and discharging state and the sense of current again, and then obtain the drive singal of the first H-bridge unit.

10. according to claim 6 based on the control method of the reactive power compensator of double-H bridge structure multilevel converter, it is characterized in that: the set-point V of each mutually upper and lower brachium pontis second H-bridge unit capacitance voltage _{h, ref}compare with the actual capacitance voltage of the second H-bridge unit, through PI controller, after it exports and is multiplied by the sign function of this bridge arm current, by the second H-bridge unit voltage signal V generated _{r, ref}with obtain PWM ripple after triangular carrier, drive corresponding power switch pipe in the second H-bridge unit, charge and discharge control carried out to the electric capacity of the second H-bridge unit, realizes the equilibrium of the second H-bridge unit capacitance voltage, wherein, r=P, N;

Described loop current suppression process comprises: compared with the reference value of circulation respectively by each phase circulation, now the reference value i of circulation _{cir, ref}=i _dc/ 3, the result obtained forms the voltage reference value of second H-bridge unit by a proportional controller, and this voltage reference value is divided into 2 parts, and the voltage signal of brachium pontis second H-bridge unit mutually upper and lower with this is added respectively, wherein i _dcfor three-phase MMC DC bus current.