CN105490285A - Reactive power compensation device of double-H-bridge modular multilevel converter (MMC) structure under three-phase unbalance and control method of reactive power compensation device - Google Patents

Abstract

The invention discloses a reactive power compensation device of a double-H-bridge modular multilevel converter (MMC) structure under three-phase unbalance and a control method of the reactive power compensation device, and relates to a reactive power compensation device of an MMC and a control method of the reactive power compensation device. The invention aims to solve the problems of poor energy mobility among three phases, low system working efficiency and poor safety and stability existing in the reactive power compensation device in the prior art when a three-phase power grid is at a voltage unbalance state. In the reactive power compensation device, a first controller of a master control part adopts a double-loop control strategy based on positive and negative sequence separation of feedforward decoupling to generate a three-phase modulation signal, a driving signal is generated by a voltage sequencing control method based on carrier phase shifting, and a second controller adopts the combination of a balance control strategy on capacitive voltage of a second H-bridge unit and a control strategy for suppressing system circulation to generate the three-phase modulation signal on the basis of positive and negative sequence separation on the circulation. By the reactive power compensation device, operation can be rapidly, safely and stably carried out under the condition of the unbalance of the power grid.

Description

The reactive power compensator of dual H-bridge MMC structure and control method thereof under three-phase imbalance

Technical field

The present invention relates to MMC reactive power compensator and control method thereof, be specifically related to reactive power compensator and the control method thereof of dual H-bridge MMC structure under three-phase imbalance, belong to reactive power compensation technology field.

Background technology

Electric energy to be lived the necessary energy as people, and its coverage and level of application represent china's overall national strength.Along with the fast development of power electronic technology, and be used in power domain on a large scale, and constantly increased in power distribution network due to uneven and nonlinear load, the quality of power supply receives and has a strong impact on, and its harm more and more seriously, must be changed.Static reacance generator (STATCOM) is a kind of compensation arrangement based on all-controlling power electronics device, the characteristic such as have that volume is little, governing speed is fast, adjustable range is wide and output current harmonics content is few, effectively can not only meet the requirement that electrical network is energy-conservation, fall damage, and be conducive to the power supply quality improving electrical network.

At present, common many level topological structure mainly contains 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 can 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.

For this reason, the scholar of university of Munich, Germany Federal Defence Forces proposes the topological structure of modularization multi-level converter (MMC).But when MMC power topology is applied in high pressure, hicap, also there are some problems: the switching frequency how reducing MMC Component units, reduce power loss; How under the prerequisite of existing hardware resource, a small amount of hardware cell that increases is to improve the level number of MMC output; And due to the imbalance of submodule capacitor voltage, the Energy distribution between each mutually upper and lower brachium pontis and in brachium pontis between submodule can be caused uneven, cause the generation of circulation, so need to carry out loop current suppression etc.

Summary of the invention

The object of the invention is reactive power compensator in order to solve prior art, to there are under three-phase power grid voltage non-equilibrium state three alternate energy flows poor, the problems such as system works efficiency is low, and safety and stability is poor.

Technical scheme of the present invention is: the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance, comprise: main control part and driver element, described main control part comprises the first controller and second controller, described first controller comprises positive-negative sequence separative element, forward-order current control ring, negative-sequence current control ring, first modulating unit and capacitance voltage sequencing unit, the positive sequence output of described positive-negative sequence separative element and negative phase-sequence output access forward-order current control ring and negative-sequence current control ring respectively, the output of forward-order current control ring is connected the first modulating unit with the output of negative-sequence current control ring, the output single pass-through capacitance voltage sequencing unit of the first modulating unit connects driver element, described second controller comprises positive sequence circulation control ring, negative phase-sequence circulation control ring and the second modulating unit, the output of positive sequence circulation control ring is connected driver element with the output of negative phase-sequence circulation control ring by the second modulating unit.

Described positive-negative sequence separative element comprises voltage positive-negative sequence separation module, electric current positive-negative sequence separation module, phase-locked loop, triangle modular converter, first coordinate transferring and the second coordinate transferring, the positive sequence voltage output of voltage positive-negative sequence separation module is connected the first coordinate transferring by phase-locked loop with triangle modular converter successively, the output of electric current positive-negative sequence modular converter connects the first coordinate transferring and the second coordinate transferring respectively, the output of the first coordinate transferring is positive sequence output, the output of the second coordinate transferring is negative phase-sequence output.

Described forward-order current control ring is identical with the structure of negative-sequence current control ring, and forward-order current control ring comprises the first comparator, second comparator, 3rd comparator, 4th comparator, 5th comparator, one PI controller, 2nd PI controller, 3rd PI controller, first reactor, second reactor and three-dimensional modular converter, the output of the first comparator connects the 3rd comparator through a PI controller, and the positive sequence output of described positive-negative sequence separative element connects the second comparator respectively, 3rd comparator first reactor and the second reactor, the output of the 3rd comparator connects the 3rd PI controller successively, 5th comparator and three-dimensional modular converter, the output of the second comparator connects the 2nd PI controller successively, 4th comparator and three-dimensional modular converter, the output of described first reactor connects the 4th comparator, the output of the second reactor connects the 5th comparator, the output of described three-dimensional modular converter is the output of forward-order current control ring, and the output of forward-order current control ring is connected the first modulating unit with the output of negative-sequence current control ring by the first comparator bank.

The positive sequence circulation control ring of described second controller is identical with the structure of negative phase-sequence circulation control ring, positive sequence circulation control ring comprises the 6th comparator, 7th comparator, 8th comparator, 9th comparator, tenth comparator, 4th PI controller, 5th PI controller, first function module, second function module, proportion adjustment module peace sub-module, the output of described 6th comparator connects the 4th PI controller successively, first function module and the 8th comparator, the output of the 7th comparator connects the 5th PI controller successively, second function module and the 9th comparator, the output of described tenth comparator connects proportion adjustment module peace sub-module successively, the output dividing module equally connects the 8th comparator and the 9th comparator respectively, the output of the 8th comparator and the 9th comparator is the output of positive sequence circulation control ring, the output of positive sequence circulation control ring is connected the second modulating unit with the output of negative phase-sequence circulation control ring by the second comparator bank.

Under described three-phase imbalance, the reactive power compensator of dual H-bridge MMC structure comprises MMC converter, described MMC converter to comprise on the identical three-phase of structure brachium pontis under brachium pontis and three-phase, every go up mutually brachium pontis include be connected in series the first H-bridge unit, second H-bridge unit and some half-bridge cells, reach suppression circulation by increasing by two H-bridge unit and make the object that output-voltage levels several times increase, it is high that dual H-bridge MMC converter has the degree of modularity, harmonic distortion is little, the feature that switching loss is low, and also have good compensation effect under unbalanced power supply condition, be applicable to being applied to high-power field, the topological structure of described MMC current transformer has common DC bus, three alternate energy can flow mutually, also can normally run during system imbalance of three-phase voltage, therefore, STATCOM based on MMC converter can realize reactive power, harmonic wave and unbalanced comprehensive compensation.

Based on the control method of the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance, the electric current detecting method that first controller of described main control part utilizes positive-negative sequence to be separated detects electric current, the Double-loop Control Strategy adopting the positive-negative sequence based on Feedforward Decoupling to be separated generates three-phase modulations signal, drive singal is generated by the voltage sequence control methods based on phase-shifting carrier wave, under imbalance of three-phase voltage state, compensating reactive power and negative-sequence current effectively; Second controller adopts and is carrying out circulation on the basis of positive-negative sequence separation, by combining with suppression circulation control strategy the Balance route of the second H-bridge unit selfcapacity voltage, generates three-phase modulations signal.

The current detecting process that described positive-negative sequence is separated comprises:

Delay method is utilized to obtain the three-phase positive sequence component of line voltage, using the three-phase positive sequence component of line voltage as with reference to coordinate, by the i of the three-phase current that current transformer exports _p-i _qdetect.

The process that described delay method obtains grid voltage three-phase positive-negative sequence component comprises:

When three-phase power grid voltage is uneven, obtained by symmetrical component method:

$\left[\begin{array}{c}{u}_a\left(t\right)\\ u_b\left(t\right)\\ u_c\left(t\right)\end{array}\right]=\left[\begin{array}{c}{u_a}^{+}\left(t\right)+{u_a}^{-}\left(t\right)+{u_a}^0\left(t\right)\\ {u_b}^{+}\left(t\right)+{u_b}^{-}\left(t\right)+{u_b}^0\left(t\right)\\ {u_c}^{+}\left(t\right)+{u_c}^{-}\left(t\right)+{u_c}^0\left(t\right)\end{array}\right]$

U in formula _x ⁺(t), u _x ^-(t), u _x ⁰t () is the positive sequence of voltage, negative phase-sequence and zero-sequence component respectively, x represents a, b, c three-phase;

Adopt Y type without center line connected mode, because above formula can obtain:

$\left[\begin{array}{c}{u}_a\\ u_b\\ u_c\end{array}\right]=\sqrt{2}{U}_{+}\left[\begin{array}{c}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})\\ \sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+}-2/3\mathrm{\π})\\ \sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+}+2/3\mathrm{\π})\end{array}\right]+\sqrt{2}{U}_{-}\left[\begin{array}{c}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{-})\\ \sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{-}-2/3\mathrm{\π})\\ \sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{-}+2/3\mathrm{\π})\end{array}\right]$

Obtained by above formula:

$\begin{array}{c}{u}_b(t+T/3)={u_b}^{+}(t+2\mathrm{\π}/3)+{u_b}^{-}(t-2\mathrm{\π}/3)=\\ \sqrt{2}{U}_{+}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})+\sqrt{2}{U}_{-}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+}-2\mathrm{\π}/3)\end{array}$

$\begin{array}{c}{u}_c(t-T/3)={u_c}^{+}(t-2\mathrm{\π}/3)+{u_b}^{-}(t-2\mathrm{\π}/3)=\\ \sqrt{2}{U}_{+}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})+\sqrt{2}{U}_{-}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+}-2\mathrm{\π}/3)\end{array}$

$\begin{array}{c}{u}_a\left(t\right)+u_b(t+T/3)+u_c(t-T/3)=\\ \sqrt{2}{U}_{+}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})+\sqrt{2}{U}_{-}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{-})+\\ \sqrt{2}{U}_{+}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})+\sqrt{2}{U}_{-}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{-}-2\mathrm{\π}/3)+\\ \sqrt{2}{U}_{+}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})+\sqrt{2}{U}_{-}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{-}+2\mathrm{\π}/3)\end{array}$

Obtain the positive-negative sequence component of three-phase voltage, be shown below;

u _a ⁺(t)＝1/3[u _a(t)-u _b(t-T/6)+u _c(t-T/3)]

u _a ^-(t)＝1/3[u _a(t)-u _b(t-T/3)+u _c(t-T/6)]

u _b ⁺(t)＝1/3[u _a(t-T/3)+u _b(t)-u _c(t-T/6)]

u _b ^-(t)＝1/3[-u _a(t-T/6)+u _b(t)-u _c(t-T/3)]

Each phase circulation is carried out positive-negative sequence separation by described second controller, the positive sequence circulation obtained and negative phase-sequence circulation respectively with circulation reference value i _{cir, ref}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 the second H-bridge unit of this mutually upper and lower brachium pontis respectively, obtain the modulating wave of a phase second H-bridge unit, also the output voltage impact of the second H-bridge unit on MMC is dropped to minimum while eliminating circulation like this.

The process that described second controller realizes the second H-bridge unit capacitance voltage equilibrium comprises: the set-point V of each mutually upper and lower brachium pontis HB2 capacitance voltage _{h2, ref}compare with the actual capacitance voltage of H-bridge unit HB2, after PI regulates, 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, by voltage given value V _{r, ref}(r=P, N) with obtain PWM ripple after triangular carrier, the power switch pipe in drive unit drives the 2nd H unit, carries out charge and discharge control to HB2 electric capacity, to realize the balance of HB2 capacitance voltage.

The present invention compared with prior art has following effect: the capacitance voltage ranking method based on phase-shifting carrier wave that the present invention first controller adopts, control simple, be easy to realize, and half-bridge cells switching unnecessary can be avoided, reduce the on-off times of power tube, reduce switching loss; By feed forward decoupling control to dq decoupler shaft, control can be made more simple, and floating adjustment can be realized by conventional linear PI adjustment.In order to carry out reactive power compensation when unbalanced power supply, finally adopting negative phase-sequence, positive sequence 2 control rings control, realizing, in unbalanced power supply situation, while carrying out reactive power compensation, carrying out negative sequence component compensation.

The present invention adopts dual H-bridge MMC converter as the main circuit of STATCOM, the capacitance voltage of first H-bridge unit added is the half of half-bridge cells capacitance voltage, MMC output level is made to bring up to 4n+1 by 2n+1, make output voltage closer to sine wave, harmonic content is little, and select suitable control algolithm can reduce the switching frequency of power tube, reduce switching loss.

Described dual H-bridge MMC topology adopts the second H-bridge unit to carry out loop current suppression, and need not carry out the negative phase-sequence coordinate transform of two frequencys multiplication, the software resource taken is fewer comparatively speaking; And the capacitance voltage of the second H-bridge unit is less, can ignore the impact that MMC exports; Utilize positive-negative sequence partition method to obtain positive sequence component and the negative sequence component of circulation, and carry out loop current suppression control respectively, effectively improve loop current suppression effect.

The described i based on instantaneous reactive power theory _p-i _qelectric current testing, the method has good real-time, can accurately detect the size of active current in electrical network and reactive current, improves the stability of system; When unbalanced source voltage, utilize positive-negative sequence partition method to carry out positive-negative sequence separation to power network current, and adopt positive and negative order double ring control strategy to make system can distinguish the idle and negative-sequence current of bucking-out system.

The present invention is a kind of novel high pressure, high-power reactive power compensator, when unbalanced power supply can fast, safety, stablely carry out work, not only can compensation network idle, support line voltage in addition, suppress the effect of circulation.

Accompanying drawing explanation

Fig. 1 dual H-bridge MMC topological structure schematic diagram;

The working state schematic representation of Fig. 2 half-bridge cells;

Fig. 3 H-bridge unit structure chart;

Fig. 4 delay method schematic diagram;

Fig. 5 i _p-i _qdetection method schematic diagram;

Positive-negative sequence current Cleaning Principle figure in Fig. 6 unbalanced power supply situation;

Fig. 7 is meritorious, reactive current control block diagram;

Fig. 8 Feedforward Decoupling equivalent control block diagram;

The control block diagram of Fig. 9 STATCOM voltage and current double closed-loop;

Figure 10 positive and negative order double ring control block diagram;

Figure 11 need drop into the determination schematic diagram of level number;

Figure 12 capacitance voltage ranking method flow chart;

The modulation strategy schematic diagram of Figure 13 dual H-bridge MMC structure;

Figure 14 first H-bridge unit HB1 control block diagram;

The equivalent model schematic diagram of Figure 15 dual H-bridge MMC topological structure;

Figure 16 second controller operation principle schematic diagram;

Figure 17 STATCOM system main-control block diagram;

Figure 18 voltage zero-crossing detection circuit schematic diagram;

Figure 19 current detecting and modulate circuit schematic diagram thereof;

Figure 20 current foldback circuit schematic diagram;

Figure 21 drive circuit schematic diagram;

Figure 22 main program flow chart;

Figure 23 capture interrupt flow chart;

Figure 24 T1 interruption subroutine flow chart;

Figure 25 error protection interruption subroutine flow chart;

Figure 26 entire system block diagram of the present invention.

Embodiment

Accompanying drawings the specific embodiment of the present invention, shown in Figure 17 and Figure 26, the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance of the present invention, comprise: main control part and driver element, described main control part comprises the first controller and second controller, described first controller comprises positive-negative sequence separative element, forward-order current control ring, negative-sequence current control ring, first modulating unit 16 and capacitance voltage sequencing unit 17, the positive sequence output of described positive-negative sequence separative element and negative phase-sequence output access forward-order current control ring and negative-sequence current control ring respectively, the output of forward-order current control ring is connected the first modulating unit 16 with the output of negative-sequence current control ring, the output single pass-through capacitance voltage sequencing unit 17 of the first modulating unit 16 connects driver element, described second controller comprises positive sequence circulation control ring, negative phase-sequence circulation control ring and the second modulating unit 33, the output of positive sequence circulation control ring is connected driver element with the output of negative phase-sequence circulation control ring by the second modulating unit 33.

As shown in Figure 6, described positive-negative sequence separative element comprises voltage positive-negative sequence separation module 18, electric current positive-negative sequence separation module 21, phase-locked loop 19, triangle modular converter 20, first coordinate transferring 11 and the second coordinate transferring 12, the positive sequence voltage output of voltage positive-negative sequence separation module 18 is connected the first coordinate transferring 11 by phase-locked loop 19 with triangle modular converter 20 successively, the output of electric current positive-negative sequence modular converter connects the first coordinate transferring 11 and the second coordinate transferring 12 respectively, the output of the first coordinate transferring 11 is positive sequence output, the output of the second coordinate transferring 12 is negative phase-sequence output.

As shown in Figure 10 and Figure 17, described forward-order current control ring is identical with the structure of negative-sequence current control ring, forward-order current control ring comprises the first comparator 1, second comparator 2, 3rd comparator 3, 4th comparator 4, 5th comparator 5, one PI controller 6, 2nd PI controller 7, 3rd PI controller 8, first reactor 9, second reactor 10 and three-dimensional modular converter 13, the output of the first comparator 1 connects the 3rd comparator 3 through a PI controller 6, the positive sequence output of described positive-negative sequence separative element connects the second comparator 2 respectively, 3rd comparator 3 first reactor 9 and the second reactor 10, the output of the 3rd comparator 3 connects the 3rd PI controller 8 successively, 5th comparator 5 and three-dimensional modular converter 13, the output of the second comparator 2 connects the 2nd PI controller 7 successively, 4th comparator 4 and three-dimensional modular converter 13, the output of described first reactor 9 connects the 4th comparator 4, the output of the second reactor 10 connects the 5th comparator 5, the output of described three-dimensional modular converter 13 is the output of forward-order current control ring, the output of forward-order current control ring is connected the first modulating unit 16 with the output of negative-sequence current control ring by the first comparator bank 15.

As shown in Figure 16 and Figure 17, the positive sequence circulation control ring of described second controller is identical with the structure of negative phase-sequence circulation control ring, and positive sequence voltage control ring comprises the 6th comparator 21, 7th comparator 22, 8th comparator 23, 9th comparator 24, tenth comparator 25, 4th PI controller 26, 5th PI controller 27, first function module 28, second function module 29, the peaceful sub-module 31 of proportion adjustment module 30, the output of described 6th comparator 21 connects the 4th PI controller 26 successively, the output of the first function module 28 and the 8th comparator the 23, seven comparator 22 connects the 5th PI controller 27 successively, second function module 29 and the 9th comparator 24, the output of described tenth comparator 25 connects the peaceful sub-module 31 of proportion adjustment module 30 successively, the output dividing module equally connects the 8th comparator 23 and the 9th comparator 24 respectively, the output of the 8th comparator 23 and the 9th comparator 24 is the output of positive sequence circulation control ring, and the output of positive sequence circulation control ring is connected the second modulating unit 33 with the output of negative phase-sequence circulation control ring by the second comparator bank 32.

Under described three-phase imbalance, the reactive power compensator of dual H-bridge MMC structure comprises MMC converter, and described dual H-bridge MMC converter is made up of 6 brachium pontis, and wherein each comprises upper and lower two brachium pontis and 2 reactor L mutually _ceach brachium pontis is made up of multiple identical half-bridge cells SM and 2 H-bridge unit, each half-bridge cells comprises two IGBT with anti-parallel diodes and 1 storage capacitor C, and H-bridge unit comprises four IGBT with anti-parallel diodes and 1 storage capacitor C; The effect of 2 H-bridge unit is multiplications that one of them H-bridge unit can realize output level number, and another H-bridge unit can realize the effective suppression to circulation; By the i based on positive-negative sequence partition method _p-i _qelectric current testing and voltage, current double closed-loop uneoupled control generate three-phase modulations signal; Adopt the capacitance voltage ranking method based on phase-shifting carrier wave, generate drive singal control MMC half-bridge cells and the first H-bridge unit respectively, realize the balance of half-bridge cells capacitance voltage and output level several times are increased; Utilize the second H-bridge unit to suppress circulation, and it can be ignored on the impact of MMC output voltage.

Dual H-bridge MMC topological structure as shown in Figure 1, MMC device is by multiple half-bridge cells, the first H-bridge unit HB1 and the second H-bridge unit HB2, wherein half-bridge cells is for controlling the first-harmonic load current in brachium pontis, a H-bridge unit can realize being multiplied to output level number, and another H-bridge unit is used for carrying out loop current suppression.

SM is half-bridge cells, and each half-bridge cells is made up of two IGBT with reverse fly-wheel diode and 1 storage capacitor C, and each half-bridge cells can only export 0 and V _dtwo kinds of voltage statuss.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.

The corresponding states table of the different on off state of table 1 half-bridge cells

As shown in Figure 2, in figure, arrow shows the flow direction of electric current to the half-bridge cells SM operating state of MMC.Half-bridge cells SM has three kinds of operating states:

1) T ₁(D ₁) open, T ₂(D ₂) turn off as input state;

As Fig. 2 a) shown in, be half-bridge cells drop into state.The output voltage of half-bridge cells is always the voltage on electric capacity, and now the charge and discharge state of half-bridge cells electric capacity depends on the flow direction of electric current.

2) T ₁(D ₁) turn off, T ₂(D ₂) open as excision state;

As Fig. 2 b) shown in, be half-bridge cells excision state.The output voltage of half-bridge cells is 0 all the time, and the flow direction of electric current does not affect the capacitance voltage of half-bridge cells.

3) T ₁and T ₂all turn off as blocking;

As Fig. 2 c) shown in, be half-bridge cells blocking.Under this state, can only to half-bridge cells capacitor charging, half-bridge cells electric capacity can not discharge, and MMC is in abnormal operational conditions.

H-bridge unit comprises the first H-bridge unit and the second H-bridge unit, and its structure chart as shown in Figure 3, has 3 kinds of on off states; 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, then the output voltage that the corresponding H-bridge unit of the on off state that 4 switching tubes are different is different and capacitor charge and discharge state, as shown in table 2.

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

Under described three-phase imbalance, the reactive power compensator of dual H-bridge MMC structure comprises voltage zero-crossing detection circuit, as shown in figure 18, described voltage zero-crossing detection circuit comprises voltage sensor 34, comparison circuit 35 and inverter 36, the input of voltage sensor 34 connects the output of three-phase alternating-current supply, the output of voltage sensor 34 connects the input of comparison circuit 35, the output of comparison circuit 35 connects DSP module by inverter 36, 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.

Under described three-phase imbalance, the reactive power compensator of dual H-bridge MMC structure comprises current detecting and modulate circuit, as shown in figure 19, described current detecting and modulate circuit comprise current sensor 37, optical isolation amplifier 38 and biasing circuit 39, the output of current sensor 37 connects optical isolation amplifier 38, the output of optical isolation amplifier 38 connects biasing circuit 39, biasing circuit 39 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 38 to isolate, the model of the optical isolation amplifier that present embodiment adopts is HCPL7840.

Under described three-phase imbalance, the reactive power compensator of dual H-bridge MMC structure comprises current foldback circuit; as shown in figure 20; described current foldback circuit comprises comparator 40 and clamp circuit 41; mid point and the DSP module of clamp circuit 41 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.

The current foldback circuit that present embodiment adopts needs to utilize the power drive protection of TMS320F2812 to interrupt PDPINTA and realizes.In the normal situation of electric current, the pwm pulse signal of DSP normally exports; But when the electric current in circuit is excessive, PDPINTA is interrupted in trigger fault protection, makes DSP block the output of all pwm pulse signals, to protect whole STATCOM system.

As shown in figure 21, isolated drive circuit adopts driving chip TLP250 to the drive circuit that present embodiment adopts, and improves the antijamming capability of system.First utilize chip 7406 couples of DSP to produce pwm signal and carry out anti-phase process, then this signal input queued switches chip TLP250 is produced the signal of driving power pipe.

The control circuit of present embodiment with the TMS320F2812 of TI company for core; realize the function such as generation, capacitance voltage sequence, generation pwm signal of the collection of electric current and voltage, modulation signal, other circuit is made up of sample circuit, drive circuit and protective circuit etc.

Based on the control method of the reactive power compensator of dual H-bridge MMC structure under described three-phase imbalance, first controller of main control part utilizes the electric current detecting method be separated based on positive-negative sequence to detect electric current, the Double-loop Control Strategy adopting the positive-negative sequence based on Feedforward Decoupling to be separated generates three-phase modulations signal, generates drive singal by the voltage sequence control methods based on phase-shifting carrier wave; Second controller adopts and is carrying out circulation on positive-negative sequence separation basis, by combining with suppression circulation control strategy the Balance route of the second H-bridge unit selfcapacity voltage, generates three-phase modulations signal.

1 three-phase power grid voltage imbalance is analyzed

When three phase network is uneven, its three-phase voltage is expressed formula and is:

$\left[\begin{array}{c}{u}_a\\ u_b\\ u_c\end{array}\right]=\left[\begin{array}{c}\sqrt{2}{U}_{as}sin(\mathrm{\ω}t+{{\mathrm{\θ}}_e}^a)\\ \sqrt{2}{U}_{bs}sin(\mathrm{\ω}t-{{\mathrm{\θ}}_e}^b)\\ \sqrt{2}{U}_{cs}sin(\mathrm{\ω}t+{{\mathrm{\θ}}_e}^c)\end{array}\right]---\left(1\right)$

In formula (1), U _as, U _bs, U _csa, b, c three-phase voltage effective value, θ _e ^a, θ _e ^b, θ _e ^cbe a, b, c voltage start-phase, in three-phase, the voltage swing of every phase all can be not identical with other two-phase with phase place, and frequency should be all power frequency 50Hz.

By symmetrical component method, these voltages can be decomposed into 3 groups of symmetrical vectors, can be expressed as shown in formula (2), u in formula _x ⁺(t), u _x ^-(t), u _x ⁰t () is the positive sequence of voltage, negative phase-sequence and zero-sequence component respectively, x represents a, b, c three-phase.

$\left[\begin{array}{c}{u}_a\left(t\right)\\ u_b\left(t\right)\\ u_c\left(t\right)\end{array}\right]=\left[\begin{array}{c}{u_a}^{+}\left(t\right)+{u_a}^{-}\left(t\right)+{u_a}^0\left(t\right)\\ {u_b}^{+}\left(t\right)+{u_b}^{-}\left(t\right)+{u_b}^0\left(t\right)\\ {u_c}^{+}\left(t\right)+{u_c}^{-}\left(t\right)+{u_c}^0\left(t\right)\end{array}\right]---\left(2\right)$

The present invention adopts Y type without the circuit of center line connected mode, and without the need to considering zero-sequence component, only need consider positive-negative sequence component, then formula (2) can be changed into:

$\left[\begin{array}{c}{u}_a\\ u_b\\ u_c\end{array}\right]=\sqrt{2}{U}_{+}\left[\begin{array}{c}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})\\ \sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+}-2/3\mathrm{\π})\\ \sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+}+2/3\mathrm{\π})\end{array}\right]+\sqrt{2}{U}_{-}\left[\begin{array}{c}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{-})\\ \sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{-}-2/3\mathrm{\π})\\ \sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{-}+2/3\mathrm{\π})\end{array}\right]---\left(3\right)$

Then coordinate transform is carried out to above formula, can obtain:

$\left[\begin{array}{c}{u}_a\\ u_b\\ u_c\end{array}\right]=C_{23}R\left(\mathrm{\θ}\right)\left[\begin{array}{c}{u_d}^{+}\\ {u_q}^{+}\end{array}\right]+C_{23}R(-\mathrm{\θ})\left[\begin{array}{c}{u_d}^{-}\\ {u_q}^{-}\end{array}\right]---\left(4\right)$

In formula (4), Matrix C ₂₃be transformation matrix voltage fastened by two-phase static coordinate in three-phase static coordinate system, R (θ) is positive-sequence coordinate transformation matrix, and R (-θ) is negative phase-sequence transformation matrix of coordinates.Then the system voltage of three-phase imbalance transforms to the formula under two-phase rotating coordinate system and is:

$\left[\begin{array}{c}{u}_d\\ u_q\end{array}\right]=\left[\begin{array}{c}{u_d}^{+}\\ {u_q}^{+}\end{array}\right]+R(-2\mathrm{\ω}t)\left[\begin{array}{c}{u_d}^{-}\\ {u_q}^{-}\end{array}\right]---\left(5\right)$

Wherein:

$\begin{array}{ccc}{C}_{32}=\left[\begin{array}{cc}1& 0\\ -1/2& \sqrt{3}/2\\ -1/2& -\sqrt{3}/2\end{array}\right]& R\left(\mathrm{\θ}\right)=\left[\begin{array}{cc}cos\mathrm{\θ}& -sin\mathrm{\θ}\\ sin\mathrm{\θ}& \cos \mathrm{\θ}\end{array}\right]& R(-\mathrm{\θ})=\left[\begin{array}{cc}cos\mathrm{\θ}& sin\mathrm{\θ}\\ -sin\mathrm{\θ}& \cos \mathrm{\θ}\end{array}\right]\end{array}$

From formula (5): in dq coordinate system, the component of voltage of the positive sequence originally in three-phase system in static abc coordinate system becomes DC quantity, and negative sequence component has become second harmonic component.

2 positive-negative sequence component detection methods

Known by above-mentioned analysis, filtering second harmonic voltage under positive sequence synchronous rotary dq coordinate system, can obtain the positive sequence voltage [u under dq axle _d ⁺, u _q ⁺] ^t, then under coordinate inverse transformation both can obtain three phase static abc coordinate system three-phase positive sequence voltage [u _a ⁺, u _b ⁺, u _c ⁺] ^t; In like manner can obtain, under negative phase-sequence synchronous rotary dq coordinate system, filtering second harmonic voltage can obtain the negative sequence voltage [u under dq axle _d ^-, u _q ^-] ^t, then under coordinate inverse transformation can obtain three phase static abc coordinate system three-phase negative/positive voltage [u _a ^-, u _b ^-, u _c ^-] ^t.

Prior art adopts the second harmonic filtering method based on low pass filter and the second harmonic filtering method based on trapper to obtain positive-negative sequence component usually, may cause dynamic lag based on the low pass filter in the second harmonic filtering method of low pass filter because frequency band is narrower; And relevant with quality factor based on the trapper in the second harmonic filtering method of trapper, want filtration result good, just need to reduce quality factor, and quality factor are too low, then can reduce to control bandwidth.

So, utilize delay method to be separated to realize positive-negative sequence in the present invention.

What delay method utilized is positive-negative sequence characteristic and symmetrical component method is to calculate positive-negative sequence component, and in computational process, used time of delay is relevant with phase sequence.

From formula (3)

$\begin{array}{c}{u}_b(t+T/3)={u_b}^{+}(t+2\mathrm{\π}/3)+{u_b}^{-}(t-2\mathrm{\π}/3)=\\ \sqrt{2}{U}_{+}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})+\sqrt{2}{U}_{-}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+}-2\mathrm{\π}/3)\end{array}---\left(6\right)$

$u_c(t-T/3)={u_c}^{+}(t-2\mathrm{\π}/3)+{u_b}^{-}(t-2\mathrm{\π}/3)=$

$\sqrt{2}{U}_{+}sin(\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})+\sqrt{2}U\_\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+}+2\mathrm{\π}/3)---\left(7\right)$

$\begin{array}{c}{u}_a\left(t\right)+u_b(t+T/3)+u_c(t-T/3)=\\ \sqrt{2}{U}_{+}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})+\sqrt{2}{U}_{-}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{-})+\\ \sqrt{2}{U}_{+}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})+\sqrt{2}{U}_{-}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{-}-2\mathrm{\π}/3)+\\ \sqrt{2}{U}_{+}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})+\sqrt{2}{U}_{-}\sin (\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{-}+2\mathrm{\π}/3)\end{array}---\left(8\right)$

In formula (8), negative sequence voltage sum is 0, then can abbreviation be:

$u_a\left(t\right)+u_b(t+T/3)+u_c(t-T/3)=3\sqrt{2}{U}_{+}sin(\mathrm{\ω}t+{{\mathrm{\θ}}_e}^{+})=3{u_a}^{+}\left(t\right)---\left(9\right)$

Can be obtained by trigonometric function relation again:

u _b(t+T/3)＝u _b(t-2T/3)＝-u _b(t-2T/3+T/2)＝-u _b(t-T/6)(10)

From formula (10), time delay 2T/3 can be replaced by time delay T/6, and consider the sampling time of control system and the operation time of DSP, this time delay can accept completely.Can be obtained by formula (9) and (10):

u _a ⁺(t)＝1/3[u _a(t)-u _b(t-T/6)+u _c(t-T/3)](11)

u _a ^-(t)＝1/3[u _a(t)-u _b(t-T/3)+u _c(t-T/6)](12)

u _b ⁺(t)＝1/3[u _a(t-T/3)+u _b(t)-u _c(t-T/6)](13)

u _b ^-(t)＝1/3[-u _a(t-T/6)+u _b(t)-u _c(t-T/3)](14)

In three-phase system, three-phase positive sequence component and negative sequence component are all symmetrical, so only demand goes out the component that two phase components can try to achieve third phase.Delay method is utilized to obtain the schematic diagram of line voltage positive-negative sequence component as shown in Figure 4.

3 electric current testings

1) i under grid balance _p-i _qdetection method:

First need to calculate the meritorious of three-phase current and idle component i _p, i _q:

$\left[\begin{array}{c}{i}_p\\ i_q\end{array}\right]=\left[\begin{array}{cc}sin\mathrm{\ω}t& -cos\mathrm{\ω}t\\ -cos\mathrm{\ω}t& -sin\mathrm{\ω}t\end{array}\right]\left[\begin{array}{c}{i}_{\mathrm{\α}}\\ i_{\mathrm{\β}}\end{array}\right]=C\left[\begin{array}{c}{i}_{\mathrm{\α}}\\ i_{\mathrm{\β}}\end{array}\right]={\mathrm{CC}}_{32}\left[\begin{array}{c}{i}_a\\ i_b\\ i_c\end{array}\right]---\left(15\right)$

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

By the i that formula (15) calculates _p, i _qafter low pass filter, obtain i _p, i _qin DC component three-phase fundamental current i is obtained again through inverse transformation _af, i _bf, i _cf.

$\left[\begin{array}{c}{i}_{af}\\ i_{bf}\\ i_{cf}\end{array}\right]=C_{23}{C}^{-1}\left[\begin{array}{c}\stackrel{\‾}{i_p}\\ \stackrel{\‾}{i_q}\end{array}\right]---\left(16\right)$

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 component i can be obtained _ah, i _bh, i _ch, Fig. 5 is i _p-i _qdetection method schematic diagram.

2) positive-negative sequence current detection method under unbalanced power supply

At i _p-i _qin method, a phase-locked loop and sine cosine generator is usually needed to produce a detection participating in below with the sine of a phase voltage homophase and cosine signal.When unbalanced power supply, by containing a large amount of negative sequence components in line voltage, and the injection of these negative sequence components also must can affect Detection results.So in the asymmetric situation of line voltage, the positive sequence component in three-phase power grid voltage can extract by we, using the positive sequence voltage of electrical network three-phase as reference synchronization coordinate, completes i _p-i _qdetection, Fig. 6 is that positive-negative sequence current in unbalanced power supply situation detects.

4, based on the system control strategy of Feedforward Decoupling

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

$\left[\begin{array}{c}{v}_{sd}\\ v_{sq}\end{array}\right]=\sqrt{3}{U}_s\left[\begin{array}{c}1\\ 0\end{array}\right],\left[\begin{array}{c}{v}_{cd}\\ v_{cq}\end{array}\right]=\frac{\sqrt{3}}{2}{\mathrm{Mu}}_{dc}\left[\begin{array}{c}1\\ 0\end{array}\right]\left[\begin{array}{c}cos\mathrm{\δ}\\ sin\mathrm{\δ}\end{array}\right]---\left(17\right)$

Wherein: δ is the phase difference of STATCOM output voltage and line voltage, M is modulation ratio, and Us is line voltage, u _dcfor 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 δ=tg ^-1(v _cd/ v _cq) 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 system.

The loss of whole current transformer is equivalent to fixed resistance R, and linked reactor and line inductance are equivalent to inductance L, then current transformer output voltage v _cdand v _cqexpression formula be

$\left[\begin{array}{c}{v}_{cd}\\ v_{cq}\end{array}\right]=\left[\begin{array}{c}{v}_{sd}\\ v_{sq}\end{array}\right]+\left[\begin{array}{cc}-R& \mathrm{\ω}L\\ -\mathrm{\ω}L& -R\end{array}\right]\left[\begin{array}{c}{i}_d\\ i_q\end{array}\right]-L\frac{d}{dt}\left[\begin{array}{c}{i}_d\\ i_q\end{array}\right]---\left(18\right)$

Gained merit according to formula (18), reactive current control block diagram as shown in Figure 7.In order to dq decoupler shaft, take feed forward decoupling control strategy.

Introduce variable x ₁, x ₂:

$\left\{\begin{array}{c}{x}_1=v_{sd}-v_{cd}+{\mathrm{\ωLi}}_q\\ x_2=v_{sq}-v_{cq}-{\mathrm{\ωLi}}_d\end{array}\right.---\left(19\right)$

Obtained by formula (18), (19)

$\left\{\begin{array}{c}{x}_1=L\frac{{\mathrm{di}}_d}{dt}+{\mathrm{Ri}}_d\\ x_2=L\frac{{\mathrm{di}}_q}{dt}+{\mathrm{Ri}}_q\end{array}\right.---\left(20\right)$

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

$\left\{\begin{array}{c}{x}_1=k_1(i_d*-i_d)+\frac{k_1}{T_1}\∫(i_d*-i_d)dt\\ x_2=k_2(i_q*-i_q)+\frac{k_2}{T_2}\∫(i_q*-i_q)dt\end{array}\right.---\left(21\right)$

Wherein k ₁, k ₂for proportionality coefficient, T ₁, T ₂for the time of integration.

Then can obtain Feedforward Decoupling equivalent control block diagram as shown in Figure 8, 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 9.

As seen from Figure 9, STATCOM system forms by by 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*; The reactive current that need record load side is on the other hand 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, through a series of computing, finally obtain the voltage v that STATCOM wants to export _cd, v _cq.

Utilize shown in Fig. 6 based on the i that positive-negative sequence is separated _p-i _qelectric current testing, then the control strategy when system balancing is before combined.Due to the present invention adopt be Y type without center line connection, without the need to considering zero-sequence component, the positive sequence component that only electric current need be obtained after positive-negative sequence is separated and negative sequence component control respectively.By above-mentioned analysis, in order to carry out reactive power compensation when unbalanced power supply, finally adopt negative phase-sequence, positive sequence 2 control rings control, positive and negative order double ring control block diagram as shown in Figure 10.

5, half-bridge cells control strategy

In MMC topological structure, each brachium pontis can be connected multiple half-bridge cells, but the DC capacitor voltage of each half-bridge cells is separate.Because the reasons such as capacitor's capacity fluctuation, power tube conduction voltage drop difference and drive singal delay can cause the imbalance of half-bridge cells DC capacitor voltage.

In order to maintain the equilibrium of MMC half-bridge cells capacitance voltage, adopt half-bridge cells capacitance voltage sequence control strategy, keep the balance of half-bridge cells capacitance voltage, its implementation procedure is as follows:

1) determination of level number need be dropped into

First, utilize CPS-SPWM modulation technique to be compared by the phase shift triangular carrier of the modulation wave signal obtained through feed forward decoupling control and each brachium pontis, obtain the level number N needed;

Then, judge obtaining level number N, and obtain new level number k;

Finally, corresponding k half-bridge cells is selected to drop into according to the flow direction of the high ordering scenario of each brachium pontis half-bridge cells capacitance voltage and bridge arm current.The determination flow chart of level number need be dropped into as shown in figure 11.

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

The flow chart of half-bridge cells capacitance voltage ranking method as shown in figure 12, wherein i _armfor bridge arm current:

(1) i is worked as _armduring >0, namely bridge arm current is to the charging of half-bridge cell capacitance, and according to half-bridge cells capacitance voltage ranking results, determine to drop into k minimum half-bridge cells of capacitance voltage, namely bridge arm current is to this k half-bridge cells capacitor charging, and its voltage is raised.

(2) i is worked as _armduring <0, namely bridge arm current is to the electric discharge of half-bridge cell capacitance, then according to half-bridge cells capacitance voltage ranking results, determine to drop into k the highest half-bridge cells of capacitance voltage, namely bridge arm current makes its voltage reduce to this k half-bridge cells capacitor discharge.

Like this, the balance of half-bridge cells capacitance voltage is just achieved.

6, the control strategy of the first H-bridge unit HB1

For traditional MMC, being located at every mutually upper and lower brachium pontis series half-bridge unit number in MMC is respectively n, and adopt phase-shifting carrier wave modulation technique, then the maximum level number of its AC output voltage is 2n+1 simultaneously.

For dual H-bridge MMC, first on the basis of traditional MMC, respectively add a H-bridge unit HB1 in each brachium pontis, its DC capacitor voltage is the half of half-bridge cells DC capacitor voltage, i.e. V _h1=V _d/ 2.From H bridge operation principle, the output level number of HB1 is 3, is+V respectively _d/ 2,0 and-V _d/ 2, therefore, utilize the output level of H-bridge unit HB1 to coordinate the switching of each brachium pontis series half-bridge unit, 2n new level can be inserted in the middle of original 2n+1 level, reach 4n+1 level to export, thus the output level number of dual H-bridge MMC is multiplied.

The implementation method that first H-bridge unit HB1 makes the output level number of dual H-bridge MMC be multiplied is:

First, utilize phase-shifting carrier wave technology to approach modulation signal, obtain level number N, then judge:

1) if level number N is original 2n+1 level, then H-bridge unit HB1 does not need to put into operation, and half-bridge cells needs the level number k=N exported, and then obtains the control signal of series half-bridge unit unit according to level number k;

2) if level number N is 2n new level, then H-bridge unit HB1 puts into operation, needs the control signal being obtained H-bridge unit HB1 by HB1 control module; Meanwhile, new level number k=N-sign (u is determined _h1), wherein u _h1it is the output voltage of the first H-bridge unit HB1; Then, the control signal of half-bridge cells unit is obtained by half-bridge cells control module.The modulation strategy block diagram of novel topological structure as shown in figure 13.

Because the input of HB1 unit and excision will coordinate input and the excision of the half-bridge cells of series connection, so the control method of HB1 is:

1) first, the electric current of brachium pontis is crossed in convection current and DC capacitor voltage detects, and the size according to the sense of current and HB1 DC capacitor voltage determines required charging and discharging state;

2) then, needed for module, charging and discharging state and the sense of current obtain output voltage state, and then obtain the drive singal of HB1.As shown in figure 14, table 3 is HB1 output voltage condition judgement table to the control flow chart of HB1.

Table 3HB1 output voltage state is determined

7, the control strategy of the second H-bridge unit HB2

1) circulation mechanism is suppressed

For dual H-bridge MMC, after each brachium pontis adds first H-bridge unit HB1, each brachium pontis respectively adds second H-bridge unit HB2, then form dual H-bridge MMC overall topology, as shown in Figure 1.As shown in figure 15, wherein DC bus current is i to the equivalent model of dual H-bridge 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 loop current suppression principle of dual H-bridge MMC topological structure, from KCL Circuit theory, a phase output current can be expressed as

i _a＝i _aP-i _aN(22)

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

$i_{aP}=i_{cir,a}+\frac{i_a}{2}---\left(23\right)$

$i_{aN}=i_{cir,a}-\frac{i_a}{2}---\left(24\right)$

Formula (23) and (24) are added, obtain

$i_{cir,a}=\frac{1}{2}(i_{aP}+i_{aN})---\left(25\right)$

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(26)

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

$i_{cir,j}=\frac{i_{dc}}{3}+{i_{zj}}^{*}---\left(27\right)$

I in formula _zj ^*, (j=a, b, c) is two frequency multiplication negative phase-sequence of acs in circulation, and convolution (23), (24) can obtain with (27)

$i_{aP}=\frac{i_{dc}}{3}+\frac{i_a}{2}+{i_{zj}}^{*}---\left(28\right)$

$i_{aN}=\frac{i_{dc}}{3}-\frac{i_a}{2}+{i_{zj}}^{*}---\left(29\right)$

Composite type (28) and (29), the two frequency multiplication negative phase-sequence alternating components that can obtain a phase circulation are

${i_{zj}}^{*}=\frac{1}{2}(i_{aP}+i_{aN})-\frac{i_{dc}}{3}---\left(30\right)$

The all half-bridge cells of each brachium pontis of MMC system can be equivalent to controlled voltage source V _jr(j=a, b, c; R=P, N), then the output voltage of the upper and lower brachium pontis of a phase can be expressed as

$V_{aP}=\underset{i=1}{\overset{N}{\&Sigma;}}{S}_i{V}_d---\left(31\right)$

$V_{aN}=\underset{i=N+1}{\overset{2N}{\&Sigma;}}{S}_i{V}_d---\left(32\right)$

In the equivalent model of Figure 15, the output voltage of H-bridge unit HB1, HB2 is V _{h1, jr}, 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 equivalent resistance of each brachium pontis is R _c, according to KVL Circuit theory, can obtain

$\frac{U_d}{2}-V_{aP}-V_{II1,aP}-V_{II2,aP}-V_a=R_c{i}_{aP}+L_c\frac{{\mathrm{di}}_{aP}}{dt}---\left(33\right)$

$\frac{U_d}{2}-V_{aN}-V_{II1,aN}-V_{II2,aN}+V_a=R_c{i}_{aN}+L_c\frac{{\mathrm{di}}_{aN}}{dt}---\left(34\right)$

Formula (33) and (34) are added, convolution (23) ~ (27), can obtain:

$2L\frac{{\mathrm{di}}_{cir,a}}{dt}+2R_c{i}_{cir,a}=(U_d-V_{aP}-V_{aN})-(V_{II1,aP}+V_{II1,aN})-(V_{II2,aP}+V_{II2,aN})---\left(35\right)$

As can be seen from formula (35), can by controlling the output voltage (V of H-bridge unit HB2 _{h2, aP}+ V _{h2, aN}) size and voltage difference (U _d-V _aP-V _aN)-(V _{h1, aP}+ V _{h1, aN}) equal thus reach and eliminate the object of circulation.

2) loop current suppression realizes

Each phase circulation is carried out positive-negative sequence separation, obtains positive sequence circulation and negative phase-sequence circulation, more respectively with circulation reference value i _{cir, ref}row compares, the result passing ratio obtained regulates the voltage reference value forming H-bridge unit HB2, this reference value is divided into 2 parts, be added in the Voltage Reference of this mutually upper and lower brachium pontis H-bridge unit HB2 respectively, like this, also the impact of HB2 on MMC output voltage is dropped to minimum while elimination circulation.The principle of loop current suppression as shown in figure 16, wherein for the positive sequence component of a phase circulation, for the negative sequence component of a phase circulation.

The process that described second controller realizes the second H-bridge unit capacitance voltage equilibrium comprises: the set-point V of each mutually upper and lower brachium pontis HB2 capacitance voltage _{h2, rrf}compare with the actual capacitance voltage of H-bridge unit HB2, after PI regulates, 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, by voltage given value V _{r, rrf}obtain PWM ripple after the triangular carrier that (r=P, N) and main control part produce, the power switch pipe in drive unit drives the 2nd H unit HB2, carries out charge and discharge control to HB2 electric capacity.

Concrete software execute process of the present invention comprises:

The present invention realizes the programming of controller DSP by the design of main program, capture interrupt subprogram, T1 cycle interruption subprogram and error protection interruption subroutine, and interruption subroutine mainly completes Phase-Locked Synchronous process, control A/D sampling A/D chip, performs main control algorithm etc.

The integrated planning of STATCOM systems soft ware has been come by program design, it is mainly configured the operational environment of dsp system, in system correlated variables initialization, each interrupt initialization, judge whether opens interrupters subprogram etc., then enter and receive and send in the circulation of data, wait for the generation of interrupt event simultaneously.When interruption is unlocked, temporarily stop major cycle, enter into corresponding interrupt service subroutine and carry out various computing and configuration pwm control signal.After having interrupted, return major cycle, continued to wait for the generation next time interrupted.Main program flow chart as shown in figure 22.

The design of capture interrupt subprogram is to realize digital phase-locked loop, with the frequency of detection of grid.The unlatching of capture interrupt subprogram is then that the rising edge produced by the zero crossing of a phase voltage signal is triggered.It should be noted that the frequency of electrical network is not unalterable 50Hz, but one among a small circle in fluctuation, electrical network normal frequency is decided to be 49.3Hz ~ 50.5Hz, if mains frequency normally, will calculate grid cycle accordingly, concrete implementation method as shown in figure 23.

The flow chart of T1 interruption subroutine as shown in figure 24; will complete the sampling of electric current and voltage, the judgement of bridge arm current polarity, half-bridge cells voltage protection, the calculating of active reactive and the calculating etc. of three-phase modulations ripple in this subprogram, the main algorithm of DSP all completes in this subprogram.

The flow chart that error protection is interrupted as shown in figure 25, if when its objective is system generation overcurrent condition, these situations are informed to control centre DSP, makes its protective circuit block generation and then the protection whole system circuit of pulse signal.

Claims (10)

1. the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance, comprise: main control part and driver element, it is characterized in that: described main control part comprises the first controller and second controller, described first controller comprises positive-negative sequence separative element, forward-order current control ring, negative-sequence current control ring, first modulating unit and capacitance voltage sequencing unit, the positive sequence output of described positive-negative sequence separative element and negative phase-sequence output access forward-order current control ring and negative-sequence current control ring respectively, the output of forward-order current control ring is connected the first modulating unit with the output of negative-sequence current control ring, the output of the first modulating unit connects driver element by capacitance voltage sequencing unit, described second controller comprises positive sequence circulation control ring, negative phase-sequence circulation control ring and the second modulating unit, the output of positive sequence circulation control ring is connected driver element with the output of negative phase-sequence circulation control ring by the second modulating unit.

2. the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance according to claim 1, it is characterized in that: described positive-negative sequence separative element comprises voltage positive-negative sequence separation module, electric current positive-negative sequence separation module, phase-locked loop, triangle modular converter, first coordinate transferring and the second coordinate transferring, the positive sequence voltage output of voltage positive-negative sequence separation module is connected the first coordinate transferring by phase-locked loop with triangle modular converter successively, the output of electric current positive-negative sequence modular converter connects the first coordinate transferring and the second coordinate transferring respectively, the output of the first coordinate transferring is positive sequence output, the output of the second coordinate transferring is negative phase-sequence output.

3. the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance according to claim 1, it is characterized in that: described forward-order current control ring is identical with the structure of negative-sequence current control ring, forward-order current control ring comprises the first comparator, second comparator, 3rd comparator, 4th comparator, 5th comparator, one PI controller, 2nd PI controller, 3rd PI controller, first reactor, second reactor and three-dimensional modular converter, the output of the first comparator connects the 3rd comparator through a PI controller, the positive sequence output of described positive-negative sequence separative element connects the second comparator respectively, 3rd comparator first reactor and the second reactor, the output of the 3rd comparator connects the 3rd PI controller successively, 5th comparator and three-dimensional modular converter, the output of the second comparator connects the 2nd PI controller successively, 4th comparator and three-dimensional modular converter, the output of described first reactor connects the 4th comparator, the output of the second reactor connects the 5th comparator, the output of described three-dimensional modular converter is the output of forward-order current control ring, the output of forward-order current control ring is connected the first modulating unit with the output of negative-sequence current control ring by the first comparator bank.

4. the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance according to claim 1, is characterized in that: the positive sequence circulation control ring of described second controller is identical with the structure of negative phase-sequence circulation control ring, and positive sequence circulation control ring comprises the 6th comparator, 7th comparator, 8th comparator, 9th comparator, tenth comparator, 4th PI controller, 5th PI controller, first function module, second function module, proportion adjustment module peace sub-module, the output of described 6th comparator connects the 4th PI controller successively, first function module and the 8th comparator, the output of the 7th comparator connects the 5th PI controller successively, second function module and the 9th comparator, the output of described tenth comparator connects proportion adjustment module peace sub-module successively, the output dividing module equally connects the 8th comparator and the 9th comparator respectively, the output of the 8th comparator and the 9th comparator is the output of positive sequence circulation control ring, and the output of positive sequence circulation control ring is connected the second modulating unit with the output of negative phase-sequence circulation control ring by the second comparator bank.

5. the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance according to claim 1, it is characterized in that: under described three-phase imbalance, the reactive power compensator of dual H-bridge MMC structure comprises MMC converter, described MMC converter to comprise on the identical three-phase of structure brachium pontis under brachium pontis and three-phase, often mutually upper and lower brachium pontis include be connected in series the first H-bridge unit, the second H-bridge unit and some half-bridge cells.

6. based on the control method of the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance described in claim 1, it is characterized in that: the electric current detecting method that the first controller of main control part utilizes positive-negative sequence to be separated detects electric current, adopt the positive-negative sequence based on Feedforward Decoupling to be separated Double-loop Control Strategy and generate three-phase modulations signal, generate drive singal by the voltage sequence control methods based on phase-shifting carrier wave; Second controller adopts and carries out on the basis of positive-negative sequence separation to circulation, by combining with suppression circulation control strategy the Balance route of the second H-bridge unit selfcapacity voltage, generates three-phase modulations signal.

7. the control method of the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance according to claim 6, is characterized in that: the current detecting process that described positive-negative sequence is separated comprises:

Utilize delay method to obtain the three-phase positive sequence component of line voltage, using the three-phase positive sequence component of line voltage as with reference to coordinate, i is carried out to the three-phase current that current transformer exports _p-i _qdetect.

8. the control method of the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance according to claim 7, is characterized in that: the process that described related method thereof obtains grid voltage three-phase positive-negative sequence component comprises:

When three-phase power grid voltage is uneven, obtained by symmetrical component method:

$\left[\begin{array}{c}{u}_a\left(t\right)\\ u_b\left(t\right)\\ u_c\left(t\right)\end{array}\right]=\left[\begin{array}{c}{u_a}^{+}\left(t\right)+{u_a}^{-}\left(t\right)+{u_a}^0\left(t\right)\\ {u_b}^{+}\left(t\right)+{u_b}^{-}\left(t\right)+{u_b}^0\left(t\right)\\ {u_c}^{+}\left(t\right)+{u_c}^{-}\left(t\right)+{u_c}^0\left(t\right)\end{array}\right]$

U in formula _x ⁺(t), u _x ^-(t), u _x ⁰t () is the positive sequence of voltage, negative phase-sequence and zero-sequence component respectively, x represents a, b, c three-phase;

Adopt Y type without center line connected mode, obtained by above formula:

$\left[\begin{array}{c}{u}_a\\ u_b\\ u_c\end{array}\right]=\sqrt{2}{U}_{+}\left[\begin{array}{c}\sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{+}\right)\\ \sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{+}-2/3\mathrm{\&pi;}\right)\\ \sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{+}+2/3\mathrm{\&pi;}\right)\end{array}\right]+\sqrt{2}{U}_{-}\left[\begin{array}{c}\sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{-}\right)\\ \sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{-}-2/3\mathrm{\&pi;}\right)\\ \sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{-}+2/3\mathrm{\&pi;}\right)\end{array}\right]$

Obtained by above formula:

$\begin{array}{c}{u}_b(t+T/3)={u_b}^{+}(t+2\mathrm{\&pi;}/3)+{u_b}^{-}(t-2\mathrm{\&pi;}/3)=\\ \sqrt{2}{U}_{+}\sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{+}\right)+\sqrt{2}{U}_{-}\sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{+}-2\mathrm{\&pi;}/3\right)\end{array}$

$\begin{array}{c}{u}_c(t-T/3)={u_c}^{+}(t-2\mathrm{\&pi;}/3)+{u_b}^{-}(t-2\mathrm{\&pi;}/3)=\\ \sqrt{2}{U}_{+}\sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{+}\right)+\sqrt{2}{U}_{-}\sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{+}+2\mathrm{\&pi;}/3\right)\end{array}$

$\begin{array}{c}{u}_a\left(t\right)+u_b\left(t+T/3\right)+u_c\left(t-T/3\right)=\\ \sqrt{2}{U}_{+}\sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{+}\right)+\sqrt{2}U\_\sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{-}\right)+\\ \sqrt{2}{U}_{+}\sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{+}\right)+\sqrt{2}U\_\sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{-}-2\mathrm{\&pi;}/3\right)+\\ \sqrt{2}{U}_{+}\sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{+}\right)+\sqrt{2}U\_\sin \left(\mathrm{\&omega;}t+{{\mathrm{\&theta;}}_e}^{-}+2\mathrm{\&pi;}/3\right)\end{array}$

Obtain the positive-negative sequence component of three-phase voltage, be shown below;

u _a ⁺(t)＝1/3[u _a(t)-u _b(t-T/6)+u _c(t-T/3)]

u _a ^-(t)＝1/3[u _a(t)-u _b(t-T/3)+u _c(t-T/6)]

u _b ⁺(t)＝1/3[u _a(t-T/3)+u _b(t)-u _c(t-T/6)]

u _b ^-(t)＝1/3[-u _a(t-T/6)+u _b(t)-u _c(t-T/3)]

9. the control method of the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance according to claim 6, it is characterized in that: each phase circulation is carried out positive-negative sequence separation by described second controller, the positive sequence circulation obtained and negative phase-sequence circulation respectively with circulation reference value i _{cir, ref}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, is added in the Voltage Reference of the second H-bridge unit of this mutually upper and lower brachium pontis respectively, obtains the modulating wave of a phase second H-bridge unit.

10. the control method of the reactive power compensator of dual H-bridge MMC structure under three-phase imbalance according to claim 6, is characterized in that: the process that described second controller realizes the second H-bridge unit capacitance voltage equilibrium comprises: the set-point V of each mutually upper and lower brachium pontis the 2nd H cell capacitance voltage _{h2, ref}compare with the actual capacitance voltage of the second H-bridge unit, after PI regulates, 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; By voltage given value V _{r, ref}(r=P, N) with obtain PWM ripple after triangular carrier, the power switch pipe in drive unit drives the 2nd H unit, carries out charge and discharge control to the second H-bridge unit electric capacity.