CN102739071B - Method for controlling direct current capacitor voltage of modular multi-level converter based on circulating current decoupling - Google Patents

Abstract

The invention discloses a method for controlling the direct current capacitor voltage of a modular multi-level converter based on circulating current decoupling. The method is characterized by comprising the following steps of: controlling the total direct current generatrix voltage by using active current which is absorbed from a power grid by the modular multi-level converter; balancing three-phase direct current voltage by using direct current components in circulating current; controlling balance of the direct current generatrixes of upper and lower bridge arms of each phase by using decoupling fundamental wave components in the circulating current; and finely adjusting the output voltage of each module along the current direction of the bridge arms, so the direct current capacitor voltage of each module in the bridge arms is balanced. The control method has the advantages that balanced control over the direct current generatrix voltage of the upper and lower bridge arms can be well realized; by using voltage control rings in other three layers, the direct current voltage of each module can be well balanced and stabilized to be in a given value range; and the method is accurate and reliable, and provides a good reference value for engineering application.

Description

Modular multi-level converter DC capacitor voltage control method based on circulation decoupling zero

Technical field

The present invention relates to the research field such as modular multilevel topological structure (MMC) mesohigh electric energy quality controller and high voltage direct current transmission (HVDC), particularly between modular multilevel brachium pontis, DC bus-bar voltage is balanced controls.

Background technology

Along with social progress and industrial development, there are two large features in modern power systems: electrical power trans mission/distribution system is huge, and idle and nonlinear load capacity increases.First, modern society is also more and more to the demand of electric power, and in order to meet user's demand growing to electric power, it is more and more huger that electric power system becomes, and coverage is also more and more wide.This brings challenges to the stability of electric power system.Meanwhile, different electric power systems often needs to be interconnected at together, to strengthen the reliability of whole electric power system, but when different electric power is interconnected, has nonsynchronous problem.Secondly, the load of modern power systems also has new feature: power electronic equipment has superior performance, is adopted in a large number according to consumption industry by industrial or agricultural.But power electronic equipment, as nonlinear load, can inject idle and harmonic wave to electrical network, along with the increase of nonlinear load capacity, it is also increasing on the impact of distribution system, system is existed dangerous, unstable hidden danger [1].Mesohigh electrical power trans mission/distribution system is carried out to idle and compensation harmonic wave, can effectively improve the stability [2] [3] of electric power system.Interconnected by HVDC (High Voltage Direct Current) transmission system (HVDC) between different electric power systems, can the asynchronous problem of resolution system, also can stop fault to spread between system, be the feasible method that improves the stability of a system and reliability.

Modular multi-level converter (MMC) just obtains scholar's research widely and engineer's strong interest after proposing.Modular multi-level converter has plurality of advantages: modularized design, low switching frequency, low-power consumption, high-quality spectral characteristic etc. [4].These advantages, to the manufacture of modular multi-level converter, are installed, and safeguard and have brought huge convenience, also make it directly hang into mesohigh electrical network without net side transformer.Modular multi-level converter has been applied to HVDC (High Voltage Direct Current) transmission system and mesohigh quality of power supply control system now, becomes effective ways [5]-[9] of improving transmission & distribution electric system Stability and dependability.

In the application of high voltage direct current transmission, mesohigh static reacance generator, mesohigh harmonic compensator and mesohigh frequency conversion speed-adjusting system, an a lot of copped wave units in series form a brachium pontis together, and six brachium pontis are connected into double star structure by linked reactor becomes 3-phase power converter.The DC side of the each copped wave unit in each brachium pontis need to be incorporated to electrolytic capacitor, need not add independently direct voltage source.Expert has carried out large quantity research to modular multi-level converter both at home and abroad at present, has proposed some control methods.

But, in actual applications, the loss of each copped wave unit is different, and the time delay of the switching signal that control circuit produces is also different, add in the dynamic process of load changing current waveform positive-negative half-cycle asymmetric, all can cause the unbalanced of DC voltage between brachium pontis or between module.If do not corrected in real time, some module DC voltages are more and more higher, and module is operated in the state that exceeds the quata, and cause working life to shorten and even directly demolish; Some module DC voltages are more and more lower, and module is operated in the volume state of owing, and can not bring into play due effect.Therefore, each module DC capacitor voltage control problem becomes the Important Problems of modular multilevel transformer application, is also difficulties [7].

For the DC bus-bar voltage control problem of modular multi-level converter, existing multiple solution now: as module row collimation method, brachium pontis energy preestimating method, circulation collocation method, [10]-[17] such as negative-sequence current equalization.These methods are all to propose for a certain concrete application, do not have versatility and systematicness, also have some shortcoming and defect of self simultaneously.Not yet see the good control strategy for modular multi-level converter.For this reason, also need the DC capacitor voltage control problem of modular multi-level converter to carry out systematic research.

Below provide the pertinent literature of retrieval

[1] He Xiangning, Chen Alian. the theory of multi-level converter and application technology. Beijing: the .2006 of China Machine Press

[2] Li Yongdong, Xiao Xi, Gao Yue etc. large capacity multi-level converter---principle, control, application. Beijing: the .2005 of Science Press

[3] Wang Zhaoan, Yang Jun, Liu Jinjun. harmonic wave suppresses and reactive power compensation [M]. Beijing: China Machine Press, 2004.

[4]Hirofumi?Akagi.“Classification,Terminology,and?Application?of?the?Modular?Multilevel?Cascade?Converter(MMCC),”IEEE?Trans.Power?Electron.,vol.26,no.11,pp.3119-31305,Jul.2011.

[5]Saeedifard,M.,Iravani,R.“Dynamic?Performance?of?a?Modular?Multileve?Back-to-Back?HVDC?System,”IEEE?Trans.Power?Delivery.,vol.25,no.4,pp.2903-2912,Jul.2010.

[6]Hirofumi?Akagi.“New?Trends?in?Medium-Voltage?Power?Converters?and?Motor?Drives,”Industrial?Electronics(ISIE),2011?IEEE?International?Symposium.pp.5-14,2011.

[7]H.Mohammadi?P.,M.Tavakoli?Bina.“A?Transformerless?Medium-Voltage?STATCOM?Topology?Based?on?Extended?Modular?Multilevel?Converters,”IEEE?Trans.Power?Electron.,vol.26,no.5,pp.1534–1545,Jul.2011.

[8]Xiaofeng?Yang,Jianghong?Li,Wenbao?Fang,et?al.“Research?on?Modular?Multilevel?Converter?Based?STATCOM,”Industrial?Electronics?and?Applications(ICIEA),2011?6th?IEEE?Conference,pp.2569-2574.2011.

[9]B.Gemmell,J.Dorn,D.Retzmann,and?D.Soerangr,“Prospects?of?multilevel?VSC?technologies?for?power?transmission,”in?Proc.Rec.IEEETDCE,Chicago,IL,2008,pp.1–16.

[10]Qing?rui?Tu,Zheng?Xu.“Impact?of?Sampling?Frequency?on?Harmonic?Distortion?for?Modular?Multilevel?Converter,”IEEE?Trans.Power?Delivery.,vol.26,no.1,pp.298–306,Ju1.2011.

[11]Chun?Gao,Jianguo?Jiang,Xingwu?Yang,“A?Novel?Topology?and?Control?Strategy?of?Modular?Multilevel?Converter(MMC),”Electrical?and?Control?Engineering(ICECE),2011International?Conference,pp.967-971,sept.2011.

[12]D.Soto-Sanchez,T.C.Gree.“Control?of?a?modular?multilevel?converter-based?HVDC?transmission?system,”Power?Electronics?and?Application(EPE?2011),Proceedings?of?the2011-14th?European?Conference,pp.1-10.Aug./Sept.2011.

[13]Makoto?Hagiwara,Ryo?Maeda,Hirofumi?Akagi.“Control?and?Analysis?of?the?Modular?Multilevel?Cascade?Converter?Based?on?Double-Star?Chopper-Cells(MMCC-DSCC),”IEEE?Trans.Power?Electron.,vol.26,no.6,pp.1649–1658,Ju1.2011.

[14]Lanhua?Zhang,Guangzhu?Wang.“Voltage?Balancing?Control?of?a?Novel?Modular?Multilevel?Converter,”Electric?Utility?Deregulation?and?Restructuring?and?Power?Technologies(DRPT),2011?4th?International?Conference,pp.109-114,Conf.2011.

[15]Makoto?Hagiwara,Hirofumi?Akagi.“Control?and?Experiment?of?Pulsewidth-Modulated?Modular?Multilevel?Converters,”IEEE?Trans.Power?Electron.,vol.24,no.7,pp.1737–1746,Jul.2009.

[16] Zhao Xin, Zhao Chengyong, Li Guangkai. adopt the modularization multi-level converter capacitance voltage balance control [J] of phase-shifting carrier wave technology. Proceedings of the CSEE, the 31st volume, the 21st phase, 48-55,2011.

[17] Guo Jie, Jiang Daozhuo, Zhou Yuebin. the modularization multi-level converter control method that alternating current-direct current side electric current difference is controlled. Automation of Electric Systems, the 35th volume, the 7th phase, 42-47,2011.

Summary of the invention

The object of the invention is to propose a kind of modular multi-level converter DC capacitor voltage control method based on circulation decoupling zero.Total be exactly specifically utilize the DC bus-bar voltage of modular multi-level converter from the active current control of electrical network absorption, make dc-voltage balance between three-phase by the flip-flop in circulation, by the balance of every mutually upper and lower two the brachium pontis DC buss of first-harmonic Composition Control of the decoupling zero in circulation, finally realize the equilibrium of DC capacitor voltage between the inner modules of brachium pontis along the output voltage of brachium pontis sense of current fine setting modules.Emphasis of the present invention is at circulation decoupling zero portion control section.

In order to achieve the above object, the present invention is by the following technical solutions:

A kind of modular multi-level converter DC capacitor voltage control method based on circulation decoupling zero, the total DC bus-bar voltage of active current control of utilizing modular multi-level converter to absorb from electrical network, make dc-voltage balance between three-phase by the flip-flop in circulation, by the balance of every mutually upper and lower two the brachium pontis DC buss of first-harmonic Composition Control of the decoupling zero in circulation, finally realize the equilibrium of DC capacitor voltage between the inner modules of brachium pontis along the output voltage of brachium pontis sense of current fine setting modules.

In order to achieve the above object, the present invention can also be by the following technical solutions:

A modular multi-level converter DC capacitor voltage control method based on circulation decoupling zero, comprises the following steps:

Step 1, the DC bus-bar voltage control that three-phase is total

Step 1.1, all copped waves of many level current transformers of detection moduleization three-phase unit DC voltage v _api, v _ani, v _bpi, v _bni, v _cpi, v _cni; Wherein i=1,2 ... N, N is natural number; Obtain A go up mutually brachium pontis DC voltage and

obtain successively A and descend mutually total DC voltage of brachium pontis and B, the each brachium pontis of C two-phase by the method $v_{c,\mathrm{na}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{ani}},$ $v_{c,\mathrm{pb}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{bpi}},$ $v_{c,\mathrm{nb}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{bni}},$ $v_{c,\mathrm{pc}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{cpi}},$ $v_{c,\mathrm{nc}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{cni}},$ Obtain total DC voltage v of each mutually upper and lower two brachium pontis _{ph, a}=v _{c, pa}+ v _{c, na}, v _{ph, b}=v _{c, pb}+ v _{c, nb}, v _{ph, c}=v _{c, pc}+ v _{c, nc}and three-phase average voltage

finally obtain the average DC voltage of module of each brachium pontis $\stackrel{\‾}{V_{\mathrm{cel},\mathrm{pa}}}=v_{c,\mathrm{pa}}/N,$ $\stackrel{\‾}{V_{\mathrm{cel},\mathrm{na}}}=v_{c,\mathrm{na}}/N,$ $\stackrel{\‾}{V_{\mathrm{cel},\mathrm{pb}}}=v_{c,\mathrm{pb}}/N,$ $\stackrel{\‾}{V_{\mathrm{cel},\mathrm{nb}}}=v_{c,\mathrm{nb}}/N,$ $\stackrel{\‾}{V_{\mathrm{cel},\mathrm{pc}}}=v_{c,\mathrm{pc}}/N,$ $\stackrel{\‾}{V_{\mathrm{cel},\mathrm{nc}}}=v_{c,\mathrm{nc}}/N.$

Step 1.2, by three-phase dc busbar voltage mean value

with DC voltage set-point

send into single channel subtracter and carry out computing, operation result is sent into single channel proportional and integral controller and is adjusted, and its output valve is as the active current of modular multi-level converter and AC network exchange

be injected into and on the current inner loop d axle based on dq decoupling zero control, control the active power that whole modular multi-level converter absorbs from electrical network as additional amount.

Step 2, the instruction of DC component in synthetic circulation

Step 2.1, utilizes step 1.1 detection limit

and v _{ph, a}, v _{ph, b}, v _{ph, c}it is poor to do, and sends into three single channel pi regulators and generates

Step 2.2, upper step computing generates

as the instruction of DC component in circulation.

Step 3, the instruction of alternating current component in synthetic circulation

Step 3.1, detects three phase network voltage v _{s, a}, v _s,b, v _{s, c};

Step 3.2, carries out three phase network voltage three phase static coordinate and is tied to the transform operation of two-phase rotating coordinate system, the numerical value of d axle after conversion is sent into single channel low pass filter and carry out filtering, and the output of filter is designated as V _pd; Then by the B detecting, C two-phase line voltage switch, carry out three phase static coordinate again and be tied to the transform operation of two-phase rotating coordinate system, after computing, d axle and q axis values are sent into respectively two single channel low pass filters and are carried out filtering, and after filtering, the numerical value of d axle and q axle is designated as respectively V _ndand V _nq; Transformation matrix in this step is:

$T_{\mathrm{abc}-\mathrm{dq}}=\frac{2}{3}\left[\begin{array}{ccc}\sin \left(\mathrm{\ωt}\right)& \sin (\mathrm{\ωt}-2\mathrm{\π}/3)& \sin (\mathrm{\ωt}+2\mathrm{\π}/3)\\ \cos \left(\mathrm{\ωt}\right)& \cos (\mathrm{\ωt}-2\mathrm{\π}/3)& \cos (\mathrm{\ωt}+2\mathrm{\π}/3)\end{array}\right]$

Step 3.3, the V that step 3.2 is generated _pd, V _ndand V _nqfor people's following relationship, generate perception and the capacitive reference direction of each phase voltage on line side:

v _s,a,1＝V _pd?cosωt+V _nd?cosωt+V _nq?sinωt

v _s,a,-1＝-V _pd?cosωt-V _nd?cosωt-V _nq?sinωt

$v_{s,b,1}=\frac{\sqrt{3}}{2}{V}_{\mathrm{pd}}\sin \mathrm{\ωt}-\frac{1}{2}{V}_{\mathrm{pd}}\cos \mathrm{\ωt}-\frac{\sqrt{3}+1}{2}{V}_{\mathrm{nd}}\cos \mathrm{\ωt}-\frac{\sqrt{3}+1}{2}{V}_{\mathrm{nq}}\sin \mathrm{\ωt}$

$v_{s,b,-1}=-\frac{\sqrt{3}}{2}{V}_{\mathrm{pd}}\sin \mathrm{\ωt}+\frac{1}{2}{V}_{\mathrm{pd}}\cos \mathrm{\ωt}+\frac{\sqrt{3}+1}{2}{V}_{\mathrm{nd}}\cos \mathrm{\ωt}+\frac{\sqrt{3}+1}{2}{V}_{\mathrm{nq}}\sin \mathrm{\ωt}$

$v_{s,c,1}=-\frac{\sqrt{3}}{2}{V}_{\mathrm{pd}}\sin \mathrm{\ωt}-\frac{1}{2}{V}_{\mathrm{pd}}\cos \mathrm{\ωt}+\frac{\sqrt{3}-1}{2}{V}_{\mathrm{nd}}\cos \mathrm{\ωt}+\frac{\sqrt{3}-1}{2}{V}_{\mathrm{nq}}\sin \mathrm{\ωt}$

$v_{s,c,-1}=\frac{\sqrt{3}}{2}{V}_{\mathrm{pd}}\sin \mathrm{\ωt}+\frac{1}{2}{V}_{\mathrm{pd}}\cos \mathrm{\ωt}-\frac{\sqrt{3}-1}{2}{V}_{\mathrm{nd}}\cos \mathrm{\ωt}-\frac{\sqrt{3}-1}{2}{V}_{\mathrm{nq}}\sin \mathrm{\ωt}$

Step 3.4, the v that step 3.3 is generated _{s, b ,-1}and v _{s, c, 1}for people following relationship-v _{s, a}=A (a) v _{s, b ,-1}+ B (a) v _{s, c, 1}equate respectively according to the coefficient of equation the right and left sin ω t and cos ω t, ask for coefficient A (a) and B (a); Similarly, foundation-v _{s, b}=A (b) v _{s, c ,-1}+ B (b) v _{s, a, 1}ask for A (b), B (b), foundation-v _s,c=A (c) v _{s, a ,-1}+ B (c) v _{s, b, 1}ask for A (c), B (c);

Step 3.5, by the numerical value v of step 1.1 _{c, pa}and v _{c, na}, v _{c, pb}and v _{c, nb}, v _{c, pc}and v _{c, nc}send into respectively three single channel subtracters, three single channel pi regulators are sent in the output of subtracter again, and the output valve of three pi regulators is defined as respectively: c _a, c _band c _c;

Step 3.6, by step 3.3,3.4,3.5 variablees that generate are for people's following relationship:

$i_{\mathrm{ac},a}^{*}=c_a\·v_{s,a}+c_b\·B\left(b\right)\·v_{s,a,1}+c_c\·A\left(c\right)\·v_{s,a,-1}$

$i_{\mathrm{ac},b}^{*}=c_a\·A\left(a\right)\·v_{s,b,-1}+c_b\·v_{s,b}+c_c\·B\left(c\right)\·v_{s,b,1}$

$i_{\mathrm{ac},c}^{*}=c_a\·B\left(a\right)\·v_{s,c,1}+c_b\·A\left(b\right)\·v_{s,c,-1}+c_c\·v_{s,c}$

Obtain the interchange circulation instruction of three-phase decoupling zero.

Step 4, the equilibrium control of DC bus-bar voltage between inner each copped wave unit of brachium pontis.

Step 4.1, the actual current i of six brachium pontis of detection _p,a, i _n,a, i _p,b, i _n,b, i _p,c, i _n,c, A is gone up mutually to the average DC voltage of brachium pontis copped wave module

go up mutually the DC voltage v of first module of brachium pontis with A _ap1send into single channel subtracter, proportional controller is sent in the output of single channel subtracter, the output valve of adjuster and the current i of upper brachium pontis _p,amultiply each other, obtain A and go up mutually the amount trimmed of first module alternating voltage of brachium pontis in like manner, A is descended mutually to the average DC voltage of brachium pontis copped wave module

descend mutually the DC voltage v of first module of brachium pontis with A _an1send into single channel subtracter, proportional controller is sent in the output of single channel subtracter, the inverse value-i of the output valve of adjuster and lower brachium pontis electric current _n,amultiply each other, obtain A and descend mutually the amount trimmed of first module alternating voltage of brachium pontis

continue to use same thinking, obtain the alternating voltage amount trimmed of B, first module of C phase upper and lower bridge arm

Step 4.2, obtains respectively A and goes up mutually brachium pontis second to the instruction of N module trim voltage

a descends brachium pontis second to the instruction of N module trim voltage mutually

b goes up brachium pontis second mutually to the instruction of N module trim voltage

b descends brachium pontis second to the instruction of N module trim voltage mutually

c goes up brachium pontis second mutually to the instruction of N module trim voltage

c descends brachium pontis second to the instruction of N module trim voltage mutually

Step 5, the instruction of copped wave module alternating voltage PWM modulating wave generates

Step 5.1, the current loop control system of modular multi-level converter output current is carried out closed loop tracking control to instruction current and output current, and its output is through obtaining three-phase output order voltage pwm modulating wave after dq inverse transformation

with

modular multi-level converter loop current ring control system using step 2.2 and the 3.5 DC loop-current instructions that generate with exchange that circulation instruction adds and as total circulation instruction, and total circulation instruction current and actual loop current are carried out to closed loop tracking control, obtain the command voltage PWM modulating wave of three-phase control circulation

with

Step 5.2, the fine setting instruction of A being gone up mutually to first H bridge module of brachium pontis PWM modulating wave

generate with step 5.1

with

by relational expression

computing, obtains A and goes up mutually the PWM modulating wave of first module of brachium pontis; A is descended mutually to the fine setting instruction of first H bridge module of brachium pontis PWM modulating wave

generate with step 5.1 with

by relational expression computing, obtains A and descends mutually the PWM modulating wave of first module of brachium pontis; By that analogy, obtain remaining in A phase upper and lower bridge arm the final PWM modulating wave of module

with

in B phase upper and lower bridge arm, remain the final PWM modulating wave of module

with

in C phase upper and lower bridge arm, remain the final PWM modulating wave of module

with

Step 5.3, compares with the triangular carrier of phase shift successively with the modulating wave of every mutually each module, generates the switching signal of modules.

The present invention is characterised in that four control rings in above-mentioned steps, and wherein step 1 is first control ring, and object is to control whole modular multi-level converter and absorbs meritorious size from electrical network, to offset the power loss of whole current transformer.Step 2, by automatically regulating the needed DC loop-current of the many level current transformers of generation moduleization, realizes the balance of three alternate DC bus-bar voltage.Step 3 object is to generate the interchange circulation instruction of mutual decoupling zero, redistributes every active power between middle upper and lower bridge arm mutually, makes the equilibrium of direct voltage between each mutually upper and lower two brachium pontis.Step 4 object is to finely tune the command voltage of each module in each brachium pontis, reconfigure the active power that modules absorbs, make the active power of this module actual absorption just can offset the loss of this module self, make the virtual voltage of each module DC side equal rated value.

With respect to prior art, the present invention has following beneficial effect: in the present invention, provide balance control method between the modular multi-level converter upper and lower bridge arm based on circulation decoupling zero, and balance control method between modules.For the feasibility of authentication control method, it is 5kVA that inventor has built capacity in laboratory, and each brachium pontis is by the small test model machine of two copped wave module series connection.From experimental waveform, can find out that this control method can realize the equilibrium control of DC bus-bar voltage between each mutually upper and lower two brachium pontis well.Coordinate the voltage control loop of other three levels, the DC voltage of modules has been realized well equilibrium and has been stabilized in set-point again.This control strategy even all has performance preferably under grid failure state under various operating modes.Experimental result has all proved between three-phase and the equilibrium control of alternate module, and the method is correct, reliable, for engineering application provides good reference value.

Accompanying drawing explanation

Fig. 1 is modular multi-level converter main circuit structure figure;

Fig. 2 is modular multi-level converter DC capacitor voltage control system block diagram; Fig. 2 (a) is total DC bus-bar voltage control; Fig. 2 (b) is balanced control between three-phase, k=a, b, c; Fig. 2 (c) is DC bus-bar voltage control between every mutually upper and lower two brachium pontis, wherein k-1=c, a, b; K=a, b, c; K+1=b, c, a; Fig. 2 (d) is DC bus-bar voltage control between the inner each module of each brachium pontis, wherein k=a, b, c; J=1,2 ... N.

Fig. 3 is modular multi-level converter current control system block diagram; Fig. 3 (a) is the output current control based on dq; Fig. 3 (b) is the circulation control based on proportional controller, k=a, b, c; The total AC output voltage instruction of each module is: $v_{p,\mathrm{kj}}^{*}=\frac{E}{2N}-\frac{v_k^{*}}{N}-\frac{v_{z,k}^{*}}{2N}+\mathrm{\Δ}{v}_{p,\mathrm{kj}}^{*};$ $v_{n,\mathrm{kj}}^{*}=\frac{E}{2N}-\frac{v_k^{*}}{N}-\frac{v_{z,k}^{*}}{2N}+\mathrm{\Δ}{v}_{n,\mathrm{kj}}^{*}.$

Fig. 4 is PWM modulation effect figure.The waveform of four passages is followed successively by, and CH1:A goes up the total AC voltage of brachium pontis mutually; CH2: upper brachium pontis electric current; CH3: lower brachium pontis electric current; CH4: total output current.

Fig. 5 is the experimental verification figure of total DC voltage control.The waveform of four passages is followed successively by, CH1:A phase line voltage; CH2:A phase output current; CH3:A goes up the total output voltage of brachium pontis mutually; CH4:A goes up first module DC voltage of brachium pontis mutually.

Fig. 6 is the balanced experimental verification figure controlling of three-phase.The waveform of four passages is followed successively by, CH1:A phase line voltage; CH2:A goes up first module DC voltage of brachium pontis mutually; CH3:B goes up first module DC voltage of brachium pontis mutually; CH4:C goes up first module DC voltage of brachium pontis mutually.

Fig. 7 is the balanced experimental verification figure controlling between every mutually inner upper and lower two brachium pontis.The waveform of four passages is followed successively by, CH1:A phase line voltage; CH2:A phase output current; CH3:A goes up first module DC voltage of brachium pontis mutually; CH4:A descends first module DC voltage of brachium pontis mutually.

Fig. 8 is the balanced experimental verification figure controlling between the inner each module of brachium pontis.The waveform of four passages is followed successively by, CH1:A phase line voltage; CH2:A phase output current; CH3:A goes up first module DC voltage of brachium pontis mutually; CH4:A goes up second module DC voltage of brachium pontis mutually.

Fig. 9 is the experimental verification figure that modular multi-level converter compensates symmetrical reactive load.The waveform of four passages is followed successively by, CH1:A phase line voltage; CH2:A phase current on line side; CH3:A phase output current; CH4:A goes up first module DC voltage of brachium pontis mutually.

Figure 10 is the experimental verification figure of modular multi-level converter compensation unbalanced nonlinear loads.Figure 10 (a) is three-phase unbalanced network voltage; Figure 10 (b) is three-phase nonlinear load electric current; Figure 10 (c) is three-phase output current; Figure 10 (d) is line voltage and DC capacitor voltage.

Embodiment

With reference to Fig. 1, the many level current transformers of connection mode blocking between three-phase power supply system and threephase load.The main circuit structure of mixed multi-level current transformer is connected by respectively with six linked reactors of six brachium pontis, then forms double star and connects.Each brachium pontis has two copped wave modules to be composed in series, and module DC side parallel has electrolytic capacitor, and switching device adopts the large power all-controlled devices such as IGBT or GTO.The selection of linked reactor parameter depends primarily on the switching frequency of H bridge module.

In each brachium pontis, serial module structure number does not have the upper limit, and value is decided by electric power system electric pressure.In order to narrate conveniently, in the present invention, be elaborated as an example of two module series connection example.Electrical network three-phase voltage is designated as u _s, that is: u _sa, u _sb, u _sc; Power supply three-phase current is designated as i _s, that is: i _sa, i _sb, i _sc; 12 unit DC voltages of series connection copped wave module are designated as respectively v _ap1, v _ap2, v _an1, v _an2, v _bp1, v _bp2, v _bn1, v _bn2, v _cp1, v _cp2, v _cn1, v _cn2; The three-phase offset current of series H-bridge multi-level grid-connected inverter output is designated as i _c, that is: i _ca, i _cb, i _cc; Threephase load electric current is designated as i _l, that is: i _ka, i _lb, i _lc.

With reference to Fig. 2, modular multi-level converter direct current bus voltage control method in the present invention, comprise four control rings, wherein step 1 is first control ring, be total AC/DC energy exchange, step 2 is second control ring, and step 3 is the 3rd control ring, step 4 is the 4th control ring, and concrete steps are as follows:

Step 1, the DC bus-bar voltage control that three-phase is total

Step 1.1, all copped waves of many level current transformers of detection moduleization three-phase unit DC voltage v _api, v _ani, v _bpi, v _bni, v _cpi, v _cni; Wherein i=1,2 ... N, N is natural number; Obtain A go up mutually brachium pontis DC voltage and

obtain successively A and descend mutually total DC voltage of brachium pontis and B, the each brachium pontis of C two-phase by the method $v_{c,\mathrm{na}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{ani}},$ $v_{c,\mathrm{pb}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{bpi}},$ $v_{c,\mathrm{nb}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{bni}},$ $v_{c,\mathrm{pc}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{cpi}},$ $v_{c,\mathrm{nc}}=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{v}_{\mathrm{cni}},$ Obtain total DC voltage v of each mutually upper and lower two brachium pontis _{ph, a}=v _{c, pa}+ v _{c, na}, v _{ph, b}=v _{c, pb}+ v _{c, nb}, v _{ph, c}=v _{c, pc}+ v _{c, nc}and three-phase average voltage

finally obtain the average DC voltage of module of each brachium pontis $\stackrel{\‾}{V_{\mathrm{cel},\mathrm{pa}}}=v_{c,\mathrm{pa}}/N,$ $\stackrel{\‾}{V_{\mathrm{cel},\mathrm{na}}}=v_{c,\mathrm{na}}/N,$ $\stackrel{\‾}{V_{\mathrm{cel},\mathrm{pb}}}=v_{c,\mathrm{pb}}/N,$ $\stackrel{\‾}{V_{\mathrm{cel},\mathrm{nb}}}=v_{c,\mathrm{nb}}/N,$ $\stackrel{\‾}{V_{\mathrm{cel},\mathrm{pc}}}=v_{c,\mathrm{pc}}/N,$ $\stackrel{\‾}{V_{\mathrm{cel},\mathrm{nc}}}=v_{c,\mathrm{nc}}/N.$

Step 1.2, by three-phase dc busbar voltage mean value

with DC voltage set-point send into single channel subtracter and carry out computing, operation result is sent into single channel proportional and integral controller and is adjusted, and its output valve is as the active current of modular multi-level converter and AC network exchange

be injected into and on the current inner loop d axle based on dq decoupling zero control, control the active power that whole modular multi-level converter absorbs from electrical network as additional amount.

Step 2, the instruction of DC component in synthetic circulation

Step 2.1, utilizes step 1.1 detection limit

respectively with v _{ph, a}, v _{ph, b}, v _{ph, c}it is poor to do, and sends into three single channel pi regulators and generates respectively

Step 2.2, upper step computing generates

as the instruction of DC component in circulation.

Step 3, the instruction of alternating current component in synthetic circulation

Step 3.1, detects three phase network voltage v _{s, a}, v _s,b, v _{s, c}.

Step 3.2, carries out three phase network voltage three phase static coordinate and is tied to the transform operation of two-phase rotating coordinate system, the numerical value of d axle after conversion is sent into single channel low pass filter and carry out filtering, and the output of filter is designated as V _pd; Then by the B detecting, C two-phase line voltage switch, carry out three phase static coordinate again and be tied to the transform operation of two-phase rotating coordinate system, after computing, d axle and q axis values are sent into respectively two single channel low pass filters and are carried out filtering, and after filtering, the numerical value of d axle and q axle is designated as respectively V _ndand V _nq.Transformation matrix in this step is:

$T_{\mathrm{abc}-\mathrm{dq}}=\frac{2}{3}\left[\begin{array}{ccc}\sin \left(\mathrm{\ωt}\right)& \sin (\mathrm{\ωt}-2\mathrm{\π}/3)& \sin (\mathrm{\ωt}+2\mathrm{\π}/3)\\ \cos \left(\mathrm{\ωt}\right)& \cos (\mathrm{\ωt}-2\mathrm{\π}/3)& \cos (\mathrm{\ωt}+2\mathrm{\π}/3)\end{array}\right]$

Step 3.3, the V that step 3.2 is generated _pd, V _ndand V _nqfor people's following relationship, generate perception and the capacitive reference direction of each phase voltage on line side:

v _s,a,1＝V _pd?cosωt+V _nd?cosωt+V _nq?sinωt

v _s,a,-1=-V _pd?cosωt-V _nd?cosωt-V _nq?sinωt

$v_{s,b,1}=\frac{\sqrt{3}}{2}{V}_{\mathrm{pd}}\sin \mathrm{\ωt}-\frac{1}{2}{V}_{\mathrm{pd}}\cos \mathrm{\ωt}-\frac{\sqrt{3}+1}{2}{V}_{\mathrm{nd}}\cos \mathrm{\ωt}-\frac{\sqrt{3}+1}{2}{V}_{\mathrm{nq}}\sin \mathrm{\ωt}$

$v_{s,b,-1}=-\frac{\sqrt{3}}{2}{V}_{\mathrm{pd}}\sin \mathrm{\ωt}+\frac{1}{2}{V}_{\mathrm{pd}}\cos \mathrm{\ωt}+\frac{\sqrt{3}+1}{2}{V}_{\mathrm{nd}}\cos \mathrm{\ωt}+\frac{\sqrt{3}+1}{2}{V}_{\mathrm{nq}}\sin \mathrm{\ωt}$

$v_{s,c,1}=-\frac{\sqrt{3}}{2}{V}_{\mathrm{pd}}\sin \mathrm{\ωt}-\frac{1}{2}{V}_{\mathrm{pd}}\cos \mathrm{\ωt}+\frac{\sqrt{3}-1}{2}{V}_{\mathrm{nd}}\cos \mathrm{\ωt}+\frac{\sqrt{3}-1}{2}{V}_{\mathrm{nq}}\sin \mathrm{\ωt}$

$v_{s,c,-1}=\frac{\sqrt{3}}{2}{V}_{\mathrm{pd}}\sin \mathrm{\ωt}+\frac{1}{2}{V}_{\mathrm{pd}}\cos \mathrm{\ωt}-\frac{\sqrt{3}-1}{2}{V}_{\mathrm{nd}}\cos \mathrm{\ωt}-\frac{\sqrt{3}-1}{2}{V}_{\mathrm{nq}}\sin \mathrm{\ωt}$

Step 3.4, the v that step 3.3 is generated _{s, b ,-1}and v _{s, c, 1}for people's following relationship

-v _{s, a}=A (a) v _{s, b ,-1}+ B (a) v _{s, c, 1}equate respectively according to the coefficient of equation the right and left sin ω t and cos ω t, ask for coefficient A (a) and B (a); Similarly, foundation-v _{s, b}=A (b) v _{s, c ,-1}+ B (b) v _{s, a, 1}ask for A (b), B (b), foundation-v _{s, c}=A (c) v _{s, a ,-1}+ B (c) v _{s, b, 1}ask for A (c), B (c);

Step 3.5, by the numerical value v of step 1.1 _{c, pa}and v _{c, na}, v _{c, pb}and v _{c, nb}, v _{c, pc}and v _{c, nc}send into respectively three single channel subtracters, three single channel pi regulators are sent in the output of subtracter again, and the output valve of three pi regulators is defined as respectively: c _a, c _band c _c.

Step 3.6, by step 3.3,3.4,3.5 variablees that generate are for people's following relationship:

$i_{\mathrm{ac},a}^{*}=c_a\·v_{s,a}+c_b\·B\left(b\right)\·v_{s,a,1}+c_c\·A\left(c\right)\·v_{s,a,-1}$

$i_{\mathrm{ac},b}^{*}=c_a\·A\left(a\right)\·v_{s,b,-1}+c_b\·v_{s,b}+c_c\·B\left(c\right)\·v_{s,b,1}$

$i_{\mathrm{ac},c}^{*}=c_a\·B\left(a\right)\·v_{s,c,1}+c_b\·A\left(b\right)\·v_{s,c,-1}+c_c\·v_{s,c}$

Obtain the interchange circulation instruction of three-phase decoupling zero.

Step 4, the equilibrium control of DC bus-bar voltage between inner each copped wave unit of brachium pontis.

Step 4.1, the actual current i of six brachium pontis of detection _p,a, i _n,a, i _p,b, i _n,b, i _p,c, i _n,c, A is gone up mutually to the average DC voltage of brachium pontis copped wave module go up mutually the DC voltage v of first module of brachium pontis with A _ap1send into single channel subtracter, proportional controller is sent in the output of single channel subtracter, the output valve of proportional controller and the current i of upper brachium pontis _p,amultiply each other, obtain A and go up mutually the amount trimmed of first module alternating voltage of brachium pontis

in like manner, A is descended mutually to the average DC voltage of brachium pontis copped wave module

descend mutually the DC voltage v of first module of brachium pontis with A _an1send into single channel subtracter, proportional controller is sent in the output of single channel subtracter, the inverse value-i of the output valve of proportional controller and lower brachium pontis electric current _n,amultiply each other, obtain A and descend mutually the amount trimmed of first module alternating voltage of brachium pontis continue to use same thinking, obtain the alternating voltage amount trimmed of B, first module of C phase upper and lower bridge arm

Step 4.2, obtains respectively A and goes up mutually brachium pontis second to the instruction of N module trim voltage

a descends brachium pontis second to the instruction of N module trim voltage mutually

b goes up brachium pontis second mutually to the instruction of N module trim voltage

b descends brachium pontis second to the instruction of N module trim voltage mutually c goes up brachium pontis second mutually to the instruction of N module trim voltage

c descends brachium pontis second to the instruction of N module trim voltage mutually

Step 5, the instruction of copped wave module alternating voltage PWM modulating wave generates.

Step 5.1, the current loop control system of modular multi-level converter output current is carried out closed loop tracking control to instruction current and output current, and its output is through obtaining three-phase output order voltage pwm modulating wave after dq inverse transformation

with

modular multi-level converter loop current ring control system using step 2.2 and the 3.5 DC loop-current instructions that generate with exchange that circulation instruction adds and as total circulation instruction, and total circulation instruction current and actual loop current are carried out to closed loop tracking control, obtain the command voltage PWM modulating wave of three-phase control circulation with

Step 5.2, the fine setting instruction of A being gone up mutually to first H bridge module of brachium pontis PWM modulating wave generate with step 5.1

with

by relational expression computing, obtains A and goes up mutually the PWM modulating wave of first module of brachium pontis; A is descended mutually to the fine setting instruction of first H bridge module of brachium pontis PWM modulating wave

generate with step 5.1

with

by relational expression

computing, obtains A and descends mutually the PWM modulating wave of first module of brachium pontis.By that analogy, obtain remaining in A phase upper and lower bridge arm the final PWM modulating wave of module

with

in B phase upper and lower bridge arm, remain the final PWM modulating wave of module with

in C phase upper and lower bridge arm, remain the final PWM modulating wave of module

with

Step 5.3, compares with the triangular carrier of phase shift successively with the modulating wave of every mutually each module, generates the switching signal of modules.

Step 1 is first control ring, and object is to control whole modular multi-level converter and absorbs and gain merit from electrical network, the loss producing to offset whole combining inverter.Step 2 is synthesized the DC component instruction in circulation by calculating, regulates the balance of DC bus-bar voltage between three-phase.Step 3 is synthesized the alternating current component instruction in the circulation of mutual decoupling zero by calculating, realize the balance of DC bus-bar voltage between each phase upper and lower bridge arm.Step 4 is by the output voltage instruction of the inner modules of fine setting brachium pontis, redistribute the active power that modules absorbs, make the active power of this module actual absorption just can offset the loss of this module self, thereby make the stable operation under specified command voltage value of each module.Step 5 is synthesized the final output voltage instruction of modules, modulates for PWM.

For the feasibility of authentication control method, it is 5kVA that inventor has built capacity in laboratory, and each brachium pontis is by the small test model machine of two copped wave module series connection.Fig. 4,5,6,7,8,9,10 has provided the experimental waveform that adopts control method in the present invention, be respectively PWM modulation effect, total DC voltage, balanced control between three-phase, between every mutually inner upper and lower two brachium pontis, DC bus-bar voltage is balanced controls, between the inner each module of each brachium pontis, DC bus-bar voltage is balanced controls, and compensation effect when compensation three phase symmetry load compensates the compensation effect of three-phase nonlinear load in electric network fault situation.From experimental waveform, can find out that this control method can realize the equilibrium control of DC bus-bar voltage between each mutually upper and lower two brachium pontis well.Coordinate the voltage control loop of other three levels, the DC voltage of modules has been realized well equilibrium and has been stabilized in set-point again.This control strategy even all has performance preferably under grid failure state under various operating modes.

Claims (1)

1. the modular multi-level converter DC capacitor voltage control method based on circulation decoupling zero, is characterized in that, comprises the following steps:

Step 1, the DC bus-bar voltage control that three-phase is total

Step 1.1, all copped waves of many level current transformers of detection moduleization three-phase unit DC voltage v _api, v _ani, v _bpi, v _bni, v _cpi, v _cni; Wherein i=1,2 ... N, N is natural number; Obtain A go up mutually brachium pontis DC voltage and

a descends total DC voltage of brachium pontis and B, the each brachium pontis of C two-phase mutually

obtain total DC voltage v of each mutually upper and lower two brachium pontis _{ph, a}=v _{c, pa}+ v _{c, na}, v _{ph, b}=v _{c, pb}+ v _{c, nb}, v _{ph, c}=v _{c, pc}+ v _{c, nc}and three-phase average voltage finally obtain the average DC voltage of module of each brachium pontis

Step 1.2, by three-phase dc busbar voltage mean value

with DC voltage set-point

send into single channel subtracter and carry out computing, operation result is sent into single channel proportional and integral controller and is adjusted, and its output valve is as the active current of modular multi-level converter and AC network exchange be injected into and on the current inner loop d axle based on dq decoupling zero control, control the active power that whole modular multi-level converter absorbs from electrical network as additional amount;

Step 2, the instruction of DC component in synthetic circulation

Step 2.1, utilizes step 1.1 detection limit

respectively with v _{ph, a}, v _{ph, b}, v _{ph, c}it is poor to do, and sends into three single channel pi regulators and generates respectively

Step 2.2, upper step computing generates as the instruction of DC component in circulation;

Step 3, the instruction of alternating current component in synthetic circulation

Step 3.1, detects three phase network voltage v _s,a, v _s,b, v _s,c;

Step 3.2, carries out three phase network voltage three phase static coordinate and is tied to the transform operation of two-phase rotating coordinate system, the numerical value of d axle after conversion is sent into single channel low pass filter and carry out filtering, and the output of filter is designated as V _pd; Then by the B detecting, C two-phase line voltage switch, carry out three phase static coordinate again and be tied to the transform operation of two-phase rotating coordinate system, after computing, d axle and q axis values are sent into respectively two single channel low pass filters and are carried out filtering, and after filtering, the numerical value of d axle and q axle is designated as respectively V _ndand V _nq; Transformation matrix in this step is:

Step 3.3, the V that step 3.2 is generated _pd, V _ndand V _nqsubstitution following relationship, generates perception and the capacitive reference direction of each phase voltage on line side:

v _s,a,1=V _pdcosωt+V _ndcosωt+V _nqsinωt

v _s,a,-1=-V _pdcosωt-V _ndcosωt-V _nqsinωt

Step 3.4, the v that step 3.3 is generated _{s, b ,-1}and v _{s, c, 1}substitution following relationship

-v _s,a=A (a) v _{s, b ,-1}+ B (a) v _{s, c, 1}equate respectively according to the coefficient of equation the right and left sin ω t and cos ω t, ask for coefficient A (a) and B (a); Similarly, foundation-v _s,b=A (b) v _{s, c ,-1}+ B (b) v _{s, a, 1}ask for A (b), B (b), foundation-v _s,c=A (c) v _{s, a ,-1}+ B (c) v _{s, b, 1}ask for A (c), B (c);

Step 3.5, by the numerical value v of step 1.1 _{c, pa}and v _{c, na}, v _{c, pb}and v _{c, nb}, v _{c, pc}and v _{c, nc}send into respectively three single channel subtracters, three single channel pi regulators are sent in the output of subtracter again, and the output valve of three pi regulators is defined as respectively: c _a, c _band c _c;

Step 3.6, by step 3.3,3.4, the 3.5 variable substitution following relationships that generate:

Obtain the interchange circulation instruction of three-phase decoupling zero;

Step 4, the equilibrium control of DC bus-bar voltage between inner each copped wave unit of brachium pontis

Step 4.1, the actual current i of six brachium pontis of detection _p,a, i _n,a, i _p,b, i _n,b, i _p,c, i _n,c, A is gone up mutually to the average DC voltage of brachium pontis copped wave module

go up mutually the DC voltage v of first module of brachium pontis with A _ap1send into single channel subtracter, proportional controller is sent in the output of single channel subtracter, the output valve of proportional controller and the current i of upper brachium pontis _p,amultiply each other, obtain A and go up mutually the amount trimmed of first module alternating voltage of brachium pontis a is descended mutually to the average DC voltage of brachium pontis copped wave module descend mutually the DC voltage v of first module of brachium pontis with A _an1send into single channel subtracter, proportional controller is sent in the output of single channel subtracter, the inverse value-i of the output valve of proportional controller and lower brachium pontis electric current _n,amultiply each other, obtain A and descend mutually the amount trimmed of first module alternating voltage of brachium pontis along using the same method, obtain the alternating voltage amount trimmed of B, first module of C phase upper and lower bridge arm

Step 4.2, obtains respectively A and goes up mutually brachium pontis second to the instruction of N module trim voltage

a descends brachium pontis second to the instruction of N module trim voltage mutually b goes up brachium pontis second mutually to the instruction of N module trim voltage

b descends brachium pontis second to the instruction of N module trim voltage mutually c goes up brachium pontis second mutually to the instruction of N module trim voltage

c descends brachium pontis second to the instruction of N module trim voltage mutually

Step 5, the instruction of copped wave module alternating voltage PWM modulating wave generates

Step 5.1, the current loop control system of modular multi-level converter output current is carried out closed loop tracking control to instruction current and output current, and its output is through obtaining three-phase output order voltage pwm modulating wave after dq inverse transformation

with

modular multi-level converter loop current ring control system using step 2.2 and the 3.5 DC loop-current instructions that generate with exchange that circulation instruction adds and as total circulation instruction, and total circulation instruction current and actual loop current are carried out to closed loop tracking control, obtain the command voltage PWM modulating wave of three-phase control circulation

with

Step 5.2, the fine setting instruction of A being gone up mutually to first H bridge module of brachium pontis PWM modulating wave

generate with step 5.1

with by relational expression

computing, obtains A and goes up mutually the PWM modulating wave of first module of brachium pontis; A is descended mutually to the fine setting instruction of first H bridge module of brachium pontis PWM modulating wave

generate with step 5.1

with

by relational expression

computing, obtains A and descends mutually the PWM modulating wave of first module of brachium pontis; By that analogy, obtain remaining in A phase upper and lower bridge arm the final PWM modulating wave of module

with

in B phase upper and lower bridge arm, remain the final PWM modulating wave of module

with

in C phase upper and lower bridge arm, remain the final PWM modulating wave of module

with

Step 5.3, compares with the triangular carrier of phase shift successively with the modulating wave of every mutually each module, generates the switching signal of modules.

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