Research on Three-level Rectifier Neutral-Point Voltage Balance. Control in Traction Power Supply System of High. Speed Train

Transcription

1 Researc on Tree-level Rectifier Neutral-Point Voltage Balance Control in Traction Power Supply System of Hig Speed Train LU XIAO-JUAN, WANG XIN-JU, GUO QI, LI SHU-YUAN Scool of Automation and Electric Engineering, Lanzou Jiaotong University, Lanzou 737 Cina Lanzou Jiaotong University, Lanzou 737, Graduate Management Team, Engineering University of Armed Police Force, Xi an 778 Cina CHINA Abstract:-For te neutral-point(np) voltage balance problem of tree-level PWM rectifier in ig speed train traction power supply system, in tis paper, a metod for controlling te neutral point voltage of tree level rectifier is proposed, wic is based on te metod of carrier amplitude conversion. By analyzing te effect of te cange of te carrier amplitude on te neutral point potential, te sinusoidal pulse widt modulation (SPWM) mode of te carrier amplitude transform is introduced into te control of te single pase tree level pulse rectifier. Te control of te neutral point voltage is acieved by te modulation of triangle carrier amplitude. Pulse conversion and carrier frequency sift are used in te control of ig speed train traction and regenerative braking. At last, te Algoritm was applied to te traction and regenerative braking control of CRH (CHINA RAILWAY HIGH-SPEED). Compared to te transient direct current metod tat used in ig speed train now, adding te pulse conversion and carrier amplitude sift control to te NP voltage control system ave better ability to balance te NP voltage under te traction and regenerative braking. But in te of transform traction to regenerative braking, control metod wit amplitude sift is proved to be better, wic illustrated te validity and superiority of carrier amplitude sift control. Key words: Hig-speed Train; Tree-level Rectifier; NP Voltage Balance; Carrier Amplitude Sift Introduction NP voltage balance in ig-speed train is a key point to ensure te train running safely and stably. Train traction drive system requires tree level NPC pulse rectifier to work bi-directionally. Fig. is te main circuit diagram of a power unit on CRH. wile in traction, te rectifier operates in te rectification state, wic gets energy from power grid. wile in regenerative braking, it works in inverter state, wic could transfer te energy tat collected from brake to te power grid. Single pase tree-level PWM rectifier works as AD-DC transformation in EMU traction drive system, but te imbalance of neutral point voltage is te inerent problem of diode clamp converter[][]. For tat, scolars in various countries ave done different researces. Literatures[3][4][5] used treelevel space vector PWM(SVPWM) algoritm m to control te NP voltage troug adjusting te reflecting time of redundant small vector. Literatures[6][7] raised tat troug injecting zero E-ISSN: 4-66X 73 Volume 5, 6

2 sequence signals to balance te NP voltage. Tese two metods are not only of great computational complexity, but also te realization of te process is complex. Tey bot considered single pase rectifier only, but didn t realize te control effect of inverter control, and te train regenerative braking needs te converter works in inverter state. Currently, except te way to balance te neutral point voltage mentioned above, tere are also metod of optimizing te topology[8]. Tis metod needs to improve te ardware, of wic te cost is too ig. Modular capacitor voltage balancing control metod[9], tis metod is to control te average voltage actually, te control precision is not enoug. Motor 4 Pantograp Traction convertor r r R e c tifie In v e rte filter capacitor R e c tifie r Traction transformer Vacuum circuit breaker Traction convertor filter capacitor r rte In v e Motor Fig.. A main circuit diagram of a power unit Te reason to te imbalance of neutral point potential is analyzed in Section. In Section 3, te transient direct current control metod is used to control te single pase tree-level PWM rectifier of ig speed train. Tis paper aimed at te imbalance of NP voltage in te ig-speed train, and used NP voltage balance control metod of pulse conversion(see in Section 4) to make researc. te metod can meet te requirement of balance neutral point voltage in traction, but te control metod can t maintain te balance of te NP voltage in te traction switc to te regenerative mode. Furter more, frequent pulse switcing will increase te switcing frequency of te switc, wic will sorten te service life of te device. Terefore, adding carrier amplitude sift[][] metod (see in Section 5)to te traction, braking and te switc of two kinds of working s of ig speed train, troug cange te magnitude of te carrier, keeps te pase constant to balance te NP voltage, and tis way ave succeed and acieved good results. Finally, using Matlab/Simulink simulated, and compared wit te pulse conversion control metod. Te simulation results sow tat tis metod applying into te traction, regeneration and te switc between two kinds of working s in ig-speed train, bot ave a good ability to balance te NP voltage. Te last section points te main conclusions of tis 4 paper. Analysis of NP voltage of single pase tree-level PWM rectifier in ig speed train Wat sown in Fig. is te topological structure of single pase tree-level diode-clamped PWM rectifier in CRH EMU[]. UN LN RN in Sa Sa S3a S4a a Sb Sb b S3b S4b i C + _ u i io o C + u _ n il RL U Fig. Te main circuit of single pase tree-level PWM rectifier As we can see, RN, LN are grid side resistance and inductance respectively, RL is equivalent load resistance, C and C are DC side support capacitors, and S a ~ S 4b are IGBT power elements. In traction, grid side power factor is close to, sowing positive resistance caracteristic. net side power factor is close to -, sowing negative resistance caracteristic. Te conduction situation of eac bridge leg is sown as follows., S i = Si = Si =, Si = S3i = i= ab, (), S3i = S4i = According to te formula, eac bridge leg equals to a switc, and eac switc as tree kinds of states,, -. Tree-level rectifier equivalent circuit[] is sown in Fig.3 u N i N b i C L N R N a + _ S a - S b - Fig.3 Tree-level rectifier equivalent circuit i i o C p o + _ n u u i L R L + _ + _ E-ISSN: 4-66X 74 Volume 5, 6

3 Eac bridge leg of tree-level rectifier main circuit as tree conduction states,,-, totally 9 working modes. Eac mode of working is sown in Table, and reflect te on and off of te switc, u, u are te voltage of capacitor C and C. Table Voltage of eac working mode S a S b u ao u bo u ab Mode u u u u - u - u u + u u - u u u 5 -u u -u -u 6 -u - u 7 -u - u 8 Tree-level rectifier covert working mode according to formula(), te input voltage u ab is sown in Fig. 4. Tere are five kinds of states to equivalent sine wave, tey are u ab /V 4 - u ± u, ±, Fig.4 Rectifier input voltage Te imbalance of NP voltage will cause current distortion of AC side, produce low order armonic wic will cause te Train generate torque ripple of te traction motor, and influence te speed regulation performance. te lifetime of te power electrolytic capacitor is sortened, so tat efficiency of te power converter is degraded and ig power factor can not be acieved. We can see in Table tat in mode,3,5,7, only one of uao, ubo is zero. At tis point, te grid side voltage source is carging to one of te capacitor and anoter capacitor is discarged troug te load. Current is injected into te neutral point. Te current tat is injected into te neutral point du du io C C dt dt = +, because C = C = C, ten got d u io = C () dt u = u u = idt C (3) According to (3), we can see te basic reason tat causes te inequality of two output voltage is te imbalance carge and discarge of two capacitors in DC side. 3 Transient direct current control of ig speed train At present, te tree level of te CRH multiple units train is te instantaneous direct current metod, te outer loop is te voltage loop and te inner loop is current loop. Te deviation between actual voltage u and given voltage u is used as te input of PI regulation. After te output of te PI regulation is syncronized wit te input voltage sync signal, we can get te reference current component i N. By calculation te voltage and te current of intermediate DC link, we can get te effective component of te given current i N. Ten te given output current i N equals i N plus i N. in = Kp( Ud Ud) + / Ti ( Ud Ud) dt i = IU / U N d d N in = in+ in uab () t = un () t ωlin cosωt RNiN sinωt K in sin ωt in () t (4) Were, K p and T i are te parameters of PI regulator, U d is te given voltage of intermediate DC side, Id, U d are current and voltage of intermediate DC link respectively, K is proportional amplification coefficient, and ω is angular frequency of grid side voltage. Fig.5 is PWM rectifier Simulink simulation, u ab is te output of transient direct current control(fig.6), wic troug SPWM modulation produce trigger pulse control te on-off of te rectifier s two bridge legs to acieve te effect of rectification. E-ISSN: 4-66X 75 Volume 5, 6

4 Fig.5 PWM rectifier simulation Fig.6 Transient direct current control simulation Te reference voltage of te CRH is 3V, grid side voltage is 5V, te frequency of single pase alternating current is 5Hz, and grid side inductance and resistance are L N =.H, R N =.Ω respectively, switc frequency f s = 5Hz. After.5s, te converter switc traction (rectifier) switc to regenerative braking (inverter). Fig.7 is metod of using transient direct current control. wen tere is no NP voltage control, te voltage of DC side tends to stable value 3V after.s and te fluctuation is less tan 4V. After.5s te traction switces to regenerative braking again, and ten state becomes steady after.5s. Fig.8 sows te voltage of two output capacitors, and te voltage deviation are more tan V under two working s. Transient direct current control metod as good regulation ability to DC side voltage wen traction, braking, and two kinds of operating s, but it don t acieve te balance of te upper and lower capacitor voltage. (u Fig U voltage waveform witout NP voltage E-ISSN: 4-66X 76 Volume 5, 6

5 balancing control (u,u Fig.8 Capacitor voltages of te tree-level rectifier witout NP voltage balancing control. 4 Pulse conversion control of ig speed train In te researcing progress, te metod of balancing te NP voltage by pulse conversion is introduced[4]. Its principle is to transform te pulse combination of switc tube tat leads to NP potential imbalanced to te pulse combination tat is conducive to balance te NP potential. Table sows tat only wen te pulse combination are (,), (,), (,-), (-,), tere is io. Among u u tem, (,) and (,-) are redundant state eac oter, and state (-,) and (,) are redundant state eac oter too. Define te relational expression of pulse conversion as (5). Pulse conversion relation in traction (rectifier) is concluded in formula (6). (inverter) is opposite. W u u u i = (5) ( ) N N ( ), (, ) W > (,) (, ) (, ) (, ) W < (, ) (,) (6) Wile u u >, un >, in >, to balance te NP voltage, we sould discarge C or carge C. So te pulse (,) sould switc to (,-), and oter conversions are similar. Fig.9 is pulse conversion control simulation. If te voltage deviation is witin te tolerance range of te comparator, ten output is te original pulse. If te voltage deviation beyond te tolerance range of te comparator, ten output is te converted pulse. Fig.9 Pulse conversion control simulation Fig. is U voltage tat added pulse conversion control. Under traction, after.3s, te DC side voltage tends to te stable value 3V witout more tan 4V fluctuations. Wen te time is.5s, switc to regenerative braking mode. Wen te time is.8s, return to steady state. Fig. is te capacitor E-ISSN: 4-66X 77 Volume 5, 6

6 voltage tat added pulse conversion control. In traction, wen te time is.3s, te two capacitor voltages tends to be close to a given value 5V, and reac a steady state. Wen te time is.5s, switc to regenerative braking mode. Wen te time is.8s, return to steady state. At te steady state, te deviation of te capacitor voltage is less tan V. Wile in switcing operating mode, te deviation is about 5V. Compared wit te transient direct current control metod, after adding pulse conversion control to te system, te fluctuation of U voltage is obviously decreased, and te problem of te capacitor voltage imbalance is improved obviously. Wile switcing to te working s, te capacitor voltage deviation is still large. Besides, te control metod of pulse conversion needs frequently switc te switc tube, te smaller te ysteresis widt, te better te control effect, but accordingly, te switcing frequency is iger, wic will ave an impact on te service life of ardware equipment. if increase te ysteresis widt, we can t reac te ideal control accuracy. (u Fig. (u,u U voltage of te tree-level rectifier wit pulse conversion control Fig. Capacitor voltages of te tree-level rectifier wit pulse conversion control 5 Carrier amplitude sift control of u u ig speed train based on Tree level SPWM modulation is tat comparison of carrier and modulation generates a trigger pulse, and te cange of te carrier wave amplitude can cange te neutral point current so as to influence te neutral point potential. Tree-level SPWM modulated carrier can be expressed by (7)[5] Ts rem(, t Ts) rem(, t Ts) < Ts C = T s rem(, t Ts) rem(, t Ts) < Ts Ts Ts rem(, t Ts) rem(, t Ts) < Ts C = Ts rem(, t Ts) rem(, t Ts) < Ts Ts Were C, C are upper and lower carrier, Ts = / fs is te carrier period, rem is remainder function. Te duty ratio of state in a carrier period is sown in Fig. and (8). u i u i - ()u i ()u i < +u i Fig. Duty ratio of te state ui, ui DiO = i= ab, (8) + ui, ui < Were u = u = u u are modulation wave. a ab b, ab Te duty ratio of working mode (,), (,-) is ua, ua < D(, ) = D(, ) = (9) ua, ua < (7) E-ISSN: 4-66X 78 Volume 5, 6

7 Te duty ratio of working mode (,), (-,) is ua, ua < D(,) = D(, ) = () + ua, < ua < u a u b t/s - - Fig.3 Amplitude sift SPWM modulation u a u b t/s Fig.3 sows te carrier Amplitude sift SPWM modulation sceme. Take = Km as te K, as carrier amplitude cange quantity, [ ] amplitude sift coefficient, m [,] as modulation ratio. Wile te carrier amplitude sift is, we can see te state of te bridge leg ab, in a carrier cycle duty ratio cange to - ()u i - u i ()u i <- +u i Fig.4 Te duty ratio of te state after te amplitude canged ui, ui + D ' io = () + ui, ui < Te duty ratio of working mode (,),(, ),(-,),(,-) respectively are ua, > ua D '(, ) = (), ua < + ua, ua D '(,) =, > ua > + ua, ua D '(, ) = +, > ua > + - (3) (4) ua, > ua D '(, ) = + (5), ua < Wen =, we still use te original carrier modulation. Wen te carrier amplitude is canged( ), state duty cycle and te four kinds of work modes tat affect te NP potential canges all. And D' (, ) D' (, ), D '(, ) D ' (, ), So te effect of adjusting te duty cycle can be acieved by canging te amplitude of te carrier. In a carrier cycle, te current flowing troug te neutral point i tat o caused te NP potential imbalance, in te mode (,), (-,), io = il and in te mode (,), (,-), io = il. In a modulated wave period ( arcsin K, π arcsin K), te voltage difference of u and u could be expressed as π arcsin K arcsin K u = i dwt C (6) Were i o is related to te duty ratio of eac state. Canging te size of can influence te regulation of te duty cycle, and acieve te effect of regulating NP voltage. Fig.5 is control module diagram based on carrier amplitude sift, in wic u, u are te voltage across te capacitors C and C. Te voltage deviation is outputted by ysteresis controller to control te amplitude range of te carrier. Te triangle carrier wic te amplitude transforms compares wit te modulation wave u ab, ten we can get te converter trigger pulse. Fig. 5 is te simulation diagram of carrier amplitude sift control u ab u a u b Amplitudescale conrol m K SPWM modulation Fig. 5 Carrier amplitude sift control u load E-ISSN: 4-66X 79 Volume 5, 6

8 Fig. 6 Te simulation diagram of carrier amplitude sift control Fig.7 is U voltage waveform of carrier amplitude sift control, under te traction, te voltage tends to be stable value 3V after.3s, and te deviation is less tan 3V. At te time.5s, after switcing to regenerative braking mode, at te time s, voltage returns to te steady state, and te voltage deviation is less tan 4V. Fig.8 is te capacitor voltages of carrier amplitude sift control. In te of traction, barking and transformation between two s, te voltage deviations be controlled witin V. Compared to te pulse conversion control, NP voltage deviation decreased significantly, and solved te problem of te large deviation wen te working canges. Fig.9 is te grid side voltage and current waveform of carrier amplitude sift control, un, i N ave te same pase under traction, and te power factor close to. un, i N are appositive pase under regenerative braking, and te power factor is close to -. Te SIMULINK results are consistent wit te teory. Tis control metod improved te problem of te NP voltage imbalance witout canging te oter system performances. (u Fig.7 U 3 3 voltage of carrier amplitude sift control (u,u Fig.8 Capacitor voltages u and u using carrier amplitude sift metod (u N,i N )/(V,A) u N i N u u Fig.9 Grid side voltage and current of carrier amplitude sift control Compared wit te pulse conversion control, te carrier amplitude sift metod don t need to switc te operation state of switc tube frequently. Witout te limitation of te switcing frequency, tis metod can acieve iger control accuracy in various operating s and te conversion of s. Fig. sows te capacitor voltage deviation of using no NP voltage balance control, pulse conversion control and carrier amplitude sift metod respectively. In terms of no NP potential control, te voltage deviation is large under traction. After.4, te deviation is about 3V. At te time.5s, te deviation increases wile te traction mode switces to te regenerative braking. After te system is stable, te deviation is about 3V. Te voltage deviation decreased significantly after added te NP potential control of pulse conversion, and te deviation is less tan V under te traction wen te system is stable. At te time of.5s, te voltage deviation reaces to 5V wen te is switced. Te deviation is controlled witin V after te system is stable. After adding te NP control of te carrier amplitude sift, te voltage deviations are all controlled witin V under traction and braking. Tere is no major fluctuation wen te E-ISSN: 4-66X 8 Volume 5, 6

9 traction is switced to te regenerative mode witin.5s. ( u ( u ( u Fig. Capacitor voltage deviation under tree control modes Table are te mean square error (MSE) of two capacitor voltages deviation of transient direct current control, pulse conversion and carrier amplitude sift control under traction and braking s respectively. Under te traction, te deviation is larger witout NP voltage control, and te MSE is.77. Te deviation is decreased after te addition of te pulse conversion control, te MSE is 7.79, and te control tecnique of pulse conversion acieved te effect of balance te NP potential in traction. te MSE decreased to 3.75 after added carrier amplitude sift control, wic is significantly better tan te first two control effects. In regenerative braking, te MSE is 8.3 witout NP potential control. After adding te pulse conversion control, because te voltage deviation is large wen operating mode canges. Te MSE increased to 5.57, and it becomes.98 after adding te carrier amplitude sift control. Te problem of NP voltage unbalance is effectively solved. Te control tecnique of te pulse conversion can reac a certain balance effect under te traction, but after switcing to regenerative braking mode, te control effect is not good. Te control metod for carrier amplitude sift as te good ability to balance te NP voltage in bot traction and braking s. Table Te mean square error of te two capacitor voltages error deviation under all kinds of control metod Control metods Conditions MSE Traction.77 Instant direct Regenerative Current control 8.3 braking Pulse Switcing Traction 7.79 control Amplitude-Sift Control 6 Conclusion Regenerative braking 5.57 Traction 3.75 Regenerative braking.98 In tis paper, CRH EMU converter is cose as te researc object, te reason to te imbalance of neutral point potential is analyzed. Based on transient direct current control, a neutral point potential control metod by adjusting te amplitude of te carrier is introduced. Comparing wit te neutral point potential control metod of pulse transformation, tis metod can not only balance te neutral point potential well wile te system is stable, control te deviation of te two capacitor voltage witin V, but also response fast and maintain neutral point potential balance wen te system s canges. Te mean square deviation of traction and regenerative braking are 3.75 and.98 respectively, wic provide an effective metod for solving te traction motor load torque ripple problem tat caused by te imbalance of neutral point potential in te ig speed train and ensure te good speed performance of te train. Te voltage deviation is controlled witin V after adding te NP control of te carrier amplitude sift. Te relationsip between te carrier amplitude cange quantity and voltage deviations will be studied in te future work in order to control NP voltage more accurately. References [] Akira Nabae, Isao Takaasi, Hirofumi Akagi, A New Neutral-point-clamped PWM Inverter, IEEE Trans.on Industry Application, Vol.7, No.5,98, pp [] Wang Wanbao, Zang Li, Hu Haibing, Researc on Mecanism of Single-pase Tree-level Half-bridge Grid-tied Inverter Voltage Unbalance of Input Capacitors and Voltage Balance Control Strategy, Proceedings of te CSEE, Vol.34, No.6,4, pp [3] Zang Conglong, Yuan Cunming, Zang Xing, Tree-level Inverter s Neutral Point Balancing in Simplified SVPWM Algoritm and Its Implementation, Electric Drive, Vol.38, E-ISSN: 4-66X 8 Volume 5, 6

10 No.,8, pp [4] Song Wenseng, FENG Xiaoyun, WANG Lijun, Researc on Single Pase Tree-level PWM Rectifier Based on SVPWM Modulation, Converter Tecnology, Vol.3, No.,8, pp [5] Pinkymol Harikrisna Raj, Ali l. Maswood, Gabriel H.P.Ooi, Ziyou Lim, Voltage balancing tecnique in a space vector modulated 5-level multiple-pole multilevel diode clamped inverter, IET Power Electron, 5, 8(7):63 7. [6] Song Qiang, LIU Wenua, A Neutral-point Potential Balancing Algoritm for Tree-level NPC Inverters by Using Analytically Injected Zero-sequence Voltage, Proceedings of te Csee, Vol.4, No.5,4, pp [7] Feng Xiaoyun, Song Wenseng, A Single Pase Tree-Level Carrier-based PWM Sceme wit Zero-sequence Voltage Injection, Transactions of Cina Electrotecnical Society, Vol.8, No.4 3, pp [8] Tan Xitang, Ceng Linkun, Zu Qinyue, Researc on Simulation of Optimization Control of Traction Converter, Computer Simulation, Vol.3, No.,5, pp [9] Z.L.Su, X.Q.He, Z.Y.Wang, D.Q.Qiu and Y.Z.Jing, Voltage Balancing Approac Diode-Clamped Multilevel Converter Using Auxiliary Capacitor-Based Circuits, IEEE Trans. Power Electronics, Vol.8, No.5, 3, pp. -4. [] Gong, Bo; Ceng, Sangmei; Qin, Yi, A Tree-Level Neutral Point Voltage Balance Control Strategy Based on Carriers of SPWM, Transactions of Cina Electro tecnical Society, Vol.8, No.6,, pp [] Gong, Bo; Ceng, Sanmei; Qin, Yi, Simple Tree-level Neutral Point Voltage Balance Control Sceme Based on Carrier Overlapping SPWM, Przeglad Elektrotecniczny, Vol.88, No.a,, pp [] LU Xiaojuan, EMU control tecnology, Soutwest Jiao Tong University Press,. [3] A.Sukla, A.Gos and A.Josi, Control and DC Capacitor Voltages in Diode-Clamped Multilevel Inverter Using Bidirectional Buck-Boost Coppers, IET Power Electron, Vol.5, No.9,, pp [4] ZHANG Zibing, ZHANG Zixue, Study of Control Strategies and Neutral Point Voltage Balancing of Single Pase Tree-level Rectifier, Electric Drive for Locomotives, No.4 8, pp [5] GONG Bo, CHENG Sanmei, QIN Yi. Novel Tree-level Neutral Point Voltage Balance Control Strategy, Electric Drive, Vol.4, No.,, pp E-ISSN: 4-66X 8 Volume 5, 6