A HIGH POWER FACTOR THREE-PHASE RECTIFIER BASED ON ADAPTIVE CURRENT INJECTION APPLYING BUCK CONVERTER

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1 9th International onference on Power Electronics Motion ontrol - EPE-PEM Košice A HIGH POWER FATOR THREE-PHASE RETIFIER BASE ON AAPTIVE URRENT INJETION APPYING BUK ONVERTER Žarko Ja, Predrag Pejović EE institute Nikola Tesla, Belgrade, YU, Koste Glavinića 8A Faculty of Electrical Engineering, Belgrade, Bul. revolucije 7 tel.: +8 97; fax: +8 98; e-ail: ja@ieent.org Košice Slovak Republic Abstract. The proble addressed in this paper is how to restore the power taken by the current injection network in the case of optial injection of third haronic, in a siple way, providing self-adjustent of the rectifier to the load current variations. Goals of the presented research are to use only one current sensor, to iprove third haronic injection technique. A new rectifier topology is proposed, as well as the iproved control algorith which odify the rectifier output current. Algorith of the rectifier design is presented. Analytical results are verified by experients. Keywords: A/ converters, Haronics, Power factor correction, Power Quality, Three phase systes, EM/EMI, Efficiency, urrent injection.. INTROUTION Extensive use of power electronics systes gained concern about the power quality. Proinent aong nonlinear power electronics loads are three-phase rectifiers. They are widely used in variable speed drives, power supplies uninterruptible power supplies systes. A ethod to reduce haronic pollution caused by the three-phase rectifiers is the third haronic current injection. The technique applies injection of the third haronic currents in the rectifier supply lines in order to odify the input current wavefor to reduce its distortion. Application of the technique in three-phase diode bridge rectifiers is presented in [] []. The rectifiers proposed in [] [] utilize passive current injection networks, suitable for application at high power levels. To provide proper current injection, a part of the input power has to be transitted to the current injection network. In the case of the optial third haronic current injection, that reduces the input current TH to about 5%, the power taken by the current injection network equals 8.57% of the input power []. The proble addressed in this paper is how to restore the power taken by the current injection network in a siple way, providing self-adjustent of the rectifier to the load current variations. A class of the rectifiers focused in this paper is suitable for application at high power levels. The rectifiers consist of a diode bridge, a current injection device, a current injection network. The current injection network consists of two resonant branches, an adaptive resistance eulation syste. In this paper a new rectifier topology that applies transforers to extract power fro the resonant branches of the current injection network a buck converter to adjust the equivalent transfer ratio of the transforers to the optial value is proposed copared to stard solutions. The analysis of the proposed topology is perfored under the assuption that the diode bridge operates in the continuous conduction ode the design guidelines are given. Finally, the analytically obtained results are confired on an experiental setup of kw of the output power. Goal is to iniize the nuber of sensors used in circuits to get as good as possible TH of input current. Also, it is iportant to use only one power converter (buck) for optial third haronic injection as well as for optial generation of current haronics at even triples of line frequency in paralel to the load. In the second section a basic principles of resistance eulation are pointed out. A novel rectifier with adaptive resistance eulation is proposed in section, using only one current sensor, while in section experiental results are expressed. In section 5 the possibility for further iproveents of input current TH is exainated. The th section contains the conclusion.. THE URRENT-INJETION NETWORKS APPYING AAPTIVE RESISTANE EMUATION Assuing that input voltages for a positive-sequence undistorted three-phase voltage syste, under the constraint that the diode bridge operates in the continuous conduction ode, voltages at the diode bridge output terinals (Fig..) are given by their Fourier series expansions as v A v B V V n n cosn t 9n n cosn t 9n The third haronic coponents of the diode bridge output terinal voltages are () () -

2 9th International onference on Power Electronics Motion ontrol - EPE-PEM Košice va, vb, V sin t () 8 they are utilized to perfor the current injection. v v v 5 v A F F I v OUT I V OUT By choosing the paraeter one can deterine values of capacitors inductors in resonant branches, keeping in ind that branches are resonant at third haronic of line frequency []. In order to acheve the adaptivity of the resistance eulation syste, it is necessaru to introduce a power converter to process the power which has to be dissipated within resonant branches []. The choice is on buck converter due its inherent ability to control the output current. () v B. THE NOVE RETIFIER TOPOOGY Fig.. A basic rectifier circuit with passive resistance eulation To iprove the rectifier efficiency, resistance eulation techniques with power recovery have to be applied. The resistance eulation systes provide the input port voltage proportional to the input port current, they transfer the power taken by the input port to the output port. Basic structure of the rectifier that applies passive resistance eulation is presented in Fig.. urrent controlled voltages are obtained at priaries of the transforers of the resistance eulation syste. Secondaries of the transforers are connected to a filter by a diode bridge. The diode bridge is applied to provide rectification of the third haronic currents, while the filter with a stiff voltage input a stiff current output is applied to transfer the power taken by the resistance eulator to the rectifier output. Voltage across the filter capacitor F is equal to the dc coponent of the output voltage, V OUT V () thus the aplitude of the square wave voltages across the transforer priaries is V (5) R V where is the equivalent transforer turns-ratio (for parallel connection of transforers secondaries). Theoretically, internal losses in the current injection network coponents could deterine the injected current aplitude, but precise realization of the transforers turns ratio is necessary []. Additional proble that has to be solved is how to copensate for variations of the coponent losses the load current. The input current TH is dependent on the resonant circuit characteristic resistance, it decreases with the characteristic resistance increase. This dependence is presented in Fig.. as a function of noralized characteristic resistance, defined as The circuit presented in Fig.. applies a diode bridge in the resistance eulation network. Secondary windings of the transforers are connected in series. This connection of the secondary windings iposes constraint that injected currents i IA i IB are equal, leading to eliination of haronic coponents of these currents at even triples of the line frequency []. 5 5 TH [%] Fig.. The input current TH versus Voltages at priaries of the transforers have a specific wavefor, shown in Fig v RA. [p.u.] electrical angle [rad] Fig.. Voltage wavefor at the transforer priary Ipedance of the current injection network branches is very sall at the triple of the line frequency, so the diode bridge output terinal voltages the controlled voltage sources have to balance each other. The optial value of the transforers turns-ratio is shown to be [] -

3 9th International onference on Power Electronics Motion ontrol - EPE-PEM Košice 5 A Tr Tr i IA i IB F Q F I O + _ OUT B N i Y Fig.. Proposed rectifier topology with adaptive resistance eulation but in practice it has to be higher to copensate for internal losses of the rectifier coponents. The proposed adaptive rectifier topology is presented in Fig.. The role of the buck converter is to keep the aplitude of the injected current at the optial level to achieve the inial TH of input currents. In order to achieve the lowest possible TH of the input current, the necessary aount of even triples of the line frequency current [5] is added to the rectifier output current by appropriate odulation of the buck converter output current (current through F ). Varying the buck converter equivalent conversion ratio, the equivalent transforer turns-ratio is varied, resulting in siple adjustent to variations of the load current coponent paraeters. The control circuitry aintain the aplitude of the injected current so that the RMS value is I Y I () in order to achieve the optial third haronic current injection. The ipleented control circuitry is based on a single insulated current sensor. The current sensor used has one hole of diaeter large enough to put two current carrying conductors in. The first conductor is carrying the dc load current in one direction, while the second one is (9) carrying the rectified injected current. The rectification of the injected current is perfored by an auxiliary diode bridge, connected in series with injection device. The output voltage of the current sensor thus represents the current error, which is further processed through PI regulator, in order to derive the duty cycle control voltage for the boost converter. On such way, the control circuitry aintains condition () to achieve the near optial current injection. onsequently, the introduced error in the injected current aplitude is.7% in coparison to an optial case, what can be considered negligible.. EXPERIMENTA RESUTS In order to verify the analytically obtained results, several siulations a nuber of experients are perfored. The adaptive rectifier fro Fig.. is built tested at different power levels. An average efficiency of 95% is achieved, while load power is varied by varying the supply voltage, due to convenience. Three load points are tested, corresponding to the phase voltage of V,.7V 8.7V, respectively. In the applied current injection network, inductors are of inductance 8.5H capacitors are of capacitance.7f. The supply voltage frequency is f 5Hz. The load resistor is connected in series to inductor of sufficient inductance to -

4 9th International onference on Power Electronics Motion ontrol - EPE-PEM Košice eulate current sink. The load resistor resistance is R. Wavefors of the corresponding input line currents are shown in Fig. 5., for three different rectifier input voltages. The correspondig wavefors of the injected currents are shown in Fig.. This adaptive control of current injection network aintains the total haronic distortion of about 8.% within the specified range of input voltages (different load levels). i t [s] Fig. 5. Input current for different supply voltages, rectifier of Fig.. i Y t [s] Fig.. Injected current for different supply voltages, rectifier of Fig.. 5. FURTHER IMPROVEMENT OF THE POWER REOVERY ONTRO The rectifier under consideration is perforing the optial third haronic injection, as described in [], the best achievable rectifier input current TH is shown to be 5.%. The question is, can be TH of the rectifier input current further iproved while using the optial third haronic injection. According to [5], in optial case the load current can not be constant, but have to contain also haronic coponents at even triples of line frequency, in order to iprove input current TH. The su of output load current haronic coponents at even triples of line frequency is expressed by (), i ( t) I sin( ) t () for range t this wavefor is periodic with period equal to one sixth of the line period. In such case it is necessary to introduce additional odulation of the buck converter output current in the proposed rectifier topology. Thanks to the fact that the extracted power in resonant branches is 8.57% of input power, or 9.7% of output rectifier power, the buck converter output current is 9.7% of load curent for lossless case, or even less if there are losses within power recovery circuit. This constant power recovery current has to be odulated with wavefor (), ( t) I I( cos( )) i t () where the range of arguent reains the sae as for (). This wavefor contains all haronics of even triples of line frequency except zeroth. The total buck converter output current should have the wavefor as shown in Fig. 7., it is obvious that the buck output current is positive all the tie. In that case the rectifier input current TH of.% can be achieved, which eans that optial third haronic current injection generation of haronic coponents at even triples of line frequency is perfored by one single buck power converter. To verify that stateent, a experiental setup is built, soe haronic injection (not optial) in paralel to load current is perfored. In real case, the power recovery buck converter have to have high efficiency to operate in continuous current ode, what ipose hard requireents on buck inductor, which have to process approxiately one tenth of output load current, odulated on described way. Paraeters of the current injection network built for experients are 5 H 5F. The rectifier operates at phase voltages of V RMS V, with the line frequency of f 5Hz. The output dc current is set to be I A. The adjusted value of equivalent transforer turns-ratio is 7. 9 it is tuned to correspond the inial TH of the input currents. The input voltage wavefor is distorted with TH. %, while wavefor of the corresponding input current is presented in Fig. 7. In Fig. 8. wavefor of the resonant branch current i IA is presented. The input current distortion of TH 8. % is achieved. The rectifier built for the experiental purposes operates at the input power level of 9 W, the output power of W is easured. This result corresponds to the rectifier efficiency of 9.%.. ONUSIONS Three-phase high power factor rectifiers that apply current injection networks with resistance eulation power recovery are analyzed in this paper. The adaptive resistance -

5 9th International onference on Power Electronics Motion ontrol - EPE-PEM Košice eulation syste is focused, with only one current sensor eployed. It is shown that low total haronic distortions of the input currents high efficiency could be achieved applying only linear eleents, diodes only one controlled switching eleent. A new rectifier topology is proposed, as well as the iproved control algorith which odify the rectifier output current. Algorith of the rectifier design is presented. Analytical results are verified by experients angle [deg] Fig. 7. The buck output current waveshape in an ideal case (relative to output current) i input tie [s] Fig. 8. Input current of the proposed rectifier, odulated buck output current case i IA - 7. REFERENES [] Ki S., Enjeti P., Packebush P., Pitel I., "A new approach to iprove power factor reduce haronics in a three-phase diode rectifier type utility interface," IEEE Transactions on Industry Applications, pp , vol., no., Nov./ec. 99. [] awrance W. B., Mielczarski W., "Haronic current reduction in a three-phase diode bridge rectifier," IEEE Transactions on Industrial Electronics, pp , vol. 9, no., ec. 99. [] Pejović P., Ja Ž., "An Analysis of Three-Phase ow-haronic Rectifiers Applying the Third- Haronic urrent Injection", IEEE Trans. on Power Electronics,Vol., No., May 999, pp 97-7 [] Ja Ž., Pejović P., "A Novel ow-haronic Three- Phase iode Rectifier Type Utility Interface Applying Passive Resistance Eulation", PEM'98, Prague, 8- Sept. 998, pp 7-,7-9 [5] Pejović P., Ja Ž, "ow-haronic Three-Phase Rectifier Applying urrent Injection", IEE Proc. - Electr. Power Appl., Vol., No. 5, Septeber 999, pp THE AUTHORS Žarko Ja: (9) received the B.S. M.S. degrees in electrical engineering fro the University of Belgrade, Belgrade, Yugoslavia, in , respectively. Since 98, he has been with the epartent of ontrol, EE Institute Nikola Tesla, where he works on high-power converters, uninterruptible power supplies high-powerfactor rectifiers. Predrag Pejović received his B.S. M.S. degrees in electrical engineering fro the University of Belgrade, his Ph.. degree fro the University of olorado, Boulder, in 99, , respectively. Since 995 he has been an Assistant Professor at the University of Belgrade. His current research interests include three-phase low-haronic rectifiers electronic easureents tie [s] Fig. 9. Injected current of the proposed rectifier, odulated buck output current case -