 Energy and Power Engineering, 2013, 5, 121-124 doi:10.4236/epe.2013.54B023 Published Online July 2013 (http://www.scirp.org/journal/epe) Control of Unit Power Factor PWM Rectifier* Meifang Xue, Mingzhi He School of Electrical Engineering, Beijing Jiaotong University, Beijing, China Email: xuemeifang12@gmail.com Received March, 2013 ABSTRACT To solve the problem of harmonic pollution to the power grid that caused by traditional diode rectifier and phase con-trolled rectifier, the unit power factor PWM rectifier is designed. The topology structure of the rectifier circuit is intro-duced and the double closed-loop control strategy in three-phase stationary coordinate system is analyzed. For the defi-ciency of control strategy, the control strategy in two-phase synchronous rotating coordinate system is proposed. This makes the independent control of active current and reactive current to be realized. The simulation model of the PWM rectifier is built and the effectiveness of the control method proposed in this paper is verified by simulation. Keywords: Control; PWM Rectifier; Unit Power Factor; d, q Coordinates 1. Introduction Unity power factor PWM rectifier has the advantages of high power factor, low harmonic content of grid side current, energy bidirectional transmission etc, is widely used in AC drive, reactive power compensation, active power filter, unified power flow control, as well as unin-terruptible power supply, etc. This paper introduces the topology of three-phase PWM rectifier, and describes the control method of the rectifier in three-phase static coordinate system. On the basis of the analysis of the advantages and disadvantages of this control method, the control method in two-phase synchronous rotating coor-dinate system is put forward. Then the mathematical model of three-phase PWM rectifier in d, q coordinates is established and the single control of active current and relative current is realized. 2. The Control Method of Three-phase Voltage Source PWM Rectifier in Three-phase Static Coordinate System Figure 1 shows the topology of three-phase voltage source PWM rectifier, abc is three-phase voltage source, C is the dc side filtering capacity and eeeLR is the load. In order to realize the control of input current and output voltage, the traditional method is controlling the three-phase input current directly. The control of the in- put current is also the control of the flow of energy, thus the control of the output voltage can be realized. The control method of PWM rectifier in the three-phase sta- tionary coordinate system is shown in Figure 2. In this control method, the outer loop controls the DC voltage. The difference value of the command signal and actual signal of DC side voltage is imported to PI regu-lator. The output value of PI regulator is DC current sig-nal mI, mI is proportional to the amplitude of AC input current. So the command signal of three-phase AC cur-rent a*I、*bI、*cIcan be obtained by separately multiplying mI by sinusoidal signal whose phase is the same as three-phase voltage. The difference value of command current and actual current is imported to PI regulator, and the sinusoidal modulation wave can be deserved. By comparing the sinusoidal modulation wave with carrier wave, the PWM wave which can control the switch can be deserved. Figure 1. The topology of three-phase voltage source PWM rectifier. sin t2sin( )3t2sin( )3t Figure 2. The control method of PWM rectifier in three- phase stationary coordinate system. *Project Supported by National Natural Science Foundation of China (51207008) Copyright © 2013 SciRes. EPE M. F. XUE, M. Z. HE 122 This control method is simple, but the command cur- rent in control system is a varying sine time-varying sig- nal which has a certain frequency, amplitude and phase angle. The effect of steady-state performance is not de- sirable, and the independent control of the active current and the reactive current can’t be achieved. 3. The Control Method of Three-phase Voltage Source PWM Rectifier in Two-phase Synchronous Rotating Coordinate System In order to realize the non-static error control of three- phase current and independent control of active current and reactive current, the control method of unit power factor PWM Rectifier in two-phase synchronous rotating coordinate system will be introduced. Figure 3 shows the control method of unity power factor PWM rectifier in dq rotating coordinate system. Through coordinate transformation, three-phase station-ary coordinate system (a, b, c), can be converted to syn-chronous rotating (d, q) coordinate system that synchro-nous rotate with the grid fundamental wave. The transformation matrix is: 32sin120 sin120sin2/3 cos cos120 cos120srC (1) The inverse transformation matrix is: 23sin cos2 / 3sin120cos120sin120 cos120rsC  (2) The most prominent advantage of this transformation is the fundamental sinusoidal quantitative in (a, b, c) co-ordinate system can be converted into a DC variable in (d, Figure 3. The control method of PWM rectifier in dq rotat-ing coordinate system. q) Coordinate system. In this transformation, the d-axis in two-phase synchronous rotating coordinate system rep-resents the active component, and the q-axis represents the reactive component. If we take the position of input voltage vector as the positive direction of d-axis, and the three-phase input voltage can be written as: coscos 120cos 120ambmbmeU wteU wteU wtm (3) Through coordinate transformation the power supply voltage in dq coordinate system can be written as: 0dqeUe (4) According to the instantaneous power theory, the in-stantaneous active power p and reactive power q of the system is: 3232dd qqdq qdpeieiqeiei (5) Because 0qe, the equation (5) can be simplified as 3232dddqpeiqei (6) If we don’t consider the fluctuations in grid voltage, d is a fixed value. So the instantaneous active power p and instantaneous reactive power q of PWM rectifier is proportional to d and q. So that by controlling d and q, the active and reactive power of PWM rectifier can be controlled. ei iiiIn three-phase PWM rectifier, the input instantaneous value of active power in the DC side is dcdc , if the loss of PWM rectifier is not considered, from equation pui(6), we can know that 32dddc dceiui p . When the grid voltage is a fixed value and the loss of rectifier is ignored, the DC side voltage dc is proportional to d, so that DC side voltage of PWM rectifier can be controlled by the control of . u idThe control method shown in Figure 3 also consists of voltage outer loop and current inner loop. Introducing DC feedback and the non-static error control of DC voltage can be realized. Due to the DC voltage can be controlled by the control of d, the output value of volt-age outer loop of PI regulator is the reference value of current inner loop, so that the active power of PWM rectifier can be adjusted. The reference value of reactive iiCopyright © 2013 SciRes. EPE M. F. XUE, M. Z. HE 123current is based on the reference value of reactive power, so when , PWM rectifier operates on unit power factor state. *0qiIn this control method, the PI regulator can realize non- static error control. Compared with the control method in three-phase static coordinate system, the steady state performance is better. At the same time, the independent control of active current and reactive current can be real- ized. 4. The Simulink Results MATLAB/SIMULINK is used to establish the simula-tion model of PWM rectifier. Simulation parameters are as follows: the voltage of power grid is 380 V/50Hz. the inductance in AC side is 0.8 mH. The given value of DC side capacitor voltage is 700 V. The resistance load is 3.72 Ω. Figure 4 shows the three-phase input current wave and its FFT analysis of unit power factor PWM rectifier under the control of three-phase static coordinate system. Figure 5 shows the three-phase input current 0.30.320.34 0.36 0.380.4-400-300-200-1000100200300400 ia ib ic (a) Three-phase input current wave under the control of three-phase static coordinate system 0200 400 600800100000.20.40.6Harmonic orderFundamental (50Hz) = 284.2 , THD= 1.41%Mag (% of Fundamental) (b) FFT analysis of input current under the control of three-phase static coordinate system Figure 4. Three-phase input current wave and its FFT analysis of unit power factor PWM rectifier under the con-trol of three-phase static coordinate system. 0.30.32 0.340.36 0.380.4-400-300-200-1000100200300400 ib icia (a) Three-phase input current wave under the control of two-phase synchronous rotating coordinate system 0200400 600800100000.20.40.6Harm onic orderFundamental (50Hz) = 284.5 , THD= 1.08%Mag (% of Fundamental) (b) FFT analysis of input current under the control of two-phase syn-chronous rotating coordinate system Figure 5. Three-phase input current wave and its FFT analysis of unit power factor PWM rectifier under the con-trol of two-phase synchronous rotating coordinate system. 0.20.250.30.350.4690695700705710 Figure 6. DC side capacitor voltage in two-phase synchro-nous rotating coordinate system. wave and its FFT analysis of unit power factor PWM rectifier under the control of two-phase synchronous ro-tating coordinate system. Figure 6 shows the control effect of DC side capacitor voltage in two-phase syn-chronous rotating coordinate system. It can be seen from the FFT analysis of current waveform that control in two- phase synchronous rotating coordinate system has a bet-ter steady state response than control in three-phase static Copyright © 2013 SciRes. EPE M. F. XUE, M. Z. HE Copyright © 2013 SciRes. EPE 124 coordinate system. 5. Conclusions This paper respectively introduced the control strategy of unit power factor PWM rectifier in three-phase static coordinate system and two-phase rotating coordinate system, and the two control method are compared. The-oretical analysis and simulation results show that the control in synchronous rotating coordinate system has better steady state performance. REFERENCES  K.-N. Areerak, S. V. Bozhko, G. M. Asher and D. W. P. Thomas, “DQ-Transformation Approach for Modelling and Stability Analysis of AC-DC Power System with Controlled PWM Rectifier and Constant Power Loads,” International Power Electronics and Motion Control Conference, 2008, pp. 2049-2054.  Z. Zheng, C. Wang and X. P. Jing, “Comparison of Two Control Strategy for Three-Phase Voltage Source PWM Rectifier,” International Conference on Computer and Communication Technologies in Agriculture Engineering, 2010, pp. 101-104. doi:10.1109/CCTAE.2010.5544851  K. Wei and F. 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