P. Sivakumar* 1 and V. Rajasekaran 2

Transcription

1 IJESC: Vol. 4, No. 1, January-June 2012, pp. 1 5 P. Sivakumar* 1 and V. Rajasekaran 2 Abstract: This project describes the design a controller for PWM boost Rectifier. This regulates the output voltage of the converter and reduces the THD in the input current to operate at unity power factor, when the system has balanced supply. This project introduced the fuzzy controllers. Fuzzy logic to derive a practical control scheme for boost rectifier with active power factor correction. The methodology integrates fuzzy logic control technique in the feedback path and linear programming rule on controlling the duty cycle of the switch for shaping the input current waveform. The proposed approach avoids complexities associated with nonlinear mathematical modeling of switching converters. The control action is primarily derived from a set of linguistic rule written in accordance to experience and intuitive reasoning. Moreover, the number of sensing element is lesser than the classical rectifier as it is unnecessary to sense the supply voltage for shaping the input current. The hysteresis control method is a feedback current control method where the actual current tracks the reference current within a hysteresis band. The hysteresis band current control is used very often because of its simplicity of implementation. Also, besides fast response current loop, the method does not need any knowledge of load parameters. However, the current control with a fixed hysteresis band has the disadvantage that the PWM frequency varies within a band because peak-to-peak current ripple is required to be controlled at all points of the fundamental frequency wave. When the system is unbalanced and/or distorted the reference current is calculated based on p-q theory. The simulation is performed by using MATLAB/SIMULINK software. Keywords: fuzzylogics, PWM sinewave rectifier, MATLAB/SIMULINK software. 1. INTRODUCTION Voltage Source Converter technology is matured at a reasonable level for ac-dc conversion with reduced harmonic currents, high power factor, and low electromagnetic interference (EMI) and radio frequency interference (RFI) at input ac mains and well regulated and good quality dc output to feed loads ranging from fraction of kilowatt to megawatt power ratings in a large number of applications, Voltage source converters were developed in the last couple of decades with varying configurations, control strategies, solid-state devices, circuit integration, varying magnetic in topologies such as boost, buck, buck-boost, and multilevel for unidirectional and bidirectional power flow. A large number of VSC configurations have been evolved to suit for various applications for maintaining a high level of quality at input ac source and output dc loads. THREE-PHASE ac-dc conversion of electric power is widely employed in adjustable-speeds drive (ASDs), uninterruptible power supplies (UPSs), HVAC systems, 1 Department of EEE, R.V.S. School of Engineering and Technology, Dindigul. 2 Department of EEE, PSNA College of Engineering, Dindigul. * Corresponding Author: impsiva@gmail.com and utility interfaces with no conventional energy sources such as solar photovoltaic systems (PVs), etc., battery energy storage systems (BESSs), in process technology such as electroplating, welding units, etc., battery charging for electric vehicles, and power supplies for telecommunication systems [1]-[25]. Traditionally, acdc converters, which are also known as rectifiers, are developed using diodes and thrusters to provide controlled and Uncontrolled unidirectional and bidirectional dc power. They have the problems of poor power quality in terms of injected current harmonics, resultant voltage distortion and poor power factor at input ac mains and slowly varying rippled dc output at load end, low efficiency, and large size of ac and dc filters. TECHNOLOGICAL advances in power electronic devices in recent years have motivated rich study on the pulse width modulation (PWM) ac/dc converters. As compared to traditional phase controlled rectifiers, PWM converter has the attractive features such as high power factor, nearly sinusoidal input current, and simplicity in control circuit and bidirectional power flow ability. To simultaneously achieve these performances, one must pay attention on the design of controller. The system is controlled by fuzzy control algorithm, in which a set of linguistic rules written in accordance to experience and intuitive reasoning. However, if the

2 2 P. Sivakumar and V. Rajasekaran control methodology is directly applied to classical acdc converters with active power factor correction (APFC) [8], it might impose considerable computation time to deal with the fast-varying current loop. This paper presents the use of fuzzy logic to derive a control scheme for boost rectifier with APFC. The methodology integrates fuzzy logic control technique in the feedback path and linear programming rule on the PWM ramp voltage to control the duty cycle of the switch for shaping the input current waveform. The proposed approach avoids complexities associated with nonlinear mathematical modeling of switching converters. The control action is primarily derived from a set of linguistic rule written in accordance to experience and intuitive reasoning. Instead of generating fast-changing PWM signal. Moreover, it is unnecessary to sense the supply voltage for shaping the input current, minimizing the number of sensors. 2. FORCE COMMUTATED PWM RECTIFIER Force-commutated PWM Rectifiers are built with semiconductors with gate-turn-off capability. The gateturn-off capability allows full control of the converter because switches can be switched ON and OFF whenever required. This allows commutation of the switches hundreds of times in one period, which is not possible with line-commutated rectifiers, where thyristor's are switched ON and OFF only once in a cycle. There are two ways to implement force-commutated three phase rectifiers: (a) As a current source rectifier, where power reversal is by dc voltage reversal Figure 1.1 shows the basic circuits for these two topologies. compared with a reference V REF. The error signal generated is used for generating PWM signals to switch ON and OFF the six switches of the rectifier. In this way, power can come from or return to the ac source according with the dc link voltage requirements. The voltage V D is measured across the capacitor C D. Figure 2.1: Operating Principle of the Voltage Source Rectifier When the current I D is positive (rectifier operation), the capacitor C D is discharged, and the error signal is given to the Control Block for more power from the ac supply. The Control block generates the appropriate PWM signals for the six switches. In such a way, more current flows from the AC to the DC side, and the capacitor voltage is recovered the PWM Control not only can manage the active power, but reactive power also. These features allow this type of rectifier to correct power factor. Besides, the ac current waveforms can be maintained almost sinusoidal, reducing harmonic contamination to the mains supply. For example, the pulse width modulation of one phase is shown in Figure. 3.1 Pulse Width Modulation of One Phase The PWM Control not only can manage the active power, but reactive power also. These features allow this type of rectifier to correct power factor. Besides, the ac current waveforms can be maintained almost sinusoidal, reducing harmonic contamination to the mains supply. For example, the pulse width modulation of one phase is shown in Figure 2.2. Figure 1.2: Voltage Source Rectifier 3. ANALYSIS OF THREE PHASE PWM COVERTER OPERATION OF THE THREE PHASE PWM CONVERTER The three phase PWM boost rectifier circuit is shown in figure 2.1. The function of boost rectifier is to regulate the dc output voltage and to shape the input current. To accomplish this task, the dc link voltage is measured and Figure 2.2: PWM Pattern and Its Fundamental V MOD

3 Design a Controller for PWM Boost Rectifier 3 Output voltage of the rectifier can be controlled and by changing its phase-shift with respect to the mains which is shown in figure 2.2 the rectifier can be controlled to operate in the four quadrants: leading power factor rectifier, lagging power factor rectifier, leading power factor inverter, and lagging power factor inverter. 4. CLOSED LOOP OPEARATION OF THREE PHASE PWM RECTIFER To have full control of the operation of the rectifier, the six diodes must be polarized negatively at all values of instantaneous ac voltage supply. Otherwise diodes will conduct and the PWM rectifier will behave like a common diode rectifier bridge. The way to keep the diodes blocked is by ensuring a dc link voltage higher than the peak dc voltage generated by the diodes alone as shown in figure 3. In this way, the diodes remain polarized negatively, and they will only conduct when at least one transistor is switched ON and favorable instantaneous ac voltage conditions are given. In the figure 3 V D represents the capacitor dc voltage, which is kept higher than the normal diodebridge rectification value V BRIDGE. To maintain this condition, the rectifier must have a control loop like the one displayed in figure 4. Figure 3: DC Link Voltage Condition for the Operation of the PWM Rectifier The closed loop control of three phase boost rectifier control is shown in figure 4. The control is achieved by measuring the instantaneous three phase currents and forcing them to follow a sinusoidal current reference template I ref. The amplitude of the current reference template, I MAX, is evaluated using the following equation: I MAX = G c (V ref V D ) Where G C is gain of the voltage compensator. The voltage compensator may be a controller such as PI, P, Fuzzy or other. The sinusoidal waveform of the template is obtained by multiplying I MAX with a sine function, with the same frequency of the mains, and with the desired phase-shift angle, as shown in figure 4. Besides, the template Iref must be synchronized with the power supply. I ref and I line are compared and the error is given to the hysteresis controller, the hysteresis controller will generate PWM pulses. These PWM pulses drives six switches of boost rectifier so as to provide a regulated dc output and also to shape the input current to follow the input voltage in order to improve the power factor. Figure 4: The Closed Loop Block Diagram of the PWM Converter 5. CONTROL TECHNIQUES 5.1 Introduction In this chapter various control techniques implemented for the PWM converter is explained. For open loop, sinusoidal pulse width modulation is used and for closed loop hysteresis controller is employed for inner current controller and for outer voltage control the performance of PI controller and fuzzy logic controller are analyzed. 5.2 PI Controller The PI controller is used for the outer voltage control which reduces the steady state error value to zero. By proper tuning of the values of Kp and Ki the error value can be reduced. The figure 5 shows the general block diagram of the PI controller in the gain K is computed from the following equation, Ki K = K p e, S Where e represents the error, K P and K i are the proportional and integral time constants. Figure 5: Block Diagram of PI Controller

4 4 P. Sivakumar and V. Rajasekaran 5.3 Structure of Fuzzy Logic Controller There are specific components characteristic of a fuzzy controller to support a design procedure. In the block diagram in Figure 3.4, the controller is between a preprocessing block and a post-processing block. The following explains the diagram block by block. Control Rule Table of the FLC ce PB PM PS ZE NS NM NM PB PM e PS ZE NS NM NB Figure 6: Structure of Fuzzy Logic Control When the input to the controller is error, the control strategy is a static mapping between input and Control signal. The control action in a fuzzy logic controller is determined by a set of linguistic rules. Although it is necessary to have a thorough understanding of the converter to be controlled, it does not require a detailed mathematical model of the whole system. Fuzzy logic control has been investigated for applications such as motor drives and dc-dc converters; available literature on fuzzy control of APFC is limited. Objectives include tight output voltage regulation, high rejection of input voltage variations and load transients. 6. FUZZIFICATION Fuzzification is to map e and ec in (6) into suitable linguistic values. Seven fuzzy levels are defined for e and ce, including negative big (NB), negative medium (NM) negative small (NS), zero (E) positive small (PS), positive medium (PM), and positive big (PB). Each input variable is assigned a membership value p to each fuzzy set, based on a corresponding membership function. The number of fuzzy levels is not fixed and depends on the input resolution of needed. The larger the number of fuzzy levels, the higher is the input resolution. Figure 7 shows the membership functions, which are triangular fuzzyset values. Figure 7: Membership Function Adopted in the FLC 7. SIMULATION RESULTS Closed Loop Model The figure shows the closed loop simulation model with outer voltage loop consist of PI controller and inner current loop with hysteresis controller Figure 8: Closed Loop Model with Hysteresis Controller and PI Controller Figure 9: Hysteresis Controller The hysteresis control is a feedback current control method where the actual current tracks the reference current within a hysteresis band the controller generates the sinusoidal reference current of desired magnitude and frequency that is compared with the actual line current. If the current exceeds the upper limit of the hysteresis band, the upper switch of the inverter arm is turned off and the lower switch is turned on.

5 Design a Controller for PWM Boost Rectifier 5 Comparison of PI and fuzzy logic controller 8. CONCLUSION The simulation results of open loop and closed loop operation of three phase PWM boost converter are presented. In closed loop operation the performance of PI controller and fuzzy logic controller are compared from the simulation we can conclude that the voltage regulation is good for both the controllers, the transient response and the dynamic response is better for fuzzy logic controller when compared to PI controller. a fuzzy-logic controller for ac-dc boost rectifier with APFC It integrates fuzzy logic control technique in the feedback path and linear programming rule on controlling the magnitude of the ramp voltage, in order to adjust the duty cycle of the switch for the input current shaping. The proposed approach avoids complexities associated with nonlinear mathematical modeling of switching converters. The control action is primarily derived from a set of linguistic rule written in accordance to experience and intuitive reasoning. Moreover, it is unnecessary to sense the supply voltage for shaping the input current. Experimental measurements show that the system under large-signal variation in the supply voltage and the output load is still stable, demonstrating the validity of the method. REFERENCES [1] B. Singh, B.N. Singh, A. Chandra, K. Al-Haddad, A. Pandey, and D. P. Kothari, A Review of Three-Phase Improved Power Quality AC-DC Converters, IEEE Trans. On Ind. Electronics, 51(3), pp , June [2] Bin Shi, Giri Venkataramanan and Naresh Sharma, Design Considerations for Reactive Elements and Control Parameters for Three Phase Boost Rectifiers, IEEE Transaction pp , June 2005 [3] B.K. Bose, An Adaptive Hysteresis-Band Current Control Technique of a Voltage-Fed PWM Inverter for Machine Drive System, IEEE Transactions on Industrial Electronics, 37(5), October 1990., pp [4] B. Tamyurek A. Ceyhan, E. Birdane, and F. Keles. A Simple DSP Based Control System Design for a Three- Phase High Power Factor Boost Rectifier, IEEE Transaction, pp , [5] Tzann-Shin Lee. Input-Output Linearization and Zero- Dynamics Control of Three-Phase AC/DC Voltage- Source Converters. IEEE Transaction on Power Electronics. 18(1), January [6] Yuan Chang, Liu Jinjun, Wang Xiaoyu, Yang Xin, Wang Zhaoan. Small-signal Modeling of Three-phase Boost Rectifier Under Non-sinusoidal Condition, IEEE Preceding, , [7] R.D. Middlebrook and S. Cuk, A General Unified Approach to Modeling Switching Converter Power Stages, in Proc. IEEE Power Electron. Special. Con$ Rec., 1976, pp