Modeling, Analysis And Control Of Single-Phase And Three-Phase PWM Rectifiers
Pulse width modulation (PWM) rectifiers are extensively used in battery charger, regulated dc voltage source, UPS systems, ac line conditioner and motor drives. The conventional control schemes for these rectifiers require PLL, transformations, and input voltage sensing, which increase the cost and complexity of the controller. Simple control schemes based on resistance emulation control are developed in this thesis work for different PWM boost rectifiers. Modeling, analysis and design methods for these rectifier systems are presented. The effect of computational delay involved in digital implementation on the performance of the above rectifier systems is studied. A single-switch boost rectifier system is presented, which operates in DCM and in CCM for an output power less than and greater than 50% rated load, respectively, exploiting the best features of both the operating modes. A generalized feedforward control is presented to improve the dynamic response of output voltage of single-phase boost rectifiers against input voltage, load current and reference voltage disturbances. Feedforward control requires additional voltage and/or current measurements. A state observer is presented for estimating the inductor current of a buck rectifier, and two disturbance observers are presented to estimate the input voltage and the load current of a boost rectifier. These observers eliminate the need of additional sensors for implementing the feedforward control. The resistance emulation control is extended to four-wire PWM rectifier. Two control methods are presented. The first method makes the input currents of the rectifier proportional to their respective input voltages, while the second one balances its input currents even under unbalanced input voltage condition. A detailed analysis of line and neutral current distortions of four-wire converter is presented. A three-carrier based PWM scheme is presented, which significantly reduces the neutral current of the rectifier compared to conventional PWM scheme, when three single-phase inductors are used, and considerably reduces both line and neutral current distortions, when a three-limb inductor is used. A regenerative test setup containing two back-to-back connected three-phase PWM converters is presented for testing high-power converters in the active and reactive power circulation mode. The proposed scheme considerably reduces the cost of testing, and hence, the overall production cost of the converters compared to load-bank testing. A mathematical model is presented for the above system. A suitable control method is presented to control the two converters of the back-to-back system. A new PWM scheme is presented, which considerably reduces the requirement of the dc bus voltage of the back-to-back system compared to conventional PWM schemes. All theoretical predictions are experimentally validated. The experimental results are presented.
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