Abstract:
New topologies of three phase controlled converters are introduced for AC-DC conversion with high performance. Single phase diode rectifiers normally use DC-DC conversion between rectifier block and the load for power factor correction. It makes input current high frequency switched. Two feedback controllers are required for shaping the input current and to regulate the output voltage of single phase PFC rectifiers. Input power factor and input current THD improvement of three phase rectifiers cannot be achieved effectively by connecting a DC-DC converter stage between the diode bridge and the load. Though regulation of output voltage is possible, due to the existence of paths for line current other than phase to phase, the input source current has shape which is difficult to filter maintaining high efficiency and high input power factor of the rectifier.
In three phase AC-DC conversion, usual practice is to use fully controlled converter comprising of six controlled switches to form the bridge network for power factor correction. Because of pulse width modulation control, these converters show good performance in terms of power factor, total harmonic distortion of input current and efficiency. Due to complex control and requirement of several controlled switching devices, the circuit is not robust. In large three phase rectifier units, harmonic current injection technique is used for power factor improvement and input current shaping. Current injection techniques require separate or derived dc source for generating harmonic current to be injected.
To make a simple 1-2 switch solution for three phase PFC rectification, a new three phase bidirectional switch comprising of 8-12 diodes and a controlled switch is introduced in this research. This switch has separate input and output terminals and can be placed between the input voltages and diode bridge to chop input current and regulate output voltage. Direct input current switching ensures current displacement with input voltage to be nearly unity. This makes it simple to maintain high input power factor and low input current THD. The efficiency of the converters remain high because of single stage AC-DC conversion.
The investigated new three phase AC-DC circuits show good performances which is validated by simulation and practical work. The topologies allow analysis of the circuits on per phase basis. As a result single phase control strategy of PFC can be used in these three phase rectifiers. The proposed three phase bi-directional switch is used to modify modular and Vienna rectifiers. It is observed that by modifying the circuit for three phase modular and Vienna rectifiers it is possible to achieve output voltage control of three phase rectifiers.
New one and two switch SEPIC, Ćuk and Boost-Buck three phase rectifiers are introduced as extension of modular three phase AC-DC rectifiers. These rectifiers have boost-buck gain characteristics and load isolation by capacitance. The proposed three switch and single switch SEPIC, Ćuk and Boost-Buck rectifiers perform well as observed from simulation and experimental results. It is expected that proposed new three phase AC-DC converters comprising of new three phase bidirectional ac switches will provide simple, low cost, high performance solution for systems requiring three phase rectification.
