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Three Phase Fully Controlled Bridge Converter

Operation and Analysis of the Three Phase Fully Controlled Bridge Converter

Instructional Objectives

On completion the student will be able to

            • Draw the circuit diagram and waveforms associated with a three phase fully controlled bridge converter.

            • Find out the average, RMS valves and the harmonic spectrum of the output voltage / current waveforms of the converter.

            • Find out the closed form expression of the output current and hence the condition for continuous conduction.

            • Find out the displacement factor, distortion factor and the power factor of the input current as well as its harmonic spectrum.

            • Analyze the operation of higher pulse number converters and dual converter.

            • Design the triggering circuit of the three phase fully controlled bridge converter.

Introduction

The three phase fully controlled bridge converter has been probably the most widely used power electronic converter in the medium to high power applications.  Three phase circuits are preferable when large power is involved.  The controlled rectifier can provide controllable out put dc voltage in a single unit instead of a three phase autotransformer and a diode bridge rectifier.  The controlled rectifier is obtained by replacing the diodes of the uncontrolled rectifier with thyristors.  Control over the output dc voltage is obtained by controlling the conduction interval of each thyristor.  This method is known as phase control and converters are also called “phase controlled converters”.  Since thyristors can block voltage in both directions it is possible to reverse the polarity of the output dc voltage and hence feed power back to the ac supply from the dc side.  Under such condition the converter is said to be operating in the “inverting mode”.  The thyristors in the converter circuit are commutated with the help of the supply voltage in the rectifying mode of operation and are known as “Line commutated converter”.  The same circuit while operating in the inverter mode requires load side counter emf. for commutation and are referred to as the “Load commutated inverter”.

In phase controlled rectifiers though the output voltage can be varied continuously the load harmonic voltage increases considerably as the average value goes down.  Of course the magnitude of harmonic voltage is lower in three phase converter compared to the single phase circuit.  Since the frequency of the harmonic voltage is higher smaller load inductance leads to continuous conduction.  Input current wave shape become rectangular and contain 5^th and higher order odd harmonics.  The displacement angle of the input current increases with firing angle.  The frequency of the harmonic voltage and current can be increased by increasing the pulse number of the converter which can be achieved by series and parallel connection of basic 6 pulse converters.  The control circuit become considerably complicated and the use of coupling transformer and / or interphase reactors become mandatory.    

With the introduction of high power IGBTs the three phase bridge converter has all but been replaced by dc link voltage source converters in the medium to moderately high power range.  However in very high power application (such as HV dc transmission system, cycloconverter drives, load commutated inverter synchronous motor drives, static scherbius drives etc.) the basic B phase bridge converter block is still used.  In this lesson the operating principle and characteristic of this very important converter topology will be discussed in source depth.