AC DC Converters

AC DC Converters:

AC DC Converters – The naturally commutated phase-controlled converter is a common type of controlled power electronic converter which has reigned supreme for the last 30 years.

Above a few tens of kW, three-phase rectifiers are used, the most common arrangements being the fully/semi-controlled bridges. Semiconverter systems, including a free-wheeling diode give better power factor than the full converter. The poor power factor is a major concern and various techniques such as asymmetrical gating, bias voltage control, etc., have been tried to improve it over the years. A number of gate-firing controllers have been used to control the firing angle in response to a variable input control voltage. Microprocessor-based control is more popular due to implementation possibility of any control law. Two phase controlled converters connected in antiparallel form a dual converter. It is also used for de motor drives.

Thyristor based ac voltage controller (same output frequency) using the phase control principle is normally used in many applications including single-phase appliance-type drives. Phase-controlled cycloconverters are used in VSCF (variable speed constant frequency) large capacity ac drives. The inherent harmonics and lagging var problems due to phase control are also present here.

The Matrix Converter

The desirable features of MCT’s and IGBT’s are expected to initiate a revolution in power converter design over the next decade. The most important will probably be the so-called matrix converter [70] which with proper switching algorithm is capable of generating an almost sinusoidal waveform at input and output and can operate at lagging, unity or even leading fundamental power factor while supplying power to a lagging power factor load at a variable frequency. Four-quadrant operation can clearly be obtained, i.e. motor or regenerating operation with either leading or lagging current. The important feature of this converter is the complete absence of a “dc link” and thus no energy storage capacitor or inductor is needed to support the link voltage or current.

DC/DC Converters

Direct dc/dc chopper converters convert fixed voltage dc source into a variable voltage de source and can be used as a dc transformer to step down (buck) or step up (boost). The force commutated thyristor chopper with voltage commutation was earlier known as oscillation chopper. Other variations are a current commutated or load commutated chopper. Choppers are normally used for traction motor control, rotor resistance control of slip ring induction motors and dc circuit breakers (with zero chopping frequency). A microprocessor based optimum control for four-quadrant operation chopper is described by Dewan and Mirbord. Multiphase choppers are also used for large traction drives requiring large load current.

DC/AC Converters

A bewildering variety of direct dc/ac converters or inverters are in use today employing different auxiliary arrangements for forced commutation or new switching devices. As mentioned, they may be either voltage source or current source type. In current source inverters, the direct current is restricted to one polarity and power reversal for regeneration is carried out by reversing the polarity of the direct voltage. In voltage source inverters, the direct voltage polarity cannot change and reversal of direct current is needed for regeneration using a feedback converter. Also, according to switching, inverters can be PWM or resonant type.

Control

The expert system (ES), ANN and fuzzy control techniques are slowly being used in the control of power electronics. ES techniques have tremendous potential in automating system design, modelling, simulation, real-time control, tests and diagonstics. Fuzzy logic deals with uncertain or imprecise situations.

Power electronic converters are able to perform almost any type of electric power conversion or control. The present research effort is directed towards reducing the cost, size, weight and losses increasing reliability and achieving better performance of various converters.

VSI

A VSI power circuit supplied from a fixed or variable dc voltage (when supplied from an ac source through a phase controlled converters or a diode rectifier followed by a chopper) with a filter capacitor and small inductor. Main applications of VSI are either in ac motor drives with variable voltage and frequency or in UPS having constant voltage and frequency.

Current Source Inverters (CSI)

The CSI is a dual of the VSI described above. A phase controlled rectifier or a diode rectifier followed by a chopper with a current loop and a large inductor provides the adjustable current source. While the major advantages of CSI over VSI are rugged and reliable power circuit, inherent four quadrant operation, etc, the limitations are lower frequency range due to commutation overlap, need of large size dc link inductance and bulky capacitors, open loop instability, torque pulsations and harmonic heating of machines when supplied from it. Various attempts[68] have been made for the last several years to obviate the limitations of CSI.