INTRODUCTION TO POWER ELECTRONICS

Power electronics refers to control and conversion of electrical power by power semiconductor devices wherein these devices operate as switches. Advent of siliconcontrolled rectifiers, abbreviated as SCRs, led to the development of a new area application called the power electronics. Prior to the introduction of SCRs, me|rcy-arc rectifiers were used for controlling electrical power, but such rectifier circuits were part of industrial electronics and the scope for applications of mercury-arc rectifiers limited. Once the SCRs were available, the application area spread to many fields such as drives, power supplies, aviation electronics, high frequency inverters and power electronics originated. Power electronics has applications that span the whole field of electrical power systems, with the power range of these applications extending from a few VA/Watts to several M VA / MW. The main task of power electronics is to control and convert electrical power from one form to another. The four main forms of conversion are:

Rectification referring to conversion of ac voltage to dc voltage,

DC-to-AC conversion,

DC-to-DC conversion and

AC-to-AC conversion.

"Electronic power converter" is the term that is used to refer to a power electronic circuit that converts voltage and current from one form to another. These converters can be classified as:

Rectifier converting an ac voltage to a dc voltage,

Inverter converting a dc voltage to an ac voltage,

Chopper or a switch-mode power supply that converts a dc voltage to another dc

voltage, and

Cycloconverter and cycloinverter converting an ac voltage to another ac voltage. In addition, SCRs and other power semiconductor devices are used as static switches

Rectification

Rectifiers can be classified as uncontrolled and controlled rectifiers, and the on-trolled rectifiers can be further divided into semi-controlled and fully-controlled rectifiers. Uncontrolled rectifier circuits are built with diodes, and fully-controlled recti­fier circuits are built with SCRs. Both diodes and SCRs are used in semi-controlled rectifier circuits.

Power rating of a single-phase rectifier tends to be lower than 10 kW. Three-phase bridge rectifiers are used for delivering higher power output, up to 500 kW at 500 V dc or even more. For low voltage, high current applications, a pair of three-phase, three-pulse rectifiers interconnected by an inter-phase transformer (IPT) is used. For a high current output, rectifiers with IPT are preferred to connecting devices directly in parallel.