Introduction to Power Electronics

Power electronics is a branch of engineering that combines the fields of electrical power, electronics, and control. It started with the introduction of the mercury arc rectifier in 1900. The grid controlled vacuum tube rectifier, ignitron, and thyratron followed later. These found extensive application in industrial power control till 1950. In the meanwhile, the invention of the transistor – a semiconductor device – in 1948 marked a revolution in the field of electronics. It also paved the way for the introduction of the silicon-controlled rectifier (SCR), which was announced in 1957 by the General Electric Company. In due course it has come to be named as the ‘thyristor’.

There is one important difference between the earlier electronic tubes and their semiconductor successors. On the low-current side the transistor is a base current controlled device whereas its predecessor, the vacuum tube, is a grid voltage-controlled device. Similarly, the thyristor, which is designed to carry high currents, is a gate current-controlled device; it has succeeded the thyratron, which is a grid voltage-controlled gas tube.

After the inception of the thyristor, which is a pnpn device, many other devices of its family came into existence. These include the DIAC, TRIAC, gate turn-off thyristor (GTO), and the MOS-controlled thyristor (MCT) among others. Subsequently, devices such as the power bipolar junction transistor (BJT), metal oxide semiconductor field effect transistor (MOSFET), and the IGBT have evolved as alternative power electronic devices, each of them being superior to the thyristor in one or more ways.

With its rapid development, power electronic equipment now forms an important part of modern technology. Power electronic applications can be broadly divided into the following five categories:

  1. Motor controls: ac (alternating current) and dc (direct current) drives used in steel, cement, and various other industries
  2. Consumer applications: heat controls, light dimmers, security systems, hand power tools, food mixers, and other home appliances
  3. Vehicle propulsions: electric locomotives used in railways, forklift trucks, and dc chopper based electric vehicles.
  4. Power system applications: applications such as static VAR control, active harmonic filtering, flexible ac transmission system (FACTS) devices, and high-voltage dc (HVDC) systems
  5. Other industrial applications: uninterruptible power supplies (UPSs), switch mode power supplies (SMPSs), battery chargers, industrial heating and melting, arc welding, electrolysis, HV supplies for electrostatic precipitators, aerospace applications, electromagnets, etc.
  6. It is important here to note that all the devices employed for power electronic applications are used in the ‘switch’ mode. The moments of switching on or off are controlled to fulfil the requirements of the circuit under consideration. Likewise, the BJT is operated as a switch.

The advantages of power electronic applications are (a) high efficiency because of low ‘ON state’ conduction losses when the power semiconductor is conducting and low ‘OFF state’ leakage losses when it is blocking the source voltage, (b) reduced maintenance, (c) long life, (d) compactness because of the facility of assembling the thyristors, diodes, and RLC elements in a common package, (e) faster dynamic response as compared to electromechanical equipment, and (f) lower acoustic noise as compared to electromagnetic controllers.

Thyristorised power controllers have some disadvantages; important among them are (i) they generate harmonics which adversely affect the loads connected to them and get injected into the supply lines, (ii) controlled rectifiers operate at low power factors and cause derating of the associated rectifier transformers, and (iii) they do not have a short-time overload capacity. However, as their advantages outnumber their demerits, they are widely used in the various applications detailed above. They have also replaced conventional controllers.

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