1 - Power switching theory

Summary

POWER FLOW CONTROL BY SWITCHES

The flow of electrical energy between a fixed voltage supply and a load is often controlled by interposing a controller, as shown in Fig. 1.1. Viewed from the supply, the apparent impedance of the load plus controller must be varied if variation of the energy flow is required. Conversely, seen from the load, the apparent properties of the supply plus controller must be adjusted. From either viewpoint, control of power flow can be realised by using a series-connected controller with the desired properties. If a current source supply is used instead of a voltage source supply, control can be realised by the parallel connection of an appropriate controller. For safety reasons the latter technique is rarely adopted.

The series-connected controller in Fig. 1.1 can take many different forms. In a.c. distribution systems where variability of power flow is a secondary requirement, transformers are often the prevalent interposing elements. The insertion of reactive elements is inconvenient because variable inductors and capacitors of appropriate size are expensive and bulky. It is easy to use a series-connected variable resistance instead, but at the expense of a considerable loss of energy. Viewing from the load side, loads that absorb significant electric power usually possess some form of energy ‘inertia’. This allows amplitude variations created by the interposed controller to be effected in an efficient manner.

Amplitude variations of the controller may be exchanged for a fractional time variation of connection and disconnection from the supply.