Chapter 6 - Automatic voltage regulators

Abstract

An alternating current (AC) generator supplied with constant excitation current produces a voltage that exhibits a pronounced droop with the application of normal ships' load, that is, at power factors between unity and zero lagging. By the means of a carefully designed compounding circuit, the voltage variation might be reduced to ± 2.5% with no control of the excitation current. This accuracy is not easy to achieve, and improvement is rendered impossible by a number of effects that are neglected by the simple theory. Among the most serious of these are magnetic saturation in the generator, and the asymmetry between direct (polar) and quadrature (interpolar) axes in salient pole generators and exciters. Some complication is involved in the parallel operation of compounded generators, because the compounding effect would increase the excitation of the generator carrying the largest reactive load, thereby transferring a larger share of this load to that particular generator. For these and other reasons, it has become a universal practice to equip AC generators with automatic voltage regulators (AVRs). This chapter discusses the fundamentals of an AVR, the way in which an AVR controls the excitation of a generator or exciter; it illustrates the power circuits of an excitation supply AVR and a compounding control AVR, analysis of a compounding circuit, and principle of quadrature droop compounding.