Analysis of Synchronous and Induction Generators in Parallel Operation Mode in an Isolated Electric System using a Ballast Load as a Regulation System under Transient Conditions

Abstract

This paper presents an analysis of a parallel connection of one synchronous generator and one self-excited induction generator that feeds a resistive load and an Induction Motor. The system voltage and frequency are controlled by a voltage control loop and a speed control loop connected to synchronous generator. The induction generator speed is controlled by its primary machine, which is fed by an autotransformer and a diodes bridge. Through by voltages applied by an adjustabletap autotransformer connected to induction generator's primary machine, it is possible widen the range of its shaft speedif compared with the shaft speed caused by only field flux variation method. Then, by the autotransformer method, it is possible to widen the speed and power limits from the induction generator what increases the induction generator contributions and relieves the power supply from synchronous generator. Analysis of generators power balance and its interactionsare presented in this paper in various operational scenarios. The results enable comparisons of the two methods of induction generator speed control, eitherby autotransformer method or by field flux variation method. The first one results in larger range of speed and power from the induction generator. Therefore, it has more confidence features of actual operational conditions. Besides, it presents a ballast load as contingence against a transient condition that consist on a greatremoval of loads considering the induction generation keeping on the same level of generation and it is higher than load demand. Synchronous generation pursues to regulate automatically decreasing its supply power but it is not enough due to high power supplied by induction generator. Then, there is a motorizing of the synchronous machine, and consequently its prime machine becomes generator what causes regenerating energy to the grid, because the supply converter of this machine operatesas inverter, occurring the field inversing. This condition causes an increase of system frequency that should be regulated by a ballast load as described in this paper.