Transient response of synchronous and asynchronous machines to asymmetrical faults in an unbalanced network

Abstract

Abstract This paper investigates the transient stability response of interconnected synchronous and asynchronous machines in an unbalanced system when subjected to asymmetrical shunt faults. The method involves the phase frame representation of the line and the full model simulation of the synchronous generator. The imbalance of the untransposed line and shunt loads is reflected in polyphase impedance matrices. Asymmetrical shunt faults are represented by a polyphase admittance matrix. Thus, the method proposed in this investigation allows a study to be made of the transient stability response of a system containing interconnected AC machines to a series of temporary or permanent shunt faults occurring anywhere in the system; the result of several case studies is presented. A significant finding is that by neglecting imbalance the swing rotor angle peaks are higher, thereby ‘failing to safety’. The running-up of the motor can cause the second peak of the rotor angle swing curve to be higher than the first peak and still the machine goes back to the prefault conditions. In such cases, it is no longer correct to judge the system's stability only on the traditional ‘first swing cycle’ of the rotor angle.