High-speed Generator-transformer Unit Backup Protection Scheme

摘要

Power systems transient stability directly depends on the operation of relay protection facilities. The shorter the relay protection operating time, the higher the stability. The main protection and first stage of backup protections are the fastest. One should note here that only their redundancy can reliably prevent the loss of synchronism. The main generator-transformer unit protection against multiphase short circuits in a generator stator winding is the longitudinal differential current protection. Backup of the main protection is effective only against asymmetric short circuits and using only one type of protection - the negative sequence current cutoff without delay and with the protected zone within the unit. In case of symmetric faults negative sequence components of currents and voltages are absent, so this protection is unable to work. All other backup protections act with delay, among them the first-zone distance relay is the fastest. The operating zone of this protection has to reliably cover the unit transformer, so its delay time must be more than the operation time of breaker failure protections of adjacent elements, i.e. no less than 0,9 s. Thus, in the case of a longitudinal differential protection fault, the generator backup protection system is unable to reliably prevent the loss of synchronism after a symmetrical fault clearance. In this report we propose to solve the specified problem by adding an additional stage (a third one) to the distance protection system, with the protection zone within the unit and acting without time delay. Several issues related to the implementation of the proposed scheme are considered.