Enhancing backup protection for thermal power generating stations using sampled values

Abstract

Evaluation of relay misoperations provides an opportunity to improve the design of backup protection schemes. Existing generator station backup protection schemes use distance protection (21) or voltage controlled/restrained overcurrent protection (51V) elements for clearing phase faults that are not cleared by either line protection or generator protection. Many schemes also use transformer ground overcurrent protection (51NG) for backup protection for ground faults on the system. Since generator protection can only respond to faults with fault resistance accurately when only there is no remote infeed from the other terminals, designing a secure backup protection for a generating station during remote infeed conditions is a challenge. Apart from the remote infeed, fast response of automatic excitation controllers also affects the current seen by the generator protection relay. Considering these effects, different vendors have different approaches for setting the 21 element. In order to analyze these recommendations, this paper studies the performance of synchronous generators for a thermal power station connected through a breaker and half scheme using Real Time Digital Simulator (RTDS) simulations. This paper proposes a new approach to enhance the performance and coverage of backup protection schemes by using sampled values of the IEC 61850 standard. In addition to that, this paper also address the challenges associated with the existing generator protection schemes based on breaker auxiliary contacts and under current elements and proposes a solution using a synchronism check element which uses sampled values. These two techniques can be used in the existing generating station protection schemes to improve the reliability and security of these schemes.