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 respond to faults with fault resistance accurately only when 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 thesis studies the performance of backup protection schemes
applied at a thermal power station connected through a breaker and half scheme
using Real Time Digital Simulator (RTDS) simulations. This thesis proposes a new
approach to enhance the performance and coverage of backup protection schemes by
using the IEC 61850 standard by using measurements from multiple VT locations
in the same relay to enhance protection security.. In addition to that, this thesis
also address the challenges associated with the existing generator breaker failure
protection schemes based on breaker auxiliary contacts and undercurrent elements
and proposes a solution using a synchronism check element which uses sampled
values. These two techniques can be used in existing generating station protection
schemes to improve the reliability and security of these schemes.