Relay Coordination of EHV, HV, MV and LV Systems of a Typical Thermal Power Station

Abstract

In any thermal power station, auxiliary equipment plays vital role in smooth operation. Majority of the auxiliaries are HT/LT motors, distribution transformers etc. Auxiliary load of thermal unit is fed through unit transformers and station transformers. Transformers have unit protections such as differential, restricted earth fault and short circuit which operates instantaneously. Over-current and earth fault protections are provided in addition to unit protections. These protection relays are coordinated with down stream relays such that relay nearest to fault will operate first. For proper protection, proper co-ordination of relays with appropriate relay set- tings is to be done. However the review of relay co-ordination is always essential since various additions/deletion of feeders and equipments will occur after the initial commissioning of plants. As power can be received from generators of power plant, the analysis becomes complex. Relay co-ordination can be done after selecting proper plug setting multiplier (PSM) and time multiplier setting(TMS) of the relay, considering maximum fault current at the relay location. Thus, for relay co-ordination it is necessary to analyze the system under steady state and having short circuit analysis. Time coordination is done in such a way that no upstream relay will operate first. With the help of IDMT over-current relay, one can calculate required time set- ting on having the short circuit currents, pickup currents and operating time required. This thesis includes the simulation of a working system of a typical thermal power station, with the short circuit analysis performed for single phase to ground fault, development of relay logic for the same and later having relay co-ordination from the obtained results.