Protection relay stability study for 115kV and 34.5kV transmission and distribution system

Some clients request the stability study of a protection system in designing power system to evaluate whether protection relays properly operate in case of an internal fault and if they do not function in the events of severe external fault. High fault currents could cause Current Transformers (CTs) to be saturated. A critical concern is the performance of fast protective relay (instantaneous or differential elements) during severe saturation of CTs. Time domain transient simulation software is widely used for the study of these types of works. In this paper, a relay stability study of 115 kV and 34.5 kV transmission and distribution systems was carried out and evaluated utilizing the PSCAD software, which provides several modeling methodologies for various electrical equipment such as transmission line, power transformer, current transformer and protective relays. For the purpose of this study, the modeling method of CT is the most important factor because it influences heavily on the protection relay operation. It is very important to model CT as close as the real characteristics of the CT. Two types of CT models provided by the PSCAD were discussed and compared with each other and an appropriate CT modeling methods were covered in this study. This study is mainly based on digital relays, which prevail recently in the relay market. Digital relay characteristics like anti-aliasing filter, digital filter and various setting function are embedded into PSCAD.