Current Transformer Performance during Transient Conditions and the Development of a Current Transformer Selection Criterion for Protection Applications

One of the most crucial requirements for the correct functioning of power system protection equipment is the optimum selection of current transformers (CTs). Therefore, when selecting CTs, the protection engineer has to pay attention to steady state performance as well as transient performance of current transformers. The transient performance of current transformers varies with both system parameters and current transformer parameters. System parameters vary with the fault level and with the inductance to resistance ratio (L/R) at the fault location. In the power system of Sri Lanka, these parameters rapidly vary due to network developments. Thus, the type of the protection relay selected, the type of the protection function and the arrangement of the switchgear have a huge influence on current transformer selection. This paper discusses the development of a current transformer selection criterion for protection applications based on the transient performance of the transformers. In addition to analyzing the current transformer transient performance, PSCAD software has been used in this study to simulate current transformer performance during fault conditions with a case study done to validate the developed selection criterion.