Comparison of current ratio errors of support and toroidal type CTs in electric power systems: A case study

A current transformer (CT) is a specialized device used to reduce high current values to safe and manageable levels in measurement and protection systems. Current ratio error (CRE) is an inevitable error in CTs due to factors such as magnetic saturation, magnetic core properties, primary and secondary winding errors, temperature effects and frequency changes, and manufacturing tolerances. Keeping this error close to zero is critical to the reliability of EPS because it directly affects the operation of protective relays and the accuracy of energy measurement systems. This study has performed a field-based quantitative comparison of absolute percentage current ratio error (AP-CRE) between S-CTs and T-CTs within ARAS’s (Erzurum/Turkey) operational electric power system (EPS). The AP-CRE measurement has been obtained from S-CTs and T-CTs in EPS for protection with accuracy class 5P. Experimental measurement tests on a total of 34 CTs, including 17 S-CT and 17 T-CT, actively used in EPS. AP-CREs have been obtained by means of the TURA-01 device. In addition, during each AP-CRE measurement, absolute phase displacement (A-PD) and excitation current values have been also measured in real time. The performance of S-CT and T-CT have been evaluated based on three main metrics: mean absolute error (MAE), root mean square error (RMSE) and standard deviation (STD) values of the AP-CRE values obtained from CTs. According to AP-CRE; T-CT: MAE=0.44, RMSE=0.47 and STD=0.15, while S-CT: MAE=0.55, RMSE=0.58 and STD=0.21. The AP-CRE performance proves that T-CTs are better than S-CTs in EPS, which increases reliability.