APPLICATION OF POLYNOMIAL FITTING TECHNIQUES TO THE CURVE m 10-Z L OF THE CURRENT TRANSFORMER

Abstract

Conventional ferromagnetic core current transformer (CT) is widely used to measure large alternating currents with lower rated instrumentations or ammeters. The improper choice of the load of secondary instrumentations (symbol in Z L , e.g., relay protection apparatus) will negatively influence the measurement accuracy. Testing Z L and multiple of current in the case of 10% error of current m 10 , and thus plotting the relationship between Z L and m 10 , is helpful to give valuable instructions for properly choosing the secondary instrumentations. The principle of calculating values of Z L and m 10, methods of operation of acquired data to determine key parameters, and technique to fit the curve m 10 -Z L resorting to least square estimator are presented. Hence the curve m 10 -Z L can be successfully plotted based on polynomial coefficients of fitting curve. Experimental results verify the validity of the proposed method, which removes most of the noise of the data, smoothes the curve, and hence increases the measurement accuracy.