L. Weibo, M. Chengxiong, L. Jiming, and Y. Xiang
m10-ZL Curve of CT, least square estimator, polynomial fitting curve
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 ZL, e.g., relay protection apparatus) will negatively influence the measurement accuracy. Testing ZL and multiple of current in the case of 10% error of current m10, and thus plotting the relationship between ZL and m10, is helpful to give valuable instructions for properly choosing the secondary instrumentations. The principle of calculating values of ZL and m10, methods of operation of acquired data to determine key parameters, and technique to fit the curve m10-ZL resorting to least square estimator are presented. Hence the curve m10-ZL 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.
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