An alternative method for testing transient response of protective current transformers

Transient behaviour of protective current transformers is of critical importance from the view-point of power system's performance. International standards prescribe direct testing of transient response of this kind of transformer. Sometimes, when we have to test units designed for big systems, it becomes rather difficult to obtain the necessary time constants in medium power laboratories, especially in low voltage ones. With the purpose of overcoming these difficulties, the authors present an alternative method of testing, exciting the transformer from its primary with a power frequency current of the required value, but with lower than needed time constant, with the addition of a DC current injected into the secondary, so as to obtain the nominal transient flux. Methods of calculation to determine the required amount of DC current are outlined. Theoretical considerations and experimental evidence are presented in the body of the article.