TRANSIENT RESPONSE OF CURRENT TRANSFORMERS.

The asymmetrical component of power-system fault current sometimes causes some degree of saturation of current transformers supplying input current to protective relaying schemes. After saturation occurs, the CT output current will show evidence of distortion and the performance of the protection scheme can be affected. The time-to-saturation, during which time the CT output current is a faithful replica of the primary current, can be readily determined from generally available power-system and CT parameters by using curves presented. The time-to-saturation may be shortened by remanence in the core, and can be lengthened by the use of air gaps. The effect of distorted current is discussed generally according to relay design classes. After the primary fault current ceases, the CT can produce unidirectional decaying output current and can have a high level of flux trapped in the core, both of which can affect the performance of protection schemes.