Current Transformers under Transient Conditions: Subject Review

This paper gives the basic theory of the transient behaviour of protective current transformers with the equations for the primary fully asymmetrical short-circuit current, the transformed secondary current and the required flux density in the core when the secondary circuit is loaded with resistance and inductive reactance at different power factors ranging from cos ψ = 1.0 to cos ψ = 0.2. The design factors M and Mo are introduced and presented in numerical and graphical forms to be used for design relations and performance evaluation of current transformers. The limits for the three basic design parameters are shown, namely the minimum core cross-sectional area, the minimum number of secondary turns and the maximum burden that must be used if it is required from the c.t. to transmit the primary fault current without saturation for a specific length of time, up to 10 cycles, or if the saturation is to be avoided completely. The effects of residual flux, if present, are included.