Influence of Current Transformer Saturation on Numerical Protection Relays – A Case Study of Circuit Breaker Failure Protection

A low impedance fault on a transmission line causes large fault currents that can lead current transformers in the switchyard that supplies the fault to saturation. If the fast automatic reclosing is implemented the current transformers additionally enter saturation. After the definitive trip, the residual magnetic energy in the current transformers is discharged via secondary circuits. The measuring inputs of the protective devices are connected to the secondary of the current transformers, and if the current is large and lasts long enough consequence can be an unwanted operation of the protective device. This paper gives an example of an unwanted operation of circuit breaker failure protection, which is a function within distributed busbar protection, as a consequence of such an event.