Frequency Response of MV Current Transformers

At present, high impedance faults (HILT) on distribution networks still present the most persistent and challenging problem to protection engineers. This is because this fault type does not produce a fault current large enough to be detectable by conventional protection devices such as over-current relays, reclosers or fuses. Prolonged arcing and flashing at the point of contact due to HIF presents a serious safety and fire risk. The current signals generated by high impedance faults consist of the fundamental (50Hz) and harmonic components and they are all time-varying because of the non-linear nature of the fault arc and intermittency. Nevertheless, it is possible to detect HIFs based on the spectral contents of the signal. The important requirement for such a UHF detector is that the input to it should be faithfully reproduced by the sensor. This paper demonstrates that existing current transformers (CT) in the distribution network are suitable for use to acquire signals as input to the HIF detection system. The frequency response of the CT up to 10 kHz is investigated. Experiments are set up in the laboratory to study the effects of large normal 50Hz load currents (several hundreds of amps) on the frequency response measurement of small amplitude HIF current (in the order of ten amps).