Current Transformer Accuracy with Asymmetric and High Frequency Fault Currents

The accuracy of two commercially available window current transformers is determined for asymmetrical 60 Hz fault currents and for lower magnitude higher frequency currents. The 4000: 5, window type CT's tested are found to be accurate to within 3 percent for frequencies up to 25 kHz for burdens of less than 10 ohms. With an antiremanence gap, such a Ct can transform a fully offset fault current of 5000 amperes and 50 millisecond time constant with less than 3 percent error for a burden resistor of 0.5 ohms