On-line calibration pulse injection through high frequency current transformers for partial discharge locations

During partial discharge (PD) measurements on high-voltage equipment, injection of calibration pulses into the terminal(s) is normally required to determine PD magnitudes and/or to locate the discharges. A technique is presented for online injection of calibration pulses into the earthed neutral through a specially designed high-frequency current transformer (CT). The CT can be clamped around the neutral lead, and pulses of 1-5-V magnitude can be produced at the neutral without disconnecting the earthing lead. This method is particularly useful for the location of PDs in transformer and generator stator windings. Tests carried out on a transformer winding in the laboratory and a hydro-generator in service have shown the validity of the technique. For PD pulses emanating from within the winding, the effect of the busbars and the equipment connected at the HV terminal under normal operating conditions can be determined and used to further improve the accuracy for locating the partial discharges