Power transformer LI tests with high frequency pulse CTs for current measurement

In HV laboratories, the insulation of power transformers is tested with 1.2/50 µs lightning impulses (LI). According to current standards, test technique consists in comparing the waveforms of the applied voltage and current through the winding checked, obtained at reduced voltage and test voltage. Voltage measurement is made with a divider and current measurement is made with a shunt. The paper analyzes the replacement of conventional shunts with high frequency pulse current transformers (HFPCT) that currently have a competitive price and performance, including the advantage that they have no point earthed and so avoid the common mode disturbances from the grounding loop. There are presented the results of the calibration measurements that were performed with reference measuring systems and comparative LI tests on several power transformers to confirm this replacement.