A Wideband Current Transformer Bridge

The development and testing results of a new current transformer (CT) test set at Physikalisch-Technische Bundesanstalt are described. The operation range for the ratio based bridge is intended for frequencies from 16.7 Hz up to 20 kHz. Equal setups of magnetically shielded current comparators with primary windings for rated currents from 100 mA up to 5 A convert the secondary currents of the standard CT and of the transformer under test (TUT) into proportional voltages. The errors of the TUT are calculated from the subsequent measurement of the complex voltage ratio. This current bridge and the voltage-ratio-based method allow comparing CTs with nonequal ratios. At 50 Hz, the measuring system offers an extended measurement uncertainty of less than 2 ppm or μrad, respectively. The bridge is verified for use with uncertainties of below 200 ppm or 400 μrad at a frequency of 20 kHz.