Testing Methods for Measuring the Effects of Stray Capacitances on High-Voltage Current Transformers

This paper evaluates different measuring systems to estimate the effect of stray capacitances between primary and secondary windings on current transformers (CTs) used in high-voltage power networks. All windings are at nearly ground potential during laboratory calibration because this is the typical configuration for almost all CT's calibration systems. However, in service, the primary winding is at high potential, so that additional currents flow through the load due to stray capacitances between primary and secondary windings, even if the CT has an electrostatic shield for reducing these capacitances. This effect changes the CT errors determined at no potential. Experimental determination of the variation of ratio and phase displacement errors between both working conditions is evaluated for a set of different CTs using different measuring systems. These systems include from the simplest low-voltage bridge capacitance measurement to the most complex one that simultaneously applies current and voltage.