Influence of inductive voltage transformers on the measurement of switching transients

The inductive voltage transformers (VTs) used on the power system are primarily designed for protection or metering at 50Hz. The leakage inductance of the VT windings and stray capacitances can form a resonant circuit. When subjected to AC voltages at frequencies well below resonance, the VT can be assumed to behave linearly giving accurate measurements. The resonance of the VT limits the bandwidth over which it may be used to provide accurate measurements. This is of particular importance when taking harmonic and transient measurements. This paper reviews the different kinds of VT model available. A model is selected for transient simulation in EMTP, and the influence of the VT model on switching transients is examined. A key observation is that when the inductive VT model is subjected to a step change in voltage, a voltage ripple is introduced at the VT's resonant frequency to its secondary winding. The aim of this work is to quantify the influence of the inductive VTs on measured transient voltage waveforms.