Electrical EngineeringPower Systems

Extreme Overvoltage and Magnetic Saturation from Open-Circuit Secondary in Current Transformers (CT)

Detailed physical and mathematical analysis of extreme overvoltage and magnetic saturation in current transformers (CT) with open-circuit secondary.

Ing. Francisco Ramírez

Physical Mechanism of Magnetic Saturation

The Current Transformer (CT) is engineered to operate with minimal secondary burden. Under standard conditions, primary current Ip induces a secondary current Is, resulting in negligible net core flux. Opening the secondary circuit forces the primary current to act entirely as magnetizing current. The governing relationship is:

ϕ(t)=∫vs(t)dt/Ns\phi (t) = \int v_{s}(t) dt / N_{s}
Without secondary counter-flux, the core enters deep saturation, causing magnetizing inductance Lm to plummet toward zero.

Generation of Peak Voltages

Deep saturation leads to high-permeability collapse, inducing massive voltage spikes. These transients easily exceed winding insulation ratings. Within Vexten, we utilize the Short-Circuit (IEC 60909) module to evaluate the primary fault currents that, if occurring with an open secondary, would immediately compromise the CT's dielectric integrity.