Ferroresonance is a phenomenon which causes extremely large overvoltages in distribution and transmission systems due to resonance of non-linear inductance and capacitance. It commonly occurs in the primary of delta-wye distribution transformers during a single phasing condition, for example when a single fuse blows. The capacitance of the open line will resonate with the inductance of the delta windings, causing an exceedingly high voltage drop across the transformer windings.
GP Technologies was commissioned to analyze the failure of a 72kV iron core voltage transformer. Initially, it was speculated that the failure was caused by ferroresonance.
After thorough simulation, it was confirmed that an exceptionally high voltage had developed across the VT windings due to the interaction of the non-linear inductance of the iron core and the grading capacitors. It was found that the specific saturation characteristics of the VT and the size of the grading capacitors enabled resonance when the circuit breaker was opened.
Multiple solutions were provided to mitigate the overvoltage issue. While the options are expensive, these included installing shunt resistors to dampen the overvoltage, or replacing the VTs with Coupling Capacitive Voltage Transformers.
Single Phase Equivalent Circuit Modeled in ATP-EMTP