Investigation of Ferroresonance Mitigation Techniques in Voltage Transformer Using ATP-EMTP Simulation

Ferroresonance is a complex nonlinear electrical phenomenon that can cause dielectric and thermal problems for electrical equipment. Electrical systems with ferroresonant behavior are nonlinear dynamical systems. The ferroresonance phenomenon may take place when the core of an inductive device becomes saturated, and its current flux characteristic becomes nonlinear. While in the case of a linear resonant circuit the resonance frequency is well defined, in the case of a nonlinear circuit, the oscillations may exist at various frequencies, depending on many factors of the particular case. In this paper, ferroresonance phenomenon and its mitigation techniques in 33 kV/110 V voltage transformers (VT) were studied using ATP-EMTP simulation. Initial investigations were carried out for the VT failures occurred at one substation in Malaysia. Physical and burn characteristics of the failed VTs were studied. Simulation results show that ferroresonance cannot be proven to have occurred at the VT due to switching operations since one precondition, namely the critical capacitance, could not have been satisfied. However, in the event of a ferroresonance occurring, several mitigation techniques such as using load resistors, proper grounding sequence, reconfiguration of VT connection, and overcurrent and overvoltage protection can be implemented.