Measurement and EMTP Simulation of Ferroresonance Phenomenon

Ferroresonance is difficult to predict because its occurrence in the power system depends on several parameters, some of which are unknown (parasitic capacities). In addition, measuring electrical conditions during high voltage ferroresonance is very difficult to achieve. In order to check the possibilities of numerical calculations of ferroresonance, the ferroresonant circuit was made as a low voltage circuit in which all parameters were known. Measurement of ferroresonant current and voltages in a single-phase low voltage circuit will be described. The system consists of an AC voltage source, a capacitor, a switch and the single-phase transformer. These electrical elements are connected in series. The magnetic flux-current points of the magnetization curve of the transformer was experimentally obtained and used in the simulation. When the switch closes, ferroresonance takes place since all conditions were met for its occurrence. The waveforms of the transformer current and the voltage at the transformer terminals were measured after switch closing. During the ferroresonance, transformer voltage and current reach relatively high values. At the beginning of the ferroresonance (during its transient part), the voltage of the transformer reaches values that are about two times higher in amplitude than the amplitude of the AC source. The primary side of the transformer was modelled with a series connection of the ohmic resistance and leakage inductance. The core was modelled with a parallel connection of the ohmic resistance and nonlinear inductance. The ohmic resistance represents the core losses while the nonlinear inductance models the magnetizing characteristic of the transformer. A comparison between simulated and experimentally obtained results shows that EMTP successfully simulates the ferroresonance phenomenon for this specific electrical system as simulated results are in a good agreement with the measured results.