Experiments on Ferroresonance at Various Line Conditions and Its Damping

Ferroresonance is a nonlinear dynamic electrical phenomenon in a power system. It may occur in ungrounded three-phase lines at transformer banks on no-load due to mal - functioning of switching devices (one pole closing at first). A laboratory set-up is developed with core-type transformers and power capacitors simulating line transformers and capacitance respectively to investigate ferroresonance. An electronic switching device is developed with variable ON/OFF time for measurement of over-voltages across transformers and capacitor during ferroresonance by a digital recorder. For any operating voltage applied to series combination of capacitance and transformer windings on no-load, the critical value of capacitance to cause ferroresonance jump is determined using Rudenberg´s graphical method. The resulting ferroresonance over-voltages across circuit components are recorded at various line conditions like stored charge magnitude and polarity. The results show the ferroresonance over-voltages reaching dangerously high peak and its dependence on stored charge of capacitor and loading at the secondary of transformers.