A systematical method for suppressing ferroresonance at neutral-grounded substations

At neutral-grounded substations, ferroresonance may happen upon switching a circuit breaker or disconnector, due to the saturated magnetizing characteristics of the bus PT. In this paper, a systematical method is presented to analyze and suppress the phenomenon. The scheme for suppressing the ferroresonance is to insert resistance, made from parallel-connected resistors, into the PT´s wye secondary circuit. For a particular substation, it is first studied theoretically whether ferroresonance can possibly happen; and if so, a theoretical critical resistance is estimated to avoid the resonance. A practical one is then achieved by EMTP simulation on the basis of the theoretical one. The scheme for switching these resistors out of the circuit is to do that not all at once, but rather, in turn, to avoid exciting ferroresonance again. The appropriate resistance value of the resistors and the schedule of switching them out are also determined by EMTP simulation. A prototype was developed to fulfill the scheme, and proved to be effective by field tests. In addition, it is demonstrated that this kind of ferroresonance cannot be suppressed by adding resistance to the PT´s delta secondary circuit.