Influence of HV inductive voltage transformers core design to the ferroresonance occurrence probability

The occurrence of ferroresonance oscillations of high voltage inductive (electromagnetic) voltage transformers (VT) has been recorded and reported in a number of papers and reports. Because of its non-linear characteristic, inductive voltage transformer has the possibility of causing ferroresonance with capacitances present in the transmission network, if initiated by a transient occurrence such as switching operation or fault. One of the solutions for ferroresonance mitigation is introducing an air gap into voltage transformer core magnetic path, thus linearizing its magnetizing characteristic and decreasing the possibility of a ferroresonance occurrence. This paper presents results of numerical ATP-EMTP simulation of typical ferroresonance situation involving inductive voltage transformers in high voltage networks with circuit breaker opening operation after which the voltage transformer remains energized through the circuit breaker grading capacitance. Main variable in calculating to the ferroresonance occurrence probability was the magnetizing characteristic change caused by the introduction of an air gap to the VT core, and separate diagrams are presented for VTs with different air gap length, including the paramount gapped transformer design – open core voltage transformers.