Ferroresonance Analysis Due to the Effect of External Faults on a 20 kV Voltage Transformer

Abstract

Medium Voltage (MV) Switchgear is an essential component in the electric power distribution system with a working voltage of 20 kV. MW Switchgear consists of Circuit Breaker (CB) and Voltage Transformer (VT). VT is one component that often fails in MW Switchgear in the power distribution system, where conditions cause the VT iron core to saturate. This saturation zone will make whatever capacitance reactance value ( ) that generated from the power system network will be the same value as the inductive reactance value of the inductance VT ( ), which causes the impedance value to be zero. A very wide frequency range will be able to trigger a ferroresonance which results in a large current flowing on the primary side of VT and has the potential to cause failure in VT and MV switchgear, characterized by an explosion. This research will focus on the main causes of ferroresonance emergence due to external disturbance, 20kV VT specification and MV Switchgear. Ferroresonance simulation is carried out by ATPDraw Software, external disturbance variations, VT and MV Switchgear specifications are given for simulation to observe the response of VT’s voltage and current. The variables studied include disturbances of CB switching operations which have an impact on the emergence of variations in network capacitance values and produce subharmonic mode ferroresonance with voltage value reaches 150% of the nominal voltage for Cg = 0,005 – 0,1 µF and 275,5% of the nominal voltage for Cs = 0,05 – 1 µF, then disturbances of lightning impulse currents which will cause ferroresonance in networks with small capacitance values, this disturbance is very dangerous because it creates ferroresonance with the amplitude of the primary voltage VT can reach 14.391% of the rated voltage, and quasi-periodic mode’s ferroresonance resulting from a single phase to ground fault which reaches 201.47 % of the rated voltage value. The choice of a VT design with a voltage factor of 1.9Un/8h and an MV Switchgear design which loads the VT burden with an inductance composition that is greater than its resistance and approaches 80% of the VT burden specification can mitigate the emergence of ferroresonance