Modelling of switching over-voltage on ship service transformers

Switching over-voltages are a normal phenomenon when a transformer is energized. This transient is experienced when switching a service or propulsion transformer on board a ship. Under some special circumstances, the over-voltage levels experienced by the transformers may be exacerbated by system interactions and can exceed the insulation levels of the transformer, leading to damage of the windings. The phenomenon of transient over-voltages during the switching of an HV/LV ship´s service transformer is analyzed in this paper. The system conditions which lead to these worst case switching over-voltages are summarized below. A combination of factors can cause the worst case switching over-voltage condition, these are: Travelling switching wave and reflections; Re-strikes of vacuum circuit breakers; Saturation of the transformer which may inversely charge the power cable; A load characteristic that causes current surge at the switching instant inversely charging the cable. It is very difficult to model the vacuum circuit breaker and its switching transient accurately, because there are a number of random and unpredictable factors which impact greatly on the switching process and dynamics. A simplified model has been developed, based on the detailed description of the characteristics of the vacuum circuit breakers (VCBs) Ref[1], to simulate the switching over-voltages. This model is then used to investigate some of the parameters associated with the worst case switching over-voltages. The main conclusions reached from this work are: The simplified model is sufficient to model predict the transient over-voltages following the switching of a transformer. The worst case over-voltage could reach levels as high as 5.0 pu; Damping filters or commercially available equipment such as a ZORC (Zinc Oxide Resistor Capacitor) suppressor, can be used to control the over-voltages to more moderate and acceptable levels, at approximately 2.0 pu.