Simulation and analysis of voltage sag during transformer energization on an offshore platform

Abstract

Typically, in offshore platforms, small reciprocating engine generators supply essential loads. At times, it may be necessary to energize a large transformer from these generators. Transformer energization draws high magnetizing current from the generators for several seconds and can have an adverse effect on the system voltage. This paper describes the voltage sag analysis of the energization of two transformers (1.5 MVA and 3.75 MVA) from a diesel engine generator rated at 625 kVA/0.48 kV/0.8 power factor and several gas engine generators rated at 938 kVA/0.48 kV/0.8 power factor using real-time dynamic simulations. The power system model of the offshore platform comprises six generators along with an excitation and governor control, two sequentially energized transformers, buses, breakers, and cables. The paper details simulation results for different energization scenarios. The simulations varied the number of generators online, the load on the system, the voltage reference set points, and the transformer parameters. Several factors that influence inrush currents and the resulting voltage sag (such as transformer air-core reactance, breaker closing instant, subsea cable capacitance, and source impedance) were considered. The results of this study aided in designing transformers and developing operational procedures and protection set points.