Transformer Inrush Currents

This article presents the harmonic analysis of transformer inrush currents in offshore power systems. Offshore production facilities interconnected by subsea cables are becoming increasingly common, either as new projects or as extensions to existing facilities. Interconnection via a subsea cable introduces a significant shunt capacitance to the source power system, giving rise to low natural or resonant frequency. One source of harmonic currents is the system transformer. When initially energized, the transformer may draw a transient inrush current that contains all harmonic components. If one of the harmonic components in the inrush current is close to the resonant frequency of the power system depending upon the damping levels, a sustained overvoltage may be produced. This has recently caused operational problems on a number of newly interconnected offshore systems. Analysis tools and methodologies are presented in this article that can be used during detailed engineering design to assess the likelihood of such dangerous events and to identify whether mitigation is required, long before the problems are encountered during system commissioning.