Resonance Excited by Transformer Inrush Current in Inter-connected Offshore Power Systems

Offshore production facilities inter-connected by sub-sea cables are becoming increasingly common, either as new build projects or as extensions to existing facilities. Inter-connection via a sub-sea cable introduces a significant shunt capacitance to the source power system giving a low natural or resonant frequency. One source of harmonic currents are the system transformers; when initially energised they 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 over-voltage may be produced. This has recently caused operational problems on a number of newly inter-connected offshore systems. Analysis tools and methodologies are presented that can be used during detailed engineering design to assess the likelihood of such dangerous events occurring and identify if mitigation is required, long before problems are encountered during system commissioning.