Analysis of harmonic overvoltages during transformer energization for mass rapid transit systems

Emergency power supply facilities and loads connected by long power cables are becoming increasingly common in industrial power systems. System facilities connection via a power cable introduces a considerably 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 emergency power systems for mass rapid transit. This paper presents the methodology and results that assist industrial power system restoration switching with regards to control of harmonic overvoltages due to transformer energization. Analysis tools and methodologies are presented in this paper 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.