Frequency domain investigation of switching transients in Offshore Wind Farms

Offshore Wind Farm (OWF) as one of the renewable energy alternatives has received much focus in recent years. The installation of wind turbines, cable interconnections and power transformers in offshore areas requires more attention to maintenance and repair. Wind Turbine Transformers (WTTs) are subjected to high stresses due to switching transients in offshore systems. In this paper, the resonance voltages on the Low Voltage (LV) terminals of a no-load WTT are surveyed. For that purpose, we apply a black box model of a 11 kV/235V, 300 kVA transformer that has been obtained by measurement followed by model extraction by Vector Fitting and implemented in ATP by a lumped circuit RLC network. Emphasis is put on the frequency domain analysis of transformer model as well as time domain cable-transformer energization to identify resonance situations. Finally, switching transients in a row of wind mills is simulated in ATP and transformer terminal voltages are investigated in both time domain and frequency domain. The simulation results indicate that dominant resonance frequency is always available on HV and LV terminal of transformer in any switching transient. Besides, in critical cable length where traveling frequency matches with the dominant resonance frequency of transformer, extreme resonance overvoltages occur on LV terminals.