Internal resonant overvoltage in wind turbine transformers- sensitivity analysis of measurement techniques

The installation of medium voltage transformers in wind turbines motivates the wind turbine manufacture, operators and researchers to review the challenges of insulation failure owing to overvoltages. Among these overvoltages, internal resonant overvoltages are most crucial and destructive. One of the protective measures, which decrease the potential of internal resonances, is the selection of appropriate winding type, i.e. disc, layer, and pancake. In this paper, the internal resonances of the three windings are compared and the least venerable one, pancake, is introduced. The focus will be on the assessment of measurement techniques and the sensitivity analysis of the obtained results on them. The motivation was to obtain high quality measurements in the range of 10 kHz-10 MHz so that the modeling and calculation results can be compared with. Measurement results show that the RG58A/U coaxial cables have loading effect due to stray capacitance and influence the voltage measurements. It is recommended to apply oscilloscope papers with x10 attenuation. Measurement results for input impedance are sensitive on both voltage and current probes. The selection of current transformer CM-100-6L-IR50 provides quite better results compared to current probe Tektronix P6021. It is also found out that the installation of foil around transformer presents more realistic results for both terminal and internal resonances.