Investigation of High Frequency Switching Transients on Wind Turbine Step Up Transformers
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Pre-mature failures of wind turbine step up (WTSU) transformers have been reported in the wind farms although, the failed transformers had previously passed all quality assurance tests and had assembled all standard requirements. Vacuum circuit breaker (VCB) initiated steep front transient impact is one of the potential causes of such insulation failures. The use of VCB as switching devices and intense cable network increases the likelihood for high frequency transient overvoltages (TOVs) in wind farms. Multiple prestrikes and restrikes of VCB in conjunction with cable capacitance and inductance of transformer give rise to fast steep front voltage transients which eventually cause insulation failures in WTSU transformer. This emphasizes the need to conduct switching transient analysis studies for wind power plants, to investigate the severe switching overvoltages experienced by WTSU transformers. In this work, high frequency modeling in a broad frequency range for major components of the wind farms and an investigation of switching transients on WTSU transformer are presented. An adaptive model of VCB capable of simulating statistical phenomena and overvoltages on circuit breaker and the components that it interacts with is developed in PSCAD/EMTDC. A high frequency phase model of single core cable, taking into account the high frequency effect of cable, i.e., electromagnetic transient propagation, skin effect and reflections is simulated in PSCAD/EMTDC. A linear wideband frequency-dependent black box model of an actual WTSU transformer based on the experimental determination of admittance matrix in a wide frequency range and subjecting the measured admittance matrix to an approximation by means of a rational function through vector fitting is used to simulate WTSU transformer. The rational function obtained for WTSU transformer can then be realized into an RLC network for time domain simulations in PSCAD/EMTDC. A test bench is simulated using the above mentioned high frequency models and replicating Type-IV wind turbine generator synchronized with the grid. Transient scenarios are investigated to understand the most severe switching transients experienced by WTSU transformers, considering the worst repeated switching transient overvoltages and the steep rate of voltage rise experienced by the WTSU transformer. Six different attributes of voltage waveforms across the WTSU transformer are used to investigate the transient behavior in the cases carried out on the proposed test bench.