Abstract
This paper investigates the high-frequency properties of a 3.14-MVA 35/1.0/0.4-kV wind turbine transformer commonly used for the doubly fed asynchronous generator. Frequency sweep measurements carried out for this transformer show that the voltage transfer from the 35-kV side to either of the two low-voltage taps is strongly dependent on the loading on the other tap. A frequency-dependent terminal equivalent model of the transformer is developed based on frequency sweep measurements and vector fitting. The model enables simulating the interaction between the 35-kV and the 1.0-kV sides with the 0.4-kV tap being open. The model accurately reproduces the high-frequency voltage transfer between the windings. To achieve an accurate power frequency initial condition, a co-simulation method is applied. Simulation case studies are conducted which investigate the high-frequency interaction of the turbine transformer with the adjacent network. These studies include the investigation of the transient recovery voltage on the HV side vacuum circuit breaker (VCB) when interrupting a three-phase fault at the LV side of the transformer, VCB restrike effects, and the impact of grid inverter-side harmonic filters on the transient overvoltages. It is shown that the use of a simplified transformer model based on lumped capacitances gives unrealistic results.