Influence of the winding design of wind turbine transformers for resonant overvoltage vulnerability

Switching transients and earth faults may lead to resonant overvoltages at wind turbine transformer terminals as well as inside High Voltage (HV) and Low Voltage (LV) windings. The winding design in a transformer could strongly influence the internal voltage distribution as function of frequency, and this has a great impact on risk of insulation failure. In this paper, resonant overvoltages in three winding designs; layer, disc and pancake, are investigated and analyzed for the application in offshore wind farm. To achieve this, a prototype 500 kVA transformer with the three winding types on the HV side and taps for voltage measurements is designed and produced. The measurements show that a HV winding of layer type gives the highest transferred voltage to the LV terminal and that this happens at 1.6 MHz which could be excited for close-up earth faults. Although disc winding seems the least vulnerable when measuring the transferred voltage to the LV terminal, it has higher potential of internal resonances in the HV winding compared to the two other types both voltage-to-ground and voltage drops. Pancake and layer windings have less vulnerability in the case of internal resonances. Pancake windings have modular design characteristic. This advantage eases the repair for offshore wind applications.