Modeling of wind turbine transformers for the analysis of resonant overvoltages

• Development of an analytical model to study resonant overvoltages in wind turbine transformers and verification by measurements.
• Comparison of the frequency response of three winding types, i.e. disc, layer and pancake, in the sense of resonant frequencies and the amplitude of the transferred voltages at these frequencies.
• Proposing the pancake winding as the least vulnerable winding to resonance and being appropriate for wind farm applications.

Switching transients and earth fault in a wind farm collection grid are two transient phenomena which can lead to resonant overvoltages at the LV terminal of the wind turbine transformers as well as inside HV and LV windings. The aim of this paper is to analyze the potential of the resonant overvoltage for various winding designs; disc, layer and pancake. In this way, the least vulnerable winding design can be recommended. For this aim, a 500 kVA transformer test object with the three aforementioned winding types has been designed and manufactured. Similar geometrical characteristics are used for all the three windings. By measuring the frequency response, the resonant frequencies can be found and the amplitude of the transferred voltage at these frequencies can be compared. The windings are also modeled in this paper in detail based on analytical functions. This RLC ladder model is verified by the measurements.The measurements and modeling results show that all winding designs have a resonant frequency around 800 kHz for the transferred voltage to LV terminal. Disc winding shows the lowest amplitude of the transferred overvoltages (6 p.u.). The layer winding, also has a resonance at 1.6 MHz with an even higher transferred overvoltage (80 p.u.). The frequency response of the pancake winding has characteristics of both disc and layer windings. In spite of having the lowest transferred overvoltage peak, the disc winding has many additional resonant frequencies in the range of 100 kHz–1 MHz. This could excite resonances in other parts of the winding. Consequently, pancake winding designs might be the most promising to minimize resonance situations.