Transformers internal voltage stress during current interruption for different wind turbine layouts

The voltage transients generated during the breaker operations in cable systems, for instance, a wind park collection grid, can reach very low rise times. The rise times of these transients can be almost 50 times shorter than the rise time of a lightning pulse. Such transients can generate a very high voltage stress on the internal transformers insulation. In this paper, a test case is studied using verified models of different types of transformers and wind turbine layouts in order to account for typical wind turbine layouts found in modern wind farms. A critical switching scenario is chosen in order to provoke the highest possible voltage transients during a current interruption. Furthermore, internal overvoltages are estimated using model of a similarly sized winding. Simulations show that the magnitude of the voltage transients is higher than the basic lightning impulse insulation level (BIL) defined by present standards. Moreover, it is found that the rise time of the voltage surges is much shorter than the rise time of the lightning pulse. The shortest rise time of 40ns is obtained in a wind turbine layout where the wind turbine breaker is placed near the transformer. Due to very short rise times of the transients, very high internal overvoltages are estimated in dry-type transformer windings. These internal overvoltages are much higher than overvoltages recorded at the basic lightning impulse level. For a wind turbine layout where a breaker is placed in the bottom of a tower and a dry-type transformer in a nacelle, the highest turn-to-turn voltage of about 1.5pu is estimated. This is almost 4 times higher turn-to-turn voltage then the voltage obtained during the BIL test. In a wind turbine layout where a breaker is placed close to the transformer, the amplitude of the turn-to-turn voltages reached 1.8pu due to lower stray capacitances and thus a shorter rise time of voltage strikes.