Neutral grounding in wind farm medium voltage collector grids

The choice of the optimum neutral grounding option for a given network always involves a trade-off between the level of permissible short-circuit current on the one hand and tolerable voltage stress at the healthy phases following a single line to ground fault on the other. Effective grounding leads to high fault currents but the concomitant voltage stress is limited. The high fault current makes the fault detection and clearance easy. For conventional power transmission and distribution networks the available options are clear, and utilities have established grounding practices. But with increasing wind energy, it is necessary to take unique features of wind farms into consideration in choosing the most suitable neutral grounding option, at least for the wind farm grids. Based on EMT type simulation using a representative 144-MW wind farm grid, the paper provides an in-depth discussion of the pros and cons of the alternative grounding strategies vis-à-vis the relevant operational requirements within a large offshore wind farm, mainly focusing on the aspects of selectivity and voltage limitation. The level of over-voltages after tripping of the affected line for the feasible grounding options will be compared and contrasted with one another. Additionally, the effects of different voltage control strategies in the wind turbines on the over-voltages will be discussed and illustrated.