Transient Over-Voltage (TOV) and its suppression for a large wind farm utility interconnection

Recently, renewable wind energy is enjoying a booming growth globally to become an important green electricity source to replace fossil fuels. Different from a classical generation plant consisting of synchronous generators, large wind farms are typically made up of many wind turbine generators (WTGs) rated up to several-MWs so far and connected through underground cables in a geographical terrain. The types of WTGs are diversified, which can be a fixed-speed induction generator, a variable-speed induction generator, a double-fed induction generator (DFIG), or a full-scale back-to-back (BtB) AC-DC-AC VSC-connected (voltage source converter) WTG. These combined unique characteristics of a wind farm can be potential factors for over-voltages which can cause surge arresters and equipment failure within a wind farm. In the paper, an investigation of transient over-voltages (TOV) during wind farm switching events and the impact of generator step-up transformer (GSU) configuration and solutions to suppress the TOV during these events was conducted on a large wind farm (300 MW). Two different types of WTGs, which are a fixed speed squirrel cage induction generator (SCIG) and a full-scale BtB VSC type WTG, were studied. Two GSU transformer configurations, Delta/Yg or Yg/Yg, were analyzed. Two methods of mitigating the TOV which were evaluated are the grounding transformer and fast grounding switch. Their performance and significance were demonstrated. The feasibility using COG (coefficient of grounding) analysis method for renewable wind generation grounding design is discussed.