Overview and Impedance-Based Stability Analyses of Bison Wind Farm: A practical example

This article presents an overview and performs impedance-based stability analyses of Bison wind farm (BWF), which is the largest wind farm in the state of North Dakota, owned and operated by Minnesota Power. An ac collector grid (34.5 kV) collects the total BWF power (496.6 MW) and sends it to the Square Butte line-commuted converter (LCC) station, where it is boosted and rectified to 250 kV for high-voltage dc (HVdc) transmission. The HVdc system is a 465-mile bipolar link that sends the power to the Arrowhead LCC substation in the Duluth industrial area, MN, USA. Statistical analyses using a Weibull distribution are performed to calculate the probability of BWF wind energy output. To perform the stability studies, small signal impedance models of the BWF in the d-q frame are developed, and the stability is evaluated by applying the Bode plot and Nyquist criterion. The overall BWF is divided into source and load impedances. An aggregated BWF is modeled as the current source supplying power to the grid, whereas a detailed model of a grid-side converter (GSC) in a wind turbine (WT) is controlled with a current controller. It is demonstrated that the BWF remains stable if and only if the ratio of the grid impedance and the GSC satisfies the Nyquist criterion. Based on the impedance model, the impact of change in the grid impedance and control parameters of the phase-locked loop (PLL) on the stability of the overall system is studied. It is observed that when the grid strength is reduced with decreasing short circuit ratio (SCR), the stability margin of the system reduces, and a small increase in the PLL bandwidth can destabilize the overall system. Time domain simulations are performed to validate the analytical models and approaches.