Stability Analyses of Bison Wind Farm Using Frequency Scan

The stability of power systems integrated with renewable energy sources is a critical aspect of their operation, and numerous approaches have been proposed to evaluate this aspect. The power grid sector advocates for the adoption of electromagnetic transient-based analysis to study the grid interaction and planning process. In this paper, a frequency scan-based approach is demonstrated to perform an impedance-based stability analysis of the Bison wind farm (BWF). The BWF, which is the largest wind farm in North Dakota and is owned and operated by Minnesota power, uses a 34.5 kV, AC collector grid and a line commutated converter (LCC) based bipolar high-voltage DC (HVDC) transmission grid rated at ±250 kV. The HVDC line transfers a total of 500 MW of green energy from the Square Butte LCC-HVDC station in North Dakota to the Arrowhead LCC-HVDC substation in Duluth industrial area, Minnesota. Given the complexity of the system, obtaining an analytical model is challenging. Therefore, an approach based on frequency scanning is employed to extract impedance and perform impedance-based stability analyses. The impedance versus frequency response of the BWF is obtained by exciting the wind farm with sinusoidal voltage perturbations during its stable operation. The generated voltage and current signals are subjected to Fast Fourier Transform (FFT) to obtain frequency components. Finally, the Bode plot analysis and Generalized Nyquist criteria (GNC) are applied considering the impedance of both the BWF and grid to perform stability analyses. From the results it is observed that the frequency scan-based analysis accurately predicts the behavior of complex system, providing valuable insights for controller design. The results emphasize the impact of variation in the grid SCR and controller parameters on the overall system stability.