Impact of Outer-Loop Control on Converter-Driven Stability: An Impedance-Based Small Signal Analysis of Grid-Following VSCs

Sammanfattning

The increasing share of power electronic converters in modern power systems has led to the introduction of converter-driven stability as a new stability category, capturing converter-induced phenomena across a wide frequency range. Traditional power system stability classifications were primarily developed for systems dominated by synchronous machines, and do not fully capture the dynamic behavior introduced by power electronic converters. Due to the inherent differences between power electronic converters and synchronous machines, as well as the presence of multiple converter control loops operating over a wide range of time scales, new stability challenges arise that require new evaluation methods. This thesis investigates the impact of outer-loop control on the small-signal converter-driven stability of a grid-following voltage source converter connected to a weak grid, using an impedance-based method in the frequency domain. The converter's control system, including the inner current controller, phase-locked loop and outer-loop control is implemented in the synchronous dq-frame to enable linear control. This control framework is then applied to a generic two-level voltage source converter modeled and simulated in PSCAD, from which impedance data are extracted to and analyzed in MATLAB. System stability is evaluated using Nyquist and Bode plot analyses in the frequency range 1-200 Hz. The results showed that the choice of control mode significantly affects the negative damping characteristics of the converter and the frequency ranges in which resonance with the grid tends to occur. When the outer-loop control on the q-axis was removed, instability occurred at the fundamental frequency. When reactive power control was applied, a symmetric region of negative damping appeared around the fundamental frequency, whereas AC voltage control shifted the negative damping to supersynchronous frequencies. In general, the d-axis control objective had a smaller influence on negative damping characteristics. However, DC-side dynamics affected the converter impedance up to twice the fundamental frequency, beyond which their influence became negligible. Furthermore, the combination of reactive power and DC-link voltage control was shown to be the most sensitive configuration when connected to a weak grid. The analysis also demonstrated that Bode plot analysis alone is insufficient for a complete assessment of converter-driven stability and that Nyquist plot analysis is necessary to ensure a complete stability assessment. Overall, the findings provide improved insight into the impact of outer-loop control strategies on converter-driven stability in grid-following voltage source converters and contribute to a better understanding of stability phenomena in weak power systems.