Low-Frequency Converter-Driven Oscillations in Weak Grids: Explanation and Damping Improvement

Low-frequency oscillations have been reported in several weak-grids-connected voltage-source-converter(VSC) systems. Although efforts have been devoted to understanding the parametric and sensitivity impact of the VSC controller gains, a general formulation of the oscillation mechanism is still missing. Using transfer function dynamic modelling approach, we find that the outer loop active power control's bandwidth mainly determines the oscillation frequency. The PLL introduces a large phase lag around the frequency of the PLL bandwidth in weak grids which decreases the oscillation damping. A simple but effective PI+Clegg integrator (CI) compensator is proposed to replace the standard outer loop active power controller compensating the PLL's phase delay and increasing the oscillation damping. The results are verified in a real time digital simulator.