Impact of the PLL Time Constant in Converter Control on the Dynamic Frequency Support

Abstract

The use of a phase-locked loop in grid-forming converter control is controversially discussed, as it implies a time delay to the converter’s response to grid disturbances. In literature, slowing down the PLL dynamics to create a retarded measured voltage and current phasor is discussed to virtually resemble synchronous generators’ inherent inertia. In this case study, the influence of the PLL time constant is investigated in a simple medium-voltage testbench with the standard cascaded control and the direct voltage control concept. The frequency, active power, direct and quadrature current of the converter-based generator are evaluated for different PLL time constants following an active power mismatch. The results show that slowing down the PLL has a small impact on the current and power infeed of both control concepts. However, results of the standard cascaded and the direct voltage control are similar and a general advantage of the slower PLL cannot be concluded in this case study.