Effects of PLL in the Damping Unit of a Gird-Connected VSG

As a promising control method for grid-forming converters, the virtual synchronous generator (VSG) control strategy was developed quickly using different damping units. A straightforward approach is to emulate the dynamics of a synchronous generator (SG), where a phase-locked loop (PLL) is employed to increase the transient damping ratio by measuring the angular frequency of the grid. Ignoring the effects of PLL, this damping method can effectively improve the active power stability of a grid-connected VSG. However, the impact and the mechanism of the introduced PLL are not distinct. Considering the effects of PLL, the dynamic model of a grid-connected VSG is established in this paper. Further investigations reveal that the PLL will introduce a pair of unexpected eigenvalues, which will trigger instability when the bandwidth of the PLL is higher than a critical value. Finally, the theoretical analysis is verified by simulation results.

I. Introduction

Since renewable power resources were developed quickly, many power converters have recently been integrated into power grids as interface equipment [1]. To overcome the shortages of lacking inertia in traditional converters, the virtual synchronous generator (VSG) control method is proposed by emulating the swing equation of synchronous generators (SGs), which can effectively provide inertia support for the grid to restrain its frequency fluctuation [2], [3]. Various damping method has been proposed, which is very important for power oscillation suppression [3]–[5]. A straightforward approach of introducing a damping term is to mimic the dynamic of damper winding of SG. Thus, a damping coefficient is introduced to adjust the damping effect and a phase-locked loop (PLL) is adopted to detect the angular frequency at the point of common coupling (PCC) [4].