Analysis of the Parallel Operation Between Synchronverters and PLL-Based Converters

The stability of a converter synchronized to the grid voltage through a phase-locked loop (PLL) is negatively affected by the presence of another converter with the same synchronization principle operating nearby, especially under weak grid conditions. However, while weak grids are critical for PLL-based converters, virtual synchronous machines (VSMs) are instead suitable for such applications. In fact, it can be proven that the robust stability of a VSM is enhanced when it is connected to a weaker grid. Recent studies have investigated the parallel operation between VSMs, but the parallel operation between a VSM and a PLL-based converter has not yet been properly addressed in the literature. In this paper, this topic is investigated by means of eigenvalue analysis, showing the positive effects on the stability of a PLL-based converter due to the presence of a synchronverter operating nearby. The model of the system under study is obtained adopting the component connection method (CCM), and time-domain simulations as well as experimental results using a power hardware-in-the-loop (PHIL) test bench are reported to demonstrate the validity of the performed analysis.