Robust Stability Investigation of the Interactions Among Grid-Forming and Grid-Following Converters

State-of-the-art grid-connected converters can be classified as “grid-following,” meaning that they require a dedicated synchronization unit in order to inject active and reactive currents into the grid. Recently, other converter control concepts have been proposed in the literature, such as the synchronverter, which can instead achieve synchronization without a dedicated unit and, within its physical limitations, make the converter behave as an ideal voltage source. Since it should be expected that the grid-connected converters having different control philosophies will coexist for many years, in this article, the interaction among the converters operating nearby are addressed. First, the component connection method (CCM) technique is introduced, as a means for obtaining the state-space representation of a complex system with several units operating nearby. Due to the complexity of the grid and the difficulty in obtaining its exact representation, μ-analysis is adopted in this article for assessing the robust stability of the converter under different operating conditions, according to a defined set of plant uncertainties. Simulation results and experimental tests in a laboratory environment by means of a power hardware-in-the-loop (PHIL) test bench are performed to demonstrate the validity of the presented analysis.

I. Introduction

State-of-the-art converters for grid-connected applications can be classified as “grid-following,” meaning that they require a dedicated synchronization unit for their operation to estimate the voltage angle at any time [1]. The influence of synchronization units, often implemented by means of phase-locked loops (PLLs), on converter stability has been recently addressed in several works [2]–[8]. In [9] and [10], the negative effects due to the interactions among the synchronization units of converters operating nearby have been investigated, showing their impact, especially under weak grid conditions.