Sizing and experimental validation of a selfsync droop-based grid forming inverter with direct voltage control

Currently, the transient stability of the grid is mainly ensured by the inertial behavior of its synchronous generators. The increase of renewable energy sources connected to the grid through grid following (GFL) inverters is severely affecting the inertia of the grid and its stability is beginning to be compromised. Moreover, in case of a large disturbance, these renewable energy sources disconnect from the grid by design, thus exacerbating the problem. The idea of mimicking the behavior of synchronous generators in an inverter and participating in the grid stability gave birth to the so-called grid forming (GFM) inverters. This implies a change in paradigm from current controlled sources (GFL) to voltage-controlled sources (GFM). Despite known stability issues in current controlled converters, a typical approach to GFM converters with LCL output filters is based on cascaded current and voltage control loops. An internal current loop controls the inverter's output currents and an external voltage loop controls the filter capacitors' voltages. The problem is that this solution is complex and is also marginally stable. Alternatively, in this paper we explore a direct voltage control, which is based on droop control. The proposed control's output is used for the direct control of the inverter's output voltage in amplitude and phase. A complete analysis is performed, and a method is proposed to determine the appropriate control parameters as well as the sizing of the inverter components. Finally, an experimental setup is presented and the effectiveness of the proposed method is shown in simulations as well as in real-life experiments.