Impact of line and control parameters on Droop Stability in Inverters for Distributed Generation

The paper analyzes stability conditions and design requirements for the control parameters in droop-controlled inverters. The analysis focusses on a single inverter unit with AC (output) filters and interface transformer, as this may be regarded as the elementary component of a distributed generation system. Through a process of thought simplifications and manipulations, the linearized full state-space model is transformed into a unique single-input, single-output system comprising several nested loops. The resulting analytical model in terms of transfer functions has enough “structure” to represent interactions between the different actors of the overall control, such as droop linear and derivative terms, virtual impedance and voltage controllers. Stability conditions for the corresponding parameters are then derived analytically. Experimental tests are carried out on the complete system to check the reliability of the proposed model.