Stability criterion of droop-controlled parallel inverters based on terminal-characteristics of individual inverters

Droop-controlled parallel inverters are widely adopted in micro-grids due to its high reliability and low cost. However, the interaction among the inverters will cause whole system to be unstable even though each individual inverter can operate stably in stand-alone mode. A lot of work has been done for analyzing the stability of parallel inverters with droop control through state-space approach originally used in conventional electrical power system. For this existing approach it is necessary to have the knowledge of inside parameters of each individual inverter in advance, while in practical applications it is not so convenient to obtain the internal structure and parameters. To overcome this problem, this paper proposes a stability analysis approach for the droop-controlled parallel inverters based on the terminal-characteristics of individual inverters. At beginning, the terminal-characteristics of individual inverter with droop control are defined in system synchronous reference frame (SRF), which can be represented by the transfer function between output current and output voltage, and the transfer function between output current and fundamental angular frequency. Then the terminal-characteristics of the whole parallel system are derived out with the ones of each individual inverter. Furthermore, a stability criterion is proposed for the parallel inverters based on the terminal-characteristics of each individual inverter according to the Generalized Nyquist Criterion. Finally, the proposed terminal-characteristics and stable criterion for droop-controlled parallel inverters are verified in frequency domain.