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Stability Criterion of Droop-Controlled Parallel Inverters Based on Terminal Characteristics of Individual Inverters

Image of typical configuration of droop-controlled parallel inverters.
Fig. 1. Typical configuration of droop-controlled parallel inverters.
Droop-controlled parallel inverters are widely adopted in micro-grids due to their high reliability and low cost. However, the interaction among the inverters can cause the whole system to be unstable, even though each individual inverter can operate stably in stand-alone mode. A lot of work has been done to analyze the stability of parallel inverters with droop control through the state-space approach originally used in conventional electrical power systems. For this existing approach, it is necessary to have the knowledge of the inside parameters of each individual in-verter in advance, while in practical applications it is inconvenient 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 the beginning, the terminal characteristics of individual inverters with droop control are defined in the system synchronous reference frame (SRF), which can be represented by the transfer function be-tween the output current and output voltage, and the transfer function between the output current and fundamental angular frequency. Then, the terminal characteristics of the whole parallel system are derived, including the characteristics of each individual inverter. Furthermore, a stability crite-rion 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 characteris-tics and stability criterion for droop-controlled parallel inverters are verified in the frequency domain.

Image of small-signal block diagram of the parallel inverters based on terminal characteristics of indi-vidual inverters.
Fig. 2. Small-signal block diagram of the parallel inverters based on terminal characteristics of individual inverters.
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