AC microgrid mainly comprise inverter-interfaced distributed generators (IIDGs), which are nonlinear complex systems with multiple time scales, including frequency control, time delay measurements, and electromagnetic transients. The droop control-based IIDG in an AC microgrid is selected as the research object in this study, which comprises power droop controller, voltage-and current-loop controllers, and filter and line. The multi-time scale characteristics of the detailed IIDG model are divided based on singular perturbation theory. In addition, the IIDG model order is reduced by neglecting the system fast dynamics. The static and transient stability consistency of the IIDG model order reduction are demonstrated by extracting features of the IIDG small signal model and using the quadratic approximation method of the stability region boundary, respectively. The dynamic response consistencies of the IIDG model order reduction are evaluated using the frequency, damping and amplitude features extracted by the Prony transformation. Results are applicable to provide a simplified model for the dynamic characteristic analysis of IIDG systems in AC microgrid. The accuracy of the proposed method is verified by using the eigenvalue comparison, the transient stability index comparison and the dynamic time-domain simulation.
In this paper, a nonlinear, bounded, distributed secondary control (DSC) method is proposed to coordinate all the distributed generators (DGs) in islanded AC microgrids (MGs). This proposed consensus-based DSC strategy can not only guarantee the restoration control of frequency and voltage but also realize an accurate active power sharing control. Through introducing a nonlinear dynamic from beta cumulative distribution function (CDF), the convergence speed of DSC is accelerated, the asymptotical convergence of DSC is ensured, and the transient overshoot of DSC is diminished compared with traditional DSC. Moreover, by ensuring the Lipchitz continuity characteristic of the control algorithm, the common chattering phenomenon in non-Lipchitz DSC scheme is eliminated. The stability and performance of the proposed DSC are also analyzed in this paper. An islanded AC microgrid test system with four inverter-based DGs is built in MATLAB/SIMULINK to further validate the effeteness of the proposed DSC strategy. INDEX TERMS Nonlinear distributed secondary control, islanded microgrid, multi-agent system, Lipchitz continuity, chattering phenomenon.
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