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An accurate, robust and efficient transport-delay based phase-lock loop (TDPLL) algorithm is proposed for single phase utility interactive inverters. The traditional TDPLL has low complexity and its transient response is smooth, but its performance is significantly affected by the change in the grid frequency. The proposed TDPLL is designed to have robustness against grid frequency variations. The proposed method uses two delays instead of one to cancel the grid frequency variation effect. Since the grid voltage could be polluted by harmonics and may suffer from sag/swell anomalies, the proposed method is enhanced further to be robust against these sources of disturbances. Moreover, a method is proposed to tune the parameters of the system. The performance of the proposed PLL is validated through simulations and experimental results.

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In this paper, nonlinear droop relations are suggested to optimize the operation of islanded microgrids. By using nonlinear droop, many aspects of the microgrid operation can be optimized in addition to the inherent advantages of the droop control. In the proposed method, the droop relations are allowed to have any nonlinear shape as long as that shape satisfies certain characteristics required for the stability and proper microgrid operation. The procedure of constructing the nonlinear droop relations that minimize the operating cost of the microgrid and share the reactive power effectively among the sources is investigated and explained in detail. The selected droop structure is a combination of integer and fractional power functions whose parameters are selected using two-stage particle swarm optimization algorithm. The effectiveness of the proposed method is verified through simulation and experimental studies.

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