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Owing to mismatched feeder impedances in an islanded microgrid, the conventional droop control method typically results in errors in reactive power sharing among distributed generation (DG) units. In this study, an improved droop control strategy based on secondary voltage control is proposed to enhance the reactive power sharing accuracy in an islanded microgrid. In a DG local controller, an integral term is introduced into the voltage droop function, in which the voltage compensation signal from the secondary voltage control is utilized as the common reactive power reference for each DG unit. Therefore, accurate reactive power sharing can be realized without any power information exchange among DG units or between DG units and the central controller. Meanwhile, the voltage deviation in the microgrid common bus is removed. Communication in the proposed strategy is simple to implement because the information of the voltage compensation signal is broadcasted from the central controller to each DG unit. The reactive power sharing accuracy is also not sensitive to time-delay mismatch in the communication channels. Simulation and experimental results are provided to validate the effectiveness of the proposed method.

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The four-switch three-phase inverters have become an effective approach for fault-tolerant reconstruction and operation of the six-switch three-phase topology. However, the conventional control strategy for four-switch three-phase inverters can result in a large number of current harmonic components, high electromagnetic acoustic noise, and electromagnetic interference (EMI). Therefore, this paper proposes a random switching frequency pulse width modulation method under the centrosymmetry period with a two-state Markov chain based on four-switch three-phase inverters (RSFPWM-CPTMC). In this method, random numbers are optimized and evenly distributed on both sides of the center frequency within a specific frequency bandwidth range, which significantly reduces the current harmonics and EMI at the switching frequency and frequency multiplication. The spectral characteristics generated by the random switching frequency under the centrosymmetry period with the two-state Markov chain are evaluated and compared to that provided by the traditional fixed switching frequency pulse width modulation (FSFPWM). Simulations and experiments are carried out to illustrate the superiority of the proposal.

In this study, networked control scheme applied in dq0 coordinate for three-phase parallel inverters of distributed generation units is presented to achieve flexible current weighting distribution. Firstly, the output impedance based on generalised state space averaging method with the consideration of dead-time effect is investigated in dq0 coordinate and proved to be inductive for fundamental components in dq0 coordinate. Therefore the weighted output current distribution can be realised conveniently with networked control applied in dq0 coordinate without the additional circuit or virtual impedance. Besides, for the sake of flexibility and autonomy of a parallel inverters system, the power proportional coefficients of different inverters can be changed automatically. In the end, power weighting coefficients in networked control are optimised by means of functional analysis. The experimental results demonstrate the effectiveness of a networked control scheme applied in dq0 coordinate on the flexible power weighting distribution for parallel inverters.