Most of the existing reactive power sharing schemes that assume parallel architecture are known to be less effective for multi-bus radial microgrids. This paper proposes an improved reactive power sharing scheme that exploits the novel concept of droop equivalent impedance into designing a consensus virtual-output-impedance-based droop control scheme. The control scheme leads to two notable improvements: (a) it proves that only either virtual resistance or virtual reactance is sufficient to restore proportionate reactive power sharing; (b) only a global coupling gain needs to be tuned and no proportional-integral controller is required. A systematic guideline that establishes the approximate range of stable coupling gain is developed. This simplifies the tuning process of the coupling gain. The power correction performance, the resulting bus voltage behavior, consensus control stability, and the robustness to time delay have been investigated in conjunction with an islanded microgrid modified from the IEEE 34 Node Test Feeder. It is shown that the consensus control scheme is capable to demonstrate accurate power sharing regardless of the changes in the network topology, network impedances, loading conditions, and communication delay.

This paper presents a comparative study of the proportional-integral-based (PI-based) synchronous current control strategy with derivative-feedback-based active damping and the finite-controlset model-predictive-control-based (FCS-MPC-based) synchronous current control strategy with costfunction-based active damping. For a fair comparison, the sensor requirement and the average switching frequency of FCS-MPC are made equivalent to that of the pulse-width-modulation-based counterpart through internal model estimation and control sampling frequency adjustment. The comparative study considers gain/weighting-factor tuning, delay compensation, switching harmonics, and active damping performance at the critical frequency operating point. The overall performance of both schemes is validated through the same experimental setup and test scenarios. The results conclude that the emerging FCS-MPC has the potential to produce similar results as the classical PI-based counterpart while carrying some practical features. These include being intuitive in active damping design and tuning, guaranteeing fast dynamics, and being sufficiently robust to grid impedance shifting. These findings essentially justify the potential of model predictive control being a viable alternative for this area of application. INDEX TERMS Model predictive control, cost function, LCL filter, active damping, derivative voltage feedback. CHEE SHEN LIM (M'14) received the B.Eng. degree (Hons.

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The subject of optimal secondary control of power-electronic-interfaced distributed energy resources (DERs) in droop-controlled microgrids has garnered significant research attention in recent years. While the feasibility of optimal secondary control based on non-linear power flow has been proven, the power flow algorithm is essentially iterative in nature. This work proposes an optimal secondary control with non-iterative power flow to regulate multi-bus voltages and DERs' reactive powers. The control scheme incorporates a modified Decoupled Linearized Power Flow that is known to be superior in terms of reactive power and bus voltage magnitude estimation, as compared to classical DC power flow, into a constrained quadratic programming. Q-V droop is integrated into the linear power flow in place of the slack bus. The proposed optimal scheme is provably accurate for maintaining reactive power sharing while regulating multiple load-bus voltages. The additional degrees of freedom enabled by the weighting factors significantly improve the control flexibility of the secondary controller. The allowable bus voltages and DER kVar capacity limits have also been considered by the control algorithm. The work is proven through an accurate co-simulation study comprising an 18-bus network and a full primary control models in PowerFactory, interfaced through industrial communication tool MatrikonOPC.INDEX TERMS Microgrid, droop control, reactive power sharing, voltage regulation, optimal secondary control, decoupled linearized power flow.

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