Abstract
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.
Highlights
Accurate reactive power sharing of distributed energy resources (DERs) in distribution microgrids can be achieved through improved droop schemes based on dispatch mechanisms [1], synchronous-axis voltage droop mechanisms [2], enhanced droop techniques [3], and virtual output impedance (VOI) mechanisms [4], [5]
Distributed control structures. [6] adopts a full-graph dispatch droop to realize average DER-voltage restoration. [7] proposed a non-linear state estimator to realize decentralized secondary DER-voltage control. [8]–[14] have investigated on consensus-based secondary controls that restore the average DER-voltage while maintaining accurate reactive power sharing. [11] proposed a voltage-consensus scheme to maintain the averaged DERbus voltage to the rated value. [12], [15] proposed a PIconsensus based secondary voltage controller for regulating averaged-DER-bus voltage without the needs of external reference
The co-simulated network has 5 DERs (3 is used in [29]) and 18 buses – these settings are chosen in order to limit the wait time due to computational burden encountered when running the network model in a standard computing workstation
Summary
Accurate reactive power sharing of distributed energy resources (DERs) in distribution microgrids can be achieved through improved droop schemes based on dispatch mechanisms [1], synchronous-axis voltage droop mechanisms [2], enhanced droop techniques [3], and virtual output impedance (VOI) mechanisms [4], [5]. In applications where bus voltages within the droop-based microgrids are to be regulated concurrently (e.g. being the critical buses or the main points of common coupling with the wider AC grid), advanced secondary voltage control schemes are relevant. These secondary control schemes can be categorized to the types of targeted buses: (i) DER-bus scheme; (ii) load-bus scheme. [8]–[14] (with [8] adopting VOI-based primary control) have investigated on consensus-based secondary controls that restore the average DER-voltage while maintaining accurate reactive power sharing.
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