Distributed Control Strategy Based on a Consensus Algorithm and on the Conservative Power Theory for Imbalance and Harmonic Sharing in 4-Wire Microgrids

Abstract

A distributed control system is proposed which uses the Conservative Power Theory (CPT) and a consensus algorithm to share imbalance and harmonics between different converters in three-phase four-wire droop-controlled Microgrids (MGs). The CPT is used to identify the balanced, unbalanced and distorted components of the currents and powers in the system. Control loops based on virtual impedance and implemented in the stationary a-b-c frame are then used to distribute these components between the various converters in the MG. The magnitudes of the virtual impedances are adaptively calculated using a novel consensus-based distributed control scheme with the aim of sharing imbalances and harmonics according to the residual VA capacity of each converter whilst regulating the imbalance and distortion at their output to meet the appropriate IEEE power quality standards. Extensive simulations show that the proposed distributed control has excellent performance, and experimental validation is provided using a laboratory-scale 4wire MG.

Single-Phase Consensus-Based Control for Regulating Voltage and Sharing Unbalanced Currents in 3-Wire Isolated AC Microgrids

“…Regarding the problem of sharing unbalanced currents, to the best of the authors' knowledge, distributed controllers have been proposed only in [24] [25] and [26]. These methods are based on the concept of the virtual impedance loop, which means that negative and/or zero sequence impedances are implemented to control the sharing of unbalanced currents between the power converters.…”

“…However, the method proposed in [25] does not limit imbalance in the voltages. This issue is addressed in [26], where a distributed control scheme is proposed to improve the sharing of imbalances in 4-wire MGs and, at the same time, regulate the imbalance in the voltage at the output of the power converters to meet the appropriate IEEE power quality standards. Experimental results are provided.…”

“…The distributed approaches reported in [24] [25] and [26] are based on the application of virtual impedance loops [27]; thus, they need to identify positive, negative and/or zero sequence current components. In [24] [25], this is performed using symmetrical component theory (SCT).…”

“…[28] [29], affecting its performance [30]. This drawback is overcome in [26], where a current transform based on the Conservative Power Theory (CPT) [8] is used. However, the implementation of the CPT algorithm represents a relatively high computational burden for the control platform [31].…”

“…In [24] [25] [26], the sharing of imbalance is achieved, controlling the magnitude of the negative sequence current supplied by the converters, but not its negative sequence phase angle. Therefore, when consensus is achieved, and all the converters are supplying the same magnitude of negative sequence currents to the load, it does not mean that in the a-bc domain the magnitude of the current in phase "a" of one converter is similar or equal to the current supplied on phase "a" of another converter (the same happens in phases "b" and "c").…”