Adaptive virtual impedance scheme for selective compensation of voltage unbalance and harmonics in microgrids

Abstract

Abstract-This paper presents a two-level hierarchical control approach for voltage source inverters used to interface Distributed Generators (DGs) in microgrid applications. The control structure comprises primary and secondary levels. The primary level is a local controller, which consists of voltage and current inner control loops in order to fix the filter capacitor voltage and a virtual impedance loop mainly for voltage harmonics and unbalance compensation. The virtual impedance is set by the central secondary controller to mitigate the voltage distortion at sensitive load bus (SLB). Secondary controller is connected to a measurement unit to obtain the data of voltage harmonics and unbalance at microgrid SLB and broadcasts the commands for adjusting the virtual impedance of each unit. A general case with a combined voltage harmonic and unbalance distortion is considered. In such a case, voltage distortion is mitigated by selective insertion of capacitive virtual impedances for negative sequence of fundamental component as well as positive and negative sequences of main harmonics. The values of virtual capacitances are determined based on the required voltage quality at the load bus; thus, this scheme is titled as adaptive virtual impedance. Experimental results are presented to demonstrate the effectiveness of the proposed control approach.

Review of Active and Reactive Power Sharing Strategies in Hierarchical Controlled Microgrids

“…Under extreme situations, the poor reactive power sharing may result in severe circulating reactive powers among the DG units and may cause system instability [38]. To share the reactive power, various droop control methods have been proposed, which include three main categories: the improved primary droop control methods [18], [20], [39][40][41], [70], [71], the improved virtual impedance methods [42][43][44][45][46][47][48][49][50][51][52] and the improved hierarchical control strategies [63][64][65][66], [72]- [79].…”

“…It is difficult to share the reactive power accurately under the mismatched feeder impedance, and nonlinear and unbalanced load conditions by the improved droop control. As a supplement of the improved droop control, the methods based on the virtual impedance or improved virtual impedance, have been proposed to share the active and reactive powers [42][43][44][45][46][47][48][49]. Although the inductive virtual impedance can enhance the capacity of the reactive power sharing under the mismatched feeder impedance condition, the reactive power cannot be shared accurately when the loads are nonlinear and unbalanced in islanded MGs.…”

A Review on Impacts of Power Quality, Control and Optimization Strategies of Integration of Renewable Energy Based Microgrid Operation

“…It does not depend on the impedance of the output voltage but may also include resistive, inductive, capacitive or complex. Generally, Adaptive virtual impedance control method [38,39] is the power flow control method in low voltage distribution grids. Fig.7 shows the basic arrangement of the adaptive virtual impedance control used to extract the positive and negative sequence at harmonic components.…”

A Composite Strategy for Harmonic Compensation in Standalone Inverter Based on Linear Active Disturbance Rejection Control

“…However, the effects of the output inductance value fluctuation on the virtual harmonic impedance compensation effect are not solved in [20][21][22]. Authors in [23] propose adaptive virtual harmonic impedance based on the required voltage quality at the load bus, which solved the uncertainties of virtual harmonic impedances to some extent. But it requires additional calculations for virtual harmonic impedances.…”