The concept of microgrid hierarchical control is presented recently. In this paper, a hierarchical scheme is proposed which includes primary and secondary control levels. The primary level comprises distributed generators (DGs) local controllers. The local controllers mainly consist of power, voltage and current controllers, and virtual impedance control loop. The central secondary controller is designed to manage the compensation of voltage unbalance at the point of common coupling (PCC) in an islanded microgrid. Unbalance compensation is achieved by sending proper control signals to the DGs local controllers. The design procedure of the control system is discussed in detail and the simulation results are presented. The results show the effectiveness of the proposed control structure in compensating the voltage unbalance.

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Secondary Control Scheme for Voltage Unbalance Compensation in an Islanded Droop-Controlled Microgrid

Recently, there has been an increasing interest in using distributed generators (DGs) not only to inject power into the grid but also to enhance the power quality. In this paper, a stationary-frame control method for voltage unbalance compensation in an islanded microgrid is proposed. This method is based on the proper control of DGs interface converters. The DGs are properly controlled to autonomously compensate for voltage unbalance while sharing the compensation effort and also active and reactive powers. The control system of the DGs mainly consists of active and reactive power droop controllers, a virtual impedance loop, voltage and current controllers, and an unbalance compensator. The design approach of the control system is discussed in detail, and simulation and experimental results are presented. The results demonstrate the effectiveness of the proposed method in the compensation of voltage unbalance.

/pdf/autonomous-voltage-unbalance-compensation-in-an-islanded-1sdy5z0fc8.pdf

Autonomous Voltage Unbalance Compensation in an Islanded Droop-Controlled Microgrid

In this paper, a current-control strategy is proposed for voltage-source inverters in microgrids. The main objective of the proposed controller is to inject a clean sinusoidal current to the grid, even in the presence of nonlinear/unbalanced loads and/or grid-voltage distortions. The repetitive control technique is adopted because it can deal with a very large number of harmonics simultaneously. The proposed current controller consists of an internal model and a stabilizing compensator, which is designed by using the H∞ control theory. It turns out that the stabilizing compensator may be simply an inductor. This leads to a very low total harmonic distortion (THD) and improved tracking performance. In order to demonstrate the improvement of performance, the proposed controller is compared with the traditional proportional-resonant, proportional-integral, and predictive deadbeat controllers. The control strategies are evaluated in the grid-connected mode with experiments under different scenarios: steady-state and transient responses without local loads, and steady-state responses with unbalanced resistive and nonlinear local loads. The proposed controller significantly outperforms the other control schemes in terms of the THD level, with the price of slightly slower dynamic responses.

https://www.sheffield.ac.uk/polopoly_fs/1.323817!/file/IEEEPE1103_CurrentRep.pdf

A Current-Control Strategy for Voltage-Source Inverters in Microgrids Based on $H^{\infty }$ and Repetitive Control

This paper presents a linear current control scheme for single-phase active power filters. The approach is based on an outer voltage loop, an inner current loop, and a resonant selective harmonic compensator. The design of the control parameters is carried out using conventional linear techniques (analysis of loop gain and other disturbance-rejection transfer functions). The performance of the proposed controller is evaluated and compared with two reference controllers: a basic control and an advanced repetitive control. In comparison with these controllers, the proposed control scheme provides additional attenuation to the harmonics coming from the load current, the grid voltage, and the reference signal, resulting in a grid current with lower harmonic distortion. Experimental results are reported in order to validate this paper.

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Selective Harmonic-Compensation Control for Single-Phase Active Power Filter With High Harmonic Rejection

The aim of this work is the application of the feedback linearization theory to a single-phase shunt active power filter, since this technique has been successfully applied to other areas of power electronic. The active filter is linearized by means of a nonlinear transformation of the system model, deduced from the application of Tellegen's theorem to the system. After that, a sliding mode controller is proposed to impose a desired dynamic behavior on the system, giving robustness and insensitivity to parameter variations. Moreover, the proposed controller ensures proper tracking of the reference signals and simplifies the overall control design. The controller was implemented into a low cost DSP. Experimental and simulation results are provided.

Feedback Linearization of a Single-Phase Active Power Filter via Sliding Mode Control

This paper presents the detailed design, analysis, and application of the controller for a shunt active power filter based on a pulsewidth modulation dc-to-ac voltage source converter. The controller is mainly tailored to compensate harmonic currents of nonlinear loads connected to the mains. However, it can also achieve reactive-power compensation and mains-current balancing when required. The controller has a two-layer structure. The outer layer generates the current references for the inner layer. The former uses a plug-in discrete-time repetitive algorithm for current-harmonic compensation, a proportional-integral algorithm to maintain the dc-capacitor voltage in spite of unmodeled losses and a reactive-power-reference generator. The inner layer uses state-feedback with integral action for current control. The repetitive controller is justified to improve the tracking of the periodic current references required by the active filters. The stability of the resulting closed-loop system is studied and some indication of the system robustness is given. The proposed controller has been tested in a prototype with balanced and unbalanced nonlinear loads. A discrete-time model of the filter has been used from the beginning. The microcomputer delay when calculating the controller output and the delay due to the anti-aliasing filters have been included in the inner system state-variable model

/pdf/application-of-a-repetitive-controller-for-a-three-phase-31zvxoy88o.pdf

Application of a Repetitive Controller for a Three-Phase Active Power Filter

This paper presents the analysis and the application of a current controller in an active power filter (APF) based on a PWM voltage-source electronic converter with three legs and four wires. The neutral wire is connected to the middle point of the DC-capacitor voltage. The controller proposed here is an extension of the one proposed for a three-wire Shunt Active Power Filter. The controller is a two-level nested controller. The outer-loop generates the reference current for the inner-loop. The latter, is a state-feedback current controller with integral action. The former consists of (i) a selective harmonic elimination technique and (ii) a DC-capacitor-voltage controller. This paper will focus on the neutral-wire current control and on the balance control of the DC-capacitor voltage. The performance of the control algorithm has been demonstrated using a test-rig with balanced and non-balanced non-linear loads.

O. Pinzon-Ardila

Papers

Application of a Repetitive Controller for a Three-Phase Active Power Filter

Control of a Shunt Active Power Filter based on a three-leg four-wire electronic converter