Control of Inverter-Interfaced Distributed Generation Units for Voltage and Current Harmonics Compensation in Grid-Connected Microgrids

Journal of Operation and Automation in Power Engineering Vol. 4, No. 1, Winter & Spring 2016, Pages: 66-82 http://joape.uma.ac.ir Control of Inverter-Interfaced Distributed Generation Units for Voltage and Current Harmonics Compensation in GridConnected Microgrids R. Ghanizadeh1, M. Ebadian1,*, G. B. Gharehpetian 2 1 2 Department of Electrical and Computer Engineering, University of Birjand, Birjand, Iran. Department of Electrical Engineering, Amirkabir University of Technology, Tehran, Iran. ABSTRACT In this paper, a new approach is proposed for voltage and current harmonics compensation in grid-connected microgrids (MGs). If sensitive loads are connected to the point of common coupling (PCC), compensation is carried out in order to reduce PCC voltage harmonics. In absence of sensitive loads at PCC, current harmonics compensation scenario is selected in order to avoid excessive injection of harmonics by the main grid. In both scenarios, compensation is performed by the interface converters of distributed generation (DG) units. Also, to decrease the asymmetry among phase impedances of MG, a novel structure is proposed to generate virtual impedance. At fundamental frequency, the proposed structure for the virtual impedance improves the control of the fundamental component of power, and at harmonic frequencies, it acts to adaptively improve nonlinear load sharing among DG units. In the structures of the proposed harmonics compensator and the proposed virtual impedance, a self-tuning filter (STF) is used for separating the fundamental component from the harmonic components. This STF decreases the number of phase locked loops (PLLs). Simulation results in MATLAB/SIMULINK environment show the efficiency of the proposed approach in improving load sharing and decreasing voltage and current harmonics. KEYWORDS: Distributed generation, Microgrid, load sharing, Voltage and current harmonics compensation, Self-tuning filter. voltage levels, has changed the voltage and current harmonics into a common problem for the power quality of MGs. These problems might have undesirable effects such as interruption in the operation of adjustable speed drives (ASDs) and protective relays, motors and transformers overheating, and errors of power factor correction capacitors [5]. DGs are usually connected to electrical systems through a power electronic converter. The main role of an interface converter is to control the active and reactive powers injected by the DG. In addition, using appropriate control approaches, these converters can also be used to compensate for power quality problems. The control system of each DG includes controllers of fundamental component of the power, controllers of current, voltage, and virtual impedance loop. The characteristics used for 1. INTRODUCTION  A microgrid (MG) is a controllable network which includes distributed generation (DG) units, energy storage systems, and distributed loads. An MG can be utilized in two modes; grid-connected mode and independent (islanded) mode [1-2]. Optimal utilization of MGs removes the need for building new transmission networks, decreases environmental pollution, reduces energy losses in transmission and distribution networks, increases power quality, and creates new approaches for using renewable energy resources [2-4]. Also, the great increase in using nonlinear loads at distribution Received: 4 Sep. 2015 Revised: 8 Nov. 2015 Accepted: 29 Jan. 2016 *Corresponding author: M. Ebadian (E-mail: ) © 2016 University of Mohaghegh Ardabili 66 R. Ghanizadeh, M. Ebadian, G. B. Gharehpetian: Control of Inverter-Interfaced … controlling the fundamental components of powers, are only capable of sharing the positive sequence of the fundamental component of the load current among DGs. But other components of the load current such as harmonic components, are shared based on the impedances between each DG and the position in which the load is installed (including the impedance of DG and line impedance). Therefore, in previous studies, some methods have been presented to improve the sharing of nonlinear loads among the interface converters of DGs. A method was presented in Ref. [6] for improving the sharing of distortion power (D) among DGs. In this approach, D is shared by adjusting the voltage control bandwidth, which is done since the voltage control gain increases as D increases and causes the nonlinear load sharing to be improved. However, this approach has a disadvantage; it decreases voltage control stability [7]. In Refs. [7-11], some methods have been presented for sharing nonlinear load among DGs, which are based on creating a virtual impedance on the path of currents of harmonic components. In Ref. [8], a control strategy was proposed for sharing harmonic power in an islanded MG. The proposed control strategy employed negative virtual harmonic impedance to compensate for the effect of line impedance on harmonic power delivery. In Ref. [7], the harmonic current of any order was proposed to be used for creating a voltage drop which could lead the current by 90 degrees. By doing so, a virtual inductance can be created at harmonic frequencies. However, a virtual resistance is usually preferred to a virtual inductance since its impedance value does not depend on frequency and on the other hand, it helps damp the system oscillations; however, inductance will have a large impedance value at high frequencies, disturbing the output voltage of DGs. In this regard, in Ref. [11], fixed harmonic resistances were used. Using fixed values for virtual impedance which has been proposed in Refs. [7-11] cannot lead to a proper nonlinear load sharing in MGs which are remarkably asymmetric from the load distribution or lines impedance points of view. In this paper, a structure is proposed for the virtual impedance, to adaptively improve the nonlinear load sharing among DGs at harmonic frequencies. In the proposed structure, virtual resistances with variable values is used for different harmonic frequencies. The value of harmonic resistance of each DG is determined based on the amount of the nonlinear load supplied by it. Therefore, the effect of the asymmetry of MG impedances is significantly decreased. However, it must be noted that a proper nonlinear load sharing among DGs causes distortion in DG outputs and consequently increases PCC harmonics. So far, several approaches have been presented for controlling MGs, which aim to compensate for voltage and current harmonics. In general, these approaches either are based on autonomous control of the interface converter of DG or use a central controller to compensate for distortions. In this regard, in Ref. [12], a single phase DG was considered which acted as a shunt active filter. In other words, DG injected harmonic currents to improve the voltage quality. In Refs. [13-14], an optimal control approach based on particle swarm optimization (PSO) algorithm has been used to improve the power qua (...truncated)