Abstract

In this paper, a coordinated harmonic compensation and voltage support scheme is presented for distributed generations’ (DGs’) interface inverters in a resistive grid-connected microgrid. Voltage support is performed by reactive power compensation which can mitigate the over/under voltage problem; furthermore, the active power curtailment is proposed in order to mitigate the overvoltage problem when the reactive power compensation is not sufficient. Harmonic compensation is achieved by using virtual admittances in selected harmonic frequencies. Reactive power and harmonic compensation currents are injected with regards to the limited capacity of the interface inverter. If necessary, the reference powers of the grid-tied inverters are changed. Voltage support and harmonic compensation can be achieved based on local or central (communication-based) measurement schemes. The effect of communication delay is also investigated in this study. Experimental and simulation results are obtained in order to demonstrate the effectiveness of the proposed method.

Introduction

Voltage Source Inverters (VSIs) are widely used for interfacing Distributed Generation (DG) systems to the grid [1]. The DG interfacing inverters can contribute in voltage support [2–۱۳] and harmonic compensation [12–۲۱] of Microgrids (MGs) and utility grids. These inverters known as multifunctional inverters inject power to the grid and the remaining capacity of the inverter can be dedicated to voltage support and power quality enhancement [10].

To fully comprehend the voltage support of the DG interfacing inverters, reactive power compensation is conventionally proposed in distribution systems [2–۷,۱۱,۱۲] and MGs [8–۱۰] in order to mitigate the over/under voltage problems. Overvoltage can be created because of high penetration of Wind Turbine (WT) and Photovoltaic (PV) systems [2]. In Refs. [2,3], the voltage rise problem caused by high penetration of DGs in an LV distribution system has been studied and a droop-based reactive power control is proposed for Current Controlled Mode (CCM) of VSIs. In these papers, the reactive power compensation by DGs interfacing inverters is considered. In Refs. [4,5], the voltage rise mitigation based on active power curtailment in distributed systems has been proposed. The power curtailment algorithms of Refs. [4,5] are based on local measurement and communication system, respectively. In Ref. [6], different reactive power methods for DG units are studied. The reactive power control approaches in Ref. [6] can be classified into distributed and central controller based schemes. The reactive power control of DG interfacing inverters is also studied in Ref. [7]. In this paper, only a grid-connected interfacing inverter is considered whereas in grid-tied MGs more DG units can exist while a coordinated control of the units is required. In Refs. [8,9], voltage and frequency support functions of utility scale PV systems have been proposed. In Ref. [8], the performance of a PV system in voltage sag/swell compensation is evaluated based on the small signal modeling of utility scale PV and power system. In addition, the frequency support is added in Ref. [9]. In Refs. [10,11], the voltage support by Voltage Control Mode (VCM) VSIs in a grid-connected MG has been discussed, while, while it is obvious that, PV and WT systems are integrated as CCM VSIs.