Control Strategy for a Grid-Connected Inverter under Unbalanced Network Conditions—A Disturbance Observer-Based Decoupled Current Approach

Emre Ozsoy,Lucian Mihet-Popa,Fiaz Ahmad,Asif Sabanovic,Viliam Fedák,Rasool Akhtar,Sanjeevikumar Padmanaban

doi:10.3390/en10071067

Abstract

This paper proposes a new approach on the novel current control strategy for grid-tied voltage-source inverters (VSIs) with circumstances of asymmetrical voltage conditions. A standard grid-connected inverter (GCI) allows the degree of freedom to integrate the renewable energy system to enhance the penetration of total utility power. However, restrictive grid codes require that renewable sources connected to the grid must support stability of the grid under grid faults. Conventional synchronously rotating frame dq current controllers are insufficient under grid faults due to the low bandwidth of proportional-integral (PI) controllers. Hence, this work proposes a proportional current controller with a first-order low-pass filter disturbance observer (DOb). The proposed controller establishes independent control on positive, as well as negative, sequence current components under asymmetrical grid voltage conditions. The approach is independent of parametric component values, as it estimates nonlinear feed-forward terms with the low-pass filter DOb. A numerical simulation model of the overall power system was implemented in a MATLAB/Simulink (2014B, MathWorks, Natick, MA, USA). Further, particular results show that double-frequency active power oscillations are suppressed by injecting appropriate negative-sequence currents. Moreover, a set of simulation results provided in the article matches the developed theoretical background for its feasibility.

Highlights

The rapid penetration of renewable energy sources (RESs) connected to the grid and distribution systems with power electronic converter topologies has changed the expected grid requirements to guarantee an appropriate performance under grid faults
It is assumed that symmetrical component decomposition of the voltage and currents is perfectly estimated, and an accurate Phase Locked Loop (PLL) voltage phase and angle estimation is achieved
The dynamic performance of positive-sequence controllers was demonstrated by applying appropriate dq current steps (Simulation C)

Summary

Introduction

The rapid penetration of renewable energy sources (RESs) connected to the grid and distribution systems with power electronic converter topologies has changed the expected grid requirements to guarantee an appropriate performance under grid faults. The main contribution of this study is a proportional decoupled current controller with a DOb are sufficient for robust operation, as the DOb accurately estimates and feeds forward uncertain fed-forwarded low-pass filter DOb, which satisfies positive-sequence power requirements by terms. The main contribution of this study is a proportional decoupled current controller with controller is to bring freedom from the sensitivity of the controllers with regard to variations in the a fed-forwarded low-pass filter DOb, which satisfies positive-sequence power requirements by grid parameters during operation for various reasons. This is the first reported study for a decoupled dq current control structure by these methods inject non-sinusoidal currents to the grid at the instant of unbalanced voltage using symmetrical component decomposition and estimating the disturbances with the DOb conditions. The study was implemented on a Matlab/Simulink (2014B, MathWorks, Natick, MA, USA) simulation platform

Dynamic

First-Order Low-Pass Filter Disturbance Observer

Instantaneous

Simulation Results

Simulation

Conclusions