Variable- and Fixed-Switching-Frequency-Based HCC Methods for Grid-Connected VSI With Active Damping and Zero Steady-State Error

This paper presents variable- and fixed-switching-frequency-based hysteresis current control (HCC) methods for single-phase grid-connected voltage-source inverters with LCL filter. The main feature of the proposed HCC methods is that the reference inverter current is generated through a proportional-resonant (PR) controller for achieving zero steady-state error in the grid current. The consequence of using the PR controller is eliminating the need for using derivative operations in generating the reference inverter current. Furthermore, the active damping method is employed to damp the LCL resonance. An equation is derived for variable-switching frequency. Fixed-switching frequency operation is achieved by modulating the hysteresis band. The performance of both HCC methods has been validated by simulation and experimentally. It is reported that the proposed HCC methods not only preserve the inherent features of the conventional HCC methods, but also damp the LCL resonance using an active damping method and guarantee zero steady-state error in the grid current.

I. Introduction

Owing to the latest improvement of power distribution systems with renewable energy sources, the control of grid-connected voltage-source inverter (VSI) systems has been studied by many researchers in recent years [1], [2]. In such systems, a coupling filter (L or LCL) is required between the VSI and grid so as to alleviate the harmonics of the grid current. In recent years, the use of an LCL filter is considered by many researchers because of its prominent features like the operation ability at lower switching frequencies, the ability to provide better harmonic attenuation, and the ability to offer lower ripple in the grid current. However, such filter suffers from three disadvantages: 1) the risk of unstable operation because of two extra poles; 2) the requirement of LCL resonance damping; and 3) the increased complexity of the control algorithm. Therefore, the control methods developed for an LCL-filter-based system should not only cope with these disadvantages, but also should achieve a set of objectives, such as sinusoidal grid current with considerably small total harmonic distortion (THD), fast transient response, high robustness against parameters variation, implementation simplicity, and achieving zero steady-state error in the grid current.