Novel control of single phase LCL filtered grid connected inverter with seamless mode transition and soft start technique using direct synthesis approach

Abstract

Grid-connected inverters (GCI) are used to inject renewable / stored energy directly into the utility grid. GCI operation warrants smoother interconnection to the grid, supply of pre-specified real and reactive power, stable operation under wide grid parameter variation, etc. Hence the design of controller for GCI needs to address multiple objectives. Several controller structures and tuning methods are proposed in the literature addressing each control objective of GCI. The aim of this Thesis is to provide a simplified and unified approach that will lead to the right choice of controller including its structure and tuning method with an insight to meet all the objectives by direct synthesis approach. The control objectives are identified as: (i) sinusoidal current tracking with prescribed power factor, (ii) harmonics attenuation, (iii) resonance damping and (iv) extended stability over parameter variation. The first objective is to track a given sinusoidal current reference by synthesizing controller in stationary reference frame. In a typical two stage GCI system, this current reference is derived from the intermediate DC link controller to maintain it at a specified voltage. The phase information of the reference current is generated from a Phase Locked Loop (PLL) unit. This ensures proper phase to be maintained with respect to the voltage at the Point of Common Coupling (PCC). In this Thesis, injection of current into the grid at unity power factor is considered. newline