Improved Design, Analysis and Current Controlled Strategy for Photovoltaic (PV) Grid Connected Converters

Abstract

To utilize the energy from the Renewable Energy Sources (RES) globally, they need to be connected to the grid using the converters. There are many topologies used for interconnection with the grid for e.g., two-level with LCL filter, three-level Neutral Point Clamped (NPC), multi-level, matrix, and interleaved etc. While interconnecting, there are many local and international standards which must be followed. To achieve this, we require a suitable control strategy. The current fed to the grid at the Point of Common Coupling (PCC), where the RES are connected to the grid must be having a Total Harmonic Distortion (THD) below 5%. The classical controllers do not give suitable Gain and Phase Margins at the utility harmonic frequencies and they fail to stabilize the system if the grid current THD>5%. In this thesis we have attempted to study the performance limitations of the classical controllers and also to identify the optimal gains for the proportional (P) and proportional integral (PI) controllers. The topic which has been explored in this thesis is an active research area. Furthermore, the effect of the active damping on the system performance has also been explored in detail. Ideally, we require a controller that has infinite gains at the harmonic frequencies. For this purpose, we have also reviewed the performance of the linear quadratic regulator (LQR), the Proportional Resonant (PR) and the Repetitive Controller. The Proportional Resonant Controller has been identified as a good candidate for our desired objective. We have considered a current-controlled Two-Level Pulse Width Modulated (PWM) Grid-Connected Converter, because the grid impedance is assumed to be fixed so we feed a constant current to the grid. The classical PI controller can give suitable stability margins under varying load impedance, but provides insufficient disturbance rejection which leads to high Total Harmonic Distortion (THD) in the current fed to the grid. We have tried to ascertain the maximum performance bounds of the classical P and PI controllers respectively under variation in grid harmonics.