الفهرس 
DC-DC Converter Functions and Requirements
. Maximum Power Point TrackingOnly 14 pages are availabe for public view
 |  | from | 118 |  |  |
| | | from | 118 | | |
Abstract
The high penetration of solar energy systems has driven researchers worldwide to direct their efforts towards improving the efficiency of these systems. This thesis is concerned with double stage grid-connected photovoltaic (PV) systems where the DC-DC converter is a very effective component.
Traditional pulse width modulated (PWM) DC-DC converters suffer from the drawbacks of hard switching which degrades the efficiency because of the increased switching losses. Also hard switching limits the range of operating frequency which results in low power density.
Resonant DC-DC converters have been introduced as an attempt to mitigate the limitations found in traditional PWM DC-DC converters. These converters offer soft switching mechanisms like zero voltage switching (ZVS) and zero current switching (ZCS) which significantly reduce the switching losses and consequently increase the efficiency. In addition, soft switching enables operation at very high frequencies increasing the power density of the converter.
A LLC resonant DC-DC converter is proposed to boost the DC voltage output of the PV array and perform the task of maximum power point tracking (MPPT). This converter has the advantages of wide input and output regulation capability, high boost ratio, high efficiency and high power density. Furthermore, the power density can be enhanced by the utilization of the magnetizing and leakage inductances of the transformer as parts of the resonant tank. The incremental conductance MPPT is applied using PI controller for its simplicity and decent accuracy. The performance of the MPPT controller is enhanced through the application of ant colony optimization (ACO) algorithm to optimally tune gain parameters of the PI controller.
A three level neutral point clamped voltage source inverter (VSI) is utilized to interface between the PV array and grid. The inverter is controlled to inject a high quality AC power into the grid at unity power factor. The control system consists of two consecutive control loops; an outer voltage loop that maintains the DC bus voltage at a fixed level, and an inner current loop for synchronization between injected currents and grid voltages and achieving unity power factor operation.
The entire grid-connected PV system was built and simulated in MATLAB/SIMULINK based on double diode model of PV cell. The PV array was subjected to different types of disturbance in solar irradiation to verify the dynamic and steady state performance of the proposed system.