13-05-2014, 03:54 PM
THREE PHASE MULTISTRING BOOST FRONT-END HYBRID MULTI-LEVEL INVERTER FOR A PHOTO-VOLTAIC SYSTEM
THREE PHASE MULTISTRING.pdf (Size: 1.32 MB / Downloads: 31)
Abstract
The proposed work describes a novel three-phase inverter topology for grid-connected
PV system. The novel inverter circuit is built by connecting a line-frequency
three-phase bridge inverter in series with three high-frequency single-phase bridge
inverters. The three-phase bridge inverter bears the main output power; three
single-phase full-bridge inverters are used to improve the system dynamic per-
formance. Multi-string PV system can provide independent control circuitry for
maximum power point tracking (MPPT), which leads to higher energy conversion
efficiency. Compared with the traditional inverter circuit, the circuit can achieve
high efficiency and low harmonics at the same time, and it can also reduce the volt-
age stress of the power switches. The circuit can be controlled easily, so it was easy
to be modularized. The proposed work consists of the study of inverter topology,
principle of operation, switching states and control circuit design. The simulation
model of the inverter can be done using MAT LAB Simulink software and the
prototype can be tested using PIC controller. The correctness and feasibility of
the theoretical analysis can be verified by simulation software.
Introduction
In order to solve the energy crisis and environmental pollution, renewable energy
has been the certain choice of sustainable development for mankind, particularly
solar and wind energy. Solar energy is one of the favourable renewable energy
source because of its distinctive advantages, such as simple configuration, easy al-
location, free of pollution, no noise, low maintenance cost, etc[1]. In recent years,
as cost of photovoltaic system declined, photovoltaic technology has progressed sig-
nificantly. Photovoltaic (PV) generation is the technique which uses photovoltaic
cell to convert solar energy to electric energy. Photovoltaic system converts dc to
ac voltage in a two-stage process. In the first stage, a dc-dc converter boosts the
dc voltage and in the second stage the increased dc voltage converted to ac voltage
using an inverter. A PV inverter which is an important element in the PV system
is used to convert DC power from the solar panels into AC power. Many different
types of PV inverters have been studied.
Layout of the Thesis
The thesis report is organized as follows. Chapter 2 describes the literature
survey which includes detailed explanation of the photovoltaic system, the brief
description of power electronic converter, earlier inverter topologies along with the
problem definition and tackling methods. Chapter 3 includes design of converter
and modelling of of PV array followed by the proposed work in chapter 4, which
includes the study of proposed inverter topology, principle of operation, switching
states and control circuit design. Chapter 5 includes Simulation models, design and
results of PV module followed by hardware details in brief in chapter 6. Chapter
7 and 8 includes the conclusion and references.
Equivalent Circuit of Photo-Voltaic System
A simplified equivalent circuit model given in fig.2.3. An ideal PV is modelled
by a current source in parallel with a diode. However no solar cell is ideal and
thereby shunt and series resistances are added to the model. Rs is the intrinsic
series resistance whose value is very small. Rp is the equivalent shunt resistance
which has a very high value [5].
PV module
Due to the low voltage generated in a PV cell (around 0.5V), several PV cells
are connected in series (for high voltage) and in parallel (for high current) to form
a PV module for desired output. Separate diodes may be needed to avoid reverse
currents, in case of partial or total shading, and at night.The current rating of the
modules depends on the area of the individual cells. Higher the cell area, higher
the current output of the cell.The higher efficiency, the smaller surface is needed
for a given power. This is important because in some applications the space is
limited and other costs and parameters of the installation depend on the installed
PV surface.
Power Electronic Converters in PV System
Due to the photo-voltaic effect, the voltage of a PV cell is not very high. Because
PV panels in a series string are constrained to all conduct the same current, the
least efficient cell sets this string current, which may spell failure when one cell
of a string is inactive. The overall efficiency of the PV array is reduced to the
efficiency of this least efficient cell. It means that PV panels in a series string must
be given the same orientation and be of identical size for obtaining a higher output
voltage. Besides, the corresponding output voltage is varied easily with respect to
the variation of loads. In order to satisfy the requirement of high-voltage demand,
a dc-dc converter with high voltage gain is one of the essential mechanisms in the
PV generation system[24].
PV Boost with Maximum Power Point Track-
ing Control
PV cell has non-linear current-voltage characteristics. The power delivered by
an array increases to a point, as the current draw rises. Any additional current
drawn from the array results in a rapid drop-off of cell voltage, thus reducing the
array power output.Thus a MPPT control to extract maximum power from the
PV arrays at real time becomes indispensable in PV generation system. Aging
effects and external factors such as temperature and partial shading of an array
make locating and tracking the maximum power point a bit more challenging.
These techniques differ widely in complexity, tracking accuracy, response time,
costs, efficiency and target implementation .