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 Single Phase-Single Stage Z-Source Solar PV
Inverter

 

Abstract

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Recently
developed Z-source inverters have the capability to provide buck and boost
functions to the preliminary stage of inverters. The current research presented
in this paper proposes a Single Phase-Single Stage (SP-SS) Z-Source SPV
inverter topology. The designed system, not only raises the voltage coming from
the solar photovoltaic arrays, but also capable of tracking the maximum power for
varying level of irradiance. In this paper difference between single stage and
dual stage system is presented. The results are obtained in order to analyse the
Total Harmonic Distortion (THD) and DC component present in the output AC
coming from the inverter.

Keywords:
Photovoltaic
(PV), Single Phase-Single Stage (SP-SS), reliability, THD

1.      
INTRODUCTION

To
get a pollution free environment renewable energy resources comprises of solar,
wind, hydro etc. are the best available options for generation of electrical
power. Over a period, solar energy for generating the electricity became a
viable and feasible option due to its availability and abundance in nature. Solar
energy is harnessed using solar panels for extracting power which requires high
initial investment. Since, the availability of solar is there only during day
time, therefore to help people during night time storing of electricity during
day through electrical storage or batteries are required. Utilizing the solar
power to its maximum and the efficient design of the inverter are the key
factors in reducing the cost. PV array’s I-V characteristic is non-linear in
nature 1 and the movement of big blue marble known as earth, encompassing the
sun entails the operation of MPPT 2.

Standalone
system can attain maximum power point tracking, but it requires battery bank to
store the power. For grid connected system multistage systems have been
reported in the literature. Most of such multistage systems comprises of two
stages. Figure 1(a) represents the conventional two stage system 3 for
conditioning the power generated through PV and then feeding it into the grid.
In the first stage, the harvested solar power boosts up and also tracks the
maximal solar power. In the second stage, DC power generated by the solar array
is inverted to good quality AC power. Generally, boost type DC-DC converter or
buck-boost are used in the first stage. The above two stages are time tested
and gives good output result. This configuration has some drawbacks too, that
is, increased number of parts, lowering in efficiency and reliability, also the
large size and high cost are added to it. So, we could attain a situation
depicted in Figure 1(b) i.e. to lower down of the power processing stages.

 

               
(a)

               
 (b)

Figure1.
Grid integrated solar photovoltaic topologies:
(a) traditional double-stage structure and (b) single-stage structure

To reduce the
multistage systems, and problems associated with it, to single stage systems
there are two simple solutions:

1)       By
using a step-up transformer with the H-bridge inverter.

2)       By
constructing an array which produces large voltage followed by a H-bridge
inverter.

The
above stated solutions can come into use, but they also suffer from hitches too
4.  In the first case, by adding a
transformer similar to the frequency of the grid, the system will become bulky
and costly and also losses will increase. In the second case, while
constructing PV array we would get large DC voltage which results in hot-spots
during partial shading leads to safety problem and probability of leakage
current to flow between the system and ground. But in both the cases we must take
care of maximum power point tracking. 5

To
overcome these problems the perfect choice is to reduce the processing stages
betwixt the DC generation and the grid as shown in Figure1(b). Such compact
systems are in demand now days, which offers high reliability and performance
with reduced cost, weight and size. As number of processing stage are less, so
it is easy to integrate the module and the devices used in the power stages are
also minimized. In this research paper, a lone stage has been presented, which
can be used both with grid connected and standalone system. 6 

In 7 author has
presented a refined a topology which is single stage and also with boosting
capability. This topology is consists of dual boost converter which are
performing in a supportive way and are interdependent. Although the topology is
not best suitable for PV applications, then also the circuit is satisfactory
for lone stage topology for grid linked PV applications. In this all the
components are hard switched in one go, leading to huge frequency, making the
structure prone to EMI issues and form more switching losses.

In 8 author has
proposed the topology by taking the idea from the Zeta & Cook converter
configuration. This topology is an improved one as it has an advantage of using
lesser count of semiconductor devices also avoids the drawback of concurrent
huge frequency procedure of all the semiconductor devices. Switch S1 operates
at higher frequency during the negative half cycle, during the same period
switches S2 & S4 are kept open (ON). Power transferred during the positive
half cycle depends on the principle of buck n boost. At the same time, switch
S2 is operated during negative half cycle at a higher frequency, although
switches S1 & S3 are kept open (ON). Therefore Power transferred meanwhile
the negative half cycle follows the boost principle. Main advantage of the
presented topology is, of having low switching losses. During the two half
cycle converter leads to asymmetrical operation which is the major drawback of
the topology.

In 9 half bridge
buck-boost inverter topology had been proposed by the author. Asymmetrical
operation at the time of two half cycles is the drawback of the topology.
Switching losses and conduction losses are minimal adds an advantage to the
topology, since usage of two power devices is found during the half cycle.
Since switches are minimal in number which operates at higher frequency, so
they help in increment in the reliability and low EMI.

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