26-02-2013, 11:29 AM
PHOTOVOLTAIC-GRID INTEGRATED SYSTEM
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ABSTRACT
This paper proposed solution for directly energizing
of ac load throughout Photovoltaic Solar Array during
the daytime by applying so called variable voltage
tracking system (VVT). The main function of VVT is
to maintain the average output chopped voltage at fixed
value irrespective of solar radiation rate, in turn the
chopped voltage is converted into ac voltage suitable
for grid-connected loads. This solution is realized by
integrating both complementary buck-boost chopper
and dc to ac converter. The ac-grid contributes to the
load in two cases, first when there is a power shortage
during the daytime due to weak irradiation rates, and
second during the night time. The power estimator unit
is used to determine the grid contribution intervals.
This solution excludes the use of battery bank
which is the main obstacle in massive use of solar
energy due to their weight, short life time, maintenance
and cost. Matlab/Simulink is used to simulate the
proposed model, where the obtained simulation results
confirm and justify the proposed approach for further
study and looking for optimized solutions for cost
reduction and energy savings.
INTRODUCTION
Photovoltaic energy resources presents alternative
and friendly to the environment sources. It presents
unique solution for providing remote area with clean
and sustainable energy during the daytime in heating,
lighting, refrigeration and water pumps systems [1-3]
without the need of battery system, while during the
night time the accumulated energy can be fully or
partially used to cover the energy domain.
The output circuit connected to the photovoltaic
system is usually dc-dc converters mainly boost
choppers in order to boost the voltage to the
predetermined levels.
The DC/DC converters are widely used in regulated
switch mode power supplies, where the input voltage to
these converters varies in wide range especially in the
case of photovoltaic (PV) supply source due to
unpredictable and sudden change in the solar
irradiation level as well as the cell operating
temperature.
Double-chopper PV Array
Regulating the output chopped voltage according to
reference or grid voltage can be realized by modifying
the conventional boost chopper into double chopper
circuit with buck converter called "Complementary
buck-boost converter" as shown in Figure 4. Power
switched Q1 and Q2 operates in complementary mode
boosting up the input PV voltage, while Q3 regulates
this output voltage toward increase or decrease
according to Vref.
PV-GRID INTEGRATED SYSTEM
According to Figure 1, the generated PV voltage is
adjusted by complementary buck-boost converter and
being applied to the load via grid selector. The power–
status estimator generates switching pulses required to
operate the grid selector. The ac-grid contribution can
be described into two approaches:
• Fully inverted circuit;
• Partially inverted circuit.
In case of fully inverted circuit, the ac-grid voltage
is converted into dc throughout grid-adapting module,
and then added to the output chopped dc voltage as
shown in Figure 7.
In partially inverted circuit, the PV voltage is
converted into ac voltage, while the ac-grid voltage is
directly connected to the load after being synchronized.
In present paper first approach will be described
hereinafter.
Grid-drop compensation
Grid-drop compensation module is required to
calculate the voltage drop of the grid circuit with
purpose generates accurat reference voltage according
to eq.(8), and generates appropriate switching PWM
pulses that drives buck chopper Q3. The simulink circuit
for this module is illustrated in Figure 10.
CONCLUSION
In this work a simulation study for PV-Grid
integrated model has been conducted, where the
following conclusions can be drawn:
- The proposed PV model consists of variable tracking
module and voltage drop compensating module that
can be used for either dc or ac loads with precise
voltage tracking procedure. The added power-status
estimator modules create new aspect to this model,
where the power shortages can be measured and
delivered from alternative sources or main ac-grid.
- The proposed model can be used for simulating
photovoltaic system individually or combined with
battery charging unit. During the daytime there is no
need of battery unit, resulting in efficiency
enhancement, reliability of the system and long life
time. Meanwhile, during the night time the load is
directly energized from the grid, which in turn
enhances the system reliability and reduces the total
cost.