06-10-2012, 02:27 PM
Maximum Power Point Tracking using Perturb and Observe Algorithm For a PV System
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Abstract
It is important to track the maximum power
point of the Photovoltaic panel, as the electrical power
supplied by solar array depends on insolation, temperature
and load. Maximum Power Point Tracking (MPPT) is a
Power electronic system that operates the photovoltaic
modules to produce maximum power. MPPT varies the
electrical operating point of the modules and enables them
to deliver maximum available power. The issue of MPPT
has been addressed in different ways in the literature but,
especially for low-cost implementations, the perturb and
observe (P&O) maximum power point tracking algorithm
is the most commonly used method due to its ease of
implementation. A new modified perturb and observation
MPPT control algorithm is proposed to track the
maximum power point. Unlike the control of PV voltage or
current in normal P&O method the duty ratio of the dc to
dc converter is directly controlled in the proposed
algorithm. The proposed MPPT has the advantages of
simplicity and high convergence speed. The algorithm is
tested under various operating conditions. The obtained
results have proven that the MPP is tracked even under
sudden change of irradiation and loading level.
INTRODUCTION
Solar power is at the forefront of clean, renewable energy,
and it is gaining momentum due to advances in solar panel
manufacturing and efficiency as well as increasingly volatile
fuel costs. Photovoltaic (PV) solar cells are the most readily
available solar technology, and they operate best on bright
days with little or no obstruction to incident sunlight.
However, frequent overcast days and partial obstructions such
as tree limbs or buildings limit the reliability of solar power in
much of among the several renewable energy sources,
Photovoltaic arrays are used in many applications such as
water pumping, battery charging, street lighting and grid
connected PV systems.
From the IV characteristics of PV array (Fig 1.a) and
(Fig1.b), the effect of variation of illumination (G) is; both
Short circuit current (Isc) and Open circuit voltage (Voc) of
PV array increase with increasing the intensity of radiation.
Short-circuit current and open circuit voltage depends linearly
and logarithmically respectively. The effect of variation of
temperature (T) is; Increase in temperature causes some
increase in short circuit current. The reverse saturation current
increases rapidly with temperature. Due to this, the cell
voltage decreases by approximately 2.2mV per C° rise in its
operating temperature. Fill factor decreases slightly with
increase in temperature. Hence on line tracking of the
maximum power point of a PV array/module is necessary in
order to maximize the utilization of solar power. A variety of
maximum power point tracking (MPPT) methods is
developed. The methods vary in implementation complexity,
sensed parameters, required number of sensors, convergence
speed, and cost [1].
Perturb and Observe
In P&O method, the MPPT algorithm is based on the
calculation of the PV output power and the power change by
sampling both the PV current and voltage. The tracker
operates by periodically incrementing or decrementing the
solar array voltage. If a given perturbation leads to an increase
(decrease) in the output power of the PV, then the subsequent
perturbation is generated in the same (opposite) direction. So,
the duty cycle of the dc chopper is changed and the process is
repeated until the maximum power point has been reached.
Actually, the system oscillates about the MPP. Reducing the
Perturbation step size can minimize the oscillation. However,
small step size slows down the MPPT. To solve this problem,
a variable perturbation size that gets smaller towards the MPP.
However, the P&O method can fail under rapidly changing
atmospheric conditions. Several research activities have been
carried out to improve the traditional Hill-climbing and P&O
methods. Reference [4] proposes a three-point weight
comparison P&O method that compares the actual power point
to the two preceding points before a decision is made about
the perturbation sign.
PROPOSED MPPT METHOD
Among the different maximum power point
algorithms, perturb and observe algorithm is simple and also
gives good results. This algorithm is selected and certain
changes are made in the present work. This is known as new
modified perturb and observe algorithm.
The proposed algorithm tracks neither the Vmpp nor
the Impp. However, it tracks directly the maximum possible
power Pmax that can be extracted from the PV through the
duty ratio of boost converter. The flowchart of the proposed
MPPT method is shown in Fig.8.
The proposed modified Perturb and Observe MPPT
algorithm starts by setting the computed power to an initial
value (50% of the duty ratio). Actual PV voltage V(k) and
current I(k) are measured. Then the instantaneous value of PV
power P (k) is computed. The error between P (k) and P(k-1)
determine the sign of perturbation step size and duty ratio of
boost converter. The output of the MPPT controller is used to
drive the IGBT of the Boost Converter such that the P(k)
tracks maximum power.
CONCLUSION
The PV array output power delivered to a load can be maximized using MPPT control systems, which consist of a power conditioner to interface the PV output to the load, and a control unit, which drives the power conditioner such that it extracts maximum power from the PV system. A highly efficient MPPT tracking system is developed with a DC motor by extending it to traction applications.