An Improved MPPT Method with Variable Step Size for PV System
Wen Sun, Lixin Zou, Yaxiang Sun, Yishen Xu
Department of Physics Science and Technology
Soochow University
Suzhou, China
jysunwen123@163.com, zoulx@suda.edu.cn, sudasyx@163.com, xys2001@suda.edu.cn
Abstract—The output power of photovoltaic (PV) power
generation system is the nonlinear function of insulation,
temperature and load, and thus Maximum Power Point
Tracking (MPPT) techniques are widely applied in PV systems
to make PV array generate peak power. Among all the MPPT
strategists, the Perturbation and Observation (P&O) method is
widely used, due to its ease of implementation. On the other
hand, its main drawbacks are the waste of energy in steady
state conditions. This paper proposes a modified variable step
size P&O MPPT algorithm, the step size is automatically tuned
according to the operating point. Simulation results show the
proposed method can effectively improve not only the steady
state performance but also the dynamic response of the PV
system.
Keywords- photovoltaic; MPPT; variable perturb step
I. INTRODUCTION
As a kind of green energy with no pollution, no noise,
inexhaustibility, solar energy gets more and more people’s
attention. There are many different applications for solar
power systems, but also many limitations to these
applications. The cost-benefit is too low for solar power
systems to be widely used for powering homes, businesses,
or even individual products. The output power of PV power
generation system is nonlinear, and it is always changing
with insulation, temperature and load. Therefore, maximum
power point tracking is necessary for PV system to increase
the output power. Fig. 1 shows the schematic Block diagram
of the MPPT (Maximum Power Point Tracking) system [3].
The solar panel will feed the DC-DC converter directly
which stores the electrical energy temporarily in an inductor
and then charges the battery. The battery then feeds the load
during sunlight as well as nighttime. The DC-DC converter
is to be operated by a MPPT converter unit. The converter
unit will be based upon a microprocessor that monitors the
voltage and current levels coming from the PV array and
controls the DC-DC converter.
In recent years, a large number of MPPT methods have
been proposed and implemented. Among the popular
tracking methods are: the Perturbation and Observation
(P&O) or hill climbing, incremental conductance, fractional
open-circuit current and so on. Although so many methods,
no one is perfect. The conductivity increment method is ideal
theoretically, but it is constraint by the measuring equipment.
Fig.1 Block diagram of the MPPT system.
Perturbation and Observation (P&O) method, which
implemented easily, is widely used to move the PV module
to the maximum power point among these methods [7].
However, for this method, a tradeoff must be made in
choosing the increment value by which the controlled
parameter such as duty cycle or reference voltage is adjusted;
large values increase the looses in steady state due to large
perturbations around MPP, while small value reduce the
dynamic behavior in situations involving quickly changing
irradiation, conditions or load. This paper presents a
modified P&O algorithm that is designed to solve the
problem. Varying the step size value reduces the oscillations
around the MPP and leads to a faster responses to reach it.
When a step change in the solar irradiance occurs, the step
size is automatically tuned according to the operating point.
If the operating point is far from the MPP, it increases the
step size which enables a fast tracking ability.
II. S
OLAR CELLS CHARACTERISTICS AND ITS MODULE
A. Solar Cells Characteristics
Solar panel is composed of a number of solar cells, and
each solar cell is a semiconductor of P-N section which
converts solar energy directly into electrical energy, so it can
be assumed that solar panel generates an independent current
source its own by the light irradiation to supply load. The
output characteristic of solar cells is nonlinear and it is
subject to light intensity, ambient temperature, load and other
factors. A typical solar panel output characteristic curve is
shown in Fig. 2. It presents that each curve only has one
MPP. Additionally, as sunlight increase, the maximum
output point voltage increase.
PV
array
DC-DC
converter
Load
MPPT converter
Unit
Battery
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978-1-61284-836-5/11/$26.00 ©2011 IEEE