Essay: MPPT CONTROLLER UNDER PARTIAL

ABSTRACT: Maximum Power Point
Tracking (MPPT) is the most important part
of an energy conversion system using
photovoltaic arrays. Maximum power point
tracking (MPPT) techniques are used in
photovoltaic (PV) systems to maximize the
PV array output power by tracking
continuously the maximum power point
(MPP) which depends on panel temperature
and on irradiance conditions. The power
voltage characteristic of PV arrays
operating under partial shading conditions
exhibits multiple local maximum power
points (LMPPs). In this paper, a review of
various characteristics curves of MPPT
controller under partial shading conditions
has been presented to analyze the
performance of MPPT controller under
such conditions.
Keywords: Maximum Power Point
Tracking (MPPT), Global Maximum Power
Point (GMPP), Local Maximum Power
Point (LMPP), Multiple Maxima, Partial
Shading, Photovoltaic (PV).
I. INTRODUCTION
A PHOTOVOLTAIC (PV) cell is an
electrical device that converts the energy of
light directly into electricity through PV
effect. PV cells have a complex relationship
between solar irradiation, temperature, and
total resistance, and exhibit a nonlinear
output efficiency characteristic known as
the P’V curve. Therefore, maximum power
point tracking (MPPT) techniques should be
developed in PV systems in order to
maximize the output power of PV systems.
Nowadays, there have been many MPPT
methods reported in the literature, such as
hill climbing, perturb and observe
incremental conductance (INC) and ripple
correction.
However, when there is multiple local
power maxima, from partially shading or
from installation on a curved surface,
conventional MPPT techniques do not
perform well. Multiple maxima may occur
due to bypass diodes, which are used to
avoid hot spots from forming when some
cells in a module or some modules in a
string receive less irradiance than others.
Without the remediation of power
electronics, the lost energy due to partial
shading can be significant. Thus, it is
imperative to utilize MPPT techniques that
reliably track the unique global power
maximum present in shaded arrays.
Some researchers have proposed global
maximum power point tracking (GMPPT)
algorithms to address the partial shading
condition. It is observed that the peaks
follow a specific trend in which the power at
a peak point continues to increase until it
reaches the GMPP, and afterward, it
continuously decreases. The proposed
algorithm incorporates an online current
measurement and periodic interruptions to
address certain challenges associated with
rapidly changing insolation and partial
shading. This method can be an effective
solution to mitigate the effect of partial
shading. The simulation results, however,
obtained by measuring environmental
parameters and the actual case will be
drastically different, because the actual
characteristic of the solar panels depends on
many factors (e.g., light intensity,
temperature,
Fig. 1 PV array under different partial
shading conditions.
ageing, dust, and partial shading). In
addition, the method increases the PV
system cost in practical commercial
applications.
II. PARTIAL SHADING
CONDITIONS
Fig. 1 shows a PV array which has
four PV modules connected in series under
uniform insolation conditions. Fig. 2(a)
illustrates typical I’V and P’V curves for
the PV array under a uniform solar
irradiance of 1000 W/m2 on all the PV
modules. The traditional MPPT algorithm
can reach this peak and continue oscillating
around the MPP. The P&O method, e.g.,
perturbs the solar array voltage in one
direction in each sampling period and tests
the power change afterward. It is assumed
that initially PV array is operating at point
A, as shown in Fig. 2(a).
An operating voltage of the PV array
is perturbed in a given direction (from A to
B), and an increase in output power is
observed (PB > PA). This means that point B
is closer to the MPP than point A, and the
operating voltage must be further perturbed
in the same direction (from B to C). On the
other hand, if the output power of the PV
array decreases (from D to E), the operating
point has moved away from the MPP, and
therefore, the direction of the operating
voltage perturbation must be reversed (from
D to C). Through constant perturbation,
eventually the operating voltage will reach
and continue oscillating around the MPP
level.
However, in some practical
conditions, the series strings of PV modules
are not under the same solar irradiance
condition. The partial shading condition is a
common situation due to the shadows of
buildings, trees, clouds, dirt, etc. Fig. 1
shows several different partial shading
situations. Under the partial shading
condition, if there is one module in a PV
string that is less illuminated, the shaded
module will dissipate some of the power
generated by the rest of the modules. It
means that the current available in a series
connected PV array is limited by the current
of the shaded module. This can be avoided
by using bypass diodes which can be placed
in parallel with the PV module.
The method of using bypass diodes
allows the array current to flow in the
correct direction even if one of the strings is
completely shadowed. Bypass diodes are
widely implemented in commercial solar
panels. Because of bypass diodes, multiple
maxima appear under the partial shading
condition. The P’V curve of PV array in
Fig. 1 possesses multiple maxima under the
partial shading condition, as shown in Fig. 2
(b). The unshaded modules in the sample
PV array are exposed to 1000 W/m2 of
solar insolation and the shaded module is
exposed to 400 W/m2 of solar insolation.
There are two observed peaks in the P’V
curve, because of the natural behavior of the
bypass diode and PV array connection
inside the module. Point A is the GMPP,
while point B the local maximum power
point (LMPP). When the area covered by
the shadow changes, the P’V curve and the
location of GMPP also changes, as shown in
Fig. 2(c) and (d). Under these conditions,
traditional algorithms can only track either
of the two MPPs, and cannot distinguish
between GMPP and LMPP.
Continuing with the P&O method as
an example, both points satisfy the
conditions to be the ‘MPP.’ If the operating
point obtained by the PV array algorithm is
LMPP, the output power is significantly
lower. Some researchers proposed a global
scan method to obtain the PV output curves.
Then a complex algorithm is required to
calculate the GMPP of the curves. This
method is able to obtain the GMPP, but it
cannot determine whether the PV cell is
operating under shading conditions, and
blindly and constantly scans for the MPP,
wasting the output energy. For these
reasons, a new improved MPPT method for
the PV system under the partial shading
condition is proposed in this paper.
Fig. 2 P’V and I’V characteristics curves
of a PV array under different partial
shading conditions
III. ANALYSIS OF
CHARACTERISTIC CURVES
UNDER PARTIAL SHADING
CONDITIONS
In order to avoid blind global scan,
methods to determine the presence of partial
shading are essential. It is noted that when a
series of PV array is under the identical
solar irradiance condition [Fig. 1], every PV
model works as a source, and all modules
are identical in their voltage, current, and
output power at any time. But this state
changes when there is shadow. Fig. 1 is an
example in the following analysis. The
models in the series array are exposed to
two different solar irradiances, and the solar
irradiation levels are 1000 and 400 W/m2,
respectively. The voltages of the modules
that are exposed to different irradiation
levels are completely different.
The two peaks on the P’V curve are
divided into two separate parts, as shown in
Fig. 2(c). Part A is the curve containing the
left peak (curved A’C), and part B is the
curve containing the right peak (curve C’B’
E). In part A, the current of the PV array IPV
is greater than the maximum current that the
PV module can
Fig. 3 Every module output voltage with
array output power.
(a) Unshaded module. (b) Shaded
module.
produce under the shade (M3 and M4);
therefore, the current will flow through the
bypass diode of each module. At this stage,
only PV M1 and M2 are supplying power,
and PV M3 and M4 have been bypassed by
the diodes. The characteristic curves of the
PV module voltage with output power are
shown in Fig. 3(a) and (b). The voltages of
PV M3 and M4 are approximately negative
0.7V (the diode’s forward voltage drop) in
part A, as shown in Fig. 3(b).
Therefore, the module voltages
being equal to the negative of the diode’s
forward voltage can be used as one effective
way to estimate partial shading condition. In
part B, all PV modules are supplying power,
but the unshaded and shaded modules are in
different working conditions. Because the
PV modules receive different amounts of
solar radiation, the voltages of the PV
modules are different. In part B (curve C’
B’E), the voltage of the unshaded modules
is greater than that of the shaded modules,
as shown in Fig. 4. It is evident that this is
another indicator to efficiently identify
partial shading. Following the above
analysis, some of the observations are listed
as follows.
1) I’V curves under partial shading
conditions have multiple steps, while the
P-V curves are characterized by multiple
peaks.
2) The number of peaks is equal to the
number of different insolation levels
irradiated on the PV array, and any peak
point may be the GMPP.
Fig 4 Array output power with unshaded
module output voltage and shaded
module output voltage.
3) The voltages of PV modules that receive
different solar radiations are different.
4) The voltage of the PV module that is
bypassed by a diode is equal to the negative
of the diode’s forward voltage drop.
CONCLUSION
In this paper, a review of concepts &
developments in the field of MPPT has been
shown. Also various partial shading
conditions have been briefly reviewed. The
comparison between this various conditions
of partial shading has been summarized with
the help of various characteristic curves.
Finally it is concluded that conventional
MPPT techniques have disadvantages like
energy loss, not able to determine partial
shading conditions, etc. Majority of these
problems can be eliminated by improved
MPPT controller method. Therefore
application of Improved MPPT controller
method now a day’s not limited up to
generation level but research work
suggested that it is having ability to replace
the conventional MPPT methods too in near
future.
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