International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
Incremental Conductance Based Maximum Power Point
Tracking for PV Multi-string Power Conditioning System
Lokin Joshi1
1
P.G Student, M.E 4
th
Semester, Sankalchand Patel College of Engineering, Visnagar.
Abstract- This paper deals with Multi-string Power
Conditioning System for PV application and Maximum Power
Point Tracking. This paper includes the results performed in
PSIM-9 software such as Performance comparison of
Incremental Conductance MPPT method with fixed step size
and variable step size under varying irradiation conditions.
Keywords—Multi-string Power Conditioning System (PCS),
Maximum Power Point Tracking (MPPT)
I. INTRODUCTION
AS PEOPLE are much concerned with the
environmental problems and the fossil fuel exhaustion
caused by the conventional power generation, Renewable
energy sources are becoming widely popular. The
photovoltaic panel is widely used in many grid connected
and stand-alone application. It has several advantages like
no noise, no installation area limitation, and no additional
maintenance cost but its installation cost is high and low
efficiency and also due to non-linear characteristic it
requires MPP Tracking. A Power Conditioning System is a
key component of the PV generation which cannot only
convert the PV panel power into AC power but also realize
the MPPT control to allow the system to yield maximum
power[1]. There are three kind of PCS systems as shown
in Fig. 1. based on the way the PV modules are connected:
the central PCS, the string PCS and the multi-string PCS.
A central PCS is connected to a dc bus cable where the
PV modules are connected both in series and in parallel to
achieve high-voltage and high-power capacity. However
the central PCS has several drawbacks such as necessity of
high dc voltage cables between the PV modules and the
PCs, low efficiency due to mutual influence of PV modules
and extra diodes in each string, severe partial shading
problems, and the mismatch problem of parallel connected
strings.
Fig. 1. Types of PCS. (a) Central PCS. (b) String PCS. (c) Multistring
PCS.
Series connected PV modules are called a string. A
string PCS uses a single string of modules to obtain high
voltage.
The string PCS can avoid most of the drawbacks of the
central PCS. However, the string PCS has a power limit
due to a limited number of series connections in order to
increase the nominal power.
A multi-string PCS consists of several dc-dc converters
and an inverter with a common dc link. The parallel
connected boost converter avoid high dc-bus voltage which
will eventually increase the size of capacitors and the
inverter cost. So, the Multistring PCS has the positive
aspects of the string PCS and the central PCS and
eliminates the drawbacks: no nominal power limitation, no
mismatch problem of parallel connected strings, no severe
partial shading problem, low specific cost of PV inverters,
etc. In Section II of this paper, an incremental Conductance
based MPPT control for the multi-string PCS is
explained[2][3][4].
645
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
PV cells have a single operating point where the values
of current and voltage result in a maximum power output
for the cell. Maximum power point tracker is basically an
electronic system that controls the duty circuit of the
converter to enable the photovoltaic module operate at
maximum operating power at all condition. The advantages
of MPPT regulators are greatest during cloudy or hazy days
or cold weather. There are different types of maximum
power point tracking methods developed over the years and
they are listed below as follows (1) Perturb and observe
method, (2)Incremental conductance method, (3) Artificial
neutral network method,
II. MULTISTRING CONVERTER
A. Circuit Configuration
The multi-string PV PCS is utilized as shown in Fig. 2.
This system is composed of two boost converters and a
single-phase full-bridge inverter. Two identical boost
converters are designed at 50% of the rated power. Each
boost converter performs its own MPPT control.
Fig. 2. Multistring PV single-phase PCS.
Fig. 3. Basic idea of IncCond method on a P-V curve of solar array.
(y axis div. 1 section=100 w)
The full-bridge inverter regulates the dc link voltage and
converts dc power into ac power. As the dc link voltage is
increased, the inverter increases the output power. On the
contrary, as the dc link voltage is decreased, the inverter
decreases the output power. The bulky 50 Hz isolation
transformer is excluded in the inverter. This transformer
less full-bridge inverter has the advantages of small size
and low cost.
Fig.4. Flow chart of INC method with fixed step size.
(4) Fuzzy logic method, (5) Peak power point method, (6)
Open circuit voltage method, and (7) Temperature method
etc.[5] The MPPT plays a very significant role because
without the MPPT the desire output electrical power will
not be achieve with changing weather conditions.
B. MPPT Systems
A maximum power point tracker is a high-efficiency
DC-DC converter, which functions as an optimal electrical
load for photovoltaic cell, most commonly used for a solar
panel or array and converts the power to a voltage or
current level which is more suitable to whatever load the
system is design to drive.
646
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
[ (
C. INC Method with fixed step size.
In incremental conductance method with fixed step size
the array terminal voltage is always adjusted according to
the MPP voltage it is based on the incremental and
instantaneous conductance of the PV module.
Fig. 3. shows that the slope of the P-V array power curve
is zero at The MPP, increasing on the left of the MPP and
decreasing on the Right hand side of the MPP. The basic
equations of this method are as follows.
At MPP
At Left of MPP
At Right of MPP
)]
(5)
At MPP,
(6)
The above equation could be written in terms of array
voltage V and array current I as,
(7)
The MPPT regulates the PWM control signal of the dc to
dc boost converter until the condition (dI/dV) + (I/V) = 0
is satisfied.
(1)
(2)
(3)
Fig.5. Variation of Normalized Power and dP/dV versus voltage
curve.
Where I and V are P-V array output current and voltage
respectively. The left hand side of equations represents
incremental conductance of P-V module and the right hand
side represents the instantaneous conductance. When the
ratio of change in output conductance is equal to the
negative output conductance, the solar array will operate at
the maximum power point. This method exploits the
assumption of the ratio of change in output conductance is
equal to the negative output Conductance Instantaneous
conductance[6].
(4)
Fig.6. Flow chart of INC method with variable step size.
D. INC Method with Variable step size.
The Incremental Conductance MPTT algorithm usually
uses a fixed iteration step size, which is determined by the
accuracy and tracking speed requirement. Thus, the
corresponding design should satisfactorily address the
tradeoff between the dynamics and steady state oscillations.
Applying the chain rule for the derivative of products
yields to,
647
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
To solve these problems, a modified INC MPPT with
variable step size is proposed in this paper. The step size is
automatically tuned according to the inherent PV array
characteristics. If the operating point is far from MPP, it
increases the step size which enables a fast tracking ability.
If the operating point is near to the MPP, the step size
becomes very small that the oscillation is well reduced
contributing to a higher efficiency and also it can
effectively improve the dynamic performance and steady
state performance simultaneously[7]. The PV output power
and slope of output power versus output voltage curves are
shown in figure. 5.
Equation for the step size is,
| |
If this cannot be satisfied, the variable step size INC
MPPT will be working with a fixed step size of the
previously set upper limiter ΔDmax. This equation
provides a simple guidance to determine the scaling factor
N of the variable step
size INC MPPT algorithm.With the satisfaction of this
equation, larger N exhibits a comparatively faster response
than a smaller N.
III. SIMULATION ON PV MULTISTRING PCS.
A simple MPPT PV system shown in Fig. 6 is simulated
in PSIM-9 to test the feasibility of the proposed method.
PV array specifications are shown in table I. A boost
converter and full bridge inverter is used as the power
interface between the PV array and the load to achieve
maximum power. the output voltage of the boost converter
can be expressed as,
(8)
(11)
TABLE I
PV Array Specification
No. of Cells(NS)
Short-Circuit Current(ISC)
Open-Circuit Voltage(VOC)
Maximum Power(PMAX)
4*48
8.32 A
117.6 V
500 W
Fig.7. PV Multistring Power Conditioning System with MPPT
control.
To ensure the convergence of the MPPT update rule, the
variable step rule must obey the following:
| |
(9)
Where, | |
operation of
obtained as,
(a) PV array Max Power and Output Power.
(y axis div. 1 section=100 w)
is the | | at fixed step size
.The scaling factor can therefore be
⁄| |
(10)
648
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
(b) Change in Duty cycle. (y axis div. 1 section=0.05 v)
(b) Change in Duty cycle. (y axis div. 1 section=0.05 v)
Fig.9. PV array Output Power and Change in duty cycle with variable
size INC MPPT. (a) PV array max power and output power. (b)
Change in Duty cycle. (N=0.1)
Fig.8. PV array Output Power and Change in duty cycle with fixed
size INC MPPT. (a) PV array max power and output power. (b)
Change in Duty cycle. (ΔD=0.05)
The duty cycle command is therefore updated every 0.05
ms. The output power performance of INC MPPT with
fixed step size 0.05 under irradiation step change
conditions are shown in Fig. 7. The irradiation was
suddenly changed from 1000 to 800 W/m2 at 0.35 s and
changed back to 1000 W/m2 at 0.71 s. For the comparative
purpose, the allowable maximum duty size ΔDmax is set as
0.05 for the proposed variable step size INC MPPT
method. The corresponding PV output power and step size
under N = 0.1 are shown in Figs. 8 respectively. The
tracking performance under both fixed and variable INC
MPPT methods are presented.
The PV array average output power with fixed step size
of 0.05 is 453.22 W. The proposed variable step size
method reduces the oscillations at stead state due to the
very small dP/dV and the PV array output power is 457.36
W. Moreover, the dynamic performance is obviously faster
than that of fixed step size of 0.05.
Where D is the Duty cycle. It can be seen that the input
dc voltage can be easily shifted to a high level. Boost
converter have higher efficiency than other topologies, and
also it can achieve maximum charging effect using
minimum number of PV modules. The very low number of
component is a security against failures and increases the
mean time before failure.
To compare the performance of the variable step size
INC MPPT method with the ordinary fixed step size INC
MPPT method, the simulation are configured under exactly
the same Condition to compare the performances. The PV
array in simulation is composed of four PV module, and the
sampling period used for MPPT algorithm is chosen as
0.05 ms.
IV. CONCLUSION
(a)
The photovoltaic Multistring power conditioning system
with MPPT control using fixed step size and variable step
size incremental conductance method have been analyzed
by simulation. The fixed step-size incremental conductance
algorithm can’t meet tracking speed and steady-state
accuracy of the PV system simultaneously, the value of
tracking step-size is determined by the system performance
requirements. In this paper, the variable step-size
incremental conductance algorithm has been presented,
which is able to automatically adjusted according to dP/dV
characteristics of PV system. If the working point is far
from MPP, step-size is increased to achieve fast dynamic
response.
PV array Max Power and Output Power.
(y axis div. 1 section=100 w)
649
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
[4]
Whereas the working point is near to the MPP, step-size
is small to reducing the oscillation and improving the
efficiency of the PV system.
[5]
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