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oscillation around MPP, resulting in a energy loss, and the tracking accuracy and
speed of response could not be taken into account [1]. To solve the above problems,
this paper proposes an improved algorithm.
3.1 The Basic Ideas of Improvement
Figure 1 shows that dP/dU curve has the following characteristics.
dP
>
0
 
left
 
of
 
MPPT
,
dU
dP
(2)
=
0
 
at
 
MPPT
,
dU
dP
<
0
 
right
 
of
 
MPPT
.
dU
Therefore, tracking algorithms could be divided into three sections. On the left of the
MPP short-circuit current method is used, around MPP variable step P&O method is
used, and on the right of the MPP open-circuit voltage method is used. The test steps
are followed as below:
(1) Detection of short-circuit current
Figure 2 shows the whole process of the search algorithm. It could be seen from the
figure dP/du decreases monotonically with the PV cells output voltage increasing.
The curve of photovoltaic cells is in the short circuit condition at A point, the battery
output current is zero, dP/du is equal to the battery's short circuit current. With the
increase of output voltage, dP/du value is slowly declined which could be seen that
the value is approximately equal to the short-circuit current in the AB Interval [6].
As the I mpp and I s have a approximate linear relationship, therefore, the output power
of PV under the method can not reach the MPP and only work around the MPP. By
this design, the algorithm could accelerate initial speed and rapidly approaching MPP.
(2) Variable step P&O method-- gradual approximation search
The tracking algorithm could be realized by variable step P&O method around MPP
which is carried out by the idea of gradual approximation step search [7].The
algorithm has a high precision and small fluctuations in a steady state. First, a larger
step is used to search in the region where the maximum power is and then go on to
search by narrowing it to half step, by this moment, searching area will be reduced to
the half and the precision will be doubled till to the MPP.
(3) Detection of open-circuit voltage
It could be seen from the figure.2, dP/di increases monotonically with the PV output
current increasing. The curve of PV is in a open state at D point , the battery output
current is zero, dP/di is equal to the absolute value of the battery's open circuit
voltage. With the battery output current increasing, dP/di value is slowly rises which
could be seen that the value is approximately equal to the open circuit voltage in the
DC Interval [8].
Open-circuit voltage method could make the right of MPP rapidly return to the
MPP. As it is an approximate point, which can only work around MPP.
Based on the above analysis, when the system is worked on the left of MPP (AB
section), short-circuit current method could be used to control, when worked around
MPP(BC section ), variable step P&O method could be used, searching the step by
 
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