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118 CHAPTER 4 Performance of MPPT Techniques of Photovoltaic Systems

1.3 CONSTANT-CURRENT MPPT
It is also observed that, the relation between the short-circuit current and the current
associated with the maximum power is approximately constant. So, it is possible to
use a constant-current MPPT algorithm that approximates the MPP current as a con-
stant ratio of the short-circuit current [10,11]. The short-circuit current is measured
and the MPP current is calculated, and the PV array output current is then adjusted
by the MPPT until the calculated MPP current is reached. This technique faces the
same problems of the constant-voltage technique. However, constant-voltage control
is normally favored because the measurement of the open-circuit voltage is much
simple than the short-circuit current. The momentary interruption in the constant
voltage or current can be avoided by using a pilot cell [12].

1.4 PERTURB AND OBSERVE TECHNIQUE
The Perturb-and-Observe (P&O) technique works by perturbing the PV system by
incrementing the array operating voltage and observing its impact on the array
output power. Because of constant step width, the system faces high oscillation espe-
cially under unstable environmental conditions. Some techniques used waiting time
to avoid high oscillation; however, it also makes the MPPT slower to respond to
weather changes. Also, this technique suffers from wrong operation, especially in
case of multiple local maxima when working in partial shading conditions. A lot
of modifications for this technique have been presented in literature [13e21].
P&O is the most frequently used technique to track the maximum power because
of its simple structure [22]. The flowchart of P&O is shown in Fig. 4.3 [23]. A com-
mon problem in this technique is that the PV module terminal voltage is perturbed
every MPPT cycle. Therefore, when the MPP is reached, the output power oscillates
around its MPP, resulting in power loss in the PV system. A modified P&O technique
has been introduced in [24] to remedy this problem by multiplying the change in the
duty ratio by dynamic constant depending on the previous change in the extracted
power. Another technique [25] used artificial neural network to predict this multi-
plying constant. These techniques increase the complexity of the system and may
cause more oscillations in stable weather conditions.

1.5 INCREMENTAL CONDUCTANCE TECHNIQUE
Among all the MPPT strategies, the incremental conductance (IncCond) technique
is widely used because of the high tracking accuracy at steady state and good adapt-
ability to the rapidly changing atmospheric conditions [26]. The IncCond method
[27e31] is based on comparing the instantaneous panel conductance with the incre-
mental panel conductance. The input impedance of the dcedc converter is matched
with optimum impedance of PV panel. This technique requires more sensing equip-
ment and sophisticated control system. The slope of the PVarray power curve is zero
at the MPP, positive for values of output power smaller than MPP, and negative for
values of the output power greater than MPP. The derivative of the PV module power