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Overview of PV Maximum Power Point Tracking Techniques 97

variability of solar irradiation and ambient temperature (see Figure 5.3), the position of the MPP
will be changed accordingly. Thus, the application of an MPPT control algorithm is required, which
is capable of guaranteeing fast convergence to the continuously moving MPP of the PV source in
order to optimize the energy production of the PV system. The operating principles of alternative
techniques, which belong to this class of MPPT methods (see Figure 5.4), along with a comparison
of their operational characteristics, are presented next.

5.3.1 Constant-Voltage and Constant-Current MPPT

The constant-voltage (also referred to as fractional open-circuit voltage) MPPT technique is
based on the assumption that the ratio of the MPP voltage to the open-circuit voltage of a PV
module remains relatively constant at 70%–85% [22, 23]. Thus, by periodically disconnecting
the power converter (see Figure 5.1) from the PV array, the output current of the PV array is set
to zero and the resulting open-circuit voltage is measured. In the constant-current (or fractional
short-circuit current) MPPT method, a similar approach is adopted [24]. In this case, the MPPT
process is based on the assumption that the MPP power is proportional to the short-circuit current,
which is measured by periodically setting the PV module/array under short-circuit conditions,
through a power switch. In both the constant-voltage and constant-current MPPT methods, the
corresponding MPP voltage is calculated by the control unit according to the measurements of
the open-circuit voltage and short-circuit current, respectively, and then the power converter is
regulated to operate at that point.
The constant-voltage and constant-current MPPT methods require only one sensor for their
implementation (i.e., a voltage and current sensor, respectively), but the periodic interrup-
tion of the PV source operation for measuring the open-circuit voltage/short-circuit current
results in power loss. In both of these methods, the accuracy of tracking the MPP is affected
by the accuracy of knowing the value of the proportionality factors between the open-circuit
voltage and short-circuit current, respectively, with the corresponding values at the MPP for the
specific PV module/array used in each installation, as well as their variations with temperature
and aging.

5.3.2 Perturbation and Observation MPPT
The perturbation and observation (P&O) MPPT method is based on the property that the derivative
of the power–voltage characteristic of the PV module/array is positive on the left side and negative
on the right side (see Figure 5.2a), while at the MPP, it holds that

∂P pv (5.3)
= 0
∂V pv

where P pv and V pv are the output power and voltage, respectively, of the PV module/array.
During the execution of the P&O MPPT process, the output voltage and current of the PV module/
array are periodically sampled at consecutive sampling steps in order to calculate the corresponding
output power and the power derivative with voltage. The MPPT process is performed by adjusting
, based on the sign
the reference signal of the power converter PWM controller (see Figure 5.1), V ref
of ∂P pv , according to the following equation:
∂V pv
V ref () = V ref ( k − ) +⋅sign   ∂ P pv k ()  (5.4)
α
k
1
 ∂ V pv 