06-11-2012, 04:37 PM
Simulation and Analysis of Existing MPPT Control Methods in a PV Generation System
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Abstract
Maximum Power Point Tracking (MPPPT) techniques are employed in PV systems to make
full utilization of PV array output power which depends on solar irradiation and temperature. Recently,
many MPPT algorithms and control schemes of PV system have been proposed, but different control
methods and different PID controller parameters lead to different output efficiency and dynamic response.
Therefore, in this paper, a PSCAD/EMTDC user defined module of the solar array is established to easily
simulate its inherent characteristics with the basic specification data. Moreover, we also compare the
advantages and weakness of the existing MPPT control methods by simulations to give a reference to the
Grid-Connected Photovoltaic System (GCPS) designers
Introduction
As the conventional energy sources are rapidly depleted,
the importance of solar Photovoltaic (PV) energy has been
emerging as an alternative energy resource. Since it is clean,
pollution-free, and inexhaustible, researches on the PV
power generation system have received much attention,
particularly, on many terrestrial applications. [1]. Maximum
Power Point Tracking (MPPT) is one of the most important
and well-known problem for all PV systems. An efficient
MPPT algorithm is very important to increase the efficiency
of PV system.
Recently, many MPPT algorithms and control schemes of
PV generation system have been proposed [2-8]. As for the
non-Artificial Intelligence method of the MPPT algorithms,
Constant Voltage Tracking (CVT) method, Fraction Open-
Circuit Voltage (OCV) method, Hill climbing and Perturb
and Observe (P&O) method, Incremental Conductance
(INC-CON) method, Variable Step Size (VSS) method
based on INC-CON method, and hybrid method which
combined above algorithms are introduced.
P&O Method
The Perturb and Observe (P&O) method is one of the
most commonly used methods in practice. The P&O
algorithms operate by periodically perturbing, i.e.
incrementing or decrementing, the array terminal voltage
and comparing the PV output power with that of the
previous perturbation cycle. If the PV array operating
voltage changes and power increases, the control system
moves the PV array operating point in that direction.
Otherwise the operating point is moved in the opposite
direction.
The logic of this algorithm and the flowchart are
explained in Fig. 2. The operating voltage of the PV system
is perturbed by a small increment of V, and this resulting
change in P. If P is positive, the perturbation of the
operating voltage needs to be in the same direction of the
increment. On the contrary, if P is negative, the obtained
system operating point moves away from the MPPT and the
operating voltage needs to move in the opposite direction of
the increment.
Variable Step Size (VSS) Method
All the above conventional tracking methods use fixed,
small iteration steps, which is determined by the accuracy
and tracking speed requirements. If the step size is
increased to speed up the tracking, the accuracy of tracking
suffers and vice versa. To overcome the above limitation, a
variable step size method has been proposed in [11]. It
offers the fast response time and good steady-state
performance.
In this method, the large step is adopted to move the PV
system to the MPP quickly when the irradiation changes
abruptly. The small one is adopted to decrease the power
oscillation when the PV module works around the MPP.
Conclusion
In this paper, an electro-magnetic transient model of two
stage grid connected PV systems was presented by using
the UDM models of the PV array and the MPPT controller
using PSCAD. Various MPPT techniques have been
reviewed and simulated with different temperatures and
irradiation environments. From the simulation results, the
validity of the developed modeling has been confirmed.
Moreover, the advantages and weaknesses of the existing
MPPT control methods were also analyzed by the
simulation results.
The summary of MPPT techniques can be beneficial in
understanding the existing MPPT control algorithms for
engineers of PV industry. The discussion and performance
summary of different MPPT methods would serve as a
useful guide in choosing the right MPPT method for
specific PV systems in the practical applications.