Edición No. 17, Issue II, Enero 2021
1. INTRODUCTION
Nowadays, wind energy has grown because of the
high demands of renewable energies worldwide. In this
context, energy efficiency must be improved in order to
obtain as much energy as possible from the source [1],
[2], [3]. For wind turbines, the transition from no load to
full load operation is crucial for the control of energy
production. Particularly, one of the characteristics that
allows more energy production is the control strategy
during partial load operation also known as MPPT.
The dynamics of the system is one of the most important
characteristics because the control strategy is designed by
using mechanical and electrical models. The mechanical
load must be modeled depending on the number of
masses, the gearbox ratio, and the friction losses along
the system. With regard to the electric characteristics, the
grid connection, the power electronic converter, and the
generator losses are included. Although there are
different types of electric generators, there are, in the
market, there two main popular wind turbines: The
Permanent Magnet Synchronous Generator (PMSG) and
the Doubly Fed Induction Generator (DFIG). PMSG,
which works through a wind turbine, is usually selected
for minimizing the cost of maintenance and
implementation whereas DFIGs are common for high
power applications [4], [5], [6]. The DFIG wind turbine
configuration has the advantage of permitting high
amounts of power extraction; thus, minimizing the cost
of power electronic devices. Further, the control strategy
can be widely adjusted since the slip angle is adaptable
in a typical range of +/-30% from the operational speed
around the synchronous speed [7]. The MPPT control is
usually designed by considering the wind speed, rotor
speed, output power, and pitch angle. One simple control
strategy consists of tracking the electromagnetic torque
vs Low Shaft Speed (LSS) curve; thereupon, allowing the
system to reach the MPPT using the natural dynamic
response. This method is also known as Indirect Speed
Control (ISC) [7], [8]. In this article, an ISC strategy for
a DFIG wind turbine in order to attain the MPPT is
presented. For this simulation, a 1,5MW wind turbine
using Matlab and FAST software has been selected [9],
[10].
In section two, the analysis and modeling of the
subsystems directly involved in the operation of the wind
turbine are detailed. Moreover, the parameters of
description that have been used for the ISC simulation are
explained. In section three, the control strategy and the
simulations based on FAST and Simulink are described.
Finally, the results are presented in section four.
2. WIND TURBINE MODEL
Basically, the wind turbine transforms the energy of
air mass movement into electrical energy. The
electromechanical model of the wind turbine depends on
the parameters related to the aerodynamics and the size
of the generator. In order to allow the maximum power
extraction, it is necessary to obtain the most accurate
mathematical model in order to implement an appropriate
MPPT control. The most relevant variables involved are
the nominal speed of the turbine, the gearbox ratio, the
electromagnetic torque of reference, the pitch angle, and
the TSR [11], [12], [13]. In Fig. 1, a description of the
basic elements of a wind turbine is detailed.
Figure 1: Basic elements of a wind turbine.
2.1 Aerodynamic Model
Aiming to produce the movement of the wind turbine
rotor, the air enters the swept rotor area which is a
circumference created by the rotation of the blades. The
rotor captures the energy of the air, causing a decrease of
pressure once the energy is transferred to the mechanism
of the wind turbine detailed in Fig. 2 [7].
Figure 2: Air flowing through the swept area
The aerodynamic model represents the energy
extraction of the rotor, and calculates the mechanical
torque as a function of the air flow in each propeller. The
wind speed can be considered as the average speed of the
incident wind in the area swept by the propellers with the
purpose of evaluating the average torque in the low speed
axis [5]. The mechanical power can be determined by the
expression in equation (1) [1], [3], [14].
(1)
However, the wind turbine can recover only part of
available power because of the Betz limit shown in (2).
Here is where the term Cp affects the wind turbine
performance due to mechanical and electrical losses.