ABSTRACT
Renewable Energy Resources have attracted worldwide attention due to soaring prices,
fast depletion of fossil fuels and climate change concerns. These are regarded as important
in enhancing energy security by reducing dependence on fossil fuels and decreasing Green
House Gas (GHG) emissions. In recent years, wide-ranging research has been carried out
to exploit the potential of renewable energy sources, particularly in wind, solar and tidal
energy. A wind power generation scheme out of all renewable energy sources is the most
competitive, environmentally clean and secures the system. During the last decades, the
concept of a variable-speed wind turbine (WT) has been receiving increasing attention due
to the fact that it is more controllable and efficient, and has good power quality.
The various generating systems considered are Wind Energy Conversion Systems
(WECSs). Different configurations of autonomous variable speed WECS, using Doubly
Fed Induction Generator (DFIG) and their control strategies are proposed to address the
issues of voltage and frequency control (VFC) with the change in wind speed. The general
outlining philosophy of the proposed systems is centered on stator windings in a DFIG are
directly connected to the grid while the rotor windings are connected to the grid via back
to back converters ac-dc-ac converter with a battery.
In this work, The complete standalone and grid-connected system has been incorporates
by the mathematical modeling of the wind speed model, the aerodynamic model of the
wind turbine, models of the DFIG, three-phase two-level PWM voltage source converter
models, transformer and the controller models are developed in MATLAB simulink
environment.
The proposed chopper-controlled external rotor resistance technique has been carried-out
on DFIG-based fixed speed wind turbine to regulate the voltage and frequency. But in this
technique the more energy is lost through external rotor resistance in the form of heat.
Therefore, voltage and frequency are not constantly maintained under the condition of
load changing conditions.
Hence in this condition another proposed technique has been implemented which is known
as Vector Control scheme by which the problem of voltage and frequency control has been
address. The vector control system is intended to demonstrate VFC output for DFIG-based
variable speed wind turbine on a stand-alone and grid-connected system. In the case of
standalone, a rotor-side and stator-side voltage source converter (VSCs) of DFIG provides
switching pulses to IGBTs through vector control scheme using unit vector generation.
With a controlled converter at rotor terminals, rotor currents are controlled through
amplitude, frequency and phase by applying suitable rotor voltages from the rotor side
converter. -oriented (SFO) frame of reference is used for decoupling the
active and reactive current control loops. The q-axis current loop controls the torque of the
machine by controlling the active power ile d-
machine by controlling . The controlled converter at stator
terminals maintained the DC link voltage by varying stator currents. In this work obtained
voltage and frequency is constant under changing of load and wind speed condition. If the
DFIG connected to grid, another problem concerning the stability analysis of low voltage
ride through capacity and voltage unbalance occurs due to variations in the load and wind
speed. To find out the solution to these problems, the feed forward vector control
technique can be used to re-establishing the voltage after the clearance of the short-circuit
fault for transient stability analysis.
Among all power quality (PQ) problems, voltage sag is the severe PQ deformation that
leads to a decrease in the efficiency of power system network and reduces the life span of
equipment connected. Recent research shows that the Unified Power Quality Conditioner
(UPQC) is an affordable custom power device used at the point of common coupling
(PCC) to protect the load from PQ distortions. A novel control strategy using Fuzzy Logic
Controller (FLC) with UPQC is proposed which eliminates the drawback of conventional
PI controller. It provides effective and efficient mitigation of both voltage sag and current
harmonics in grid connected wind power system. The topology of UPQC control is used to
enhance the power quality of grid connected DFIG system.
Consequently, all the developed simulation models of DFIG based wind turbines are
simulated at different operating conditions in MATLAB Simulink and the results are
presented in the thesis. The performance of DFIG with SWECS in MATLAB simulation
is also validated and executed in real time through the use of OPAL-RT real time
simulator. Therefore in this thesis the problem of voltage regulation and frequency for
power quality improvement in both the standalone and grid connected DFIG based WECS
have been addressed.
