08-10-2012, 11:51 AM
DIRECT TORQUE CONTROL OF DOUBLY FED INDUCTION GENERATOR BASED WIND TURBINE UNDER VOLTAGE DIPS
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ABSTRACT
This paper focuses the analysis on the control of doubly fed induction generator (DFIG) based high-power wind
turbines when they operate under presence of voltage dips. The main objective of the control strategy proposed
for doubly fed induction generator based wind turbines is to eliminate the necessity of the crowbar protection
when low-depth voltage dips occurs. A direct torque control fed induction generator control is divided into two
different control blocks. The first block that controls the machine’s torque and the rotor flux amplitude with
high dynamic capacity by Direct Torque Control. The second block that generates the rotor flux amplitude
reference in order to handle with the voltage dips. A direct torque control strategy that provides fast dynamic
response accompanies the overall control of the wind turbine. The proposed control does not totally eliminate
the necessity of the typical crowbar protection for turbines it eliminates the activation of this protection during
low depth voltage dips. Due to voltage dip in the wind turbine causes three main problems they are control
difficulties, disturbance in the stator flux, increase of voltage and currents in the rotor of the machine. The DC
bus voltage available in the back-to-back converter determines the voltage dips depth that can be kept under
control. The modeling of the complete system is done in MATLAB-SIMULINK. Simulation results show the
proposed control strategy that mitigates the necessity of the crowbar protection during low depth voltage dips.
INTRODUCTION
With exhausting of traditional energy resources and increasing concern of environment, renewable
and clean energy is attracting more attention all over the world to overcome the increasing power
demand. Out of all the renewable energy sources, wind energy and solar energy are reliable energy
sources. Now a day, wind power is gaining a lot of importance because it is cost- effective,
environmentally clean and safe renewable power source compared to fossil fuel and nuclear power
generation. Asynchronous generators are more commonly used in systems upto 2MW, beyond which
direct-driven permanent magnet synchronous machines are A Wind Energy Conversion System
(WECS) can vary in size from a few hundred kilowatts to several megawatts. The size of the WECS
mostly determines the choice of the preferred. A grid connected WECS should generate power at
constant electrical frequency which is determined by the grid. Generally Squirrel cage rotor induction
generators are used in medium power level grid-connected systems. The induction generator runs at
near synchronous speed and draws the magnetizing current from the mains when it is connected to the
constant frequency network, which results in Constant Speed Constant Frequency (CSCF) operation
of generator. However the power capture due to fluctuating wind speed can be substantially improved
if there is flexibility in varying the shaft speed.
MATHEMATICAL MODELING OF DFIG
The wind generation system studied in this paper consists of two components: the Doubly-Fed
Induction Generator (DFIG) and the variable speed wind turbine. A detailed description of these two
components is given below. The DFIG may be regarded as a slip-ring induction machine, whose
stator winding is directly connected to the grid, and whose rotor winding is connected to the grid
through a bidirectional frequency converter using back-to-back PWM voltage-source converters [9].
The electrical part of the DFIG is represented by a fourth-order state space model, which is
constructed using the synchronously rotating reference frame (dq-frame), where the d-axis is oriented
along the stator-flux vector position. The relation between the three phase quantities and the dq
components is defined by Park’s transformation.
DIRECT TORQUE CONTROL
The Direct Torque Control (DTC) method is basically a performance enhanced scalar control method.
The main features of DTC are direct control of flux and torque by the selection of optimum inverter
switching vector, indirect control of stator current and voltages, approximately sinusoidal stator flux
and stator currents and high dynamic performance even at standstill. The advantages of DTC are
minimal torque response time, absence of coordinate transformations which are required in most of
vector controlled drive implementation and absence of separate voltage modulation block which is
required in vector controlled drives. The disadvantages of DTC are inherent torque and stator flux
ripple and requirement for flux and torque estimators implying the consequent parameters
identification.
The complete block diagram of DTC is shown in Figure 2. There are two hysteresis control loops, one
for the control of torque and the other for the control of flux. The flux controller controls the machine
operating flux to maintain the magnitude of the operating flux at the rated value till the rated speed
and at a value decided by the field weakening block for speeds above the rated speeds. Torque control
loop maintains the torque value to the torque demand.
CONCLUSION
The direct torque control of doubly fed induction machine is used to generate the required rotor pulses
using the rotor flux reference generation strategy. The proposed control strategy is used during the
low depth voltage dips for higher voltage dips it is necessary to use crowbar protection. The DC bus
voltage available in the back-to-back converter determines the voltage dips depth that can be kept
under control.
The direct torque control combines the benefits of vector control and direct self-control into a sensorless
variable-frequency drive that does not require a PWM modulator. In steady state there is a ripple
in the torque. This ripple depends on the switching frequency of the inverter which is determined by
the torque and flux band. At the time of starting DTC draws high current. The switching frequency of
the inverter varies over a wide range because of using hysteresis controllers. The magnitude of the
stator flux can be maintained constant and several bright spots show the points where stator flux halts.
In the transient state, the highest torque response can be obtained by selecting the fastest accelerating
voltage vector to produce the maximum slip frequency. In steady state, by selecting the acceleration
vector and the zero voltage vectors alternatively, the torque can be maintained constant. Since the flux
ripples are relatively small and minor loops are not observed in the locus, harmonic losses and
acoustic noise of the machine may be effectively decreased. The transient response of the drive is fast
with independent control of flux and the torque.