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28-02-2013, 09:44 AM
Post: #1
DIRECT TORQUE CONTROL FOR DOUBLY FED INDUCTION MACHINE-BASED WIND TURBINES
DIRECT TORQUE CONTROL FOR DOUBLY FED INDUCTION MACHINE-BASED WIND TURBINES UNDER VOLTAGES DIPS AND WITHOUT CROWBAR PROTECTION


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Abstract

This letter proposes a rotor flux amplitude reference generation strategy for doubly fed induction machine based wind
turbines. It is specially designed to address perturbations, such as voltage dips, keeping controlled the torque of the wind turbine,
and considerably reducing the stator and rotor over currents during faults. In addition, a direct torque control strategy that provides
fast dynamic response accompanies the overall control of the wind turbine. Despite the fact that the proposed control does not totally
eliminate the necessity of the typical crowbar protection for this kind of turbines, it eliminates the activation of this protection during
low voltage dips..

INTRODUCTION

This letter focuses the analysis on the control of
doubly fed induction machine (DFIM) based high
power wind turbines when they operate under presence
of voltage dips. Most of the wind turbine manufacturers
build this kind of wind turbines with back to back
converter sized to approximately 30% of the nominal
power [1]. This reduced converter design provokes that
when the machine is affected by voltages dips, it needs
a special crowbar protection [2] in order to avoid
damages in the wind turbine and meet the grid code
requirements.
The main objective of the control strategy proposed
in this letter is to eliminate the necessity of the crowbar
protection when a low-depth voltage dip occurs. Hence,
by using direct torque control (DTC), with a proper
rotor flux generation strategy, during the fault it will be
possible to maintain the machine connected to the grid,
generating power from the wind, reducing over
currents, and eliminating the torque oscillations that
normally produce such voltage dips. In Fig.1 the wind
turbine generation system together with the proposed
control block diagram is illustrated. The DFIM is
supplied by a back-to-back converter through the rotor,
while the stator is directly connected to the grid.

SIMULATION RESULTS

The simulated wind turbine is a 2 MW, 690
V, 1⁄3 and two pair of poles DFIM. The main
objective of this simulated validation is to show the
DFIM behavior when a low depth [in this case 30%, as
illustrated in Fig.2 (a)] symmetric voltage dip occurs
with and without the proposed flux reference generation
strategy and at nearly constant speed. The simulations
are performed in MATLAB/Simulink.
During the dip, it is desired to maintain the torque
controlled to the required value (20%), allowing to
eliminate mechanical stresses to the wind turbine. This
issue is achieved, as shown in Fig.2 (b) and (g), only if
the oscillatory rotor flux is generated. For this purpose,
the rotor flux is generated according to the block
diagram of Fig.3, generating an equivalent oscillation to
the stator flux amplitude [see Fig.2 (h)]. It must be
pointed out that DTC during faults is a well-suited
control strategy to reach quick flux control dynamics, as
well as to dominate the situation, eliminating torque
perturbations and avoiding mechanical stresses.
Consequently, the proposed control schema maintains
the stator and rotor currents under their safety limits,
avoiding high over currents, as shown in Fig.2(i) and (j)
, either in the voltage fall or rise. However, as predicted
in theory, it is hard to avoid deterioration of the quality
of these currents.

CONCULSION

Simulation results have shown that the proposed
control strategy mitigates the necessity of the crowbar
protection during low depth voltage dips. In fact, the dc
bus voltage available in the back-to-back converter,
determines the voltage dips depth that can be kept under
control.
For future work, it would be interesting to explore
the possibility to generate a modified reference of rotor
flux and torque, in order to be able to address deeper
voltage dips without crowbar protection.
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Marked Categories : project ideas for torque, direct torque control of induction machine, schema crowbar, direct torque control wind, project on direct torque control of doubly fed induction machines, direct torque control method for double fed induction motor used in wind turbine under voltage dips without crowbar protectio, direct torque control of doubly fed induction machine based wind turbines under voltage dips and without crowbar protection, direct torque control of doubly fed induction machine based wind turbine under voltage dips with out crowbar protection,

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