ABSTRACT:
This
paper proposes a new soft-stalling control strategy for grid-connected small
wind turbines operating in the high and very high wind speed conditions. The
proposed method is driven by the the rated current/torque limits of the electrical
machine and/or the power converter, instead of the rated power of the connected
load, which is the limiting factor in other methods. The developed strategy
additionally deals with the problem of system startup preventing the generator
from accelerating to an uncontrollable operating point under a high wind speed
situation. This is accomplished using only voltage and current sensors, not
being required direct measurements of the wind speed nor the generator speed.
The proposed method is applied to a small wind turbine system consisting of a
permanent magnet synchronous generator and a simple power converter topology.
Simulation and experimental results are included to demonstrate the performance
of the proposed method. The paper also shows the limitations of using the
stator back-emf to estimate the rotor speed in permanent magnet synchronous generators
connected to a rectifier, due to significant d-axis current at high load.
SOFTWARE: MATLAB/SIMULINK
Fig.
1. Schematic representation of the wind energy generation system: a) Wind
turbine, generator and power converter; b) Block diagram of the boost converter
control system; c) Block diagram of the H-bridge converter control system.
Fig.
2. Simulation result showing the behavior of the proposed method under
increasing wind conditions (10 m/s, 17 m/s from 10 s, and 33 m/s from 13s): a)
rectifier voltage command (v_ r ), rectifier voltage (vr) and minimum rectifier
voltage command (v_ r min); b) boost current (ib), filtered boost current (~i
b),
current limit (ilimit) and MPPT current target (imppt); c) turbine torque (Tt)
and generator torque (Tg); d) mechanical rotor speed (!rm).
Fig.
3. Simulation result showing the behavior of the proposed method under
decreasing wind conditions (30 m/s, 21 m/s from 4.5 s, and 8.5 m/s from 7s): a)
rectifier voltage command (v_ r ), rectifier voltage (vr) and minimum rectifier
voltage command (v_ r min); b) boost current (ib), filtered boost current (~I b),
current limit (ilimit) and MPPT current target (imppt); c) turbine torque (Tt)
and generator torque (Tg); d) mechanical rotor speed
(!rm).
Fig.
4. Experimental results showing the behavior of the propose method under
increasing wind conditions (10 m/s, 17 m/s from 10 s, and 33 m/s from 13 s): a)
rectifier voltage command (v_ r ), rectifier voltage (vr) and minimum rectifier
voltage command (vr min); b) boost current (ib), filtered boost current (~I b),
current limit (ilimit) and MPPT current target (imppt); c) mechanical rotor
speed (!rm).
Fig.
5. Experimental results showing the behavior of the propose method under
decreasing wind conditions (30 m/s, 21 m/s from 4.5 s, and 8.5 m/s from 9 s):
a) rectifier voltage command (v_ r ), rectifier voltage (vr) and minimum
rectifier voltage command (vr min); b) boost current (ib),filtered boost
current (~I b), current limit (ilimit) and MPPT current target (imppt); c)
mechanical rotor speed (!rm).
CONCLUSION:
The
operation of small wind turbines for domestic or small business use is driven
by two factors: cost and almost unsupervised operation. Specially important is
the turbine operation and protection under high wind speeds, where the turbine
torque can exceed the rated torque of the generator. This paper proposes a
soft-stall method to decrease the turbine torque if a high wind speed arises
and, as a unique feature, the method is able to early detect a high wind condition
at startup keeping the turbine/generator running at low rotor speed avoiding
successive start and stop cycles. The proposed method uses only voltage and
current sensors typically found in small turbines making it an affordable
solution. Both simulation and experimental results demonstrate the validity of
the proposed concepts. This paper also shows that commonly used machine and rectifier
models assuming unity power factor do not provide accurate estimations of the
generator speed in loaded conditions, even if the resistive and inductive
voltage drop are decoupled, due to the significant circulation of d-axis current
if a PMSG is used. This paper proposes using a pre-commissioned look-up table
whose inputs are both the rectifier output voltage and the boost current.
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