ABSTRACT:
To
meet the augmented load power demand, the doubly-fed induction generator (DFIG)
based wind electrical power conversion system (WECS) is a better alternative.
Further, to enhance the power flow capability and raise security margin in the
power system, the STATCOM type FACTS devices can be adopted as an external
reactive power source. In this paper, a three-level STATCOM coordinates the
system with its dc terminal voltage is connected to the common back-to-back
converters. Hence, a lookup table-based control scheme in the outer control
loops is adopted in the Rotor Side Converter (RSC) and the grid side converter (GSC)
of DFIG to improve power flow transfer and better dynamic as well as transient
stability. Moreover, the DC capacitor bank of the STATCOM and DFIG converters
connected to a common dc point. The main objectives of the work are to improve
voltage mitigation, operation of DFIG during symmetrical and asymmetrical
faults, and limit surge currents. The DFIG parameters like winding currents, torque,
rotor speed are examined at 50%, 80% and 100% comparing with earlier works.
Further, we studied the DFIG system performance at 30%, 60%, and 80%
symmetrical voltage dip. Zero-voltage fault ride through is investigated with
proposed technique under symmetrical and asymmetrical LG fault for
super-synchronous (1.2 p.u.) speed and sub-synchronous (0.8 p.u.) rotor speed.
Finally, the DFIG system performance is studied with different phases to ground
faults with and without a three-level STATCOM.
KEYWORDS:
1. Doubly-fed
induction generator (DFIG)
2. Field
oriented control (FOC)
3. Common-capacitor
based STATCOM
4. Voltage
compensation
5. Balanced
and unbalanced faults
6. Zero-voltage
fault ride through
SOFTWARE: MATLAB/SIMULINK
CIRCUIT DIAGRAM:
Figure 1. Grid-Connected DFIG
With Three Levels Statcom Converter.
EXPECTED SIMULATION RESULTS:
Figure 2. DFIG Operation With 50%
Voltage Dip (I) Using Method In [27]. And (Ii) Using Proposed Method.
Figure 3. DFIG Operation With 50%
Voltage Dip (I) Using Method In [28] And (Ii) Using Proposed Method.
Figure 4. DFIG Operation With (I)
30% Dip, (Ii) 60% Dip And (Iii) 80% Dip In Grid Voltage.
Figure 5. Rotor, Stator Gsc And
Grid Terminal Current Waveforms With The Proposed Technique With Slg Fault.
CONCLUSION:
A generalized DFIG wind energy conversion system based
test-bed system connected to the grid is considered in the paper. The work
tested in the starting cases with two different research papers works with
proposed method under an 80% dip. Later, proposed methodology compared under
30%, 60%, and 80% dip, and the DFIG behavior is examined. Further, under three
different cases, LG, LLG and LLG faults without and STATCOM are compared to show
STATCOM controller's effectiveness. An improved field-oriented control scheme
for the DFIG with real and reactive power lookup-based control in the outer
control loops. It is observed that, there is a rapid development in the back
emf and decoupled current regime in the paper's inner control loops is
proposed. A three-level SATCOM is used in this paper, with the rectifier end
dc-link connected to the common capacitor between the DFIG back-to-back converters.
A better damping factor is observed for torque, powers, current, voltage, and
speed at 60%, 80%, and 100% dip with the proposed scheme.
The proposed method employs the adjustment of
external real and reactive powers using the optimal lookup table method as
shown in Table 3, rotor speed, and terminal voltage in the outer control loops
of both RSC and GSC. The inner control loop is fast-acting current control and
back emf- based voltage injection near the decoupling voltage loop. The
strategy works on decoupled real and reactive power flow controls in
synchronous rotating frames leads to individual power control. This technique
improves performance under normal conditions and during grid faults, with
better rotor voltage control, rotor speed, and damping. The post-fault behavior
of an overall system improved using the proposed technique.
Further improvement in the system behavior is
observed with the common- dc link STATCOM. The rectifier end dc link is
connected to the capacitor between the DFIG converters, which will reduce the
cost for capacitor and measurement sensors. This paper demonstrates the
DFIG-based WECS with better active and reactive power and EMT damping, surge
current reduction, speed control, and effective LVRT capability. There are
distortions in the rotor current waveform during the zero-voltage fault ride
during the fault and considerably more when the rotor speed is at super synchronous
speed. When the rotor speed is beyond the synchronous speed, the rotor current
is injected into the stator terminal from the rotor side windings with RSC
control scheme. Under this condition, the fault inrush current from the dc-link
capacitor will pass through this rotor terminal and reach the stator windings.
Under sub-synchronous speed, the rotor winding will receive the current for the
stator windings, so the fault effect is less influenced at lower speeds than
with higher speeds. The rotor voltage is maintained at both speeds during the
LG fault. However, waveform is less distorted with lower rotor speed.
The dc-link voltage distortions during the fault are
more with super- synchronous speed than sub-synchronous speed operation for
zero voltage ride through. The dc-link voltage is more stubborn and stable when
the rotor speed is lesser than the synchronous speed. The STATCOM current is observed
to be more in faulty phase than with other two healthy phases. The reason and
analysis are the same as that with the symmetrical fault study. The deviation
of the fault current at the STATCOM terminal is re-injected to the grid via the
closed path with the dc-link capacitor terminal. The post-fault performance is
superior with a serious 100% voltage dip case and also found better dynamic
response because of the RSC and GSC proposed technique and the STATCOM
controller. Further, an effective operation is experienced with a common link
dc capacitor STATCOM than with a conventional topology. Hence, simulated
results show better performance and profitable operation during and after the
faults than the earlier famous methods.
With the proposed method, rotor and stator current
during fault are maintained, not getting zero value and limiting surge currents
to a dangerous value. However, stator and rotor current is not supported to
their pre-fault value during the fault period. The torque reduction to a
smaller value observed increases the grid fault dip value, but there are no
surges and oscillations with the proposed method. The rotor speed is also maintained
almost constant even for significant voltage dip. As a result, the post fault
recovery in the DFIG is smooth and instantaneous, observed for winding
voltages, currents, EMT, active and reactive powers, dc-link capacitor voltage,
and rotor speed.
All the objectives specified in the Introduction
section are met 1) rotor and stator current surges are limited, current surges
ate within 1.5 times the operating value, mitigation in the rotor voltage
observed. Furthermore, the reactive power support by STATCOM, RSC and GSC
improved the DFIG WECS during and after the fault. Thereby 1) enhancement in
overall dynamic and transient stability is observed. 2) The rotor speed is almost
constant even for a significant grid voltage dip which is better than many
research papers. 3) The electromagnetic torque (EMT) and active and reactive
power flow oscillations are damped completely, and sustainable control observed
with the technique. 4) The proposed method is suitable for grid faults like
symmetrical, asymmetrical, and recurring faults. Better DFIG performance is
expected with LVRT capability for symmetrical and asymmetrical faults with
future research activities.
REFERENCES:
[1]
H. A. Mohammadpour, A. Ghaderi, H. Mohammadpour, and E. Santi, ``SSR damping in
wind farms using observed-state feedback control of DFIG converters,'' Electr.
Power Syst. Res., vol. 123, pp. 57_66, Jun. 2015.
[2]
F. Blaabjerg and K. Ma, ``Future on power electronics for wind turbine systems,''
IEEE J. Emerg. Sel. Topics Power Electron., vol. 1, no. 3, pp. 139_152,
Sep. 2013.
[3]
T. D. Vrionis, X. I. Koutiva, and N. A. Vovos, ``A genetic algorithm based low
voltage ride-through control strategy for grid connected doubly fed induction
wind generators,'' IEEE Trans. Power Syst., vol. 29, no. 3, pp.
1325_1334, May 2014.
[4]
A. M. Eltamaly and H. M. Farh, ``Maximum power extraction from wind energy
system based on fuzzy logic control,'' Electr. Power Syst. Res., vol.
97, pp. 144_150, Apr. 2013.
[5]
Y. Weng and Y. Hsu, ``Sliding mode regulator for maximum power tracking and
copper loss minimisation of a doubly fed induction generator,'' IET Renew.
Power Gener., vol. 9, no. 4, pp. 297_305, May 2015.
