Abstract- This paper describes a Wind turbine coupled doubly-fed initiation generator ( DFIG ) for modern air current energy transition strategy. The restriction of dc-link electromotive force i¬‚uctuation for a consecutive convertor is analysed. An improved control scheme with the instantaneous rotor power feedback is proposed to restrict the i¬‚uctuation scope of the dc-link electromotive force. A decoupled P-Q accountant to modulate the DC nexus electromotive force is implemented utilizing MATLAB/SIMULINK, and the consequence of fluctuations in grid side electromotive force on the convertor public presentation is examined.
Index Terms- Doubly fed initiation generator ( DFIG ) , dc-link electromotive force, d-q vector control, instantaneous power feedback, grid side accountant.
With turning concerns over environmental pollution and energy deficit, great attempts have been taken around the universe to implement renewable energy based plans. Particularly with air current energy, that has been undergoing a important development with bettering techniques, cut downing costs and low environmental impacts. The demands of power controllability during the normal operation to let some grade of control over the existent and reactive power production have increased the range for the development of a new coevals called air current energy transition system ( WECS ) .
A doubly-fed initiation generator ( DFIG ) is an adjustable-speed initiation machine widely employed in modern air current power industry [ 1, 2 ] . Wind turbine makers are progressively traveling to variable velocity constructs because of the undermentioned grounds. 1 ) Variable velocity air current turbines offer a higher energy output in comparing to constant velocity turbines. 2 ) The decrease of mechanical tonss and simpler pitch control can be achieved by variable velocity operation. 3 ) Variable velocity air current turbines offer extended controllability of both active and reactive power. 4 ) Variable velocity air current turbines show less fluctuation in end product power [ 3, 4 ] . However, the operation and public presentation of a DFIG depends non merely on the initiation machine but besides on the two dorsum to endorse convertors every bit good as how they are controlled, necessitating incorporating all the constituents together for an probe. The most preferable agencies of transition of AC-DC-AC is done with a DC-link. However, the operation and public presentation of a DFIG depends non merely on the initiation machine but besides on the two dorsum to endorse convertors every bit good as how they are controlled, necessitating incorporating all the constituents together for an probe. The most preferable agencies of transition of AC-DC-AC is done with a DC-link. The presence of the DC nexus decouples the rectifier and the inverter, and therefore each can be operated independently utilizing standard PWM techniques. These systems besides provide regenerative capacity which allows extra energy from the load side to be fed into the grid which makes it utile in applications like a Doubly Fed Induction Generator [ 5, 6 ] where bidirectional power flow is required as shown in Fig. 1. With the assistance of the frequence convertors, a four-quadrant operation is being demonstrated [ 7 ] . The grid-side convertor controls the dc-link electromotive force and the reactive power absorbed from the grid by the convertor. The general control technique for the grid-side convertor control, which is widely used in wind power industry, is a decoupled d-q control attack that uses the direct axis current constituent for existent power control and quadrature axis current constituent for reactive power control [ 8, 9 ] .
Fig. 1 Schematic of air current turbine coupled DFIG
A doubly-fed initiation generator is a criterion, lesion rotor initiation machine with its stator twists straight connected to the grid and its rotor twists connected to the grid through an AC/DC/AC frequence convertor. In modern DFIG designs, the frequence convertor is built by two self-commutated PWM convertors, a rotor-side convertor and a grid-side convertor, with an intermediate DC electromotive force nexus. By commanding the convertors on both sides, the DFIG features can be adjusted so as to accomplish upper limit of effectual power transition or capturing capableness for a air current turbine and to command its power coevals with less fluctuation.
Many different d-q control algorithms have been proposed and used for commanding the DFIG rotor- and grid-side convertors for certain dynamic and transeunt public presentation accomplishments of DFIGs. Most of them are based on a existent and reactive power control construct, a popular DFIG convertor control mechanism used in modern air current turbines. This control constellation is normally divided into rotor- and grid-side convertor controls. The rotor-side convertor controls the active and reactive power of the DFIG independently, and the grid-side convertor is controlled in such a manner as to keep the DC-link capacitance electromotive force in a set value and to keep the convertor operation with a coveted power factor [ 10 ] .
The rotor side accountant, dwelling of a reactive power accountant and an active power accountant, is normally a two-stage accountant as shown in Fig. 2. It operates in either stator-flux or stator-voltage oriented mention frame and therefore the q-axis current constituent represents active power and the d-axis constituent represents reactive power. The two accountants in the rotor-side accountant determine inverter d- and q-voltages by comparing the d- and q-current set points to the existent d- and q- currents of the initiation machine.
Fig. 2 Schematic of Rotor side convertor
The grid-side convertor regulates DC-voltage and reactive power. It is besides a two-stage accountant operating in a grid AC electromotive force mention frame as shown by Fig. 3. Traditionally, the d-axis current constituent is used for active power control, and the q-axis current constituent is used for reactive power control. The two feed-forward waies in the grid-side accountant determine convertor d- and q-voltages by comparing the d- and q-current set points to the existent d- and q-currents supplied to the grid.
Fig. 3 Schematic of Grid side convertor
The conventional control mechanism for DFIG grid-side convertor is based upon the decoupled d-q vector control concept [ 11, 12 ] . Normally, a decoupled d-q vector control attack is used, with the d-q mention frame oriented along the stator ( or supply ) electromotive force vector place.
Fig. 4 shows the schematic of the grid-side convertor for double fed initiation generator utilizing consecutive PWM convertors [ 7 ] . In the figure, a DC-link capacitance is on the left and a three-phase grid electromotive force is on the right. The electromotive force balance across the inductances is
( 1 )
Fig. 4 Schematic of Grid Side Converter
where L and R are the line induction and opposition of the transformer or the grid filter. When transforming ( 1 ) to the d-q mention frame that has the same velocity as that of the grid electromotive force, ( 1 ) becomes ( 2 ) where I‰s is the angular frequence of the grid electromotive force
( 2 )
In the d-q mention frame, the active and reactive power absorbed from the grid in per unit is
( 3 )
Aligning the d-axis of the mention frame along the stator-voltage place Vq is zero and since the amplitude of the supply electromotive force is changeless, Vd is changeless. Therefore, the active and reactive power will be relative to id and iq severally. This is the conventional foundation for the decoupled d-q controls, where the grid-side convertor is current regulated, with the direct axis current used to modulate the DC-link electromotive force and the quadrature axis current constituent used to modulate the reactive power.
The scheme for the conventional decoupled d-q control of the grid-side convertor is illustrated in Fig. 5. When d-q mention frame has the same velocity as that of the grid electromotive force, I?e=I‰st, the transportation map for the current control cringle is obtained from ( 2 ) and is given in ( 4 ) . This transportation map could be different depending on how the accountant is designed. The vitamin D and q mention electromotive forces, V*d1 and V*q1 are computed from the mistake signals of the vitamin D and Q currents, severally, as shown in Fig. 5. The I± and I? mention electromotive forces, V*I±1 and
Fig. 5 Decoupled d-q vector control construction for grid-side convertor
V*I?1, are obtained from the d-q mention electromotive forces, correspondingly, through a vector rotary motion of ejI‰st. The two I± and I? electromotive forces together are so used to bring forth the three-phase sinusoidal mention electromotive force signal for control of the grid-side PWM convertor [ 12 ] . Note that this control constellation really intends to command the existent and reactive powers through the decoupled vitamin D and q mention electromotive forces, severally. Although there is an iq constituent in V*d1 equation, it is non contributed in the manner of typical close-loop control construct. The same is true for id constituent in V*q1 equation.
= . + ( 4 )
, ( 5 )
The control of the grid-side convertor depends on the vitamin D and q mention electromotive forces V*d1 and V*q1, that are obtained from the mistake signals of the vitamin D and q currents as shown in Fig. 5. The combined vitamin D and q mention electromotive forces affect the convertor end product phasor electromotive force on the grid side by changing its amplitude and hold angle [ 10, 12 ] . This converter-injected electromotive force is linearly relative to the three-phase sinusoidal thrust signal in normal convertor additive transition manner. Normally, the grid-side convertor demands to be controlled in such a manner as to keep a changeless dc-link electromotive force, which requires that the existent power outputted from one convertor ( rotor- or grid-side convertor ) should be the power entered in another convertor ( grid- or rotor-side convertor ) when presuming no loss in the PWM convertors.
By seting the firing strategy of the convertor, we can run convertor in both rectifier and inverter manner. If I± = 0, the convertor behaves as an uncontrolled convertor whose end product electromotive force can non be adjusted. When I± = 90, the mean electromotive force of the convertor becomes zero. If the hold angle is increased farther, the mean value of the DC electromotive force becomes negative. In this manner of operation, the convertor operates as an inverter. By seting the fire angle, the convertor behavior as a burden can be studied as shown in the tabular array 1below.
CONVERTER BEHAVIOUR AS A LOAD
Converter behavior as a burden
I± = 0
Behaves like a resistive burden
0 & lt ; I± & lt ; 90
Behaves like a resistive/inductive burden and absorbs active power
I± = 90
Behaves like an inductive burden with no active power drawn
I± & gt ; 90
Behaves like an inductive burden but is besides a beginning of active power
This chapter deals with survey on the decoupled d-q vector control techniques for control of DFIG back-to-back PWM convertor both analytically and through simulation.
Fig. 6 Wind power features
The air current power characteristic shows that as the speed of air current additions, the power end product of turbine besides increases. By seting the blade pitch angle, we can accomplish an optimal end product power. As we increase the supply electromotive force, the DC nexus electromotive force besides increases. From the Fig. 8 it is clear that the electromotive force degree settees at 550 Volts, therefore the DC nexus mention electromotive force is set as 550 Volts severally.
Fig. 7 Pitch angle control
Fig. 8 Variation of DC link electromotive force with supply electromotive force
Fig. 9 Real and reactive power with normal burden
Fig. 10 Real and reactive power with PQ burden
REACTIVE POWER VS DC LINK VOLTAGE
DC nexus electromotive force
Real power in Watts
2.03 x 106
1.21 x 106
1.15 x 106
6.42 x 105
5.06 x 105
3.02 x 105
To keep a changeless dc-link electromotive force, the power passed to the rotor-side convertor from DFIG rotor should be to the power delivered to the grid from the grid-side convertor when pretermiting the convertor losingss. Therefore, while a existent power flows from the DFIG rotor to the rotor-side convertor, the grid-side convertor should be operated as an inverter and controlled in such a manner as to present the same sum of the existent power to the grid.
Fig. 11 First quadrant operation of existent & A ; reactive power
Fig. 12 Second quadrant operation of existent & A ; reactive power
Fig. 13 Third quadrant operation of existent & A ; reactive power
Fig. 14 Fourth quadrant operation of existent & A ; reactive power
Two different techniques are being adopted for convertors viz. SPWM and SVM. In comparing to SPWM and SVM techniques, it was noted that the SVM technique outputs better public presentation than SPWM as shown in the tabular array 3.
COMPARISON OF CONVERTER TECHNIQUES
THD in %
A simulation survey on the power coevals features of incorporate DFIG and its consecutive convertor with SPWM and SVM techniques is carried out. The general intent of grid-side convertor control is to maintain the dc-link capacitance electromotive force changeless by equilibrating the existent power at rotor-side and grid-side convertor, and to counterbalance DFIG reactive power every bit much as possible. Therefore, while a existent power flows from the DFIG rotor to the rotor-side convertor, the grid-side convertor should be operated as an inverter and controlled in such a manner as to present the same sum of the existent power to the grid. While a existent power flows from the rotor-side convertor to the DFIG rotor, the grid-side convertor should be operated as a rectifier and controlled in such a manner as to have the same sum of the power from the grid. Therefore in order to obtain synchronism, it is necessary to keep appropriate reactive power across the grid.
Parameters OF DFIG
RS ( Stator Resistance )
Rr ( Rotor Resistance )
LS ( Stator Inductance )
Lr ( Rotor Inductance )
Lm ( Magnetizing Inductance )
Number of pole braces