Analysis Of Multi Level Converters On Dfig Based Wecs Engineering Essay

Multilevel inverters are considered today as the most suited power convertors for high-voltage-capability and high-power-quality demanding applications. Voltage operation above authoritative semiconducting material bounds, lower common-mode electromotive forces, near-sinusoidal end products, together with little dv/dtaˆYs, are some of the features that have made these power convertors popular for industry and research. The general construction of the multilevel inverter is to synthesise near sinusoidal electromotive force from several degrees of District of Columbia electromotive forces, typically obtained from capacitance electromotive force beginnings.

Diode clamped multilevel inverter consists of capacitances, exchanging devices, dc electromotive force beginning and clamping rectifying tubes. An thousand degree rectifying tube clamped inverter typically consists of ( m-1 ) capacitances on the District of Columbia coach and produces m degrees on the stage electromotive force. Fig 2.1 ( a ) and 2.2 ( a ) shows one leg of a three degree rectifying tube clamped inverter and three degree rectifying tube clamped inverter severally. An m-level inverter requires ( m-1 ) capacitances, 2 ( Garand rifle ) shift devices and ( Garand rifle ) ten ( m-2 ) clamping rectifying tubes.

2.1. 3-level multilevel convertors:

fig 2.1 ( a ) Three degree multilevel inverter

Fig. 2.1 ( a ) shows a three-level inverter. Its end product electromotive force Va0 ( one stage leg ) i.e. fig 2.1 ( degree Celsius ) has three provinces: Vdc/2, 0 and -Vdc/2. To acquire Vdc/2, the two upper switches need to be ON. To acquire a nothing, the two in-between switches need to be ON and for -Vdc/2 the two lower switches need to be ON. One difference between a conventional two-level inverter is the portion in Fig. 2.12 ( a ) that is called D1 and D1 ‘ , mentioning to the two rectifying tubes. The needed sum of rectifying tubes can be calculated as ( m?1 ) – ( m?2 ) , where m stands for sum of degrees. So in the three-level instance two rectifying tubes are needed for each stage. The two rectifying tubes clamps the shift electromotive force to half of the DC-bus electromotive force and the difference between Va0 ( for an illustration when S1 and S2 is on, the electromotive force across a and 0 is VDC, giving Va0 = VDC ) and Van gives the electromotive force across one capacitance ( Vdc/2 ) . It is of import to add that the upper and lower shift braces are complementary. This means that S1 and S1 ‘ or S2 and S2 ‘ ne’er can be ON at the same clip. 3levelcar162.jpg

V0

S1

S2

Vdc/2

1

1

0

0

0

0

1

1

0

0

1

0

0

1

-Vdc/2

0

0

1

1

3level pole.jpg

3levelline.jpg

Table

fig 2.1 ( B )

fig 2.1 ( vitamin D )

fig 2.1 ( degree Celsius )

fig 2.1. ( B ) Source electromotive force with bearer wave forms ( degree Celsius ) end product Pole electromotive force ( vitamin D ) Output line electromotive force

2.2. 5-level multilevel convertors:

5level.jpg

fig 2.2 ( a ) five degree multilevel inverter

Table V0

Vdc/2

1

1

1

1

0

0

0

0

Vdc/4

0

1

1

1

1

0

0

0

0

0

0

1

1

1

1

0

0

-Vdc/4

0

0

0

1

1

1

1

0

-Vdc/2

0

0

0

0

1

1

1

1

fig 2.2 ( B ) A 5-level rectifying tube clamped multilevel inverter is shown in Fig. 2.2 ( a ) . Table 2 lists the end product electromotive force degrees and their corresponding exchanging provinces for five-level rectifying tube clamped inverter. if province value is 1 means the switch is ON else, switch is OFF. See the centre pat is zero electromotive force mention point. if S1, S2, S3 and S4 are ON To acquire the end product electromotive force Vdc/2 all the upper switches has to ON. if all the lower switches are ON so the end product electromotive force is -Vdc/2. For end product electromotive force Vdc/4 all the upper switches except S1 has to ON. To acquire the end product voltage-Vdc/4 all the lower switches except has to ON. In the staying shift spiels the end product electromotive force is zero.5level162car.jpg

5levline162.jpg5levpole162.jpg

Fig 2.2 ( degree Celsius ) fig 2.2 ( vitamin D )

fig 2.2. ( B ) Source electromotive force with bearer wave forms ( degree Celsius ) end product Pole electromotive force ( vitamin D ) Output line electromotive force

Comparison of entire harmonic deformation with different transition indices for three degree and five degree multilevel inverters has been presented below.

Transition indices ( m )

Three degree multilevel inverter

five degree multilevel inverter

Cardinal constituent

% THD

Cardinal constituent

% THD

m=1.1

168.7

39.07

184.4

16.10

m=1.0

158.6

41.57

174.1

16.63

m=0.9

142.2

45.57

155.4

17.48

m=0.8

126.2

48.80

138.9

21.71

m=0.7

111.0

50.84

121.1

24.33

3. Mold of DFIG:

Modeling is really utile for analyzing the transient and dynamic behaviour of any electrical machines and of interrelated electrical machine system.

3.1 Wind turbine:

The power extracted from the air watercourse by the turbine blades can be characterized by Equation

( 1 )

Where

? – air denseness ( kgm-3 )

V – air current velocity ( ms-1 )

A -Turbine swept country ( )

? – Blade pitch angle ( deg )

? – Tip velocity ratio

Cp itself is non a invariable for a given aerofoil, but instead is dependent on tip-speed ratio ( ? ) , which is the ratio of the velocity of the tip of the blade to the velocity of the traveling air watercourse and blade pitch angle ( ? ) , here pitch angle is normally around zero when the air current velocity is below rated velocity.

Using the Concordia and Park transmutation allows composing a dynamic theoretical account in a d-q mention frame from the traditional a-b-c frame as follows.

Stator electromotive forces and rotor electromotive forces

( 2 )

( 3 )

( 4 )

( 5 )

The stator and rotor flux are

( 6 )

( 7 )

( 8 )

( 9 )

Electromagnetic torsion

( 10 )

The gesture of the generator is

( 11 )

Active and reactive powers of stator

( 12 )

Control Schemes:

3.2 RSC control

Stator flux orientation technique is used to acquire the decoupled control of active and reactive powers, i.e. s = Ds, qs = 0.

,

( 13 )

( 14 )

Where

The active and reactive power becomes

( 15 )

( 16 )

PI accountant

eq 15

PI accountant

eq 16

3.3 GSC Control

Stator flux orientation technique is used to acquire the decoupled control of active and reactive powers,

i.e. s = Ds, qs = 0.

( 17 )

( 18 )

+

PI accountant

PI accountant

( 19 )

PI accountant

eq 17

4. DFIG with dorsum to endorse convertors:

In DFIG constellation two dorsum to back affiliated convertors are used. One is Rotor side convertor ( RSC ) and another is grid side convertor ( GSC ) as shown below. In sub synchronal operation RSC acts as inverter and GSC acts as rectifier. In instance of super synchronal mode action of these convertors alterations frailty versa. The side convertor ( GSC ) is implemented utilizing Sinusoidal PWM technique and rotor side convertor ( RSC ) is implemented with multilevel convertors. The responses of DFIG with 3level and 5level multi level convertor based RSC is presented below.

4.1 DFIG with 3-level multilevel inverter:

The responses of the electromagnetic torsion, the rotor velocity, rotor d-axis, q-axis currents, rotor electromotive force, and generator active and reactive power are shown in fig 4.1.1 to 4.1.8. The initial active power mention is 1000W ; it changes to 2000W at 2sec. The initial reactive power mention is 0var ; it changes to 500var at 3sec. The input wind velocity alterations from 12m/s to 13m/s at 4sec which means that the input mechanical power besides changes at 4sec.It is observed from the fig4.1.7 and 4.1.8 Stator Active and Reactive power is controlled independently. From the rotor per stage electromotive force wave form 4.1.6 we can detect that the no of degrees is three.fig 4.2.5 is the stator electromotive forces of DFIG.

Fig 4.1.1 Torque

wr

Fig 4.1.2 Speed

isabc

Fig 4.1.3 Stator currents

irabc

Fig 4.1.4 Rotor currents

vsabc4

Fig 4.1.5 Stator electromotive forces

var3

Fig 4.1.6 Rotor stage electromotive force

Postscript

Fig 4.1.7 Active power of stator

Q

Fig 4.1.8 Reactive power of stator

4.2 DFIG with 5-level multilevel inverter:

The responses of the electromagnetic torsion, the rotor velocity, rotor d-axis, q-axis currents, rotor electromotive force, and generator active and reactive power are shown in fig 4.2.1 to 4.2.7. The initial active power mention is 1000W ; it changes to 2000W at 2sec. The initial reactive power mention is 0var ; it changes to 500var at 3sec. The input wind velocity alterations from 12m/s to 13m/s at 4sec which means that the input mechanical power besides changes at 4sec.It is observed from the fig 4.2.6 and 4.2.7 Stator Active and Reactive power is controlled independently. From the rotor per stage electromotive force wave form 4.2.5 we can detect that the no of degrees is five.

1te Fig 4.2.1 Torque

2wr Fig 4.2.2 Speed

3isabc.jpg

Fig 4.2.3 stator currents

4irabc

Fig 4.2.4 rotor currents

7vra3.jpg

Fig 4.2.5 rotor stage electromotive force

9Ps

Fig 4.2.6 Active Power of stator

Q

Fig 4.2.7 Reactive Power of stator

Decision:

Three degree multilevel convertors

Five degree multilevel convertor

Rotor currents

5.00

2.13

Stator currents

4.727

2.65