Analysis Of Multi Level Converters On Dfig Based Wecs Engineering Essay

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Multilevel inverters are considered today as the most suitable power converters for high-voltage-capability and high-power-quality demanding applications. Voltage operation above classic semiconductor limits, lower common-mode voltages, near-sinusoidal outputs, together with small dv/dt‟s, are some of the characteristics that have made these power converters popular for industry and research. The general structure of the multilevel inverter is to synthesize near sinusoidal voltage from several levels of dc voltages, typically obtained from capacitor voltage sources.

Diode clamped multilevel inverter consists of capacitors, switching devices,dc voltage source and clamping diodes. An m level diode clamped inverter typically consists of (m-1) capacitors on the dc bus and produces m levels on the phase voltage. Fig 2.1(a) and 2.2(a) shows one leg of a three level diode clamped inverter and three level diode clamped inverter respectively. An m-level inverter requires (m-1) capacitors, 2(m-1) switching devices and (m-1)x(m-2) clamping diodes.

2.1. 3-level multilevel converters:

fig 2.1(a) Three level multilevel inverter

Fig. 2.1 (a) shows a three-level inverter. Its output voltage Va0 (one phase leg) i.e. fig 2.1(c) has three states: Vdc/2, 0 and -Vdc/2. To get Vdc/2, the two upper switches need to be ON. To get a zero, the two middle 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 part in Fig. 2.12(a) that is called D1 and D1', referring to the two diodes. The required amount of diodes can be calculated as (m−1) - (m−2), where m stands for amount of levels. So in the three-level case two diodes are needed for each phase. The two diodes clamps the switching voltage to half of the DC-bus voltage and the difference between Va0 (for an example when S1 and S2 is on, the voltage across a and 0 is VDC, giving Va0 = VDC) and Van gives the voltage across one capacitor (Vdc/2). It is important to add that the upper and lower switching pairs are complementary. This means that S1 and S1' or S2 and S2' never can be ON at the same time. 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(d)

fig 2.1(c)

fig 2.1. (b) Source voltage with carrier waveforms (c) output Pole voltage (d) Output line voltage

2.2. 5-level multilevel converters:

5level.jpg

fig 2.2 (a) five level 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 diode clamped multilevel inverter is shown in Fig. 2.2(a). Table 2 lists the output voltage levels and their corresponding switching states for five-level diode clamped inverter. if state value is 1 means the switch is ON else, switch is OFF. Consider the center tap is zero voltage reference point. if S1,S2,S3 and S4 are ON To get the output voltage Vdc/2 all the upper switches has to ON. if all the lower switches are ON then the output voltage is -Vdc/2. For output voltage Vdc/4 all the upper switches except S1 has to ON. To get the output voltage-Vdc/4 all the lower switches except has to ON. In the remaining switching patters the output voltage is zero.5level162car.jpg

5levline162.jpg5levpole162.jpg

Fig 2.2 (c) fig 2.2 (d)

fig 2.2. (b) Source voltage with carrier waveforms (c) output Pole voltage (d) Output line voltage

Comparison of total harmonic distortion with different modulation indices for three level and five level multilevel inverters has been presented below.

Modulation indices (m)

Three level multilevel inverter

five level multilevel inverter

Fundamental component

%THD

Fundamental component

%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. Modeling of DFIG:

Modeling is very useful for studying the transient and dynamic behavior of any electrical machines and of interconnected electrical machine system.

3.1 Wind turbine:

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

(1)

Where

ρ - air density (kgm-3)

V - wind speed (ms-1)

A -Turbine swept area ()

β - Blade pitch angle (deg)

λ - Tip speed ratio

Cp itself is not a constant for a given airfoil, but rather is dependent on tip-speed ratio (λ), which is the ratio of the speed of the tip of the blade to the speed of the moving air stream and blade pitch angle (β), here pitch angle is usually around zero when the wind speed is below rated speed.

Using the Concordia and Park transformation allows writing a dynamic model in a d-q reference frame from the traditional a-b-c frame as follows.

Stator voltages and rotor voltages

(2)

(3)

(4)

(5)

The stator and rotor flux are

(6)

(7)

(8)

(9)

Electromagnetic torque

(10)

The motion of the generator is

(11)

Active and reactive powers of stator

(12)

Control Strategies:

3.2 RSC control

Stator flux orientation technique is used to get 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 controller

eq 15

PI controller

eq 16

3.3 GSC Control

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

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

(17)

(18)

+

PI controller

PI controller

(19)

PI controller

eq 17

4. DFIG with back to back converters:

In DFIG configuration two back to back connected converters are used. One is Rotor side converter (RSC) and another is grid side converter (GSC) as shown below. In sub synchronous operation RSC acts as inverter and GSC acts as rectifier. In case of super synchronous mode action of these converters changes vice versa. The side converter (GSC) is implemented using Sinusoidal PWM technique and rotor side converter (RSC) is implemented with multilevel converters. The responses of DFIG with 3level and 5level multi level converter based RSC is presented below.

4.1 DFIG with 3-level multilevel inverter:

The responses of the electromagnetic torque, the rotor speed, rotor d-axis, q-axis currents, rotor voltage, and generator active and reactive power are shown in fig 4.1.1 to 4.1.8. The initial active power reference is 1000W; it changes to 2000W at 2sec. The initial reactive power reference is 0var; it changes to 500var at 3sec. The input wind speed changes from 12m/s to 13m/s at 4sec which means that the input mechanical power also 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 phase voltage waveform 4.1.6 we can observe that the no of levels is three.fig 4.2.5 is the stator voltages 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 voltages

var3

Fig 4.1.6 Rotor phase voltage

Ps

Fig 4.1.7 Active power of stator

Qs

Fig 4.1.8 Reactive power of stator

4.2 DFIG with 5-level multilevel inverter:

The responses of the electromagnetic torque, the rotor speed, rotor d-axis, q-axis currents, rotor voltage, and generator active and reactive power are shown in fig 4.2.1 to 4.2.7. The initial active power reference is 1000W; it changes to 2000W at 2sec. The initial reactive power reference is 0var; it changes to 500var at 3sec. The input wind speed changes from 12m/s to 13m/s at 4sec which means that the input mechanical power also 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 phase voltage waveform 4.2.5 we can observe that the no of levels 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 phase voltage

9Ps

Fig 4.2.6 Active Power of stator

Qs

Fig 4.2.7 Reactive Power of stator

CONCLUSIONS:

Three level multilevel converters

Five level multilevel converter

Rotor currents

5.00

2.13

Stator currents

4.727

2.65

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