power electronic topologies for wind turbines zaragoza power electronic topologies for wind turbines...

Jordi Zaragoza

Power electronic topologies

for wind turbines

Source: ALSTOM WIND

Author: Jordi Zaragoza

XIX JORNADAS de CONFERENCIAS

JCEE’14

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Presentation of the wind systems: large numbers

Annual offshore wind installation (2011)

Size evolution of wind turbines over time

Source: EWEA

Source: EWEA

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Local wind turbine grid

Collecting

point

Wind farm

grid

interface

Transmission

system PCC

WT

General wind farm layout

Wind farm elements:

Wind turbines (WT). WT connected in parallel to radials.

Local wind turbine grid.

Collecting point. To increase the voltage for transmission.

Transmission system. HVAC or HVDC transmission.

Wind farm interface. To adapt the voltage, frequency and the reactive

power demand of the grid in the PCC.

Point of common connection.

Rules for connecting wind farms to the grid

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Fixed and variable speed wind turbines

Wind Turbine configurations

a) Wind turbine using an induction generation (IG).

b) Wind turbine using a doubly-fed induction generator (DFIG).

c) Wind turbine using a permanent magnet synchronous generator (PMSG).

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Fixed-speed wind turbines

Wind Turbine configurations

The electrical generator is connected directly to the grid.

An induction generator is normally used.

Since the grid frequency is fixed, the speed of the wind turbine is settled by the

ratio of the gearbox and by the number of poles in the generator.

fn

60

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Induction generator operating at fixed speed

Advantages:

- Robust design.

- No need for maintenance.

- Well enclosed.

- Produced in large series.

- Low price.

- Can withstand overloads.

Disadvantages:

- Uncontrollable reactive power consumption.

- Fixed speed means more mechanical stress.

Source: AEROSTAR

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Capacitor banks


Capacitors banks compensate for reactive power from the induction generator.

Maxim use of the electrical grid is done operating at unity power factor.

C C C

Voltage

Current

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If you connected (or disconnected) a large wind

turbine generator to the grid with a normal

switch, you would be quite likely to damage

both the generator and the gearbox. Also large

currents in the neighborhood grid would be

produced.

- To prevent this situation, wind turbines connect

and disconnect gradually to the grid using

thyristors.

- To avoid thyristor losses under normal

operation mode, a bypass switch is activated

(main contactor).


Soft-starter

8

Ind

uctio

n

Gen

erator

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Variable-speed wind turbines

Doubly-fed induction generator

A frequency converter control the currents in the rotor.

The slip of the rotor can change within a wider range.

Power converters have to stand only a fraction the nominal power (20% or 30%).

It is the most common topology produced by large manufacturers nowadays.

Less expensive compared to the full power converter.

Source: ALSTOM

WIND

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Multipole synchronous generator

Multipole synchronous generators may not need a gearbox (these generators

have a large diameter).

This is expected to be the most common wind turbine configuration in the

future.

ENERCON E-126 (7 MW)

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Variable speed

The frequency of the generator voltages can be different from the

electrical grid (50-60 Hz) and therefore the turbine speed can change.

Advantages:

- More energy production.

- Less mechanical stress.

- Reduce power fluctuation.

- Capacity of noise reduction.

- May have more control on the grid currents.

Drawbacks:

- The system requires power electronic converters.

- More expensive.


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Wind turbine components

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Permanent magnet synchronous generator

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Wind turbine system based on PMSG

Wind Turbine Modeling

Variation of Power coefficient Cp with Tip Speed Ratio

(for fixed values of β)

2.5 7.5 10 12.5 15

Cp

5 0

0.1

0.2

0.3

0.4

0.5

Tip speed ratio λ 0

ß=0°

ß=2.5°

ß=5°

ß=7.5°

ß=10°

ß=12.5°

ß=15°

ß=17.5°

ß=20°

Cpmax

λ opt

= 0.48

= 8.1

wind

mechanicalp

P

PC

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Second interval variable speed and variable pitch

Third interval fixed speed and variable pitch

First interval variable speed and fixed pitch

Area of different control operation

Control operation

PN

Vmin Vmax

Wind speed (m/s)

Po

we

r (k

W)

1 2 3

Control regions for the WT

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Source: L.M. Tolbert, “High Power Electronics for a Sustainable 21st Century,” NSF

Workshop for Sustainable Energy Systems, The University of Tennessee, Dec.

2000, Atlanta, Georgia.

Power electronic Semiconductors

Power Electronic Converter topologies of a

wind turbine

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AC/DC/AC

CvC2

C

r

Vdc s t

a

b

c

Wind-Turbine Two-Level Converter

vr

vs

vt

3*L

Multipole

Synchronous

Wind Turbine

Electrical

Grid

vC1

PMSG

Grid-Connected Two-Level Converter


wind turbine

Back-to-back-connected conventional two-level

The maximum voltage that the transistors have to withstand is the total dc-

link voltage (Vdc).

Low quality output voltage spectra. This implies large values of the reactive

components to filter the output currents

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The modulation stage in a power electronic converter is

responsible for defining the state of the switches from

continuous (or sampled) control variables.

Modulation Strategies


wind turbine

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Half-Bridge Inverter

+

-

+

-

2

dcV

)0(

C2

C2

T ( )a as

T ' ( )a as

2

dcV

0avv )(a

Vdc

To avoid shortcircuits, only one transistor is activated at any time.

The control functions define the state of the transistors. They can

take two values sa=0,1 meaning “0”=off and “1”=on.


wind turbine


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If the upper switch is activated (ON state), the output

voltage becomes va0=+Vdc/2 independently of the current

direction.

02

dca

Vv v

+

-

+

-

2

dcV

)0(

C2

C2

aT

aT '

2

dcV

)(aVdc

02

dca

Vv v

+

-

+

-

2

dcV

)0(

C2

C2

aT

aT '

2

dcV

)(aVdc

The upper transistor

carries the output current

The upper diode



wind turbine


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If the lower switch is activated (ON state), the output

voltage becomes va0=-Vdc/2 independently of the current

direction.

02

dca

Vv v

+

-

+

-

2

dcV

)0(

C2

C2

aT

aT '

2

dcV

)(aVdc

02

dca

Vv v

+

-

+

-

2

dcV

)0(

C2

C2

aT

aT '

2

dcV

)(aVdc

The lower transistor


The lower diode



wind turbine


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Only one transistor from the leg can be activated at any time

(ON state).

The transistor activated defines the output voltage level,

independently of the output current direction.

Only two voltage levels can be obtained at the output

(va0=Vdc/2).

If sinusoidal waveforms are needed, the output variable

(voltage and/or current) has to be filtered.

The switching process is crucial (calculation of the ON and

OFF times) to achieve a good output voltage spectrum

which can be easily filtered. This process is determined by

the modulation strategy.

Remarks


wind turbine

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Modulation signal (sinusoidal):

vs(t)=VSMsinst

Transistors control:

If vsvT Ta ON v=va0=+Vdc/2

If vs<vT Ta’ ON v=va0=-Vdc/2

0ast

Vdc/2

VSM

VTM

vs (modulation) vT (carrier)

p 2p

v1 Fondamental component <> “instantaneous” mean value

0ast

p 2p

v (output voltage)

-Vdc/2

+

-

+

-

2

dcV

)0(

C2

C2

aT

2

dcV

0avv )(a

Vdc

aT '


wind turbine

Sinusoidal PWM (SPWM)

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Back-to-back-connected conventional three-level


wind turbine

(NP)

C vC2

vC1

C

r Vd

c s

t

a

b

c

Wind-Turbine NPC Converter Grid-Connected NPC Converter

vr

vs

vt

3*Lg iwt ig

Multipole Synchronous Wind Turbine

Electrical Grid

AC/DC/AC

The transistors have to withstand only half of the total dc-link voltage (Vdc/2).

The output voltage spectra have better quality.

More switching frequency in the power devices

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1

-1

a b c

Vdc

vC2

0

C vC1

C vC2

-1

0

1

c

b

a ia

ib

ic

sc-1

sc1

sc0

sb-1

sb1

sb0

sa-1

sa0

sa1

C vC1

Vdc

C

Functional diagram

Three-level or neutral-point-clamped (NPC) converter

Two consecutive switches must be in on-state

1

-1

a b c

Vdc

vC2

0

C vC1

C vC2

-1

0

1

c

b

a ia

ib

ic

sc-1

sc1

sc0

sb-1

sb1

sb0

sa-1

sa0

sa1

C vC1

Vdc

C

1

-1

a b c

Vdc

vC2

0

C vC1

C vC2

-1

0

1

c

b

a ia

ib

ic

sc-1

sc1

sc0

sb-1

sb1

sb0

sa-1

sa0

sa1

C vC1

Vdc

C

1

-1

a b c

Vdc

vC2

0

C vC1

C vC2

-1

0

1

c

b

a ia

ib

ic

sc-1

sc1

sc0

sb-1

sb1

sb0

sa-1

sa0

sa1

C vC1

Vdc

C


wind turbine

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It is based on comparing sinusoidal modulation signals with a triangular

carrier of significantly larger frequency.

temps/T

0 0.005 0.01 0.015 0.02

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Sinusoidal pulse-width modulation SPWM

Vdc

i0

0

(NP)

C vC2

C

a

b c

vC1

sa4

sa3

sa2

sa1

sb4

sb3

sb2

sb1

sc4

sc3

sc2

sc1

-1

1

Control applied to define the state of the output voltages:

Reference above both carriers High level output (1).

Reference between both carriers Medium level output (0).

Reference below both carriers Low level output (-1).


wind turbine

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Simulation Results

m=0.8

-Wye-connected load

R=1 Ω and L=2 mH.

-C=2200 µF

-Vdc=1800 V

-f =50 Hz


wind turbine

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