07/04/2016

1

Wind Generation and its Grid

Conection

J.B. EkanayakePhD, FIET, SMIEEE

Department of Electrical and Electronic Eng.,

University of Peradeniya

Content

• Wind turbine basics

• Wind generators

• Why variable speed?

• Grid Code requirements

• Concluding remarks

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Introduction

http://www.wwindea.org/hyr2015/

124

67.8

42.423.8

23

13.3

10.2

87.8

China USA Germany India Spain UK Canada RoW

Worldwide wind capacity as at mid of 2015 = 393 GW

WIND TURBINE BASICS

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How wind turbine works?

5

How wind turbine works?

6

•Air incident on the

airfoil produces

plenty of lift which

aid the rotation of

the blade.

•The drag tries to

bend the blade

towards the tower.

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Power available in a wind stream

• The kinetic energy in a flow of

air = per unit mass

• Mass flow rate= (kg/s)

– is the air density in kg/m3

• Power available in the wind

stream =

21

1

2U

1AUρ

ρ

31

1

2AUρ

Energy- extracted by the wind turbine

• Power extracted by the aerodynamic rotor = Cp x

Power available

• Cp is the coefficient of performance

• The Betz limit: Maximum value of the coefficient

of performance Cp

is 59%.

• Cp depends on the tip speed ratio

Velocity at rotor tip

Wind velocity

R

U

ωλ = =

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Cp vs Tip speed ratio

Variable speed operation

To extract maximum power ωr should vary with the wind speed

0 0.2 0.4 0.6 0.8 1 1.20

0.2

0.4

0.6

0.8

1

1.2

1.4

Generator rotational speed (pu)

Po

we

r (p

u)

5 m/s6 m/s

7 m/s

8 m/s

9 m/s

10 m/s

11 m/s

12 m/s

13 m/sGenerator speed in pu

1440/1800 = 0.8 pu

Maximum power that can

be extracted

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Evolution of Wind Turbine Technologies

Available wind turbines

Turbine Capacity Generator Rotor diameter Geared

Vestas V164 8 MW FPC 164 m Yes

Enercon E126 7.5 MW FPC 127 m No

Repower 6M 6 MW DFIG 126 m Yes

Siemens SWT-6.0 150 6 MW FPC - PMG 154 m No

Alstom Haliade 150 6 MW FPC - PMG 150 m No

Areva M5000 5 MW FPC - PMG 135 m Yes

Gamesa G128 5 MW FPC - PMG 128 m Yes

http://en.wind-turbine-models.com/

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HTS - DD

http://www.amsc.com/documents/hts-generator-solutions-brochure/

Power curve of modern wind turbines

2 MW

Power

Wind

speed12 m/s 25 m/s

Cut-in wind

speed

Rated wind

speed

3.5 m/s

Cut-out wind

speed

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Control of modern wind generators

� Wind speed 7 to 15 m/s Electronic control

� Wind speed above 15 m/s Pitch control

� Input aerodynamic power is reduced by increasing the pitch angle at

high wind speed

3.5 m/s

WIND GENERATORS

16

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Fixed speed wind turbine

Variable speed wind turbines

Turbine

Gear

box

Wound rotor

induction

generator

Back-to-Back

VSCs connected

to the rotor

Induction or

synchronous

generator

Diode bridge and

a VSC connected

to the stator

Multi-pole

permanent magnet

synchronous

generator

Doubly-Fed Induction

Generator

Geared Full Power

Converter

Gearless Full Power

Converter

Back-to-Back

VSCs connected

to the stator

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Doubly-fed arrangement

• Variable speed operation is possible by absorbing or injecting slip power using an external means.

• The sub-synchronous or super-synchronous speeds can be obtained by feeding in or taking out electric power to/from the rotor.

• This is normally done by injecting a voltage into the rotor circuit through slip rings.

Zero rotor injection

Operates as a fixed speed machine

jXrRr/s

sE2

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 -15

-10

-5

0

5

10

Slip (pu)

To

rqu

e (p

u)

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Negative injected voltage

• Will deliver power from the rotor through

the converters to the network.

jsXrRr

sE2 P

ωr > ωs

sf Vr

Negative injected voltage

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 -15

-10

-5

0

5

10

Slip (pu)

To

rqu

e (

pu

)

(1)

(2)

A

The DFIG wind turbine running at super-synchronous speed

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Positive injected voltage

The DFIG rotor absorbs power.

jsXrRr

sE2 P

ωr < ωs

sf Vr

Slip – torque curve

The DFIG wind turbine running at sub-synchronous speed

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 -15

-10

-5

0

5

10

Slip (pu)

Torq

ue

(pu

)

(1)

(2)

(3)

AB

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Doubly fed induction generator (DFIG)

25

rv

ri

sv

si a

i

gi

Controller

ac / dc dc / ac

C1 C2

av

ggjQP +

DFIG

• Converter C1 inverts dc voltage into slip

frequency ac

• It can inject both positive and negative

voltages by properly controlling the

switching signals

• Converter C2 maintains a constant voltage

on the DC capacitor

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Control of converter C1

• By injecting proper voltage through

converter C1

– Speed can be controlled for optimum power

extraction.

– No – load power factor of the generator can be

controlled.

– Terminal voltage of the generator can be

controlled.

� Torque control in q-axis

� Voltage control in d-axis

ωrated

ωr

+

−+ I

P

KK

s β

pitch demand

� Pitch controller

Maintaining the turbine operation

point on the maximum power curve

is by means of controlling the

generator torque

Generator terminal voltage is controlled

by manipulating the reactive power

supply from the generator

Orientation of the turbine blades are

Physically moved to control the

aerodynamic torque.

Control of DFIG wind turbines

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Fully rated converter (FRC) wind

turbines

�

�

� DC-link totally decouple the generator from

the gridGrid frequency is decoupled, wind turbine can

operate at any rotor speed

Grid voltage is decoupled, change in grid voltage

does not affect the generator dynamics

� Gearbox can be avoided if a multi-pole

synchronous generator is used, e.g.

Enercon turbines with 64 poles

Control of FRC wind turbines

• In order to ensure maximum power extraction

and wide speed range operation, the controller

of machine side converter varies the operating

frequency.

• This shifts the torque-speed curve and thus

moves the operating point to match the

maximum power extraction curve.

Machine side converter control

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Control of FRC wind turbines

1.12510.8750.75 0.6250.5 0.3750.250.125-1.8

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

Speed, pu

Torq

ue,

pu

Torque-speed curve

for maximum power

50 Hz

45 Hz

39 Hz

33 Hz

23 Hz

(0.46, -0.2)

(0.65, -0.4)

(0.8, -0.6)

(0.9, -0.8)

(1.02, -1.0)

Control of FRC wind turbines

s qsL iω ′

ss dsL iω

1

rr

r

refqs

L refdsR

i

i

1referefds

T

k i

rω

dsv

qsv

sω

ω

qsid sirω

refqsi

refeT

ω sωrω

refqsi

refdsi

ω

dsi

qsi

ω

refdsi

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WHY VARIABLE SPEED?

Variable speed operation

To extract maximum power ωr should vary with the wind speed

0 0.2 0.4 0.6 0.8 1 1.20

0.2

0.4

0.6

0.8

1

1.2

1.4

Generator rotational speed (pu)

Po

we

r (p

u)

5 m/s6 m/s

7 m/s

8 m/s

9 m/s

10 m/s

11 m/s

12 m/s

13 m/sGenerator speed in pu

1440/1800 = 0.8 pu

Maximum power that can

be extracted

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Power output

Forces on wind turbines

Blade Flapwise bending

Aerodynamic forces in flapwise

direction

Blade Edgewise bending

Gravity forces

Aerodynamic forces in edgewise

direction

Loads on Blades

Loads on Rotor Hub

In-plane bending moments of the blades

Out-of-plane bending moments of the

blades

Drive train interaction – torque

fluctuations due to control action

Loads on Tower

Axial bending moment

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Forces on wind turbines

Blade x direction

Blade y direction

Blade z direction

Fixed speed

Variable speed

GRID CODE REQUIREMENTS

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39

• Grid Codes specify the mandatory minimum technical

requirements that a power plant should fulfil and

additional support that may be called on to maintain

the second-by-second power balance and maintain

the required level of quality and security of the system.

• Grid Codes for wind farm connections demand

requirements at the point of connection of the wind

farm not at the individual wind turbine generator

terminals.

Grid Codes

General Requirements

46 48 5047 49 51 52

Voltage [%]

Frequency [Hz]

1 min

Con

tin

uou

s o

pe

ratio

n

AVH

VL

B

30 s

60 m

in

As most of the controllers employed in

modern wind turbines are digital, the

maximum allowable voltage limit for

ICT should be considered.

As all the wind turbines and PV parks

are connected to the collector

network through a transformer, a

typical transformer V/f characteristic

should be considered.

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General Requirements

ITI (CBEMA) Curve

V/f characteristics of transformers

Active power control

Ireland, India, Denmark and

Germany

UK

Active p

ow

er

ou

tput a

s a

% o

f

ava

ilable

pow

er

Frequency

PA

PB

PD

Ireland 47 (min) 49.5 50.5 52

Denmark 47 (min) 49.9 50.1 53

India 50.3

Germany 50.2

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Reactive Power and Voltage Control

Reactive power requirements

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Reactive power requirements

Reactive power requirements• If reactive power requirement is not satisfied then a source

of reactive power is required.

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• With the penetration of wind generation increasing, Grid Codes now

generally demand Fault Ride-Through capability for wind turbines

connected to transmission networks.

• What is FRT?

Fault Ride-Through (FRT)

Pre

• Wind turbines operate normally and generate electricity

Fault

• Fault occurs and power generated from wind turbines can not be supplied.

• Generator speeds-up or DC cap voltage rise

Post

• Fault is cleared by a circuit breaker

• Generators should come back to normalcy

Grid disturbances and DFIG

J

J

Before a fault

� During a fault, terminal voltage of

the generator goes to very low value.

In order to maintain the power flows,

the controller increases the rotor

currents.

� Crowbar is employed to protect

the converter when the rotor current

increases beyond its maximum limit.

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Grid disturbances and Full range

GEpGRpCp

dcv

C

DCR

DCRp

J

Chopper resistor at the DC-link

� During a fault, power to the grid is limited

� DC-link voltage rises rapidly

� Input power has to be reduced or excessive

power has to be dissipated

LVRT capability

The wind farm and any constituent wind turbine

generating unit must remain transiently stable and

connected to the system without tripping for balanced

voltage dips and associated durations anywhere on or

above the heavy black line

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LVRT capability requirements

FRT capability of wind turbines

Fast pitching

De-loading

Braking resistor

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Primary response

f T∆d

dt

spTrω

2fK

1fK

f∆

+−

+

+

maxT

maxT

J

Frequency support from DFIG wind turbine

� Stored kinetic energy of the rotor is high and can

be used to support the power system.

Inertia support

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Concluding remarks

• Penetration of wind generation is increasing

• Large wind turbines and new technologies are

emerging

• Utilities now expect wind farms to perform

exactly like a large synchronous generator

– This demands extra plants to be connected at the

point of connection

– In turn the CAPEX will increase

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