This document discusses how wind turbines can provide grid support functions similar to Flexible AC Transmission Systems (FACTS) devices through advanced controls of the power electronics and generator systems. It describes how doubly-fed induction generator wind turbines can regulate voltage and reactive power at the point of interconnection, provide inertial response during frequency disturbances, limit ramp rates during changes in wind speed, and improve transient stability compared to synchronous generators. The document argues that these grid support functions allow high levels of wind power to be integrated onto the grid in a reliable manner without compromising system performance.

1. GE Energy
Facts On Grid Friendly Wind
Plants
Nicholas W. Miller
Paul E. Marken
July, 2010
IEEE GPM Minneapolis

2. 2
GE Energy
Wind Generators are Different
Wind Turbines Need to have some degree of
speed variability due to the need to:
1. Control mechanical loads
2. Maximize energy capture (efficiency)

3. 3
GE Energy
Structure of wind turbine generator
systems - Overall structure
Rotor
NacelleHub
Tower
Tower base

4. 4
GE Energy
Structure of wind turbine generator
systems - Nacelle
Nose
cone
Rotor shaft
Pitch drive
Oil cooler
Hub
Main bearing
Obstruction
lights
Control
Cooler
Casing
Generator
Ground frame
Noise decoupling Yaw drives
Coupling
Gearbox

5. 5
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Aerodynamic Fundamentals
Rotor power: P = 1/2 cp ρAvw
3
cp - rotor power coefficient
ρ - air density
A - rotor swept area
Ideal cp = 0.593 (Betz factor)
Tip speed ratio: λ = vt / vw
cp = f(λ)
max for 3 blade rotor: cp = 0.47 at λ = 7
max for 2 blade rotor: cp = 0.45 at λ = 10
• to operate at peak cp requires variable rotor speed
• 2 blade machine will spin 30% faster, slightly lower cp
• diminishing returns for more than 3 blades
“Windturbines: Fundamentals, Technologies, Application and
Economics”, Erich Hau, ISBN: 3540570640; (April 30, 2000)
Wind Speed
Tip Speed Ratio λ
PowerCoefficeintCp
λ ∼ 7

6. 6
GE Energy
Wind Turbine Generator
Electrical Conversion Technologies
• Type 1 – Squirrel-cage induction generator
• Type 2 – Wound rotor induction generator with variable rotor resistance
• Type 3 – Doubly-fed asynchronous generator
• Type 4 – Full converter interface
generator
Plant
Feeders
PF control
capacitors
generator
Slip power
as heat loss
Plant
Feeders
PF control
capacitors
ac
to
dc
generator
partial power
Plant
Feeders
ac
to
dc
dc
to
ac
generator
full power
Plant
Feeders
ac
to
dc
dc
to
ac

7. 7
GE Energy
What Really is a Doubly-Fed
Generator?
• Physically – the machine resembles an induction
generator
• Conceptually – it is like a variable speed,
“synchronous” generator with bus-fed excitation
• Functionally – it tends to operate more like a
current-regulated electronic converter
It is not just a type of “induction generator”;
operationally, there is little in common with an
induction generator

8. 8
GE Energy
Doubly-Fed Asynchronous Generators
• Machine is wound-rotor induction-type generator with 3-phase rotor winding
• AC “excitation” supplied to rotor
• Excitation supplied by a line-fed pulse-width modulated (PWM) converter
Wind Turbine
Generator
Side
Converter
Grid Side
Converter
Generator
AC/DC/AC Power Converter
To GridRotor
Stator
Generator
Side
Converter
Generator
Side
Converter
Grid Side
Converter
Generator
AC/DC/AC Power Converter
To GridRotor
Stator

9. 9
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DFG Frequency and Speed - 1
To Grid
Stator
60 Hz
Stator MMF rotates in
airgap at 60hz:
e.g. 1200 RPM

10. 10
GE Energy
DFG Frequency and Speed - 2
To Grid
Rotor
Rotor mechanically
rotates NOT at 60hz:
e.g. 1440 RPM ~
72hz

11. 11
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DFG Frequency and Speed - 3
Generator
Side
Converter
Grid Side
Converter
AC/DC/AC Power Converter
To GridRotor
“AC” excitation on rotor electrically
rotates FLUX NOT at 60hz, to
cancel DIFFERENCE between
mechancial speed of rotor and
electrical speed of stator flux:
e.g. -240 RPM ~
-12hz

12. 12
GE Energy
Generator
Side
Converter
Grid Side
Converter
Generator
AC/DC/AC Power Converter
To GridRotor
Stator
Generator
Side
Converter
Generator
Side
Converter
Grid Side
Converter
Generator
AC/DC/AC Power Converter
To GridRotor
Stator
DFG Frequency and Speed - 4
Net of Rotor
mechanical angular
speed and electrical
angular speed is
synchronous:
phasor diagram
looks like a
synchronous
machine
Ei
Vt

13. 13
GE Energy
• Wind plants have a greater impact on the grid
• Displace other generation, so wind needs to provide
its share of system support
• Continuity of wind generation contribution becomes
essential to grid reliability
• Behavior during disturbances needs to be
predictable
As Wind Power Plants increase in size and
number …
Plant Level and WTG Level Controls
enable FACTS-like behaviors

14. Not trip during faults and other system disturbances …
FACTs – like behavior…

15. 15
GE Energy
3-phase 18.5% retained voltage, 700ms fault:
P, Q (Mw,Mvar)
Reactive Power well
behaved: like a
STATCOM - supports
grid during voltage
depression
Field Test Results (2.5 unit)

16. Not trip during faults and other system disturbances …
ride through capability
Regulate plant voltage & reactive power
Provide reactive power all the time
FACTs – like behavior…

17. 17
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WTG Reactive Power Capability
• Full leading and lagging range over full power range
• Faster reactive response than synch. generator
• Capability of reactive compensation with no wind
• Behavior like a FACTS devices

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Voltage at POI
Wind Plant Power Output
Voltage & Reactive Power Controls
Actual measurements from a
162MW wind plant
Wind Plant Voltage
• Regulates Grid Voltage at
Point of Interconnection
• Minimizes Grid Voltage
Fluctuations Even Under
Varying Wind Conditions
• Regulates Total Wind Plant
Reactive Power through
Control of Individual
Turbines
Average Wind Speed
Voltage and Reactive Power Regulation
Like A Conventional Power Plant…with FACTS speed

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TurbinekVAR
75
Time (seconds)
1500
0
1500
TurbinekVAR
75
Time (seconds)
1500
0
1500
WindFREE Reactive Power
• Wind Turbine converter can
deliver reactive power
(kVAR) without wind (kW)
• Benefits weak grids and
systems with high wind
penetration
• Voltage support continues
without active power
generation…even following
trips
Active
Power
(zero)
Reactive
Power
Field Test Results (2.5 unit)
Reactive Power - even without wind:
A valuable option – An unreasonable requirement

20. Not trip during faults and other system disturbances …
ride through capability
Regulate plant voltage and reactive power
Provide reactive power all the time
Limits the amount and/or rate of change of power
from variations in wind speed … Ramp Rate Control
React to changes in grid frequency … Frequency Droop
FACTs – like behavior…

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0
5000
10000
15000
20000
25000
30000
35000
4:30:00
AM
4:38:00
AM
4:46:00
AM
4:54:00
AM
5:02:00
AM
5:10:00
AM
5:18:00
AM
5:26:00
AM
5:34:00
AM
5:42:00
AM
5:50:00
AM
5:58:00
AM
6:06:00
AM
6:14:00
AM
6:22:00
AM
6:30:00
AM
Time
PowerkW
0
5
10
15
20
Actual power
Wind speed
Curtailment and Ramp Rate Limits
(Example 30 MW Plant)
Ramp rate limit enforced
when curtailment is
released
Curtailed to 10
MW: regulation is
very tight

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Over-Frequency Droop Response
2% Frequency Increase
50% Power Reduction
Power(kW)
Frequency(Hz)
Function is Grid Friendly with Little Opportunity Cost:
A Reasonable Request
Initial Steady-State
Conditions:
• 25% power reduction
for 1% frequency
increase
Test:
• 2% frequency ramp-
up @ 0.125 Hz/sec
• 50% reduction in
plant watts with 2%
over-frequency
Equivalent Under-frequency Function has High
Opportunity Cost: To Be Used Sparingly

23. Not trip during faults and other system disturbances …
ride through capability
Regulate plant voltage and reactive power
Provide reactive power all the time
Limits the amount and/or rate of change of power
from variations in wind speed … Ramp Rate Control
React to changes in grid frequency … Frequency
Droop
Provide inertial response to large under-frequency
events
FACTs – like behavior…

24. 24
GE Energy
Why Inertial Response: System Needs
• Increasing Dependence on Wind Power
– Large Grids with Significant Penetration of Wind Power
• Modern variable speed wind turbine-generators do not
contribute to system inertia
• System inertia declines as wind generation displaces
synchronous generators (which are de-committed)
• Result is deeper frequency excursions for system
disturbances
• Increased risk of
– Under-frequency load shedding (UFLS)
– Cascading outages
Inertial response enabled by power electronics
enables CONTROLLED performance
(not just Park’s equations!)

25. 25
GE Energy
How does it work?
Wind
Electrical Torque is a function of:
• Converter Control
• Commands from Turbine Control
Mechanical Torque is a function of:
• Wind Speed
• Blade Pitch
• Blade Speed ( α Rotor Speed)
Converter controls used to increase electric power
during the initial stages of a significant downward
frequency event
Wind Turbine
f
rotor
P
rotor
f
net
P
stator
3 φ AC Windings
Converter
P rotor
F rotor
P conv
F network
Electrical Power Delivered to Grid
Wound-Rotor
Generator
Machine Terminals
Wind

26. 26
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58.0
58.5
59.0
59.5
60.0
60.5
0 10 20 30
Time (Seconds)
WindPlantPOIBusFrequency(Hz)
1000 MW Synchronous Machine
1000 MW Wind without WindINERTIA
1000 MW Wind with Simple WindINERTIA (Wind Speed above Rated)
1000 MW Wind with Simple WindINERTIA Model (Rated Wind Speed)
With Inertial Control
frequency excursion is
~21-23% better
An Example:
Range of possible
recovery
characteristics
Performance is a function of wind and other conditions: not perfectly
deterministic like synchronous machine inertial response
0
300
600
900
1200
1500
1800
0 10 20 30 40 50 60 70 80
Time (Seconds)
Power(kW)
8 m/s Avg Meas 10 m/s Avg Meas 14 m/s Avg Meas
Field Tests

27. Transient Stability
Combination of Controls and Power Electronics
of Double Fed Machines radically changes
transient stability behavior:
FACTS-like synchronization…

28. 28
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Transient Stability
DFAG wind plants are more stable than conventional
synchronous generators.

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Transient Stability
In fact, wind plant will survive some disturbances that
trip conventional synchronous generators.

30. GE Energy
Thanks