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Automatic control
(of wind turbines and wind farms in the power system)
- WIND TURBINE CONTROLS - WIND FARM CONTROL - POWER SYSTEM INTEGRATION
MARE WINT Opening Lectures NTNU, 4. September 2013
Kjetil Uhlen NTNU
2
Overview TECHNOLOGY
Review on relevant wind turbine technologies and configurations (energy conversion principles)
CONTROL
Overview of various control systems and the purpose of those: Power control (speed and torque) Voltage control (and reactive power compensation) Other controls
SYSTEM INTEGRATION
Relevant issues on network integration and system operation System requirements and grid codes
3
Overview TECHNOLOGY
Review on relevant wind turbine technologies and configurations (energy conversion principles)
CONTROL
Overview of various control systems and the purpose of those: Power control (speed and torque) Voltage control (and reactive power compensation) Other controls
SYSTEM INTEGRATION
Relevant issues on network integration and system operation System requirements and grid codes
4
Technology – Wind turbines/power plants
Horizontal axis (three-bladed) wind turbines for electric power generation Electro-mechanical configurations Automatic controls
Foto: Hydro
5
Wind turbine power conversion
Type BVariable slip
Type CDoubly-fed IG
Gear box IG
Control system
Gear box IG
Control system
Gear box
Control system
Gear box
Control system
Type DFull converter (IG/PM/SG)
Gear box G
Control system ~~
Gear box DFIG
Controlsystem ~~
Gear box DFIG
Controlsystem ~~~~
Type AFixed speed
6
Relevant electro-mechanical configurations 1. Induction (asynchronous) generator + Passive control
(”stall-control”) 2. Induction (asynchronous) generator + Rotor angle control
(”pitch-control” or ”active stall”) 3. Induction (asynchronous) generator with variable slip +
Rotor angle control (”opti-slip”) 4. Doubly-fed induction generator (DFIG) + Rotor angle
control (”opti-speed”) 5. Induction generator with frequency converter + Rotor
angle control 6. Synchronous generator with frequency converter + Rotor
angle control
7
Gear-box
Pmvw
Pel
ωPowergridfn
β = const.
AGx
Gear-boxGear-box
Pmvw
Pel
ωPowergridfn
β = const.
AGxx
Gear-box
Pmvw
Pel
ωPowergridfn
β
AGx
Gear-boxGear-box
Pmvw
Pel
ωPowergridfn
β
AGx
Gear-box
vwPowergridfn
β
AG
R
ω fn (1 + s)x
Gear-boxGear-box
vwPowergridfn
β
AG
R
ω fn (1 + s)ω fn (1 + s)xx
Powergrid
fn
Gear-box
f1
vw
β
AG
ω f1
Powergrid
fn
Gear-boxGear-box
f1
vw
β
vw
β
AG
ω f1 ω f1
f1
vw
β
Powergrid
fn
ω f1
uf
f1
vw
β
Powergrid
fn
ω f1 ω f1
uf
1)
2)
3)
4)
5)
6)
Power grid
fn
Gear-box
vw
β
DFIG
ω fn- fr fr
Power grid
fn
Gear-boxGear-box
vw
β
vw
β
DFIG
ω fn- fr ω fn- fr frfr
8
Electro-mechanical configurations of major wind turbine manufacturers Vestas (DK)
Originally referred to as “Opti-slip” and “Opti-speed” NTE: Vikna og Hundhammerfjellet
SIEMENS (DK) Traditional AG/active stall (recently with frequency converter) Statkraft: Smøla (150 MW), Hitra (55 MW) and Kjøllefjord
Enercon (DE) Multi-pole synchronous generator, direct drive TE: Valsneset and Bessakerfjellet
Nordex (DE) DFIG Havøygavlen: 16 x 2.5 MW
GE wind (USA) DFIG and frequency converter
9
How they look..
Schematic of Siemens 2.3 MW Her mangler det et bilde av en ”konvensjonell” vindturbingenerator
Schematic of Vestas V80-2MW
10
How they look..
Stator in Enercon’s 4.5 MW
Her mangler det et bilde av en ”konvensjonell” vindturbingenerator
Schematic of Enercon 2 MW
11
Overview TECHNOLOGY
Review on relevant wind turbine technologies and configurations (energy conversion principles)
CONTROL
Overview of various control systems and the purpose of those: Power control (speed and torque) Voltage control (and reactive power compensation) Other controls
SYSTEM INTEGRATION
Relevant issues on network integration and system operation System requirements and grid codes
12
What is special about wind power?
The wind power plant lacks a controllable ”energy storage” on the input Generation planning becomes more difficult
Grid G Energy input: -Fuel -Reservoir
Active power Frequency
Voltage
Reactive power
Grid
vw G Energi input:
-Wind
Active power Frequency
Voltage
Reactive power
13
What is special about wind power?
More degrees of freedom with power electronics and variable speed operation
Grid G Energy input: -Fuel -Reservoir
Active power Frequency
Voltage
Reactive power
Grid
vw G Energi input:
-Wind
Active power
Speed Torque
Voltage Reactive power
14
Three main control objectives
Optimization of power output and energy yield To track the optimal power setpoint.
Minimize mechanical load and wear Active damping of (mechanical) resonant modes.
Contribute to power system control Power-frequency and voltage control
Wind turbine controls Control objectives (more specifically):
Optimization of power output and energy yield Power output limitation Minimization of power fluctuations and mechanical
loads, due to: Rapid variations in windspeed (and waves) Structural modes, 3P-variations, etc. (hydrodynamic forces) (Disturbances from the grid)
Maintain voltage quality requirements Damping of fast power variations (impacting grid voltage). Reduce voltage flicker Reactive power support / voltage control
(optionally) active power-frequency control support
16
Other control and monitoring functions Protections and breaking systems
Start and stop functions
Operational control and monitoring (communication
with control centre)
Mostly relevant in autonomous systems or weak grids: Frequency control Energy storage
Other controls / protections
Start/stop procedures (normal) Grid synchronization and breakers ”Soft starter”
(Start)/stop procedures (emergency) Protection: Overspeed, low voltage (fault ride through) Safety: brakes and breaking system (mechanical, fail safe
pitch control, braking resistor / dump load, electrical breakers)
“Storm” control
Yaw control
(Additional) Wind farm controls
Control of power output from wind farm. Setpoint control within the available power
range (for congestion management and balancing purposes)
Power optimization
Frequency and voltage control Control functionality enabling wind farms to
contribute with primary active and reactive reserves
19
Development trends Increasing demand on controllability:
To reduce mechanical loads
Lighter components Cost reduction (cost/kWh)
To optimize wind turbine design and performance Cost reduction (cost/kWh)
To ensure voltage quality Eliminate local grid constraints
Increasing focus on system requirements (as specified in grid codes)
Wind farms are treated more and more as conventional power plants Must be able to offer system services (contribute to system frequency and voltage
control) Improve system stability and maintain system security
Floating turbines
Trend towards very large turbines
Low frequency tower oscillations Damping (stability) relies on control system
20
Summary control objectives
Load Mitigation
Power Quality
Electrical disturbances
(FRT)
Frequency control
(Primary)
Frequency control
(Secondary)
Voltage/ Reactive
power Control
Active Power
Forecast
Time
Impact
System wide
Local
Wind Turbine Level
Wind Farm Level
Power System Level
...........ms..............seconds ................ minutes ............hours..........
Wakes & Turbulence
Ref. Olimpo Anaya-Lara
22
Power and energy from wind
312 windwind p air rotorP C A vρ=
Turbine power:
www.windpower.org
1
2
3vv
=
Betz-Lanchester: Maximum efficiency for an ideal rotor: Cpmax= 0.59 if
Max efficiency for three-bladed wind turbines today is around Cpmax=0.5.
Max efficiency depends on rotor design and the number of blades in the rotor
23
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0 5 10 15 20Tip-speed ratio
Cp
-101510
"Pitch"-vinkel[grader]
Virkningsgradskurver, Cp (λ ,β )
24
Wind turbine controls Power control Possibilities depend on configuration (turbine and
power conversion system) Main principles for power control:
(Passive stall) Pitch control Variable speed
Through frequency converter Through induction generator and variable slip
Yaw control
25
Turbine power: PW = ½ Cp(λ ,β )ρ A vw
3 ,”Tip speed ratio” λ = ω r / vw - Speed - Pitch - Yaw
Gear- box Nett
PW vw
Pel
ω f1 f2
β
26
Power control “Pitch” versus “stall” and speed control
Power is a function of torque and speed: P = T · ω Turbine speed is determined by grid frequency, gear ratio and slip of
induction generator. ”STALL”: Passive torque regulation, determined by the turbine’s
aerodynamic properties. ”PITCH”: Active torque control through pitching of rotor blades
(applied for both optimization and power output limitation)
Gear- box
PW vw
Pel
ω Nett fn
β
AG
27
Induction motor Torque versus speed curves
TEL
ngen
Motor
Generator
ngen ≈ fn(1-s)
28
Power versus windspeed curves
0
20
40
60
80
100
120
0 5 10 15 20 25 30Wind speed (m/s)
Pow
er (%
)
Pitch controlled
Stall regulated
29
Power control “Pitch” versus “stall” and speed control
Source: Lubosny
www.windpower.org
30
Conventional pitch control (rated @ 1500 kW)
-15
-10
-5
0
5
10
15
20
25
0 5 10 15 20 25
Windspeed [m/s]
Pitc
h an
gle
[deg
rees
]
3000 kW2500 kW2000 kW1500 kW1000 kW 500 kW 0 kW
Power limitationOptimisation
31
Active stall control
-15
-10
-5
0
5
10
15
20
25
0 5 10 15 20 25
Windspeed [m/s]
Pitc
h an
gle
[deg
rees
]
3000 kW2500 kW2000 kW1500 kW1000 kW 500 kW 0 kW
Power limitationOptimisation
32
0
500
1000
1500
2000
2500
0 5 10 15 20 25
Windspeed [m/s]
Pow
er [k
W]
-101271025
"Pitch"-vinkel[grader]
-1
0 1
2
7 10 25 16 Pitch controlled Active stall control
33
10-2 10-1 100 10110-6
10-4
10-2
100
102
104
106
Frequency (Hz)
PSD
(kW
2 /Hz)
((m
/s)2 /H
z)PowerWind speed
Power spectral density from measurements
34
Power control Variable speed operation There are limitations to pitch control with regard to
actuator speed (bandwidth) and performance. Additional achievements by variable speed control:
Further optimization of efficiency (one more degree of freedom). Energy of rotor inertia can be exploited (short-term energy
storage). Enables faster and more precise control.
Implementation of variable speed control is possible by using: Induction generator with variable slip Doubly-fed induction generator (DFIG) Full-scale frequency converter (independent of generator type)
35
Optimization by variable speed control
[Ref: Heier]
Optimization by variable speed control Control regions
3max ,
3
1( )2wind p opt windP v C Av
K
ρ
ω
=
=
37
Induction generator with variable slip
Gear- box
vw
Grid fn
β
AG
R
ω fn (1 - s)
Grid
fn
Gir- box
vw
β
DFIG
ω fn- fr fr
Doubly-fed induction generator (DFIG) (with frequency converter controlling rotor voltage)
38
Induction motor Torque versus speed curves
TEL
ngen
39
Power balance of a DFIG
Super-synchronous speed:
Sub-synchronous speed:
DFIG
AC-DC-AC
Pmek Pel
DFIG
AC-DC-AC
Pmek Pel
Pr
Ps
Ps
Pr
Pr s = (ωr- ωN)/ωN
40
Frequency converter at part load
Full frequency converter
f1
vw
β
Grid
fn
ω f1
Grid
fn
Gear- box
f1
vw
β
AG
ω f1(1-s)
41
Wind turbine controls Voltage control
Possibilities depend on configuration (power conversion system and grid connection)
Main principles for voltage control: Synchronous generator with AVR (not widely used in WTs) Switched capacitor banks (Mechanically or thyristor based) Using power electronics (converter based)
(Internally) as part of the wind turbine’s power conversion system
(Externally) as separate solutions within the wind farm transformer station (point of common connection)
42
Synchronous generator with AVR (not a common solution)
Gear- box
vw
ω Grid Un SG
AVR
Gear- box
vw
ω Grid Un AG
Induction generator with switched capacitor banks (conventional solution)
x x
43
Full frequency converter
Converter based compensator (Statcom) Uset
vw Grid
Un
Grid Un
Gear- box
Uset
vw AG
44
Voltage control at point of common connection (using an SVC)
Gear- box
vw
ω Grid
Un AG
x x
Gear- box
vw
ω AG
SVC
: :
45
Voltage control at point of common connection (using Statcom)
Gear- box
vw
ω Grid
Un AG
Gear- box
vw
ω AG
: :
STATCOM
Xt
46
Control structures
Control objectives Choice of measurement signals Choice of control variables (actuators) Controller implementations – Choice of:
Control loops Control structures and tuning
47
Control structures for active and reactive power
Converter controls for variable speed turbines
Pitch control
General control structure (for active power)
fG
vw
β
fn
ω fG
Network
Pel
ω
Pref ω
ωmax Pitch controller
Power/torque controller
-
- Max power tracking
τG
ω
ω Pref
Max power tracking (for active power)
vw
fG
β
fn
ω fG
Network
Pel
ω
Pref
ωmax Pitch controller
Power/torque controller
-
- τG
ω
ω
Max power tracking
ω
Pref
ω
50
Rotor powerConverter
Grid
Gear
Inductionmachine
Transformer
PrimaryController
SecondaryController
Example: Overall control structure for DFIG (active and reactive power)
Pset
Uset
Un
Uset
fset αset Qset
Pel Udc
ω
α
vwind
Speed Power
51
Controllers and control loops in a DFIG-model
ir
vs
ac / dc dc / ac
C1 C2 Rotor position and
speed
Coordinate Transformation
Control of grid side converter
PWM PWM
Inverse Coordinate
Transformation
θr
rω
Tsp
+
refqri
qri
qrv
Torque to current
transformation (K’)
PI Controller
rω
dri
+
drv
PI Controller
Magnitude and angle of voltage
vector
Slip-frequency estimation
θs
V
vdc
Voltage or power factor
control Vref refdri
Control challenges Development trends Lighter turbines and offshore turbines:
More flexible constructions More dependent on control systems
for reduction of structural loads for stabilisation
Wind power impact on power system increasingly important Wind farms must contribute to power system frequency and
voltage control Must be able to ride through faults and provide support
during disturbances
Offshore floating turbines: Stabilisation of tower movements Drag forces:
Stabilisation of tower movements
fG
vw
β
fn
ω fG
Network
Pel
ω
Pref ω
ωmax Pitch controller
Power/torque controller
-
- Max power tracking
τG
ω
ω Pref
Additional measurements of position/speed/acceleration
?
55
Control structures for active active and reactive power
Breakingresistor
~
~=
=
Generator sideconverter
Generator
Grid sideconverter
DC link
AC gridShaft
Turbine
PIω βref
-ωmax
-
Ratelimit
Anglelimitτpitch
β
Gainscheduling
Pitch actuator
MId MId MIq MIq
β
56
...with active tower stabilisation
Breakingresistor
~
~=
=
Generator sideconverter
Generator
Grid sideconverter
DC link
AC gridShaft
Turbine
PIω βref
-ωmax
-
Ratelimit
Anglelimitτpitch
β
Gainscheduling
Pitch actuator
MId MId MIq MIq
β
ω BPPSSTSt α
Stabilisation and load reduction
fG
vw
β
fn
ω fG
Network
Pel
ω
Pref ω
ωmax Pitch controller
Power/torque controller
-
- Max power tracking
τG
ω
ω Pref
(Both high and low frequency dynamics) Additional measurements
58
Normal converter controls
f(ω) PIω Popt
PMId-
Converter interface & control
LP
id-
PI
PIMIq-
Converter interface & control
iq-
PI
Q
Qref
U
QU
Voltage droop
Speed / Active power
Voltage / Reactive power
59
Possibilities for active damping
PIMId-
Converter interface & control
id-
PI
UDC
UDCref
ω
-
BPPSS
Indirectly through modulation of DC
voltage
f(ω) PIω Popt
PMId-
Converter interface & control
LP
id-
PI
BPPSS
Through modulation of power control
Independent pitch control
fG
vw
β1
fn
ω fG
Network
Pel
ω
Pref ω
ωmax Pitch controller
Power/torque controller
-
- Max power tracking
τG
ω
ω Pref
β3 β2
Additional measurements
Independent pitch control Reduction of loads
f [Hz] Source: Bossanyi
1p. collective(dependent) pitch
Independent pitch + different control realisations
Fault ride through and power system support
fG
vw
β
fn
ω fG
Network
Pel
ω
Pref ω
ωmax Pitch controller
Power/torque/voltage controller
-
- Max power tracking
τG
ω
ω Pref
Additional measurements
U,f id,set
Power optimization Wakes and turbulence
( )1 1
max ( ) max ( )n turb n turb
i ii i
P t P t= =
>
∑ ∑Question:
Picture: © Vattenfall, Horns Rev 1 owned by Vattenfall. Photographer Christian Steiness
Fault ride through..
Source: ABB
65
Overview TECHNOLOGY
Review on relevant wind turbine technologies and configurations (energy conversion principles)
CONTROL
Overview of various control systems and the purpose of those: Power control (speed and torque) Voltage control (and reactive power compensation) Other controls
SYSTEM INTEGRATION
Relevant issues on network integration and system operation System requirements and grid codes
66
Large-scale wind power integration
SYSTEM REQUIREMENTS
Kjetil Uhlen NTNU
Problem areas and need of system requirements (”grid codes”) for grid
integration of large wind farms.
67
Content Problem areas in system operation
General overview (of system problems and control requirements)
System requirements (and examples illustrating relevant system problems)
Offshore grids
68
System operation – problem areas
Power balance (frequency control) Power system security (contingencies, congestions) Thermal limitations (Lines, cables, transformers,..) Voltage stability Stability (electro-mechanical)
Transient stability Damping (power oscillations)
Voltage quality Requirements on equipment
Control requirements Protection, limits on tolerance
etc.
69
System requirements - Motivation
That wind power plants will (or should) face similar requirements as conventional power plants regarding controllability and ability to provide relevant system services
Two key questions: What are the system performance requirements (technical issue)? Which system services should be provided by wind farms
(economic issue)?
70
”Grid codes” The aim is to ensure system control properties that are essential for
the reliability of supply, security in operation and power quality in the short end long term. General laws, regulations and agreements apply (”Entso-E Grid Codes”)
Specific grid codes for large scale wind power are being established in
Germany, Denmark, UK, Ireland, Spain, Sweden, .. Norway..
Process to harmonise in Europe through Entso-E
71
Grid codes for wind power .. Power control
Ability to control power output Frequency control (primary reserves) Start and stop, limits on power gradients,..
Frequency and voltage deviations Frequency and voltage limits where wind farms shall operate and when they shall stop
Reactive power and Voltage control Reactive compensation Control Requirements (Mvar-control, cosΦ-control, Voltage control, etc.) Voltage quality (Voltage variations, dips, flicker, harmonics, etc.)
Response to grid faults Stability requirements (transient) (Various types of faults)
Protection of the wind farm against grid faults Responsibility Tolerance.
Communication (between wind farm and grid operator, ..) Responsibility for providing information, operational data, etc.
Requirements regarding documentation, analysis, testing, etc.
Fault Ride Thourgh (FRT) requirements German FRT Grid Code requirements (BDEW) for MV
U/U c
t [ms]
100%
70%
45%
15%
0 150 700 1500 3000
Not disconnect
Disconnect
Examples of LVRT requirements
Fault Ride Thourgh (FRT) requirements
U/U c
t [ms]
100%
70%
45%
15%
0 150 700 1500 3000
Not disconnect
Disconnect
Examples of LVRT requirements
German FRT Grid Code requirements (BDEW) for MV Norwegian grid code (FIKS) for HV (> 220 kV)
1000
25%
Fault Ride Thourgh (FRT) requirements
U/U c
t [ms]
100%
70%
45%
15%
0 150 700 1500 3000
Disconnect
Examples of LVRT requirements
Not disconnect
German FRT Grid Code requirements (BDEW) for MV Norwegian grid code (FIKS) for HV ( >= 220 kV) Norwegian grid code (FIKS) for MV/HV (< 220 kV)
400 1000
75
Example illustrating problems related to congestion management and balancing control (frequency control) Frequency control reserves Congestion management Balancing services Reserves
Power capacity related problems
Example is from 8. January 2005
More than 1700 MW wind power discionnects due to severe storm in Southern Scandinavia
http://www.eltra.dk/composite-15751.htm
76
Western Denmark (Eltra)
Key counts of the power system of Eltra for the year 2003 (Source: Eltra)
MW GWh Central power plants 3,516 16,161 Decentralised CHP units 1,567 6,839 Decentralised wind turbines 2,374 4,363 Offshore wind farm Horns Rev A 160 Consumption 21,043 Maximum load 3,780 Minimum load 1,246 Capacity export to UCTE 1,200 Capacity import from UCTE 800 Capacity export to Nordel 1,560 Capacity import from Nordel 1,610
77
Elspot areas and transmission capacities
NO1
DK1
SE
NO2 FI
To Germany
DK2
950 MW 1000 MW
800 MW 1200 MW
78
8 January 2005
-1000
-750
-500
-250
0
250
500
750
1000
1250
1500
1750
2000
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
MW
h/h
Exchange DK1 -> NO1Balancing power (NO1)Windpower DK1
Source: ELTRA / NORDPOOL
79
Power and frequency control
Primary frequency control and primary reserves Relevant in systems with very large scale wind integration In parts of the grid where wind power is the dominating
Power control (secondary control)
Important for congestion management and to avoid too strong or too fast power variations
80
Power-frequency control Frequency droop and primary reserves
50.0
49.9
49.5
HZ
MW
R = 100 MW/Hz
Pa
10 MW min 15 MW
Pn
FR DR
R = 2Pn/δ δ = 6 % Pn = 300 MW
81
Wind farm power control (active and reactive power reserves)
Time
Pow
er
Set-point power
Available power
Frequency
Pow
er
droop
Voltage
Rea
ctiv
e po
wer
droop
Time
Reserve power
Available power
Pow
er
Main challenges in operation and control
Primary control: Less primary reserves if new generation provide less
frequency response Secondary control: More need for secondary reserves with more variable
generation Tertiary control: Benefits with larger control areas and exchange of
reserves.
New possibilities with an offshore Multi-terminal HVDC grid!
Offshore Multi-terminal HVDC
MTDC has the potential to fully integrate power markets between asynchronous areas.
Can be operated in a similar manner as ac grids.
With the dc voltage droop control, no need of fast communication between converter terminals.
Primary reserves can be traded between asynchronous areas (with frequency droop on the converter)
87
Summary
Basic wind power technology: Power versus windspeed Pitch control Impact of variable speed operation
Turbine- and generator technologies 6 different power conversion configurations
Control systems Power, torque and speed control Voltage control
Grid connection and system operation issues Problem areas, grid codes and network analysis
