1. Wind Turbines Technology
Cataldo Pignatale
Product Support Manager
Vestas Italia S.r.l.
Desire-Net Project

2. Session Contents
Aim: at the end of this session participants will have an overview of the wind turbine generators technologies developed over the years and implemented on the modern wind turbines
Duration: 35-40min

3. Agenda Wind turbines characteristics Control of power
Type of generators
Connection to grid
Control systems
Grid integration of wind trubines
Construction technologies of a modern wind turbine
This is the programme of today.

4. Wind turbines characteristics

5. Wind Turbine Generator
Definition: Machine capable to convert the kinetic energy of a wind tube into electrical energy.
“Betz' law’’’: less than 16/27 (or 59%) of the kinetic energy in the wind can be converted to mechanical energy using a wind turbine (Betz' law was first formulated by the German Physicist Albert Betz in 1919)
Question: How much of the kinetic energy can be converted into electrical energy?
His book "Wind-Energie" published in 1926 gives a good account of the knowledge of wind energy and wind turbines at that moment. It is quite surprising that one can make such a sweeping, general statement which applies to any wind turbine with a disc-like rotor.

6. Main parts of a modern wind turbine
Blade
Hub
Nacelle
Tower
The central part of the turbine is called nacelle and it contains generator, gearbox and other machinery.
The Nacelle is bolted together with the tower which again is bolted to the foundation. The assembly of hub and blades is called the rotor and is bolted to the main shaft or gearbox in the nacelle.
The tower consists of two or more sections which are assembled by bolted flange connections.
Foundation

7. Wind Turbines Characteristics
Rotor axis: horizontal, vertical;
Alignment to the wind: upwind, downwind;
Alignment to the wind: active (forced) or passive (free) yawing system;
Number of blades: even, odd; 3, 2, 1;
Control of power: pitch, stall, active stall, yaw;
Rotation transmission: with or without gearbox;
Type of generator: synchronous, asynchronous;
Grid connection: direct, indirect;
With gerabox
Horizontal axis rotor
Upwind turbine
Vertical axis rotor
Downwind turbine
Without gearbox
3 blades
2 blades
1 blade
Active yaw mechanism
Free yaw mechanism
Pitch control
Vertical or Horizontal Axis
The only vertical axis turbine which has ever been manufactured commercially at any volume is the Darrieus machine, named after the French engineer Georges Darrieus who patented the design in 1931.
The basic theoretical advantages of a vertical axis machine are 1) you may place the generator, gearbox etc. on the ground, and you may not need a tower for the machine. 2) you do not need a yaw mechanism to turn the rotor against the wind. The basic disadvantages are 1) Wind speeds are very low close to ground level, so although you may save a tower, your wind speeds will be very low on the lower part of your rotor. 2) The overall efficiency of the vertical axis machines is not impressive. 3) The machine is not self-starting (e.g. a Darrieus machine will need a "push" before it starts. This is only a minor inconvenience for a grid connected turbine, however, since you may use the generator as a motor drawing current from the grid to to start the machine). 4) The machine may need guy wires to hold it up, but guy wires are impractical in heavily farmed areas. 5) Replacing the main bearing for the rotor necessitates removing the rotor on both a horizontal and a vertical axis machine. In the case of the latter, it means tearing the whole machine down. (That is why EOLE 4 in the picture is standing idle).
Upwind or Downwind
Upwind machines have the rotor facing the wind. The basic advantage of upwind designs is that one avoids the wind shade behind the tower. On the other hand, there is also some wind shade in front of the tower, i.e. the wind starts bending away from the tower before it reaches the tower itself, even if the tower is round and smooth. Therefore, each time the rotor passes the tower, the power from the wind turbine drops slightly.
Downwind machines have the rotor placed on the lee side of the tower. They have the theoretical advantage that they may be built without a yaw mechanism, if the rotor and nacelle have a suitable design that makes the nacelle follow the wind passively.
Active or Passive Yaw Mechanism
The wind turbine is said to have a yaw error, if the rotor is not perpendicular to the wind. A yaw error implies that a lower share of the energy in the wind will be running through the rotor area. (The share will drop to the cosine of the yaw error, for those of you who know math).
Number of Blades
Why Not an Even Number of Blades? Modern wind turbine engineers avoid building large machines with an even number of rotor blades. The most important reason is the stability of the turbine. A rotor with an odd number of rotor blades (and at least three blades) can be considered to be similar to a disc when calculating the dynamic properties of the machine. A rotor with an even number of blades will give stability problems for a machine with a stiff structure. The reason is that at the very moment when the uppermost blade bends backwards, because it gets the maximum power from the wind, the lowermost blade passes into the wind shade in front of the tower.
The Danish Three-Bladed Concept Most modern wind turbines are three-bladed designs with the rotor position maintained upwind (on the windy side of the tower) using electrical motors in their yaw mechanism. This design is usually called the classical Danish concept, and tends to be a standard against which other concepts are evaluated. The vast majority of the turbines sold in world markets have this design
Two-Bladed (Teetering) Concept Two-bladed wind turbine designs have the advantage of saving the cost of one rotor blade and its weight, of course. However, they tend to have difficulty in penetrating the market, partly because they require higher rotational speed to yield the same energy output. This is a disadvantage both in regard to noise and visual intrusion. Lately, several traditional manufacturers of two-bladed machines have switched to three-bladed designs. Two- and one-bladed machines require a more complex design with a hinged (teetering hub) rotor as shown in the picture, i.e. the rotor has to be able to tilt in order to avoid too heavy shocks to the turbine when a rotor blades passes the tower. The rotor is therefore fitted onto a shaft which is perpendicular to the main shaft, and which rotates along with the main shaft. This arrangement may require additional shock absorbers to prevent the rotor blade from hitting the tower.
One-Bladed Concept Yes, one-bladed wind turbines do exist, and indeed, they save the cost of another rotor blade! If anything can be built, engineers will do it. One-bladed wind turbines are not very widespread commercially, however, because the same problems that are mentioned under the two-bladed design apply to an even larger extent to one-bladed machines. In addition to higher rotational speed, and the noise and visual intrusion problems, they require a counterweight to be placed on the other side of the hub from the rotor blade in order to balance the rotor. This obviously negates the savings on weight compared to a two-bladed design.

8. Control of power

9. Control of power Reducing the power at high windspeed
At high wind the power is reduced by pitching the blades. This can be done in two ways.
Reducing the lift and over speeding called Pitch variable speed
Reducing the lift by generating stall
Wind attack point
Flow on upper and lower surface equal  no lift
At rated power the pitch variable speed turbines turn the blades clockwise when seen from the tip as on the figure. This rotation moves the point of attack and makes the way above and below the profile more even and hereby the lift and power through the gearbox. When a wind gust hit the turbine the rotor will speed up and as the rotational speed increase the point of attack moves and reduce the lift.
The variable speed pitch turbines is the most common on the market and in the program today. The brand names for the control system is VCS (Vestas Control System) and opti speed.
An other Principle of reducing the lift is to let stall occur. If the blades are fixed, stall will occur automatically when the wind speed reach rated power and the drive train will not be overloaded. In order to stop the turbine it is needed to turn the blades or at least the tips. Many of the old turbines are passive stall turbines with fixed blades and tips for braking, but they are not produced much more as they are sensible to the air density and dirt on the blades.
In the programme we have active stall turbines which has turntable blades use for stopping and optimizing the production.
Wind attack point

10. Control of power Pitching
Low wind High wind Stop
Pitch variable speed and optislip
Passive stall
This diagram show how the blades are pitched for the three basic types of turbines depending on the situation. The pitch variable turbines are turned clock wise to reduce the power and the angels are relatively large. The passive stall turbine does not pitch the blades at all. Tip brakes are activated to stop the turbine.
The active stall turbine pitch the blades a few degrees to optimize production and pitch the blade 90° anti clock wise to stop
Active stall

11. Control of power Wind Power and Power Curves
Pitch variable speed
Active stall
Rated power
Passive stall
Max Power = ½ · A · v3 ·  · Cp
If you look at the energy in the wind it grow as the wind speed in third power. At low wind the wind turbine have to be as effective as possible, but when the rated power is reached the rotor must be in efficient in order to get rid of the power before it reach the gearbox. The gearbox and the rest of the drive train is designed to rated power only to reduce the costs. The power curve for a passive stall turbine will fall after rated power has been reached and that has to do with the iso power curves shown earlier.
‘A’ is area
‘v’ is velocity (wind speed)
‘’ is air density
‘Cp’ power coefficient
m/s

12. Control of power Iso-power map wind speed and pitch angle
0 kW
500 kW
1000 kW
1500 kW
2000 kW
2500 kW
Wind speed m/s 5 25 15 10 20
-10
+10
+20
+30
Pitch angle (deg)
-20
― Stall control
― Pitch control
This diagram show how much the turbine is able to produce depending on the pitch angles and wind speed. Other blades and dimensions will have different diagrams but the principles will be the same.
The red and green lines show the pitch angle for a pitch regulated and a stall regulated turbine. Up to rated power the pitch angle is identical for the two different systems.
From this diagram you can see it is possible to limit the maximum power to 2 MW on a stall turbine just by setting the pitch angle to -4° without adjusting. To reach the same by pitching it is needed to pitch from 0 to +25° this means the demands to the pitch system is high in order to avoid power peaks when the wind is increasing. If the wind is increasing from 17 to 19 m/s and the pitch is not adjusted the power is increased from 2 to 3 MW which means 50 % overload.
72 m rotor 2MW turbine

13. Control of power Pitching mechanism
Electrical
Pinion
Blade turning gear
Battery bank
Many of our competitors use an electrical motor with a pinion and internal gearing in a system like the one we use in the yaw system shown later. The electrical pitch system eliminate the risk of a leaking hydraulic system. However it create new problems like keeping the battery backup system alive and wear of the teeth on a 20 years life time. The hub here is used on a NM 92 turbine.
On all the other Vestas turbines the pitching is done individually by one hydraulic pitch cylinder per blade. The cylinders are feed by high pressure hydraulic oil from the nacelle through the hollow main shaft.
Hydraulic

14. Type of generators

15. Type of generator Synchronous Asynchronous
Wind turbines which use synchronous generators normally use electromagnets in the rotor which are fed by direct current from the electrical grid. Since the grid supplies alternating current, they first have to convert alternating current to direct current before sending it into the coil windings around the electromagnets in the rotor. The rotor electromagnets are connected to the current by using brushes and slip rings on the axle (shaft) of the generator.
Most wind turbines in the world use a so-called three phase asynchronous (cage wound) generator, also called an induction generator to generate alternating current. This type of generator is not widely used outside the wind turbine industry, and in small hydropower units, but the world has a lot of experience in dealing with it anyway:
Synchronous
Asynchronous

16. Type of generator Fixed speed asynchronous generator
50 Hz
rpm
1000
+ kW
(generator)
- kW
(motor)
When the generator like the one shown is rotated at a speed slightly above 1000 rpm it will generate power. If it is rotating slightly below 1000 rpm it will act as a motor. A generator like this can operate with variation in rotational speed of approx. +/- 1% When the rotor run up to 1 % faster than the stator it is called slip and the higher the slip is the larger is the generated power.
6-poled stator
Rotational speed
60 x frequency
number of pole pairs
rpm =

17. Type of generator Variable speed asynchronous generators
50 Hz
Stator field = 1000 rpm
Rotor mechanically = 1100 rpm DC AC AC DC
For the variable speed turbines it is not enough to have a speed variation of 1%. It is possible to increase the variation window by manipulating the frequency of the rotor. By changing the frequency on the rotor by -100 rpm to the rotational speed of the rotor field, the mechanical rotation speed of the rotor is still 1100 rpm, but the rotor field now rotates with 1000 rpm and is synchronous with the stator field.
The manipulation is done by rotating the magnets in the rotor as it rotates. This can be done optically in opti speed or electrically by slip rings for the double feed systems VCS. The optical solution is not able to transport energy out from the rotor and the energy generated in the rotor is wasted.
The system which manipulate the frequency of the rotor in the VCS system is called a frequency converter. On the US market GE has a patent on using a frequency converter to bring energy from the rotor to the grid making it difficult for Vestas and other suppliers to deliver turbines on the US market.

18. Connection to the grid

19. Connection to grid Direct
PCC
Grid frequency AC
Grid frequency AC

20. Connection to grid Indirect
Rectifier
Inverter
PCC
Variable frequency AC
(e.g. from synchronous generator) DC
Irregular switched AC
Grid frequency AC
Advantages of Indirect Grid Connection: Variable Speed The advantage of indirect grid connection is that it is possible to run the wind turbine at variable speed. The primary advantage is that gusts of wind can be allowed to make the rotor turn faster, thus storing part of the excess energy as rotational energy until the gust is over. Obviously, this requires an intelligent control strategy, since we have to be able to differentiate between gusts and higher wind speed in general. Thus it is possible to reduce the peak torque (reducing wear on the gearbox and generator), and we may also reduce the fatigue loads on the tower and rotor blades. The secondary advantage is that with power electronics one may control reactive power (i.e. the phase shifting of current relative to voltage in the AC grid), so as to improve the power quality in the electrical grid. This may be useful, particularly if a turbine is running on a weak electrical grid. Theoretically, variable speed may also give a slight advantage in terms of annual production, since it is possible to run the machine at an optimal rotational speed, depending on the wind speed. From an economic point of view that advantage is so small, however, that it is hardly worth mentioning.
Disadvantages of Indirect Grid Connection The basic disadvantage of indirect grid connection is cost. As we just learned, the turbine will need a rectifier and two inverters, one to control the stator current, and another to generate the output current. Presently, it seems that the cost of power electronics exceeds the gains to be made in building lighter turbines, but that may change as the cost of power electronics decreases. Looking at operating statistics from wind turbines using power electronics (published by the the German ISET Institute), it also seems that availability rates for these machines tend to be somewhat lower than conventional machines, due to failures in the power electronics. Other disadvantages are the energy lost in the AC-DC-AC conversion process, and the fact that power electronics may introduce harmonic distortion of the alternating current in the electrical grid, thus reducing power quality. The problem of harmonic distortion arises because the filtering process mentioned above is not perfect, and it may leave some "overtones" (multiples of the grid frequency) in the output current.

21. Control systems

22. Control systems Fixed speed
Getriebe 1:50
Parking
brake
Rotor
bearing
Bypass
contactor
Soft start
equipment
WTG
control
Asynchronous generator
kV, f = 50 Hz/
,5 kV, f = 60 Hz
Step-up
transformer HV
switchgear
ABB drawing
Passive Stall
Gearbox
Generator AC
f = constant
n = costant
The fixed speed turbine use a asynchronous generator connected directly to the grid. Depending on the number of poles in the generator the speed will typically be 1000 or 1500 rpm for 50Hz conditions. The generator is fixing the speed of the turbine and the blades will stall limiting the power. If the grid disappears the turbine will accelerate and it have to be stopped by using the blades or blade tips.

23. Control systems Fixed speed
Getriebe 1:50
Parking
brake
Rotor
bearing
Bypass
contactor
Soft start
equipment
WTG
control
Asynchronous generator
Step-up
transformer HV
switchgear
ABB drawing
Active Stall, Pitch Control
Gearbox
Generator AC
f = constant
n = costant
Pitch
drive
kV, f = 50 Hz/
,5 kV, f = 60 Hz
The fixed speed turbine use a asynchronous generator connected directly to the grid. Depending on the number of poles in the generator the speed will typically be 1000 or 1500 rpm for 50Hz conditions. The generator is fixing the speed of the turbine and the blades will stall limiting the power. If the grid disappears the turbine will accelerate and it have to be stopped by using the blades or blade tips.

24. Control systems Semi-variable speed
ABB drawing
Variable slip, pitch control
Getriebe 1:50
Parking
brake
Rotor
bearing
Bypass
contactor
Soft start
equipment
WTG
control
Asynchronous generator
Step-up
transformer HV
switchgear
Gearbox
Generator AC
f = constant
n = semi-variable
Pitch
drive
RCC
unit
HEAT
kV, f = 50 Hz/
,5 kV, f = 60 Hz

25. Control system Variable speed
ABB drawing
Variable speed control DFIG (doubly fed induction generator)
Getriebe 1:50
Parking
brake
Rotor
bearing
WTG
control
Doubly-fed
asynchronous generator
Step-up
transformer HV
switchgear
Gearbox
Generator AC
f = constant
n = variable
Pitch
drive
side
converter
Grid
Converter
kV, f = 50 Hz/
,5 kV, f = 60 Hz

26. Control system Variable speed
ABB drawing
Variable speed control with full scale converter
Getriebe 1:50
Parking
brake
Rotor
bearing
WTG
control
Step-up
transformer HV
switchgear
Gearbox
Generator AC
f = variable
n = variable
Pitch
drive
Converter
kV, f = 50 Hz/
,5 kV, f = 60 Hz
Asynchronous or synchrounous generator

27. Control system Generator layout
Pitch/Stall/Active stall
Semi-variable speed
Stator
Rotor
Grid
IGBT
Grid
Capacitor battery
Stator
Rotor
1-2% slip
1-10 % slip
Variable speed (DFIG)
Variable speed, full scale converter
Grid DC Ac dc
Stator
Rotor
Grid
Stator
Rotor DC Ac dc

28. Grid integration of wind turbines

29. Grid integration of wind turbines Electric power path to consumers
Power station
400,000V
20,000V
Transformer station
400/ 230 V
Consumer
150,000V
Transformer station
Transformer station
20,000V

30. Grid integration of wind turbines Medium and high voltage components
Generator
Main contactors
Transformer
The power produced in the generator is going through the main contactors to the transformer. In the transformer the voltage is transformed from the generator voltage which typically is 690 or 1000 V. The other side of the transformer has the grid voltage which typically is 20 kV or more. Between the transformer and the grid the switchgear act as a heavy duty switch. The high voltage makes it difficult to do the switching.
Switchgear
Grid

31. Grid integration of wind turbines Step-up transformer location
External housing
Nacelle housing
Inside tower housing

32. Grid integration of wind turbines Connection of wind turbines

33. Grid integration of wind turbines
The wind turbines operate as a part of an integrated power system with other production sources and consumers. Therefore there is a mutual influence between the wind turbines and the grid.
The following issues have to be considered:
Layout of grid-connecting infrastructure
Power quality assessment
Electrical system stability issues

34. Grid integration of wind turbines Power quality assessment
Operation of wind turbine can be disturbed if following grid parameter are not within defined limits:
Voltage
Frequency
Voltage unbalance
Harmonics level
Wind turbine connection shall not reduce existing power quality on the grid

35. Grid integration of wind turbines Parameters relevant for correct operation of wind turbines
Voltage limits:
Regime limits
Slow transient limits
Frequency limits:
Normal operation limits
Admitted transient limits
Voltage unbalance:
Admitted operational limits
Harmonics level:
Recommended maximum value: As defined in EN 50160

36. Grid integration of wind turbines Possible negative impacts of WT to the power quality on electrical grid
Wind turbines can cause the following negative impact on the grid:
Stationary voltage increase
High in-rush current
Flicker
Harmonics and inter-harmonics
Generally, the wind turbines´ impact on the grid depends on:
Wind turbines characteristics
The grid characteristics at the connection point (PCC)
Strong grids can accept more wind turbine without negative consequences on power quality.
Weak grids can accept limited number of wind turbines, or the grid has to be reinforced.

37. Grid integration of wind turbines Flicker
Flicker describes the effects of rapid voltage variations on electrical light. The flicker level can be measured with an instrument called flicker-meter.
Flicker during continuous operation
Flicker due to generator switching
Limits are defined at PCC and global effect has to be calculated as aggregated contribution of all the installed wind turbines.
Wind turbine´s performances concerning flicker emission are characterised by:
flicker coeficient cf
flicker step factor kf

38. Grid integration of wind turbines Harmonics and inter-harmonics
Voltage deviations from the perfect sinus shaped 50 Hz curve result in harmonics.
Harmonics are not wanted on the grid because they cause increased losses and in serious cases it may lead to an overloading of the capacitors, trans-formers and electrical appliances as well as disturbances of communication systems and control equipment.
It is differed between:
Even harmonics e.g. 100, 200, 300… Hz
Odd harmonics e.g. 150, 250, 350,550 … Hz
Inter-armonics (50 multiplied with decimal numbers) e.g. 165 Hz, 2525 Hz etc.

39. Grid integration of wind turbines Standards and recommendations
All units that deliver electrical power to electrical system shall respect relevant power quality standards.
The most relevant documents for wind turbines are:
IEC standard:
“Power quality requirements for grid connected wind turbines”
IEC standard:
“ EMC limits. Limitation of emissions of harmonic currents for equipment connected to medium and high voltage power supply systems”
Local requirements

40. Grid integration of wind turbines System stability issue
Large wind farms can influence not only locally grid but also a large part of whole power supply system
Dynamic grid stability may be a limiting factor to the grid connection of large wind farms
Grid stability analyses are needed
Data for modeling or models of Wind Turbines may be requested
Each country can issue local grid code requirements that have to be duly considered in designing wind parks.
Fulfilment of grid code requirements might require installation of additional equipments (capacitor banks, static VAR compensators, dynamic VAR compensators).

41. Coonstruction tecnologies of a modern wind turbine

42. Main parts of a modern wind turbine
Blade
Hub
Nacelle
Tower
The central part of the turbine is called nacelle and it contains generator, gearbox and other machinery.
The Nacelle is bolted together with the tower which again is bolted to the foundation. The assembly of hub and blades is called the rotor and is bolted to the main shaft or gearbox in the nacelle.
The tower consists of two or more sections which are assembled by bolted flange connections.
Foundation

43. Onshore foundation Gravity concrete foundation Rock anchor foundation
The typical foundation is a concrete slab foundation cast on the site in due time before the turbine is erected. The foundation will normally be covered by earth and only the embedment will be visible. The embedment is the lowest part of the tower which cast into the foundation and integrated with the reinforcement.
Rock anchor foundation

44. Offshore foundation Monopile Tripod Gravity Floating
The most common foundation at least for Vestas is the monopile. A heavy steel pipe is hammered approximately 20 meters into the seabed and a transition piece is mounted on top of it. The costs for monopiles are relatively low compared to gravity foundations or tripod foundations but it requires low top head masses (nacelle and rotor weight) to avoid frequency problems. If the top head mass is to large it will require a very stiff tower and foundation which means the tower and monopile need to a very large diameter or a more stiff foundation.

45. The tower Tubular Steel plates are rolled and welded
Flanges at each section
Shot blasted and coated with paint
Lattice
Bars are prepared in factory and assembled on site
Bolted junctions
Hot galvanized steel
The tower is made from heavy steel plates which are rolled into conic subsections which are welded together to form a section. The sections are basically made as large as possible but still transportable. Each section has a flange in each end used for bolting it to the other sections and foundation. The tower is normally conic to comply with the loads in every level. The loads are mainly the torque from the wind trust on the rotor. This torque is largest at the bottom and is then reduced gradually with the height.

46. Blade concepts Supporting carbon spar and glass fiber airfoil shells
Wood carbon strong shell technology
Today we have two blade concepts. One is the traditional Vestas solution with a supporting spar bonded in between two thin airfoil shells. The other one is the old Aero laminates concept where the shells are strong and kept together by thin walls. This concept use wood and carbon as strengthening materials and will be concept used in the future.

47. Supporting carbon spar concept
The supporting spar with a rectangular section
The airfoil shells with sandwich construction at the rear
This concept use a supporting spar which is hollow and mainly rectangular profile made from carbon and epoxy. This spar is strong and carry most of the loads. This is bonded in between two airfoil shells made from glass fiber and epoxy. At the rear of the shell profile a foam sandwich construction is used to make the shell stiff.

48. Wood carbon concept
Plywood and carbon rods are used where high strength is needed
Balsa or foam is used where only stiffness is needed
In the wood carbon concept the strength is obtained by using plywood combined with rods of carbon. The advantage of this combination is that plywood and carbon has more similar elasticity than carbon and glass and can be used side by side. Light materials like foam and balsa wood is used in a sandwich construction in the areas where the high strength is not needed but only stiffness of the large surfaces like in the tail end is required.
The blade is produced as two separate shells and glued together in the front, in the tail and in the middle to create a stiff structure.

49. Main components in the nacelle
Main bearings/Main shaft
Anemometer
Hub
Pitch system
Gearbox
This is a typical layout of a Vestas V80/2MW turbine
The pitch system turns the blades
The hub is the connection between main shaft and blades.
Main shaft is the connection between hub and gearbox.
Main bearings takes the loads from the rotor.
Gearbox maintains the correct rotational speed.
The yaw system turns the nacelle to keep the rotor in the correct angle compared to the wind direction.
The hydraulic station powers the pitch system and the brake system.
The coupling connects the gearbox and the generator.
The disc brake is used for ”parking” the rotor.
The anemometer measure the wind speed and direction
The following slides will describe the components in details.
Hydraulic station
Yaw system
Generator
Disc brake
Coupling

50. Questions?