Wind Turbine Control Methods & Strategies

8/10/2019 Wind Turbine Control Methods & Strategies

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PRACTICAL: 3WIND TURBINE CONTROL METHODS AND

STRATEGIES

1.1 INTRODUCTION:

Controlled power generation is an essential requirement for the efficient

working of any power plant, not only for the personnel but also for the safety of the

generating plant itself and the environment. Wind power plants (WPPs) are not an

exception in this regard. Power control means that the WPP limits the share of theavailable power in the wind using both the aerodynamic control and the electrical

generator control. The major goal of wind power control strategies discussed in this

chapter is to maximise the energy production and at the same time keep the WPP

operation within safe limits.

1.2 POWER CONTROL CLASSIFICATION:

There are two major strategies for the power control of WPPs. Although

following two control strategies are independent, they can be closely interrelated.

Aerodynamic control.

Power electronic control.

Aerodynamic control method of limiting the power from the wind is generally

adapted when the power available in the wind is higher than the power for which the

WPP has been designed. This method control is also called as principle of positive feed

forward control. The acceleration and deceleration of the rotor blades have to be

controlled to limit the electrical load on the generator and the mechanical stresses on

the rotor blades, the hub (to some extent) and the rest of the drive train as well.

However, continuous control of the rotor speed by pitching of the blades leads to

continuous fluctuation of the power output to the grid which is not desirable. If a quick

variation of speed is possible by this control when there is large difference between

the input power and output power, than the stress on the blades is increased on

account of the large torque needed.

Power electronic control is the ability of WPP to adapt its electrical generator

rotor speed during normal energy production operation. This control depends on the

type of electric generator use in the WPP and is called principle of negative feedback


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control. This method of speed control technique offers a smooth operation as it doesnot involve any mechanical action. Power electronic control is applicable when there is

power electronic convertor (PEC) interface between the electric generator and local

grid.

Control strategies are different for constant speed and variable speed WPPs.

There controls in coordination with aerodynamic and power electronic control are

depicted in figure. It is seen that the control strategies for the WPPs are different for

condition. A when the wind speed is

By the PEC the value of??can be controlled. WPP power control algorithms for

variable speed WPPs at above rated wind speed conditions are for rotor speedregulations by pitch actuation as well as foe electrical power production by generator

torque control. Rotor speed regulation is achieved by the feedback of the proportional

derivative (PD) control and pitch angle is achieved by the feedback of the proportional

integration (PI)control.

1.3 INEGRATED AERODYNAMIC AND ELECTRIC CONTROL STRATEGIES:

For an economical WPP design, the maximum performance of the generator and

gearbox needs to be limited to an appropriate level for the overall WPPs safe

operating environment. The ideal situation for the wind turbine rotor is to be able to

extract as much power as possible from the wind till the rated power of the electric

generator is reached and then limit the power extraction at that rated level even if the

wind speed increases further. For this to happen the WPPs are designed to work at

maximum aerodynamic efficiency between cut-in speed and rated speed in region 2

(see Figure 1.3). For higher than rated wind speeds in region 3 but below cut-out

speed, the WPP is controlled by stall, pitch or active stall action to maintain the

loading of electrical generating rating. The following control strategies are the

combinations of aerodynamic and electric control which render it possible to manage

the WPP functionally for different strategies of operations.

Constant speed generator fixed pitch(CSG-FP)

Constant speed generator variable pitch (CSG-VP)

Variable speed generator fixed pitch (VSG-FP)

Variable speed generator variable pitch (VSG-VP)

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Figure 1.3: Integrated Aerodynamics and Electric Control of WPPs: It can be seen that

out of the different WPP topologies, a variable speed WPP combined with pitch

control is the best strategy because even at lower than rated wind speeds, more

power can be captured.

1.3.1 Constant speed Generator fixed pitch (CSG-FP) configuration:

In this strategy, the electrical generator speed is constant and rotor blades are

permanently fixed at an angle of attack for the most optimum blade position based

on the wind data of that particular site. In this topology where the induction generator

is used in the WPP, the stator is directly connected to the electric grid causing the

generator speed to be locked to the electrical power line frequency to keep the RPM

fixed. It runs at approximately constant speed even in high winds producing more

power yet achieving this without any change to the rotor geometry. The grid behaves

like a large flywheel holding the speed of the WPP nearly constant, irrespective of the

changes in the wind speed.

1.3.2 Constant speed Generator variable pitch (CSG-VP)

configuration:

In this strategy, the electrical generator speed is constant but the blade pitch

angles are varied continuously (see Figure 1.3) based on the wind speed. However, it

can be noted that the feathering of the blade takes significant amount of control

design but stalling action also increases the unwanted thrust force, as the wind speed

increases.

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Below the rated wind speed, this type of WPP has a near optimum efficiency inthe region 2, as seen by the solid line in the power curves (see Figure 1.3). To limit the

captured power at full load rating when operating above the rated wind speed (which

is generally around 12 m/s to 15 m/s for large WPPs ) , the blade pitch is continuously

adjusted at 10 minutes average for IEC wind class 1 and 2 WPPs. It does not pay to

design for very strong wind speeds beyond the rated speed, as they are quite rare. For

IEC wind class 3 wind turbines, cut-off wind speed value is in the range of 17 m/s-

20m/s. However, microcomputer initiates the emergency stop in case of damage or

other critical results.

1.3.3 Variable speed generator fixed pitch (VSG-FP) configuration:

In this type of WPP,the electrical generator runs at a variable speed (see Figure

1.3) but blade pitch angles are fixed at the most optimum angle of attack relevant to

that wind site. To limit the power captured, small control method (as in JeumontJ48

WPP ) is adapted which relies heavily upon the aerofoil blade design to limit the

electrical power generated through passive stalling.

The power control in such WPPs is by the PECs to regulate the generator

electromagnetic torque. By regulating the generator torque, the blade rotor speed can

be adjusted and the WPP can be operated at the point of optimal TSR within the

generator and the rotor design operational constraints. When the maximum design

blade rotor speed is reached, the WPP is operated at a constant speed mode with

passive stall regulation.

1.3.4 Variable speed Generator Variable pitch (VSG-VP)

Configuration:

VSG-VP is the only control strategy that theoretically achieves the ideal power

curve. In this strategy, the electrical generator runs at a variable speed (see Figure 1.3)

and simultaneously the pitch angle vary depending on the wind speed in coordination

with the synchronous generator speed (or doubly fed induction generator speed)

through PEC to maximise the energy capture (see Figure 1.3.4) and improve the power

quality. Most of the type D WPPs is operated in this mode.

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Figure 1.3.4: Variable Speed Generator and variable Pitch Operation of a Pitch

Regulated WPP.

1.4 AERODYNAMIC POWER REGULATION:

The power regulation of WPPs is done by aerodynamic, electric and electronic

control. The following aerodynamic control methods are popularly being adapted by

the manufacturers to regulate the power of large wind turbine rotors, although other

types of aerodynamic control are still under experimentation and research:

Stall Control:

It is also called stall regulated. In this, the stall profiled rotor blades are mounted

at a fixed angle on the hub.

Active stall control:

It is also called active stall regulated. In this, the stall profiled rotor blades arepivotable for few angles in the longitudinal axis.

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Pitch Control:It is also called pith regulated. In this, the rotor blades are almost infinitely

pivotable in the opposite direction to the active stall blades from 0 to 90 in

longitudinal axis.

The selection of any of three types of power regulation significantly affects the

wind power plant technologies adapted by different manufacturers and the design of

the blade aerofoils as well.

1.5 STALL CONTROLLED WPP:

The WPP technological revolution in late 1970s and 1980s was begun by the

adaption of the simplest Danish concept, three-bladed, fixed speed, stall regulated

WPP with squirrel cage induction generator. Stall is a potentially fatal event for an

aircraft wing aerofoil, whereas WPPs make purposeful use of stall as a means of

limiting power and loads in high wind speeds. This passive control of the WPP is based

on the fixed blade inherent characteristics. Stall, from a functional standpoint, is the

breakdown of the normally powerful lifting force when the angle of attack over a

blade aerofoil becomes too steep.

1.5.1 STALLING ACTION:

Stall method of power regulation is simple, reliable and efficient. It gives the

lowest possible dynamic loads on the WPP. The rotor blades have to be installed at the

correct angle of attack during the installation of the WPP for that particular location

based on the wind resources data. The wind turbine rotor works on the fact that the

angle of attack increases with the increase in a wind speeds in such a manner so that

at a certain angle corresponding to a particular pre-designed wind speed, stalling

effect gets initiated which is usually a little earlier than the rated wind speed. In other

words, the rotor blade is completely passive while it is the wind speed on its own that

causes the stall effect and the power regulation to happen. In doing so the

performance coefficient Cp falls at a higher rate beyond the nominal wind speed of the

WPP.

Since the blade speed is constant and the WPP is connected to the Grid, the

variable wind speed determines the angle of attack . This angle proportionally

increases with the increase in wind speed till the critical angle is reached (see Figure

1.5.1). After this point the lift force begins to decrease on its own for increasing wind

speeds due to the design of the blade profile.

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Depending on the particular aerofoil adapted and other factors, the criticalangle could vary usually between 14 to 16 degree. This critical angle is referred as the

Stall Angle. Stall regulated blade limit power in a high winds which reduces drive train

loads. Stall regulation is well understood technically and much simpler mechanically

then the competing pitch regulation technique.

Figure 1.5.1: Typical Stall actions of a WPP Blade: (a) At an angle of attack, say 2

degree, the air flow is laminar. The lift force is greater than the drag force and the

rotor produces some power; (b) At a greater angle of attack, say 9 degree, the air flow

continuous to be laminar and the rotor produces more power; (c) At critical angle, stall


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action gradually sets in but lift force is the highest, and (d) At this angle, drag force isgreater than the lift force and the blade rotation stops.

When the wind impinges on the wind turbine rotor blade at an angle of attack,

say 2 degree (see the Figure 1.5.1), the air flow over the aerofoil is laminar and hence

sticks to the blade surface from the leading edge to the trailing edge. It is this laminar

flow that creates a lift force on the blade aerofoil and the blade starts rotating, as the

blade has only 1 degree of freedom and free to move only in that plane.

At the wind speed goes on increasing, for a constant speed rotor, the angle of

attack (see Figure 1.5.1) on the stall blades also goes on increasing, say upto 09

degree. This laminar flow over the blade continues to be there creating a greater liftforce that applies a greater pressure on the rotor blade to turn the generator in order

to produce more power.

The aerodynamic profile and properties of the rotor blade are designed in such a

manner so that when the wind speed exceeds a certain limit, the lift action gets spoilt

due to the non-laminar wind flow turbulence beginning at the trailing edge and goes

on increasing to the leading edge till the blade stalls or stops the rotation of the rotor

as the wind speed goes on increasing beyond the designed limit. The stall blade is also

designed in such a way so that the stall effect and the turbulence starts to set in a little

earlier to bring in graduate stall rather than abrupt stall. The turbulent flow causes

drag and decreases the efficiency of the Aerofoil. Hence it can be said that stall action

Arises due to separation of laminar flow from aerofoil.

Results in decrease in lift coefficient with the increasing angle of attack.

1.5.2 STALL POWER CURVE:

Since the wind speed V0 and the air density cannot be controlled and the

radius of the blades R is fixed, the performance coefficient Cp is the only means for

torque control. Hence, this wind turbine rotor blade profile is shaped in such a way so

that Cp steeply falls at the start of the predetermined high wind speed at the stallangle. The rotor of this type cannot rely on aerodynamics to increase the rotor speed.

As a consequence, starting is done by connecting the induction generator to the grid

and starting it as a motor upto the operation speed.

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Figure1.5.2: Peaky Power Curve of Stall Controlled WPP: The stall slowly sets in much

earlier than the cut out speed. Here, it is around 16 m/s and the shaded area depicts

the lost power at higher wind speeds even before the cut out speed is reached.

As the blade angle is fixed in stall controlled WPPs, no external control is

possible for this type of WPP. It is to be noted that the output power just depends on

the wind speed which does not exactly reach a constant value at the rated power level

for which it is designed. During wind gusts, these passive stall WPPs get overloaded for

brief periods as depicted by the peaky power curve (see Figure 1.5.2) characteristics.

In fact, as the wind speed increases, it overshoots the rated power (hence, the peak of

the power curve) for a short period which then comes down as the wind speed

increases further. The WPP then starts the pitch controlled WPP.

The design requirements of stall regulations have led to new aerofoil

developments and also the use of devices such as Vortex generators, stall strips,

fences and gurney flaps for fine tuning rotor blade performances.

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1.5.3 FEATURES OF STALL CONNTROLLED WPP:

The following salient features of the stall regulated WPP continue its popularity

even today:

1) Bases on the simplest wind turbine technology.

2) Since the power is always controlled aerodynamically, they produce less

fluctuating power output as compared to that produced by a pitch regulated

WPP.

3) Cheapest among all types of WPPs.

4)

Time tested WPP for its reliability and robustness.

There are many demerits as well which are given below:1) Limited control of reactive power makes it more difficult to control network

voltages.

2) During network disturbances (such as a sudden fault in the network), such a

WPP is likely to aggravate the situation.

3) They are not self-starting.

4)

They are suited for relatively stronger grids.

5) The power of the electrical generator must be over dimensioned so that it

does not lose synchronism during wind gusts.

6) It has lower efficiency at lower wind speeds.

7) Stall regulation is not preferred much for megawatt capacity WPP as the

pitchable tip become unwieldy and other associated problems for activating

them with ease.

1.6 PITCH CONTROLLED WPP:

The pitch controlled (also called pitch regulated) WPP which got developed in

1990s, turn the whole length of the rotor blades in and out of the wind along the

longitudinal blade axis to regulate the power extracted from the wind. Pitch control

can maximise the energy captured even below the rated wind speeds. Due to greater

control (a feature demanded by the electric grid operators) many in the wind industry

who prefer large WPPs have a greater preference for this more complex method ofaerodynamic torque regulation which prevents mechanical overspeed in addition to

other benefits.

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1.6.1 Pitching Action:

The angle that the blade cord makes with the rotor disc is the pitch angle. Pitch

control can be undertaken to reduce as well as to increase the angle of attack towards

stall and modified the lift and drag values to regulate the power harness from the

wind.

The pitch action of each blade is achieved independently by geared electric

motors mounted on each blade bearing or group controlled by hydraulically operated

lever mechanisms. Pitch regulation allows the energy captured to be optimized for a

constant and variable speed operation of WPPs and at the same time provides over

speed protection as well by large pitch angle adjustments (see Figure 1.6.1). The

power coefficient Cp of a WPP is thus charged by adjusting the blade angle to optimize

the angle of attack of the blade. Almost all variable speed WPPs use pitch control.

Figure 1.6.1: Pitch controlled WPP. (a) It shows an operating pitch controlled WPP.

Based on the wind speed, 2.5 MW blades are continuously pitched at optimum angles

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to maximise the harnessing of energy. (b) This 2.5 MW wind turbine is in stoppedposition with the leading edge of the blade facing the wind at 90 degree to the wind.

Conventionally blade pitch is often control by a simple proportional, integral (PI)

based collective blade pitch controller which receives its input signals from the error in

electrical generator speed. The microprocessor (or microcomputer) based electronic

controller checks the various parameters received from the sensors of the operating

WPP several times per second. The microcomputer adjust the pitch of the blades to a

few degree to maintain the rotor blades at an optimum angle in order to maximise the

output power for all wind speeds. Blade adjustment is generally not continuous but at

some small fixed intervals based on certain logic (otherwise the pitch mechanism

would be worn out quickly and maintenance would be high).

1.6.2 Pitch Power Curve:

A typical pitch regulated WPP has a flattened power curve (see Figure 1.6.2) at a

rated power unlike a peaky stall WPP power curve as seen earlier. The bumps are

smoothen out due to the controlled pitching action of the blades resulting in lesser

stresses on the various mechanical and electrical components as well as on the

electrical grid. The power curve of WPP can be divided into following four categories:

1)

Region 1 (Low wind)WPP is not connected to Grid

2)

Region 2 (Medium Wind) WPP is connected but produces less than the rated

power.

3) Region 3 (Higher Wind)WPP is connected and produces only rated power.

4) Region 4 (Cut out Wind) WPP is disconnected and stop.

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Figure 1.6.2: Pitch Controlled WPP Power Curve: Unlike the peaky power curve

of the stall regulated WPP, the power curve of pitch control WPP is flat-topped due to

the pitching action of the blades resulting in lesser stresses on the various mechanical

and electrical components.

In region 1 of the power curve (see Figure 1.6.2) the wind speed is very low and

the rotor does not rotate, the pitch angle of the blade are turned approximately at 45

degree and the generator is not connected to the grid. This gives a maximum start

moment to rotor when the wind speed increases. At low wind speeds with a constant

TSR, the speed variation (dP/d of the output power P from the electric generator is

a function of the Rotor speed and is quite small.

In region 2 of the power curve (see Figure 1.6.2) when the wind speed increases

and the rotor rotates, the electronic controller pitches the blades to 0 degrees in to

the wind and the cut in speed the generator is connected to the grid. When the

electrical generator produces power, it causes a torque in an opposition to the

mechanical torque of the wind turbine rotor. As the wind speed increases the rotor

speed increases and the generator power also increases. When the wind speed below

the rated speed of the WPP the main goal is to extract maximum power from the wind

for which optimal power coefficient Cp is necessary. For this condition constant TSR is

to be maintained and hence, blade pitching is not used but kept at a constant value

(almost 0 degree) for maximum power extraction. To achieve this constant TSR

electrical generator torque is used to control the rotor speed curve (see Figure 1.6.2).

At a moderate wind speeds and with constant speed operation (dP/d may be quite

high.

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In region 3 of the power curve (see Figure 1.6.2),the main goal is to limit thepower output. Once the rated power is reached the combined action of the generator

torque on the pitch is used to control the electrical power output keeping it at a rated

value P(rated) as well as to control the blade rotor speed and maintain it within the

acceptable limits around the rated speed. For high wind speeds (dP/d is close to

zero, since the output power is kept constant. Practically what happens in this region is

given below:

1) By pitching, also called feathering of the blades.

Angle of attack decreases

Aerodynamic forces decrease

Rotor blades spill away the extra power

2) Error signal, e = gen) - rated)

3) Proportional integral (PI) controller is used to drive e to zero.

In a region 4 of the power curve (see Figure 1.6.2) when the wind becomes too

strong (above 25 m/sec) rotor blades are feathered to 90 degree to shut down the

WPP and the generator is disconnected from the Grid.

Due to pitching action of the blades the starting current in the electrical

generator can be controlled within its limits, unlike the electrical generator in the stall

controlled WPPs.

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Figure 1.6.2.1: Comparison of Stall and Pitch controlled WPP starting currents

1.6.3 Features of Pitch Controlled WPPs:

The salient features of pitch controlled WPPs are as follows:

I. The greatest advantage is the increased energy captured than the stall WPPs.

II. These are self-starting and controlled start up is possible.

III. In a contrast to stall regulated WPPs, the rotor blade profile for pitch regulated

WPPs is not so critical.

IV.

Pitching helps to reduce the aerodynamic loads, peak torques and hence lowers

the fatigue loads.

V. It helps the wind farm to withstand voltage dips, as pitching limits the

mechanical power on the main shaft resulting in greater grid elasticity. In other

words pitching partially damps the mechanical power variations resulting in a

lesser voltage variations being passed on to the grid.

However there are some limitations as well which are given below:

I. For the same rating practically the extra energy that can be obtained is only

about 2 to 4 % of compared to the stall WPP.

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II.

Its main limitation is that during high wind speeds, even small speed variationsresults in large variations in a power output to which the pitch mechanism is fast

enough to respond. To limit the power excursions especially during the gusts the

pitch changing has to act rapidly say 6 to 7 degree per second or even better.

III. The fatigue loading is also higher than that of the stall regulated WPPs due to

the rate of change of the lift coefficient.

IV. The hub of pitch regulated wind turbine is more sophisticated as it has to hold

the pitch bearing and pitching mechanism.

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