Intelligent blade monitoring – the benefits

Feature article

50 renewable energy focus January/February 2008

Intelligent blade monitoring – the benefi ts STRUCTURAL HEALTH MONITORING SHM OF WIND TURBINE DRIVE

TRAINS IS NOW REGARDED AS AN ESSENTIAL PART OF ANY MW CLASS OF

WIND TURBINE AND HAS A DUAL PAYOFF FOR WIND ENERGY, AS GEORGE

MARSH FOUND OUT WHEN HE EXAMINED AN INNOVATIVE SOLUTION

FROM INSENSYS. George Marsh

The above comment is especially true for larger machines and those

located off shore. Operators value SHM, insurers may well insist on it and

there are a number of proven systems on the market. The technology

enables owners and operators to save fi nancially by maximising the

performance of their assets and dynamically scheduling inspection and

maintenance.

However, there is currently one section of the turbine – the turbine rotor

– that is not routinely monitored, yet can provide much information

about the turbine’s operational performance, health and energy yield.

Insensys Ltd, based near Southampton, UK, has developed an innovative

fi bre optical blade load sensing approach that not only provides an inval-

uable layer of SHM, but also facilitates Individual Pitch Control (IPC). This

is potentially rather signifi cant. As Phil Rhead, business development

manager, wind energy, explains, “Using blade load sensing elements to

enable the pitch of a standard wind turbine’s blades to be controlled indi-

vidually, rather than collectively as generally happens at present, secures

a better balance of conditions around very large rotors. This allows turbine

designers to specify larger rotors for a given machine than would other-

wise be viable. As a result, such turbines can capture more energy from

the wind”.

Alternatively, says Rhead, the designer can trade this advantage for a

lighter, cheaper structure to capture the same amount of energy or, in a

retrofi t situation, achieve greater reliability and longer life in an existing

installation. Insensys’ ceo Martin Jones, who fi ve years ago founded the

company together with Damon Roberts, technical board member, adds

that this form of advanced turbine control can enhance installation safety

and reduce costs per kWh.

Non-uniform

Rotors for large wind turbines today can be 90 to 130 metres in diameter,

and the areas swept are consequently huge. This can result in drive trains

and structures being subjected to eff ects that are scarcely evident on

smaller machines. In particular, the wind fi eld over the swept area is far

less uniform. Wind gradient, the rise in wind speed that naturally occurs

with increasing distance from the ground, means that greater wind speeds

are seen at the top of the rotor than at the bottom. Add to this the vari-

ations in gust strength and turbulence that may occur over a sizeable

area, and it is clear why one blade can encounter conditions that are

substantially diff erent to those at another.

If the wind load at each blade could be sensed and the blade could be

pitched independently to accommodate it, then the rotor could be

operated in a more balanced condition, avoiding the imposition of

out-of-balance forces on the hub, drive train and tower.

These adverse forces can unduly stress downstream structures and

components, and exacerbate the cumulative eff ects of fatigue. Reducing

the stress reduces the potential for damage over time and prolongs the

turbine’s operational life. At the same time, the ability to make key compo-

nents lighter and smaller enables designers to counter the seemingly

inexorable rise in mass that is a feature of modern turbine evolution. Plus,

of course, increasing wind energy capture and cost reduction remain

constant aims for turbine designers.

Whilst growing in size and mass, turbines have also become more diffi cult

to inspect, service and repair. SHM can facilitate remote inspection and,

by detecting faults in their earliest stages, enable measures to be taken

that counter their further development, thereby avoiding expensive fail-

ures later. Run to failure is not an operational option with large multi-

megawatt machines today. Insensys deploys monitoring technology that

signifi cantly extends the capability of conventional techniques based

mainly on accelerometers and FFT (fast Fourier transform) analysis.

At the core of the Insensys approach is the use of optical fibre sensors

to measure load rather than conventional electrical strain gauges. This

has major advantages. FO sensors are small, light and easily embedded

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renewable energy focus January/February 2008 51

in composite structures or bonded to components. They are physically

compatible with glass fibre reinforced plastic (GRP), the material most

turbine blades plus certain hub and nacelle components are made

from, and do not disrupt composite properties. They are easily incor-

porated into the laminate as part of any of the normal production

processes – hand lay-up, vacuum infusion, pre-preg etc. FO sensors do

not conduct electricity and are not susceptible to electromagnetic

interference (EMI) or damage through lightning strikes. They have

excellent long-term static and dynamic fatigue performance, and are

able to withstand high levels of shock and vibration. They can be

produced in volume inexpensively.

Optical heart

Fibre optical cables can be regarded as conductors of light rather than

electricity. They can do this because they are made up of concentric layers

of glass having diff erent properties such that light is internally refl ected

back into the core rather than escaping. If the fi bre bends, the way

diff erent wavelengths within the light passing through the optic are

refl ected and refracted is altered and this results in a change in the wave-

length (colour) spectrum emerging from the end of the fi bre. The amount

of this alteration is a measure of the amount of bend in the fi bre.

If the fi bre optic (FO) is embedded within a wind turbine blade, it will

bend as the blade bends in response to applied loads. Thus, if the diff er-

ences between the original unaltered light and that emerging from a

bending fi bre could be analysed, this would provide a measure of the

strain the turbine blade is experiencing due to wind strength.

One way of examining this diff erence is to bring the altered light together

with a sample of the unaltered light such that the two interfere with each

other. Studies of the various wavelength relationships within the consequent

interference pattern as it changes in response to bending loads can reveal

even the slightest amount of bend. Because the system is so sensitive, it can

be used to measure the strain in any component to which a FO strain gauge

is bonded, not just those of composite material.

Analysing interference fringe patterns is an activity for scientifi c laborato-

ries, not a function easily incorporated into industrial processes. A way is

needed by which the strain-induced spectral change can be more easily

quantifi ed, preferably electronically.

The answer is to engineer diff raction gratings into the sensors. Fibre

Bragg gratings (FBGs), so called after the scientist who pioneered use of

the eff ect, emit light at a single wavelength (colour) when broadband

(white) light is directed at them. The precise wavelength (colour) of the

emission varies according to the amount of strain to which the fi bre

containing the FBG is subjected.

Because the FBG wavelength increases linearly with the applied load, it

off ers a direct measurement of strain. Measurement accuracy is high over

a large strain range. The Insensys OEM-1000 Series Load Monitoring

System provides a measurement resolution of 0.8 microstrain over a range

of +/- 4500 microstrain, which is more than adequate to cover all likely

rotor blade load conditions. Measurements are made in real time at a

frequency of 500Hz.

FBGs are easily incorporated into optical fibres as part of the fibre

manufacturing process. Over 100 sensing elements can be written into

a single 250-micron diameter optical fibre – a simple way of providing

multiple sensor installation and connectivity. Each grating consists of

a series of very closely spaced bands of glass having different refrac-

tive indices, such that the boundaries between the bands can reflect

incident light. When light from a source located at one end of the

optical fibre encounters the FBG, some of the incident light is reflected

back from the boundaries. If the inter-band separation distances and

the light wavelength ‘tally’, the reflections from successive bands are

additive and the FBG transmits the single resulting wavelength,

An Insensys technician at work on a rotor hub


Wind/O&M


suppressing others. This emission is analogous to the ‘interference

wavelength’ – the peak-to-peak distance of the interference waveform

mentioned above.

When the fi bre is strained, the relationship between the Bragg grating

fringes or separations and the wavelength of the refl ected light alters.

Consequently, the wavelength emitted by the FBG changes. The monochro-

matic Bragg emission is readily converted to an electrical analogue suitable

for subsequent digital electronic processing. Unfortunately, FBG sensors are

aff ected by temperature as well as strain, so temperature-compensating

electronics have to be part of the subsequent processing chain.

System

An Insensys system for application to a wind turbine rotor comprises

three channels, one per blade. Four strain sensors (typically) located

mutually at right angles at the root of each blade provide two pairs of

strain measurements, one pair each for edgewise and fl apwise bending

moments. This number is considered optimum for most in-service systems,

though as many as 60 sensors might be fi tted to a prototype system for

test purposes. Sensors are either embedded during blade manufacture or

retrofi tted, and are connected to looped optical fi bres that are similarly

embedded or bonded to the blade. For each blade, a connector box

embedded or retrofi tted at the blade root provides all necessary connec-

tions between the blade and the hub, as well as light sources for projecting

light along the optical fi bres.

An interrogator unit in the blade hub interrogates the fi bre sensors and

receives the responses. The system operates on a time division multiple

access (TDMA) basis, an electronic technology that maximises channel

effi ciency by allocating capacity on the basis of time slots. Sensor

responses are passed to a processor that converts the results into a form

suitable for input to the WT’s programmable logic controller (PLC) or

blade pitch controller. As a result, the PLC or computer commands blade

pitch movements, both individually and collectively, according to the

wind conditions prevailing at each blade.

Combining the Insensys-instigated pitch command terms with those

emanating from the WT manufacturer’s own control system to best eff ect,

can be a complex multi-dimensional problem.

Here, Insensys has relied on the expertise of a leading provider of pitch

control algorithms, Garrad Hassan, whose Bristol offi ce has developed the

necessary advanced individual pitch control algorithm for incorporation

into the overall control logic. The two companies, whose collaborative

relationship extends back several years, have worked closely together to

ensure that the Insensys system design meets the requirements of the

algorithm.

Tony Mercer, head of control system activities at Garrad Hassan comments,

“we regard independent pitch control as a must have for the next gener-

ation of larger turbines. By using the latest control techniques, combined

with reliable load measurement throughout the life of the wind turbine,

we can improve its structural effi ciency, cope with a wider range of

adverse fl ow conditions and permit a larger, higher-yield rotor for a given

nacelle and support structure.”

Insensys has also worked directly with a number of wind turbine manu-

facturers to integrate its system with their own proprietary control

systems, so augmenting them with independent blade control capability.

Health monitoring

Although WT rotors are an obvious focus for the Insensys load measure-

ment technology, Phil Rhead points out that application is not limited to

these. “We have put FBG strain gauges on turbine hubs, shafts, nacelles

Overview of drive train monitoring


Wind/O&M


and even towers,” he says. “In fact you can use an optical strain gauge

wherever you can put an electrical one. This makes it a practical and desir-

able system for structural health monitoring.”

He adds that, nevertheless, rotors are one of the areas of the turbine that

can most benefi t from monitoring. Among the most expensive turbine

components, rotors are hard to inspect and still rely largely on manual

inspection. Moreover, they are responsible for some of the heaviest and

potentially most destructive loads seen by the drive train, especially when

they are out of balance. They present, though, a challenging environment

for any monitoring technology. Phil Rhead highlights in particular the fact

that lightning strikes are not infrequent.

“All the signal paths within a fi bre optical system are resistant to electro-

magnetic interference and substantially lightning proof,” he declares.

“Compare that with conventional systems where, for example, lightning

frequently tracks back to the in-hub amplifi ers, which then have to be

replaced”.

Rhead states that, while electrical (resistive) strain gauging is still the

dominant technology for test and measurement companies, the convic-

tion has grown over the last fi ve years that optical technology is an inher-

ently superior alternative. Industry insiders feel that it is only a matter of

time before it becomes the measurement standard for all applications.

“I believe this is the only reliable way to do it [measure strain]” asserts

Rhead. “Lightning strikes frequently destroy and de-bond conventional

electrical sensors as well as damaging their associated electronics. The

chances of a strike increase with turbine size and height, and with off shore

location”. And he adds, “we’ve had our system trialled by leading turbine

manufacturers, test houses and research labs and have not yet had a

sensor fail during operation”.

While conceding that FO sensors are more expensive than conventional

electrical strain gauges, Rhead argues that, by the time installation time

and cost are taken into account, costings for commissioned systems are

similar for both solutions. Experience with certain prototype WTs fi tted

with dozens of FO sensors for test and qualifi cation purposes suggests

that the economics are acceptable and that large amounts of data can be

harvested successfully by this method.

The Insensys technology interfaces readily with other manufacturers’

monitoring and control systems. This has been amply demonstrated in a

collaboration with SKF Condition Monitoring, a company that specialises

in monitoring gears and rolling bearings, components that account for a

signifi cant proportion of wind turbine failures. Catching faults early is

important since an unnoticed drive train defect in a US$1500 bearing, say,

could ultimately lead to a US$100,000 gearbox replacement, a US$50,000

generator rewind, and a US$70,000 bill for accessing the failed compo-

nents. ProCon is the company’s condition monitoring system, intended to

avoid expensive repairs.

SKF contends that monitoring both blade health and drive train health

with the same integrated system improves the quality of SHM overall by

relating drive train degradation to the rotor-induced forcing loads that

cause it. A bonus from relating cause and eff ect in this way, the company

points out, is that results recorded over time can help manufacturers to

improve the design of turbine components and help operators to devise

eff ective load reduction strategies.

A number of SKF clients, such as renewable energy provider Enertag, use

ProCon to monitor their drive train components. Installed in each turbine’s

machine compartment, the system continually monitors levels of vibra-

tion at specifi c drive train gears and bearings. Data analysis then estab-

lishes the condition of the monitored component, identifying faults such

as pitting or spalling (bearings) or damaged teeth (gears). Adding the

ability to sense drive train input loads, both static and cyclic, in real time

augments the system’s power to diagnose a range of defects such as

mechanical imbalance or looseness, shaft bending, failing couplings,

structural resonances and even weak foundations.

Monitoring a dozen or more sensors in a rotor 30 times each second, the

Insensys instrumentation rapidly generates large amounts of data. The

system performs statistical analyses on blade bending data in both time

and frequency domains to extract key information and passes this

‘summary’ data to the condition monitoring system.

Installing optical fi bre strain instrumentation inside a rotor hub.


Wind/O&M

renewable energy focus January/February 2008 55

Alternatively, it is passed to a data logger for subsequent retrieval and

analysis. From the data, the system can calculate individual blade loads

and, further, can make inferences about blade condition – especially if

more sensors are installed in the blade than the two pairs at the root.

Storing and analysing load service histories for the blades enables their

residual fatigue lives to be predicted. Though this is not yet an exact

science for composites, due to the still limited knowledge of the long-

term degradation behaviour of these highly variable materials, the experi-

ence base is constantly growing, permitting on-going refi nement of the

predictive algorithm.

Resolving the edgewise and flapwise loads in the plane of the rotor

enables the input torque to the drive shaft to be calculated. Plotting

this over time shows the magnitude and variability of the drive torque

and indicates whether recommended maximum levels are being

exceeded, reducing intended service life. Resolving blade root bending

moments in the horizontal and vertical directions enables offset loads

on the drive shaft to be determined. Shaft bearings are designed to

accommodate axial loads and continual excessive dynamic loading

offset from this plane can accelerate degradation. Calculation of a

resultant load vector will indicate whether or not there is prolonged

offset loading.

Currently Insensys is working hard to expand its SHM capability, having

seriously entered the condition monitoring (as distinct from individual

blade pitch control) business only in the last 18 months. It off ers a range

of standard solutions suitable for new-build or retrofi t applications. These

include load monitoring units designed for hub PLC/pitch cabinet instal-

lation, stand-alone hub installation or R&D applications. The company

also welcomes the opportunity to customise solutions for specifi c turbines

and manufacturers.

There is a growing focus on software development. Says Rhead, “we’re

progressively bringing in automatic analysis. For instance, we now do

trend analysis and are adding fatigue and lifetime prediction to our own

data reduction software. Future rotor monitoring software will typically

encompass cumulative load counting, residual lifetime estimation, critical

event monitoring and frequency analysis”.

He claims signifi cant interest from turbine manufacturers and operators

and reports that the present technology is currently on 13 diff erent types

of turbine produced by 10 manufacturers. He predicts major market

uptake, pointing out that the optically-based solution is under active

consideration for many turbines now in their early conceptual or design

stages.

“Sophisticated electrical power utilities are used to having lots of data,”

argues Rhead. “Wind energy has hitherto operated with relatively little;

we believe that, as the scale of wind energy grows, particularly in the

off shore sector, turbine manufacturers will fi nd it in their interests to

provide more data. Strain measurements, obtained through reliable

optical means, will be an essential part of this”.

CEO Martin Jones agrees, adding that Insensys is well placed to succeed

with its leading-edge technology because of its dual contribution to reli-

able, economic wind farm operation. “Using the same technology to

capture a larger proportion of the available wind energy while also

expanding the scope of structural health monitoring is our unique

selling proposition,” he says. “We think it’s one that has wide market

appeal”.


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Intelligent blade monitoring – the benefits