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NTN TECHNICAL REVIEW No.80（2012）

[ Technical Article ]

Application of Condition Monitoring Systemfor Wind Turbines

1. Introduction

Japan is 13th 1) in the world, with wind powergeneration capacity of 2.5 GW in 2011. It is expectedthat it will significantly increase with the import tariffprogram that started in July this year. On the otherhand, as the fixed price will be reviewed annually,further efforts are required, including reduction of themaintenance cost and improvement of capacity usage. The main components of windmills such as blades,

main bearings, step-up gears, and generators arelocated at the top of the tower. Therefore, repair and/orreplacement of the equipment requires a supersizedcrane. In addition, since most of the windmills arelocated in the mountainous areas where access ischallenging, maintenance cost is high. Further, theshutdown period may be prolonged, resulting in lowavailability if replacement components are not securedbefore the shutdown caused by failure. Condition monitoring systems (CMS) for wind power

generators target early detection of componentdeterioration from symptoms such as vibration andtemperature, preventing proliferation of damage andenabling earlier ordering of replacement componentsfor increased uptime. In addition, maintenance costscan be saved since the windmills are monitoredremotely, reducing the need for site visits.Since adequate sites for wind power generation are

becoming scarce on-shore, it is expected that offshore

NTN has developed a Condition Monitoring System for windturbines. CMS can detect the failure of main bearings, gearboxes,generators, and many mechanical components of wind turbines intheir early stages. One feature of CMS is that users can monitor

from a remote location. This system includes a data acquisition module, data management softwareand monitoring and analysis software for a client PC and was the first in Japan to acquire GLcertification. In this paper, the structure and the diagnostic method of NTN’s CMS will be introduced.

Akitoshi TAKEUCHI*Takashi HASEBA*Hiroshi IKEDA*

*New Energy Engineering Department Industrial Machinery Division

LAN

Operation informationsensor (rotation/current)

Data acquisition module

Condition monitoringsensor (vibration)

installations will increase 2). Access to these locationsis even more difficult than on-shore installation fortransporting replacement components, which willcause longer shut down when major failures areencountered. Therefore, Germanischer Lloyd (GL)recommends the use of GL approved CMS foroffshore wind turbines. In order to contribute to the advancement of

maintenance, NTN has developed CMS based on itsown vibration diagnostics technology. The system wasthe first in Japan to acquire GL-type approval.

2. System configuration

NTN’s CMS consists of (1) a data acquisitionmodule and measurement sensors, (2) datamanagement software, (3) monitor/analysis software,as shown in Fig. 1. “(1) Data acquisition module”measures vibration, etc., with the sensors installed onthe wind turbine and sends the data to the serverthrough the Internet for storage. “(2) Datamanagement software” performs a primary diagnosisof the data and issues an alarm if the data exceedsthe threshold. In addition, when “(3) Monitor/analysis software” is

installed on a PC connected to the Internet, alarmsand data can be monitored at a remote location fromthe wind turbine. This software also has an analysisfunction, enabling detailed diagnostics.

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(2) Data management software

ServerData

User or NTN

(1) Data acquisition moduleStep-up gears

Main bearings

BladeNacelle

Generator

(3) Monitor/analysis software

Measurement sensor

Fig. 1 Structure of NTN’s CMS

Fig. 2 Data acquisition module

3. Data acquisition module

Table 1 shows the specifications for “(1) Dataacquisition module” and Fig. 2 shows its externalview. The module is the world’s smallest, for easierinstallation into the nacelle, even on existing windturbines. Since it has high dustproof/waterproofcapabilities and a wider operating temperature range,it can be used in a wide variety of environments,including offshore applications. Different sensors for vibration, rotation, current,

temperature, etc., can be used and changing of thenumber of channels can also be accomplished easily.

4. Flow of operation and diagnosisphilosophy

4.1 Observation mode operationFig. 3 shows the flow of CMS from installation to

operation. As the operating conditions of wind turbineschanges from moment to moment, the cause must bedetermined if the change of vibration comes from thechange of operation conditions or damage to thebearings and gears. In order to eliminate the influenceof vibration of operating conditions, after CMS isinstalled, a preliminary measurement called“observation mode operation” is conducted as apreparation of diagnosis, to determine diagnosticoperating conditions for the production stage.With NTN’s CMS, rotational speed and generated

power are used as parameters to indicate theoperation conditions. In “observation mode operation,”the data acquisition module is regularly activated, andthe rotational speed and generated power arerecorded. Fig. 4 shows an example of the main shaft

rotational speed and generated power. In general,condition 1, which has large power output around therated rotation speed, is considered diagnosisoperating condition. For diagnosis of lower speed in a

Item

Measuredsignals

Specification

AC90 ～ 260V (50/60Hz)

250 × 290 × 108 mm

5.1 kg

-20 ～ +60˚C

Vibration (built-in acceleration sensor in the amplifier)

Voltage (AC, DC, current sensor)

Rotational speed (proximity sensor)

Power supply

Case dimension

WeightOperatingtemperatureProtection ofenclosureElectromagneticcompatibility

Temperature (thermocouple)

max16ch

max 4ch

1ch

max 6ch

Degree of protection of electric machinesand appliances

Complied with EN Standards,EN61000-6-2 and EN61000-6-4

Table 1 Specification of data acquisition module

NTN TECHNICAL REVIEW No.80（2012）

Installation

Observation-modeoperation

Learning-modeoperation

Production-modeoperation

Primary diagnosis: automatic diagnosis, “Caution”and “Alert” alarm issuedPrecise diagnosis: diagnosis by experts

⇒ Determination of diagnosis operation condition

⇒ Determination of thresholds

Fig. 3 Flow of NTN’s CMS operation

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Application of Condition Monitoring System for Wind Turbines

period of weaker wind conditions, condition 2 withslow rotation speed may be added as a diagnosisoperation condition.

4.1 Learning mode operationAfter the diagnosis operation conditions are

determined, “learning mode operation” is conducted todetermine the diagnostic thresholds. In this mode,diagnosis parameters in normal conditions arecollected. The data acquisition module is regularlyactivated in the same way as “observation modeoperation,” data is collected only when the rotationspeed and generated power match the diagnosisoperation conditions.

NTN’s CMS automatically calculates the thresholdsfor “Caution” and “Alert.” From the relationshipbetween the diagnosis parameters in normalconditions and those in damaged conditions obtainedfrom the past failure cases and the data in normalconditions measured in “learning mode operation,” thethresholds for the diagnosis parameters for thefollowing section 5 are automatically established. Inaddition, the thresholds can be manually set, as well.

4.3 Production mode operationUnder “production mode operation,” CMS regularly

conducts measurement and if a diagnosis parameterexceeds the threshold, it issues two types of alarms,“Caution” and “Alert,” as a “primary diagnosis” in Fig.3. The alarm can be confirmed on “(3)Monitor/analysis software.” It is also able toautomatically notify predetermined personnel viaemails. “Precise diagnosis” in Fig. 3 is diagnosis provided

by an expert using analytical tool of “(3)Monitor/analysis software.” “(3) Monitor/analysissoftware” displays diagnosis parameters, trendanalysis, and thresholds, for easy determination of thediagnosis parameter trend. It also provides filterprocessing, envelope processing, and FFT processingof time waveforms, as well as a function to display

frequencies caused by defects of bearings and gearson the spectrum. It is also possible to compare pastwaveforms with the current ones.

5. Diagnosis parameters

The main components of wind turbines are theblades, main bearings, step-up gears, and generators.NTN’s CMS addresses damage to any of thesecomponents. Table 2 shows diagnosis parameterscorresponding to the failure modes. The relationbetween the failure modes and diagnosis parametersare discussed in detail in the following:(1) Imbalance of bladesWhen blades are damaged, imbalance of mass willcause an increase in vibration. Imbalance appearsas a peak in rotational frequency. Imbalanceanomaly is detected taking the primary to tertiarycomponents of rotational frequency of the mainshaft as the diagnosis parameters.

(2) Abnormal vibration of nacelle/towerThis indicates a large vibration out of norm whenthey receive strong wind, etc. The vibration consistsof low frequencies; the sum of spectrum in lowfrequency band among acceleration vibration isused as the diagnosis parameters.

(3) Anomaly in bearingsIt is caused by separation or abnormal wear on theraceway surface or rolling surface. The effectivevalue and modulation factor are used as diagnosisparameters. The effective value is an average ofvibration and the modulation factor is an effectivevalue of the AC component of accelerated vibrationafter envelope processing and can be used fordetermination of vibration caused by a shock.

(4) Anomaly in gearsIt indicates anomalies of tooth contact surface, suchas the pitching and abnormal wear of tooth flank. Itis diagnosed by tooth meshing frequency of primaryto tertiary components and the sum of spectra ofhigh-frequency band in the acceleration vibrationspectra.

00 5 10 15

Rotational speed of main shaft min-1

Condition 1

Condition 2

20 25

400

800

1200

1600

Powe

r out

put

kW

Rated

rotat

ional

spee

d

Fig. 4 Main shaft rotational speed and electric power

Failure mode

Imbalance of blades

Abnormal vibration of nacelle/tower

Diagnosis parameters

Anomaly in bearings (main bearings, step-up gears,generator)

Anomaly of gears (step-up gears)

Primary to tertiary frequencycomponents of the main shaft rotation

Effective value and modulation factorof vibration in the proximity of bearings

Primary to tertiary components of toothmeshing frequency and vibrationcomponents of high frequency bandcalculated from specification of the gears

Low frequency components of nacelle

Table 2 Failure modes and diagnosis parameters

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NTN TECHNICAL REVIEW No.80（2012）

Fig. 7 Outer raceway failure by electrical pitting

Fig. 6 Envelope spectrum of vibration

00 200 400 600Frequency Hz

F0：Frequencies of defective outer ring

F0×1 F0×2 F0×3

800 1000

1

2

3

4

Ampli

tude

m

/s2

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6. Case study of diagnosis forgenerator bearings

Effective values, which are diagnosis parameters,were obtained from the measurement of vibration inbearings with electrical pitting and normal bearings ofthe actual generator. Fig. 5 shows the relationbetween main shaft rotational speed of the windturbine and vibration effective values around thebearings. The effective values are significantly largerdue to electrical pitting. The difference from thenormal bearings is more significant as the rotationalspeed becomes higher. Fig. 6 shows the result of envelope spectrum of

vibration. The outer ring defect frequency, which is afrequency that a point on the outer ring racewaysurface is in contact with the rolling element, and itshigher order components are observed. It is thenpossible to assume damage on the outer ring racewaysurface. This envelope spectrum evaluation result wasproven correct after this bearing unit with electricalpitting was disassembled and examined, as it showedrepetitive ridge marks on the outer ring raceway, asshown in Fig. 7.

00 5 10 15Rotational speed of main shaft min-1

20 25

5

10

15

20

Effe

ctive

value

m

/s2

Normal bearings

Bearings with electrical pitting

Fig. 5 Main shaft rotational speed and root mean squarevalue of vibration

Photo of authors

Takashi HASEBANew Energy Engineering

DepartmentIndustrial Machinery Division

Akitoshi TAKEUCHINew Energy Engineering

DepartmentIndustrial Machinery Division

Hiroshi IKEDANew Energy Engineering

DepartmentIndustrial Machinery Division

7. Conclusion

Functions expected of CMS for wind turbinesinclude not only an early detection of damages butalso the prediction of remaining life. For improving thecapacity uptime, it is important to minimize the periodof shutdown, and therefore, it is critical for the users toknow how long certain components can be used afterdamage is detected. For highly precise prediction ofthe remaining life, vibration measurement on theactual machines and accumulation of data fromexaminations through disassembly are necessary. We will work together with different players such as

wind power providers, windmill facility manufacturersand maintenance service providers to accumulatediagnostic know-how so that the system can providemore useful information for expansion of wind powerutilization.

Reference1）Global Wind Energy Council：Global Wind ReportAnnual Market Update 2011

2) Takeshi Ishihara: Current situation and futureperspective of offshore wind power generation, TheJournal of the Institute of Electrical Engineers ofJapan, Vol. 131 No. 7, 2011

3）Germanischer Lloyd Industrial Services GmbH：Guideline for the Certification of Condition MonitoringSystems for Wind Turbines,2007

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