NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Duration Test Report for the 

Entegrity EW50 Wind Turbine

Joe Smith, Arlinda Huskey, Dave Jager, and 

Jerry Hur

National Renewable Energy Laboratory

Technical Report

NREL/TP-5000-57141

December 2012
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

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Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Duration Test Report for the 

Entegrity EW50 Wind Turbine

Joe Smith, Arlinda Huskey, Dave Jager, and 

Jerry Hur

National Renewable Energy Laboratory

Prepared under Task No. WE110308

Technical Report

NREL/TP-5000-57141

December 2012
NOTICE

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Page 1 of 36

Duration Test Report

for the

Entegrity EW50 Wind Turbine

Conducted for

Wind Energy Program

DOE/NREL

Conducted by

National Wind Technology Center

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

Joe Smith, Arlinda Huskey, Dave Jager, and Jerry Hur

25 October 2012
Page 2 of 36

Notice

This report was prepared by the National Renewable Energy Laboratory (NREL), operated for

the United States Department of Energy (DOE) by the Alliance for Sustainable Energy, LLC 

(Alliance), as an account of work sponsored by the United States government. The test results 

documented in this report define the characteristics of the test article as configured and under 

the conditions tested.

THIS REPORT IS PROVIDED "AS IS" AND NEITHER THE GOVERNMENT, ALLIANCE, NREL NOR 

ANY OF THEIR EMPLOYEES, MAKES ANY WARRANTY, EXPRESS OR IMPLIED, INCLUDING THE 

WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR 

ASSUMES ANY LEGAL LIABILITY OR RESPONSIBILITY FOR THE ACCURACY, COMPLETENESS, 

OR USEFULNESS OF ANY SUCH INFORMATION DISCLOSED IN THE REPORT, OR OF ANY 

APPARATUS, PRODUCT, OR PROCESS DISCLOSED, OR REPRESENTS THAT ITS USE WOULD 

NOT INFRINGE PRIVATELY OWNED RIGHTS. 

Neither Alliance nor the U. S. Government shall be liable for special, consequential or incidental 

damages. Reference herein to any specific commercial product, process, or service by trade 

name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its 

endorsement, recommendation, or favoring by the United States government or any agency 

thereof. The views and opinions of the authors expressed herein do not necessarily state or 

reflect those of the United States government or any agency thereof or Alliance. 

NREL is a DOE Laboratory, and as an adjunct of the United States government, cannot certify 

wind turbines. The information in this report is limited to NREL’s knowledge and understanding 

as of this date. 

NREL is accredited by the American Association for Laboratory Accreditation (A2LA) and the 

results shown in this test report have been determined in accordance with the NREL’s terms of 

accreditation unless stated otherwise in the report.

This report shall not be reproduced, except in full, without the written approval of Alliance or 

successor operator of NREL.

Approval By: __________________________________________________________

Jeroen van Dam, NREL Test Engineer Date

Review By: ___________________________________________________________

Arlinda Huskey, NREL Test Engineer Date
Page 3 of 36

Table of Contents

1. Background ............................................................................................................................5

2. Test Objective and Requirements ........................................................................................5

3. Description of Test Turbine ..................................................................................................6

4. Description of Test Site.........................................................................................................8

5. Description of Instrumentation.............................................................................................9

6. Results ..................................................................................................................................12

6.1. Period of Testing ............................................................................................................12

6.2. Months of Operation.......................................................................................................12

6.3. Hours of Power Production ............................................................................................12

6.4. Operational Time Fraction..............................................................................................13

6.5. Environmental Conditions ..............................................................................................14

6.6. Power Degradation Checks ...........................................................................................14

6.7. Dynamic Behavior..........................................................................................................16

6.8. Post-test Inspection........................................................................................................16

7. Uncertainty............................................................................................................................17

7.1. Hours of Power Production ............................................................................................17

7.2. Operational Time Fraction..............................................................................................18

7.3. Environmental Conditions ..............................................................................................18

8. Deviations and Exceptions .................................................................................................18

8.1. Deviations from the Standard.........................................................................................18

8.2. Deviations from Quality Assurance................................................................................18

A. Appendix: Pictures of Post-Test Inspection .....................................................................19

B. Appendix: Equipment Calibration Sheets .........................................................................22
Page 4 of 36

Table of Tables

Table 1: Test Turbine Configuration and Operational Data.............................................................7

Table 2: Equipment List for Duration Test .....................................................................................11

Table 3: Monthly and Overall Results of the Entegrity EW50 Duration Test.................................13

Table of Figures

Figure 1. Entegrity EW50 test turbine at the NWTC (PIX# 22243)..................................................6

Figure 2. Electrical single-line drawing of Entegrity EW50 installation ............................................8

Figure 3. Map of area surrounding Entegrity’s EW50 at NWTC’s 1.E1 test site .............................9

Figure 4. Meteorological tower and instruments............................................................................10

Figure 5. Power in each wind speed bin [m/s] vs. time..................................................................15

Figure 6. Tip brake #1 bushing has disintegrated (PIX# 22238) ...................................................19

Figure 7. Tip brake #3, the rubber bushing is still intact (PIX# 22239)..........................................19

Figure 8. Rust and oil marks near the generator and gearbox connection (PIX# 22248) .............20

Figure 9. Corrosion around electrical connector to tip brake’s electromagnet (PIX# 22241)........20

Figure 10. Draining earbox oil; significant contaminints were seen (PIX# 22246) ........................21

Figure 11. Anemometer #1; installed on 1 April 2009, used until 24 February 2010.....................23

Figure 12. Anemometer #2; installed on 24 February 2010, used until 23 July 2010 ...................24

Figure 13. Anemometer #3; installed on 23 July 2010, used until end of test...............................25

Figure 14. Power transducer calibration #1; installed 25 February 2009 ......................................26

Figure 15. Power transducer calibration #2; installed 9 February 2010 ........................................27

Figure 16. NI 9229 data acquisition module #1; installed 25 February 2009 ................................28

Figure 17. NI 9229 data acquisition module #1; post-test calibration............................................29

Figure 18. NI 9229 data acquisition module #2; installed 18 January 2010..................................30

Figure 19. NI 9217 data acquisition module #1; installed 25 February 2009 ................................31

Figure 20. NI 9217 data acquisition module #1; post-test calibration............................................32

Figure 21. NI 9217 data acquisition module #2; installed 18 January 2010..................................33

Figure 22. NI 9205 data acquisition module #1; installed 25 February 2009 ................................34

Figure 23. NI 9205 data acquisition module #1; post-test calibration............................................35

Figure 24. NI 9205 data acquisition module #2; installed 18 January 2010..................................36
Page 5 of 36

1. Background

This test was conducted as part of the U.S. Department of Energy’s (DOE) Independent Testing 

project. This project was established to help reduce the barriers to wind energy expansion by 

providing independent testing results for small turbines. Five turbines were tested at the 

National Wind Technology Center (NWTC) as a part of round one of this project. Duration

testing is one of up to five tests that may be performed on the turbines, including safety & 

function, power performance, noise, and power quality tests. Test results provide manufacturers 

with reports that can be used to fulfill part of the requirements for small wind turbine certification.

The test equipment included a grid connected Entegrity Wind System’s EW50 wind turbine 

mounted on a 30.5-m (100-ft) monopole.

2. Test Objective and Requirements

The objective of this test was to assess the following aspects of the Entegrity EW50 in 

accordance with Clause 9.4 of the International Electrotechnical Commission’s (IEC) standard, 

Wind turbines – Part 2: Design requirements for small wind turbines, IEC 61400-2 Ed. 2.0:2006-

03 (throughout the report referred to as the Standard).

The major parts of this test are to investigate:

• Structural integrity and material degradation 

• Quality of environmental protection 

• The dynamic behavior

The wind turbine will pass the duration test when it has achieved reliable operation for:

• 6 months of operation

• 2,500 hours of power production in winds of any velocity

• 250 hours of power production in winds of 1.2*Vave (10.2 m/s) and above

• 25 hours of power production in winds of 1.8*Vave (15.3 m/s) and above.

Reliable operation means:

• Operational time fraction of at least 90%

• No major failure of the turbine or components in the turbine system

• No significant wear, corrosion, or damage to turbine components 

• No significant degradation of produced power at comparable wind speeds.

Based on the parameters defined in the Standard for small wind turbine classes, Entegrity Wind 

Systems identified the Entegrity EW50 turbine as class II. This corresponds to a Vave of 8.5 m/s.
Page 6 of 36

In addition, this test was conducted in accordance with NREL’s quality system procedures; this 

report will meet the full requirements of its accreditation by A2LA. NREL’s quality system 

requires that this report meet all applicable requirements specified by A2LA and ISO/IEC 17025 

or to note any exceptions in the test report.

3. Description of Test Turbine

The test turbine was an Entegrity EW50 wind turbine. It has an advertised rated power of 50

kilowatts (kW). The EW50 is manufactured by Entegrity Wind Systems Inc. and is a downwind, 

three-blade, passive-yaw, horizontal-axis wind turbine. The blades are fixed pitch, and the 

turbine employs an asynchronous generator that operates at a fixed speed to deliver threephase electric power at 60 Hz. Table 1 lists the configuration of the Entegrity EW50 that was 

tested at the NWTC. Figure 1 is a picture of the EW50 at the NWTC.

Figure 1. Entegrity EW50 test turbine at the NWTC (PIX# 22243)

The EW50 wind turbine was mounted on a 30.5-m (100-ft) monopole tower manufactured for 

the Entegrity EW50 by Maico Industries, Inc. The test turbine used a controller manufactured by 
Page 7 of 36

Orbital A/S. The concrete mat foundation was installed per Entegrity designs by a third party, 

under contract to NREL. The system was installed in March 2009 by the NWTC Site Operations 

group with guidance and assistance from several Entegrity personnel.

The following components were considered part of the test turbine system:

• Tower and foundation designed for installation at the NWTC test site 1.E1

• All control components, including wiring between the up-tower components and the 

down-tower control panel

• All wiring and components on the turbine side of the subpanel, which connects the 

turbine to the NWTC site electrical grid at the 1.E1 data shed.

Table 1: Test Turbine Configuration and Operational Data

Turbine manufacturer and address

Entegrity Wind Systems Inc.

4855 Riverbend Rd.; Ste 100

Boulder, CO 80301

Model name EW50

Gearbox serial number 543132-007

Generator serial number C0812180024

Production date January 2009

Design nominal voltage at terminals 480 Vac

Maximum current at terminals 108 A

Design frequency at terminals 60 Hz

SWT class II

Design 50-year extreme wind speed, Ve50 59.5 m/s

Rotor Diameter 14.9 m

Hub Height (vertical center of rotor) 31.1 m

Tower Type 30.5m (100') freestanding monopole

Rated Electrical Power 50 kW

Rated Wind Speed (lowest wind speed at 

which turbine produces rated power) 11.3 m/s

Rated rotor speed (lowest rotor speed at 

which turbine produces rated power) 65 rpm

Rotor speed range 64-70 rpm

Cut-out Wind speed (m/s) 25

Fixed or variable pitch Fixed w/ tip brake

Number of Blades 3

Blade Tip Pitch Angle (deg) 5.14 ̊ to 5.17 ̊ at 75% span

Blade make, type, serial number Entegrity, epoxy/glass fiber, 7.2m, 150kg, 

s/n: 284, 285, 283

Description of control system (device & 

software version)

TMC microprocessor by Orbital A/S in 

Entegrity enclosure version: “EW15 2.031”

Figure 3 shows the general electrical arrangement of the test. The wire run from the controller at 

the base of the tower, to the point of grid connection at the data shed was approximately 100 m.

The connection was made using 1/0 American Wire Gauge (AWG) wire for the three hot lines 

plus the neutral line, and a #2 AWG for ground line. The data shed housed the power 

instrumentation, disconnect switch, and data acquisition system (DAS). The transformer was 
Page 8 of 36

located outside and adjacent to the data shed, which first stepped the voltage up to 480 V and 

then to 13.2 kV for the NREL grid.

Figure 2. Electrical single-line drawing of Entegrity EW50 installation

4. Description of Test Site

The test turbine was located at site 1.E1 at the National Wind Technology Center, located 8 km 

(5 miles) south of Boulder, Colorado. The terrain primarily consists of mostly flat terrain with 

short vegetation. The test site has prevailing wind bearing 292° relative to true north. Figure 3 

shows the turbine and meteorological tower locations. This figure also shows nearby 

obstructions and topographical features of the site.
Page 9 of 36

Figure 3. Map of area surrounding Entegrity’s EW50 at NWTC’s 1.E1 test site

5. Description of Instrumentation

Test instrumentation was installed in accordance to IEC 61400-12-1 for measuring wind speed, 

wind direction, turbine power, air temperature and air pressure. For duration testing, there were 

additional signals to monitor the rotor speed, generator speed, brake status, and overall turbine 

system availability. Figure 4 shows the location of the meteorological tower instruments and 

Table 2 provides an equipment list with the specifications for each of the instruments used. 
Page 10 of 36

Figure 4. Meteorological tower and instruments
Page 11 of 36

Table 2: Equipment List for Duration Test

Instrument Make and Model Serial Number Calibration Due Dates

Power transducer Ohio Semitronics, DMT 

1040E 07070473 12 Feb 10 (recalibrated)9 Feb 11

Current transformers Ohio Semitronics, 12974

001293045

001235428

001293049

Calibrated with power 

transducer

Primary anemometer Thies, First Class

0707884

0707885

0609010

24 Feb 10

23 Jul 10

22 Jul 11

Reference 

anemometer NRG, Max 40 179500049025 –

Wind vane Met One, 020C with 

Aluminum Vane U1477 25 Feb 10(post-test calibrated)

Pressure sensor Vaisala, PTB101B

T0740016

C1020015

T3330002

9 Aug 09

3 Sept 10

10 Aug 11

Temperature sensor Met One, T200 0549828 10 Oct 09 (recalibrated)

10 Feb 11

Precipitation sensor Campbell Scientific, 237 None In situ

Data acquisition 

system

Compact DAQ w/LabView 

cDAQ backplane (9172)

NI 9229

NI 9217

NI 9205

 Second set of modules

NI 9229 

NI 9217

NI 9205

12E4CEB

13DEC38

13FAE1C

13E3D05

12CB7A

12C73B4

12ECB77

N/A

10 Nov 09

16 Dec 09

12 Nov 09

18 Jan 11

18 Jan 11

18 Jan 11

The power transducers were not compliant with class 0.5 (or better) specifications of the IEC 

60688 “Electrical measuring transducers for converting alternating current (AC) electrical 

quantities to analogue or digital signals,” but the transducers exceeded the minimum accuracy 

required by the Standard.

The current transformers were not compliant with class 0.5 (or better) specifications of the IEC 

60044-1 “Instrumentation transformers – Part 1: Current transformers,” but the transformers

exceeded the minimum accuracy required by the Standard.

The data acquisition modules were used beyond the calibration due date. The modules were 

post-test calibrated and found to be in compliance within the specifications. Appendix B includes 

the post-test calibration sheets.
Page 12 of 36

6. Results

The turbine was delivered to the NWTC in February 2009. The Entegrity EW50 was installed in 

March 2009. After a commissioning/shake-down period, which included replacing the parking 

brake solenoid, testing began on 2 April 2009.

6.1. Period of Testing

The Entegrity EW50 was under testing from 2 April 2009 until 23 August 2010, and from 14 

October 2010 until 17 January 2011.

On 23 August 2010, the test team found capacitors in the power factor correction unit to be 

faulty and suspended the testing, preventing the EW50 from operating. On 14 October 2010,

the test team decided to restart testing after determining that the failed capacitors did not 

constitute a major failure with reference to the Standard.

6.2. Months of Operation

The duration test was conducted over a period of approximately 18.5 months (not including the 

two months of suspended testing), exceeding the six month minimum required by the Standard.

The end of testing was determined by the need to recalibrate DAS modules. On 20 January,

2011 the Entegrity EW50 tip brakes would consistently deploy as soon as the rotor reached 

operational speed stopping the turbine. On that day, the Entegrity EW50 controller was turned 

off and the turbine did not operate again.

6.3. Hours of Power Production

The hours of power production at any wind speed: 2,807.6 hours (2,500 hours required)

The hours of power production above 1.2*Vave (10.2 m/s): 686.2 hours (250 hours required)

The hours of power production above 1.8*Vave (15.3 m/s): 161.9 hours (25 hours required)

The turbine therefore met the requirements for hours of power production during the test. Table 

3 shows the overall and month-by-month results of the duration test.
Page 13 of 36

Table 3: Monthly and Overall Results of the Entegrity EW50 Duration Test

6.4. Operational Time Fraction

The operational time fraction is defined as follows:

×100% − −

− − − =

T U E

T N U E

T T T

T T T T O

where:

TT = total time period under consideration

TN = time during which the turbine is known to be non-operational

TU = time during which the turbine status is unknown

TE = time which is excluded in the analysis

The overall operational time fraction of the combined wind turbine system in the total test period 

was 98.9%. The final column of Table 3 shows the operational time fraction per month. 

The total test time (TT) was 15,012 hours, not including the 1,250 hours in which testing was 

suspended from August to October 2010. The total environmental exposure from beginning of 

test to completion was approximately 16,262 hours.

Month 0 m/s 1.2Vavg10.2 m/s

1.8Vavg

15.3 m/s max gust TI @ 15 

m/s TT TU TE TN O [%]

Totals 2807.6 686.2 161.9 44.2 19.7 15012.0 1222.1 522.1 148.8 98.9

Minimum Met? Yes Yes Yes Yes Yes

Apr 2009 164.2 29.3 5.5 31 18.2 684 0.1 5.7 0 100

May 148.2 9.8 0 21.5 744 0.5 6 0 100

Jun 105.7 11.2 1.2 30 17.6 720 0 0.2 0 100

Jul 81.5 7.7 0.7 28.1 24 744 14.3 0 0 100

Aug 32.5 0.5 0 18 744 370.9 14.3 70.7 80.3

Sep 70.8 13.3 1.3 27.4 16.4 720 134.6 31.5 0 100

Oct 163.7 44.2 1.7 30.5 18.1 744 11.7 49.2 0.5 99.9

Nov 121.2 8 0 22.3 720 -0.9 6.3 0 100

Dec 207.5 62.7 12.2 36.1 19 744 0.7 0 0 100

Jan 2010 111.7 46.2 14.2 33.2 21.6 744 29.2 2.7 68.8 90.3

Feb 38.3 12.2 3.2 28.9 19.7 672 393.8 0.3 0 100

Mar 137.7 21 0.3 25.1 18.1 744 63.6 0.3 0 100

Apr 223 62.7 19.7 32.7 19 720 2.7 0 8.5 98.8

May 169.2 51 17.8 39 18.4 744 4.1 0.7 0 100

Jun 100 6.7 0 22.9 16.9 720 0.1 0 0 100

Jul 136.5 10.5 0.3 26.9 19.3 744 1.2 1.5 0 100

Aug 105.2 17.2 1.7 28.8 18 744 29.2 213.8 0 100

Sep 0 0 0 0 0 0 0 0 0 0

Oct 134.8 54.3 18.7 38 20.2 744 142.1 186.3 0 100

Nov 169.5 55.7 16 44.2 20.6 720 -0.5 0.8 0 100

Dec 184.2 57 20.2 35.1 19.4 744 2.5 2.5 0.3 100

Jan 2011 202.2 105 27.2 42.9 20.9 408 22.2 0 0 100

Hours of power production in Environmental conditions Operational time fraction
Page 14 of 36

The Entegrity EW50 turbine system experienced non-operational time, or downtime (TN), for 

several reasons. The significant events are detailed below:

• 4 August, 2009 

Four months into the testing, the controller universal power supply (UPS) failure resulted 

in three days of down time. To restore the EW50 to an operational state, Entegrity 

personnel bypassed the UPS and the EW50 was returned to normal operation.

• 15 January, 2010

A surge suppressor and a fuse failed. These were replaced by Entegrity personnel and 

the turbine was put back into service.

• April 2010

Two generator over-speed faults resulted in a total of about eight hours and 45 minutes 

of non-operational time. In each case this required a manual reset. NWTC personnel 

reset the turbine per EW50 operating instructions.

• 12 December, 2010 

Over-current fault. This fault was automatically reset by the EW50 controller, and the 

EW50 resumed normal operation within 10 minutes from the time of the fault.

The main reasons for excluding time (TE) in the duration test were:

• Suspension of testing between 23 August 2010 and 14 October 2010, as mentioned 

above in section 6.1 Period of Testing

• Time during power outages that prevented the turbine from running

• Noise or safety and function testing that required the turbine to be shutdown

• NWTC initiated inspections of the EW50.

If no reliable measurements were available, the time was classified as unknown (TU) because 

the turbine’s status was unknown. These were primarily when the DAS was off (maintenance or 

power outage) or when both anemometers were iced.

6.5. Environmental Conditions

During the test period, the highest instantaneous wind speed was 44 m/s during November 

2010. The average turbulence intensity at 15 m/s during the test was 19.7%.

6.6. Power Degradation Checks

For the power degradation analysis, the average power for each wind speed bin is plotted as a 

function of time over the whole test period. This plot is analyzed for any obvious trends in power 

production. This plot is shown in Figure 5.
Page 15 of 36

Figure 5. Power in each wind speed bin [m/s] vs. time

Figure 5 shows the power level in individual wind speed bins for each month. The apparent 

degradation in power at 22 m/s for November 2010 is due to the EW50 reaching its high-wind 

cut-out twice in one wind event. This cut-out data was not filtered-out and the 10-minute delay 

before resuming power production lowered the 22-m/s average power value.

The lower power production for wind speed above 17 m/s in January 2011 is attributed to highwind cut-outs. During January 2011, there were seven instances in which winds exceeded the

EW50’s high-wind cut-out (25 m/s) several times during each event. During each cut-out, the 

EW50 did not operate for at least 10 minutes. If the average wind speed during the 10-minute 

shut-down period was less than 20 m/s, the turbine would resume operation. However, these 

non-operational periods were not filtered out from the power degradation plot because they are 

not considered faults since the turbine is designed to shut down in very high winds and wait for 

winds below 20 m/s. Inclusion of these shut-down periods therefore lowered the average power 

for several wind speed bins.

Figure 5 illustrates some power degradation emerging in January 2011.

-10

0

10

20

30

40

50

60

70

Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan

Power (kW)

Month

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

2009 2010 2011
Page 16 of 36

6.7. Dynamic Behavior

NWTC personnel observed the turbine operation for at least five minutes at wind speeds of 

approximately 5 m/s, 10 m/s, 15 m/s, and 20 m/s, for a total observation time of at least 

one hour. 

NWTC recorded the following observations during this time:

• The EW50 produces a significant gearbox noise. When the generator was operating 

exactly at synchronous speed, the turbine was relatively quiet, but was not quiet when 

operating sub-synchronously or super-synchronously. When the brakes were deployed,

there was a significant amount of noise. The parking brake deployment brought the rotor 

to stop very quickly and slammed the tip brakes back to their stow/operate position,

causing a shudder through the entire tower. Although vibrations could be felt in the tower 

and sometimes seen, the vibrations were not deemed excessive. (Accelerations on the 

EW50 were not measured.)

• Yaw oscillations can be severe at times, particularly in gusty winds. The rotor would 

occasionally swing 60° in approximately one second.

• In gusty winds, the rotor can get stuck upwind, which, in winds below approximately 10

m/s, appeared almost as dynamically stable as downwind. In winds above 15 m/s the 

turbine rarely operated in the upwind position.

• In winds around 4.5 m/s, the brakes release. This could be heard near the tower. The 

turbine requires winds greater than approximately 5.5 m/s to freewheel up to 

synchronous speed.

• In winds around 5 m/s, the generator may motor up the rotor. Witnessed motoring up 

events lasted up to about three seconds. These were loud and caused some vibration to 

the turbine. With the inertia of a spinning rotor, if winds were above 6 m/s the turbine 

rotor could quickly increase to operational speed. If winds were less than 5.5 m/s the 

rotor speed gradually fell, never reaching synchronous speed.

6.8. Post-test Inspection

NWTC conducted part of the post-test inspection on 25 January 2011, while the EW50 was still 

installed. The rest of the post test inspection was conducted after 18 August 2011, when the 

EW50 was decommissioned and partially disassembled. 

The EW50 tip-brakes are aluminum and a circular steel plate attached to the tip brake is 

attracted to the electromagnetic, which keeps the brakes in place during normal operation. On 

25 January 2011, the rubber bushing that connects the tip-brake to the steel plate was 

discovered to have deteriorated on one tip brake (see Figure 7 and Figure 8 in Appendix A).

Without a properly fitting rubber bushing, the tip brake can move a few degrees around its hinge 

point, which changes its aerodynamics. However, when the bushings were replaced at the end 

of January 2011, the EW50 still experienced premature tip brake deployment.

During the 18 August 2011 tear-down and inspection, corrosion was found in the electrical 

connections to the tip brake’s electrical magnet. This could be the cause of the early tip brake 
Page 17 of 36

deployment failures, but this was not tested. The blades were in good condition and no cracks 

were found.

On 24 August 2011, the gearbox oil was drained. The oil contained significant contamination.

The gears were not inspected because it was difficult to access them without taking the gearbox 

apart.

Pictures of the inspections are provided in Appendix A.

7. Uncertainty

The uncertainty is estimated for the following parameters:

• Hours of power production 

• Operational time fraction

• Environmental conditions

No uncertainty analysis was done for the power degradation results. These results were used 

only to find relative trends, which might indicate deterioration in the turbine’s performance.

7.1. Hours of Power Production

The turbine can be assumed to be producing power for the entire 10-minute period whenever 

the average power for that period is positive and winds are below the 25 m/s threshold. This 

method overestimates time for power production in average wind speeds around 6 m/s. When 

the 10-minute average wind speed is close to 6 m/s, the turbine may have been producing 

power for less than half of the time recorded. At higher wind speeds, this method produces less 

of an overestimate. However, this overestimate is offset because in low wind speeds, the EW50 

continues to operate at rated rotor speed by consuming power for 10 minutes before the 

controller disconnects the generator form the grid. Therefore, any 10-minute period that has 

positive power and winds below cut-out represents at least a continuous 10-minute period in 

which the EW50 was operating. While the EW50 was operating (generating or consuming 

power), wear accumulated on the entire turbine system. 

For the hours of power production above 10.2 m/s and 15.3 m/s, the uncertainty in the wind 

speed is assumed to be the dominant uncertainty factor. Assuming an uncertainty in wind speed 

of 0.3 m/s, the hours of power production reduces to 664.67 (above 10.5 m/s) and 157.67

(above 15.6 m/s); this is still well over the 250 and 25 hours that are required.

The tool used at the NWTC does not accurately account for switching from standard to daylight 

savings times, even though the time stamp on the data reflects this switch. It is assumed the net 

effect on results is negligible, even though its affects can be seen in the values of TU for 

November 2009 and 2010 and March 2010.
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7.2. Operational Time Fraction

If there was a failure at any time during a 10-minute period, the entire 10-minute period was 

classified as downtime (TN). This means that the 98.9% is the lower bound of the operational 

time fraction.

7.3. Environmental Conditions

The maximum deviation in anemometer measurements for the calibration range is 0.032 m/s, 

which results in a standard uncertainty of 0.02 m/s. The calibration range was only to 16 m/s; 

thus, an extrapolation to 44 m/s could be unreliable. However, it is assumed that the standard 

uncertainty of calibration is consistent at 0.02 m/s. Combining the calibration uncertainty with

operational characteristics (0.052 m/s + 0.52%), mounting effects (1%), and terrain effects (2%), 

while assuming DAS uncertainty is negligible for the maximum instantaneous gust of 44 m/s, 

the resulting cumulative uncertainty is 1 m/s.

Standard deviation of wind speed is used for the Turbulence Intensity (TI) calculation. Average 

wind speed at 15 m/s has an uncertainty of 0.4 m/s, resulting in a TI estimate of between 19.2%

and 20.2%.

8. Deviations and Exceptions

8.1. Deviations from the Standard

As explained in Section 5, Description of Instrumentation:

• The power transducer was not compliant with IEC 60688, but exceeded the minimum 

accuracy required by the Standard. This should have no effect on results or uncertainty, 

but it is in exception to the Standard.

• The current transformers were not IEC 60044-1, but exceeded the minimum accuracy 

required by the Standard. This should have no effect on results or uncertainty, but it is in 

exception to the Standard.

8.2. Deviations from Quality Assurance

The data acquisition modules were used beyond the calibration due date. They were post-test 

calibrated and found to be in compliance within the specifications. Appendix B includes the posttest calibration sheets.
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A. Appendix: Pictures of Post-Test Inspection

Figure 6. Tip brake #1 bushing has disintegrated, allowing for some play in the tip brake

(PIX# 22238)

Figure 7. Tip brake #3, the rubber bushing is still intact (PIX# 22239)
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Figure 8. Rust and oil marks near the generator and gearbox connection (PIX# 22248)

Figure 9. Corrosion around electrical connector to tip brake’s electromagnet (PIX# 22241)
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Figure 10. Draining earbox oil; significant contaminints were seen, such as the dark 

spots inside the clear tube (PIX# 22246)
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B. Appendix: Equipment Calibration Sheets

11. Anemometer #1; installed on 1 April 2009, used until 24 February 2010

12. Anemometer #2; installed on 24 February 2010, used until 23 July 2010

13. Anemometer #3; installed on 23 July 2010, used until end of test

14. Power transducer calibration #1; installed 25 February 2009, removed 8

February 2010

15. Power transducer calibration #2; installed 9 February 2010, used until end of test 

16. NI 9229 data acquisition module #1; installed 25 February 2009, removed 3

February 2010

17. NI 9229 data acquisition module #1; post-test calibration 

18. NI 9229 data acquisition module #2; installed 18 January 2010 used until end of 

test

19. NI 9217 data acquisition module #1; installed 25 February 2009, removed 3

February 2010

20. NI 9217 data acquisition module #1; post-test calibration 

21. NI 9217 data acquisition module #2; installed 18 January 2010 used until end of 

test

22. NI 9205 data acquisition module #1; installed 25 February 2009, removed 3

February 2010

23. NI 9205 data acquisition module #1; post-test calibration 

24. NI 9205 data acquisition module #2; installed 18 January 2010 used until end of 

test
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Figure 11. Anemometer #1; installed on 1 April 2009, used until 24 February 2010
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Figure 12. Anemometer #2; installed on 24 February 2010, used until 23 July 2010
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Figure 13. Anemometer #3; installed on 23 July 2010, used until end of test
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Figure 14. Power transducer calibration #1; installed 25 February 2009, removed 8

February 2010
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Figure 15. Power transducer calibration #2; installed 9 February 2010, used until 

end of test
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Figure 16. NI 9229 data acquisition module #1; installed 25 February 2009, 

removed 3 February 2010
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Figure 17. NI 9229 data acquisition module #1; post-test calibration
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Figure 18. NI 9229 data acquisition module #2; installed 18 January 2010 used 

until end of test
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Figure 19. NI 9217 data acquisition module #1; installed 25 February 2009, 

removed 3 February 2010
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Figure 20. NI 9217 data acquisition module #1; post-test calibration
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Figure 21. NI 9217 data acquisition module #2; installed 18 January 2010 used 

until end of test
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Figure 22. NI 9205 data acquisition module #1; installed 25 February 2009, 

removed 3 February 2010
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Figure 23. NI 9205 data acquisition module #1; post-test calibration
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Figure 24. NI 9205 data acquisition module #2; installed 18 January 2010 used 

until end of test