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

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This report is available at no cost from the National Renewable Energy

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Contract No. DE-AC36-08GO28308

Duration Test Report for the 

Viryd CS8 Wind Turbine

J. Roadman, M. Murphy, and J. van Dam

National Renewable Energy Laboratory

Technical Report

NREL/TP-5000-58970

June 2013
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.

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Duration Test Report for the 

Viryd CS8 Wind Turbine

J. Roadman, M. Murphy, and J. van Dam

National Renewable Energy Laboratory

Prepared under Task No. WE11.0206

Technical Report

NREL/TP-5000-58970

June 2013
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. 

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Cover Photos: (left to right) photo by Pat Corkery, NREL 16416, photo from SunEdison, NREL 17423, photo by Pat Corkery, NREL 

16560, photo by Dennis Schroeder, NREL 17613, photo by Dean Armstrong, NREL 17436, photo by Pat Corkery, NREL 17721.

 Printed on paper containing at least 50% wastepaper, including 10% post consumer waste. 
Viryd Duration Test Report 130604 Page 4 of 54

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Duration Test Report 

for the

Viryd CS8 Wind Turbine

at the

National Wind Technology Center 

in Boulder, Colorado

Conducted for

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, Colorado 80401

Conducted by

National Wind Technology Center

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, Colorado 80401 

Jason Roadman

Mark Murphy

Jeroen van Dam

30 May 2013
Viryd Duration Test Report 130604 Page 

5 of 54

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at www.nrel.gov/publications.
Viryd Duration Test Report 130604 Page 6 of 54

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at www.nrel.gov/publications.

Table of Contents

List of Figures ............................................................................................................................................. 7

List of Tables ............................................................................................................................................... 7

1 Background ........................................................................................................................................... 8

2 Test Objective and Requirements....................................................................................................... 8

3 Description of Test Turbine................................................................................................................. 9

4 Test Site Description.......................................................................................................................... 12

5 Description of Instrumentation ......................................................................................................... 14

6 Results................................................................................................................................................. 17

6.1 Operation Time .............................................................................................................................. 17

6.2 Hours of Power Production............................................................................................................ 17

6.3 Operational Time Fraction............................................................................................................. 18

Failed Anemometer........................................................................................................................ 19

Failed Power Supply...................................................................................................................... 19

Failed Compressor ......................................................................................................................... 19

Remaining TN................................................................................................................................. 19

Excluded Time TE .......................................................................................................................... 19

Unknown Time TU ......................................................................................................................... 19

6.4 Environmental Conditions............................................................................................................. 19

6.5 Power Degradation Checks............................................................................................................ 20

6.6 Dynamic Behavior......................................................................................................................... 21

7 Uncertainty .......................................................................................................................................... 23

7.1 Hours of Power Production............................................................................................................ 23

7.2 Operational Time Fraction............................................................................................................. 23

7.3 Highest Instantaneous Wind Speed ............................................................................................... 23

8 Deviations and Exceptions................................................................................................................ 24

8.1 Deviations from the Standard ........................................................................................................ 24

8.2 Deviations from Quality Assurance............................................................................................... 24

References................................................................................................................................................. 25

Appendix A: Instrument Calibration Certificates................................................................................... 26

Appendix B: Turbine Commissioning Checklist.................................................................................... 47

Appendix C: Post-Test Teardown Inspection Report............................................................................ 49

Nose cone ............................................................................................................................................. 49

Nacelle cover........................................................................................................................................ 49

Blades................................................................................................................................................... 49

Brake system ........................................................................................................................................ 49

Gearbox ................................................................................................................................................ 49

Yaw system .......................................................................................................................................... 49

Generator.............................................................................................................................................. 50

Tail assembly........................................................................................................................................ 50

Up-tower Electrical components.......................................................................................................... 50

Tower.................................................................................................................................................... 50

Foundation ..................................................................................................................................... 50

Controller.............................................................................................................................................. 50
Viryd Duration Test Report 130604 Page 7 of 54

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List of Figures

Figure 1. Viryd CS8 test turbine at the NWTC........................................................................................ 10

Figure 2. Electrical schematic of turbine installation............................................................................ 12

Figure 3. Map of the test site.................................................................................................................... 13

Figure 4. Close up of test site around the test turbine ......................................................................... 13

Figure 5. View of instruments on the meteorological tower................................................................. 16

Figure 6. Operational time fraction for each month .............................................................................. 18

Figure 7. Scatter plot of power versus wind speed (10-minute averages).......................................... 20

Figure 8. Power level in several wind speed bins (m/s) as a function of time.................................... 21

Figure 9. Power level in several wind speed bins (m/s) as a function of time with known nonoperational time removed.................................................................................................................. 21

Figure A1. Calibration sheet for primary anemometer used through 12 September 2012................ 26

Figure A2. Calibration sheet for primary anemometer used from 17 September 2012 on................ 27

Figure A3. Calibration sheet for the wind vane used through 12 September 2012 ........................... 28

Figure A4. Calibration sheet for the wind vane used from 17 September 2012 on............................ 29

Figure A5. Page 1 of power transducer calibration sheet used through 12 September 2012 .......... 30

Figure A6. Page 2 of power transducer calibration sheet used through 12 September 2012 .......... 31

Figure A7. Page 1 of power transducer calibration sheet used from 17 September 2012 on........... 32

Figure A8. Calibration sheet for the pressure transducer used from 15 May 2012 on...................... 33

Figure A9. Post-test calibration sheet for the pressure transducer used through 15 May 2012...... 34

Figure A10. Calibration sheet for the pressure transducer used from 16 May 2012 on.................... 35

Figure A11. Calibration sheet for temperature sensor used through 12 September 2012................ 36

Figure A12. Calibration sheet for temperature sensor used from 17 September 2012 on................ 37

Figure A13. Calibration sheet for signal conditioning module 13DEC38 used through 26 July 201238

Figure A14. Post-test calibration sheet for signal conditioning module 13DEC38 used through 26 

July 2012.............................................................................................................................................. 39

Figure A15. Calibration sheet for signal conditioning module 12A2037 used from 26 July 2012 on40

Figure A16. Calibration sheet for signal conditioning module 13FAE1C used through 26 July 201241

Figure A17. Post-test calibration sheet for signal conditioning module 13FAE1C used through 26 

July 2012.............................................................................................................................................. 42

Figure A18. Calibration sheet for signal conditioning module 12BFEE2 used from 26 July 2012 on43

Figure A19. Calibration sheet for signal conditioning module 13E3D05 used through 26 July 201244

Figure A20. Post-test calibration sheet for signal conditioning module 13E3D05 used through 26 

July 2012.............................................................................................................................................. 45

Figure A21. Calibration sheet for signal conditioning module 14DA726 used from 26 July 2012 on46

Figure B1. Viryd CS8 commissioning checklist, page 1....................................................................... 47

Figure B2. Viryd CS8 commissioning checklist, page 2....................................................................... 48

Figure C1. Scratches on Viryd nacelle cover......................................................................................... 51

Figure C2. Chip discovered on leading edge of blade .......................................................................... 51

Figure C3. Blade root................................................................................................................................ 52

Figure C4. Loosened brake caliper bolt.................................................................................................. 52

Figure C5. Inspection inside gearbox..................................................................................................... 53

Figure C6. Rusty fasteners on tail........................................................................................................... 53

Figure C7. Failed run capacitor ............................................................................................................... 54

List of Tables

Table 1. Test Turbine Configuration and Operational Data.................................................................. 11

Table 2. Measured Blade Pitch Angle Relative to the Hub Plate.......................................................... 11

Table 3. List of Channels and Measurement Instruments .................................................................... 15

Table 4. Overall and Monthly Results of the Viryd CS8 Duration Test................................................ 17
Viryd Duration Test Report 130604 Page 8 of 54

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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 of wind energy expansion by 

providing independent testing results for small turbines. Several turbines were selected for 

testing at the National Wind Technology Center (NWTC) at the National Renewable Energy 

Laboratory (NREL) as a part of the Small Wind Turbine Independent Testing project. Duration

testing is one of up to five tests that may be performed on the turbines. Other tests include safety 

and function, power performance, acoustic noise, and power quality. Viryd Technologies, Inc. of 

Austin, Texas, was the recipient of the DOE grant and provided the turbine for testing. 

Researchers at the NWTC conducted this test in accordance with its quality system procedures to 

ensure that this final test report meets the full accreditation requirements of A2LA. NREL’s 

quality system requires that the test meet all applicable requirements specified by A2LA and 

ISO/IEC 17025 (or to note any exceptions in the test report). 

2 Test Objective and Requirements

The objective of this test was to assess the following aspects of the Viryd CS8 wind turbine 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 (hereafter referred to as the “Standard”). 

NREL tested the turbine’s:

• Structural integrity and material degradation

• Quality of environmental protection 

• Dynamic behavior. 

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

• At least 6 months of operation 

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

• At least 250 hours of power production in winds of 1.2 Vave, annual average wind speed 

at hub height,(10.2 m/s) and above 

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

Reliable operation means:

• An 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, Viryd identified 

the test turbine to be a class II. This corresponds to a Vave of 8.5 m/s. 
Viryd Duration Test Report 130604 Page 9 of 54

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3 Description of Test Turbine

The test turbine, shown in Figure 1, is a three-bladed, upwind, passive yaw turbine with a rated 

power of 8 kW. Table 1 lists the basic turbine configuration and operational data provided by 

the manufacturer. As part of Viryd’s standard commissioning procedure, small shims were 

inserted between the hub plate and the blade root, thereby pitching the leading edge of the blades 

into the wind. The pitch angle of each blade with respect to the hub plate is fixed. The angle was 

measured and provided in Table 2. 

Figure 2 shows the one-line diagram for the test turbine installation.

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

• A tower and foundation that were designed for installation at the NWTC test site 3.3a 

• The wiring and components on the turbine side of the test shed’s electrical panel, which 

connects the turbine system to the electrical grid at the test site

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

down-tower control panel. 
Viryd Duration Test Report 130604 Page 10 of 54

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Figure 1. Viryd CS8 test turbine at the NWTC 

(Photo by Mark Murphy, NREL 22258) 
Viryd Duration Test Report 130604 Page 11 of 54

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Table 1. Test Turbine Configuration and Operational Data

Turbine Manufacturer and Address

Viryd Technologies, Inc.

9701 Metric Blvd. Suite 200

Austin, TX 78758

Model Viryd CS8

Serial number CS008100X

Rotor diameter (m) 8.5

Hub height (m) 25

Tower type U.S. tower, guyed, tilt-up lattice

Rated electrical power (kW) 8

Rated wind speed (m/s) 10

Small wind turbine class II

Rotor speed range (rpm) 115−125

Fixed or variable pitch Fixed

Power regulation (active or 

passive)

Passive

Number of blades 3

Blade pitch angle (deg) See Table 2

Blade make, type, serial number Viryd proprietary design, serial numbers 

not provided

Direction of rotation Clockwise viewed from upwind

Description of control system 

(device and software version)

Proprietary – PCB

Tower make, type, height U.S. Tower, tilt-up, guyed lattice, 24.4 m

Electrical output 240 VAC single phase

Table 2. Measured Blade Pitch Angle Relative to the Hub Plate

Blade Pitch Angle

1 1.4°

2 1.1°

3 1.4°

The test configuration consisted of the turbine mounted on a lattice tower, the controller, 

meteorological tower, associated wiring and junction boxes, and a data shed containing the data 

acquisition instrumentation. The turbine was installed on a guyed 24.4 m, tilt-up lattice tower. 

The wire run from the base of the tower to the data shed is approximately 98 meters of #6 

American Wire Gauge (AWG) wire. Inside the test shed, the turbine was connected to a 240-volt 

(V) panel, where 240/480-V and 0.480/13.2 kilovolt (kV) transformers allowed the turbine to 

connect to the site’s 13.2-kV grid. Grid tolerances were 1% for frequency and 5% for voltage. 

This general electrical arrangement is shown in Figure 2. 
Viryd Duration Test Report 130604 Page 12 of 54

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Viryd Control

Panel

Supplemental

Junction Box

Uptower 

J-box

Met tower jbox

Power 

measurement box

60A 600V 

disconnect

kWh 

meter

Power panel 3-3L 

120/240V

L1

datashed

3*6AWG

>8*20AWG

4* 8AWG

Ground (8AWG)

Ground (6AWG)

Ground rod on the north 

side of datashed

2 pole 60A 

breaker 

no.27&29

Everything between power 

box and panel 3-3 L 

6AWG

N

L2

60A 600V 

disconnect

LO/TO 

Disconnect

3* 4AWG

Viryd control wires

Viryd DAS 

Box>8*20AWG

3* 6AWG

Figure 2. Electrical schematic of turbine installation

(Source: NREL 2012)

4 Test Site Description

The Viryd CS8 wind turbine was located at test site 3.3a of the NWTC, approximately 8 km 

south of Boulder, Colorado. The site consists of mostly flat terrain with short vegetation and 

prevailing winds bearing 292° relative to true north. Figure 3 and Figure 4 show the turbine and 

meteorological tower locations, as well as nearby turbines. NREL engineers limited assessments 

of power degradation to data that were obtained when winds were within the measurement sector

of 211° to 38°. In this measurement sector, the influence of terrain and obstructions on the 

anemometer is small and meets the requirements of IEC 61400-12-1 without having to conduct a 

site calibration test. 
Viryd Duration Test Report 130604 Page 13 of 54

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Figure 3. Map of the test site 

(Source: NREL 2012)

Figure 4. Close up of test site around the test turbine 

(Source: Map data ©2012 Google earth)

Skystream

Test turbine

Met tower

50 m
Viryd Duration Test Report 130604 Page 14 of 54

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5 Description of Instrumentation

Equipment used for duration testing differs only slightly from the equipment used for power 

performance testing. Normal power performance requires measurements of wind speed, wind 

direction, turbine power, air temperature, air pressure, precipitation, and overall turbine system 

availability. For duration testing, NREL added voltage signals to monitor the status lightemitting diodes (LEDs) on the controller. The green LED indicates when the turbine is 

producing power; the blue LED indicates when the turbine is on standby: “waiting for wind” 

(solid), or experiencing a cut-out wind condition (blinking); and the red LED indicates when the 

turbine has faulted. The red LED blinks various times at various duty cycles, based on whether 

the turbine is experiencing a recoverable or unrecoverable fault or startup/shutdown sequences. 

However, Viryd informed NREL that the number of blinks alone was not sufficient to determine 

the cause of a fault. A computer was required to log into the controller and determine the exact 

cause. As a whole, this level of diagnostics was deemed beyond the scope of this test. Instead, 

NREL engineers set the threshold for examining whether the turbine was faulted based on the 

10-minute average of the red LED signal being higher than 1% of its full-scale voltage when 

solidly lit. Every identified sequence was manually examined and classified as a fault or not 

based on additional signals and turbine behavior. For true faults, the NREL team worked under 

the guidance of Viryd to diagnose and fix the cause. Many of these faults are described below. 

Figure 5 provides the location of the meteorological tower instruments and Table 3 provides the 

specifications for each of the instruments used. The first set of data acquisition modules were 

used beyond their calibration due date; they were post-calibrated and found to be within 

tolerance. The pressure transducer was out of calibration and was post-test calibrated. Residuals 

between the two calibrations were found to be 0.006 kPa at most, a negligible amount in the 

context of this test. No data was collected between 12 September 2012 and 17 September 2012 

due to an instrumentation change. Post-test calibration sheets are provided in Appendix A. 
Viryd Duration Test Report 130604 Page 15 of 54

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Table 3. List of Channels and Measurement Instruments

Signal Location Sensor Make Model Serial Number Cal Due Date

Primary Wind 

Speed

24.9 m Thies First Class 0707884

0609006

12 Sept. 2012

17 Sept. 2013

Reference Wind 

Speed

23 m Met One, 010C U2643 NA

Wind Direction 23 m Met One, 020 U1475

W5515

13 Sept. 2012

17 Sept. 2013

Air Pressure 22.1 m Vaisala, PTB101B T0740016

C1040014

5 April 2012

13 Feb. 2013

Air Temperature 22.4 m Met One, T-200 0566229

0603-1

15 Sept. 2012

17 Sept. 2013

Precipitation Data Shed Campbell Scientific 237 NA NA

Active Power Data Shed Secondwind Phaser 5-4A20

with OSI pn. 12973 CT’s

01091

01091

15 Sept. 2012

15 Sept. 2013

Turbine Status Turbine 

Controller

Turbine Controller Lights/ 

Brake Solenoid

NA NA

Rotor Speed Turbine 

Controller

Phoenix Contact MCR-f-UIDC67472901 3 Oct. 2012

Data Acquisition 

Modules

Data Shed National Instruments NI 9229

National Instruments NI 9217

National Instruments NI 9205

13DEC38

12A2037

13FAE1C

12BFEE2

13E3D05

14DA726

24 June 2012

27 June 2013

24 June 2012

27 June 2013

24 June 2012

27 June 2013
Viryd Duration Test Report 130604 Page 16 of 54

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Figure 5. View of instruments on the meteorological tower 

(Source: NREL 2012)

Reference Wind Speed

Air Pressure

Wind Direction

Primary Wind Speed

Air Temperature
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6 Results

6.1 Operation Time

The test turbine system was installed on 22 September 2011 (the commissioning checklist from 

the installation can be found in Appendix B). The turbine was ready for its initial shakedown 

period on 12 October 2011. The duration test began on 1 November 2011 and was terminated on 

1 February 2013, after the turbine no longer produced power following a suspected and 

confirmed failure of the air compressor. Although the test period was more than 6 months, as 

required by the Standard, the turbine did not meet the Standard’s criteria for reliable operation. 

NREL engineers analyzed operational time up until the last time the turbine produced power on 

1 February 2013. The turbine was decommissioned on 7 May 2013.

6.2 Hours of Power Production

During the test, engineers recorded the following number of hours of power production: 

• At any wind speed − 1,268.9 hours (2,500 hours required)

• Above 1.2*Vave (10.2 m/s) − 442.7 hours (250 hours required) 

• Above 1.8*Vave (15.3 m/s) − 81.7 hours (25 hours required). 

The turbine did not meet the requirement for hours of power production during the test. Table 4

shows the overall and monthly results of the duration test. 

Table 4. Overall and Monthly Results of the Viryd CS8 Duration Test

Month 0 m/s 10 m/s 15 m/s

max 

gust 

[m/s]

TI@15 

m/s

 [%]

TT TU TE TN O [%]

Overall 1268.9 442.7 81.7 47.7 18.3 11014.0 912.4 1067.9 1487.2 83.5%

Min. Met? No Yes Yes Yes No

Nov 2011 90.2 30.8 3.8 38.3 18.4 720 15.4 353.2 1.7 99.5

Dec 71.2 18.2 1.3 46.6 17.4 744 2 0.3 92.2 87.6

Jan 2012 0 0 0 46.2 17.9 744 172.5 146 425.5 0

Feb 79.5 32 14 47.7 23.3 696 71.2 15.8 431 29.2

Mar 156.7 67.3 26.5 41.9 19.5 744 12.8 1 29.2 96

Apr 96.7 33.8 4.3 30.1 17.3 720 136.8 12.2 42.3 92.6

May 78.8 27.2 0.8 28.2 19.6 744 3 12.7 10.3 98.6

Jun 129.7 36.3 1.2 25.2 15.1 720 0.8 0 0.2 100

Jul 54.5 16.8 0.2 34.1 14.6 744 94 1.5 48 92.6

Aug 83.3 20 0.8 30.3 11.9 744 157.8 0 0 100

Sep 38.7 7.8 0 21.1 16.9 720 200.3 0.5 0 100

Oct 82 31.3 5.7 34 19.3 744 45.6 202.7 29.5 94

Nov 75.5 17.8 1.3 35.2 18.2 720 0 116.7 82 86.4

Dec 100.8 39.5 6.5 33.2 19.2 744 0.1 167.8 145.3 74.8

Jan 2013 124.3 62.7 15.3 34.6 18.8 744 0 36.3 142.5 79.9

Feb 6.8 1.2 0 32.5 16.7 22 0.1 1.2 7.5 63.4

Hours of power Environmental Operational time fraction
Viryd Duration Test Report 130604 Page 18 of 54

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6.3 Operational Time Fraction

The operational time fraction is defined as follows:

× %100 −−

−−− =

EUT

EUNT

TTT

TTTT O

where

TT = the total time period under consideration 

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

TU = the time in which the turbine status is unknown 

TE = the time that is excluded in the analysis. 

The overall operational time fraction of the combined wind turbine systemincluding the wind 

turbine, tower, and controllerin the total test period was 83.5%. Table 4 and Figure 6 show the 

operational time fraction per month. 

Figure 6. Operational time fraction for each month

0

10

20

30

40

50

60

70

80

90

100

Nov-11Dec-11

Jan-12

Feb-12

Mar-12

Apr-12

May-12

Jun-12

Jul-12

Aug-12Sep-12

Oct-12

Nov-12Dec-12

Jan-13

Feb-13

Mar-13
Viryd Duration Test Report 130604 Page 19 of 54

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System Downtime

The main reasons for wind turbine system downtime (TN) during the test period were the failed 

anemometer, failed power supply, and failed compressor. 

Failed Anemometer

On 31 December 2011, the turbine’s anemometer failed. On 23 January 2012, a replacement 

anemometer from Viryd arrived and was installed on 31 January 2012. The downtime until the 

replacement anemometer arrived was classified as non-operational time (TN), and the time 

between the arrival and installation of the anemometer was classified as excluded time (TE), 

because the lack of available NREL personnel affected the timing of the installation. 

Failed Power Supply

On 4 February 2012, the 24-VDC power supply for the controller failed. The controller was 

replaced on 21 February 2012 and downtime was classified as non-operational. 

Failed Compressor

Analysis of the turbine duration data showed that the turbine produced power for the last time on 

1 February 2013. After that, the turbine faulted every time the wind speed approached cut-in. 

During the post-test inspection, the pneumatic compressor for the brake system was found to be 

inoperative. The NREL team believes that this final series of faults may have been caused by the 

failed compressor. 

Remaining TN

Over the course of the test, the turbine experienced several faults of unknown origin. The NREL

team was unable to log into the controller to determine the exact cause, but worked directly with 

Viryd to troubleshoot and reset the faults. The resulting downtime from the faults was classified 

as non-operational unless NREL engineers were unable to get to the issue in a timely manner, at 

which point excess faulted time was excluded.

Excluded Time TE

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

• Time during power outages that prevented the turbine from running 

• Noise testing that required the turbine to be shut down 

• Resetting of faults that took longer than normal because of personnel availability. 

Unknown Time TU

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

turbine’s status could not be explicitly determined. 

6.4 Environmental Conditions

As an indication of the environmental conditions that the turbine experienced during the duration 

test, the highest instantaneous wind speed gust was 47.7 m/s (during February 2012) and the 

average turbulence intensity at 15 m/s was 18.3% based on 10-minute statistics.
Viryd Duration Test Report 130604 Page 20 of 54

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6.5 Power Degradation Checks

A factor of reliable operation is that the turbine should experience no significant power 

degradation. Figure 7 shows a scatter plot of 10-minute averaged power versus wind speed over 

the course of the test. Figure 8 shows the power degradation plot, which provides the power level 

in individual wind speed bins for each month. These plots are then analyzed for any obvious 

trends in power production. Variations in the power levels from season to season are caused by 

air density variations. Stronger dips in power are present as a result of the known faulted 

conditions or excluded times (i.e., grid outages) being part of the data set, per the Standard. Thus, 

any unknown faults are hidden by the known time when the turbine is non-operational. The 

power degradation is more clearly observed Figure 9 than Figure 8, where the known offline 

turbine time is removed. 

In Figure 7, two distinct power curves are observed, corresponding to a sharp decrease in power 

and power factor that was observed around 20 May 2012. It is worth noting that both run 

capacitors were found to be faulty during the post-test inspection (detailed in Appendix C). The 

NREL team suspect the failure of these capacitors caused the power degradation. 

Figure 7. Scatter plot of power versus wind speed (10-minute averages)
Viryd Duration Test Report 130604 Page 21 of 54

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Figure 8. Power level in several wind speed bins (m/s) as a function of time

Figure 9. Power level in several wind speed bins (m/s) as a function of time with known nonoperational time removed

6.6 Dynamic Behavior

The turbine’s operation was observed by NWTC personnel for several minutes on a weekly basis 

over the course of the test. Observed wind speeds ranged from less than 5 m/s (below cut-in) to 

more than 26 m/s. Total observation time was well over 1 hour. The turbine was observed to cutin between 4 and 8 m/s, smoothly motoring up and switching to producing power. The exact 

wind speed for cut-in varied, as the turbine controller utilizes a wind-speed-weighted counter to 

determine when to start. The turbine passively tracked the wind well, both when running and 

parked with only minor vibration of the tail. Occasionally, the turbine would run downwind 

(detailed below). Running downwind always initiated a preprogrammed detection and shutdown 

0

1

2

3

4

5

6

7

Sep-11 Nov-11 Jan-12 Mar-12 May-12 Jul-12 Sep-12 Nov-12 Jan-13 Mar-13

Power [kW]

4

5

6

7

8

9

10

11

12

13

14

15
Viryd Duration Test Report 130604 Page 22 of 54

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algorithm, followed by the tail passively turning the turbine back into the wind, thereby allowing 

the turbine to start back up. No excessive vibration of any components was observed throughout 

the test. The following observation excerpts were recorded in the log book during the test. 

21 February 2012 Wind Speed: 10-15 m/s Wind direction: WNW

“No defects noted. Operation is fine. Returned to run mode (after fault). Stand-by light 

illuminated. Awaiting normal operation. 

Witnessed normal operation of turbine. Continued operation.”

22 February 2012 Wind Speed: 10-25 m/s Wind direction: WNW

“Witnessed several shutdowns this morning. They’ve been accomplished by winds gusting 

greater than 25 m/s. The turbine stops and alternately flashes yellow and red LEDs. Blue LED 

also flashes.1 After several minutes, the turbine enters standby and run modes.”

6 June 2012 Wind Speed: 4-7 m/s Wind direction: NNE

“Observed the Viryd running downwind. Videos on my phone...heard it all the way from site 1.2. 

Ran for at least 10 minutes downwind, more likely 15. Observed the turbine go through its 

shutdown routine and yaw into the wind. After 5 minutes, it still is in standby/waiting for wind. 

Winds are low, not sure if it’ll start up again.” 

“Turbine started back up at 19:47 MT.” 

18 October 2012 Wind Speed: 5-18 m/s Wind direction: WNW−NW

“Turbine running all day in high winds.” 

1 November 2012 Wind Speed: 3-5 m/s Wind direction: SE

“Reset turbine through manual reset and had turbine run through start up check. No odd noises 

or vibrations during check. Standby LED came back after check and fault LED went away.” 

12 November 2012 Wind Speed: 5-15 m/s Wind direction: W 

“Ran turbine through startup check. Slight metallic noise, perhaps from tail fin, but might be 

worth checking before wind season picks up.2

 Standby LED is back on and fault LED cleared.” 

 

1 This LED combination is in line with the manual’s description of the turbine behavior during high-wind cut-out: a 

recoverable fault state with a 5-minute wait period accompanied by a flashing blue standby LED indicating the high 

wind event.

2 Turbine was inspected and found to be in good working condition.
Viryd Duration Test Report 130604 Page 23 of 54

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7 Uncertainty

NREL researchers estimated turbine uncertainty for the following parameters:

• Hours of power production 

• Operational time fraction

• Highest instantaneous wind speed. 

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

to find relative trends that might indicate hidden faults in the turbine. 

7.1 Hours of Power Production

NREL researchers assumed that the turbine was producing power for the entire 10-minute period 

when the average power for that period was positive. This method overestimated the time for 

power production in wind speeds between 5 and 10 m/s. At these wind speeds, the turbine may 

have been producing power for about 90% of the time. At higher wind speeds, this method 

would produce less of an overestimate. NREL estimates that the reported time of power 

production in wind speeds greater than 0 m/s may be 5% less than calculated. 

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

speed was assumed to be the dominant factor. Using the uncertainties in wind speed detailed 

below, uncertainty in wind speed at 15.6 m/s is 0.4 m/s. When considering this uncertainty, the 

hours of power production would at most reduce to 392.2 hours (above 10.6 m/s) and 71.7 hours 

(above 15.7 m/s), which still exceeds the 250 and 25 hours that the Standard requires.

7.2 Operational Time Fraction

The total test time was 11,014.0 hours. Even if the classification of TN was off by 5% (which is a 

conservative assumption), the operational time fraction would be 84.4%. 

7.3 Highest Instantaneous Wind Speed

The uncertainties in the wind speed measurements were 0.06 m/s calibration uncertainty, 0.03 

m/s + 0.26% operational characteristics, 1% mounting effects, and 2% terrain effects. For the 

maximum instantaneous gust of 47.7 m/s, the uncertainty was 1.1 m/s.
Viryd Duration Test Report 130604 Page 24 of 54

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8 Deviations and Exceptions

8.1 Deviations from the Standard

There were no deviations or exceptions to the Standard. 

8.2 Deviations from Quality Assurance

The first set of data acquisition modules were used beyond their calibration due date; however,

they were post-calibrated and found to be within tolerance. The post-test calibration sheets are 

provided in Appendix A. 

In addition, the pressure transducer was used beyond its calibration due date (see Appendix A for 

the post-test calibration sheet). Residuals between the pre- and post-test calibrations were found 

to be 0.006 kPa at most, a negligible amount in the context of this test. 
Viryd Duration Test Report 130604 Page 25 of 54

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References

International Electrotechnical Commission (IEC). (2006). Wind Turbines – Part 2: Design 

requirements for small wind turbines, IEC 61400-2, Ed 2.0, Clause 9.4, 2006-03, Geneva, 

Switzerland.
Viryd Duration Test Report 130604 Page 26 of 54

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Appendix A. Instrument Calibration Certificates

Figure A1. Calibration sheet for primary anemometer used through 12 September 2012
Viryd Duration Test Report 130604 Page 27 of 54

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Figure A2. Calibration sheet for primary anemometer used from 17 September 2012 on
Viryd Duration Test Report 130604 Page 28 of 54

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Figure A3. Calibration sheet for the wind vane used through 12 September 2012
Viryd Duration Test Report 130604 Page 29 of 54

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Figure A4. Calibration sheet for the wind vane used from 17 September 2012 on
Viryd Duration Test Report 130604 Page 30 of 54

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Figure A5. Page 1 of power transducer calibration sheet used through 12 September 2012
Viryd Duration Test Report 130604 Page 31 of 54

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Figure A6. Page 2 of power transducer calibration sheet used through 12 September 2012
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Figure A7. Page 1 of power transducer calibration sheet used from 17 September 2012 on
Viryd Duration Test Report 130604 Page 33 of 54

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Figure A8. Calibration sheet for the pressure transducer used from 15 May 2012 on 
Viryd Duration Test Report 130604 Page 34 of 54

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Figure A9. Post-test calibration sheet for the pressure transducer used through 15 May 2012
Viryd Duration Test Report 130604 Page 35 of 54

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Figure A10. Calibration sheet for the pressure transducer used from 16 May 2012 on
Viryd Duration Test Report 130604 Page 36 of 54

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Figure A11. Calibration sheet for temperature sensor used through 12 September 2012
Viryd Duration Test Report 130604 Page 37 of 54

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Figure A12. Calibration sheet for temperature sensor used from 17 September 2012 on
Viryd Duration Test Report 130604 Page 38 of 54

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Figure A13. Calibration sheet for signal conditioning module 13DEC38 used through 26 July 2012
Viryd Duration Test Report 130604 Page 39 of 54

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Figure A14. Post-test calibration sheet for signal conditioning module 13DEC38 used through 

26 July 2012
Viryd Duration Test Report 130604 Page 40 of 54

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Figure A15. Calibration sheet for signal conditioning module 12A2037 used from 26 July 2012 on
Viryd Duration Test Report 130604 Page 41 of 54

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Figure A16. Calibration sheet for signal conditioning module 13FAE1C used through 26 July 2012
Viryd Duration Test Report 130604 Page 42 of 54

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Figure A17. Post-test calibration sheet for signal conditioning module 13FAE1C used through 

26 July 2012
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Figure A18. Calibration sheet for signal conditioning module 12BFEE2 used from 26 July 2012 on
Viryd Duration Test Report 130604 Page 44 of 54

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Figure A19. Calibration sheet for signal conditioning module 13E3D05 used through 26 July 2012
Viryd Duration Test Report 130604 Page 45 of 54

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Figure A20. Post-test calibration sheet for signal conditioning module 13E3D05 used through 26 

July 2012
Viryd Duration Test Report 130604 Page 46 of 54

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Figure A21. Calibration sheet for signal conditioning module 14DA726 used from 26 July 2012 on
Viryd Duration Test Report 130604 Page 47 of 54

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Appendix B. Turbine Commissioning Checklist

Figure B1. Viryd CS8 commissioning checklist, page 1
Viryd Duration Test Report 130604 Page 48 of 54

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Figure B2. Viryd CS8 commissioning checklist, page 2
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Appendix C. Post-Test Tear Down Inspection

The Viryd CS8 turbine was taken down from site 3.3a on 07 May 2013 after NREL engineers 

completed all testing activities as part of the U.S. DOE’s Independent Testing project. A tear 

down inspection was performed as part of the duration test, and the report included the following 

observations. 

Nose cone

The nose cone did not show any signs of wear. No cracks were found near any of the bolt holes. 

Nacelle cover

The nacelle cover was found to be in good condition. Minor cracking could be seen in some of 

the gel coat, but no structural cracking was observed. Minor scrapes in the paint (Figure C1) 

were also found, caused by the turbine yawing with the covers lowered during the up-tower

inspections. 

Blades

The turbine blades were found to be in good shape. A small chip was observed on the leading 

edge of one of the blades (Figure C2). All blade roots showed signs of light rubbing on the hub 

clamping brackets (Figure C3). 

Brake system

The brake system consists of two normally closed, pneumatically actuated brake calipers. The 

brake disk showed no signs of significant wear. The guide pins appeared in good condition, 

however, one was found to have loosened several turns (Figure C4). Although this loosening had 

not yet caused any wear or problems, it easily could have if it had continued to operate 

unchecked. All air hoses appeared in good condition. Brake pad thickness was measured to be 

0.31” for both pads on the rotor side. Brake pad thickness was measured to be 0.31” and 0.31” on 

the downwind side. The compressor was supplied with 24 VDC during the inspection and was 

found to be inoperative. NREL engineers believe that the failure of the compressor led to the 

turbine’s inability to produce power after 1 February 2013 and the subsequent termination of the 

test. 

Gearbox

The gearbox and oil were visually inspected. No cracking, chipping, or excessive wear were 

observed on any of the gearbox components. Minor amounts of grease were found inside the 

gearbox (Figure C5); however, the oil was in good condition. 

Yaw system

All welds on the yaw head were visually inspected and no cracks were found. The yaw bearing 

rotated smoothly and showed no signs of excessive play. 
Viryd Duration Test Report 130604 Page 50 of 54

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Generator

The generator was not disassembled to the point where the windings could be inspected. The 

outside of the housing and cooling fan appeared in good condition. 

Tail assembly

The tail vane was inspected visually, and no cracks were found. The tail boom did not have any 

cracks or signs of wear. The fasteners attaching the tail were rusted (Figure C6).

Up-tower electrical components

The up-tower electrical components included: 

• Start and run capacitors. The start and (one of two) run capacitors appeared in good 

shape. The second run capacitor, shown in Figure C7, had a hole in the can that was close 

to where the mechanical retaining strap contacted the case. Electrolyte leakage was 

observed inside the junction box containing the capacitors. Tests of the capacitors 

showed that both run capacitors had failed. 

• Slipring assembly. The slipring assembly did not show any unexpected wear, signs of 

arcing, or overheating. 

• Terminal blocks. The terminal blocks were intact with all connections tight.

• General wiring. All wiring appeared in good condition, with no signs of wear or 

overheating on any of the loom or insulation.3

Tower

The tower was visually inspected, and no cracked welds were found. All fasteners and locknuts 

were present. During the test, following strong rain storms, water was found in the loop at the 

bottom of the down tower conduit. Improved waterproofing of the control anemometer junction 

box was conducted at Viryd’s request, thereby alleviating ingress of water. 

The winch, gin-pole, and associated rigging were all found to be in good condition at the end of 

the test. The system was used to lower the turbine during decommissioning and performed as 

expected.

Foundation

The foundation and guy wire attachment plates were visually inspected. The concrete was in 

good condition, with no cracking or excessive corrosion, and all fasteners were still tight.

Controller

The controller was visually inspected. All wires were in good condition, with no signs of wear or 

overheating. Connections showed no evidence of damage caused by the elements.

 

3 The condition of the wiring inside the generator was not inspected.
Viryd Duration Test Report 130604 Page 51 of 54

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Figure C1. Scratches on Viryd nacelle cover

Figure C2. Chip discovered on leading edge of blade 
Viryd Duration Test Report 130604 Page 52 of 54

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Figure C3. Blade root

Figure C4. Loosened brake caliper bolt
Viryd Duration Test Report 130604 Page 53 of 54

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Figure C5. Inspection inside gearbox

Figure C6. Rusty fasteners on tail
Viryd Duration Test Report 130604 Page 54 of 54

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Figure C7. Failed run capacitor