Conference Paper

NREL/CP-500-48089

April 2010

Small Wind Turbine Testing 

Results from the National 

Renewable Energy Laboratory 

Preprint 

Amy Bowen, Arlinda Huskey, Hal Link, Karin 

Sinclair, Trudy Forsyth, and David Jager

National Renewable Energy Laboratory

Jeroen van Dam and Joe Smith

Windward Engineering

Presented at the International Small Wind Conference

Glasgow, Scotland

April 27-28, 2010
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1

Small Wind Turbine Testing Results from the

National Renewable Energy Lab

Amy Bowen, Arlinda Huskey, Hal Link, Karin Sinclair, Trudy Forsyth, David Jager

National Renewable Energy Laboratory

Mailstop: 3911; National Renewable Energy Laboratory; 1617 Cole Blvd.; Golden, CO 80401; USA

Amy.Bowen@nrel.gov, Arlinda.Huskey@nrel.gov, Hal.Link@nrel.gov, Karin.Sinclair@nrel.gov, 

Trudy.Forsyth@nrel.gov, David.Jager@nrel.gov

Jeroen van Dam, Joe Smith

Windward Engineering

Mailstop: 3911; National Renewable Energy Laboratory; 1617 Cole Blvd.; Golden, CO 80401; USA

Jeroen.van.Dam@nrel.gov, Joe.Smith@nrel.gov

Abstract

In 2008, the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) began testing 

small wind turbines (SWTs) through the Independent Testing project. Using competitive solicitation, five SWTs 

were selected for testing at the National Wind Technology Center (NWTC). NREL’s NWTC is accredited by the 

American Association of Laboratory Accreditation (A2LA) to conduct duration, power performance, safety and 

function, power quality, and noise tests to International Electrotechnical Commission (IEC) standards. Results of the 

tests conducted on each of the SWTs are or will be available to the public on the NREL website. The results could 

be used by their manufacturers in the certification of the turbines or state agencies to decide which turbines are 

eligible for state incentives.

1. INTRODUCTION

The Independent Testing project was established at the National Renewable Energy Laboratory’s National Wind 

Technology Center (NWTC) to help reduce barriers to wind energy expansion. Among these barriers is a lack of 

independent testing results for small turbines. Independent testing results will meet partial requirements for turbine 

certification. Certified turbines increase consumer confidence in small turbine technology and certification will help 

consumers in the selection of turbines.

The turbines selected in the first round of the independent testing project were the Mariah Power Windspire, the 

Gaia-Wind 11 kW, the Abundant Renewable Energy ARE 442, the Entegrity EW50, and Ventera VT10. Figure 1 

shows the selected turbines installed at the NWTC. Power performance, duration, noise, and safety and function 

tests are performed on all turbines. Power quality testing is performed only on three-phase turbines, as the IEC 

Standard 61400-21 only applies to turbines with a three-phase grid connection.

2. TURBINES SELECTED

The five turbines are listed and described below in the order of their installation dates.

The Mariah Power Windspire was installed on May 5, 2008. It is a vertical-axis turbine mounted on a monopole 

tower, and has a rotor height of 6.1 m and a rotor area of 1.2 m by 6.1 m. The Windspire uses a 120 VAC, singlephase, grid-connected, permanent-magnet generator wind turbine rated at 1 kW. Testing on the Windspire was 

terminated on January 14, 2009 after repeated turbine problems.

The Gaia-Wind 11 kW turbine was installed on May 13, 2008. It is a downwind, two-bladed, horizontal-axis 

turbine. The Gaia-Wind 11 kW has a 133 m2 swept area and is mounted on an 18.2 m monopole tower. Its threephase 11-kW induction generator delivers 480 VAC to the grid. At the time of this writing, data collection is 

complete with the exception of a post duration test inspection.
2

The Abundant Renewable Energy ARE 442 was installed on June 11, 2008. It is a horizontal-axis, three-bladed 

turbine with a swept area of 41 m2

. It operates upwind and uses a furling mechanism for power control. The ARE 

442 is mounted on a 30.5 m lattice tower. The turbine uses a single-phase, grid-connected, permanent-magnet 

generator that is rated at 10 kW at 240 VAC. Testing is complete.

The Entegrity EW50 was installed on March 3, 2009. It is a horizontal-axis, downwind machine. It has a swept area 

is 177 m2 and is mounted on a 30.5 m monopole tower. The EW50’s three-phase induction generator produces 50 

kW at 480 VAC. At the time of this writing, data collection for power performance and acoustic noise is complete 

and undergoing analysis. Data collection continues for duration testing, safety and function, and power quality.

The Ventera VT10 completed installation on March 23, 2010. It is a downwind, horizontal-axis, three-blade turbine. 

Its swept area is 35 m2

, and it is mounted on a 24.7 m lattice tower. The VT10 uses a single-phase, grid-connected, 

permanent-magnet generator that is rated at 10 kW at 240 VAC. Pitchable tips govern the rotor speed. At the time of 

this writing, data collection is in progress for all tests.

 

Figure 1. From left to right: Windspire, Gaia-Wind 11 kW, ARE 442, EW50, and VT10. PIX numbers left to right: 

15705, 15733, 16390, 15704, 16948

3. RESULTS

The available results to date are presented below, though some are preliminary and subject to change.

3.1. Duration Testing

The duration test is conducted according to section 9.4 of the IEC Standard 61400-2: Design Requirements for 

Small Wind Turbines. Duration testing provides information about the turbine’s structural integrity, quality of 

environmental protection, and dynamic behavior. The test requires a minimum of 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 and greater, 

and 25 hours of power production in winds of 1.8 Vave and greater. Section 6.2 of IEC Standard 61400-2 defines 

Vave, which depends on the small wind turbine class identified by the manufacturer and based on the wind speeds in 

which the turbine was designed to operate; “class I” turbines are designed for strong wind environments, and “class 

IV” turbines are designed for lower wind environments, with a “class S” for other designs specified by the 

manufacturer. The standard requires that the turbine must not experience any major failures during the test period 

and it must achieve an operational time fraction of 90% or greater. 
3

The operational time fraction is defined by the following:

Where TT is the total test time, TN is the time attributed to turbine faults and manufacturer-mandated inspections and 

maintenance, TU is the time during which the turbine status is unknown due to lost data or data-acquisition failure 

and maintenance, and TE is the time that is excluded from analysis due to grid faults and laboratory-mandated 

inspections or stops.

The dynamic behavior of the turbine is assessed by observing the turbine in a range of operating conditions. The 

turbine is observed at wind-speed intervals from cut-in wind speed to 20 m/s for at least one hour in total. Tower 

vibrations, noise, yaw behavior, and tail movement all are documented in the logbook and included in the durationtest report.

Another factor of reliable operation is that the turbine should experience no significant power degradation. Each 

month the average power is plotted for each wind-speed bin and analyzed for any obvious trends in power 

production. Examination of power degradation plots indicated no apparent power degradation for the Gaia-Wind 

11kW, ARE 442, or EW50.

Part of the reliable-operation requirement for the duration test includes no significant wear, corrosion, or damage to 

turbine components. The structural integrity and material degradation are investigated through inspections of the 

turbine before, during, and after the testing period. Blades, welds, and other turbine components were visually 

inspected and photographed before the test and any apparent abnormalities documented. After the required test data 

is collected, the turbine is lowered and disassembled for inspection of components. Routine inspections of the ARE 

442, Gaia-Wind 11 kW, and EW50 before and during the tests did not reveal any abnormalities. Post-test 

inspections for the Gaia-Wind 11 kW or EW50 have not occurred. The post-test inspection of the ARE 442 

revealed the turbine system to be in generally good condition, except for cracks in each blade.

The IEC standard requires reporting maximum wind speed and Turbulence Intensity (TI). TI is the ratio of the 

wind-speed standard deviation to the average wind speed. These values characterize the wind resource the test 

turbine experienced. The NWTC has a relatively high maximum wind speeds and high TI.

3.1.1. Windspire

Duration testing on the Windspire was terminated on October 14, 2008. The turbine had experienced repeated 

problems. These problems included repeated loose nuts at the base of the turbine, a broken washer at the base of the 

turbine, broken welds at the top of the turbine, airfoil displacement in the struts, and an inverter failure. The 

Windspire is a class IV turbine.

The following dynamic observations were made: “The turbine was exhibiting unsteady behavior periodically. There 

was a noise that seemed to come from the tower approximately three times per revolution. The washers at the 

foundation base were loose and the bolts and nuts on the foundation had shifted.”

3.1.2. Gaia-Wind 11kW

Table 1 shows the final duration results for the Gaia-Wind 11 kW. The turbine accumulated 2,705 hours of total run 

time, with an operational time fraction of 90.8%. The Gaia-Wind 11 kW is a class III turbine.

The low operational time fraction for August and September of 2008 was caused by the failure of two contactors in 

the controller. Investigations suggest that the 2-pin flat connectors used to wire the contactors were poorly connected 

when installed by Gaia-Wind’s electrical supplier. Additionally, the Gaia-Wind 11 kW turbine controller originally 

was designed for a 50-Hz grid, and it is possible that the original contactors were underrated for the 60-Hz grid at 

the NWTC. It now is standard for all contactors in the controller to be installed with tube connectors for a more 

secure connection and all turbine controllers on a 60-Hz grid are installed with the higher-rated contactors. Since the 

replacement of the contactor and pin connectors, the turbine has run with a high operational time fraction.

The majority of the remaining time, classified as TN during the test, is attributed to braking-time faults, vibration 

errors, and maintenance. The braking-time faults occurred when the turbine took longer to brake than was specific in 
4

its design; this usually occurred during high winds. The vibration errors are expected to have occurred from birds 

nesting in the nacelle. With guidance from Gaia-Wind, NREL installed a screen over the opening in the nacelle to 

prevent birds from entering. Since the installation of the screen, the turbine has run without vibration errors.

Table 1. Final Duration Results for the Gaia-Wind 11 kW

Note on Total time TT: times are recorded in local time, and daylight/standard time changes in November & March. 

This results in an extra hour in November and a missing hour in March. This is seen in Tables 1, 2, and 3.

The following dynamic behavior observations were made: “The turbine tracks winds well in all observed wind 

speeds. No excessive vibration was observed. There is an occasional, slight audible thumping noise as blades pass 

behind the tower.”

3.1.3. ARE 442

Table 2 shows the final results for the ARE 442. The ARE 442 accumulated 3,241 hours of total run time with an 

operational time fraction of 91.2%. The ARE 442 is a class II turbine.

The low operational time fraction that occurred in November 2008 was caused by failure of the turbine’s insulatedgate bipolar transistors (IGBTs) during a simulated grid fault for safety and function testing. The majority of the 

remaining time, classified as TN during the test, is attributed to over-temperature and overvoltage faults that the 

turbine experiences in high winds. The diversion loads for the turbine were originally located inside the data shed 

and did not dissipate heat properly, which resulted in temperature faults during high power production. The 

diversion loads were moved outside of the data shed in February 2009, which eliminated the over-temperature 

faults. However, the turbine then experienced overvoltage faults.

Table 2. Final Duration Results for the ARE 442

The following dynamic observations were made: “During high winds, the rotor operates at yaw errors of between 

approximately 30 degrees and 60 degrees and the furl movements excite the tower slightly. Overall, it appears that 

no excessive vibrations are occurring. In winds of between 8 m/s and 12 m/s the turbine tracks the wind well.”

3.1.4. EW50

Table 3 shows the preliminary results for the EW50. At the time of this writing, the turbine has accumulated 1,383

hours of total run time with an operational time fraction of 98.3%. The EW50 is a class II turbine.

Max TI @ 15 T T T U T E T N O

Month 0 m/s 9 m/s 13.5 m/s m/s (%) (hours) (hours) (hours) (hours) (%)

Overall 2704.9 710.6 215.0 41.9 19.0 7094 172.5 152.0 624.6 90.8

Jun 2008 238.2 36.2 3.8 28.6 18.5 518 11.3 7.8 3.3 99.3

Jul 256.0 8.5 0.3 23.9 - 744 78.2 2.2 38.8 94.1

Aug 115.8 4.5 0.0 19.2 - 744 6.3 20.0 323.0 55.0

Sep 120.5 11.7 1.8 22.4 - 720 36.2 30.3 174.7 73.3

Oct 236.0 45.0 12.2 32.8 17.3 744 0.7 1.3 0.0 100.0

Nov 348.0 98.7 22.5 37.0 20.9 721 22.1 0.0 0.0 100.0

Dec 339.7 160.5 54.8 41.4 17.4 744 7.9 27.2 32.8 95.4

Jan 2009 385.0 155.5 56.0 38.8 19.9 744 4.9 32.0 36.5 94.8

Feb 333.2 107.3 36.8 41.9 20.0 672 3.2 27.0 0.0 100.0

Mar 332.5 82.7 26.8 36.7 18.0 743 1.7 4.2 15.5 97.9

Hours of Power Production Above:

TI @ 15 T T T U T E T N O

Month 0 m/s 10.2 m/s 15.3 m/s m/s (%) (hours) (hours) (hours) (hours) (%)

Overall 3240.6 552.6 156.7 42.9 18.9 7249 99.7 214.8 612.0 91.2

Jul 2008 295.8 7.5 0.0 27.8 15.8 735 3.1 152.2 0.0 100.0

Aug 286.8 9.5 0.0 26.5 16.9 744 31.0 4.0 0.0 100.0

Sep 217.5 8.8 0.7 23.2 13.8 687 49.6 5.3 0.3 99.9

Oct 279.7 35.7 6.7 34.0 16.9 744 0.9 5.0 9.5 98.7

Nov 156.0 8.2 0.0 34.3 19.5 721 0.0 0.1 332.8 53.8

Dec 379.2 131.8 41.8 42.9 18.2 744 1.0 10.2 124.2 83.1

Jan 2009 466.5 146.3 44.3 42.9 20.0 744 0.7 1.8 79.8 89.2

Feb 389.3 104.5 46.3 39.5 19.7 672 4.0 31.7 42.3 93.3

Mar 416.8 68.3 13.2 34.9 18.4 743 3.9 1.5 20.3 97.2

Apr 353.0 32.0 3.7 28.4 18.0 720 43.5 3.0 2.8 99.6

Hours of Power Production Above: Max 

Gust
5

A low operational time fraction occurred in August 2009 as a result of the failure of the controller’s UPS after a grid 

outage. Entegrity service personnel bypassed the UPS and returned the EW50 to operation. A low operational time 

fraction in January 2010 was caused by a grid fault which blew a fuse in the control panel.

Table 3. Preliminary Duration Results for the EW50

The following dynamic behavior observations were made: “In light winds the EW50 was observed yawing to face 

upwind, and remain stable in the upwind position. In the upwind position the EW50 consumed power, depending on 

the wind speed. After 10 minutes of consuming power, the EW50 controller shut down and act of stopping the EW50 

returned it to the downwind position. After 10 minutes, the EW50 sensed sufficient wind, and the EW50 released the 

brakes and started again.”

3.1.5. VT10

As of this writing the VT10 has less than one month of operation. It is a class III turbine.

The following dynamic behavior observations were made: “The turbine tracks the wind well. Its blade tips govern 

rotor speed by pitching to stall, as designed. The turbine rotor emits more noise while governing than during normal 

operation, but no excessive vibrations observed during governing or normal operation.”

3.2. Power Performance Testing

Power performance testing is conducted per IEC standard 61400-12-1, Power Performance Measurements of 

Electricity Producing Wind Turbines, referencing Annex H for small wind turbines. Products of the test include a 

measured power curve and an estimation of annual energy production (AEP).

For small turbines, statistical data is collected in 1-minute sets and sorted into 0.5-m/s-wide wind speed bins. Data 

collection is complete when the wind speed bins (between 1 m/s below cut-in and 14 m/s) contain 10 minutes of 

data each. The total database consists of at least 60 hours of data. Wind speed bins are plotted against the 

corresponding bin power to produce a power curve. Power curves are normalized to sea-level air density; the NWTC 

is approximately 1,850 m above sea level. Therefore, the site-specific air density at the NWTC is relatively low,

averaging slightly less 1.0 kg/m3

.

Figure 2 shows the power curve for all five small turbines tested. The EW50 is scaled on the right axis, the rest are

scaled on the left axis. The Gaia-Wind 11 kW and ARE 442 are the final curves. The EW50, VT10 curves are 

preliminary. The original inverter on the Windspire was optimized for power performance and failed after several 

months of operation. After the failure, a production model inverter was installed and operated until testing on the 

Windspire was suspended. The required amount of data was not collected on either inverter due to failures. Figure 2 

shows the incomplete power curve for the second, production model inverter.

TI @ 15 T T T U T E T N O

Month 0 m/s 10.2 m/s 15.3 m/s m/s (%) (hours) (hours) (hours) (hours) (%)

Overall 1383.0 266.1 40.3 36.1 19.5 8706 1000.5 116.5 140.0 98.2

Apr 2009 164.2 29.3 5.5 31.0 18.2 684 0.1 5.7 0.0 100.0

May 148.2 9.8 0.0 21.5 744 0.5 6.0 0.0 100.0

Jun 105.7 11.2 1.2 30.0 17.6 720 0.0 0.2 0.0 100.0

Jul 81.5 7.7 0.7 28.1 24.0 744 14.3 0.0 0.0 100.0

Aug 32.5 0.5 0.0 18.0 744 370.9 14.3 70.7 80.3

Sep 70.8 13.3 1.3 27.4 16.4 720 144.6 31.5 0.0 100.0

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 0.0 22.3 721 0.1 6.3 0.0 100.0

Dec 207.5 62.7 12.2 36.1 19.0 744 0.7 0.0 0.0 100.0

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 0.0 100.0

Mar 137.7 21.0 0.3 25.1 18.1 743 63.6 0.3 0.0 100.0

Hours of Power Production Above: Max 

Gust
6

Figure 2. Power Curves of the Five Turbines Tested

Annual energy production (AEP) is estimated by applying the power curve generated from power performance 

testing to a Rayleigh distributed wind resource. The AEP is calculated for annual average wind speeds at hub height

for 4 m/s to 11 m/s. In Table 3 below, the measurements reported assume no energy production beyond the highest 

filled bin in the power performance test. The AEP for the EW50 has not been published, and the AEP for the VT10

is preliminary. The AEP is not reported for the Windspire because the required amount of data was not collected.

Table 3. Measured AEP for the Gaia-Wind 11 kW, ARE 442, EW50, and VT10

3.3. Safety and Function Testing

Safety and function testing is conducted according to section 9.6 of the IEC Standard 61400-2: Design Requirements 

for Small Wind Turbines. Its intent is to test the essential functions of the turbine system. For each turbine, NREL 

collects data to characterize the turbine’s power control, rotor-speed control, behavior upon loss of load, normal 

start-up, normal shutdown, and emergency shutdown. Additionally, NREL performs turbine-specific tests to verify 

the turbine controller’s function and predicted behavior. Although safety and function testing examines the essential 

functions of the turbine, it does not certify whether a turbine is safe to operate. The following tables in this section 

are summaries of the Safety & Function test.

Table 4 shows the final safety and function data summary for the Gaia-Wind 11 kW. The turbine performed as 

designed with one exception. When the turbine was shut down manually using the disconnect switch and then was 

restarted, an overspeed error was present on the controller. The error had to be reset before the turbine could be 

started again.

Gaia-Wind 

11kW ARE 442 EW50 VT10

AEP-measured AEP-measured AEP-measured AEP-measured

m/s kWh kWh kWh kWh

4 17,716 7,884 11,006 5,213

5 32,122 15,327 41,796 11,318

6 46,292 23,516 83,543 18,688

7 58,690 30,967 130,785 26,267

8 68,525 36,718 178,132 33,129

9 75,474 40,459 220,869 38,639

10 79,617 42,350 255,754 42,531

11 81,326 42,770 281,396 44,854

Hub height 

annual average 

wind speed
7

Table 4. Final Safety and Function Test Summary for the Gaia-Wind 11 kW

Test Method Comment Complies 

with Design

Power control Turbine controls power output per design Yes

Rotor speed control Turbine controls rpm to 61, per design Yes

Normal start-up Turbine starts after several motor pulses in design wind speed and above, 

and below cut-out; over-speed error on start-up after manual shutdown

Partially

Normal shutdown Turbine shuts down normally in winds less than cut-in and greater than

cut-out

Yes

Emergency stop Turbine stops within 2 to 3 seconds of pressing emergency stop button Yes

Loss of grid Turbine brakes immediately and stops within 2 to 3 seconds of load loss Yes

Undervoltage / 

overvoltage

In an overvoltage simulation the turbine brakes immediately Yes

High wind speed 

shutdown

Turbine stops in winds greater than 25 m/s and waits for start-up per 

the design

Yes

Rotor overspeed Turbine brakes immediately in simulated 10% overspeed and deploys tip 

brakes at 15% simulated overspeed

Yes

Generator overcharge Turbine brakes immediately in simulated generator overcharge Yes

Excessive vibration Vibration error registers on turbine controller after activating vibration 

sensor

Yes

Cable twist Cable-twist error registers on turbine controller after lifting cable-twist arm Yes

Table 5 shows the final safety and function data summary for the ARE 442. The turbine’s two diversion loads were 

originally installed inside the data shed according to the manufacturer’s design. They later were moved to an 

enclosure outside of the data shed after it was determined that their placement was causing repeated overtemperature faults. When the diversion loads were installed in the data shed, the over-temperature faults would 

occur in high-wind conditions as heat built up inside the data shed. Temperatures measured near the turbine’s 

sensors indicated that the turbine shut down near or below its set point.

Table 5. Final Safety and Function Test Summary for the ARE 442

Test Method Comment Complies 

with Design

Power control Power is limited by the capacity of the inverters, these max out at 

12 kW; after that, power is diverted to the diversion loads

Yes

Rotor speed control During start up in high winds, measurements indicate a much higher 

rotor speed then what was indicated by the manufacturer

Appears noncompliant

Yaw control The turbine tracks the wind under all conditions; due to the furl 

mechanism, the rotor almost always has a yaw error

Yes

Normal start-up Turbine starts in any winds ranging from cut-in to 25 m/s Yes

Emergency stop Turbine stops when stop button is pushed on the voltage clamp; this 

has been tested for a wide range of wind speeds

Yes

Loss of grid Disconnecting the grid causes an immediate shutdown; in two events 

where a grid outage occurred in high-wind conditions, the IGBT’s in the 

voltage clamp failed, although the turbine still shut down

Partially

Overvoltage fault In high winds the turbine currently experiences the overvoltage fault, 

demonstrating that this feature works

Yes

Over-temperature fault Temperatures measured near the turbines sensors indicate that the 

turbine shuts down near or below its set points

Yes

Table 6 shows the preliminary safety and function data summary for the EW50. One notable exception was found 

in the controller input. When the turbine’s control panel manual three-way switch is set to “Off” and the grid 

experiences an outage, the EW50 will be in a “Run” state once power is restored, thereby overriding the control 

panel’s manual switch.
8

Table 6. Preliminary Safety and Function Test Summary for the EW50

Test Method Comment Complies 

with Design

Power control Turbine controls power output per design Yes

Rotor speed control Turbine controls rpm per design Yes

Normal start-up The turbine starts normally. If the anemometer mounted on the 

turbine indicates enough wind, and the rotor is not up to speed, the 

generator will motor start the rotor.

Yes

Normal shutdown The turbine shuts down normally as winds drop below cut-out, The 

tip breaks deploy and when the rotor slows to approximately 18rpm 

the parking brake on the high speed shaft engages

Yes

Emergency stop In moderate winds the turbine stops in approximatly15 seconds of 

opening the disconnect switch

Yes

Loss of grid In moderate winds the turbine stops in approximately 15 seconds 

of load loss

Yes

Rotor overspeed The controller has halted the turbine in very strong winds due to a

“generator overspeed” fault

Yes

Overvoltage/undervoltage Test pending —

Table 7 shows the preliminary safety and function data summary for the VT10. The design of the VT10 allows for 

the rotor to spin when disconnected from the grid. The rotor is governed mechanically as the tips pitch to stall. The 

designed governing speed is 260 to 280 rpm, but the turbine at the NREL site governs at 220 to 240. Governing 

happens at 13m/s when the turbine is connected to the grid and the inverter is extracting power, and it governs at

approximately 7 m/s when it is not connected to the grid. When the rotor speed drops below the governing speed, 

springs return the rotor to its normal operating state. The VT10 is designed to spin at all times, even when there is a 

grid outage. If the rotor must be stopped, a parking brake is available and may be engaged when the rotor is below 

governing speeds.

Table 7. Preliminary Safety and Function Test Summary for the VT10

Test Method Comment Complies 

with Design

Power control Turbine controls power output per design Yes

Rotor speed control Turbine controls rpm, but approximately 40 rpm lower than 

designed

Yes

Normal start-up The turbine starts normally Yes

Emergency stop The inverter stops, when commanded. However the rotor does not 

stop. As designed, when the generator is disconnected from the 

grid, the rotor governs itself in winds above 7m/s

Yes

Loss of grid As designed the rotor governs itself when disconnected from the 

grid.

Yes

Rotor overspeed Test pending —

Overvoltage/undervoltage Test pending —

Table 8 shows the partial final safety and function data summary for the Windspire. Safety and function testing was 

not completed for the Windspire before the test program was terminated due to turbine problems. The Windspire 

normally experiences two resonances, before it reaches its rated speed of 180 rpm. 

Table 8. Final Safety and Function Test Summary for the Windspire, publication pending.

Test Method Comment Complies 

with Design

Power control Turbine controls power output per design Yes

Rotor speed control Turbine controls rpm per design; maximize at 390 rpm Yes

Normal start-up The turbine starts normally; it experiences two resonance modes Yes

Normal shutdown The turbine shuts down normally as winds drop below cut-out, 

however it maintain a low rpm (3–10) when braked; the turbine was 

not designed to shut down in high winds

Yes
9

Emergency stop Turbine stops within 2 to 3 seconds of opening the disconnect 

switch

Yes

Loss of grid Turbine stops within 2 to 3 seconds of load loss Yes

Rotor overspeed Data was not collected for this test Unknown

Overvoltage/undervoltage Data was not collected for this test Unknown

3.4. Power Quality

IEC standard 61400-21, Measurement and Assessment of Power Quality Characteristics of Grid Connected Wind 

Turbines, applies to three-phase turbines. Power quality tests are performed for the Gaia-Wind 11 kW and the 

Entegrity EW50 because they are three-phase systems. Measurements include reactive power, flicker, voltage 

fluctuations, and harmonics. At the time of this writing, only partial results for the EW50 exist and final results for 

the Gaia-Wind 11 kW are awaiting publication; published test report will be posted on NREL’s Independent Testing 

website: http://www.nrel.gov/wind/smallwind/independent_testing.html

3.5. Noise Testing

IEC standard 61400-11, Acoustic Noise Measurement Techniques, does not contain information specific to small 

wind turbines, however noise will be addressed specifically in the next revision of this standard. One result of the 

noise test is to characterize emissions from a turbine in terms of sound power level. For small wind turbines, the IEC 

standard is followed with some modifications. Ten-second averages are used instead of 1-minute averages to better 

characterize the more-dynamic nature of small wind turbines. Also, to determine the sound power levels at the 

integer wind speeds, binning data is used instead of regression analysis. Figure 7 shows the sound power levels for 

the Gaia-Wind 11 kW, ARE 442, and EW50. Testing on the Windspire was not completed, and testing on the VT10 

has not begun.

Figure 7. Preliminary Sound Power level for Gaia-Wind 11 kW, ARE 442, and EW50

4. Conclusion

This paper presents the partial results of International Electrotechnical Commission tests on five small wind turbines 

at the National Renewable Energy Laboratory (NREL). The final results and complete test reports will be provided 

to the public through the NREL website: http://www.nrel.gov/wind/smallwind/independent_testing.html. These 

results may be used by the public, certifying bodies, and state agencies to incentivize small wind turbines.
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April 2010

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Conference Paper

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4. TITLE AND SUBTITLE

Small Wind Turbine Testing Results from the National Renewable 

Energy Laboratory: Preprint

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DE-AC36-08-GO28308

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6. AUTHOR(S)

A. Bowen, A. Huskey, H. Link, K. Sinclair, T. Forsyth, D. Jager

J. van Dam, and J. Smith

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NREL/CP-500-48089

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WE10.1161

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National Renewable Energy Laboratory

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14. ABSTRACT (Maximum 200 Words)

In 2008, the U.S. Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) began testing small 

wind turbines (SWTs) through the Independent Testing project. Using competitive solicitation, five SWTs were 

selected for testing at the National Wind Technology Center (NWTC). NREL's NWTC is accredited by the American 

Association of Laboratory Accreditation (A2LA) to conduct duration, power performance, safety and function, power 

quality, and noise tests to International Electrotechnical Commission (IEC) standards. Results of the tests conducted 

on each of the SWTs are or will be available to the public on the NREL website. The results could be used by their 

manufacturers in the certification of the turbines or state agencies to decide which turbines are eligible for state 

incentives.

15. SUBJECT TERMS

small wind turbine certification; IEC standards; testing small wind turbines

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