NREL/PR-5000-62887

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Wind Turbine Drivetrain Testing

and Research at the

National Renewable Energy Laboratory

Jonathan Keller

jonathan.keller@nrel.gov

Pennsylvania State University

University Park, Pennsylvania

September 15, 2014
2

Wind Power Industry Overview

• Wind energy today...

o Multibillion dollar industry with multinational corporations

o 60+ gigawatts (GW) deployed (~5% of U.S. electricity) 

o Land-based wind @ 7-8 ₵/kilowatt-hour (kWh); beating coal

o Leads all renewable technology deployment.

• Tremendous opportunity remains...

o Achieve parity with

natural gas

at 5–6 ₵/kWh

o Establish offshore wind

o Provide research and 

development (R&D)

to achieve 30% wind.

Source: AWEA
National Wind Technology 

Center Overview
4

U.S. Department of Energy National Wind Technology Center

• Turbine testing since 1977

• Leader in development of design and analysis codes

• Pioneers in component testing

• Modern utility-scale turbines

• 130–160 personnel on-site

• Partnerships with industry, CRADAs, and work-for-others

• Leadership roles for international standards

• Offshore wind and marine and hydrokinetic technology development.

• Unique test facilities:

- Megawatt-scale turbines and sites

- Blade testing

- Dynamometers

- Controls research turbines

Photo by Dennis Schroeder, NREL 30764
5

Current Test Facilities

• Field

o Field test sites and infrastructure

o DOE 1.5, industry megawatt (MW) turbines, 

small turbines

o Controls Advanced Research Turbines (CARTs)

o Research-grade inflow meteorological data.

• Drivetrain

o 225-kW, 2.5-MW, and 5-MW dynamometers 

o Dynamic torque testing of fully integrated 

drivetrain systems (gearboxes, generators, PE)

o Model-in-the-loop (to simulate turbulence).

o Non-torque loading to simulate rotor loads

• Blade

o Static testing and resonant fatigue testing to 50 

meters (m)

o Accredited (A2LA) fatigue (resonant) and static 

testing of blades to IEC standards, modal and blade 

property measurements, profiling

o Innovative blade test methodology R&D

o Massachusetts large blade test facility.

• ISO 17025, A2LA-accredited to IEC Standards

o Power performance, power quality, acoustic 

emissions, structural loads, duration, and structural 

and fatigue testing.

 Photo by Lee Jay Fingersh, NREL 14688

Photo by Dennis Schroeder, 

Photo by Lee Jay Fingersh, NREL 30780

NREL 16269
6

Multimegawatt Turbines

• GE 1.5 MW

o Model: GE 1.5 SLE

o Tower height: 80 m; rotor diameter: 77 m

o DOE-owned; used for research and education

o Turbine commissioned September 2009.

• Siemens 2.3 MW

o Model: SWT-2.3-101

o Tower height: 80 m; rotor diameter: 101 m.

• Alstom 3 MW 

o Model: ECO 110

o Tower height: 90 m; rotor diameter: 110 m.

• Gamesa 2 MW

o Model G97

o Tower height: 90 m; rotor diameter: 97 m.

Photo by Dennis Schroeder, NREL 30770
7

New 5-MW Dynamometer

• 5.8 MW (low-speed shaft)

• 12–4 rpm (at full power) 

• 4.6 MNm (3.4 M ft-lb) max 

torque

• Non-torque loads in 5 DOF

• Dual 75-ton cranes

• 20 m x 12 m test bay

• 5 degree drive table tilt

2.5-MW Dynamometer

• 2.5 MW (low-speed shaft)

• 17–31 rpm (at full power)

• 1.64 MNm (1.2 M ft-lb) max 

torque

• Non-torque loads in 3 

degrees of freedom (DOF)

• Single 50-ton crane

• 12.2 m x 15.2 m test bay

• 0–6 degree drive table tilt

Dynamometer Facilities

Designed to test entire integrated wind turbine drivetrain system

Photo by Lee Jay Fingersh, 

NREL 16267

Photo by Robb Wallen, 

NREL 17400

Photo by Mark McDade, NREL 27249
8

Non-torque loading

system

• Emulate rotor loading on drivetrain

• Produce forces in 5 DOF on test article 

low-speed shaft while rotating

– Up to 7.2 MNm (5.5 M ft-lb) bending moment

– Up to 3.5 MN (800,000 lb) axial force

• Hydraulic force actuators

• High priority for stakeholders.

5-MW Dynamometer Non-Torque Loading System (NTL)

Dynamometer motor 6 MW 

(8,000 HP)

Dyno non-torque 

loading system 

(NTL)

Dynamometer gearbox

75:1, 3-stage

Test article

Photo by Mark McDade, 

NREL 28218

Illustration by Scott Lambert, NREL
9

NWTC Controllable Grid Interface (CGI) Facility

o CGI connected to 

both dynos and 

some turbines

o Command voltage 

profiles to emulate 

real power-line faults

o 7 megavolts-ampere 

(MVA) continuous 

with 28 MVA short 

circuit capacity

Dyno motor 6 MW 

(8,000 HP)

Dyno nontorque loading 

system

Dyno variable 

frequency 

drive Test article 

power 

converter

Dyno gearbox

75:1, 3-stage Test article 

drivetrain

Test article 

transformer

Photo by Dennis Schroeder, NREL 25890

Illustration by Scott Lambert, NREL
Gearbox Reliability 

Collaborative

http://www.nrel.gov/wind/grc/index.html

Sponsor: DOE EERE Wind
11

Wind Turbine Reliability

• Plant availability often 95%+

o Adjusted for curtailment

• Subsystem reliability improving, 

but still an operation and 

maintenance (O&M) cost driver

o Decrease in downtime of gearbox, 

generator, and electrical system

o Gearboxes continue to be the largest 

source of downtime

o Generator, power conversion, and 

main bearing not far behind

• Need to maintain availability 

with less cost and effort.

Source: NREL, Wind Stats

Source: GL Garrad Hassan
12

Typical Wind Turbine Drivetrain Architecture

• Modular, three-point mounting system

o Main bearing and gearbox torque arms

• Three-stage gearbox

o Ratio of 80 or 100:1 

o Three-planet stage(s)

o Floating sun

o Parallel stage(s).

Generator

Main 

Bearing Main Shaft

Brake

Generator Shaft

Gearbox

Bed Plate

Hub

Torque Arms
13

NREL Gearbox Failure Database

• Gearbox failure event data highlighting damaged 

components, failure modes, and possible root causes

o Twenty partners involved in operating ~30% of the U.S. capacity

o Number of gearbox failure incidents = 289.

• Observations

o Bearings 70%

o Gears 26%

o Parallel section faults

most common

o High-speed and

intermediate-speed

stage (HSS and IMS)

bearing axial cracking

most common failure mode.

HSS Bearing Helical Gear

IMS

Bearing

Planet

Bearing

Planet

Gear
14

Photo by Jeroen van Dam, 

NREL 19257

Gearbox Reliability Collaborative

Test Validate Analyze Redesign Propagate

• Potential causes:

o Underspecified loads?

o Inaccurate design tools?

o Inadequate design practice?

o Insufficient testing?

• Solution process:

o Conduct field and dynamometer 

testing to measure responses

(150+ signals)

o Validate modeling approaches (multibody, finite 

element analysis)

o Analyze design deficiencies (cylindrical roller bearing 

[CRB] clearance)

o Redesign gearbox to correct (preloaded tapered roller 

bearings [TRBs])

o Propagate lessons learned to industry and standards.

Field test

Dynamometer test

Analysis

• Load cases

• System loads

• Internal loads

NREL dynamometer. Photo by Lee Jay Fingersh, NREL 16913
15

GRC Implementation

• Use smaller 750-kW drivetrain to control costs

• Lack of public models → re-design/re-build commercial gearbox (GB)

o First iteration (GB 1&2) brings to state of the art circa 2007

o Second iteration (GB 3) brings to state of the art circa 2012.

• Significant internal and external instrumentation

o Main shaft, gearbox, coupling, and generator displacements

o Planetary section loads, motions, and temperatures

o HSS, pinion, and bearing loads recently added.

NREL dynamometer. Photo by Lee Jay Fingersh, NREL 16913
16

New HSS Instrumentation

Slip rings Pinion tooth toot 

strain for Gear Khβ

Bearing strain for relative loading

(four axial slots, two Poisson gauges per slot,)

and temperature (both bearings)

Shaft bending strain

(two axes, three axial locations,)

and shaft torsion for torque

Encoder for azimuth 

and speed

Photo by Scott 

Naucler, NREL 26259

Photo by Jon Keller, 

NREL 27895

Photo by Scott Naucler, 

NREL 30252

Photo by Scott Naucler, 

NREL 30250

Photo by Scott Naucler, 

NREL 30251
17

Test Summary

• Phase 1 (300+ hours of data)

o GB 1 field load test

– Oil loss event led to GB damage

• Phase 2 (700+ hours of data)

o GB 2 dynamometer static load test

o GB 1 dynamometer test

– Condition monitoring and GB teardown

• Phase 3 (underway)

o GB 2 dyno re-test

– HSS loads, dynamic field loads, and generator misalignment

o Improved GB 3 test.

Apr

2009

Oct

2009

July 

2010

Nov

2010

Jan 

2011

Jan 

2013

Jan 

2015

Ponnequin Wind Farm. Photo 

by Jeroen van Dam, NREL 

19257
18

Gearbox 1 & 2 Design Deficiencies

• Non-Torque and Planetary Loads

o Significant effect in three-point configuration

o Cause edge loading on planetary gear teeth

– Misalignment between carrier and ring gear 

because of operating radial clearance in 

carrier bearings

o Cause poor load share between planets and

upwind and downwind planetary bearings

– Carrier wind-up and planetary pin 

deflection with operating radial clearance 

in the CRBs

– High bearing skidding risks at low torque.

Photo by Ed Overly, 

NREL 19494
19

Carrier Bearings

Planet Bearings

Ring/Torque 

Arm/Housing 

Joint

Ring Gear

Hollow Shaft 

Bearings Improved 

Reliability

Lubrication

Gearbox 3 Design Improvements

Illustration by Romax Technology

Project Team:

Romax Technology

Powertrain Engineers

Timken

Brad Foote Gearing
20

TRBs used to 

improve planetary 

alignments with 

load variation

Robust bolt system 

to accommodate 

increased housing 

load from TRBs

Nitrided to 

improve fatigue life

Integral TRBs; 

improve load share 

and eliminate 

outer race fretting

X-arrangement; 

tighter inner rings 

to avoid slip

Carrier radial feed;

three feeds center 

plate;

spray all gears;

high-speed TRBs axial 

feed

Gearbox 3 Design Improvements

Illustration by Romax Technology
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Next Generation Drivetrain

Sponsor: DOE EERE Wind

Project Team: NREL

CREE

DNV KEMA

Romax Technology

Vattenfall Windpower

http://apps1.eere.energy.gov/wind/newsletter/detail.cfm/articleId=189
22

Next Generation Drivetrain

• Project Concept

o Medium-speed, medium-voltage drivetrain incorporating 

advances in the gearbox and power converter that increase 

efficiency, reliability, and annual energy production (AEP); and 

reduce O&M costs and cost of energy (COE) from wind

o Paper study proved the benefits and led to competitively 

selected funding for testing of key technologies

– 5% increase in AEP and 13% decrease in COE at 5 MW.

70%

75%

80%

85%

90%

95%

100%

0% 50% 100%

Drivetrain Efficiency (%)

Rotor Power (%)

Next generation 

drivetrain

Three stage, 

doubly fed 

drivetrain

+10%

+5.6%

Illustration by Al Hicks, NREL
23

Technology Development and Testing

• Gearbox journal bearing and 

flex pin robustness

o Ferrium C61 explored on paper

• Converter grid fault control 

software effectiveness

• Reuse WindPACT generator

• Medium voltage, hybrid 

Si/SiC module performance

Dynamometer Power Converter Modules

650-kW drivetrain technology test bed

Medium voltage 

Hybrid Si/SiC 

Module test bed

Photo by CREE

Photo by DNV KEMA
24

Current Activities and Results

• Gearbox in manufacture at Brad Foote Gearing

• Converter software in development at DNV

o NERC/FERC grid interconnectivity requirements emerging

• Hybrid Si/SiC modules in manufacture at Powerex

o High-efficiency, medium-voltage power conversion a focus in

many industries

• Drivetrain and module testing complete early 2015.
Wrap-Up
26

Opportunities for Researchers and Students

• Researchers

o Research Participant Program (RPP)

– http://www.nrel.gov/rpp/

– https://www.youtube.com/watch?v=0TBeFWGU0T8

• Students and Educators

o Science Undergraduate Laboratory Internships (SULI)

– http://www.nrel.gov/education/suli_intern.html

– https://www.youtube.com/watch?v=tAppljjGXLM

o Graduate Fellowship Program

– http://www.nrel.gov/education/scgsr.html

o Visiting Faculty Program

– http://www.nrel.gov/education/teachers.html#fast

• Full-time Employment

o http://www.nrel.gov/employment/