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

Contract No. DE-AC36-08GO28308

Wind Turbine Generator System

Power Quality Test Report for the

Gaia Wind 11-kW Wind Turbine 

Amy Curtis and Vahan Gevorgian

Technical Report

NREL/TP-5000-51477

July 2011
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Page 1 of 35

Wind Turbine Generator System 

Power Quality Test Report 

for the

Gaia Wind 11-kW Wind Turbine

in

Boulder, CO 

Conducted for

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401 

Conducted by

Wind Energy Program 

DOE / NREL

Amy Curtis, Vahan Gevorgian

October 12, 2010

Page 3 of 35

Table of Contents

1.0 Background ................................................................................................................... 5

2.0 Test Summary ............................................................................................................... 5

3.0 Continuous Operation Test Results .............................................................................. 6

3.1 Wind Turbine Data .................................................................................................... 6

3.2 Active Power, Reactive Power and Power Factor ..................................................... 7

3.3 Line Voltage Distortions ......................................................................................... 10

3.4 Current Harmonics .................................................................................................. 11

3.5 Current Interharmonics ........................................................................................... 13

3.6 Current Distortions .................................................................................................. 14

3.7 Current and Voltage Imbalance ............................................................................... 15

3.8 Continuous Operation Flicker ................................................................................. 16

4.0 Switching Operation Tests Results ............................................................................. 18

4.1 Turbine starts and stops ........................................................................................... 18

5.0 Exceptions ................................................................................................................... 28

Appendix A. Test Matrix .................................................................................................. 29

Appendix B. Post Calibration Report ............................................................................... 30
Page 4 of 35

List of Tables

Table 1: Gaia Wind 11-kW wind turbine general data ....................................................... 6

Table 2: Wind turbine rated data ........................................................................................ 6

Table 3: Maximum permitted power .................................................................................. 6

Table 4: Number of data points per 1 m/s wind speed bins ................................................ 7

Table 5: Data binned by real power .................................................................................. 10

Table 6: Maximum instantaneous real and reactive power .............................................. 10

Table 7: Maximum Current Harmonics (10 min averages) .............................................. 11

Table 8: Current Interharmonics ....................................................................................... 13

Table 9: Current Distortions (2 - 9 kHz range) ................................................................. 14

Table 10: Resulting flicker coefficients in continuous operation ..................................... 18

Table 11: Table of Figures for transient waveforms ......................................................... 18

Table 12. Characteristics of start at cut-in wind speed ..................................................... 27

Table 13. Characteristics of stop at cut-in wind speed ..................................................... 27

Table 14. Characteristics of start at rated wind speed ...................................................... 27

Table 15. Characteristics of stop at rated wind speed ....................................................... 28

List of Figures

Figure 1: Turbulence intensity vs. wind speed ................................................................... 7

Figure 2: Active and reactive power vs. wind speed .......................................................... 8

Figure 3: Power factor vs. active power ............................................................................. 9

Figure 4: Reactive Vs. Active Power .................................................................................. 9

Figure 5: Current TDD Vs. Power .................................................................................... 12

Figure 6: Voltage Imbalance Vs. Power ........................................................................... 15

Figure 7: Current imbalance Vs. Power ............................................................................ 15

Figure 8: Pst vs. power for 30° network impedance angle ................................................. 16

Figure 9: Pst vs. power for 50° network impedance angle ................................................ 16

Figure 10: Pst vs. power for 70° network impedance angle ............................................... 17

Figure 11: Pst vs. power for 85° network impedance angle ............................................... 17

Figure 12: Cut-in wind (4.8 m/s) start: current waveforms (August 19, 2009) ................ 19

Figure 13: Cut-in wind (4.8 m/s) start: power and RMS voltage (August 19, 2009) ....... 20

Figure 14: Rated wind (10.5 m/s) start, current waveforms (December 31, 2009) .......... 21

Figure 15: Rated wind (10.5 m/s) start, power, and RMS voltage (December 31, 2009) 22

Figure 16: Cut-in wind (3.4 m/s) stop, current waveforms (August 19, 2009) ................ 23

Figure 17: Cut-in wind (3.4 m/s) stop, power, and RMS voltage (August 19, 2009) ...... 24

Figure 18: Rated wind (10.0 m/s) stop: current waveforms (December 31, 2009) .......... 25

Figure 19: Rated wind (10.0 m/s) stop: power and RMS voltage (December 31, 2009) . 26

1 
Page 5 of 35

1.0 Background

This test is being 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. In 

total, five turbines are being tested at the NWTC as a part of this project. Power quality 

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

performance, safety and function, noise, and duration tests. The results of the testing will 

provide the manufacturers with reports that may be used for small wind turbine 

certification.

2.0 Test Summary

This test was conducted in accordance with the International Electrotechnical 

Commission’s (IEC) standard, Measurement and Assessment of Power Quality 

Characteristics of Grid Connected Wind Turbines, IEC61400-21, First Edition, 2001-12.

The power quality test for the Gaia Wind 11-kW wind turbine began on July 23, 2009 

and ended on January 15, 2010. About 9,995 10-min intervals of data were collected 

while the turbine was available. The IEC 61400-21 Power Quality Standard requires at 

least five 10-min average data points for each 1 m/s wind speed bin for the wind 

turbulence intensity between 8 and 16%. This condition reduced the valid data to 1,021 

10-min intervals. The highest bin filled (with no wind speed normalization) was the 14 

m/s bin. When the data is binned by percent of rated power, more than five data points 

were recorded for each bin. According to the IEC 61400-21, not enough data was 

collected to fill the 15 m/s bin due to high turbulence intensity for the data points 

collected in the 15 m/s bin. The test matrix is given in Appendix A. 

The test was conducted in accordance IEC 61400-21 Power Quality Standard and 

MEASNET procedures, and the following are the test results required by the standard and 

are included in this report: 

• Maximum measured power (60-sec and 0.2-sec), reactive power demand

• Voltage fluctuations (flicker)

• Voltage harmonics, current harmonics, interharmonics and distortions 

• Turbine start and stop tests 
Page 6 of 35

3.0 Continuous Operation Test Results

3.1 Wind Turbine Data

Table 1: Gaia Wind 11-kW wind turbine general data

Turbine make, model, serial number, production year Gaia-Wind 11-kW, 10711114, 2007 

Wind turbine type Horizontal axis

Number of blades 2

Hub height (m) 18.2

Blade control None

Rotor diameter (m) 13

Speed control Constant speed

Generator type and rating Induction, 11 kW

Converter type n/a

Table 2: Wind turbine rated data

Rated power, Pn (kW) 11

Rated wind speed, Vn (m/s) 9.5

Rated apparent power, Sn (kVA) 13

Rated reactive power, Qn (kvar) 9.6

Rated current, In (A) 16

Rated voltage, Vn (V) 480

Table 3: Maximum permitted power

Peak continuous power, Pmc (kW) 20

Normalized value, pmc=Pmc/Pn 1.2
Page 7 of 35

3.2 Active Power, Reactive Power and Power Factor

The relationship between active and reactive power was measured at the wind turbine 

low voltage (480 VAC) terminals. Only 10-min data points, with turbulence intensity 

within 8-16% range, were used for this analysis (Figure 1). Power measurements were 

sampled during continuous operation only, and taken so that at least five 10-min time 

series of power were collected for each 1 m/s wind speed (according to IEC 61400-21). 

The sampled data was transferred to 10-minute average data by applying block averaging 

for each 10-min period (Table 4 and Figure 2). The power factor (Figure 3) was 

calculated using active and reactive power values. The reactive power demand is shown 

in Figure 4. 

0

2

4

6

8

10

12

14

16

18

0 2 4 6 8 10 12 14 16 18

Wind Speed (m/s)

Turbulence Intensity

Figure 1: Turbulence intensity vs. wind speed

Table 4: Number of data points per 1 m/s wind speed bins

Wind Speed Bin, m/s Number of Valid 10 min 

Data Points

0 to 1 0

1 to 2 0

2 to 3 0

3 to 4 142

4 to 5 346

5 to 6 220

6 to 7 112
Page 8 of 35

7 to 8 61

8 to 9 54

9 to 10 29

10 to 11 15

11 to 12 17

12 to 13 16

13 to 14 6

14 to 15 1

Power Curve

-15

-10

-5

0

5

10

15

0 5 10 15

Wind Speed (m/s)

Power (kW)

ACTIVE POWER (kW)

REACTIVE POWER (kVAR)

Figure 2: Active and reactive power vs. wind speed
Page 9 of 35

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 2 4 6 8 10 12 14 16

Acitve Power (kW)

Power Factor

Figure 3: Power factor vs. active power

-12

-10

-8

-6

-4

-2

0

0 2 4 6 8 10 12 14 16

Active power (kW)

Reactive Power (kVAR)

Figure 4: Reactive Vs. Active Power

The 10-min average data was sorted according to the method of bins so that the reactive 

power could be specified for 0, 10, ...., 90, 100% of rated power (Table 5). The 

maximum active power was measured both as 60-sec (P60) and 0.2-sec average (P0.2)
Page 10 of 35

values. The reactive power at Pmc , P60, and P0.2 was determined by extrapolation of the 

measured relation between the active and reactive power (Table 6).

Table 5: Data binned by real power

Power Bin Number of 10-min Output power, Reactive power,

(% of rated power) data points per bin bin-mean-value (kW) bin-mean-value (kVAR)

-5 to 5 161 0.238 -6.118

5 to 15 223 1.111 -5.695

15 to 25 182 2.180 -5.682

25 to 35 97 3.266 -5.844

35 to 45 86 4.325 -6.088

45 to 55 59 5.473 -6.493

55 to 65 43 6.594 -6.870

65 to 75 35 7.677 -7.116

75 to 85 38 8.795 -7.383

85 to 95 24 9.909 -7.807

95 to 105 25 10.932 -8.202

105 to 115 24 12.210 -8.792

Table 6: Maximum instantaneous real and reactive power

Max. permitted power, 

Pmc: 

60-sec average: 0.2-sec average:

Pmc , kW

 15.00 

pmc=Pmc/Pn 

 1.36

P60 , kW

 13.94 

p60=P60/Pn

 1.27 

P0.2 , kW

 19.62 

p0.2=P0.2/Pn

 1.78 

Qmc at Pmc, 

kVAR

-10.51 

qmc=Qmc/Pn

 -0.96

Q60 at P60 , 

kVAR

 -9.78 

q60=Q60/Pn

 -0.89 

Q0.2 at P0.2 , 

kVAR

-15.77 

q0.2=Q0.2/Pn

 -1.43 

Note 1: Data were not normalized to sea-level density. 

Note 2: The active power curve shown in Figure 2 may not be used for the Gaia Wind 11-

kW wind turbine power performance evaluation. The official power curve will be 

published in the Gaia Wind 11-kW Power Performance Test Report. 

3.3 Line Voltage Distortions

The average voltage Total Harmonic Distortions (THD) measured during the test are 

shown below for each phase: 

Phase A – 2.028 % 

Phase B – 1.798 % 

Phase C – 1.734 % 
Page 11 of 35

The maximum voltage THDs measured during the test are shown below for each phase:

Phase A – 4.163 % 

Phase B – 4.028 % 

Phase C – 3.595 % 

All voltage THDs were calculated from harmonic voltages. The harmonic voltages were 

subgrouped according to Section 5.6 of IEC 61000-4-7/CDV. The window width used 

during measurements Tw=12.

3.4 Current Harmonics

The individual harmonic currents during continuous operation were measured as 10-min 

average data for each harmonic order (up to 50th) at the output power giving the 

maximum individual harmonic current. The harmonic data and the maximum current 

Total Demand Distortion (TDD) values as a percentage of nominal current In are shown 

in Table 7. Figure 5 shows the plots of 10-min average data for maximum current TDD 

(% of In) as a function of output power. 

Table 7: Maximum Current Harmonics (10 min averages) 

Phase A Phase B Phase C

Order Power Harm. current Power Harm. current Power Harm. current

(kW) (%) (kW) (%) (kW) (%) 

1 11.665 115.845 11.665 115.995 11.665 115.717

2 1.424 1.267 13.228 0.597 1.424 1.507

3 1.424 9.443 1.424 10.264 1.721 4.519

4 1.424 0.702 1.424 0.479 1.424 0.412

5 6.304 5.410 7.088 4.198 2.307 6.006

6 1.424 0.486 1.424 0.273 1.424 0.424

7 7.752 4.204 7.752 4.482 7.552 3.781 

8 1.424 0.372 1.424 0.292 4.310 0.217

9 1.721 1.103 1.037 1.312 1.721 0.990

10 11.446 0.257 11.446 0.222 11.446 0.208

11 4.513 6.854 4.513 5.849 4.513 5.838

12 0.956 0.226 1.669 0.213 1.970 0.215

13 4.109 6.692 7.782 4.816 4.109 6.492

14 3.504 0.532 3.504 0.449 3.504 0.512

15 2.256 1.563 1.410 1.533 4.709 0.898

16 4.310 0.681 4.310 0.665 4.310 0.728

17 7.782 2.590 7.782 3.611 6.759 3.639

18 4.310 1.588 4.310 1.624 4.310 1.781

19 4.310 4.030 4.310 3.817 4.310 3.915

20 4.310 3.112 4.310 3.603 4.310 3.475 

21 4.310 4.473 4.310 4.992 4.310 4.747

22 4.310 5.335 4.310 5.438 4.310 5.855

23 4.310 7.574 4.310 7.016 4.310 6.085

24 4.310 5.017 4.310 4.138 4.310 4.303

25 4.310 7.064 4.310 4.774 4.310 6.356

26 4.310 3.218 4.310 2.731 4.310 2.568

27 4.310 2.729 4.310 2.323 4.310 2.058

28 4.310 2.334 4.310 1.935 4.310 1.933
Page 12 of 35

29 4.310 2.444 4.310 2.054 4.310 1.937

30 4.310 1.775 4.310 1.494 4.310 1.518

31 4.310 1.634 4.310 1.344 4.310 1.361

32 4.310 1.488 4.310 1.188 4.310 1.239

33 4.310 1.378 4.310 0.998 4.310 1.083 

34 4.310 1.255 4.310 0.901 4.310 1.010

35 4.310 1.354 0.189 1.195 4.310 1.023

36 4.310 0.920 4.310 0.674 4.310 0.755

37 4.310 0.758 4.310 0.596 4.310 0.654

38 4.310 0.636 4.310 0.508 4.310 0.531

39 4.310 0.500 4.310 0.441 4.310 0.389

40 4.310 0.441 4.310 0.379 4.310 0.373

41 7.771 0.555 4.447 0.454 4.447 0.536

42 4.310 0.303 4.310 0.273 4.310 0.251

43 12.112 0.288 12.112 0.288 4.310 0.212

44 4.310 0.216 4.310 0.198 4.310 0.179

45 12.112 0.207 12.112 0.201 4.310 0.137

46 4.310 0.157 4.310 0.145 4.310 0.128 

47 4.447 0.232 1.627 0.199 0.845 0.252

48 4.310 0.114 4.310 0.109 4.310 0.091

49 11.579 0.126 11.579 0.149 4.622 0.112

50 4.310 0.086 4.310 0.082 6.731 0.079

Max. phase A current TDD (% of In): 16.823 Output power at max current THD (kW): 12.780

Max. phase B current TDD (% of In): 15.113 Output power at max current THD (kW): 12.780

Max. phase C current TDD (% of In): 15.316 Output power at max current THD (kW): 12.780

The harmonic currents were subgrouped according to Section 5.6 of IEC 61000-4-

7/CDV. The window width used during measurements was Tw=12.

0

2

4

6

8

10

12

14

16

18

20

0 2 4 6 8 10 12 14 16

Power (kW))

Current TDD (%)

CURRENT A

CURRENT B

CURRENT C

Figure 5: Current TDD Vs. Power
Page 13 of 35

3.5 Current Interharmonics

The individual interharmonic currents below 2 kHz are given in Table 8 as 10-min 

average values for each frequency at the output power giving the maximum individual 

interharmonic current.

Table 8: Current Interharmonics

Phase A Phase B Phase C

f Power Interhar. 

current

Power Interhar. 

current

Power Interhar. 

current

(Hz) (kW) (%) (kW) (%) (kW) (%)

90 10.868 2.399 10.868 2.428 10.868 2.410

150 0.280 0.728 0.280 0.664 0.280 0.814

210 0.280 0.539 0.280 0.542 0.280 0.457

270 0.280 0.393 0.280 0.332 0.280 0.454 

330 0.280 0.328 0.280 0.296 0.280 0.448

390 2.141 0.313 2.141 0.279 0.280 0.290

450 0.736 0.404 0.736 0.380 0.736 0.381

510 0.736 0.271 0.736 0.260 0.736 0.257

570 0.736 0.208 0.736 0.198 0.736 0.201

630 2.735 0.260 2.735 0.241 2.735 0.244

690 2.735 0.321 2.735 0.282 2.735 0.291

750 6.127 0.549 6.127 0.496 6.127 0.499

810 6.127 0.628 6.127 0.559 4.257 0.463

870 0.736 0.442 0.736 0.411 0.736 0.431

930 0.736 0.592 0.736 0.551 0.736 0.605

990 0.736 0.827 0.736 0.787 0.736 0.862

1050 0.736 1.231 0.736 1.199 0.736 1.315

1110 0.736 1.745 0.736 1.832 0.736 1.917

1170 0.736 2.380 0.736 2.733 0.736 2.636

1230 0.736 3.299 0.736 3.855 0.736 3.759

1290 0.736 4.413 0.736 4.923 0.736 5.014

1350 0.736 5.123 0.736 4.980 0.736 5.221

1410 0.736 5.086 0.736 4.348 0.736 4.464

1470 0.736 4.210 0.736 3.548 0.736 3.341

1530 0.736 3.417 0.736 2.884 0.736 2.609

1590 0.736 2.817 0.736 2.402 0.736 2.136

1650 0.736 2.434 0.736 2.065 0.736 1.898

1710 0.736 2.091 0.736 1.768 0.736 1.664

1770 0.736 1.821 0.736 1.552 0.736 1.499

1830 0.736 1.597 0.736 1.343 0.736 1.343

1890 0.736 1.452 0.736 1.189 0.736 1.221

1950 0.736 1.324 0.736 1.054 0.736 1.104

2010 0.736 1.234 0.736 0.927 0.736 1.001

2070 0.736 1.082 0.736 0.798 0.736 0.866

2130 0.736 0.958 0.736 0.696 0.736 0.777

2190 0.736 0.812 0.736 0.606 0.736 0.654

2250 0.736 0.672 0.736 0.528 0.736 0.551 

2310 0.736 0.554 0.736 0.460 0.736 0.459

2370 0.736 0.458 0.736 0.397 0.736 0.382

2430 0.736 0.374 0.736 0.341 0.736 0.317

2490 0.736 0.323 0.736 0.297 0.736 0.276

2550 0.736 0.271 0.736 0.252 0.736 0.235

2610 0.736 0.233 0.736 0.216 0.736 0.200

2670 12.778 1.091 12.778 1.049 12.778 1.013 

2730 0.736 0.175 0.736 0.164 6.127 0.163

2790 12.778 1.216 12.778 1.302 12.778 1.066
Page 14 of 35

2850 6.127 0.146 6.127 0.135 6.127 0.153

2910 6.127 0.141 6.127 0.131 6.127 0.149

2970 6.127 0.138 6.127 0.128 6.127 0.145

3030 12.778 0.817 12.778 0.734 12.778 0.700

The interharmonic currents were subgrouped according to Annex A of IEC 61000-4-

7/CDV. The window width used during measurements Tw=12.

3.6 Current Distortions

The individual current distortions in the range 2 kHz up to 9 kHz are given in Table 9 as 

10-min average values for each frequency at the output power giving the maximum 

individual current distortion.

Table 9: Current Distortions (2 - 9 kHz range)

Phase A Phase B Phase C

f Power Interhar. current Power Interhar. current Power Interhar. current

(kHz) (kW) (%) (kW) (%) (kW) (%)

0.1 12.793 117.017 12.793 117.322 12.793 119.634

0.3 4.644 6.863 4.644 5.452 4.644 6.972

0.5 7.752 4.236 7.752 4.502 7.552 3.803

0.7 1.037 8.403 1.037 6.910 4.109 7.523

0.9 2.149 3.642 2.149 3.749 2.149 3.807

1.1 1.231 11.490 1.231 12.014 1.231 11.859

1.3 2.149 47.437 2.149 56.479 2.149 40.754

1.5 2.149 38.814 2.149 29.895 2.149 31.273

1.7 1.231 11.770 1.231 10.320 1.231 8.660

1.9 1.231 6.658 1.231 5.841 1.231 5.816

2.1 2.149 14.726 2.149 13.327 2.149 14.368

2.3 2.149 14.279 2.149 11.802 2.149 11.549

2.5 2.149 2.349 2.149 1.851 2.149 1.890

2.7 1.231 2.275 1.231 2.106 1.231 2.077

2.9 2.149 7.109 2.149 6.443 2.149 6.227

3.1 1.231 1.477 1.231 1.445 1.231 1.358

3.3 2.149 0.463 2.149 0.378 2.149 0.417

3.5 2.149 1.189 2.149 1.062 2.149 1.146

3.7 2.149 0.911 1.231 0.784 2.149 0.792

3.9 2.149 0.265 2.149 0.234 2.149 0.197

4.1 2.149 0.346 2.149 0.256 1.231 0.276

4.3 1.231 1.019 1.231 0.735 1.231 0.862

4.5 2.149 0.307 1.231 0.284 2.149 0.239

4.7 1.231 0.254 1.231 0.267 1.231 0.208

4.9 1.231 0.610 1.231 0.659 1.231 0.656

5.1 1.231 0.621 1.231 0.619 1.231 0.424

5.3 1.231 0.145 1.231 0.162 1.231 0.151

5.5 1.231 0.193 1.231 0.198 1.231 0.169

5.7 1.231 0.322 1.231 0.273 1.231 0.301

5.9 1.231 0.268 1.231 0.289 1.231 0.196

6.1 1.231 0.135 1.231 0.113 1.231 0.111

6.3 1.231 0.090 1.231 0.100 1.231 0.086

6.5 1.231 0.128 1.231 0.136 1.231 0.129

6.7 2.149 0.070 2.149 0.074 1.231 0.060

6.9 2.149 0.065 1.231 0.065 1.231 0.055

7.1 2.149 0.078 2.149 0.079 2.904 0.077

7.3 2.149 0.086 2.149 0.073 2.149 0.069

7.5 2.149 0.060 2.149 0.061 1.231 0.050

7.7 2.149 0.064 2.149 0.063 2.149 0.055

7.9 2.149 0.087 2.149 0.083 2.149 0.076

8.1 2.149 0.062 2.149 0.065 2.149 0.054

8.3 2.149 0.057 1.231 0.059 1.231 0.049

8.5 1.231 0.064 2.149 0.064 2.149 0.063
Page 15 of 35

8.7 2.149 0.064 2.149 0.063 2.149 0.055

8.9 2.149 0.053 2.149 0.054 1.231 0.047

The current distortions were measured and evaluated according to Annex B of IEC 

61000-4-7/CDV. The window width used during measurements Tw=12. 

3.7 Current and Voltage Imbalance

Figure 6 and Figure 7 show the voltage and current imbalance observed during the 

testing. The current and voltage imbalance was calculated for each 10-min period. The 

values are plotted against the average power of each 10-min data set.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12 14 16

Acitve Power (kW)

Voltage Imbalance (%)

Figure 6: Voltage Imbalance Vs. Power

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10 12 14 16

Acitve Power (kW)

Current Imbalance (%)

Figure 7: Current imbalance Vs. Power
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3.8 Continuous Operation Flicker

The flicker coefficients C(Ψk ,Va) were determined according to procedures listed in 

IEC61400-21. The flicker measuring and analysis methods are described in the standard. 

Total number of 10-min interval data collected for this test: Nbin=1021. 

Figure 8, Figure 9, Figure 10, and Figure 11 show the graphs of fictitious grid flicker 

coefficients as a function of 10-min average active power for four different impedance 

phase angles (30°, 50°, 70° and 85°). The resulting flicker coefficients for continuous 

operation are shown in Table 10. The ratio between three-phase short circuit apparent 

power of the fictitious grid Sk,fic and turbine rated apparent power Sn used for the analysis 

is 50. During the power quality test there was a nearby dynamometer test and a 

neighboring turbine test that were both known to affect the power quality on the grid. 

These tests may have contributed to the variations in Pst. 

0

1

2

3

4

5

6

-5 0 5 10 15

POWER (kW)

Pst

Figure 8: Pst vs. power for 30° network impedance angle

0

1

2

3

4

5

6

-5 0 5 10 15

POWER (kW)

Pst

Figure 9: Pst vs. power for 50° network impedance angle
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0

1

2

3

4

5

6

-5 0 5 10 15

POWER (kW)

Pst

Figure 10: Pst vs. power for 70° network impedance angle

0

1

2

3

4

5

6

-5 0 5 10 15

POWER (kW)

Pst

Figure 11: Pst vs. power for 85° network impedance angle

The simulated flicker values Pst depend on Sk.fic and the grid impedance angle. To reduce 

this dependence, IEC 61400-21 defines flicker coefficients C(Ψk ,Va) which are 

calculated as a normalized quantity according to Equation 6 of the standard (Ψk - network 

impedance phase angle, Va – annual average wind speed). The flicker coefficient of the 

wind turbine for the actual Ψk and Va at the site, may be found from the Table 10 by 

applying linear interpolation. 
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Table 10: Resulting flicker coefficients in continuous operation

Network impedance phase 

angle, Ψk (deg) 30° 50° 70° 85° 

Annual average wind 

speed, Va (m/s)

Flicker coefficients, C(Ψk ,Va)

6.0 98.42 116.35 125.85 122.40

7.5 97.77 116.35 122.18 119.08

8.5 97.18 116.35 121.93 118.65

10.0 96.37 116.35 120.92 117.95

4.0 Switching Operation Tests Results

4.1 Turbine starts and stops

Normal start tests were conducted at cut-in, moderate, and rated wind speeds. The wind 

speed was recorded during the tests along with voltage and current waveforms, and 10-

min average wind speed during the switching operation was within ± 2 m/s of the 

required wind speed. The measurements were taken for a period long enough to ensure 

that the transient of the switching operation was abated, though limited to exclude 

possible power fluctuations due to turbulence. Table 11 lists the figure numbers for the 

current waveforms for all starting and stopping cases. 

Table 11: Table of Figures for transient waveforms

Wind

Regime

Starts Stops

Cut-in Figure 12, Figure 13 Figure 16, Figure 17

Rated Figure 14, Figure 15 Figure 18, Figure 19

The active and reactive powers were calculated for each start and stop case. The time 

series RMS voltage for Phase A also are shown for each case to illustrate voltage 

fluctuations during switching operations. 
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The voltage and current time series were combined in Equation 1 of the standard to 

simulate fictitious voltage time series ufic(t) for four different network impedance angles. 

The flicker step factors and voltage change factors were determined for each switching 

operation for four impedance angles (Table 12-15). 

Figure 12: Cut-in wind (4.8 m/s) start: current waveforms (August 19, 2009)
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Figure 13: Cut-in wind (4.8 m/s) start: power and RMS voltage (August 19, 2009) 
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Figure 14: Rated wind (10.5 m/s) start, current waveforms (December 31, 2009) 
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Figure 15: Rated wind (10.5 m/s) start, power, and RMS voltage (December 31, 2009) 
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Figure 16: Cut-in wind (3.4 m/s) stop, current waveforms (August 19, 2009) 
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Figure 17: Cut-in wind (3.4 m/s) stop, power, and RMS voltage (August 19, 2009) 
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Figure 18: Rated wind (10.0 m/s) stop: current waveforms (December 31, 2009) 
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Figure 19: Rated wind (10.0 m/s) stop: power and RMS voltage (December 31, 2009) 
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The maximum number of switching operations for 10-min and 120-min intervals (N10 and 

N120) for each type of switching operation was not provided by the turbine manufacturer. 

So, the N10 and N120 values were assumed based on the method given by IEC61400-21. 

The flicker step factor and voltage change factors were determined as the average results 

of five values are shown in the tables below. 

Table 12. Characteristics of start at cut-in wind speed

Case of switching operation:

Start at cut-in wind speed

Maximum number of switching operations, N10: 10

Maximum number of switching operations, N120: 120

Network impedance angle, Ψk 30° 50° 70° 85°

Flicker step factor, kf(Ψk): 4.165 4.956 5.128 4.856 

Voltage change factor, kU(Ψk): 0.095 0.071 0.049 0.056 

Table 13. Characteristics of stop at cut-in wind speed

Case of switching operation:

Stop at cut-in wind speed

Maximum number of switching operations, N10: 10

Maximum number of switching operations, N120: 120

Network impedance angle, Ψk 30° 50° 70° 85°

Flicker step factor, kf(Ψk): 1.167 1.067 0.892 0.753 

Voltage change factor, kU(Ψk): 0.029 0.039 0.048 0.048 

Table 14. Characteristics of start at rated wind speed

Case of switching operation:

Start at rated wind speed

Maximum number of switching operations, N10: 1

Maximum number of switching operations, N120: 12

Network impedance angle, Ψk 30° 50° 70° 85°

Flicker step factor, kf(Ψk): 4.100 4.851 5.022 4.773 

Voltage change factor, kU(Ψk): 0.082 0.064 0.051 0.066 
Page 28 of 35

Table 15. Characteristics of stop at rated wind speed

Case of switching operation:

Stop at rated wind speed

Maximum number of switching operations, N10: 1

Maximum number of switching operations, N120: 12

Network impedance angle, Ψk 30° 50° 70° 85°

Flicker step factor, kf(Ψk): 1.544 1.171 1.001 1.046 

Voltage change factor, kU(Ψk): 0.056 0.062 0.066 0.067 

The flicker step factor and voltage change factor of the wind turbine for the actual Ψk at 

the site can be found from the above tables by applying linear interpolation. 

5.0 Exceptions

Exceptions to IEC61400-21: 

• The current and voltage sensors used for this testing do not meet the Standard 

requirements for compliance with the IEC 60044-1 and IEC 60186 respectively, but 

they do exceed the minimum accuracy required by the Standard. This should have no 

effect on results or uncertainty. 

• The 15 m/s wind speed bin was not filled because it is difficult for our site to have 

high wind speed within the 8% to 16% turbulence intensity required in the standard 

due to our diverse terrain. 
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Appendix A. Test Matrix

Test Type Measured Required by IEC 61400-21 and 

MEASNET

Continuous operation:

number of 10 min intervals 5 for each bin, 

except 15 m/s 

bin 

At least 5 for each 1 m/s wind speed 

bin 

Switching operation:

number of wind turbine starts (cutin wind speed) 5 At least 5

Switching operation:

number of wind turbine stops (cutin wind speed) 5 At least 5

Switching operation:

number of wind turbine starts 

(rated wind speed)

5 At least 5

Switching operation:

number of wind turbine stops 

(rated wind speed)

5 At least 5
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Appendix B. Post Calibration Report
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