Wind Turbine Generator System

Power Quality Test Report for the

Gaia Wind 11-kW Wind Turbine
Amy Curtis and Vahan Gevorgian

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.

Technical Report
NREL/TP-5000-51477
July 2011

Contract No. DE-AC36-08GO28308

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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 1 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 3 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 4 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 5 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 6 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.

18

16

14
Turbulence Intensity

12

10

0
0 2 4 6 8 10 12 14 16 18

Wind Speed (m/s)

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 7 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
Power (kW)

ACTIVE POWER (kW)

0
0 5 10 15 REACTIVE POWER (kVAR)

-5

-10

-15
Wind Speed (m/s)

Figure 2: Active and reactive power vs. wind speed

Page 8 of 35
 1.0

0.9

0.8

0.7
Power Factor

0.6

0.5

0.4

0.3

0.2

0.1

0.0
0 2 4 6 8 10 12 14 16

Acitve Power (kW)

Figure 3: Power factor vs. active power

0
0 2 4 6 8 10 12 14 16

-2
Reactive Power (kVAR)

-4

-6

-8

-10

-12

Active power (kW)

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 9 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, 60-sec average: 0.2-sec average:
Pmc:

Pmc , kW pmc=Pmc/Pn P60 , kW p60=P60/Pn P0.2 , kW p0.2=P0.2/Pn

15.00 1.36 13.94 1.27 19.62 1.78
Qmc at Pmc, qmc=Qmc/Pn Q60 at P60 , q60=Q60/Pn Q0.2 at P0.2 , q0.2=Q0.2/Pn
kVAR -0.96 kVAR -0.89 kVAR -1.43
-10.51 -9.78 -15.77

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 10 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 11 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.

20

18

16

14
Current TDD (%)

12 CURRENT A
10 CURRENT B
CURRENT C
8

0
0 2 4 6 8 10 12 14 16
Power (kW))

Figure 5: Current TDD Vs. Power

Page 12 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. Power Interhar. Power Interhar.
current current 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 13 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 14 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.

1.2

1.0
Voltage Imbalance (%)

0.8

0.6

0.4

0.2

0.0
0 2 4 6 8 10 12 14 16

Acitve Power (kW)

Figure 6: Voltage Imbalance Vs. Power

80

70
Current Imbalance (%)

60

50

40

30

20

10

0
0 2 4 6 8 10 12 14 16

Acitve Power (kW)

Figure 7: Current imbalance Vs. Power

Page 15 of 35
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.

6
5
4
3
Pst

2
1
0
-5 0 5 10 15
POWER (kW)

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

6
5
4
3
Pst

2
1
0
-5 0 5 10 15
POWER (kW)

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

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 6
5
4
3
Pst

2
1
0
-5 0 5 10 15
POWER (kW)

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

6
5
4
3
Pst

2
1
0
-5 0 5 10 15
POWER (kW)

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 Flicker coefficients, C(Ψk ,Va)

speed, Va (m/s)

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 Starts Stops

Regime

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

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 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, At least 5 for each 1 m/s wind speed
except 15 m/s bin
bin

Switching operation:

number of wind turbine starts (cut- 5 At least 5

in wind speed)

Switching operation:

number of wind turbine stops (cut- 5 At least 5

in wind speed)

Switching operation:

number of wind turbine starts 5 At least 5

(rated wind speed)

Switching operation:

number of wind turbine stops 5 At least 5

(rated wind speed)

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Appendix B. Post Calibration Report

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