IEC TS 61400-21-4:2025

IEC TS 61400-21-4 ®
Edition 1.0 2025-04
TECHNICAL
SPECIFICATION
Wind energy generation systems –
Part 21-4: Measurement and assessment of electrical characteristics – Wind
turbine components and subsystems

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IEC TS 61400-21-4 ®
Edition 1.0 2025-04
TECHNICAL
SPECIFICATION
Wind energy generation systems –

Part 21-4: Measurement and assessment of electrical characteristics – Wind

turbine components and subsystems

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.180  ISBN 978-2-8327-0314-4

– 2 – IEC TS 61400-21-4:2025 © IEC 2025
CONTENTS
FOREWORD . 12
INTRODUCTION . 14
1 Scope . 18
2 Normative references . 19
3 Terms, definitions, symbols, units and abbreviated terms . 20
3.1 Terms and definitions . 20
3.2 Symbols, units and abbreviated terms . 29
4 Overview of tests . 31
5 Definitions of minimum DUT, components and subsystems . 33
5.1 General . 33
5.2 Description of components and subsystems . 33
6 Test bench systems . 35
6.1 General . 35
6.2 Nacelle test benches . 37
6.2.1 General. 37
6.2.2 Nacelle test benches – closed-loop testing (1a). 38
6.2.3 Nacelle test benches – open-loop testing (1b) . 39
6.3 Electrical generation test benches (2a and 2b) . 40
6.3.1 General. 40
6.3.2 Electrical generation test benches – closed-loop testing (2a) . 40
6.3.3 Electrical generation test benches – open-loop testing (2b) . 41
6.4 Converter test benches (3a and 3b) . 42
6.4.1 General. 42
6.4.2 Converter test benches – closed-loop testing (3a) . 42
6.4.3 Converter test benches – open-loop testing (3b) . 43
6.5 Controller test benches (4a) and (4b) . 44
6.5.1 General. 44
6.5.2 Controller test benches – closed-loop testing (4a) . 45
6.5.3 Controller test benches – open-loop testing (4b) . 45
6.6 Auxiliary test benches (5) . 46
6.6.1 General. 46
6.6.2 Requirements for auxiliary test benches (5) . 47
6.7 Test bench equipment . 47
6.7.1 General. 47
6.7.2 HiL systems . 47
6.7.3 Prime mover for test benches . 56
6.7.4 Examples of UVRT/OVRT equipment for test benches . 56
6.7.5 Full converter based grid emulator . 57
6.7.6 Measurement systems for test benches . 60
6.7.7 Measurement uncertainty . 60
7 Measurement and test of electrical characteristics as defined in IEC 61400-21-1 . 60
7.1 General . 60
7.2 Power quality aspects . 60
7.2.1 Flicker during continuous operation . 60
7.2.2 Flicker and voltage change during switching operations . 60
7.2.3 Harmonics . 61

7.3 Steady state operation . 64
7.3.1 General. 64
7.3.2 Observation of active power against wind speed . 64
7.3.3 Maximum power . 65
7.3.4 Reactive power characteristic (Q = 0) . 65
7.3.5 Reactive power capability . 66
7.3.6 Voltage dependency of PQ diagram . 67
7.3.7 Unbalance factor. 67
7.4 Control performance . 68
7.4.1 Active power control . 68
7.4.2 Active power ramp rate limitation . 70
7.4.3 Frequency control . 73
7.4.4 Synthetic inertia . 75
7.4.5 Reactive power control . 78
7.5 Dynamic performance – Voltage fault ride-through . 79
7.5.1 General. 79
7.5.2 Testing according to Strategy 1 . 79
7.5.3 Testing according to Strategy 2 . 86
7.6 Disconnection from the grid . 89
7.6.1 General. 89
7.6.2 Grid protection . 89
7.6.3 RoCoF (df/dt) protection . 90
7.6.4 Reconnection time . 91
8 Additional measurement and test of electrical characteristics under controllable
test conditions . 92
8.1 General . 92
8.2 Power quality aspects . 93
8.2.1 Flicker control . 93
8.2.2 Flicker and voltage change during switching operations . 95
8.2.3 Active filter / sink for harmonics . 96
8.2.4 Frequency dependent impedance measurement . 98
8.3 Steady state operation . 103
8.3.1 Voltage capability . 103
8.3.2 Frequency capability . 104
8.3.3 Current unbalance factor capability . 106
8.4 Control performance . 107
8.4.1 Fundamental frequency grid impedance variations . 107
8.4.2 Island operation . 109
8.5 Dynamic performance . 111
8.5.1 RoCoF – real df/dt – capability . 111
8.5.2 Phase jump . 112
Annex A (informative) Report template. 114
A.1 Overview . 114
A.2 General . 114
A.3 Measurement and test of electrical characteristics . 117
A.3.1 Power quality aspects. 117
A.3.2 Steady state operation. 130
A.3.3 Control performance . 135
A.3.4 Voltage fault ride-through . 136

– 4 – IEC TS 61400-21-4:2025 © IEC 2025
A.3.5 Disconnection from the grid. 142
A.4 Additional measurement and test of electrical characteristics . 147
A.4.1 Power quality aspects. 147
A.4.2 Steady state operation. 157
A.4.3 Control performance . 162
A.4.4 Dynamic performance . 164
Annex B (informative) General information on test strategies and subsystems
overview . 166
B.1 General . 166
B.2 Guideline test strategies – functional, capability and performance test . 166
B.3 Overview of components, subsystems and control functions . 167
Annex C (informative) Modification and replacement of components . 170
C.1 General . 170
C.2 Definition of changes . 170
C.3 Workflow replacement of component . 170
C.4 Test & measurement procedure. 174
Annex D (informative) Transferability examples . 178
D.1 Overview . 178
D.2 Introduction to the results for test bench type 1a . 178
D.2.1 General. 178
D.2.2 Power quality aspects. 178
D.2.3 Steady state operation. 179
D.2.4 Control performance . 180
D.2.5 Dynamic performance . 181
D.3 Introduction to the results for test bench type 2a . 184
D.3.1 General. 184
D.3.2 Dynamic performance . 184
D.4 Introduction to the results for test bench type 3a . 186
D.4.1 General. 186
D.4.2 Dynamic performance . 186
D.4.3 Reactive power capability . 187
D.4.4 Impedance scan . 188
Annex E (informative) Harmonic assessment . 190
Annex F (informative) Examples of FRT functionalities . 198
Annex G (informative) Variants of HiL . 201
Annex H (informative) Voltage fault types. 203
H.1 Overview . 203
H.2 Phase-to-neutral voltage for a Type C fault . 204
H.3 Phase-to-neutral voltage for a Type D fault:. 206
Annex I (informative) Summary of grid emulator requirements . 208
Annex J (informative) Grid adaptability test using grid emulator . 211
J.1 Overview . 211
J.2 Grid adaptability test setup . 211
J.3 Grid adaptability requirements . 212
Bibliography . 215

Figure 1 – Overview of TC 88 – Standards related to grid connection. 14

Figure 2 – Overview of performance-, capability- and functionality test, and their
relation to the field tests . 16
Figure 3 – Example of step response . 25
Figure 4 – Simulated equivalent circuit of the grid emulator and the DUT . 28
Figure 5 – Generic structure of WT types . 33
Figure 6 – General overview of test bench systems included in this TS, including the
logical distinction between closed-loop and open-loop testing of wind turbines . 36
Figure 7 – Hierarchy of the different test bench types . 37
Figure 8 – Example of a nacelle test bench and a Type IV WT setup for closed-loop
testing . 38
Figure 9 – Example of a nacelle test bench and a type IV WT setup for open-loop
testing . 39
Figure 10 – Example of an electrical generation test bench and a Type IV WT setup for

closed-loop testing . 40
Figure 11 – Example of an electrical generation test bench and a Type IV WT setup for
open-loop testing . 41
Figure 12 – Example of a converter test bench and a Type IV WT setup for closed-loop
testing . 42
Figure 13 – Example of a converter test bench and a Type IV WT setup for open-loop

testing . 44
Figure 14 – Example of a controller test bench for closed-loop testing . 45
Figure 15 – Illustration of a controller test bench for open-loop testing . 46
Figure 16 – Example of auxiliary test bench . 47
Figure 17 – Block diagrams of the HiL systems for different test bench types. 48
Figure 18 – Overview of the process from offline simulation models to real time
suitable models used within the HiL system and the data used for verification . 53
Figure 19 – Suggested reporting form for comparison between HiL-operated test bench
results and offline simulation results . 54
Figure 20 – Example Power Spectral Density (PSD) of power from simulation and

experiment. 55
Figure 21 – Example structure of a typical grid emulator . 57
Figure 22 – Example of active power response step . 69
Figure 23 – Active power dynamic step response . 70
Figure 24 – Example of available active power and active power in ramp rate limitation
mode. 72
Figure 25 – Example of an active power control function P = f(f), with the different
measurement points and related steps of frequency . 74
Figure 26 – Synthetic inertia – example response and definition . 76
Figure 27 – Tolerance of the positive sequence voltage for the undervoltage event with

disconnected DUT under test [16] . 80
Figure 28 – Example of an undervoltage test chart . 81
Figure 29 – Tolerance of the positive sequence voltage for the overvoltage event with
disconnected DUT . 82
Figure 30 – Example of an over voltage capability chart . 83
Figure 31 – Example FRT impedance profile . 85
Figure 32 – Variable voltage and impedance grid emulator (case a). 93
Figure 33 – Constant voltage and impedance grid emulator with controllable load
(case b) . 94

– 6 – IEC TS 61400-21-4:2025 © IEC 2025
Figure 34 – Generic topology of (a) current and (b) voltage perturbation tests[12] . 99
Figure 35 – Example of a grid emulator structure for voltage perturbation
application (1) . 99
Figure 36 – Impedance measurement test methodology for wind turbines using
perturbation tests . 100
Figure 37 – Impedance variation – example of steady state procedure & stepwise
impedance increase . 108
Figure 38 – Single line diagram of Impedance load (Z-load) connected with DUT . 109
Figure A.1 – Maximum integer harmonic voltages versus harmonic order (background
noise measurement) . 121
Figure A.2 –Maximum interharmonic voltages versus frequency (background noise
measurement) . 121
Figure A.3 –Maximum higher frequency voltage components versus frequency
(background noise measurement) . 121
th
Figure A.4 – Maximum (Option 1) or maximum of the 95 percentiles (Option 2) of

integer harmonic currents versus harmonic order . 129
th
Figure A.5 – Maximum (Option 1) or maximum of the 95 percentiles (Option 2) of
interharmonic currents versus frequency . 129
th
Figure A.6 – Maximum (Option 1) or maximum of the 95 percentiles (Option 2) of
higher frequency current components versus frequency . 129
th
Figure A.7 – Maximum (Option 1) or maximum of the 95 percentiles (Option 2) of
integer harmonic voltages versus harmonic order . 129
th
Figure A.8 – Maximum (Option 1) or maximum of the 95 percentiles (Option 2) of
interharmonic voltages versus frequency . 130
th
Figure A.9 – Maximum (Option 1) or maximum of the 95 percentiles (Option 2) of
higher frequency voltage components versus frequency . 130
Figure A.10 – Reactive power versus active power . 131
Figure A.11 – PQ diagram at nominal voltage . 132
Figure A.12 – PQ diagram at maximum voltage . 133
Figure A.13 – PQ diagram at minimum voltage . 134
Figure A.14 – IUF-P diagram . 135
Figure A.15 – Wave shape of 3-phase voltages during entrance of voltage dip/swell
when the DUT is not connected . 137
Figure A.16 – Wave shape of 3-phase voltages during clearance of voltage dip/swell
when the DUT is not connected . 137
Figure A.17 – 3-phase voltages as RMS (1 line period) during the test when the DUT is
not connected . 138
Figure A.18 – Positive sequence voltage during the test when the DUT is not
connected . 138
Figure A.19 – Negative sequence voltage during the test when the DUT is not
connected . 138
Figure A.20 – Wave shape of 3-phase voltages during entrance of the voltage dip/swell
when the DUT is connected . 140
Figure A.21 – Wave shape of 3-phase voltages during clearance of the voltage
dip/swell when the DUT is connected. 140
Figure A.22 – 3-phase voltages as RMS (1 line period) during the test when the DUT is
connected . 140
Figure A.23 – Positive and negative sequence fundamental voltage during the test
when the DUT is connected . 141

Figure A.24 – 3-phase currents as RMS (1 line period) during the test when the DUT is
connected . 141
Figure A.25 – Pos. and neg. sequence fundamental current during the test when the
DUT is connected . 141
Figure A.26 – Pos. sequence fundamental active power during the test when the DUT

is connected . 141
Figure A.27 – Pos. sequence fundamental reactive power during the test when the
DUT is connected . 141
Figure A.28 – Pos. sequence fundamental active current during the test when the DUT
is connected . 142
Figure A.29 – Pos. sequence fundamental reactive current during the test when the

DUT is connected . 142
Figure A.30 – Wind speed or available power during the test when the DUT is
connected . 142
Figure A.31 – Voltage during the reconnection test of 10 s . 146
Figure A.32 – Active power during the reconnection test of 10 s, including the recovery . 146
Figure A.33 – Time-series of measured wind speed during the reconnection
test of 10 s . 146
Figure A.34 – Voltage during the reconnection test of 60 s . 146
Figure A.35 – Active power during the reconnection test of 60 s, including the recovery . 146
Figure A.36 – Time-series of measured wind speed during the reconnection
test of 60 s . 146
Figure A.37 – Voltage during the reconnection test of 600 s . 147
Figure A.38 – Active power during the reconnection test of 600 s, including the
recovery . 147
Figure A.39 – Time-series of measured wind speed during the reconnection test of
600 s . 147
th
Figure A.40 – Maximum (Grid emulator) or Maximum of the 95 percentiles (Public
grid) of integer harmonic currents versus harmonic order without AF . 155
th
Figure A.41 – Maximum (Grid emulator) or Maximum of the 95 percentiles (Public
grid) of integer harmonic currents versus harmonic order with AF . 155
th
Figure A.42 – Maximum (Grid emulator) or Maximum of the 95 percentiles (Public
grid) of integer harmonic voltages versus harmonic order without AF . 155
th
Figure A.43 – Maximum (Grid emulator) or Maximum of the 95 percentiles (Public
grid) of integer harmonic voltages versus harmonic order with AF . 155
Figure A.44 – Voltage during the minimum voltage test . 157
Figure A.45 – Active power during the minimum voltage test . 157
Figure A.46 – Reactive power during the minimum voltage test . 158
Figure A.47 – Voltage during the nominal voltage test . 158
Figure A.48 – Active power during the nominal voltage test . 158
Figure A.49 – Reactive power during the nominal voltage test . 158
Figure A.50 – Voltage during the maximum voltage test. 158
Figure A.51 – Active power during the maximum voltage test . 159
Figure A.52 – Reactive power during the maximum voltage test . 159
Figure A.53 – Frequency during the minimum frequency test . 159
Figure A.54 – Voltage during the minimum frequency test . 159
Figure A.55 – Active power during the minimum frequency test . 160
Figure A.56 – Reactive power during the minimum frequency test . 160
Figure A.57 – Frequency during the nominal frequency test . 160

– 8 – IEC TS 61400-21-4:2025 © IEC 2025
Figure A.58 – Voltage during the nominal frequency test . 160
Figure A.59 – Active power during the nominal frequency test . 160
Figure A.60 – Reactive power during the nominal frequency test . 161
Figure A.61 – Frequency during the maximum frequency test . 161
Figure A.62 – Voltage during the maximum frequency test . 161
Figure A.63 – Active power during the maximum frequency test . 161
Figure A.64 – Reactive power during the maximum frequency test . 161
Figure A.65 – Time series of the positive sequence voltage measured at the DUT
terminals . 162
Figure A.66 – Time series of the measured positive and negative sequence current . 162
Figure A.67 – Time series of the measured active and reactive power. 163
Figure A.68 – Time series of measured frequency at the DUT . 163
Figure A.69 – Time series of positive and negative sequence voltage at the DUT . 163
Figure A.70 – Time series of positive and negative sequence current . 163
Figure A.71 – Time series of active and reactive power from the DUT . 163
Figure A.72 – Time series of measured frequency or frequency reference value . 164
Figure A.73 – Time series of positive sequence active power output . 164
Figure A.74 – Time series of measured positive and negative sequence voltage
at the DUT . 165
Figure A.75 – Time series of measured positive and negative sequence current . 165
Figure A.76 – Time series of measured active and reactive power from the DUT . 165
Figure A.77 – Instantaneous voltage and current measurements from 20 ms before the
phase jump event until min 200 ms after the event . 165
Figure B.1 – Overview of three different test strategies. 167
Figure C.1 – Flowchart of the procedure to handle a hardware or software update . 171
Figure C.2 – Illustration of a set-up on a test field, when testing a complete wind
turbine with the in- and outputs, the parameter, references, measurements, the grid
and disturbances including a certain component type A . 175
Figure C.3 – Illustration of a set-up on a test bench according to the second step in
this procedure . 176
Figure C.4 – Illustration of the above described comparison of one component A tested
in the field and on the test rig, whereas the component B is only tested on a test rig . 177
Figure D.1 – Flicker comparison under different operating conditions . 178
Figure D.2 – Comparison of maximum active power in normal operation, observed in
the field and at the test benches [14] . 179
Figure D.3 – Comparison of Reactive Power Capability Test Results [14] . 180
Figure D.4 – Static error of the active power control . 180
Figure D.5 – Reactive power controls results derived at the test bench and in the field . 181
Figure D.6 – Positive sequence voltage for a three-phase dip to 25 % U during WT
N
full load operation [14] . 182
Figure D.7 – Observed phase angle during different 2-phase voltage dips . 182
Figure D.8 – Comparison of reactive current injection during three-phase dip to 25 %
U with the WT being in full load operation [14]. 183
N
Figure D.9 – Transient voltage transition for a two-phase fault with the WT being in No-
Load operation . 183

Figure D.10 – UVRT-event: Comparison of dynamic behaviour MoWiT simulation vs.
test bench [23] . 184
Figure D.11 – complete positive (solid), negative (dash-dotted), and zero (dotted)
sequence components of the MV quantities for a symmetrical 20 % UVRT in field
(orange) and on the test bench (green): PCC voltage (a), active current (b), reactive
current (c) . 185
Figure D.12 – detailed sections of positive – (solid), negative- (dash-dotted), and zero-
(dotted) sequence components of the medium voltage quantities in case of a
asymmetrical 0 % UVRT in field (orange) and on the test bench (green): PCC voltage
(a), active current (b), reactive current (c) [24] . 185
Figure D.13 – UVRT test results . 187
Figure D.14 – Reactive power capability – comparison of test bench and field
measurements . 188
Figure D.15 – Comparison of the G-CTR and frequency domain model impedance scan . 189
Figure E.1 – Test setup . 190
Figure E.2 – Equivalent circuit of experiment setup . 191
Figure E.3 – Thevenin model of the test set-up . 191
Figure E.4 – Sample results from averaging the results of 10 combinations of 5
experiments with varying filter capacitance; v (f) and z (f) are absolute values . 193
d d
Figure E.5 – Sample results from averaging the results of 10 combinations of 5
experiments with varying filter capacitance after outlier detection; v (f) and z (f) are
d d
absolute values . 194
Figure E.6 – Sample results from averaging the results of 10 combinations of 5
experiments after filtering of the Thevenin impedance; v (f) and z (f) are absolute
d d
values . 195
Figure E.7 – Harmonic measurement and PAR calculation of a wind turbine field
measurement with asynchronous pulse pattern . 196
Figure F.1 – Example for FRT detection and voltage base determination . 198
Figure F.2 – Example for detection of threshold and dead band of current support . 198
Figure F.3 – Example for variation of fault current contribution functionality . 199
Figure F.4 – Example for current priority based on positive and negative voltage
sequence . 199
Figure F.5 – Example for current limitations functionality . 200
Figure F.6 – Example for active power ramp rates after FRT event . 200
Figure G.1 – Different HiL systems . 202
Figure H.1 – Power system fault classification according to [13] . 203
Figure H.2 – Mains phasor diagram phase-to-neutral voltage fault type C . 204
Figure H.3 – Phasor diagram phase-to-neutral voltage fault type D . 206
Figure J.1 – Recommended grid adaptability test setup . 211

Table 1 – Overview of tests according to Clause 7 . 31
Table 2 – Overview of tests according to Clause 8 . 32
Table 3 – Overview of subsystems and main functions . 34
Table 4 – Functions of the rotor and structural dynamic model and related
requirements . 50
Table 5 – Recommended rotor model used for different tests on closed-loop test
benches . 50
Table 6 – Functions of the electrical generator model and related requirements . 51

– 10 – IEC TS 61400-21-4:2025 © IEC 2025
Table 7 – List of system of required sensor, actuator and interfaces models . 52
Table 8 – Static requirements for converter-based grid emulators . 58
Table 9 – Dynamic requirements for a converter-based grid emulators . 58
Table 10 – Harmonic voltage emission limits of the grid emulator at no-load
(disconnected DUT) .
...