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 .
...