IEC 60076-21:2018

IEC 60076-21 ®
Edition 2.0 2018-12
™
IEEE Std C57.15
INTERNATIONAL
STANDARD
Power transformers –
Part 21: Standard requirements, terminology, and test code for step-voltage
regulators
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IEC 60076-21 ®
Edition 2.0 2018-12
IEEE Std C57.15™
INTERNATIONAL
STANDARD
Power transformers –
Part 21: Standard requirements, terminology, and test code for step-voltage

regulators
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.180 ISBN 978-2-8322-6260-3

– 2 – IEC 60076-21:2018
IEEE Std C57.15-2017
CONTENTS
FOREWORD . 9
1 Scope . 11
2 Normative references . 11
2.1 IEC references . 11
2.2 IEEE references . 11
2.3 SAE references . 12
3 Terms and definitions . 12
4 Use of normative references . 16
5 Service conditions . 17
5.1 Usual service conditions . 17
5.1.1 General . 17
5.1.2 Temperature . 17
5.1.3 Altitude . 17
5.1.4 Supply voltage . 17
5.1.5 Load current . 17
5.1.6 Outdoor operation . 17
5.1.7 Tank or enclosure finish. 17
5.2 Loading at other than rated conditions . 18
5.3 Unusual service conditions . 18
5.3.1 General . 18
5.3.2 Unusual temperature and altitude conditions . 18
5.3.3 Insulation at high altitude . 18
5.3.4 Other unusual service conditions . 19
6 Rating data . 20
6.1 Cooling classes of voltage regulators . 20
6.1.1 General . 20
6.1.2 Liquid-immersed (fire point ≤ 300 °C) air-cooled . 20
6.1.3 Liquid-immersed (fire point > 300 °C) air-cooled . 20
6.2 Ratings . 20
6.2.1 General . 20
6.2.2 Terms in which rating is expressed . 21
6.2.3 Preferred ratings . 21
6.2.4 Supplementary voltage ratings . 26
6.3 Supplementary continuous-current ratings . 27
6.3.1 General . 27
6.3.2 Optional forced-air ratings . 27
6.4 Taps . 28
6.5 Voltage supply ratios . 28
6.6 Insulation levels . 28
6.7 Losses . 29
6.7.1 General . 29
6.7.2 Total loss . 29
6.7.3 Tolerance for losses . 29
6.7.4 Determination of losses and excitation current . 29
6.8 Short-circuit requirements . 30
Published by IEC under licence from IEEE. © 2018 IEEE. All rights reserved.

IEEE Std C57.15-2017
6.8.1 General . 30
6.8.2 Mechanical capability demonstration . 31
6.8.3 Thermal capability of voltage regulators for short-circuit conditions . 31
6.9 Sound pressure level for liquid-immersed voltage regulators . 31
6.10 Tests . 32
6.10.1 General . 32
6.10.2 Routine tests . 32
6.10.3 Type tests . 32
7 Construction . 33
7.1 Bushings . 33
7.2 External dielectric clearances . 34
7.3 Terminal markings . 34
7.4 Diagram of connections . 35
7.5 Nameplates. 36
7.6 Tank construction . 37
7.6.1 General . 37
7.6.2 Pressure-relief valve . 37
7.6.3 Cover assembly . 37
7.6.4 Sudden pressure relay . 38
7.6.5 Lifting lugs . 38
7.6.6 Support lugs . 38
7.6.7 Substation bases . 40
7.6.8 Tank grounding provisions . 40
7.7 Components and accessories . 41
7.7.1 Components for full automatic control and operation. 41
7.7.2 Accessories for single-phase step-voltage regulators. 41
7.7.3 Accessories for three-phase step-voltage regulators . 42
8 Other requirements . 42
8.1 General . 42
8.2 Other components and accessories . 42
8.2.1 General . 42
8.2.2 Single- and three-phase voltage regulators . 42
8.2.3 Three-phase voltage regulators . 43
9 Test code . 43
9.1 General . 43
9.2 Resistance measurements . 43
9.2.1 General . 43
9.2.2 Determination of cold temperature . 43
9.2.3 Conversion of resistance measurements. 44
9.2.4 Resistance measurement methods . 44
9.3 Polarity test . 45
9.3.1 General . 45
9.3.2 Polarity by inductive kick . 46
9.3.3 Polarity by ratio meter. 46
9.4 Ratio test . 46
9.4.1 General . 46
9.4.2 Taps . 46
Published by IEC under licence from IEEE. © 2018 IEEE. All rights reserved.

– 4 – IEC 60076-21:2018
IEEE Std C57.15-2017
9.4.3 Voltage and frequency . 47
9.4.4 Three-phase voltage regulators . 47
9.4.5 Tolerance for ratio . 47
9.4.6 Ratio test methods . 47
9.5 No-load loss and excitation current . 49
9.5.1 General . 49
9.5.2 No-load loss test . 50
9.5.3 Waveform correction of no-load loss . 51
9.5.4 Test methods for three-phase voltage regulators . 52
9.5.5 Determination of excitation (no-load) current . 52
9.5.6 Measurements . 53
9.5.7 Correction of loss measurement due to metering phase-angle errors . 53
9.6 Load loss and impedance voltage . 54
9.6.1 General . 54
9.6.2 Factors affecting the values of load loss and impedance voltage . 55
9.6.3 Tests for measuring load loss and impedance voltage . 56
9.6.4 Calculation of load loss and impedance voltage from test data . 58
9.7 Dielectric tests . 60
9.7.1 General . 60
9.7.2 Lightning impulse type test . 61
9.7.3 Lightning impulse routine test . 67
9.7.4 Applied-voltage test . 69
9.7.5 Induced-voltage test . 69
9.7.6 Insulation power factor tests . 71
9.7.7 Insulation resistance tests . 72
9.8 On-load tap-changer routine tests . 73
9.8.1 General . 73
9.8.2 Mechanical test . 74
9.8.3 Auxiliary circuits insulation test . 74
9.9 Control system routine tests . 74
9.9.1 Applied voltage . 74
9.9.2 Operation . 74
9.10 Temperature-rise test . 74
9.10.1 General . 74
9.10.2 Test methods . 75
9.10.3 Resistance measurements . 78
9.10.4 Temperature measurements . 79
9.10.5 Correction of temperature-rise test results . 82
9.11 Short-circuit test . 83
9.11.1 General . 83
9.11.2 Test connections . 83
9.11.3 Test requirements . 84
9.11.4 Test procedure . 84
9.11.5 Proof of satisfactory performance . 86
9.12 Determination of sound level . 87
9.12.1 General . 87
9.12.2 Applicability . 88
Published by IEC under licence from IEEE. © 2018 IEEE. All rights reserved.

IEEE Std C57.15-2017
9.12.3 Instrumentation . 88
9.12.4 Test conditions . 88
9.12.5 Microphone positions . 90
9.12.6 Sound level measurements . 91
9.12.7 Determination of sound level of a voltage regulator . 95
9.12.8 Presentation of results . 97
9.13 Calculated data . 98
9.13.1 Reference temperature . 98
9.13.2 Loss and excitation current . 99
9.13.3 Efficiency . 99
9.13.4 Calculation of winding temperature during a short-circuit . 99
9.13.5 Certified test data . 101
10 Component tests . 102
10.1 General . 102
10.2 Enclosure integrity . 102
10.2.1 General . 102
10.2.2 Static pressure . 102
10.2.3 Dynamic pressure . 103
10.2.4 Type test for fault current capability of a voltage regulator enclosure . 103
10.3 On-load tap-changer . 104
10.3.1 General . 104
10.3.2 Type tests . 104
10.4 Control system . 109
10.4.1 General . 109
10.4.2 Control device construction. 110
10.4.3 Accuracy . 110
10.4.4 Type tests . 112
11 Universal interface . 116
11.1 Connection between control enclosure and apparatus . 116
11.2 Universal interface connector . 117
Annex A (informative) Unusual temperature and altitude conditions . 120
A.1 Unusual temperatures and altitude service conditions . 120
A.2 Effects of altitude on temperature-rise . 120
A.3 Operation at rated kVA . 120
A.4 Operation at less than rated kVA . 120
Annex B (informative) Field dielectric tests . 121
B.1 Tests on bushings . 121
B.2 Dielectric tests in the field . 121
Annex C (informative) Step-voltage regulator construction . 122
C.1 General . 122
C.2 Type A . 123
C.3 Type B . 123
C.4 Series transformer construction . 124
C.5 Reactor circuit . 124
C.6 Equalizer winding . 124
Annex D (informative) Hazards of Bypass off Neutral . 126
Published by IEC under licence from IEEE. © 2018 IEEE. All rights reserved.

– 6 – IEC 60076-21:2018
IEEE Std C57.15-2017
Annex E (informative) Overloading of step-voltage regulators . 130
Annex F (informative) Power capacitor and distributed generation compatibility . 134
F.1 Power capacitor application issues . 134
F.1.1 General . 134
F.1.2 Power circuit for consideration . 134
F.1.3 Voltage regulator incorporating line-drop compensation (LDC) in the
control . 134
F.1.4 Voltage regulator incorporating line current compensation (LCC) in the
control . 137
F.2 Distributed generation application issues . 137
F.2.1 General . 137
F.2.2 Control operation with power reversal recognition . 138
F.2.3 Power circuit for consideration . 139
F.2.4 Distributed generator alternatives . 139
F.2.5 P-Q summary . 140
F.2.6 Example system with distribution generation (DG) . 140
F.2.7 Expanded example, distributed generation mode . 142
F.2.8 Caveats . 142
F.2.9 Conclusions . 142
Bibliography . 143

Figure 1 – Single-phase voltage regulators . 35
Figure 2 – Three-phase voltage regulators with two arrangements of bushings . 35
Figure 3 – Type-B support lugs . 39
Figure 4 – Type-C support lugs . 40
Figure 5 – Connections for voltmeter-ammeter method of resistance measurement . 44
Figure 6 – Voltage regulator connected for polarity testing – Voltage regulator in
Neutral position. 46
Figure 7 – Voltmeter arranged to read the difference between the two output side
voltages . 48
Figure 8 – Voltmeters arranged to read the two series winding voltages . 48
Figure 9 – Basic circuit of ratio meter . 49
Figure 10 – Connection for no-load loss test of single-phase voltage regulator without

instrument transformers . 50
Figure 11 – Connections for no-load loss test of a single-phase voltage regulator with
instrument transformers . 51
Figure 12 – Three-phase voltage regulator connections for no-load loss and excitation
current test using three-wattmeter method . 53
Figure 13 – Single-phase voltage regulator connections for load loss and impedance

voltage test without instrument transformers . 57
Figure 14 – Single-phase voltage regulator connections for load loss and impedance
voltage test with instrument transformers . 57
Figure 15 – Three-phase voltage regulator connections for load loss and impedance
voltage test using the three-wattmeter method . 58
Figure 16 – Example of loading back method: single-phase . 76
Figure 17 – Example of loading back method: three-phase . 77
Published by IEC under licence from IEEE. © 2018 IEEE. All rights reserved.

IEEE Std C57.15-2017
Figure 18 – Microphone location for measuring sound level . 90
Figure 19 – Sound reflection correction factor "K" calculated as per Equation (29) . 94
Figure 20 – Measurements using the sound pressure method . 98
Figure 21 – Measurements using the sound intensity method . 98
Figure 22 – Universal interface specification . 117
Figure 23 – Socket/pin detail for universal interface . 117
Figure 24 – Universal interface locations . 119
Figure C.1 – Basic diagram of single-phase, Type A, step-voltage regulator . 122
Figure C.2 – Basic diagram of single-phase, Type B, step-voltage regulator . 122
Figure C.3 – Type A . 123
Figure C.4 – Type B . 123
Figure C.5 – Example of series transformer construction . 124
Figure C.6 – Equalizer winding and reactor circuitry – Non-bridging tap position . 125
Figure C.7 – Equalizer winding and reactor circuitry – Bridging tap position . 125
Figure D.1 – "Bypass off Neutral" power circuit . 127
Figure D.2 – Example of "Bypass off Neutral" RMS symmetrical current pattern of a
Type A design . 128
Figure D.3 – Example of "Bypass off Neutral" RMS symmetrical current pattern of a
Type B design . 129
Figure E.1 – Example of overload capability by tap position . 131
Figure E.2 – Example of Type A load loss vs tap position . 131
Figure E.3 – Example of Type B load loss vs tap position . 132
Figure E.4 – Tap-changer arc interruption envelope . 132
Figure E.5 – Contact wear . 133
Figure F.1 – Power distribution substation and representative distribution feeder. 134
Figure F.2 – Power distribution system with distributed generation . 139
Figure F.3 – P-Q diagram quadrant relationships . 140

Table 1 – Dielectric strength correction factors for altitudes greater than 1 000 m
(3 300 ft) . 19
Table 2 – Limits of temperature-rise . 21
Table 3 – Ratings for liquid-immersed 60 Hz step-voltage regulators (single-phase) . 22
Table 4 – Ratings for liquid-immersed 50 Hz step-voltage regulators (single-phase) . 23
Table 5 – Ratings for liquid-immersed 60 Hz step-voltage regulators (three-phase) . 25
Table 6 – Ratings for liquid-immersed 50 Hz step-voltage regulators (three-phase) . 26
Table 7 – Supplementary voltage ratings . 26
Table 8 – Supplementary continuous-current ratings . 27
Table 9 – Forced-air ratings relationship . 28
Table 10 – Values of voltage supply ratios . 28
Table 11 – Interrelationships of dielectric insulation levels for voltage regulators . 29
Table 12 – Values of k . 30
Table 13 – Maximum no-load (excitation) sound pressure levels . 31
Published by IEC under licence from IEEE. © 2018 IEEE. All rights reserved.

– 8 – IEC 60076-21:2018
IEEE Std C57.15-2017
Table 14 – Electrical characteristics of voltage regulator bushings . 34
Table 15 – External dielectric clearances . 34
Table 16 – Bushing terminal applications . 41
Table 17 – Requirements for phase-angle error correction . 54
Table 18 – Measurements to be made in insulation power factor tests . 72
Table 19 – Ambient sound pressure level correction . 92
Table 20 – Approximate values of the average acoustic absorption coefficient . 93
Table 21 – Voltage level values for select line-drop compensation . 112
Table 22 – Control supply voltage . 115
Table 23 – Socket pin identification for connector . 118
a
Table A.1 – Maximum allowable average temperature of cooling air for rated kVA . 120
Table A.2 – Rated kVA correction factors for altitudes greater than 1 000 m (3 300 ft) . 120
Table F.1 – Relevant system voltages and currents with capacitor location . 136
Table F.2 – System and voltage regulator control response with example distributed
generation (DG), no line-drop compensation . 141

Published by IEC under licence from IEEE. © 2018 IEEE. All rights reserved.

IEEE Std C57.15-2017
POWER TRANSFORMERS –
Part 21: Standard requirements, terminology,
and test code for step-voltage regulators

FOREWORD
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– 10 – IEC 60076-21:2018
IEEE Std C57.15-2017
International Standard IEC 60076-21/IEEE Std C57.15-2017 has been prepared by IEC
technical committee 14: Power transformers, in cooperation with the Transformers Committee
of the IEEE Power and Energy
...