CONTENTS
FOREWORD 7
1Scope 11
2Normative references 11
3Basic physics of sound 11
3.1Sound pressure, p 11
3.2Particle velocity, u 13
3.3Sound intensity, I 13
3.4Sound power, W 13
3.5Sound fields 15
4Sources and characteristics of transformer and reactor sound 17
4.1General 17
4.2Sources 17
4.3Vibration transmission 27
4.4Sound radiation 29
5Measuring principles 29
5.1General 29
5.2Sound pressure level measurement 31
5.3Sound intensity measurements 31
5.4Guidance on narrow-band measurements 33
6Comparison of measuring methods 39
6.1General 39
6.2Sensitivity of the sound pressure method to the test environment 41
6.3Sensitivity of the sound intensity method to test environment 43
6.4Guidance on method selection 47
7Practical aspects of making sound measurements 47
7.1General 47
7.2Orientation of the test object 47
7.3Number of measurement points on a measuring surface 47
7.4Choice of microphone spacer for sound intensity measurements 49
7.5Impact of background noise on sound intensity measurements 51
7.6Measurements in the presence of sound-proofing screens 53
8Difference between factory tests and field sound level measurements 53
8.1General 53
8.2Load power factor 53
8.3Load current 55
8.4Operating voltage 55
8.5Operating temperature 55
8.6Harmonics in the load current and voltage 57
8.7DC magnetization 57
8.8Effect of remanent flux 57
8.9Sound level build-up due to reflections 57
8.10Influence of distance when making on-site measurements 59
8.11Converter transformers with saturable reactors and/or interphase
transformers 59
9Specifying transformer and reactor sound levels 61
9.1General 61
9.2Guarantee sound levels 61
9.3Choice of test method 63
9.4Load conditions 65
9.5Auxiliary cooling equipment 67
9.6Voltage regulation 67
9.7On-site operating conditions 67
9.8Example noise specification for power transformer and cooling auxiliaries
(see Annex A) 67
9.9Example noise specification for a distribution transformer (see Annex B) 69
Annex A (informative) Worked example: Power transformer with cooling auxiliaries mounted on a separate structure >3 m from the principal radiating surface of the
transformer – Sound power level determined via sound pressure method 71
Annex B (informative) Worked example: Distribution transformer, sound power
determined via time-synchronous sound intensity method 91
Figure 1 – Example curves showing relative change in length for one type of core
lamination during complete cycles of applied 50 Hz a.c. induction up to different peak
flux densities Bmax = 1,2 T – 1,9 T 19
Figure 2 – Induction (smooth line) and relative change in lamination length (dotted) as
a function of time due to applied a.c. induction: 1,8 T, 50 Hz – no d.c. bias 21
Figure 3 – Example curve showing relative change in lamination length during one complete cycle of applied a.c. induction with a small d.c. bias: 1,8 T, 50 Hz and 0,1 T, 0 Hz 21
Figure 4 – Induction (smooth line) and relative change in lamination length (dotted) as a function of time due to applied a.c. induction with a small d.c. bias: 1,8 T, 50 Hz and 0,1 T, 0 Hz 23
Figure 5 – Sound level increase with d.c. current in the windings 23
Figure 6 – Typical load current sound spectrum measured under short-circuit conditions 25
Figure 7 – Microphone arrangement 33
Figure 8 – Test environment 41
Figure 9 – Distribution of disturbances to sound pressure in the test environment 43
Figure 10 – Sketch of dry-type transformer showing measurement points 49
Figure 11 – Illustration of background sound passing through test area and sound radiated from the test object. Microphone pair positions indicated by open
(microphone A) and full (microphone B) circles 51
Table 1 – Values of A-weighting as a function of frequency 37
IEC 60076 consists of the following parts, under the general title Power transformers:
Part 1: General
Part 2: Temperature rise
Part 3: Insulation levels, dielectric tests and external clearances in air
Part 4: Guide to the lightning impulse and switching impulse testing – Power transformers and reactors
Part 5: Ability to withstand short circuit Part 6: Reactors (under consideration)
Part 7: Loading guide for oil-immersed power transformers Part 8: Application guide
Part 10: Determination of sound levels
Part 10-1: Determination of sound levels – Application guide Part 11: Dry-type transformers
Part 12: Loading guide for dry-type power transformers (under consideration) Part 13: Self-protected liquid filled transformers
Part 14: Design and application of liquid-immersed power transformers using high- temperature insulation materials
Part 15: Gas-filled-type power transformers (under consideration).
POWER TRANSFORMERS –
Part 10-1: Determination of sound levels – Application guide
1 Scope
This part of IEC 60076 provides supporting information to help both manufacturers and purchasers apply the measurement techniques described in IEC 60076-10. The sources and characteristics of transformer and reactor sound are described. Practical guidance on making measurements is given, and factors that may influence the accuracy of the methods are discussed. This application guide also clarifies those factors which should be agreed between manufacturer and purchaser when specifying a transformer or reactor, and indicates why values measured in the factory may differ from those measured on site.
This application guide is applicable to transformers and reactors together with their associated cooling auxiliaries.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
IEC 60076-10:2005, Power transformers – Part 10: Determination of sound levels