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ICS 29.180
SANS 780:2004
ISBN 0-626-15673-4 Edition 3.2
Any reference to SABS 780 is deemed
to be a reference to this standard
(Government Notice No. 1373 of 8 November 2002)
SOUTH AFRICAN NATIONAL STANDARD
Distribution transformers
Published by Standards South Africa
1 dr lategan road groenkloof private bag x191 pretoria 0001
tel: 012 428 7911 fax: 012 344 1568 international code + 27 12
www.stansa.co.za
© Standards South Africa
This standard may only be used and printed by approved subscription and freemailing clients of the SABS.
SANS 780:2004
Edition 3.2
Table of changes
Change No. Date Scope
Amdt 1 2003 Amended to update normative references and bibliography.
Amdt 2 2004 Amended to change constructional requirements for radiators,
bushings and mounting brackets and to change tap ratings.
Abstract
Specifies requirements for single-phase and three-phase distribution transformers of the oil-immersed
type.
Keywords
distribution transformers, electrical equipment, oil-immersed transformers, power transformers, tappings
(electrical), three-phase transformers, transformers, windings.
Foreword
This South African standard was approved by National Committee StanSA SC 67E, Electricity
distribution systems and components - Electrical distribution, in accordance with procedures of
Standards South Africa, in compliance with annex 3 of the WTO/TBT agreement.
This edition cancels and replaces edition 3.1 (SANS 780:2003).
A vertical line in the margin shows where the text has been modified by amendment No. 2.
Annex A forms an integral part of this standard. Annexes B, C and D are for information only.
This standard may only be used and printed by approved subscription and freemailing clients of the SABS.
SANS 780:2004
Edition 3.2
Contents
Page
Abstract
Keywords
Foreword
1 Scope ... ............................................................................................................................................... 3
2 Normative references ... ...................................................................................................................... 3
3 Definitions ... ........................................................................................................................................ 5
4 Constructional requirements ... ........................................................................................................... 5
4.1 General ... .............................................................................................................................. 5
4.2 Standard dimensions of three-phase transformers ... .......................................................... 5
4.3 Cores ... .................................................................................................................................. 5
4.4 Windings ... ............................................................................................................................ 6
4.5 Off-circuit tapping switch and external tapping connections ... ............................................ 6
4.6 Tank and tank cover ... .......................................................................................................... 6
4.7 Conservator ... ....................................................................................................................... 8
4.8 Gaskets ... ............................................................................................................................. 8
4.9 Bushings, cable boxes and terminal arrangement ... .......................................................... 9
4.10 Dehydrating breathers and breather pipes ......................................................................... 10
4.11 Oil-level gauge ... .................................................................................................................. 11
4.12 Earthing terminals ... ............................................................................................................. 11
4.13 Pole-mounting brackets ... .................................................................................................... 11
4.14 Underbase ... ........................................................................................................................ 12
4.15 Jacking pads ... ..................................................................................................................... 12
4.16 Lifting lugs ............................................................................................................................. 12
4.17 Protection against corrosion ... ............................................................................................ 13
5 Physical and electrical requirements ... ........................................................................................... 15
5.1 Insulating materials ... .......................................................................................................... 15
5.2 Methods of cooling .............................................................................................................. 15
5.3 Temperature rise ... ............................................................................................................. 15
5.4 Rating ... ............................................................................................................................... 15
5.5 No-load voltages ... .............................................................................................................. 16
5.6 Tappings ... .......................................................................................................................... 16
5.7 Vector symbol ... .................................................................................................................. 16
5.8 Losses ... ............................................................................................................................. 16
5.9 Impedance voltage ... ......................................................................................................... 17
5.10 Loading cycle ... .................................................................................................................... 17
5.11 Complete self-protecting (CSP) features ... ......................................................................... 17
5.12 Insulation level ... .................................................................................................................. 17
5.13 Ability to withstand short-circuit............................................................................................. 17
5.14 Audio-sound level ... ............................................................................................................. 18
1
This standard may only be used and printed by approved subscription and freemailing clients of the SABS.
SANS 780:2004
Edition 3.2
Contents (concluded)
Page
6 Inspection and methods of test ... .................................................................................................... 18
6.1 General... ............................................................................................................................... 18
6.2 Measurement of paint thickness (routine test) ... ................................................................. 18
6.3 Test for effectiveness of sealing (routine test) ... ................................................................. 18
6.4 Tank stiffness test (type test) ... ............................................................................................ 19
6.5 Corrugated tank fatigue test (type test) ... ............................................................................ 19
6.6 Thermal trip test (type test) ... .............................................................................................. 20
6.7 Short-circuit trip test and transformer short-circuit withstand test (type test) ... ................. 20
7 Marking ... ......................................................................................................................................... 20
7.1 Rating and diagram plates ... ................................................................................................ 20
7.2 Terminals ... ........................................................................................................................... 20
Tables 1-9 ... .................................................................................................................................. ..21-25
Figures 1-6... ................................................................................................................................... .26-30
Annex A (normative) Non-preferred fittings ... .................................................................................. 31
Annex B (informative) Recommendations with regard to responsibility for testing... ...................... 35
Annex C (informative) Guide to purchasers on preparing an enquiry... .......................................... 36
Annex D (informative) Bibliography ... .............................................................................................. 41
2
This standard may only be used and printed by approved subscription and freemailing clients of the SABS.
SANS 780:2004
Edition 3.2
Distribution transformers
1 Scope
1.1 This standard specifies requirements for single-phase and three-phase distribution transformers of the
oil-immersed type, that
a) are category-sized in accordance with SANS 60076-1,
b) have rated maximum voltages that do not exceed 36 kV,
c) have uniformly insulated windings,
d) have a high-voltage and a low-voltage winding or, in the case of an auto-transformer, a series and a
common winding and, in each case, if necessary, a stabilizing winding or an auxiliary winding, and
e) are suitable for off-circuit tap changing only.
NOTE Although distribution transformers are generally designed to receive power at a voltage that exceeds
3,3 V and to deliver power to systems that have a nominal voltage of 230/400 V or 525 V, this standard covers
transformers that have any combination of rated primary and secondary voltages.
1.2 This standard is not applicable to transformers that have more than two windings, other than those
referred to in 1.1(d).
1.3 It applies to distribution transformers that are designed for use, at full continuous rating, under the
conditions specified in SANS 60076-1, and also under the following conditions:
a) at an altitude that does not exceed 1 800 m;
b) at a system voltage frequency of 50 Hz; and
c) under the conditions to which the electrical installations being served could be exposed.
NOTE Requirements that have to be specified by the purchaser and those that have to be agreed upon by the
manufacturer and the purchaser are listed in annex C.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of
this standard. All standards are subject to revision and, since any reference to a standard is deemed to
be a reference to the latest edition of that standard, parties to agreements based on this standard are
encouraged to take steps to ensure the use of the most recent editions of the standards indicated below.
Information on currently valid national and international standards can be obtained from Standards
South Africa.
3
This standard may only be used and printed by approved subscription and freemailing clients of the SABS.
SANS 780:2004
Edition 3.2
BS 2562, Specification for cable boxes for transformers and reactors.
BS EN 22063, Metallic and other inorganic coatings - Thermal spraying - Zinc, aluminium and their
alloys.
IEC 60085, Thermal evaluation and classification of electrical insulation.
IEC 60354, Loading guide for oil-immersed power transformers.
ISO 8501-1, Preparation of steel substrates before application of paints and related products - Visual
assessment of surface cleanliness - Part 1: Rust grades and preparation grades of uncoated steel
substrates and of steel substrates after overall removal of previous coatings.
SANS 32/EN 10240 (SABS EN 10240), Internal and/or external protective coatings for steal tubes -
Specification for hot dip galvanized coatings applied in automatic plants. Amdt 1
SANS 121/ISO 1461 (SABS ISO 1461), Hot dip galvanized coatings on fabricated iron and steel
articles - Specifications and test methods. Amdt 1
SANS 555 (SABS 555), Unused and reclaimed mineral insulating oils for transformers and switchgear.
SANS 1019 (SABS 1019), Standard voltages, currents and insulation levels for electricity supply. SANS
1029 (SABS 1029), Miniature substations.
SANS 1037 (SABS 1037), Standard transformer bushings.
SANS 1091 (SABS 1091), National colour standard.
SANS 1507-1 (SABS 1507-1), Electric cables with extruded solid dielectric insulation for fixed
installations (300/500 V to 1 900/3 300 V) - Part 1: General. Amdt 1
SANS 1507-2 (SABS 1507-2), Electric cables with extruded solid dielectric insulation for fixed
installations (300/500 V to 1 900/3 300 V) - Part 2: Wiring cables. Amdt 1
SANS 1507-3 (SABS 1507-3), Electric cables with extruded solid dielectric insulation for fixed
installations (300/500 V to 1 900/3 300 V) - Part 3: PVC Distribution cables. Amdt 1
SANS 1507-4 (SABS 1507-4), Electric cables with extruded solid dielectric insulation for fixed
installations (300/500 V to 1 900/3 300 V) - Part 4: XLPE Distribution cables. Amdt 1
SANS 1507-5 (SABS 1507-5), Electric cables with extruded solid dielectric insulation for fixed
installations (300/500 V to 1 900/3 300 V) - Part 5: Halogen-free distribution cables. Amdt 1
SANS 1507-6 (SABS 1507-6), Electric cables with extruded solid dielectric insulation for fixed
installations (300/500 V to 1 900/3 300 V) - Part 6: Service cables. Amdt 1
SANS 2808/ISO 2808 (SABS ISO 2808), Paints and varnishes - Determination of film thickness.
SANS 10064 (SABS 064), The preparation of steel surfaces for coating.
SANS 60076-1/IEC 60076-1 (SABS IEC 60076-1), Power transformers - Part 1: General.
4
This standard may only be used and printed by approved subscription and freemailing clients of the SABS.
SANS 780:2004
Edition 3.2
SANS 60076-2/IEC 60076-2 (SABS IEC 60076-2), Power transformers - Part 2: Temperature rise.
SANS 60076-5/IEC 60076-5 (SABS IEC 60076-5), Power transformers - Part 5: Ability to withstand
short circuit.
SANS 60137/IEC 60137, Insulated bushings for alternating voltages above 1 000 V.
SANS 60815/IEC 60815 (SABS IEC 60815), Guide for the selection of insulators in respect of polluted
conditions.
3 Definitions
For the purposes of this standard, the definitions given in SANS 60076-1 apply.
4 Constructional requirements
4.1 General
4.1.1 Unless otherwise agreed upon (see annex C), a transformer shall comply with the appropriate
constructional requirements contained in this clause. Constructional details and fittings shall comply with the
requirements given in table 1 and in the related subclauses listed in table 1.
4.1.2 All fittings (or their extensions or terminations) to which access might be required, shall be
mounted on the fitting-end of a transformer or adjacent to it if there is insufficient space on the fitting-end (i.e.
on one of the short sides of the tank).
On both an outdoor and an indoor transformer, the fitting-end shall be the left end when the transformer is
viewed from the high-voltage side.
4.2 Standard dimensions of three-phase transformers
The overall dimensions of a three-phase transformer shall not exceed the applicable values given in
table 2 or table 3 for a transformer of which the primary winding has a rated voltage of 22 kV or less, or 11
kV or less, respectively.
4.3 Cores
4.3.1 Earthing of magnetic circuit
The magnetic circuit shall be earthed to the core-clamping structure, at one point only, through a
removable connection that is in an accessible position at the top of the core structure.
4.3.2 Core-clamping structures
Clamping bolts, if fitted, shall be effectively locked. Core-clamping structures shall be of metal or other
acceptable material and shall clamp the core effectively. The core-clamping structure shall be insulated
from the core and this insulation shall be able to withstand an r.m.s. test voltage of 2 kV at 50 Hz for
60 s, or shall have an insulation resistance of at least 500 MS when measured at a d.c. voltage of 2 kV.
4.3.3 Core-lifting lugs
Core-lifting lugs or other means shall be provided for lifting the core, and these shall be so positioned
that, during lifting, no stress is imposed on any clamping bolt or its insulation.
