nrs 030 - voltage transformers
DESCRIPTION
VT'sTRANSCRIPT
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ICS 01-11-22-28.180NRS 030:2001
ISBN 0626-13390-4 Second edition
ELECTRICITY DISTRIBUTION
Inductive voltage transformers for rateda.c. voltages from 3,6 kv up to andincluding 145 kv for indoor and outdoorapplications
Preferred requirements for applications inthe Electricity Supply Industry
N R S
Rationalized User Specification
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This Rationalized User Specification isissued by the NRS Project
on behalf of theUser Group given in the foreword
and is not a standard as contemplated in the Standards Act, 1993 (Act 29 of 1993).
Rationalized user specifications allow userorganizations to define the performance and quality
requirements of relevant equipment.
Rationalized user specifications may, after a certainapplication period, be introduced as national standards.
Amendments issued since publicationAmdt No. Date Text affected
Correspondence to be directed to Printed copies obtainable from
South African Bureau of Standards South African Bureau of Standards(Electrotechnical Standards) Private Bag X191Private Bag X191 Pretoria 0001Pretoria 0001
Telephone: (012) 428-7911Fax: (012) 344-1568E-mail: [email protected]: http://www.sabs.co.za
COPYRIGHT RESERVED
Printed on behalf of the NRS Project in the Republic of South Africaby the South African Bureau of Standards1 Dr Lategan Road, Groenkloof, Pretoria
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NRS 030:20011
Contents
Page
Foreword ............................................................................................................... 2
Introduction ........................................................................................................... 3
Key words ............................................................................................................. 3
1 Scope ............................................................................................................... 5
2 Normative references .......................................................................................... 5
3 Definitions and abbreviation ................................................................................ 6
4 Requirements ..................................................................................................... 6
4.1 General requirements .................................................................................. 74.2 Design Details ............................................................................................. 94.3 Construction requirements ........................................................................... 114.4 Primary terminals ........................................................................................ 134.5 Earthing terminals ........................................................................................ 144.6 Rating plates and diagram plates ................................................................... 144.7 Drawings and instruction manual .................................................................. 154.8 Spare fuses ................................................................................................ 17
5 Tests
5.1 Type tests ................................................................................................... 185.2 Routine tests ............................................................................................... 195.3 Special tests ............................................................................................... 225.4 Tests certificates ......................................... 26
6 Marking, labelling and packaging
Annexes
A (informative) Notes on system neutral earthing ...................................................... 27
B (informative) Guide to purchasers on preparing an enquiry ................................... 29
C (informative) Model form for schedules A and B ................................................... 31
Bibliography .......................................................................................................... 37
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NRS 030:2001 2
Foreword
This specification has been prepared on behalf of the Electricity Suppliers Liaison Committee(ESLC) and approved by it for use by supply authorities.This specification is based onSABS IEC 60044-2:1997.
This specification was prepared by a Working Group which, at the time of publication, comprised thefollowing members:
Dr F Mariani (Chairman) City Power, JohannesburgM J Hyde Cape Town Electricity DepartmentJ P Boshoff Eskom TransmissionA J Claasen SABSC B Clark Distribution Technology, EskomD G Duncan Ron Slatem AssociatesB de Jager Mangaung ElectricityJ A Ehrich Tshwane ElectricityF M Lukacsovics eThekwini Metropolitan ElectricityV Sewchand (Project Leader) NRS ProjectM T Outram Nelson Mandela Metropolitan MunicipalityH Smit Observer
A Manufacturers Interest Group (MIG) structured through the Electrical Engineering and AlliedIndustries Association (EEAIA) was consulted on the contents of this specification and its commentswere incorporated where the working group was in agreement. The MIG comprised the followingmembers:
M Barbolini Transformer ManufacturersA Calvino ETCK Gilbert GEC AlsthomR Constable HTSAP Goulooze ConelectricE S Mokholo ABB PowertechB Scholtz Current ElectricB Warburton InstruformA Wolmarans XAMAX
The Working Group was appointed by the ESLC, which, at the time of approval, comprised thefollowing members:
R Wienand (Chairman) eThekwini Metropolitan Council, AMEUM N Bailey Eskom Distribution TechnologyA J Claasen Electrotechnical Engineering Standards, SABSN Croucher City of Cape Town, AMEUP Crowdy Eskom Distribution TechnologyW G H Dykman City of Tshwane, AMEUA H L Fortmann eKurhuleni, AMEUP A Johnson Eskom Technology StandardizationJ G Louw City of Cape Town (Tygerberg Administration)D Michie Nelson Mandela Metropolitan Electricity, AMEUA J van der Merwe Mangaung Electricity, AMEUJ S van Heerden SABS NEFTAP van Niekerk City Power Johannesburg, AMEU
ISBN 0626-13390-4
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NRS 030:20013
Annexes A to C are for information only.
Recommendations for corrections, additions or deletions should be addressed to the NRS ProjectManager, c/o SABS, Private Bag X191, Pretoria, 0001.
Introduction
This specification was prepared to establish and promote uniform requirements for inductive voltagetransformers for rated a.c. voltages from 3,6 kV up to and including 145 kV, for use with electricalmeasuring instruments, electrical protection devices, or both. This specification is intended to enablepurchasers to acquire the specified equipment without the need for detailed and extensive contractdocuments.
The ESLC expresses the wish that, in the national interest and in support of government policy tofoster local manufacture and to stimulate export, all purchasers adopt the requirements of thisspecification insofar as their particular conditions will allow. Any differences between thisspecification and the corresponding purchasers requirements should, as far as possible, be clearlyindicated in schedules A and B attached to this specification and where appropriate, be submittedfor consideration in future revision thereof.
Key words
Voltage transformers; Inductive; Measuring; Protection.
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NRS 030:2001 4
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NRS 030:20015
SPECIFICATION
Inductive voltage transformers for rated a.c. voltages from 3,6 kV up toand including 145 kV for indoor and outdoor applications
Preferred requirements for applications in the Electricity Supply Industry
1 Scope
This specification is based on SABS IEC 60044-2 for single-phase voltage transformers andBS 7729 for three-phase voltage transformers and covers the requirements for inductive voltagetransformers for rated a.c. voltages from 3,6 kV up to and including 145 kV for indoor and outdoorapplications, for use with electrical measuring instruments and electrical protection devices, or both,at a service frequency of 50 Hz.
2 Normative references
The following standards contain provisions which, through reference in the text, constitute provisionsof NRS 030. At the time of publication, the editions indicated were valid. All standards andspecifications are subject to revision, and parties to purchase agreements based on NRS 030 areencouraged to investigate the possibility of applying the most recent editions of the standards listedbelow. Information on currently valid national and international standards and specifications can beobtained from the South African Bureau of Standards.
IEC 60060-2:1994, High voltage test techniques Part 2: Measuring systems.
IEC 60269-2:1986, Low voltage fuses Part 2: Supplementary requirements for fuses for use byauthorized persons (fuses mainly for industrial application).
IEC 60270:1981, Partial discharge measurements.
IEC 60282-1:1994, High voltage fuses Part 1: Current limiting fuses.(Amendment No. 1, 1996)
IEC 60296:1982, Specification for unused mineral insulating oils for transformers and switchgear.(Amendment No. 1, 1986)
IEC 60376:1971, Specification and acceptance for new sulphur hexafluoride.
BS 1872: 1984, Specification for electroplated coatings of tin.
BS 7729: 1994, Instrument transformers: Three-phase voltage transformers for voltage levelhaving Um up to 52kV.
SABS EN 10240:1997, Internal and/or external protective coatings for steel tubes Specificationfor hot-dipped galvanised coatings applied in automatic plants.
SABS IEC 60044-2:1997, Instrument Transformers Part 2: Inductive voltage transformers.
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NRS 030:2001 6
SABS IEC 60298:1990, A.C. metal-enclosed switchgear and controlgear for rated voltages above1 kV and up to and including 52 kV.
SABS IEC 60529: 1989, Degrees of protection provided by enclosures (IP code).
SABS IEC 60694: 1996, Common specifications for high-voltage switch-gear and control gearstandards.
SABS ISO 1461: 2000, Hot-dipped galvanised coatings on fabricated iron and steel articles Specification and test methods.
SABS 156:1977, Moulded-case circuit-breakers.
SABS 555:1985, Mineral insulating oil for transformers and switchgear (uninhibited).
SABS 1885:2001, Metal-clad switch-gear for rated a.c. voltages above 1kV and up to and including36kV Part 1: General requirements and methods of test.
3 Definitions and abbreviation
3.1 Definitions
For the purposes of this specification, the definitions given in SABS IEC 60044-2 and the followingdefinitions apply:
NOTE The terms used in this specification are generally consistent with the definitions given in the International Electro-technical Vocabulary (IEV). Where the terms are directly extracted from IEV or other sources, this is indicated.
3.1.1 approved (approval): Approved in writing by the purchaser.
3.1.2 creepage distance: The shortest distance, along the surface of the insulating materialbetween two conductive parts. [IEV 151-03-37]
3.1.3 discrimination (in protection): The ability of protective devices to disconnect only thatsection of a power system upon which a fault has occurred.
3.1.4 residual voltage winding: The winding of a transformer between the terminals of which isproduced the residual voltage or one of the three component voltages which when added togetherconstitute the residual voltage. [IEV 321-03-11]
NOTE: For single-phase voltage transformers used in a three-phase system, refer to IEC 60044-2, paragraph 2.2.
3.1.5 valid test certificates: A certified copy of the original test certificate of a test conducted byapproved testing authority.
3.1.6 special tests: Tests to be performed on request from the customer.
3.2 Abbreviation
For the purposes of this specification the following abbreviation apply:
3.2.1 MCB: miniature circuit breaker
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NRS 030:20017
4 Requirements
4.1 General requirements
a) Unless otherwise specified in schedule A, the following standard service conditions apply:
1) ambient air temperature: -10 C to 40 C;
2) altitude: not exceeding 1 800 m;
3) average humidity: not exceeding 95 %;
4) wind pressure: not exceeding 700 Pa (equivalent to 34 m/s).
5) level of atmospheric pollution: heavy; and
6) special conditions as specified in schedule A.
For voltage transformers that are intended for service at altitude exceeding the altitudes at which thetype tests and routine tests had been conducted or are to be conducted, corrections shall be appliedin accordance with SABS IEC 60044-2 and BS 7729.
b) The following will be specified in schedule A: whether the voltage transformer is required eitherindoors or outdoors; whether it is required for measuring or protection applications; the ratedburden, at the rated accuracy class; the nominal system phase-to-phase voltage (Un) or phase-to-neutral voltage; the maximum r.m.s. phase-to-phase equipment voltage (Um); the number of phases;the nominal supply frequency; the method of system earthing; single-phase or three-phasetransformer; symmetrical three-phase fault current and other relevant details.
If so specified in schedule A, the voltage transformer shall be provided with a power winding thatcomplies with the voltage rating and the continuous current rating specified in schedule A.
c) The name of the manufacturer, the type reference of the voltage transformer offered, and allother information required shall be stated in schedule B.
NOTE For Schedules A and B, see annex B2.
4.1.1 Creepage distance
The standard creepage distance between phase and earth shall be the heavy pollution level inaccordance with table 8 of SABS IEC 60044-2 i.e. 25 mm/kV of the highest r.m.s. phase-to-phasevoltage Um for equipment. Where other pollution conditions are specified in schedule A, thecreepage distance shall be as specified in table 8 of SABS IEC 60044-2, and shall be stated inschedule B.
4.1.2 Insulation levels
4.1.2.1 The rated insulation levels of the voltage transformers, shall comply with Table 1.
4.1.2.2 The voltage transformers shall be suitable for operation on a system that has a basicinsulation level (BIL) as specified in schedule A.
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NRS 030:2001 8
Table 1 Standard voltages and insulation levels of voltage transformers
1 2 3 4
Highest r.m.s.voltage for
equipment Um
Nominal system r.m.s.phase-to-phase
voltage Un
Rated lightning impulsepeak
withstand voltage
Rated short-time power-frequency r.m.s.
withstand voltage (seeIEC 60044-2)
kV kV kV kV
3,6
7,2
12
24
36
3,3
6,6
11
22
33
45
75
95
150
200
16
22
28
50
70
52
72,5
100
145
44
66
88
132
250
350
450
650
95
140
185
275
4.1.3 General requirements for insulation materials
Insulation materials shall withstand the conditions of service in normal use. Only service experienceor adequate approved tests shall be the basis for assigning rational temperature limits for theinsulation.
Where new materials and new systems are involved, appropriate functional tests shall be the basisfor the selection. Details of the insulation materials shall be stated in schedule B.
NOTE For guidance on the use of recognized systems of thermal classes of electrical insulation, and the procedures forevaluation of new insulation systems see IEC 60085.
4.1.4 Oil-insulated voltage transformers
4.1.4.1 The oil used in voltage transformers shall comply with SABS 555, or with IEC 60296, withthe requirement that at the moment of the initial filling the moisture in the oil shall not exceed10 mg/kg.
4.1.4.2 The oil shall be new (virgin), shall have no additives and shall have a naphthenic base.Details of the type of oil shall be stated in schedule B.
4.1.4.3 The oil shall be certified to contain no Polychlorinated Biphenyls (PCB) i.e. zero count.
4.1.4.4 The quantity of oil shall be stated in schedule B.
4.1.4.5 Oil insulated voltage transformers shall be hermetically sealed.
4.1.4.6 The method used to allow for the expansion of the insulating oil shall be submitted forapproval and if bellows are used they shall be of stainless steel.
4.1.4.7 All gasket joints shall be below the minimum oil level and shall not leak oil when thetransformer is tested in accordance with 5.2.5 and shall remain so under normal operating serviceconditions for the duration of the expected service life.
NOTE Where the manufacturers design requires specially designed gasketted joints to be above the oil level, machinedsurfaces and O-rings shall be used. Details of such shall be submitted for approval.
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4.1.4.8 Facilities for oil filling, draining and sampling shall be provided subject to approval. Thesefacilities shall be sealed by gasket joints and shall be below the normal operating oil level.
4.1.4.9 Oil-level indicators shall be provided on voltage transformers and shall be so arranged as tobe readable from ground level, with the voltage transformer mounted on its structure as in service.Dipstick level indicators may be offered for 11 kV and 22 kV voltage transformers. Details of the oil-level indicators shall be submitted for approval.
Oil gauges shall be flush mounted, and shall be securely attached along their entire perimeter, to thetank or expansion chamber. The sight glasses shall be resistant to ultraviolet light and shall beprotected from accidental damage.
4.1.4.10 If so specified in schedule A, oil sample valves will be provided. Details of the oil samplevalves shall be submitted for approval before manufacture is undertaken.
4.1.4.11 The rating plate shall be marked to indicate the type designation of the oil used to fill thevoltage transformer.
4.1.5 Gas-insulated voltage transformers
4.1.5.1 The voltage transformer shall be clearly labelled to indicate that it is filled with gas(SF6, etc.) and where applicable, that gas is under pressure.
4.1.5.2 Facilities shall be provided at the base of voltage transformers to allow for filling, evacuatingand topping up of gas.
4.1.5.3 Sulphur hexafluoride (SF6) gas shall comply with the requirements of IEC 60376.
4.1.5.4 Where gas is under pressure, permanent gas-density monitoring devices shall be providedat the base of the voltage transformer. Gas-density monitoring devices shall contain first stage andsecond stage alarm signalling contacts, wired to the secondary terminal strips, as well as a visibleindication of the pressure/density in the voltage transformer.
4.1.5.5 The method of sealing of the voltage transformer and any need for gas replenishment shallbe stated by the supplier in schedule B. Refer to SABS IEC 60298 for details of leakage rates andexpected life before replenishment.
4.1.6 Dry type voltage transformers
Dry type voltage transformers shall have resin-encapsulated cores and windings.
4.2 Design details
4.2.1 General
4.2.1.1 The voltage transformers shall be capable of operating continuously under the serviceconditions stated, without exceeding the temperature limits specified in SABS IEC 60044-2.
NOTE For an explanation of the effectively and non-effectively earthed systems, see annex A.
4.2.1.2 The voltage factor for isolated-neutral voltage transformers shall be at least 1,2 (continuous).
4.2.1.3 The voltage factor for earthed-neutral voltage transformers shall be at least:
a) 1,2 (continuous) for effectively earthed systems, and
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NRS 030:2001 10
b) 1,9 (30 s) for non-effectively earthed systems.
4.2.1.4 Where the purchaser requires a higher voltage factor than those specified in 4.2.1.2 and4.2.1.3, the voltage factor shall be 2,1 for 30 s or continuous for any system, as specified inschedule A.
NOTE 1 For special cases, where continuous operation with an earth fault can occur, the client must decide on the voltagefactor to be used, and this will be specified in schedule A.
NOTE 2 The standard secondary voltage for utilities in South Africa is 110V (Refer IEC 60044-2 Section 5.1.2(a)) . Where avoltage other than 110V is required, this will be specified in schedule A.
4.2.2 Accuracy class designation
4.2.2.1 The accuracy class designation required for the voltage transformer shall be as specified inschedule A in accordance with Tables 11 and 12 of SABS IEC 60044-2.
4.2.2.2 For specific requirements, refer to Table 2 and 3 below:
Table 2 Single phase outdoor post-type voltage transformers with 2 secondary windings
1 2 3
Windingnumber
Accuracy class Burden perphase
VA
1 Measuring - 0,2 100
2 Protection - 3 P 100
Table 3 Three phase indoor post-type voltage transformers with 1 secondary winding
1 2 3 4 5
Windingnumber
Accuracy class Burden perphase
VA
Voltage factorand rated time
Type
1 Measuring - 0,2 50 1,9 continuous 4 wire - 5 limb
Protection - 3 P star point internally earthed
4.2.3 Short-circuit protection
4.2.3.1 When so specified in schedule A, protection against short-circuits will be provided bymeans of fuses in the primary winding circuit, and by fuses or MCBs in the secondary windingcircuits.
4.2.3.2 For indoor voltage transformers (VTs), fuses on the primary side shall:
a) comply with the requirements of IEC 60282-1,
b) be capable of safely and effectively interrupting the current resulting from the short-circuit faultlevel of the power system as specified in schedule A,
c) preferably be mounted inside the primary bushings with access facility. If fuses are to bemounted externally, the necessary fuse holders and supports shall be provided and constructedas part of the voltage transformer. When so specified in schedule A, fuses and attachments willbe suitable for mounting in the enclosure,
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d) fuse in less than one second for a short-circuit on the primary or secondary winding up to theoutgoing secondary terminals. In addition, the primary fuses shall be so rated that timediscrimination with the secondary fuses or MCBs is possible,
e) have a rating, as stated in schedule B, that takes into account the magnetizing inrush current,and
f) have interlocking and accessibility according to SABS 1885 (Clause 4.9).
4.2.3.3 Secondary fuses or MCBs shall comply with the requirements of IEC 60269-2 or SABS 156respectively, and shall also comply with the following requirements:
a) for single-phase VTs , they shall be mounted in the secondary terminal box (see 4.3.3);
b) for single-phase VTs, the fuses or MCBs shall be fitted in the phase conductor and a bolted linkshall be fitted in the neutral conductor;
c) the operation of the fuse or MCB shall occur in less than 1 s for a secondary short-circuit; and
d) the secondary fuses or MCBs shall discriminate with the primary fuses, where the latter aresupplied.
4.2.3.4 The supplier shall state, in schedule B, the calculated primary and secondary short-circuitcurrents for a short-circuit on the secondary terminals, assuming zero source impedance.
4.2.4 Interchangeability
All voltage transformers of the same type and rating, from one manufacturer, shall have fullyinterchangeable components and shall have an identical electromagnetic performance.
4.3 Constructional requirements
4.3.1 Windings and connections
4.3.1.1 All windings and connections shall be supported to maintain clearances between oneanother and to earthed metal under short-circuit conditions and during transportation. The windingsand connections shall be braced to withstand vibration in normal service.
4.3.1.2 All winding insulation shall be treated to ensure that there will be no appreciable shrinkage ordistortion in normal service.
4.3.1.3 Where a residual voltage winding has been specified in schedule A, the primary neutralpoint will be brought out to a terminal suitable for earthing and connecting to an earthing conductorof size suitable for the specified short-circuit primary current.
4.3.1.4 Where tapped windings for providing both rated secondary residual voltages are specified inschedule A, the connections will be so brought out that an open delta connection can be made foreither secondary residual voltage. In the case of draw-out voltage transformers, the tapping neednot be brought out through plug-in contacts.
4.3.1.5 The connections for the primary, secondary and tertiary windings shall be as specified in SABS IEC 60044-2.
4.3.1.6 Details of the primary winding neutral (earth) terminal shall be stated in schedule B.
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NRS 030:2001 12
4.3.2 Cores
4.3.2.1 The core shall be connected to earth by a direct single-point connection. In the case of oil-insulated voltage transformers, the core may be connected to an earthing terminal inside the voltagetransformer tank.
4.3.2.2 Where a three-phase transformer with a residual voltage output is specified, the core shallhave 5 limbs, as specified in schedule A, to provide an iron path for zero-sequence fluxes.
4.3.2.3 Core lifting arrangements shall be provided for mechanical handling for units where coresexceed 15 kg. Lifting arrangements shall be so designed that no stress is imposed on the insulationof any of the cores.
4.3.2.4 Details of the core steel i.e. grade and thickness of lamina shall be stated in schedule B.
4.3.2.5 Details of the B/H (and not the voltage/excitation current) curve of core materials (stateannealed or not annealed, as used by the manufacturer) with all B values up to 2,3 Tesla shall alsobe stated in schedule B.
4.3.3 Secondary terminals
4.3.3.1 Unless approved the bushings used for bringing the secondary connections through the tankinto the secondary terminal box shall not be used as the secondary terminals for serviceconnections.
4.3.3.2 Rail-mounted type secondary terminals shall be either the screw clamp type or the spring-loaded insertion type.
4.3.3.3 Studs shall have centre distances of not less than 25 mm. If less than 25mm, barriersinterposed at these positions shall be provided. A minimum creepage distance of 13mm shall bemaintained between conducting parts of terminals.
4.3.3.4 Not more than two conductors may be connected to any side of a terminal.
4.3.4 Secondary terminal box
4.3.4.1 A secondary terminal box shall be provided to contain secondary winding fuses or MCBswhen these are specified. For three-phase VTs only, a bolted link shall be provided if specified inschedule A between the secondary neutral and earth. Details of fuses or MCBs and the earthingterminal shall be stated in schedule B.
4.3.4.2 The earth end of the high-voltage winding will, unless otherwise specified in schedule A, bebrought out to a terminal in the terminal box, with a link to an earth terminal. The terminal shall beclearly labelled to indicate that it has to be solidly earthed during service.
4.3.4.3 The secondary terminal box shall be located in an accessible position and shall be providedwith an easily removable (preferably slip-on) weather-proof cover. The cover shall be secured to thecorresponding terminal box.
4.3.4.4 The dimensions of the terminal box shall be stated in schedule B and shall either be:
a) integrally cast with the voltage transformer case, and approved (see 3.1); or
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b) a suitable metal box .
4.3.4.5 The terminal box with the cover fixed in place shall have a degree of protection of at leastIP54 in accordance with SABS IEC 60529.
4.3.4.6 The secondary terminal box shall have an opening, at the bottom, for vertical entry of thesecondary control cables. The opening shall be covered externally by an undrilled, removable glandplate of brass for a steel box or of aluminium for an aluminium box. Unless otherwise specified inschedule A, this gland plate, as well as the opening, shall have an effective area of at least 75 mm by50 mm. This area shall be stated in schedule B. The vertical centre-line of the opening shall projectat least 25 mm beyond any part of the unit beneath the terminal box, to facilitate vertical entry of thecable.
4.3.4.7 The distance between the bottom terminals and the gland plate shall be at least 75 mm.
4.3.4.8 The terminal box shall be fitted with a breathing vent of diameter at least 10 mm. This ventshall be situated in the bottom of the box, shall be made of non-corroding material and shall bedesigned to prevent the entry of insects.
4.3.4.9 An earth stud shall be provided for earthing inside the terminal box. The earth stud shall beof diameter at least 6 mm and shall have an external connection to the main earthing system.
4.3.4.10 The beginning and the end of each secondary winding with all secondary taps, if any, shallbe wired to suitable terminals accommodated in the terminal box.
4.3.4.11 For removable VTs, the specified terminal box requirements shall apply to the fixed portionof the racking system.
4.3.5 Mounting arrangement for outdoor voltage transformers
The mounting arrangement for the complete assembly of an outdoor voltage transformer shall besuch that it can be bolted to a support structure, with bolts arranged on the corners of a square ofdimensions not exceeding those specified in schedule A.
4.3.6 Metal finish
4.3.6.1 Unless otherwise approved, all ferrous parts associated with voltage transformers shallbe hot-dip galvanised in accordance with SABS EN 10240 and SABS ISO 1461 except that thegalvanized layer shall be of thickness at least 90 mm. Metal spraying to a thickness of at least of80 mm is also acceptable, if so specified in schedule A. In the case of metal spraying, metallizationshall be followed by a base coat and a top coat of a high quality approved paint.
4.3.6.2 Non-ferrous parts shall be made of corrosion resistant materials.
4.3.6.3 Corrugated tanks are not acceptable.
NOTE Tanks and fittings shall be of such a shape that water cannot collect at any point of the outside surfaces.
4.4 Primary terminals
Primary terminals and their associated parts, mounted on the voltage transformers as in service,shall be able to withstand the mechanical and sealing tests of 5.3.4 without leakage, distortion,cracking or other failure. Primary terminals will be of the type specified in schedule A, and details ofthe terminals shall be stated in schedule B.
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NRS 030:2001 14
Primary terminals and their components shall be of an approved material of adequate conductivityand suitably corrosion proofed. Non ferrous terminals shall be electro-tinned to comply with,classification Cu/Sn/12/f of BS 1872 without subsequent heat treatment or machining. Ferrousterminals shall be galvanized.
Where a threaded stem is specified, it shall be of thread size M16 and shall be supplied with two locknuts, two washers, and two nuts, all of the same material as the stem. Where plain cylindricalterminals are specified, these shall have a terminal length of 125 mm and a diameter of 26 mm.
Only copper terminals may be screwed directly into the bushing head, provided they are positivelysecured and that the threaded portion of the latter constitutes a blind hole. In no case shall agasketted joint be interposed in a current path.
Terminals may be fabricated integrally with the bushing head. Fabricated terminal arrangements willbe accepted subject to approval (See 3.1), but fabrication that might cause annealing of any yellowmetals used, is not acceptable.
Terminal arrangements for post type, or line to ground, voltage transformers shall be testedmechanically in accordance with clause 5.3.4. In regard to this test, the design factor of safety shallbe not less than 2.
Terminal orientation will be as specified in schedule A.
4.5 Earthing terminals
The apparatus earthing terminal or clamp arrangement shall be suitable for accommodating a 50 mm 3 mm copper earthing strap.
4.6 Rating plates and diagram plates
4.6.1 Rating plates and diagram plates shall be engraved, or have the information stamped into anintrinsically corrosion-resistant material and shall be externally mounted. Rating plates and diagramplates shall be fixed to the main body of the voltage transformer and not to any removable part. Therating plates and diagram plates shall be so positioned that they may easily be read by personnelwithout their lives being endangered.
The information to be displayed shall be as specified in SABS IEC 60044-2, except that theSABS IEC number shall be replaced by the number NRS 030:2000. In addition, the rating plate shalldisplay:
a) serial number and type designation,
b) rated voltage factor and corresponding rated time,
c) manufacturing date(month/year).
If fluid insulated voltage transformers are used, then the fluid type designation (see 4.1.4.11) shall bedisplayed. If gas-insulated voltage transformers are used, then the type designation(see 4.1.5.1) and pressure, shall be displayed.
4.6.2 In the case of voltage transformers built into circuit-breakers, each circuit-breaker shall beprovided with a rating plate and a diagram plate giving the ratings, polarity, terminal markings and theconnections of the voltage transformer windings with respect to the circuit-breaker terminals andmechanism box.
4.6.3 Diagram plates shall show the terminal markings and the relative physical arrangement of thevoltage transformer secondary windings with respect to the primary terminals.
4.6.4 Anodized aluminium shall not be used.
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NRS 030:200115
4.6.5 The material of, and the method of fixing the plates, shall be stated in schedule B.
4.7 Drawings and instruction manuals
Both paper and electronic formats are acceptable.
If electronic data and software are required by the purchaser, this will be as specified in schedule A.
4.7.1 Tender drawings and literature
In response to a tender, the following drawings and literature where applicable, shall be submitted.
4.7.1.1 For each winding of the voltage transformer:
a) the B/H (and not the voltage/excitation current) curve of core materials (state annealed or notannealed, as used by the manufacturer) with all B values up to 2,3 Tesla;
b) the resistance of the secondary winding at 75 C for the lowest and the highest ratio; and
c) if so specified in schedule A, the core steel details.
4.7.1.2 For each type of voltage transformer and specified voltage:
a) typical outline, general arrangement and dimensioned drawings, which should include thefollowing:
1) Mounting details;
2) Primary terminal markings;
3) Overall height;
4) Maximum width;
5) All other leading dimensions;
6) Position of the earthing terminal;
7) Height of the gland plate in the secondary terminal box above the base, and the distance ofits centre-line from the centre-line of the voltage transformer (see also 4.3.4.6);
8) The dimensions of the primary terminals; and
9) The mass of the complete voltage transformer, including oil, when applicable (also to bestated in schedule B).
b) Sectional arrangement drawing, which should show the following details:
1) Relative position of the cores;
2) Primary terminal markings and primary insulation;
3) Sectional drawings of the bushings and fuses (if the HV fuses are placed within thebushings);
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NRS 030:2001 16
4) Full details of test performed on proposed fuses;
5) Details of gasket joints;
6) Oil-sealing or gas-sealing arrangements;
7) In the case of oil-insulated voltage transformers, the method used to accommodateexpansion and contraction of the insulating oil (see also 4.1.4.6); and
8) Pressure relief devices (if any).
c) Details of the secondary terminal box, which may either be shown on the general arrangementdrawing or on a separate drawing. This should detail the following:
1) The cover and the method of fixing the cover and the gland plates;
2) Secondary terminals, their arrangement and clearance, creepage barriers and markings;and
3) Venting and draining arrangements.
d) Sectional drawing of the bushing shed profile.
e) A descriptive pamphlet and instruction book.
4.7.2 Contract drawings
Drawings of the quality, type and size specified in the conditions of contract shall be submitted induplicate, for approval.
4.7.2.1 The following drawings and diagrams shall be supplied for post-type voltage transformers:
a) an outline dimension, mounting detail, main terminal and clamp drawing. This drawing shall showthe main terminal markings so that the physical arrangement can be correlated with the desiredelectrical schematic arrangement;
b) a drawing giving details of secondary terminal boxes, covers, cover screws and gland plates,showing the physical arrangement of the secondary terminals and the secondary earthingterminals, protection devices, barriers, etc. The secondary terminal markings shall be shown onthis drawing;
c) the rating and diagram plate detail drawings showing the technical performance and connectiondata which will actually appear on these plates;
d) in the case of a cascade connected VT, a drawing showing the schematic connections of coresand windings; and
e) the excitation curve.
4.7.2.2 The following drawings and diagrams shall be provided for three-phase voltage transformersas well as units built into circuit-breakers:
a) a connection diagram showing the terminal markings, ratios, relative polarity and physicalarrangement of the voltage transformer windings with respect to one another, the primaryterminal markings and the circuit-breaker mechanism box. This information may be included onthe circuit-breaker connection diagram, or may take the form of the final diagram plate;
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NRS 030:200117
b) a drawing giving details of secondary terminal boxes, covers, cover screws and gland plates,showing the physical arrangement of secondary terminals and secondary earthing terminals,barriers etc. The secondary terminal markings shall be shown on this drawing; and
c) the rating plate and the diagram plate detail showing the technical performance and connectiondata which will actually appear on these plates.
4.7.3 Descriptive pamphlets and instruction books
a) Unless otherwise specified in schedule A, three sets of descriptive pamphlets and instructionbooks covering the equipment offered will be supplied at the purchasers address as soon aspossible, but not later than the delivery date. A complete set of test certificates, as specified in5.4.1 and 5.4.2, shall be included in each instruction book.
b) One additional set per order shall be supplied to each delivery site.
c) The instruction books shall include details of the recommended maintenance, inspection and oil-sampling procedures during the life of the equipment together with the necessary sectional andother sketches of oil-sampling devices provided.
4.8 Spare fuses
Spare fuses will be provided, as specified in schedule A.
5 Tests
NOTE The tests are classified as type tests, routine tests and special tests as follows:
Type tests:
a) temperature rise test;
b) short-circuit withstand capability test;
c) excitation curve;
d) impulse test on primary winding;
e) wet test for outdoor type transformers; and
f) radio interference voltage.
Routine tests:
a) verification of terminal markings;
b) high-voltage power-frequency dry withstand tests;
c) accuracy test;
d) test for effectiveness of sealing;
e) dielectric dissipation factor test (tangent delta);
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NRS 030:2001 18
f) partial discharge test; and
g) winding resistance measurements.
Special tests:
a) chopped impulse test on primary winding;
b) switching impulse test;
c) dielectric dissipation factor test (tangent delta) at ambient temperature and 90C;
d) mechanical strength and sealing test on primary terminals; and
e) long duration voltage test.
5.1 Type tests
5.1.1 General
5.1.1.1 Unless valid (see 3.6) and approved (see 3.1) type test certificates for all type testsspecified in SABS IEC 60044-2 and in schedule A, are available, carry out type tests on one fullyassembled voltage transformer of each type and rating at an approved test facility stated inschedule B, the certificates of which shall be included in the test reports. Type tests shall be followedby the routine tests.
5.1.1.2 Test voltages shall be in accordance with the insulation levels specified in table 1.
NOTE For details of system neutral earthing conditions, see annex A.
5.1.2 Temperature rise test
The temperature rise test shall be performed in accordance with SABS IEC 60044-2 with the testvoltage applied as per table 1.
5.1.3 Short-circuit withstand capability test
The short-circuit withstand test shall be performed in accordance with SABS IEC 60044-2 but use anapplied test voltage of 1,2 Um where Um is the highest r.m.s voltage. The primary and secondaryshort-circuit currents shall be measured and the results recorded.
5.1.4 Excitation curve
An excitation curve shall be produced, covering a primary voltage range from zero up to andincluding at least that point where an increase of 10 % in voltage results in an increase of 100 % incurrent. The results shall be given in tabulated form and graphical form. The knee-point beingdefined as an increase of 10 % in voltage resulting in an increase of 50 % in current, shall be clearlymarked on the curves and the saturated and unsaturated region shall be accurately depicted, statingalso whether the core is annealed or not.
For voltage transformers having a voltage factor of 2,1 the curve shall confirm that the requirementsof 4.2.1.4 are met.
5.1.5 Radio interference voltage (RIV) test
The requirements of SABS IEC 60694 shall apply.
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NRS 030:200119
The test voltage Ut shall be corrected for the effect of the relative air density during the test inaccordance with:
Ut = adR18.1 3mU
where
Rad is the relative air density; and
Um is the highest r.m.s. voltage for equipment.
The test report covering the RIV test shall contain full details of the temperature, barometric pressure,correction factor applied, RIV test value and the humidity, although no correction for humidity shallbe applied.
The value of the radio interference voltage shall not exceed 500 V.
5.2 Routine tests
5.2.1 General
All voltage transformers shall undergo routine tests at the manufacturers works to prove compliancewith this specification. The tests shall be performed on each fully assembled voltage transformerready for despatch.
5.2.2 Verification of the terminal markings
Each voltage transformer shall have terminal markings in accordance with all relative sections ofSABS IEC 60044-2.
5.2.3 High-voltage Power-frequency dry withstand test
The power-frequency dry withstand test shall be performed:
a) on primary windings in accordance with clause 9.2 of SABS IEC 60044-2 but at the withstandvoltage given in table 1 of this specification; and
b) on secondary windings in accordance with clause 9.3 of SABS IEC 60044-2. For a highervoltage requirement (e.g.4,5 kV), this will be specified in Schedule A .
5.2.4 Accuracy test
Each secondary winding of a voltage transformer shall be tested for accuracy in accordance withSABS IEC 60044-2.
5.2.5 Test for effectiveness of sealing
5.2.5.1 This test shall be performed concurrently with 5.3.4 when required.
5.2.5.2 Oil-insulated voltage transformers
A test pressure shall be applied for a period of 12 h, to each voltage transformer, filled with oil to atleast the normal level and with all sealed fittings in place, but, where necessary, with pressure limitingdevices blocked or removed. Ensure that the test pressure measured at the top of the voltage
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NRS 030:2001 20
transformer is at least 70 kPa or twice the maximum operating pressure at an ambient airtemperature of 40 oC, whichever is the greater. Check for compliance with 4.1.4.7.
5.2.5.3 Gas-insulated voltage transformers
The rate of gas leakage shall be determined in accordance with the voltage transformermanufacturers approved method.
It must be ensured that each individual SF6 gas-filled compartment is leak-tested after sealing andthat the rate of gas leakage is such as to confirm compliance with 4.1.5.5.
5.2.6 Dielectric dissipation factor test (tangent delta)
This test shall be performed after the power-frequency test and it applies only to liquid immersedinsulation units having Um 72,5 kV. The test shall be performed at ambient temperature which shallbe recorded. Raise the voltage, applied between primary terminal and the earth screen terminal, to120 % of Um and, while the voltage is being raised, record tangent delta measurements at voltages of5 %, 15 %, 30 %, 60 %, 75 %, 100 % and 120 % of Um. Then reduce the voltage to zero and, whilethe voltage is being reduced, record the tangent delta measurements at the identical voltages usedpreviously.
During both excursions, record tangent delta values at 10 kV. The voltage transformer is deemed tohave passed the test when the tangent delta readings at the same voltage, during both excursions,are not different by more than 0,5 % and the difference, in absolute value, between the reading atthe voltage of 120 % Um and that at the voltage of 5 % of Um is not more than 0,1 %.
NOTE: Should this test be required for units less than 72,5 kV, then primary terminals shall be bonded together and themethod of testing shall be as above for three-phase units.
5.2.7 Partial discharge test
After all dielectric tests have been completed, partial discharge tests shall be performed on voltagetransformers with Um 7,2 kV to demonstrate that the magnitude q in picocoulombs of thedischarge, does not exceed that permitted at the specified measuring voltage Us in Table 4.
5.2.7.1 Condition of voltage transformers.
In addition to the requirements of 6.2 of IEC 60270, the voltage transformers shall be fully assembledin the service condition, prior to testing.
5.2.7.2 Methods
The methods used shall be those specified in IEC 60270.
5.2.7.3 Test circuit and instrumentation
For test circuit and instrumentation, refer to clause 9.2.4 of SABS IEC 60044-2.
5.2.7.4 Sensitivity of measurement
The level of disturbance (i.e. background level of external discharge) shall not exceed half the valueof the maximum permissible partial discharge level, except when test procedure 2 (refer to table 4) isfollowed for which this level shall not exceed 1 pC. The sensitivity of the test apparatus shall be lessthan 1 pC. Both the disturbance level and the sensitivity level shall be measured and recorded.
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NRS 030:200121
5.2.7.5 Test procedure and measurement
The test shall be performed by means of one of the two test procedures for dry-type units and onetest procedure for fluid immersed units. For dry-type units, the manufacturer may choose eitherprocedure 1 or procedure 2, unless otherwise specified in schedule A
Test procedure 1 (Applicable to both fluid and dry-type units):
A voltage applied between primary terminals, and corresponding to the requirements ofIEC 60060-2, shall be raised to the pre-stress value Up (refer table 4).
During this excursion the partial discharge inception voltage, Ui as defined in IEC 60270, shall benoted and recorded; thereafter the applied voltage shall be increased in steps of 0,1 Up until Up isreached.
While the voltage is being raised from Ui to Up the partial discharge measurements shall be taken ateach step.
The Up voltage shall be maintained for the specified duration (refer table 4), then reduced to zeroand partial discharge measurements shall be taken at the identical previous voltage steps until theextinction voltage, Ue as defined in IEC 60270, is reached and recorded. The extinction voltage shall,in no case, be less than 1,1 Um .
During the return excursion, the partial discharge measurement at Us kept for the specified duration,shall be taken and shall not be greater than the maximum permissible value reflected in table 4.
Test procedure 2 (Applicable to dry-type units only):
A voltage applied between primary terminals, and corresponding to the requirements ofIEC 60060-2, shall be raised to the pre-stressed value Up and maintained there for the specifiedminimum duration (refer table 4), during which partial discharge level shall be noted and recorded.During this excursion the inception voltage UI shall be noted and recorded. The voltage shall then bereduced to the measuring voltage Us and kept there for the required duration, during which time themeasured partial discharge shall not exceed the maximum permissible value. Afterwards the voltageshall be reduced to zero during which the extinction voltage Ue shall be noted and recorded. In no
case, shall Ue be less than ( ) m321,1 U .NOTE The partial discharge may be determined at the same time as that for the sixty second power-frequency dry withstandtest, since the pre-stress voltage Up is equal to Ut whilst both are maintained for 60 s.
Table 4 Partial discharge test levels for voltage transformers
1 2 3 4 5 6 7
Type of maininsulation
Testprocedure
Pre-stressvoltage
Up
kV
Minimumduration of
Up
s
Measuringvoltage
Us
kV
Minimumduration
of
Us
s
Maximumpermissible
partialdischarge qabove the
backgroundlevel
pC
Fluid (for example,oil/gas)
- 60 1,3 Um 60 10
1 60 ( ) m325,1 U 60 10Dry(for example,cast resin)
2
Power-frequencyvoltage asper table 1
60 ( ) m322,1 U 180 1
5.2.8 Winding resistance measurements
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NRS 030:2001 22
The resistance shall be measured at ambient temperature of each primary and secondary winding,then the values, in ohms and corrected to 75 C, shall be recorded in the routine test certificates.
5.3 Special tests
Where so specified in schedule A, special tests will be performed and may be specified as typetests or routine tests. The special tests can be required during the tender adjudication, during themanufacturing process or prior to the installation of the VT.
NOTE Valid test certificates, if available for identical voltage transformers, may be accepted in lieu of the special tests.
5.3.1 Chopped impulse test
Routine tests shall be performed and recorded before and after the chopped impulse test.
5.3.1.1 The outdoor voltage transformers rated at Um 72,5 kV for fluid-type units and Um 7,2 kVfor dry-type units shall withstand 600 or 100 consecutive chopped impulse voltages preceded andfollowed by one full impulse voltage.
The manufacturer may choose either 600 impulses or 100 impulses. The impulse shall comply withthe chopping circuit in figure 1.
For fluid-type units, a dissolved gas analysis (DGA) test is required before, just after and three daysafter the completion of the test. The analyses shall be done by an approved laboratory.
Only negative polarity impulses shall be used combined with the negative polarity full lightningimpulse test described below.
The chopping circuit is arranged so that the chopped impulse voltage shall be fixed between 65 %and 80 % of the peak of the lightning impulse given in table 1, and chopped before the crest. Theamplitude of the first opposite polarity oscillation shall be limited to between 25 % and 50 % of thechopped impulse voltage level, so that the total swing shall be equal to 100 % of the standard impulsepeak value. The rise time, from zero to chopping, and the collapse time, from chopping to zero, ofthe impulse, shall be in accordance with table 5.
The test impulse sequence comprises
a) one full impulse,
b) 600 or 100 chopped impulses, and
c) one full impulse.
The time interval between successive impulses shall be as short as possible and shall be equal to orless than 1 min. The impulse wave shapes of the two full voltage impulses together with 12 or 10chopped impulses each at every 50 or every 10 chopped impulses depending on the number used,600 or 100 respectively, shall be recorded.
Flashovers along self-restoring external insulation during chopped impulse applications shall bedisregarded but each such application shall be repeated.
Any one of the following indicates an internal fault and constitutes failure of the unit:
a) any difference between the two full impulse wave shapes;
b) any difference between chopped impulse wave shapes;
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NRS 030:200123
c) the increase of dissolved gas concentration in the oil, between the sample analysed before thetest and that analysed three days after , exceeding the limits indicated in Table 6 ; and
d) any significant difference between all routine tests performed before and after completion of thetest.
Figure 1 Typical chopped wave test circuit
where:
G Impulse generator;
S Spheroid spark gap when 600 chopped impulses are selected, or SF6 spark gap when100 chopped impulses are selected;
T Object under test; and
D Voltage divider.
Table 5 Setting times for chopped wave impulse test
Number of chopped impulses Maximum rise time
(from zero to chopping)
mm s
Maximum collapse time
(from chopping to zero)
mm s
600 As for standard lightning impulsesspecified in schedule A
0,5
100 0,25 0,25
Table 6 Increase of dissolved gas concentration of oil samples taken before and three daysafter the application of 600 or 100 chopped impulses
Detectionthreshold
Significant minimum increase ingas concentration
Maximum permissible increasein gas concentration
Gas
g/g g/g g/g
Hydrogen (H2) 2 4 5
Methane (CH4) 0,4 1 3
Ethane (C2H6) 0,4 1 3
Ethylene ( C2H4) 0,4 1 2
Acetylene (C2H2) 0,4 1 2
NOTE Although the unit passes the test based on the values in table 6, deterioration of the insulation may have occurred. Avisual inspection of the unit shall be agreed between the manufacturer and purchaser.
G S T D
to recorder
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NRS 030:2001 24
5.3.2 Switching impulse test
5.3.2.1 Outdoor type transformers shall be subjected to wet tests only. Dry tests are not required.
5.3.2.2 The test voltages shall have the appropriate values given in table 1, depending on the highestvoltage for equipment and the specified insulation level.
5.3.2.3 The test voltage shall be applied between the line terminal of the primary winding and earth.
5.3.2.4 The earth terminal of the primary winding or the non-tested line terminal in the case of anunearthed voltage transformer, one terminal of the secondary winding(s), the frame, case (if any),and core (if there is a special earth terminal) shall be connected together and to earth. At the optionof the manufacturer the connection to earth may be made through a suitable current recordingdevice. The non-earthed secondary terminals may be left open or connected to a high impedancedevice for recording the voltage wave appearing across the secondary winding(s) during the test. Tocounteract the effect of core saturation, it is permissible, between consecutive impulses, to modifythe magnetic status of the core by a suitable procedure.
5.3.2.5 The test shall be performed with both positive and negative polarities. Fifteen consecutiveimpulses of each polarity, corrected for atmospheric conditions, shall be applied.
5.3.2.6 The transformer has passed the test if
a) no disruptive discharge occurs in the non-self-restoring internal insulation,
b) no flashovers occur along the non-self-restoring external insulation,
c) no more than two flashovers occur across the self-restoring external insulation, and
d) no other evidence of insulation failure is detected (i.e. variations in the wave-shape of therecorded quantities).
5.3.2.7 The dielectric type tests shall be carried out on the same transformer which, afterwards,shall be subjected to all routine tests.
5.3.3 Dielectric dissipation factor test (tangent delta) at ambient temperature and 90C.
The test applies only to the liquid immersed insulation units having Um 72,5 kV. The measurementsof the dielectric loss angle (tangent delta) on the primary insulation, as a special test, shall be madeafter the impulse test, switching impulse test and power frequency test, but before the long-durationvoltage test. The special test measurements of tangent delta on the completed unit consist of twoperformances:
a) at ambient temperature, the voltage applied between primary winding terminals shall be raised to120 % of the highest system voltage. While the voltage is being raised, tangent deltameasurements shall be taken at 5 %, 10 %, 15 %, 20 %, 30 %, 60 %, 75 % 100 % and 120 % ofUm . This voltage shall then be reduced to zero and tangent delta measurements shall be taken asthe voltage is being reduced at 100 %, 75 %, 60 %, 30 %, 20 %, 15 %, 10 %, and 5 % of Um;and
b) at a temperature of 90 C, tangent delta measurements shall be taken at 60 %, 75 %, 100 % and120 % of Um.
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NRS 030:200125
Ambient temperature shall be recorded. All measurements at both temperatures and at each stepshall be recorded. The voltage transformer has passed the test if the temperature coefficient 'a ' foreach step is less than or equal to 0,01/C.
Values of a of 0,02/C or larger are unacceptable. Intermediate values of a (between 0,01/C and0,02/C) may be accepted if they pass the long-duration test of 5.3.5.
)(at
90n
a tan tan
I t _ 90
1
d
da =
where:
a is the temperature coefficient;
ta is the ambient temperature;
tan d90 is the tangent delta at 90 C;
tan dta is the tangent delta at ambient temperature; and
ln is the natural logarithm.
5.3.4 Mechanical strength and sealing test on primary terminals
The mechanical strength test shall be performed before the sealing test.
The sealing test is applicable where oil and gas-insulating mediums are used.
The test shall be performed on primary terminals and their associated parts, mounted on the voltagetransformer as in service, on units fitted with each type and size of primary terminal arrangement.
Procedure:
a) first apply the force Fm at right angles to the axis of the terminal stem and at the tip of the stemfor a duration of tm; and
b) thereafter, apply the force Fi as for Fm above for the duration of ti and in the case of oil-insulatedor gas-insulated voltage transformers, concurrently with the test for effectiveness of sealing asdetailed in clause 5.2.5.
Fm, tm, Fi and ti are given in table 7, in accordance with type of insulation medium.
NOTE An acceptable method agreed with the manufacturer shall be used to check compliance with 4.4.
Table 7 Mechanical strength and sealing tests on primary terminals
1 2 3 4 5 6
Primary terminalstem diameter
Mechanical strength test Insulation medium ofvoltage transformer
Sealing test
Force Fm Duration tm Force Fi Durationti
mm N min N h
3 Fluid (for example,oil/gas)
750 12 26 1 500
60 Resin only Not applicable
5.3.5 Long duration voltage test
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NRS 030:2001 26
This test shall be performed to determine the thermal stability of voltage transformers havingUm 7,2 kV in respect of resin insulated units and Um 72,5 kV in respect of oil insulated units.
5.3.5.1 Test procedure for oil insulated units
A test voltage of 33,1 mU shall be applied to the voltage transformer's primary insulation for 48 hat the ambient temperature of 40 C 2 C. The value of tangent delta measured at the test voltageduring the last 10 h, at regular intervals of 1 h, shall not exceed the value at 75 % of Um during thetangent delta test in 5.3.3, and it shall be constant within a tolerance of 0,02 %. If this condition isnot achieved, the test shall be continued up to tangent delta stabilisation or to insulation failure.Duringthe specified time interval and the applied test voltage, the apparent charge of the partial dischargeshall be recorded and shall be well below the value measured at Us during the partial discharge test(see table 4) and the apparent charge shall be constant(with insignificant variations).
5.3.5.2 Test procedure for resin insulated units
After having raised the voltage in the primary side up to 34,1 mU and maintained for not less than
10 s it shall be reduced to 33,1 mU and then maintained at that value for 48 h.The values of the partial discharge, measured and recorded during the last 10 h at regular intervalsof 1 h, shall be well below the value measured at Us during the partial discharge test (see table 4)and the partial discharge shall be constant (with insignificant variations).
5.4 Test certificates
5.4.1 General
All tests shall be fully documented in English and shall be signed and stamped by the purchasersinspector(s), and copies shall be forwarded to the purchaser.
The copies shall include test certificates, bound together in one volume, recording the results of typetests, routine tests and special tests (if applicable), carried out on one fully assembled voltagetransformer of each type and rating.
5.4.2 Routine and sample test certificates (supplied with each voltage transformer)
Each voltage transformer shall be delivered with one copy of all routine test certificates together witha copy of the excitation curve as specified in 5.1.4. The certificates and curves shall be enclosed ina water-resistant packaging and housed inside the terminal box of each voltage transformer.
6. Marking, labelling and packaging
The following is required:
a) long term storage of spare voltage transformers;
b) handling/preparation for transport with details of lifting and support positions; and
c) correct handling and slinging methods.
Annex A
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NRS 030:200127
(informative)
Notes on system neutral earthing
A.1 For effectively earthed systems:
R0 X1 and X0 3 X1 (at the VT location)
where:
R0 is the zero sequence resistance;
X0 is the zero sequence reactance; and
X1 is the positive sequence reactance.
In practice, this means that
a) For a phase-to-earth fault, the r.m.s. phase voltage to earth of any healthy phase does notexceed 0,8 Um, where Um is the highest r.m.s. phase-to-phase voltage of equipment.
b) 80 % arresters can be applied.
c) Earth-fault currents approach the level of three-phase fault currents.
d) All power transformers must have their Y winding neutrals solidly earthed and have to be providedwith delta windings, such that the criterion X0 3 X1 is satisfied.
Typical examples are
1) Two winding transformers: Ynd, Dyn.
2) Three winding transformers: Yndyn.
3) Auto-transformers: Yynod.
A.2 For non-effectively earthed systems :
a) For a phase-to-earth fault, the r.m.s. phase voltage to earth of any sound phase equals Um.
b) 100 % arresters need to be applied.
c) Earth-fault currents are significantly less than the three-phase fault current levels at thevoltage transformer location.
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NRS 030:2001 28
Annex A(concluded)
Typical examples of non-effectively earthed systems:
1) Yy transformers without a delta winding.
2) Yy auto-transformers without a delta winding.
3) Neutral electromagnetic couplers (NECs) with
a) Solidly earthed neutrals (reactance earthing of 2 000 A to 5 000 A).
b) Low-resistance earthing (typically 300 A to 500 A).
c) High-resistance earthing (typically 10 A to 50 A).
4) Dyn transformers with
a) Low-resistance earthing (typically 300 A to 500 A).
b) High-resistance earthing (typically 10 A to 50 A).
5) Unearthed or arc suppression (Petersen) coil earthed systems.
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NRS 030:200129
Annex B(informative)
Guide to purchasers on preparing an enquiry
B.1 General
A model form is given in Annex C to provide the purchaser with a convenient aid to purchasing. Thepurchaser shall provide a standard form, based on the model form in Annex C.
The purchaser needs only specify compliance with this specification, provide the tenderers withdetails of his/her particular requirements, and set out the information he/she requires the tenderer toprovide, as indicated below.
B.2 Schedules
The model form in Annex C provides the purchaser with examples of a schedule A and a schedule B.
B.2.1 Schedule A
Schedule A lists the requirements to be specified by the purchaser in enquiries and orders. Theserequirements include references to the relevant sub-clauses in this specification to assist incompiling the schedules.
B.2.2 Schedule B
The tenderer shall complete Schedule B. By doing this, the tenderer will be stating compliance withthis specification and will provide the information the purchaser has requested.
NOTE 1 Where this specification allows the purchaser to make a choice, the example of schedule A (in the model form) liststhe preferred items/values/quantities. In the interests of standardization, purchasers are encouraged not to deviate from thesepreferences.
NOTE 2 The purchaser needs only include those items that he considers to be relevant or necessary when preparing his ownschedule A and schedule B from the examples in the model form.
NOTE 3 These schedules, when completed, become normative annexes to the enquiry specification.
B.3 Commercial conditions
A purchaser will need to indicate the commercial conditions applicable and draw up a priceschedule. Requirements for delivery, storage, packing and marking should be attended to in this partof the enquiry.
B.4 Quality assurance
This specification does not cover the purchasers possible requirements in respect of qualityassurance, quality control, inspections, etc., since each purchaser needs to consider the criticalityof the application of each component and his own policy towards these matters, etc. Purchasers arereferred to SABS ISO 9001:2000 for guidance.
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NRS 030:2001 30
Annex B(concluded)
B.5 Testing
Attention should be paid to the subject of tests, and their related costs. Tests should be carried outby a competent party and tenderers should be requested to provide assurances on this point. Priceschedules should be so drawn up and covering letters so worded that the costs of all services suchas tests, delivery and spares are declared and allowed for in the tender.
Before type tests, routine tests and sample tests are carried out, the number of samples used andthe frequency of sampling should be agreed upon with the supplier.
B.6 Revision of standards used as normative references
This specification, as has been indicated, is based on a set of defined standards which may havebeen revised or amended. Most purchasers will, in principle, wish to employ the latest standards. Itis recommended that an approach to this question be to secure an undertaking from a supplier toreview the latest versions and amendments and to incorporate these where possible and agreeable toboth parties. A blanket commitment to work to the latest versions of standards creates legaldifficulties of interpretation and risks for both parties and should be properly assessed. Thisinvariably cannot be done in the time available.
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NRS 030:200131
Annex C(informative)
Model form for schedules A and B
This model form is provided as a convenient aid to purchasing. Guidance on preparing an enquiryusing this form is given in annex B.
Schedule A: Purchasers specific requirementsSchedule B: Particulars of equipment to be supplied by tenderer
Item Clause Description Schedule A Schedule B
4.1 General requirements
1 4.1a) Service conditions (if non-standard)
a) ambient air temperature (average) C ________ xxxxxxxx
b) altitude m ________ xxxxxxxx
c) average humidity % ________ xxxxxxxx
d) wind pressure Pa ________ xxxxxxxx
e) level of pollution equipment is subject to, ifother than heavy, (for example, light, mediumor very heavy) ________ xxxxxxxx
f) special conditions, for example:
1) lightning area
2) if yes, flash density to SABS 0313
Yes/No
________
xxxxxxxx
xxxxxxxx
Number of voltage transformers required ________ xxxxxxxx
2 4.1b) System details
a) indoor or outdoor use ________ xxxxxxxx
b) nominal r.m.s voltage(Un) kV ________ xxxxxxxxx
c) number of phases ________ xxxxxxxx
d) frequency Hz 50 xxxxxxxx
e) method of earthing: effective or non-effective ________ xxxxxxxx
f) single-phase or 3-phase voltagetransformer? ________ xxxxxxxx
g) symmetrical 3-phase fault current kA ________ xxxxxxxx
4.1.2.2 h) basic insulation level kV ________ xxxxxxxx
3 4.1c) Manufacture of VTs
a) manufacturer xxxxxxxx ________
b) manufacturers type designation xxxxxxxx ________
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NRS 030:2001 32
Annex C(continued)
Item Clause Description Schedule A Schedule B
4 4.1.3 Details of insulation materials
Manufacturer of HV porcelain insulators xxxxxxxx ________
Detailed drawing of insulator Yes xxxxxxxx
4.1.4.2 Insulation medium (gas,oil,resin) xxxxxxxx ________
5 4.1.4 Details of oil-insulated voltagetransformers to be submitted forapproval:
4.1.4.2 Type of oil xxxxxxxx ________
4.1.4.3 Oil to be certified to contain no PCBs (zerocount)? Yes xxxxxxxx
4.1.4.4 Quantity of oil required l xxxxxxxx ________
4.1.4.6 Expansion accommodation method xxxxxxxx ________
4.1.4.7 Details of sealing arrangements Yes ________
4.1.4.9 Oil-level indicators Yes ________
4.1.4.10 Oil sample valve required? Yes/No xxxxxxxx
4.1.5 Gas-insulated voltage transformers
4.1.5.5 Method of sealing xxxxxxxx ________
4.1.6 Dry type voltage transformers
Is the core included in the encapsulation? xxxxxxxx Yes/No
If no: details of core treatment xxxxxxxx ________
6 4.2 Design details
4.2.1.2/3/4
Voltage factor ________ ________
Time for voltage factor s ________ ________
Primary voltage V ________ ________
Secondary voltage V ________ ________
Measuring or protection application: ________ xxxxxxxx
rated burden per phase VA ________ xxxxxxxx
accuracy class (4.2.2) ________
HV neutral earthed? Yes/No xxxxxxxx
4.1 Is a power winding required? Yes/No xxxxxxxx
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NRS 030:200133
Annex C(continued)
Item Clause Description Schedule A Schedule B
If Yes:
a) rated secondary voltage V 220/110 xxxxxxxx
b) rated continuous current ofsecondary winding
A________ ________
4.2.3 Short-circuit protection Yes/No xxxxxxxx
4.2.3.1/2 Are primary fuses required? Yes/No xxxxxxxx
If Yes:
a) rating A ________ ________
b) location xxxxxxxx ________
4.2.3.1/3 Is protection of secondary windingrequired? Yes/No xxxxxxxx
If Yes:
a) by means of fuses/MCBs? ________ xxxxxxxx
b) current rating A ________ ________
c) make and type of protection xxxxxxxx ________
d) white phase or neutral to be earthed _______ xxxxxxxxx
4.2.3.4 Short-circuit currents
a) calculated secondary short-circuitcurrent
A xxxxxxxx ________
b) maximum permissible duration ofsecondary short-circuit current
s 1 xxxxxxxx
c) operating time for secondary short-circuit xxxxxxxx ________
d) calculated primary current for asecondary short-circuit assuming zerosource impedance
s
xxxxxxxx ________
e) drawing showing method of mountingsecondary fuses/MCBs
Axxxxxxxx ________
4.3.2.2 Number of core limbs: 5 xxxxxxxx ________
Draw-out (rail ) type or hinged type Yes/No xxxxxxxx
7 4.3 Constructional requirements
4.3.1 Windings and connections
4.3.1.3 Is a residual voltage winding required? Yes/No xxxxxxxx
If Yes:
a) rated burden VA ________ xxxxxxxx
b) rated secondary voltage V
3
110 xxxxxxxx
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NRS 030:2001 34
Annex C(continued)
Item Clause Description Schedule A Schedule B
4.3.1.4 Are tapped windings for providing both
rated secondary residual voltages
required?
Yes/No xxxxxxxx
4.3.1.6 Details of primary winding neutral (earth)terminal
xxxxxxxx ________
8 4.3.4 Secondary terminal box
4.3.4.1 Number of fuses/MCBs xxxxxxxx ________
Manufacturer of fuses/MCBs xxxxxxxx ________
Type of fuses/MCBs xxxxxxxx ________
Current rating of fuses/MCBs A xxxxxxxx ________
Diameter of secondary earthing terminal mm xxxxxxxx ________
4.3.4.2 HV winding earth end to be brought out,with link to earth terminal
Yes/No ________
4.3.4.4 Dimension of terminal box:
a) length mm xxxxxxxx ________
b) height mm xxxxxxxx ________
c) depth mm xxxxxxxx ________
4.3.4.6 Minimum effective gland area ofsecondary terminal box/gland plate; length width
mm 75 50 ________
4.3.4.1 Bolted link or removable link? ________ xxxxxxxx
9 4.3.5 Mounting arrangement
Outline drawing Yes xxxxxxxx
Holding down bolts to be arranged to fallwithin a square of maximum dimensions mm ________ xxxxxxxx
10 4.3.6 Metal finish
4.3.6.1 Is metal spraying to a thickness of at leastof 80 mm acceptable?
Yes/No xxxxxxxx
Finish offered on ferrous parts xxxxxxxx ________
Finish offered on non-ferrous parts xxxxxxxx ________
11 4.4 Primary terminals (Post-type voltagetransformer)
1. Materials:
a) aluminium or ________ xxxxxxxx
b) electrotinned copper stem ________ xxxxxxxx
2. Type of primary terminal Stem/pad ________
Dimensions and orientation of stem type:
a) diameter mm ________ xxxxxxxx
-
NRS 030:200135
Annex C(continued)
Item Clause Description Schedule A Schedule B
b) minimum length mm ________ xxxxxxxx
c) orientation Horizontal/Vertical xxxxxxxx
Details of pad type terminal:
a) hole arrangement ________ ________
b) hole spacing mm ________ ________
c) length mm ________ ________
d) thickness mm ________ ________
4.5 Diameter of apparatus earthing terminal mm xxxxxxxx ________
12 4.6 Rating plates and diagram plates
4.6.5 a) materials used for rating plates xxxxxxxx ________
b) material used for diagram plates xxxxxxxx ________
c) method of fixing diagram plates andrating plates
xxxxxxxx ________
13 4.3.2 Cores
4.3.2.4 Core steel details Yes xxxxxxxx
4.3.2.4a) a) grade xxxxxxxx ________
b) thickness of lamina mm xxxxxxxx ________
4.3.2.4b) a) annealed? xxxxxxxx ________
b) B/H curve Yes xxxxxxxx
14 4.7 Drawings and instruction manuals
4.7.1 Quantity of drawings and literaturerequired with tender
________ xxxxxxxx
Electronic data Yes/No xxxxxxxx
Software for electronic data ________ xxxxxxxx
4.7.1.2a)9 Mass of complete transformer kg xxxxxxxx ________
4.7.3 Number of instruction books anddescriptive pamphlets per order, if otherthan three.
________ xxxxxxxx
4.7.2.1 Drawing numbers for post-type voltagetransformers:
a) outline dimension xxxxxxxx ________
b) assembly xxxxxxxx ________
c) terminal box xxxxxxxx ________
d) terminal marking xxxxxxxx ________
e) rating plate xxxxxxxx ________
f) diagram plate xxxxxxxx ________
Quantity required ________ xxxxxxxx
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NRS 030:2001 36
Annex C(concluded)
Item Clause Description Schedule A Schedule B
15 4.7.2.2 Drawing numbers for three-phase VTs and units built into circuit breakers
4.7.2.2 c) rating plate xxxxxxxx ________
diagram plate xxxxxxxx ________
Quantity required ________ xxxxxxxx
4.7.2.1(e) Drawing number of magnetization curveshowing voltage/exciting current.
xxxxxxxx ________
16 5.4.2 Test certificates of individual routine testsare required and shall be placed in theterminal box
Yes xxxxxxxx
17 4.8 Spare fuses
Primary fuses quantity required ________ xxxxxxxx
Secondary fuses quantity required ________ xxxxxxxx
18 5.1.1.1 Type tests
Are valid type test results available? xxxxxxx Yes/No
If No, are type tests to be performed? Yes/No xxxxxxx
If required, where are type tests to becarried out?
xxxxxxxx ________
19 5.2 Routine tests
5.2.3(b) Applied r.m.s voltage other than 3kV kV __________ __________
5.2.5 Does leakage rate comply with 4.1.5.5 or4.1.4.7?
xxxxxxxx Yes/No
5.2.5.3 Submit details for the determination of therate of gas leakage for approval
Yes ________
5.2.7.5 Test procedure for cast resin-encapsulated voltage transformers
xxxxxxxx Procedure1/2
20 5.3 Special tests
5.3.1 Chopped impulse tests Yes/No xxxxxxxx
5.3.2 Switching impulse test Yes/No xxxxxxxx
5.3.3 Dielectric dissipation factor test at ambienttemperature and 90oC
Yes/No xxxxxxxx
5.3.4 Mechanical strength and sealing test Yes/No xxxxxxxx
5.3.5 Long duration voltage test Yes/No xxxxxxxx
5.1.1 Are valid test certificates for the specialtests available?
xxxxxxxx Yes/No
If No, are the special tests to beperformed?
Yes/No xxxxxxxx
If required, where are the special tests tobe carried out?
xxxxxxxx ________
-
NRS 030:200137
Bibliography
The following documents were a source of reference in compiling this specification. They do notconstitute provisions of this specification but are referenced for further information:
IEC 60050:1983, International Electrotechnical Vocabulary (IEV) - Chapter 446: Electrical relays.
IEC 60060-1:1989, High-voltage test techniques Part 1: General definitions and testrequirements.
IEC 60060-2:1994, High-voltage test techniques Part 3: Measuring systems.
IEC 60071-1:1976, Insulation co-ordination Part 1: Definitions, principles and rules.
IEC 60085:1984, Thermal evaluation and classification of electrical insulation.
IEC 60269-1:1986, Low-voltage fuses Part 1: General requirements.
IEC 60507:1991(Report), Artificial pollution tests on high-voltage insulators to be used on a.c.systems.
IEC 60815:1986, Guide for the selection of insulators in respect of polluted conditions.
SABS 0313:1999, Protection of structures against lightning.
SABS ISO 9001:2000, Quality management systems: Requirements
sabs pta