MANUAL
ON
TRANSFORMERS
CENTRAL BOARD OF IRRIGATION AND POWER
2006
MARCH
Publication No. 295
Editors
G.N. Mathur
R.S. Chadha
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(ii)
"Reproduction of articles in publication in any form is permissible subject to proper acknowledgement and intimation to the publishers. The publishers have taken utmost care to avoid errors in the publication. However, the publishers are in no way responsible for the authenticity of data or information given by the contributors."
ISBN 81-7336-302-1
Central Board of Irrigation and Power Malcha Marg, Chanakyapuri, New Delhi 110 021 Phone : 2687 5017/2687 6567 Fax : 2611 6347
(iii) Chairman
Shri R.C. Agrawal Chief (T&D), TBG Bharat Heavy Electricals Ltd.
Bhopal Members
Shri M.M. Goswami
AGM (Engg.)
Power Grid Corp. of India Ltd., Gurgaon
Shri Anil Chanana
DGM
Power Grid Corporation of India Ltd.
Shri Kamal Sarkar
DGM (OS)
Power Grid Corp. of India Ltd., Gurgaon
Shri B.N. De Bhowmick
DGM (OS)
Power Grid Corp. of India Ltd., Gurgaon
Shri V.K. Bhaskar
Chief Manager (OS)
Power Grid Corp. of India Ltd., Gurgaon
Shri Hirdesh Gupta
Dy. Chief Design Engineer
National Thermal Power Corp. Ltd., Noida
Shri S.K. Malik
Suptd. Engineer (O&M Circle)
Bhakra Beas Management Board, Panipat
Shri R.K. Garg
Bhakra Beas Management Board, Chandigarh
Shri J.S. Batra
Bhakra Beas Management Board, Chandigarh
Shri R.M. Malhotra
Delhi Transco, New Delhi
Shri S.N. Katakwar
Superintending Engineer (Dist.)
Maharashtra State Electricity Board Mumbai
Shri M.Z.M.A. Sayed
Divisional Engineer
BEST, Mumbai
Shri Sanjay Kar Chowdhury
Assistant Manager (Sub-Station)
CESC Ltd., Kolkata
Shri Ranjit Singh Nain
Executive Engineer/PTRW
Haryana Vidyut Prasaran Nigam Ltd. Ballabhgarh, Haryana
Shri K.K. Bhatia
Addl. Chief Engineer
Gujarat Energy Transmission Corp. Ltd. Vadodara
Shri K.S. Kattigehallimatt
Chief Engineer
KTPCL, Bangalore
Shri D. Majumdar
Dy. Chief Engineer (CTC)
Damodar Valley Corporation, Jharkhand
Shri P.K. Kognolkar
Addl. Director
Central Power Research Institute, Bhopal
Shri S.C. Bhageria
AGM, TRE
Bharat Heavy Electricals Ltd., Bhopal
Shri R.K. Tiwari
AGM
Bharat Heavy Electricals Ltd. Bhopal
(iv) Bharat Heavy Electricals Ltd.
Jhansi
Shri C.P. Deshmukh
Senior DGM
Bharat Heavy Electricals Ltd. Bhopal
Shri R.K. Mohapatra
Deputy General Manager, TRE
Bharat Heavy Electricals Ltd. Jhansi
Shri B. Naik
Manager (T)
Bharat Heavy Electricals Ltd. Jhansi
Shri A.K. lkka
Bharat Heavy Electricals Ltd. Bhopal
Shri R.K. Agrawal
Bharat Heavy Electricals Ltd. Bhopal
Shri M. Vijaykumaran
Technology Expert
ALSTOM, Allahabad
Shri K. Bheema Prakash
General Manager ALSTOM, Allahabad Shri K. Raghuraman General Manager ALSTOM, Allahabad Shri Mohan Manager ALSTOM, AITPL Bangalore Shri V.K. Lakhiani GM (Q&A) Crompton Greaves Ltd. Mumbai
Crompton Greaves Ltd., Gwalior
Shri Dharam Vir
DGM, Transformer Division Crompton Greaves Ltd., Mumbai
Shri Pramod Rao
Crompton Greaves Ltd., Nasik
Shri P. Ramchandran
Asstt. Vice President
ABB Ltd., Vadodara
Shri M.L. Jain
Vice-President, Technology
EMCO Ltd., Thane
Shri Gautam Mazumdar
Manager
EMCO Ltd., Thane
Shri K. Samba Murthy
General Manager (Q&A)
Vijay Electricals Ltd., Hyderabad
Shri S.K. Mahajan
Consultant
Voltamp Transformer Pvt. Ltd., Vadodara
Shri S.R. Karkhanis
Deputy General Manager
The Tata Power Company Ltd., Mumbai
Ms. C.R. Bhonslay
Manager (Engineering Department)
The Tata power Company Ltd., Mumbai
Shri M.L. Mittal
GM (Retd.)
BHEL, Habibganj, Bhopal
Late Shri A.K. Kapur
ED (Retd.)
Power Grid Corp. of India Ltd. A-55, East of Kailash
( v )
10.3.2006
FOREWORD
India had a meager installed power generating capacity
of approximately 1,300 MW at the time of
independence in 1947. Since then we have made rapid
progress and on date this figure stands at 1,23,668 MW.
Matching with the generating capacity, transmission
and distribution networks have also grown. However,
despite this spectacular growth of power sector, still
we have about 8% shortage in energy and about 12%
shortage in peak power. Due to these shortages, despite best efforts the
voltage and frequency in different grids sometimes varies beyond
acceptable limits and lead to power cuts.
Transformer is a vital link for taking power supply from generating station
to consumer premises. Matching with the growth in power sector, about
56,000 MVA power transformers and 26,000 MVA transformers for
distribution network in the country are being manufactured annually. The
manufacturers have also upgraded the manufacturing technology matching
with the increase in the complexity of the grid besides their impressive
R&D setup to support the technology and meeting export requirement. In
transmission and distribution sector, cost of the power transformer
represents the largest portion of the capital investment and financial
consequences of failure of transformer lead to considerable loss of
revenue besides break down of power supply.
Central Board of Irrigation & Power (CBIP) has been playing a key role
to disseminate the latest technological advancement information covering
almost all aspects of power sector. In early 70s, it was felt that industry
should have detailed reference specifications for transformers.
(vi)
Transformer" was formed with representatives from users and
manufacturers to prepare detailed specifications for transformers. In 1976,
CBIP issued the specifications as Technical Report 1, "Manual on
Transformers" under Research Scheme on Power for the first time. This
manual was subsequently revised in 1987 and in 1999. The transformer
manual issued by CBIP is being widely used by power engineers as a
reference book covering all aspects of transformers for almost all
applications. Almost all the utilities in the country are referring the CBIP
Manual on Transformers while formulating their own purchase
specifications on the basis of technical information contained there in.
I am happy to note that this manual has been revised and updated now with
the help of Working Group experts from all eminent organizations, and
contains the latest technological information including installation, testing,
commissioning and repairs of transformers besides additional chapters
on 800 kV transformers, Condition Monitoring and Diagnostic Techniques
and chapters on various components of the transformers of all sizes and
voltages including dry type transformers.
I congratulate CBIP and all experts of the Working Group for bringing
out this manual covering latest state-of-art technology and I am sure that
this document will be of great use to engineering fraternity as a reference
book.
(vii)
Transformer is a vital component of all electric power
systems and every pulse generated, passes through
several transformers before it is finally consumed in
any appliance or equipment or industry. In our country
the transformers are subjected to severe stress
condition due to wide variation in voltage and
frequency in system due to huge gap in demand and
supply causing higher failure rate of transformers as
compared to western countries. With the escalations in the cost of material
and other inputs into its manufacture, the requirement of un-interrupted
power and above all the steep rise in the cost of power has forced the
manufacturers to compete in innovations and improvise designs as well
as manufacturing practices to deliver a low-loss, competitive costing
transformers with built-in monitoring features to ensure its long
trouble-free service life. On the other hand power utilities are also under immense
pressure to maintain and upgrade their system to meet the rising demand
in power with no tolerance for any breakdown in supply. Moreover with
the introduction of 'Availability Based Tariff, setting-up of the 'Electricity
Regulatory Commissions' and the 'Consumer Grievance re-dressal Forums'
all over the country, the essentiality of uninterrupted power now has a
large commercial value.
In line with our quality policy, to promote and coordinate professional
excellence in water and power sector CBIP has prepared more than 300
Manuals on all important subjects. To disseminate the latest developments
among the engineer's fraternity, CBIP has updated these manuals from
time to time. First edition of the Transformer manual was published in
1976 and it was updated in 1987 and again in 1999.
To incorporate the latest developments and facilitate the professional
engineers associated in planning, designs, procurement, testing, erection
& commissioning of the entire range of Distribution and Power
transformers upto 800 kV and also to assist in maintenance / monitoring
of the key parameters of the installed transformers, this Manual has been
(viii)
also been added. This trend of repair of transformer at site is becoming
popular since it saves in to and fro transportation time as well as HV test
cost since some of the tests including HV Test are not to be repeated.
For this purpose CBIP had constituted a working group, under the
Chairmanship of Shri R.C. Aggarwal, General Manager (T), BHEL, of
highly experienced engineers from large power utilities, designs
organisation, manufacturers and testing stations from NHPC, PGCIL,
NTPC, BHEL, ABB, ALSTOM, CGL, EMCO, VOLT AMP, CEA, WBSEB,
KPCL etc., who have put in their knowledge and experience in bringing
out this updated transformer manual. I thank all experts of the working
group for their valuable contribution.
I am sure that this Manual shall be of immense value and provide good
reference document to the practising engineers especially those looking
after planning, procurement, design, maintenance, testing and
commissioning of power systems including transformers.
G.N. Mathur
Secretary
Central Board of Irrigation and Power
New Delhi 110021
(ix)
Foreword (v)
Preface (vii)
SECTION A General 1
SECTION B Specifications for Three Phase 11 kV/433 - 250V Class 65 Distribution Transformers (upto and including 100 kVA)
SECTION B1 Specifications for Single Phase 11 kV / 250V and 11 kV/√3/ 77 250V Distribution Transformers (10, 15 & 25 kVA Ratings)
SECTION C Specifications for Three Phase Distribution Transformers 87 (above 100 kVA and upto 33 kV class)
SECTION D Specifications for Power Transformers of Voltage Class below 145 kV 99 SECTION E Specifications for 145 kV Class Power Transformers 107 SECTION F Specifications for 245 kV Class Power Transformers 115 SECTION G Specifications for 420 kV Class Power Transformers 123 SECTION G1 Specifications for 800 kV Class Power Transformers 135 SECTION H Specification for Earthing Transformers 147 SECTION I Specifications for Valves for Transformers 157
SECTION J Test Requirements for Transformers 165
SECTION K Erection, Commissioning and Maintenance 223 SECTION K1 Condition Monitoring and Diagnostic Techniques for 295
Power Transformers and Reactors
SECTION L Capitalisation Formula for Transformer Losses 319 SECTION M Specifications for Protective Schemes for Power and 323
Distribution Transformers
SECTION N Specifications for Voltage Control of Power Transformers 337 SECTION O Specifications for Fire Protection of Power Transformers 353
(x)
SECTION Q Specifications for Dry Type Transformers 375 SECTION R Cable Boxes for SF6 Gas Insulated Transformer Terminations for 383
Rated Voltages of 72.5 kV and above
SECTION S Guidelines for Repair of Power Transformers at Site 397
General
1.0 GENERAL DESIGN OF APPARATUS 1.1 Compliance with Specifications1.1.1 Except where otherwise specified or implied herein, the transformers shall comply with the latest edition of Indian Standard 2026 (hereinafter referred to as "IS").
1.2 Design and Standardisation
1.2.1 The transformer and accessories shall be designed to facilitate operation, inspection, maintenance and repairs. All apparatus shall also be designed to ensure satisfactory operation under such sudden variations of load and voltage as may be met with under working conditions on the system, including those due to short circuits.
1.2.2 The design shall incorporate every reasonable precaution and provision for the safety of all those concerned in the operation and maintenance of the equipment keeping in view the requirements of Indian Electricity Rules.
1.2.3 All material used shall be of the best quality and of the class most suitable for working under the conditions specified and shall withstand the variations of temperatures and atmospheric conditions arising under working conditions without undue distortion or deterioration or the setting up of undue stresses in any part, and also without affecting the strength and suitability of the various parts for the work which they have to perform.
1.2.4 Corresponding parts liable to be replaced shall be interchangeable.
1.2.5 Cast iron shall not be used for chambers of oil filled apparatus or for any part of the equipment which is in tension or subject to impact stresses. This clause is not intended to prohibit the use of suitable grades of cast iron for parts where service experience has shown it to be satisfactory, e.g., large valve bodies.
1.2.6 All outdoor apparatus, including bushing insulators with their mountings, shall be des igned so as to avoid pocket in which water can collect.
1.2.7 Means shall be provided for the easy lubrication of all bearings and where necessary of any mechanism or moving part, that is not oil immersed.
1.2.8 All mechanism shall, where necessary, be constructed of stainless steel, brass or gun-metal to prevent sticking due to rust or corrosion.
1.2.9 All taper pins used in any mechanism shall be of the split type complying with IS: 2393 for these items.
1.2.10 All connections and contacts shall be of amp le section and surface for carrying continuously the specified currents without undue heating and fixed connections shall be secured by bolts or set screws of ample size, adequately locked. Lock nuts shall be used on stud connections carrying current. All leads from the winding to the terminal board and bushings shall be adequately supported to prevent injury from vibration including a systematical pull under short circuit conditions. Guide pulls shall be used where practicable. 1.2.11 All apparatus shall be designed to minimise the risk or accidental short-circuit caused by animals, birds or vermin.
1.2.12 Provision shall be made to fix safety fence around top cover of transformers of rating 100 MVA and above, for safe working during installation and servicing for large capacity transformers.
1.2.13 In tank on load tap changers shall be located such that the space above the diverter switch chamber will be free of inter connecting pipes etc. for lifting the diverter switch unit for inspection and maintenance purposes.
1.2.14 Dryness of the insulation may be ensured by measuring the water extraction during vacuum drying. The water extraction per tonne of insulation per hour may be limited to 50 grams maximum. Alternatively dryness can be judged by dew point measurement.
1.3 Galvanising
1.3.1 Galvanising where specified shall be applied by the hot-dipped process or by electro-galvanising process and for all parts other than steel wires shall consist of a thickness of zinc coating equivalent to not less than 610 gm of zinc per square meter of surface. The zinc coating shall be smooth, clean and of uniform thickness and free from defects. The preparation of galvanising and the galvanising itself shall not adversely affect the mechanical properties of the coated material. The quality will be established by tests as per IS: 2633. Alternative to galvanising, zinc spraying or aluminising can also be considered.
1.3.2 All drilling, punching, cutting, bending and welding of parts shall be completed, and all burrs shall be removed before the galvanising process is applied.
1.3.3 Galvanising of wires shall be applied by the hot-dipped process and shall meet the requirements of the relevant Indian Standard. The zinc coating shall be smooth, clean and of uniform thickness and free from defects. The preparation for galvanising itself shall not adversely affect the mechanical properties of the wire.
1.3.4 Surfaces which are in contact with oil shall not be electrogalvanised/cadmium plated. 1.4 Labels
1.4.1 Labels shall be provided for all apparatus such as relays, switches, fuses, contained in any cubicle or marshalling kiosks as shown in Fig. 1.
1.4.2 Descriptive labels for mounting indoors or inside cubicles and kiosks shall be of material that will ensure permanence of the lettering. A matt or satin finish shall be provided to avoid dazzle from reflected light. Labels mounted on dark surfaces shall have white
lettering on a black background. Danger notices shall have red lettering on a white background.
1.4.3 All plates shall be of incorrodible material.
1.4.4 Labels shall be attached to panels with brass screws or with stainless steel screws or these can be stuck with suitable adhesive also.
1.5 Bolts and Nuts
1.5.1 Steel bolts and nuts exposed to atmosphere shall be of following material :
l Size 12 mm or below – stainless steel
l Above 12 mm – steel with suitable finish like electrogalvanised with passivation.
1.5.2 All nuts, bolts and pins shall be locked in position with the exception of those external to the transformer, under gasket pressure.
1.5.3 All bolts, nuts and washers exposed to atmosphere and in contact with non-ferrous parts which carry current shall be of phosphor bronze.
1.5.4 If bolts and nuts are placed so that they are inaccessible by means of ordinary spanners, suitable special spanners shall be provided by the supplier.
1.5.5 Bolts and nuts shall not be less than 8 mm in diameter except when used for small wiring terminals.
1.6 Cleaning and Painting
1.6.1 Before painting or filling with oil or compound, all ungalvanised parts shall be completely clean and free from rust, scale and grease, and all external surface cavities on castings shall be filled by metal deposition.
1.6.2 All blast cleaned surfaces (except machined faces that have to be protected) must be cleaned in accordance with ISO specification no. ISO 8501 Part 1(This standard specification is based on and now supersedes Swedish Standard SIS 05 59 00) to a minimum standard of ‘ASa2½’ or ‘BSa2½’ prior to paint application.
1.6.3 External and internal surfaces of all transformer tanks and chambers and other fabricated steel items shall be cleaned of scale, rust and surface dirt by blast cleaning or other suitable approved method. After cleaning, these surfaces should be immediately covered with paint. Hot oil resistant varnish on white synthetic enamel/epoxy paint is to be used for painting the inside of all oil filled chambers, including transformer tanks. Only one thin layer ( ≅ 25 microns) of this is to be applied.
1.6.4 Except for hardware, which may have to be removed at site, all external surfaces shall receive at least four coats of paint. The type and thickness of paint shall be chosen to suit pollution level at site.
1.6.5 Selection of paint system for different environmental conditions shall be in line with ISO : 12944.
1.6.6 For rural or mild atmosphere, alkyd enamel primer and finish system may be used in four coats to give a total dry film thickness of at least 80 microns.
1.6.7 For urban or industrial situation two coats of epoxy zinc phosphate or zinc chromate primer topped with two coats of aliphatic polyurethane glossy finish paint is recommended. The total dry film thickness should preferably be between 100 and 130 microns.
1.6.8 In case of highly polluted area, chemical atmosphere or at a place very near the sea coast, paint as above with one intermediate coat of high build MIO (Micaceous iron oxide) as an intermediate coat may be used to give a total dry film thickness of 150 to 180 microns. 1.6.9 All interior surfaces of chambers or kiosks that are in contact with air shall receive at least three coats of paint, of which the topcoat shall be of a light shade. If heaters are not provided in the chamber, then the top coat should be of anti condensation type.
1.6.10 Any scratch, bruise or paint damage incurred during transportation and unloading at site should be made good by the purchaser as soon as the damage is detected. This is to be done by thoroughly cleaning the damaged area and applying the full number of coats as was applied originally. Manufacturer should supply the necessary paint for this touch up painting at site.
1.6.11 One coat of additional paint shall be given at site over all external surfaces, including hardware, after erection by the purchaser. Supplier shall furnish necessary information on the make and grade of the top-coat paint. In general, it is possible to apply enamel paint over epoxy polyurethane coating and the vice versa is not recommended. As far as possible the make and grade of the recoat shall be same as the original coat.
1.7 Oil
1.7.1 The transformers and all associated oil-filled equipment shall normally be supplied alongwith the first filling of oil and 10 percent excess quantity of oil shall also be supplied in non-returnable drums. The oil shall conform to IS : 335. Alternatively, if the purchaser so desires, oil may be supplied in tankers directly from the refinery for transformers which are despatched from factory to site in gas filled condition.
1.8 Prevention of Acidity
1.8.1 The design and all materials and processes used in the manufacture of the transformer, shall be such as to reduce to a minimum the risk of the development of acidity in the oil. Special measures, such as nitrogen sealing or the use of inhibited oil shall not be resorted to, unless otherwise specified by the purchaser.
2.0 ELECTRICAL CHARACTERISTICS AND PERFORMANCE 2.1 Type of Transformers and Operating Conditions
2.1.1 All transformers, unless otherwise specified shall be oil immersed and may be either core or shell type and shall be suitable for outdoor installation. Normally oil immersed transformer shall be provided with conservator vessels. Where sealed transformers are specified, there shall be no conservator but adequate space shall be provided for expansion of oil without developing undue pressure. The types of cooling shall be as stated in the relevant specifications.
Fig. 1
2.1.2 Transformers designed for mixed cooling shall be capable of operating under the natural cooled condition upto the specified load. The forced cooling equipment shall come into operation by pre -set contacts in WTI and the transformer will operate as a forced cooled unit.
2.1.3 Transformer shall be capable of remaining in operation at full load for 10 minutes after failure of the oil and/or water circulating pumps or blowers without the calculated winding hot-spot temperature exceeding l50oC. Transformer fitted with two coolers each capable of dissipating 50 percent of the losses at Continuous Maximum Rating (CMR) shall be capable of remaining in operation for 20 minutes in the event of failure of the oil and/or water circulating pumps or blowers associated with one cooler without the estimated winding hot-spot temperature exceeding l50oC.
2.2 Continuous Maximum Rating and Overloads
2.2.1 Transformers provided with mixed cooling shall comply, as regards its rating, temperature rise and overloads, with the appropriate requirements of IS : 2026 when operating with natural cooling and with mixed cooling.
2.2.2 All transformers, except where stated shall be capable of operation continuously, in accordance with IS loading guide at their CMR and at any ratio. In case bi-directional flow of power is required, that shall be specifically stated by the purchaser.
2.2.3 Temperature rise test shall be performed at the tapping as desired by the purchaser. If nothing has been stated by the purchaser, the test shall be carried out at the tapping with the highest load losses.
2.2.4 The transformer may be operated without danger on any particular tapping at the rated kVA provided that the voltage does not vary by more than + 10 percent of the voltage corresponding to the tapping.
2.2.5 The transformer shall be suitable for continuous operation with a frequency variation of + 3% from normal 50 Hz. Combined voltage and frequency variation should not exceed the rated V/f ratio by 10%.
Note : Operation of a transformer at rated kVA at reduced voltage may give rise to excessive losses and temperature rise
2.3 Voltage Ratio
2.3.1 The voltage between phases on the higher and lower voltage windings of each transformer measured at no-load and corresponding to the normal ratio of transformation shall be those stated in the ordering schedule.
2.3.2 Means shall be provided in accordance with clauses 8 and 9 for varying the normal ratio of transformation.
2.4 Electrical Connections
2.4.1 Transformers shall be connected in accordance with the IS vector symbol specified in ordering schedule of the requirements.
2.4.2 Auto connected and star/star connected transformers shall have delta connected stabilising windings if specified in the order. Two leads from one open corner of the delta connection shall be brought out to separate bushings. Links shall be provided for joining together the two terminals so as to complete the delta connection and earthing it external to the tank.
2.5 Duty under Fault Conditions
2.5.1 Except where modified below, it is to be assumed that the capacity of generating plants simultaneously connected is such that normal voltage will be maintained on one side of any transformer when there is a short-circuit between phases or to earth on the other side. Any transformer may be directly connected to an underground or overhead transmission line and switched into and out of service together with its associated transmission line.
2.5.2 All transformers shall be capable of withstanding any external short-circuit according to IS : 2026 without damage.
2.5.3 Transformers with tertiary windings shall be capable of withstanding the mechanical and thermal effects of any external short-circuit to earth with the short-circuit MVA available at the terminals not exceeding the values given in the ordering schedule with the neutral points on both HV and LV windings directly connected to earth as per the requirements of IS : 2026.
2.5.4 Transformers directly connected to generator (generator step-up transformers) shall be designed for exceptional circumstances arising due to sudden disconnection of the load and shall be capable of operating at approximately 25 percent above normal rated voltage for a period not exceeding one minute and 40 percent above normal rated voltage for a period of 5 seconds. However, the purchaser will install the over fluxing protection device in case of generator step-up transformers.
Note : All inter-connected transformers of 50MVA and above shall also be provided with over fluxing protection device by the purchaser.
2.6 Stabilising Windings
2.6.1 If specified in the order, the stabilising winding shall be capable of carrying continuously the load specified therein .
2.6.2 The design of stabilising winding shall be such as to take care of the effect of transferred surges and the tenderer shall offer suitable surge protection wherever necessary.
2.7 Losses
2.7.1 The accepted losses of each transformer shall be stated in the order. The tolerance on the losses of each transformer shall be in accordance with IS : 2026.
2.8 Regulation and Impedance
2.8.1 The impedance voltage at principal tap and rated kVA shall be stated in the order and tolerance shall be in accordance with IS : 2026.
2.8.2 For all transformers, the value of impedance on any other tapping shall be generally subject to the approval of the purchaser at the time of order. Any specific requirement may be mentioned at the time of enquiry as a prequalification instead of at the time of order. 2.9 Flux Density
2.9.1 The maximum flux density in any part of the core and yokes, of each transformer at normal voltage and frequency shall be such that the flux density in over-voltage condition as per clause 2.2.5 shall not exceed 1.9 Tesla (19,000 lines per cm2)
However, in case of transformers with variable flux the voltage variation which would affect flux density at every tap shall be kept in view while designing transformers.
2.10 Vibration and Noise
2.10.1 Every care shall be taken to ensure that the design and manufacture of all transformers and auxiliary plant shall be such as to have minimum noise and vibration levels following good modern manufacturing practices.
2.10.2 The manufacturers will ensure that the noise level shall not exceed the figures as per Table 0 -1 of NEMA Pub. No. TR - 1.
2.11 Suppression of Harmonics
2.11.1 All the transformers shall be designed with particular attention to the suppression of harmonic voltage, especially the third and fifth, so as to eliminate wave-form distortion and from any possibility of high frequency disturbances, inductive effects or of circulating currents between the neutral points at different transforming stations reaching such a magnitude as to cause interference with communication circuits.
3.0 CORES
The cores shall be constructed from high grade cold rolled non-ageing grain oriented silicon steel laminations.
3.1 Magnetic Circuit
3.1.1 The design of the magnetic circuit shall be such as to avoid static discharges, development of short-circuit paths within itself or to the earthed clamping structure and the production of flux components at right angles to the plane of the laminations which may cause local heating.
3.1.2 Every care shall be excercised in the selection, treatment and handling of core steel to ensure that as far as is practicable, the laminations are flat and the finally assembled core is free from distortion.
3.1.3 Adequate oxide/silicate coating is to be given on the core steel. However, laminations can be insulated by the manufactures if considered necessary.
3.1.4 Oil ducts shall be provided where necessary to ensure adequate cooling. The winding structure and major insulation shall not obstruct the free flow of oil through such ducts. Where the magnetic circuit is divided into pockets by cooling ducts parallel to the planes of the laminations or by insulating material above 0.25 mm thick, tinned copper strip bridging pieces shall be inserted to maintain electrical continuity between pockets.
3.1.5 The framework and clamping arrangements shall be earthed in accordance with clause 5.2.
3.1.6 When insulation is provided for the core to core bolts and core to clamp plates, the same shall withstand a voltage of 2000 V AC for one minute.
3.1.7 For consideration of overfluxing the transformer shall be suitable for continuous operation for values of overfluxing factor upto 1.1, this factor being v/vm X fn/f. The
manufacturer shall state the overfluxing capability and corresponding withstand durations for the transformers for factors 1.1, 1.25 and 1.4.
3.2 Mechanical Construction of Cores
3.2.1 All parts of the cores shall be of robust design capable of withstanding any shocks to which they may be subjected during lifting, transport, installation and service.
3.2.2 All steel sections used for supporting the core shall be thoroughly sand blasted or shot blasted after cutting, drilling and welding. Any non-magnetic or high resistance alloy shall be of established quality.
3.2.3 Adequate lifting lugs shall be provided to enable the core and windings to be lifted. 3.2.4 Adequate provision shall be made to prevent movement of the core and winding relative to the tank during transport and installation or while in service.
3.2.5 The supporting framework of the cores shall be so designed as to avoid the presence of pockets which would prevent complete emptying of the tank through the drain valve, or cause trapping of air during filling.
4.0 WINDINGS 4.1 General
4.1.1 All star connected windings for system of 66 kV and above shall have graded insulation as defined in IS : 2026. All windings for system voltages lower than 66 kV shall be fully insulated. All neutral points shall be insulated for the voltages specified in IS : 2026.
4.1.2 Power transformers shall be designed to withstand the impulse and power frequency test voltages as specified in IS : 2026.
4.1.3 The windings shall be designed to reduce to a minimum the out-of-balance forces in the transformer at all voltage ratios.
4.1.4 The insulation of transformer windings and connection shall be free from insulating composition liable to soften, ooze out, shrink or collapse and be non-catalytic and chemically inactive in transformer oil during service.
4.1.5 The stacks of windings shall receive adequate shrinkage treatment before final assembly. Adjustable devices shall be provided for taking up any possible shrinkage of coils in service.
4.1.6 The coil clamping arrangement and the finished dimensions of any oil ducts shall be such as will not impede the free circulation of oil through the ducts.
4.1.7 No strip conductor wound on edge shall have a width exceeding generally six times its thickness.
4.1.8 The conductors shall be transposed at sufficient intervals in order to minimise eddy currents and equalise the distribution of currents and temperatures along the windings. 4.2 Bracing of Windings
4.2.1 The windings and connections of all transformers shall be braced to withstand shocks which may occur during transport, or due to switching short -circuit and other transient conditions during service.
4.2.2 Coil clamping rings, if provided, shall be of steel or of suitable insulating material. 5.0 INTERNAL EARTHING ARRANGEMENTS
5.1 General
5.1.1 All metal parts of the transformer with the exception of the individual core laminations, core bolts and associated individual clamping plates shall be maintained at same fixed potential.
5.2 Earthing of Core Clamping Structure
5.2.1 The top main core clamping structure shall be connected to the tank body by a copper strap. The bottom clamping structure shall be earthed by one or more of the following methods:
(a) By connection through vertical tie-rods to the top structure
(b) By a connection to the top structure on the same side of the core as the main earth connection to the tank
5.3 Earthing of Magnetic Circuit
5.3.1 The magnetic circuit shall be earthed to the clamping structure at one point only through a link placed in an accessible position beneath an inspection opening in the tank cover. The connection to the link shall be on the same side of the core as the main earth connection. The link should be brought out using bushing/terminal board on all transformers above 31.5 MVA.
5.3.2 When magnetic circuits are subdivided into separate isolated sections by ducts perpendicular to the plane of laminations all such sections should be earthed.
5.4 Earthing of Coil Clamping Rings
5.4.1 Where coil clamping rings are of metal at earth potential, each ring shall be connected to the adjacent core clamping structure on the same side of tra nsformer as the main earth connections.
5.5 Size of Earthing Connections
5.5.1 All earthing connections with the exception of those from the individual coil clamping rings shall have a cross-sectional area of not less than 0.8 cm2. Connections inserted between laminations of different sections of core as per clause 5.3.2 shall have a cross-sectional area of not less than 0.2 cm2.
6.0 TANKS
6.1 Tank Construction
(a) All transformer reactor tanks should generally be of conventional type i.e., tank body with top cover, Bell shaped construction can be specified for 100 MVA and higher rating transformer unless otherwise specified.
(b) Top cover of conventional type transformer and Bell type construction may be bolted or welded to the tank body rim. Inspection covers shall always be bolted type.
6.1.1 The transformer tank and cover shall be fabricated from low carbon steel suitable for welding and of adequate thickness. The tanks of all transformers shall be complete with all accessories and shall be designed so as to allow the complete transformer in the tank and filled with oil, to be lifted by crane or jacks, transportation by road, rail or ship/boat without over straining any joints and without causing leakage of oil.
6.1.2 The transformer conservator tank, if equipped with an air cell, need not be designed for full vacuum but a vacuum-tight valve should be provided in the Buchholz relay pipe connection.
Alternatively an equalising connection may be provided between the inside of air cell and conservator for evacuating the conservator along with air cell, which may be removed after evacuation and oil filling.
6.1.3 The main tank body excluding tap-changing compartments, radiators and coolers shall be capable of withstanding vacuum given in the following table :
Vacuum
Highest system MVA rating gauge (mm of Hg)
voltage kV pressure kN/m2
Up to 72 kV Up to 1.6 34.7 250
above 1.6 and up to 20 68.0 500
above 20 100.64 760
Above 72 kV for all MVA
ratings 100.64 760
6.1.4 The base of each tank shall be so designed that it shall be possible to move the complete transformer unit by skidding in any direction without any damage when using plates or rails.
6.1.5 Normally a detachable underbase will be used, but in case transport facilities permit, a fixed underbase can be used.
6.1.6 Where the base is of a channel construction, it shall be designed to prevent retention of water.
6.1.7 Tank stiffners shall be designed to prevent retention of water.
6.1.8 Wherever possible the transformer tank and its accessories shall be designed without pockets where gas many collect. Where pockets cannot be avoided, pipes shall be provided to vent the gas into the main expansion pipe. The vent pipes shall have a minimum inside diameter of 15 mm except for short branch pipes which may be 6 mm minimum inside diameter.
6.1.9 All joints other than those which may have to be broken shall be welded when required they shall be double welded. All bolted joints to the tank shall be fitted with suitable oil-tight gaskets which shall give a satisfactory service under the operating conditions and guaranteed temperature rise conditions. Special attention shall be given to the methods of making hot oil tight joints between the tank and the cover as also between the cover and the bushing and all other outlets to ensure that the joints can be remade satisfactorily at site and with ease with the help of semi-skilled labour.
6.2 Lifting and Haulage Facilities 6.2.1 Each tank shall be provided with :
(a) Lifting lugs suitable for lifting the transformer complete with oil
(b) A minimum of four jacking lugs, in accessible positions to enable the transformer complete with oil, to be raised or lowered using hydraulic or screw jacks. The minimum height of the lugs above the base shall be:
- Transformers upto and including 10 tonnes weight - 300 mm (approx.) so as to accommodate suitable jacks beneath the jacking parts
- Transformers above 10 tonnes weight - 500 mm (approx.) so as to accommodate suitable jacks beneath the jacking lugs
(c) Suitable haulage holes shall be provided
6.2.2 To facilitate safe handling at site, the longitudinal and transverse axes and the centre of gravity of main transformer tank should be marked permanently on all four sides.
6.3 Tank Cover
6.3.1 Each tank cover shall be of adequate strength, and shall not distort when lifted. Inspection openings shall be provided as necessary to give easy access to bushings or changing ratio or testing the earth connection. Each inspection opening shall be of ample size for the purpose for which it is provided and at least two openings one at each end of the tank, shall be provided.
6.3.2 The tank cover and inspection covers shall be provided with suitable lifting arrangements. Unless otherwise approved inspection covers shall not weigh more than 25 kg each.
6.3.3 The tank cover shall be fitted with pockets for a thermometer and for the bulbs of oil and winding temperature indicators. Protection shall be provided, where necessary, for each capillary tube.
6.3.4 The thermometer pocket shall be fitted with a captive screwed top to prevent the ingress of water.
6.3.5 The pockets shall be located in the position of maximum oil temperature at CMR and it shall be possible to remove the instrument bulbs without lowering the oil in the tank. 6.4 Axles and Wheels
6.4.1 Requirement of the roller will be specified for plinth mounted transformers. If required only one set of roller of each size to be asked for.
6.4.2 If specified, transformers are to be provided with wheels and axles. They shall be of such dimensions and so supported that under any service conditions they shall not deflect sufficiently to interfere with the movement of the transformer. Suitable locking arrangements will be provided to prevent the accidential movement of the transformer.
6.4.3 All wheels should be detachable and shall be made of cast iron or steel as required. 6.4.4 Wherever specified, flanged wheels shall be provided suitable for use on gauge track as specified in the detailed specification and shall be so placed that pinchbar can be used to move the transformer.
6.4.6 If wheels are required to swivel, they shall be arranged so that they can be turned through an angle of 90° when the tank is jacked up clear of the rails or floor. Means shall be provided for locking the swivel movements in positions parallel to and at right angles to the longitudinal axis of the tank.
6.5 Conservator Vessels, Oil Gauges and Breathers
6.5.1 A conservator complete with sump and drain valve shall be provided in such a position as not to obstruct the electrical connections to the transformer having a capacity between highest and lowest visible levels of 7.5% of the total cold oil volume in the transformer and cooling equipment. The minimum indicated oil level shall be with the feed pipe from the main tank covered with not less than 15 mm depth of oil and the indicated range of oil level shall be from minimum to maximum.
6.5.2 If the sump is formed by extending the feed pipe inside the conservator vessel, this extension shall be for at least 25 mm. The conservator shall be designed so that it can be completely drained by means of the drain valve provided, when mounted as in service. 6.5.3 One end of the conservator shall be bolted into position so that it can be removed for cleaning purposes.
6.5.4 Normally one oil gauge, magnetic/prismatic/plain type as specified shall be provided. 6.5.5 The oil level at 30°C shall be marked on the gauge.
6.5.6 Taps or valves shall not be fitted to oil gauge.
6.5.7 The oil connection from the transformer tank to the conservator vessel shall be arranged at a rising angle of 3 to 9 degrees to the horizontal up to the Buchholz Relay and shall consist of :
(a) For transformers up to and including 1000 kVA 25 mm inside diameter pipes as per IS : 3639
(b) For transformers from 1001 to 10,000 kVA 50 mm inside diameter pipes as per IS: 3639
(c) For transformers of over 10,000 kVA 80 mm inside diameter pipes as per IS : 3639 6.5.8 A valve shall be provided at the conservator to cut-off the oil supply to the transformer, after providing a straight run of pipe for at least a length of five times the internal diameter of the pipe on the tank side of the gas and oil actuated relay and at least three times the internal diameter of the pipe on the conservator side of the gas and oil actuated relay.
6.5.9 Each conservator vessel shall be fitted with a breather in which silica gel is the dehydrating agent and designed so that :
(b) The external atmosphere is not continuously in contact with the silica gel.
(c) The moisture absorption indicated by a change in colour of the tinted crystals, can be easily observed from distance.
(d) All breathers shall be mounted at approximately 1,400 mm above ground level.
(e) Self indicating (blue) silica gel contains the dye cobalt chloride which has potential health hazards.
An alternative to the blue self indicating silica gel is SILICA GEL ORANGE with an organic indicator. The colour changes from orange to light yellow as it absorbs moisture.
6.5.10 One non-return valve, which may automatically cut off the flow of oil from conservator towards the main tank may be provided in the pipe connection between the Buchholz relay and conservator for transformers of 100 MVA and above.
6.6 Filter and Drain Valves sampling Devices and Air Release plugs 6.6.1 Each transformer shall be fitted with the following :
(a) The filter and drain valves as specified.
(b) A drain valve as specified below shall be fitted to each conservator. For diameter up to 650 mm: Size of the valve 15 mm.
For diameter above 650 mm : Size of the valve 25 mm.
(c) A robust oil sampling device shall be provided at the top and bottom of the main tank. The sampling device shall not be fitted on the filter valves specified under (a) above. (d) One 15 mm air release plug.
(e) For transformers above 100 MVA rating, one 100 mm bore valve shall be provided for attaching vacuum connection and with provisions for attaching a vacuum gauge, a pressure gauge or an oil level indicator.
6.6.2 All other valves opening to atmosphere shall be fitted with blank flanges. 6.7 Cooler and Radiator Connections
Valves and valve mountings shall be provided as specified under "Cooling Plant" Clause 7. 6.7.1 All valves shall be of metal or cast steel or may have cast iron bodies with gun-metal fittings. They shall be of full way type with internal screw and shall be opened by turning counter clock-wise when facing the handwheel.
6.7.2 Means shall be provided for padlocking the bottom valves in the open and closed positions. This is required for the valves where opening device like hand-wheel, keys, etc., are the integral part.
6.7.3 Every valve shall be provided with an indicator to show clearly the position of the valve.
6.7.4 All valves shall be provided with flanges having machined faces.
6.7.5 The drilling of valve flanges shall comply with the requirements of IS : 3639. 6.8 Pressure Relief Device
6.8.1 The pressure relief device shall be provided of sufficient sizes for rapid release of any pressure that may be generated within the tank, and which might result in damage to the equipment. The device shall operate at a static pressure of less than the hydraulic test pressure for transformer tank. Means shall be provided to prevent the ingress of rain water. 6.8.2 Unless otherwise approved the relief device shall be mounted on the main tank, and, if on the cover, shall be fitted with skirt projecting 25 mm inside the tank and of such a design to prevent gas accumulation.
6.8.3 If a diphragm is used it shall be of suitable design and material and situated above maximum oil level.
6.8.4 If a diaphragm is put at the base of pipe, an oil gauge is required on the stand pipe for indicating fracture of diaphragm.
6.8.5 One of the following methods shall be used for relieving or equalising the pressure in the pressure relief device:
(a) An equaliser pipe connecting the pressure relief device to the conservator, or
(b) The fitting of a silica gel breather to the pressure relief device. The breather being mounted in a suitable position for access at ground level.
6.8.6 If specified, the pressure relief valve (spring operated type) capable of releasing the pressure in the tank when it rises above a predetermined safe limit, shall be provided. It shall be provided with a microswitch for actuating trip contact when it operates. It shall also give a visual indication of valve operation by raising a flag. The flag and the switch shall remain operated until they are reset manually. The operating pressure of the pressure relief valve shall always be less than the tank test pressure. The microswitch shall have IP 55 protection and the fasteners shall be of rust proof material.
6.8.7 An oil splashguard shall be provided to the pressure relief device to restrict spillage of hot oil in the event of operation of the pressure relief device.
6.9 Accommodation for Auxiliary Apparatus
6.9.1 If specified, facilities shall be provided for the mounting of internal/external neutral current transformer(s) adjacent to the neutral terminal(s) and tank.
6.10 Earthing Terminal
6.10.1 Two earthing terminals capable of carrying for 4 seconds the full lower voltage, short-circuit current of the transformer. Provision shall be made at positions close to each of the bottom two corners of the tank for bolting the earthing terminals to the tank structure to suit local conditions. The design of earthing terminals shall be as per IS 3639 - Part 3 (Fittings and accessories for Power Transformers Part 3 : Earth Terminals.
6.11 Rating, Diagram and Property Plates
6.11.1 The following plates shall be fixed to the transformer tank at an average height of about 1750 mm above ground level as shown in Figs. 2 and 3.
(a) A rating plate bearing the data specified in the appropriate clauses of IS : 2026
(b) A diagram plate showing the internal connections and also the voltage vector relationship of the several windings in accordance with IS: 2026 and in addition a plan view of the transformer, giving the correct physical relationship of the terminals. When links are provided in accordance with clause 2.3 for changing the transformer ratio, then approved means shall be provided for clearly indicating ratio for which the transformer is connected. No load voltage shall be indicated for each tap.
(c) Where specified a plate showing the location and function of all valves and air release cocks or plugs is to be provided. This plate shall also warn operators to refer to the maintenance instructions before applying the vacuum treatment for drying (Fig. 4). 6.11.2 The above plates shall be of material capable to withstanding continuous outdoor service.
6.12 Joints and Gaskets
6.12.1 All gaskets used for making oil tight joints shall be of proven material such as granulated cork bonded with synthetic rubber or synthetic rubber gaskets conforming to IS : 4253, unless otherwise specified.
Fig. 4 Typical valve schedule for power transformer
7.0 COOLING PLANT 7.1 General
7.1.1 Radiators and coolers shall be so designed as to avoid pockets in which moisture may collect and shall withstand the pressure tests.
7.1.2 Unless the pipe work is shielded by adequate earthed metal the clearance between all pipe work and live parts shall be more than the clearance for live parts to earth.
7.2 Radiators Mounted Directly to the Tank/Banked 7.2.1 Detachable radiators as per section I of this manual.
7.2.2 Valves shall be provided on the tank at each point of connection to the tank.
7.2.3 Where separate radiator banks are provided, the conservator vessels specified in clause 6.5 can be mounted thereon.
7.2.4 All coolers shall be suitable for mounting on a flat concrete base. 7.2.5 The oil circuit of all coolers shall be provided with the following: (a) A valve at each point of connection to the transformer tank
(b) Removable blanking plates to permit the blanking off the main oil connection of each cooler.
(c) A drain valve of 25 mm at the lowest point of each bank of cooler
(d) A thermometer pocket fitted with a captive screwed cap on the inlet and outlet oil branches of each separately mounted cooler bank.
(e) A filter valve as specified in clause 6.6 at the top and bottom of each cooler bank of cooler.
(f) Air release plugs of 15 mm.
7.2.6 In addition the following are to be provided only with water cooled oil coolers which shall be as per IS : 6088 :
(a) A suitable differential pressure gauge or equivalent suitable device fitted with electrical contacts to give an alarm when differential pressure between cooler oil outlet and water inlet pressure drops below a preset value.
(b) Oil and water flow switches, fitted with electrical contacts, in the pipework adjacent to the coolers.
7.2.7 The disposition of flow indicators is to be as shown in Fig. 5.
7.2.8 Water cooled oil coolers shall be double tube type in which water shall circulate through the inner tube and oil in between the outer tube and shell. The design of shell and tube assembly shall be such as to facilitate cleaning without any risk of water mixing with the oil. The material of the tube plates and tube shall be such that corrosion shall not take place due to galvanic action. A water analysis report shall be furnished, in time, to enable supplier to ensure a suitable material for tube and tube plates.
7.2.9 Any leakage which may take place in the oil cooler shall be of the oil into the water and not the reverse, and means shall be provided to ensure that the pressure of the oil in the cooler is always greater than the pressure of the water. The water pressure in the cooler will be kept as low as possible. Further, the cooling water discharge should be free to the atmosphere to reduce the pressure in the cooler.
7.3 Oil Piping and Flanges
7.3.1 The necessary oil piping shall be provided for connecting each transformer to the coolers and oil pumps. The oil piping shall be with flanged gasketed joints. Cast iron shall not be used.
7.3.2 The drilling of all water and oil pipe flanges shall comply with IS: 3639 and IS: 1536 (Section I -specification for valves for transformers.)
7.3.3 A suitable expansion piece shall be provided in each oil pipe connection between the transformer and the separately mounted oil coolers.
7.3.4 Drain valves/plugs shall be provided in order that each section of pipework can be drained independently.
7.4 Oil Pumps
7.4.1 Each forced oil cooler shall be provided with a motor driven oil pump of the submerged motor type and of adequate capacity. It shall be possible t o remove the pump and motor from the oil circuit without having to lower the level of oil in the transformer or coolers and without having to disturb the pump foundation fixing. Oil pump shall be capable of dealing with the maximum output of transformer and total head which may occur in service and with the varying head due to changes in the viscosity of the oil.
7.4.2 Each pump assembly shall be furnished with oil flow indicator with alarm contacts to indicate normal pump operation and oil flow.
7.4.3 For mixed type cooling, the pump should be of axial flow type to permit oil circulation when pump is idle.
7.4.5 Under no circumstances, the degree of forced circulation create a static electrification hazard in any part of a transformer under any operating condition.
7.5 Air Blowers and Ducts
7.5.1 Air blowers for use with oil coolers or for air blast cooling shall be motor driven. They shall be suitable for continuous operation outdoors and capable of dealing with the maximum output and total head required in service. The bearings shall be of sealed type, which does not require frequent lubrication.
7.5.2 Air blowers shall be capable of withstanding the stresses imposed when brought up to full speed by the direct application of full line voltage to the motor.
7.5.3 Air blowers shall be complete with all necessary air ducting and coolers shall be designed so that they operate with a minimum of noise or drumming. In order to reduce the transmission of noise and vibration the blowers shall be either mounted independently from the coolers or, alternatively, an approved form of antivibration mounting shall be adopted. It shall be possible to remove the blower complete with motor without disturbing or dismantling the cooler structure framework.
7.5.4 Blades or runners fabricated to form hollow sections shall not be used. 7.5.5 Blades shall be suitably painted for outdoor use.
7.5.6 If fans are mounted at a height less than 2.5 m suitably painted wire-mesh guards with a mesh not greater than 25 mm shall be provided to prevent accidental contact with the blades. Fans mounted at more than 2.5 m height shall be provided with outside guards against birdage. Guards shall be provided over all moving shaft and couplings.
7.6 Motors
7.6.1 Motors shall be of the squirrel cage totally enclosed weather-proof type and shall comply with Indian Standards as applicable for continuously rated machine. The motors shall be capable of operating at all loads without undue vibration and with a minimum of noise. They shall be suitable for direct starting and for continuous running from 415-240 volts three-phase, 4 wire 50 Hz supply.
7.6.2 All motors shall be capable of continuous operation at any frequency between 48 and 51 Hz, together with any voltage within 5 percent of the nominal value. Motors upon which the primary equipment depends for its continued operation at full load shall also be capable of continuous operation at 85 percent of the nominal voltage at normal frequency without injurious over-heating.
7.6.3 All motors shall have ball or roller bearings and grease lubricators shall be fitted with hexagon nipples to relevant Indian Standard.
7.6.4 Vertical spindle motors shall have bearings capable of withstanding the thrust due to the weight of the moving parts and the action of impeller.
7.6.5 The stator windings shall be adequately braced and suitably impregnated to render them non-hygroscopic and oil resistant. Weather-proof motors shall be provided with suitable means of breathing and drainage to prevent accumulation of water.
7.6.6 Each terminal box shall be fitted with means of terminating the external wiring for outdoor use.
7.6.7 Varnished cambric or glass insulation shall be used for connections from the winding to the terminals. All motor terminals shall be of the stud type and totally enclosed.
7.6.8 Each pump, or blower and its motor shall be mounted on a common base plate and the drive shall be direct.
7.7. Cooler Control
7.7.1 Each motor or group of motors shall be provided with a three pole electrically operated contactor and with control gear of suitable design both for starting and stopping the motor manually and also automatically from the contacts on the winding temperature indicating device specified in clause 13. Additional terminals for remote manual electrical control of motors shall be provided. Overload and single phasing protection shall be provided but no-volt release shall not be fitted. HRC fuses shall be provided for the main supply. This equipment shall be accommodated in the marshalling box specified in clause 15.
7.7.2 Where small motors are connected in groups, the group protection shall be arranged so that it operates satisfactorily in the event of a fault occurring on a single motor.
7.7.3 Where blowers and oil pumps are provided, the connections shall be arranged as to allow the motors or groups of motors to be started up and shutdown either collectively or individually.
7.7.4 All motor contactors and their associated apparatus shall be capable of holding in and operating satisfactorily and without over heating for a period of ten minutes if the supply
voltage falls for the period, to 75 per cent of normal at normal frequency. The motor contactors and associated apparatus shall be capable of normal operation with a supply voltage of 85 per cent of the normal value and at normal frequency.
7.7.5 All contacts and other parts which may require renewal, adjustment or inspection shall be readilly accessible.
7.7.6 The control arrangements are to be so designed as to prevent the simultaneous starting of motors of a total rating of more than 20 HP.
7.7.7 Alarm indication for failure of group of fans and oil pump shall be provided. 7.7.8 Alarm indication shall be provided to indicate failure of power supply.
7.7.9 The start up or shut down of any pump or combination of pumps must not cause maloperation of any gas and oil actuated relay.
7.7.10 For transformers with OFWF cooling required to meet peak load requirements and are thus switched on or off during the day, the oil pump shall be kept running when the transformer is off for a short period but water circuit is switched off. In case the transformer is switched off for a longer time, the oil pump can also be switched off but it shall be run at least one hour earlier before the transformer is energised again.
8.0 VOLTAGE CONTROL (OFF-CIRCUIT TYPE)
Voltage Control (off-circuit type) should conform to section N of the specification. 9.0 VOLTAGE CONTROL (ON-LOAD TYPE)
Voltage control (on-load type) should conform to section N of the specification.
10.0 PARALLEL OPERATION OF TRANSFORMERS WITH ON-LOAD TAPCHANGER
10.1 Besides the local and remote electrical control specified in clause 9, on-load tapchangers, when specified, should be suitable for remote electrical parallel control as in clause 10.2.
10.2 Remote Electrical Parallel Control
10.2.1 In addition to the methods of control as in clause 9, the following additional provision shall be made.
10.2.2 Suitable selector switch be provided, so that any one transformer of the group can at a time be selected as "Master", "Follower" or "Independent".
10.2.3 Necessary interlock blocking independent control when the units are in parallel, shall be provided.
10.2.4 The scheme will be such that only one transformer of a group can be selected as "Master".
10.2.5 An out-of-step device shall be provided for each transformer which shall be arranged to prevent further tapchanging when transformers in a group operating in "Parallel control" are one tap out-of-step.
11.0 BUSHING INSULATORS AND TERMINALS
The bushing should comply with IS 2099, IS 12676 and section P of this specification. The over voltage power frequency test level or the BIL of bushings should be one step higher than that of the windings.
11.1 Transformers shall be fitted either with bushing insulators or with cable boxes, as stated in order. Where accommodation for current transformers is required on 72.5 kV bushings and above, the requisite details will be notified to the supplier at the time of tendering.
11.2 Special precautions shall be taken to avoid ingress of moisture into paper insulation during manufacture, assembly, transport and erection.
11.3 Each porcelain bushing or insulator, and paper bushing shall have marked upon it the manufacturer's identification mark, and such other mark as may be required to assist in the representative selection of batches for the purposes of the sample tests.
11.4 Clamps and fittings made of steel or malleable iron shall be hot dip galvanised. All fasteners of size 12 mm and above shall be hot dip galvanised and fasteners of size less than 12 mm shall be of stainless steel.
11.5 The bushing flanges shall not be of re-entrant shape which may trap air.
11.6 Bushing turrets shall be provided with vent pipes which shall be connected to route any gas collection through the Buchholz relay. The take off point of the vent pipes shall be the top most point on the bushing turret so that there will not be any air trapped in the bushing turret.
11.7 The minimum clearances in air between live conductive parts and conductive parts to earthed structure shall be as follows:
Minimum clearances Rated System Voltage Basic Insulation level Phase to Phase to
kV kV peak phase (mm) earth (mm)
11 75 280 140 22 125 330 230 33 170 350 320 47 250 530 480 66 352 700 660 110/132 550 1220 1050 132 650 1430 1270 220 950 2000 1800 220 1050 2350 2150 400 1425 4000 3500 800 1950 5800 5000
Note : 1. These clearances are applicable for transformers to be installed up to an altitude of 1000 m above mean sea level.
2. For altitude exceeding 1000 m the clearance should be increased by 3 percent for every additional 300 m.
3. Air clearance of 3500 mm between phase to earth for 400 kV system can be relaxed by maximum 200 mm as fas as air release pipe emanating from bushing turret is concerned.
12.0 CABLE BOXES AND DISCONNECTING CHAMBERS
12.1 Cable boxes shall be suitable for terminating the cables directly or alternatively shall be in the form of sealing end-chambers for accommodation sealing ends into which the cable will be terminated, as specified in the order.
12.2 Cable boxes shall be designed to accommodate all the cable joint fittings or sealing ends required by the manufacturers of the cables, including stress/cones or other approved means for grading the voltage stress on the terminal insulation of cables operating at voltages of 22 kV and above, between phases. They shall also be provided with expansion chambers for the filling medium and means of preventing the formation of air spaces when filling. Drain plugs of ample size shall be provided to enable the filling medium to be removed quickly.
12.3 The cable boxes shall be fitted with suitable non-ferrous wiping glands with combined armour and earthing clamps. The ends of all wiping glands shall be tinned before despatch to site. Wiping glands for single core cables shall be insultated from the box. Wiping glands insulation cables shall be capable of withstanding a dry high voltage test of 2,000 volts AC for one minute. Air insulated cable boxes for PVC cables may be provided with compression glands. Sufficient wiping glands shall be provided for the termination of required number of cables.
12.4 Where cable boxes are provided for three core cables, the seating sockets on the two outer phases shall preferably be inclined towards the centre to minimise bending of the cable cores. Where there is more than one core per phase, the socket block shall be so designed as to minimise bending of the cable cores.
12.5 Where cables for 1 kV and above are terminated in the cable box, if specified an oil-filled disconnecting chamber with removable links shall be provided for testing purposes. A barrier shall be provided on both sides of the disconnecting chamber to prevent ingress of the oil used for filling the chamber into the cable box or the transformer. It shall only be necessary to remove part of the oil in the chamber itself when making the necessary testing connections.
12.6 Where sealing end chambers are provided, the disconnecting chamber may be omitted and the facilities for testing shall be provided in the sealing end chamber itself. A barrier shall then be provided between the sealing end chamber and the main tank subject to the provision of the next paragraph.
12.7 The barrier between the main tank and the disconnecting or cable sealing end chamber may be omitted, where the design is such that the cover of the disconnecting or cable sealing end chamber can be removed without lowering any oil level other than in the chamber itself, in order to make the necessary testing connections.
12.8 The disconnecting or sealing end chamber shall have a removable cover and the design of the chamber shall be such that ample clearances are provided to enable either the transformer or each cable to be subjected separately to high voltage tests when filled with transformer oil.
