SPECIFICATION FOR OIL FILLED TRANSFORMER
CONTENTS
1. SCOPE
2. CODES AND STANDARDS
3. GENERAL REQUIREMENTS
4 AMBIENT CONDITIONS
5. CONSTRUCTION
6. TERMINALS AND MARSHALLING BOX
7. COOLING
8. OIL
9. ACCESSORIES
10. CORE AND WINDING
11. TAPPINGS AND CONTROL
12. NEUTRAL GROUNDING RESISTOR (NGR)
13. NOISE
14. INSPECTION, TESTING AND ACCEPTANCE
15. PACKING AND DESPATCH
Abbreviations:
ONAN : Oil Natural Air Natural
ONAF : Oil Natural Air Forced
OLTC: On Load Tap Changer
OTI : Oil Temperature Indicator
WTI : Winding Temperature indicator
RTCC: Remote Tap Changer Control
1.0 SCOPE
The intent of this standard specification is to define the requirements of design, engineering,
manufacture and testing of oil filled power/distribution transformers.
2.0 CODES AND STANDARDS
2.1 The transformer shall comply with the latest edition of the following and other relevant Indian
Standards/Manual:
IS:335 - Insulating oil
IS:1271 - Thermal evaluation and Classification of electrical insulation.
IS:2026 - Power transformers
IS:2099 - Bushing for Alternative voltages above 1000 V.
IS:2705 - Current transformers.
IS:3347 - Dimensions for porcelain Transformer Bushings
IS:3637 - Gas operated relays.
IS:3639 - Fitting & accessories for power transformers
IS:4201 - Application guide for CTs.
IS:8478 - Application guide for ON-load tap changers
IS:8468 - On-load tap changers (Amendment 1)
IS:13947- LV switchgear and control gear - Part 1 General rules.
IEEE:32 – Neutral Grounding Devices- Standard Requirements, Terminology and Test Procedure
ASTM standard A240-304
CBIP Manual on Transformers
2.2
In case of any contradiction between various referred standards/ specifications/data sheets and
statutory regulations, the following order of decreasing priority shall govern:
- Statutory regulations
- Data sheets
- Job specifications
- This specification
- Codes and standards.
2.3
Transformer shall also conform to the provisions of the latest revisions of the Indian Electricity
rules and any other statutory regulations currently in force.
3.0
3.1
GENERAL REQUIREMENTS
The offered equipment shall be brand new with a proven field track record. No prototype
equipment shall be offered.
The vendor shall be responsible for design, engineering and manufacturing of the equipment to
fully meet the intent and requirements of this specification and attached data sheets.
3.2
3.3
All equipment and accessories required for completeness of the equipment, whether specifically
mentioned or not, but considered essential for satisfactory performance shall be included as a part
of the offered equipment
4.0
AMBIENT CONDITIONS
Transformer and all its accessories and allied systems shall be suitable for operating satisfactorily
in humid and corrosive atmospheres found in fertilizer plants, refineries and petrochemical plants.
Service conditions shall be as specified in the data sheets. If not specifically mentioned therein,
design temperature of 40°C and an altitude not exceeding 1000m above mean sea level shall be
considered.
5.0
5.1
CONSTRUCTION
Transformer tank shall be of welded sheet steel construction and provided with gasketed steel
cover plates. Base shall be suitably reinforced to prevent any distortion during lifting. Base
channels shall be provided with skids and pulling eyes to facilitate handling. Transformer tank
shall be tested as per clause 13.0 of this specification.
5.2
All covers and seals shall be oil and airtight and shall not be affected by mineral or synthetic oil
action. Detachable radiators equipped with air vent, drain plug and lifting lugs shall be provided
with shut-off valves to permit removal of any radiator unit without emptying the tank. Radiators
shall be securely braced to prevent undue vibration. For transformers of ratings less than 500
KVA, fixed radiators welded to the tank may be provided.
5.3
All fasteners and bolts etc. shall be cadmium plated. All surfaces to be painted shall be
thoroughly cleaned, de-scaled, made free from rust and transformer shall be epoxy painted. Paint
shall be suitable to withstand specified atmospheric conditions.
5.4
Transformer shall be suitable for using emulsifiers as part of fire protection system.
5.5
Different parts of transformers shall be connected by copper flexibles for earth continuity
purpose.
6.0
TERMINALS AND MARSHALLING BOX
6.1
Windings shall be brought out and terminated on outdoor bushings, cable boxes or bus-duct
chamber, which will be located as specified on the data sheet.
6.2
When outdoor bushings are specified they shall be supplied complete with terminal connectors
suitable for the specified conductor size.
6.3
Cable boxes shall be supplied with cable lugs and glands.
6.3.1
For MV PVC Power/control cables double compression cable glands and crimped type tinned
copper cable lugs shall be supplied. Gland plate shall be removable type. For single core cables,
gland plate shall be of non-magnetic material. HV cable box shall be suitable for termination of
specified size of XLPE insulated cable .The head-room available between cable gland plate and
terminals shall not be less than 600mm for cable upto 11 kV, and 900 mm for 22 kV and 33 kV
cables. Cable box and disconnecting chamber shall be air insulated.
6.3.2
Cable box shall be weatherproof to IP-55. For fixed portion of cable box, an inspection cover
with a lifting handle shall be provided.
6.3.3
Primary cable box (where applicable) shall be able to withstand specified primary system fault
level for 0.20 secs.
6.4
Terminal chamber for bus-duct termination shall have a gasketed cover plate bolted to it and shall
be weatherproof to IP-55. A separate inspection cover with lifting handle shall be provided to
facilitate connection and inspection. Phase sequence of the bus bars shall be as specified in the
data sheets.
6.5
Marshalling box shall be weatherproof to IP-55. All protective devices and neutral CTs shall be
wired by means of PVC insulated copper conductor armoured cables upto the marshalling box.
Terminals shall be clamp type. Removable gland plate with double compression type glands shall
be provided. Lamp with switch & socket shall be provided in the marshalling box. Preferably,
marshalling box shall be located on the front side of transformer.
6.6
For transformers having provisions for terminating TPN bus duct on the 433V side, neutral of the
star connected secondary winding shall be brought out to a secondary terminal chamber. A CT
shall be mounted (if specified) on the neutral terminal with CT secondary wired up to the
marshalling box. A separate neutral bushing shall be provided for neutral earthing of
transformers. The neutral CT shall be mounted before bifurcation of the neutral. Supporting
arrangement for GI strip/cable as applicable shall be provided for connection of neutral bushing
to earth/NGR.
7.0
7.1
COOLING
Type of cooling shall be in accordance with data sheet.
8.0
OIL
Transformer shall be supplied with first filling of oil and 10% extra oil in non-returnable drums.
9.0
9.1
9.1.1
9.1.2
9.1.3
9.1.4
9.1.5
ACCESSORIES
The following accessories shall be provided as a minimum:
Rating Plate
Terminal marking Plate
Two earthing terminals
De-hydrating breather
Conservator (For transformer rated 50 kVA and above, for rated voltage 11 kV and below, and all
ratings above 11 kV)
Air release Device (for transformers with conservator)
Oil filling hole with cover
Oil Level indicator with minimum marking
Thermometer pocket
Dial type thermo meter with contacts for OTI & WTI
Oil temperature indicator
Winding temperature indicator (for transformers of rating 500 KVA and above)
Pressure relief valve
Sampling valve
Conservator drain valve
Top oil filter valve
9.1.6
9.1.7
9.1.8
9.1.9
9.1.10
9.1.11
9.1.12
9.1.13
9.1.14
9.1.15
9.1.16
9.1.17 Drain cum bottom filter valve
9.1.18 Double float Buchholz relay
9.1.19 Separate neutral bushing outside terminal box with connector assembly
9.1.20
9.1.21
9.1.22
9.1.23
9.1.24
9.1.25
9.2
Inspection cover
Terminal box
Marshalling box (IP-55)
Lifting lugs
Jacking lugs
Cross channels with towing lugs
Conservator shall be complete with oil filling plug and cap, oil drain valve, oil level gauge in
addition to magnetic oil level gauge. A flexible oil resistance air bag shall be provided for
conservator as specified in data sheet. Air bag shall be designed to withstand repeated expansion
and contraction due to changes in oil level.
9.3
Pressure relief valve (PRV) shall be provided with alarm contact.
9.4
Thermometer pocket for thermometer shall be provided.
9.5
Separate buchholz relay shall be provided for main tank and OLTC chamber. This shall be double
float type for the main tank with separate normally open trip and alarm contacts. For the OLTC
chamber the relay shall be of single float type or oil surge relay with one trip contact. Isolating
valve shall be provided on either side of the relay and distance piece shall be supplied.
9.6
Oil temperature indicator shall be 150 mm dial type with two normally open contacts for tripping
and alarm. It shall be provided with maximum reading pointer.
9.7
Winding temperature indicator shall have normally open contacts one each for tripping, alarm and
Starting /stopping of cooling fan motors. It shall be provided with maximum reading pointer.
9.8
Contact rating of all auxiliary devices shall be 0.5A, 110V/220V DC.
9.9
For Transformer rating upto 1600 kVA the Size of valves for drain and filter shall be 25mm and
that for sampling shall be 15mm.
10.0
CORE AND WINDINGS
10.1
Transformer shall be double wound, core type with high grade cold rolled non-aging grain
oriented low loss, high permeability silicon steel laminations (M-4 or better grade) perfectly
insulated and clamped to minimise vibration and noise. Care shall be taken to insulate core
fastening bolts to reduce losses and avoid hot spots. All parts of magnetic circuit shall be bonded
to earth system.
10.2
Windings shall be of copper and shall be designed to withstand the applicable thermal and
dynamic short circuit stresses.
11.0
TAPPINGS & CONTROL
11.1
These shall be provided on high voltage side and connected to off circuit or on-load tap changing
gear as specified on data sheet. Under conditions of external short circuit, the tap changing
equipment shall be capable of carrying the same current as the windings.
11.2
Off circuit tap changing gear:
Off circuit tap changing gear shall have an external operating handle mounted on the transformer
side and shall meet the following requirements:
- Positive snap-action contact changing
- The mechanism shall be such that it is impossible for the contacts to be set in a position
whereby the windings remain open-circuited or partly short-circuited.
- Mechanical stops at the ends shall be provided to prevent overrun.
- The driving rod through cover or tank wall shall be properly sealed against oil leakage
under all service conditions.
The handle shall be metallic and adequately sized in order to allow operation without the need
of tools and be located in a directly accessible position.
The handle shall be provided with padlock facilities to lock the tap changer in the desired
position.
Tap positions shall be clearly marked in line with the data given on the rating plate.
11.3
On Load Tap Changer
11.3.1 High speed on load tap changing gear with number of steps as specified on the data sheet shall be
provided and mounted on the transformer. The OLTC gear shall have diverter resistance and the
current diverting contacts shall be housed in a separate oil chamber segregated from the main
tank of the transformer. The contacts shall be accessible for inspection and their tips shall be
replaceable. OLTC oil chamber shall have oil filling, drain and sampling facility. It shall be
provided with oil-level indicator, connection orifices, valves and silica gel breather shall be
provided.
11.3.2 OLTC shall be provided with local and remote controls.
Local Control
11.3.3 Manual-Mechanical Control
The cranking device for operation of the OLTC gear shall be removable and located at a height
not exceeding 1500mm above ground level for easy operation. The mechanism shall be complete
with normal accessories including at least the following:- A mechanical tap position indicator (Rated tap voltages shall be marked on the diagram
plate).
- A mechanical operation counter.
- Mechanical stops to prevent over cranking of the mechanism beyond extreme tap
positions.
11.3.4 Electrical Control
Control circuit shall incorporate the following:
a) Local/remote manual electrical operation.
b) Device to ensure a positive and full completion of tap change once it is initiated even if
there is loss of power.
c) An interlock to cut-off electrical control automatically upon recourse being taken to
manual mechanical control in emergency.
d) Electrical interlock to cut-off a counter impulse for a reverse tap change, being initiated
during a progressive tap change and until the mechanism comes to rest and resets circuits
for a fresh operation.
e) All auxiliaries and devices for electrical control of OLTC gear should be housed in a
weather-proof cabinet mounted on the transformer and shall include:
- Local tap position indicator
- 5 digit operation counter
- Cubicle lighting
- Thermostatically controlled space heater.
- Miniature circuit breaker with magnetic and thermal overload devices for controlling the
incoming supply to the OLTC motor.
- Padlocking arrangement for the hinged cabinet door.
- Removable plate with cable glands.
- Inside tag with control scheme indelibly marked.
11.3.5 Remote Tap Changer Control Panel
Remote Tap Changer control panel shall comprise of the following:
- Individual/parallel control on Master follower sequence selector switch.
- Raise/lower control switch.
- Potentiometer type tap position indicator.
- Out of step relay.
- Time delay relay.
- Indicating lamp for out of step.
- Out of step buzzer.
- Indicating lamp for tap changer supply available.
- Indicating lamp for tap change in progress.
RTCC panel shall be dust & vermin proof, floor mounting, and freestanding type. The enclosure
shall be cold rolled sheet of 2.5 mm for front and back and 2.5mm thick for the rest. All doors
and opening shall be provided with neoprene gaskets.
11.3.6 Automatic tap changing control shall be provided complete with voltage sensing relay if
specified on data sheet.
11.3.7 OLTC wherever called for shall be suitable for bi-directional power flow.
11.3.8 OLTC shall also be rated for basic insulation level value as specified for the transformer in
the data sheet.
12.0
Neutral Grounding Resistor (NGR)
12.1
Resistors:
12.1.1 The resistor elements shall be made of unbreakable, corrosion-proof, joint-less stainless steel
grids conforming to ASTM standard A240-304. The grids shall be in punched form.
12.1.2 The resistor shall be sized for carrying rated current specified in the data sheet for 10 seconds
with maximum temperature limited to 790 deg C for stainless steel resistor elements while
limiting the temperature of Aluminium conductor / bus bar to 350 deg C.
12.1.3 If specified in the data sheets the resistors shall also be sized to carry 10% of rated current
continuously with the maximum operating temperature of resistor elements limited to 415 deg C.
Further, the temperature rise of resistor elements when they carry 10% of rated current followed
by the specified rated current for 10 seconds shall not exceed the temperature limit specified in
the above clause. Under steady-state conditions when the NGR carries 10% of rated current the
surface temperature of the enclosure, excluding the roof but including the separate canopy shall
be limited to a maximum of 70 deg C.
12.1.4 Grids shall be mounted on steel rods insulated by special heat-resistant insulating materials,
suitable for the above temperatures. Ceramic/ Porcelain insulators shall be used to insulate the
resistor elements from the enclosure. The insulators and terminal bushings shall have adequate
minimum creepage values (total and protected) for the required voltage grade.
12.1.5 Resistors shall be provided in parallel and series combinations to achieve the overall resistance
value. Minimum two banks in parallel shall be provided in the system unless specified otherwise.
12.2
Enclosure:
12.2.1 The resistor elements shall be housed in naturally ventilated sheet steel enclosure with minimum
IP-31 degree of ingress protection and suitable for outdoor installation.
12.2.2 The enclosure thickness shall not be less than 3mm. The NGR shall have suitable arrangements
for fixing it on concrete foundations / grouting in floor.
12.2.3 The terminals for neutral and earthing connections shall be housed in a separate vermin-proof,
weather-proof terminal box with minimum IP-55 degree of ingress protection. The terminal box
shall be provided with a bolted removable undrilled gland plate of non-magnetic material. This
shall be drilled at site to suit the cable entry. A nickel plated brass double compression gland of
suitable size shall be provided along with the terminal box.
12.2.4 A separate canopy shall be provided above the enclosure roof with an air gap between them. It
shall also cover the terminal compartment. Hooks or handle shall be provided to lift the canopy.
12.2.5 It shall be possible to periodically clean the resistor elements and enclosure after removing the
bolted side covers.
12.2.6 Facility shall be provided to earth the enclosure at two points. For this purpose, suitably sized
studs shall be provided on the sides of the enclosure to accommodate the connection of GI strip.
12.2.7 The bottom of the enclosure shall be suitably sloped and provided with a drain plug to remove
water that may get collected in the enclosure.
12.3
Terminals
12.3.1 The two ends of the resistor shall be brought to suitable epoxy / porcelain bushing type terminals
of adequate rating for the neutral and earth connections. The terminals shall be suitable for
terminating the specified size of cables / earthing strip and shall be supplied complete with
tinned-copper lugs. Bolted type link shall be provided for isolating the resistor at the neutral end.
12.3.2 Suitable washers shall be provided between the grid element ends and the terminals to maintain
good electrical contact.
12.3.3 The connection between transformer neutral and NGR shall be carried out by means of single
core cable of size and type as specified in the data sheet.
12.3.4 The connection between NGR and earth pits is carried out using GI earthing strips / cable as
specified in the data sheet.
12.4
Space Heater
A suitable anti-condensation space heater shall be provided inside the NGR enclosure to prevent
condensation of moisture. This shall be rated to operate from a 240V, 50 Hz single phase supply.
The terminals of the space heater shall be brought out to a separate vermin-proof, weather –proof
terminal box. The terminal box shall have minimum IP-55 degree of ingress protection and shall
be complete with tinned-copper lugs and nickel plated brass double compression glands.
The space heater shall be controlled using a double pole switch, HRC fuse, neutral link and
thermostat provided in the terminal box. The switch shall be operable from the front of the
enclosure without opening the terminal box cover.
12.5
The current transformer, if specified, shall be cast in resin type suitable for the applicable voltage
grade and temperature conditions. The secondary connections of the current transformer shall be
brought out to suitable terminals provided with shorting link.
12.6
Cooling
The NGR shall be natural air cooled. Forced cooling shall not be used. If required, louvers can be
provided in the enclosure provided they are completely guarded on the inside with a fine wire
mesh.
12.7
Nameplate Marking
The equipment shall be provided with stainless steel nameplate located predominantly indicating
particulars such as make, tag number of the equipment, rating and other relevant details.
12.8
Painting
After manufacture, all surfaces to be painted shall be cleaned, descaled made free from rust and
given one coat of primer and two coats of epoxy paint of shade 632 or as specified in the data
sheet.
13.0
NOISE
Audible Noise level shall not exceed the limits indicated in CBIP manual.
14.0
INSPECTION, TESTING AND ACCEPTANCE
14.1
Owner's representative shall be given free access in the works from time to time for stage-wise
inspection and progress reporting. Four weeks advance notice shall be given to witness the final
routine test as per IS: 2026 and other tests as agreed upon. These tests shall be performed on the
complete assembly at manufacturer's works. Test certificates duly signed by owner's
representative shall be issued as part of final document.
14.2
Routine Tests: Routine tests as per IS: 2026 shall be carried out.
14.3
Additional Tests:
14.3.1 Oil leakage Test - All tanks and oil filled compartments shall be tested for oil tightness by being
completely filled with air/oil of a viscosity not greater than that of insulating oil to IS: 335 at an
ambient temperature and subjected to a pressure equal to the normal pressure plus 35 kN/m2
measured at the base of the tank. This pressure shall be maintained for a period of not less than 12
hours for oil and 1 hour for air, during which time no leakage shall occur.
14.3.2 Vacuum Test – The transformer tank shall be subjected to a vaccum of 34.7kN/m2 gauge
pressure (250 mm Hg) for one hour. The permanent deflection of flat plates after the vacuum has
been released shall not exceed the value specified below without affecting the performance of the
transformer.
Horizontal length of flat plate (in mm)
Upto & including 750mm
750 to 1250
1251 to 1750
1751 to 2000
2001 to 2250
2251 to 2500
2501 to 3000
Above 3000
Permanent deflection (in mm)
5
6.5
8.0
9.5
11.0
12.5
16.0
19.0
14.3.3 Pressure Test - One transformer tank of each size shall be subjected to a pressure corresponding
to twice the normal head of oil or to the normal pressure plus 35kN/m2 whichever is lower as
measured at the base of the tank and will be maintained for one hour. The permanent deflection of
the flat plates after the excess pressure is released shall not exceed the figures specified above.
14.3.4 The transformer shall be subjected to heat run test, if specified in data sheet.
14.3.5 Impulse test, if specified in the data sheet, shall be carried out on all three limbs of the
transformer.
14.3.6 Transformer shall be subjected to short circuit test, if specified in the data sheet.
14.3.7 The following tests shall be conducted for the NGR.
a. Measurement of resistance value at the prevailing ambient temperature by Kelvin Double
Bridge.
b. One minute power frequency high voltage withstand test.
c. Insulation resistance measurement, both before and after the power frequency high voltage
test.
d. Heat run test.
The temperature rise / heat run test may be conducted by passing the rated current for rated time
on a limited resistor section in a proportionately sized test enclosure. The temperature rise shall
be measured under simulated conditions ensuring that heat produced (I squared RT) in the test
enclosure is more than or equal to the calculated value of energy dissipation required for the test
enclosure in proportion to the complete Neutral Grounding Resistor enclosure as per the thermal
model adopted. Details of the calculation /modeling considered shall be furnished.
15.0
PACKING AND DESPATCH
All the equipment shall be divided into multiple sections for protection and ease of handling
during transportation. The equipment shall be properly packed for the selected mode of
transportation, i.e. by ship, rail or trailer. The equipment shall be wrapped in polythene sheets
before being placed in crates/ cases to prevent damage to finish. The crates / cases shall have skid
bottoms for handling. Special notations such as ‘Fragile’, ‘This side up’, ‘Centre of gravity’,
‘Weight’, ‘volume’ ‘Owner’s particulars’, ‘P.O. no.’ etc. shall be clearly and indelibly marked on
the packages together with other details as per purchase order. The equipment may be stored
outdoors for long periods before installation. The packing shall be completely suitable for outdoor
storage in areas with heavy rains and high ambient temperature unless otherwise agreed. In order
to prevent movement of equipment/components within the crates, proper packing supports shall
be provided.