315 MVA 400/220KV AUTO
Construction Details
Tank Cover
Temperature Indicators
Insulating Oil
Terminal Arrangements
Tap Changing equipment
Inspection and Testing
Factory Tests
Tank Tests
Technical Parameters
Equalization Formula
Bushing Current Transformer
Oil Storage Tank
Oil Sampling Bottle
This specification covers design, engineering, manufacture, testing at
manufacturer’s works, delivery at site including all materials, accessories, spares,
unloading, handling, proper storage at site, erection, testing and commissioning of
the equipment specified.
The autotransformers shall in general have constant ohmic impedance between
HV and IV on all taps. However, in case of parallel operation with the existing
The impedance, vector group, OLTC connection & range etc. of the
transformer is to be matched with that of the existing transformer
Necessary provision is to be kept in the transformer control scheme for
parallel operation with the existing Master/Follower/Independent/Off type
OLTC control scheme.
Matching of physical dimension, orientation etc. to facilitate
interchangeability with the existing spare single phase transformer, if
External or internal reactors shall not be used to achieve the HV/LV and IV/LV
impedance specified.
Reference Drawings
The lit of drawings indicated below form a part of this specification
Standard dimensions for OIP
condenser bushings (Lower portion)
Standard foundation for
Auto Transformer
Typical arrangement of auto
transformer with regulating
winding at the end of series winding
The Contractor shall dispatch the transformer filled with oil or in an atmosphere
of nitrogen or dry air. In the former case the contractor shall take care of the
weight limitation on transport and handling facility at site. In the latter case,
necessary arrangement shall be ensured by the contractor to take care of pressure
drop of nitrogen or dry air during transit and storage till completion of oil filing
during erection. A gas pressure testing valve with necessary pressure gauge and
adaptor valve shall be provided. Transformer shall also be fitted with sufficient
number of impact recorders during transportation to measure the movement due
to impact in all three directions.
The autotransformers shall be used for bi-directional flow of rated power.
Transformers shall be capable of operating under natural cooled condition upto
the specified load. The forced cooling equipment shall come into operation by
pre-set contacts of winding temperature indicator and the transformer shall
operate as a forced cooling unit initially as ONAF up to specified load and then as
OFAG. Cooling shall be so designed that during total failure of power supply to
cooling fans and oil pumps, the transformer shall be able to operate at full load for
at least ten (10) minutes without the calculated winding hot spot temperature
exceeding 1500C. Transformers fitted with two coolers, each capable of
dissipating 50 per cent of the loss at continuous maximum rating, shall be capable
of operating for 20 minutes in the event of failure of the oil circulating pump or
blowers associated with one cooler without the calculated winding hot spot
temperature exceeding 1500C at continuous max. rating. The contractor shall
submit supporting calculations for the above for Employer’s approval.
The transformers shall be capable of being operated, without danger, on any
tapping at the rated MVA with voltage variation pf + 10% corresponding to the
voltage of the tapping.
The maximum flux density in any part of the core and yoke at the rated MVA,
voltage and frequency shall be such under 10 per cent continuous overvoltage
condition it does not exceed 1.9 Tesla.
DGA of oil shall be periodically monitored by the Employer and the interpretation
of DGA results will be as per IEC-599.
Radio interference and Noise Level.
2.6.1 The transformers shall be designed with particular attention to the suppression of
maximum harmonic voltage, especially the third and fifth so as to minimize
interference with communication circuit.
2.6.2 The noise level of transformer, when energized at normal voltage and frequency
with fans and pumps running shall not exceed, when measured under standard
conditions, the values specified in NEMA standard publication TR-1.
The transformers shall be capable of being loaded in accordance with
IS:6600/IEC-354. There shall be no limitation imposed by bushings, tap changers
etc. or any other associated equipment.
The transformer and all its accessories including CTs etc, shall be designed to
withstand without injury, the thermal and mechanical effects of any external short
circuit to earth and of short circuits at the terminals of any winding for a period of
3 secs. The short circuit level of the HV & LV system to which the subject
transformers will be connected is 40kA (sym, rms, 3 phase fault on 400 kV & 220
kV) & 31.5 kA (sym, rms, 3 phase fault on 132 kV).
Transformer shall be capable of withstanding thermal and mechanical stresses
caused by symmetrical or asymmetrical faults on any winding.
Transformers shall withstand, without injurious heating, combined voltage and
frequency fluctuations which produce the following over fluxing conditions:i)
125% for 1 – minute
140% for 5 – seconds
Bidder shall indicate 150% and 170% over voltage withstand time.
The air core reactance of HV winding of transformer shall not be less than 20%.
Tertiary windings (for autotransformer above 100 MVA)
2.12.1 The tertiary windings shall be suitable for connection of reactors or capacitors
which would be subjected to frequent switching. All the windings shall be capable
of withstanding these stresses that may be caused by such switching.
2.12.2 The tertiary winding shall be designed to withstand mechanical and thermal
stresses due to dead short circuit on its terminals.
Construction Details
The features and construction details of each power transformer shall be in
accordance with the requirement stated hereunder.
Tank and Tank Accessories
3.1.1 Tank Tank shall preferably be welded construction and fabricated from tested quality
low carbon steel of adequate thickness.
-4- All seams and those joints not required to be opened at site shall be factory
welded, and wherever possible they shall be double welded. After completion of
tank and before painting, dye penetration test shall be carried out on welded parts
of jacking bosses, lifting lugs and all load bearing members. The requirement of
post weld heat treatment of tank/stress relieving shall be based on
recommendation of BS-5500 table Tank stiffeners shall be provided for general rigidity and these shall be designed
to prevent retention of water. The transformer shall be of bell type tank with joint at about 500mm above the
bottom of the tank. In case the joint is welded it shall be provided with flanges
suitable for repeated welding. The joint shall be provided with a suitable gasket to
prevent weld splatter inside the tank. Proper tank shielding shall be done to
prevent excessive temperature rise of the joint. Each tank shall be provided with:
Lifting lugs suitable for lifting the equipment complete with oil.
A minimum of four jacking pads in accessible position to enable
the transformer complete with oil to be raised or lowered using
hydraulic jacks.
Suitable haulage holes shall be provided. The tank shall be designed in such a way that it can be mounted on the plinth
directly. 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 injury when using
plates or rails.
3.1.2 Tank Cover The tank cover shall preferably be sloped to prevent retention of rain water and
shall not distort when lifted. At least two adequately sized inspection openings one at each end of the tank,
shall be provided for easy access to bushings and earth connections. The
inspection covers shall not weigh more than 25kg. Handles shall be provided on
the inspection cover to facilitate lifting. The tank covers shall be fitted with pockets at the position of maximum oil
temperature at maximum continuous rating for bulbs of oil and winding
temperature indicators. It shall be possible to remove these bulbs without
lowering the oil in the tank. The thermometer shall be fitted with a captive screw
to prevent the ingress of water.
-5- Bushing turrets, covers of inspection openings, thermometer pockets etc.shall be
designed to prevent ingress of water into or leakage of oil from the tank. All bolted connections shall be fitted with weather proof, hot oil resistant,
resilient gasket in between for complete oil tightness. If gasket is compressible,
metallic stops/other suitable means shall be provided to prevent overcompression.
3.1.3 Axles and Wheels The transformer shall be mounted either on rollers or on concrete plinth
foundation directly, as per manufacturer’s standard practice. The roller mounted transformers are to be provided with flanged bi-directional
wheels and axles. This set of wheels and axles shall be suitable for fixing to the
under carriage of transformer to facilitate its movement on rail track. Suitable
locking arrangement along with foundation bolts shall be provided for the wheels
to prevent accidental movement of transformer. The rail track gauge shall be 1676 mm. Bidder can supply one set of trolleys in place of wheel assembly rollers for
movement of transformer per sub-station for plinth mounted transformer
3.1.4 Foundation and Anti Earthquake Clamping Device To prevent transformer movement during earthquake, suitable clamping devices
shall be provided for fixing the transformer to the foundation.
3.1.5 Conservator & Oil Preservation System Main conservator shall have air cell type constant oil pressure system to prevent
oxidation and contamination of oil due to contact with moisture, and shall be
fitted with magnetic oil level gauge with low oil level potential free contacts. OLTC shall have conventional type conservator with prismatic oil level gauge. Conservator tank Conservator tank shall have adequate capacity with highest and lowest visiblelevels to meet the requirements of expansion of total cold oil volume in the
transformer and cooling equipment from minimum ambient temperature to
-6- The conservator shall be fitted in such a position so that it can be removed for
cleaning purposes. Suitable provision shall be kept to replace air cell wherever
applicable. Conservator shall be positioned so as not to obstruct any electrical connection
to transformer
Oil Preservation Equipment
The requirement of air cell type oil sealing system are given below:
Contact of the oil with atmosphere is prohibited by using a flexible air cell of
nitrile rubber reinforced with nylon cloth.
The temperature of oil is likely to rise upto 1000C during operation. As such
air cell used shall be suitable for operating continuously at 1000C .
Air cell of conservator shall be able to withstand the vacuum during
installation/maintenance periods. Otherwise provision shall be kept to isolate
the conservator from the main tank when the latter is under vacuum by
providing a vacuum sealing valve or other suitable means in the pipe
connecting main tank with the conservator.
The connection of air cell to the top of the conservator is by air proof seal
preventing entrance of air into the conservator.
Dehydrating Filter Breather
Conservator shall be fitted with a dehydrating filter breather. It shall be so
designed that:a) Passage of air is through silicagel.
b) Silicagel is isolated from atmosphere by an oil seal.
c) Moisture absorption indicated by a change in colour of the tinted crystals
can be easily observed from a distance.
d) Breather is mounted not more than 1200mm above rail top level.
e) To minimize the ingress of moisture following shall be provided.
Three breathers (of identical size) shall be connected in series
for main tank conservator.
Two breathers (each of 2.5 litres minimum volume) shall be
connected in series for OLTC tank conservator.
Pressure Relief Device
Adequate number of pressure relief devices shall be provided at suitable locations.
Theses shall be of sufficient size for rapid release of any pressure that may be
generated in the tank and which may result in damage to equipment. The device
shall operate at a static pressure less than the hydraulic test pressure of the
transformer tank. It shall be mounted directly on the tank. One set of electrically
insulated contacts shall be provided for alarm/tripping. Discharge of pressure
relief device shall be properly taken through pipes and directed away from the
transformer/other equipment and this shall be prevented from spraying on the
tank. Following routine tests shall be conducted on PRD.
Air pressure test
Liquid pressure test
Leakage test
Contact test
Dielectric test
3.1.7 Buchholz Relay
A double float/reed type Buchholz relay shall be provided. Any gas evolved in the
transformer shall collect in this relay. The relay shall be provided with a test cock
suitable for a flexible pipe connection for checking its operation and taking gas
sample. A copper/stainless steel tube shall be connected from the gas collector to
a valve located about 1200mm above ground level to facilitate sampling with the
transformer in service. The device shall be provided with two electrically
independent ungrounded contacts, one for alarm on gas accumulation and the
other for tripping on sudden rise of pressure.
3.1.8 Temperature Indicators Oil Temperature Indicator (OTI)
All Transformer shall be provided with a 150mm dial type thermometer for top
oil temperature indication. The thermometer shall have adjustable, electrically
independent ungrounded alarm and trip contacts, maximum reading pointer and
resetting device shall be provided in the OTI. a temperature sensing element
suitably located in a pocket on top oil shall be furnished. This shall be connected
to the OTI by means of capillary tubing. Temperature indicator dials shall have
linear gradations to clearly read atleast every 20C. Accuracy class of OTI shall be
+ 1.5% or better. Winding Temperature Indicator (WTI)
A device for measuring the hot spot temperature of each winding shall be
provided (HV, IV and LV). It shall comprise the following:
Temperature sensing element
Image coil
Auxiliary CTs, if required to match the image coil, shall be furnished and
mounted in the cooler control cabinet.
150mm dia local indicating instrument with maximum reading pointer and
two adjustable electrically independent, ungrounded contacts; besides that
required for control of cooling equipment if any, one for high winding
temperature alarm and one for trip. Temperature indicator dials shall have
linear gradations to clearly read at least every 20C.
Calibration device
Accuracy class of WTI shall be + 1.5% or better.
In addition to the above, the following equipment shall be provided for
remote indication of winding temperature for each of the winding:
Signal transmitter for each winding
Signal transmitter shall have additional facility to transmit signal for
recording winding temperature at Employer’s data acquisition system, for
which a duplex platinum RTD with nominal resistance of 100ohms at zero
degree centigrade shall be supplied. The RTD shall be three wire
ungrounded system. The Calibration shall be as per SAMA (USA)
standard or equivalent. The RTD may be placed in the pocket containing
temperature sensing element and image coil for WTI system which will be
used for both remote WTI and DAS. Necessary equipment for sending the
signal to remote WTI and DAS shall be provided. In lieu, separate RTD
for each of the functions shall be provided.
Remote winding temperature indicator
It shall be suitable for flush mounting on Employer’s panel. This
shall not be repeater dial of local WTI and will operate by signal
Any special cable required for shielding purpose, for connection between
cooler control cabinet and remote WTI control circuit, shall be in scope of
Contractor. Only one RWTI with a four point selector switch shall be
provided for all the three windings (HV, IV and LV).
3.1.9 Earthing Terminals Two (2) earthing pads (each complete with two (2) nos. holes, M 10 bolts plain
and spring washers) suitable for connection to 75x12mm galvanized steel
grounding flat shall be provided each at position close to earth of the two (2)
diagonally opposite bottom corners of the tank.
-9- Two earthing terminals suitable for connection to 72x12 mm galvanized steel flat
shall also be provided on cooler, marshalling box and any other equipment
mounted separately.
3.2.1 The core shall be constructed from high grade, non-ageing, cold rolled, super
grain oriented, silicon steel laminations.
3.2.2 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 production of flux component at right angles to the plane of
laminations which may cause local heating.
3.2.3 The insulation of core to bolts and core to clamp plates shall be able to withstand
a voltage of 2 kV (rms) for 1 minute.
3.2.4 Core and winding shall be capable of withstanding the shock during transport,
installation and service. Adequate provision shall be made to prevent movement
of core and winding relative to tank during these conditions.
3.2.5 All steel sections used for supporting the core shall be thoroughly sand blasted
after cutting, drilling and welding.
3.2.6 Each core laminations shall be insulated with a material that will not deteriorate
due to pressure and hot oil.
3.2.7 The supporting frame work of the core shall be so designed as to avoid presence
of pockets which would prevent complete emptying of tank through drain valve or
cause trapping of air during oil filling.
3.2.8 Adequate lifting lugs will be provided to enable the core and windings to be
3.2.9 All the CRGO vendors having BIS certification and approval from PGCIL shall
be considered.
3.3.1 The contractor shall ensure that windings of all 400 KV Class transformers are
made in dust proof and conditioned atmosphere.
3.3.2 The conductors shall be of electrolytic grade copper free from scales and burrs.
- 10 -
3.3.3 The insulation of the transformer windings and connections shall be free from
insulating compounds which are likely to soften, ooze out, shrink or collapse and
be non-catalytic and chemically inactive in transformer oil during service.
3.3.4 Coil assembly and insulating spacers shall be so arranged as to ensure free
circulation of oil and to reduce the hot spot of the winding.
3.3.5 The coils would be made up, shaped and braced to provide for expansion and
contraction due to temperature changes.
3.3.6 The conductor shall be transposed at sufficient intervals in order to minimize eddy
currents and to equalize the distribution of currents and temperature along the
3.3.7 The purity test of copper shall be got conducted on sample of copper for assessing
its quality, from two independent NABL accredited laboratories.
Insulating Oil
3.4.1 The insulating oil shall conform to all parameters specified below, while tested at
supplier’s premises. No inhibitors shall be used in oil. The contractor shall furnish
test certificates from the supplier against their acceptance norms as mentioned
below, prior to dispatch of oil from refinery to side.
Method of test
The oil shall be clear and
transparent and free from
suspended matter or
sample of the oil
shall be examined
in a 100mm thick
layer, at ambient
Density at 29.50C
0.89 gm/cm3
IS: 1448
Kinematic viscocity at 27 cSt
29.50C (Max.)
Interfacial tension at 0.04 N/m
Flash point penskey 1400C
marten (closed) (Min.)
Pour point (Max.)
value 0.03 mg KOH/gm
(total acidity) (Max.)
- 11 -
IS:335 Appendix-A
Corrosive sulphur (in Non-Corrosive
terms of classification
of copper strip)
IS:335 Appendix-A
a) New untreated oil
30 kV (rms) (if this value IS:6792
is not attained the oil
shall be treated)
b) After treatment
60 kV (rms)
Dielectric dissipation 0.002
factor (tan delta) at
900C (Max.)
(ohm cm)
(a) 900C
(b) 270C
Oxidation stability
Neutralization 0.40mg KOH/gm
value after oxidation
(b) Total sludge after 0.10 percent by weight
oxidation (Max)
Presence of oxidation The oil shall not contain IS:335 Appendix inhibitor
anti-oxidant additives
Water content (Max.)
(i) New untreated oil
50 ppm
(ii) After treatment
15 ppm
- 12 -
Ageing Characteristics
after 96 hrs as per
with catalyst (copper)
(a) Resistivity (Min) 2.5x 1012
(ohm cm) at 270C at 0.2x1012
(b) Ten delta at 900C 0.2
(c) Total acidity (Max)
0.05 mg KOH/gm
(d) Sludge content wt. 0.05% (By Weight)
PCB content
Less than 2 ppm
3.4.2 Subsequently oil samples shall be drawn
Prior to filling in main tank at site and shall be tested for
Moisture content
The acceptance norms for above parameters shall be as per CI.3.4.1 above.
Prior to energisation at site and shall be tested for following properties &
acceptance norms:
60 kV (min)
Moisture Content
15 ppm (max.)
Tan-Delta at 900C
0.05 (max)
Resistivity at 900C
1x1012 ohm-cm (min)
Interfacial Tension
0.03N/m (min.)
3.4.3 At manufacturer’s works oil sample shall be drawn before and after heat run test
and shall be tested for following:-
- 13 -
60 kV (min)
Moisture Content
15 ppm
Dissolved gas analysis
Samples for DGA shall be taken from sampling device within 24 hours prior to
commencement of temperature rise test and immediately after this test. The
acceptance norms with reference to various gas generation rates during the
temperature rise test shall be as per IS: 10593 (based on IEC-599).
3.4.4 Sufficient quantity of oil necessary for maintaining required oil level in tank,
radiators, conservator etc till commissioning shall be supplied.
Terminal Arrangements
3.5.1 Bushings The electrical and mechanical characteristics of bushings shall be in accordance
with IS: 2099 and IS: 3347. Bushings for voltage for 420 kV, 245 kV , 145 kV and 52 kV shall be of the oil
filled condenser type. 36 kV bushing shall be solid porcelain or oil
communicating type. Mounting dimensions of 420kV, 245 kV, 145 kV and 52 kV
bushing shall be as per drawing No. 0000-T-E-A-001. Dimensions of 36 kV
bushing shall conform to IS:3347 Part-V. Oil filled condenser type bushing shall be provided with at least the following
Oil level gauge.
Oil filling plug and drain valve if not hermetically sealed.
Tap for capacitance and tan delta test. Where current transformers are specified, the bushings shall be removable
without disturbing the current transformers. Bushings of identical rating shall be interchangeable. Porcelain used in bushing manufacture shall be homogenous, free from
lamination, cavities and other flaws or imperfections that might affect the
mechanical or dielectric quality and shall be thoroughly vitrified, tough and
impervious to moisture. Clamps and fittings shall be of hot dip galvanized steel.
- 14 - Bushing turrets shall be provided with vent pipes to route any gas collection
through the Buchholz relay. No arcing horns shall be provided on the bushings. Suitable insulating cap (preferably of porcelain) shall be provided on the
terminal of Bushing of tertiary winding to avoid accidental external short
Terminal Marking
The terminal marking and their physical position shall be as per IS:2026.
Neutral Earthing Arrangement
3.7.1 The neutral terminal of auto transformer shall be brought to the ground level by a
brass/tinned copper ground bar, supported from the tank by using porcelain
insulators. The end of the brass/tinned copper bar shall be brought to the bottom
of the tank, at a convenient point, for making bolted connection to two (2) 72x12
mm galvanized steel flats connected to Employer’s grounding mat.
Cooling Equipment and its Control
3.8.1 Cooling Equipment The cooler shall be designed using 2x50% radiator banks. Design of cooling
system shall satisfy the performance requirements. Each radiator bank shall have its own cooling fans, oil pumps, oil flow indicator,
shut off valves at the top and bottom (80mm size), lifting lugs, top and bottom oil
filling valves, air release plug at the top, a drain and sampling valve and
thermometer pocket fitted with captive screw cap on the inlet and outlet. Required number of standby fans of approximately 205 capacity shall also be
provided with each radiator bank. Cooling fans shall not be directly mounted on radiator bank which may cause
undue vibration. These shall be located so as to prevent ingress of rain water.
Each fan shall be suitably protected by galvanized wire guard. The exhaust air
flow from cooling fan shall not be directed towards the main tank in any case. Two (2), 100% centrifugal or axial on line oil pumps (out of which one pump
shall be standby), shall be provided with each radiator bank. Measures shall be
taken to prevent maloperation of Buchholz relay when all oil pumps are
simultaneously put into service. The pump shall be so designed that upon failure
- 15 -
of power supply to the pump motor, the pump impeller will not limit the natural
circulation of oil. An oil flow indicator shall be provided for the confirmation of the oil pump
operating in a normal state. An indication shall be provided in the flow indicator
to indicate reverse flow of oil/loss of oil flow. Cooling fans and oil pump motors shall be suitable for operation from 415 volts,
three phase 50 Hz power supply and shall conform to IS:325. Each cooling fans
and oil pump motors shall be provided with starter thermal overload and short
circuit protection. The motor winding insulation shall be conventional class ‘B’
type. Motors shall have hose proof enclosure equivalent to IP:55 as per IS:4691. The cooler and its accessories shall preferably be hot dip galvanized or corrosion
resistant paint should be applied to it. Expansion joint shall be provided, one each on top and bottom cooler pipe
Air release device and oil plug shall be provided on oil pipe connections.
Drain valves shall be provided in order that each section of pipe work can be
drained independently.
3.8.2 Cooling equipment control (ONAN/ONAF/OFAF COOLING) Automatic operation control of fans/pumps shall be provided (with temperature
change) from contacts of winding temperature indicator. The Contractor shall
recommend the setting of WTI for automatic change over of cooler control from
ONAN to ONAF and then to OFAF. The setting shall be such that hunting i.e.
frequent start-up operations for small temperature differential do not occur. Suitable manual control facility for cooler fans and oil pumps shall be provided. The changeover to standby oil pump in case of failure of service oil pump shall be
automatic. Selector switches and push buttons, shall also be provided in the cooler control
cabinet to disconnect the automatic control and start/stop the fans and pump
3.8.3 Indicating Devices
Following lamp indications shall be provided in cooler control cabinet:
Control Supply Failure
Cooling fans failure for each bank.
- 16 -
Cooling pump failure for each pump
No oil flow/reverse oil flow for pumps
Common thermal overload trip.
One potential free initiating contact for all the above conditions shall be wired
independently to the terminal blocks of cooler control cabinet.
3.8.4 Valves All valves upto and including 100mm shall be of gun metal or of cast steel/cast
iron. Larger valves may be of gun metal or may have cast iron bodies with gun
metal fittings. They shall be of full way type with internal screw and shall open
when turned counter clock wise when facing the hand wheel. Suitable means shall be provided for locking the valves in the open and close
positions. Provision is not required for locking individual radiator valves. Each valve shall be provided with the indicator to show clearly the position of the
valve. All valve flanges shall have machined faces. All valves in oil line shall be suitable for continuous operation with transformer
oil at 1000C. The oil sampling point for main tank shall have two identical valves to be put in
series. Oil sampling valve shall have provision to fix rubber hose of 10 mm size to
facilitate oil sampling. A valve or other suitable means shall be provided to fix the on line dissolved gas
monitoring system to facilitate continuous dissolved gas analysis. The location &
size of the same shall be finalized during detail engineering stage. Suitable valves shall be provided to take sample of oil from the OLTC chamber
during operation of the transformer. After testing, inside surface of all cast iron valves coming in contact with oil shall
be applied with one coat of oil resisting paint/varnish with two coats of red oxide
zinc chromate primer followed by two coats of fully gloosy finishing paint
conforming to IS:2932 and of a shade (preferably red or yellow) distinct and
different from that of main tank surface. Outside surface except gasket setting
surface of butterfly valves shall be painted with two coats of red oxide zinc
chromate conforming to IS:2074 followed by two coats of fully glossy finishing
- 17 -
All hardware used shall be cadmium plated/electro galvansied
Tap Changing Equipment
Tap change switch general requirement
OLTC shall be motor operated for local as well as remote operation. An
external handle shall be provided for local manual operation. This handle
shall be suitable for operation by a man standing at ground level.
On Load Tap Changing Gear (OLTC)
Each single/three phase transformer shall be provided with voltage control
equipment of the tap changing type for varying its effective transformation
ratio whilst the transformers are on load and without producing phase
The requirements of on load tap changing equipment are given here
The current diverting contacts shall be housed in a separate oil
chamber not communicating with the oil in main tank of the
The contacts shall be accessible for inspection without lowering oil
level in the main tank and the contact tips shall be replaceable.
The Bidder shall indicate the safeguards in order to avoid harmful
arcing at the current diverting contacts in the event of operation of
the OLTC gear under overload conditions of the transformer.
Necessary tools and tackles shall be furnished for maintenance of
OLTC gear.
The OLTC oil chamber shall have oil filling and drain plug, oil
sampling valve, relief vent and level glass. It shall also be fitted
with a oil surge relay the outlet of which shall be connected to a
separate conservator tank.
The diverter switch or arcing switch shall be designed so as to
ensure that its operation once commenced shall be completed
independently of the control relays or switches, failure of ancillary
supplies etc. To meet any contingency which may result in
incomplete operation of the diverter switch, adequate means shall
be provided to safeguard the transformer and its ancillary
Tap change shall be so mounted that bell cover of transformer can
be lifted without removing connections between windings and tap
changer .
Local OLTC control cabinet shall be mounted on the tank in
accessible Position. It should be adequately ventilated and
provided with anti-condensation metal clad heaters. All contactors
- 18 -
relay coils and other parts shall be protected against corrosion,
deterioration due to condensation, fungi etc.
Operating mechanism for on load tap changer shall be designed to
go through one step of tap change per command. Subsequent tap
changes shall be initiated only by a new or repeat command.
On load tap changer shall be equipped with a time delayed
INCOMPLETE STEP alarm consisting of a normally open contact
which closes, if the tap changer fails to make a complete tap
change. The alarm shall not operate for momentary loss of
auxiliary power.
The selsyn units or approved equivalents shall be installed in the
local OLTC control cabinet to provide tap position indication for
the transformer. The Bidder shall also provide a set of instruments
for tap position indication in the control room. Complete mounting
details shall be included in the approved diagram.
Transformer on load tap shall be equipped with a fixed resistor
network capable of providing discrete voltage steps for input to the
supervisory system.
Limit switches shall be provided to prevent overrunning of the
mechanism and shall be directly connected in the circuit of the
operating motor. In addition, a mechanical stop shall be provided
to prevent over-running of the mechanism under any condition.
Limit switches may be connected in the control circuit of the
operating motor provided that a mechanical de-clutching
mechanism is incorporated.
Thermal device or other means shall be provided to protect the
motor and control circuit. All relays, switches, fuses etc. shall be
mounted in the local OLTC control cabinet and shall be clearly
marked for the purpose of identification.
A permanently legible lubrication chart if required, shall be fitted
within the local OLTC control cabinet.
Any ‘DROP DOWN’ tanks associated with the tap changing
apparatus shall be fitted with guide rod to control the movements
during lifting or lowering.
A five digit counter shall be fitted to the tap changing equipment to
indicate the number of operations completed.
All relays and operating devices shall operate correctly at any
voltage between the limits specified.
- 19 -
It shall not be possible to operate the electric drive when the
manual operating gear is in use.
It shall not be possible for any two controls to be in operation at
the same time.
The equipment shall be suitable for supervisory control and
indication with make before break multi-way switch, having one
potential free contact for each tap position. This switch shall be
provided in addition to any other switch/switches which may be
required for remote tap position indication.
Operation from the local or remote control switch shall cause one
tap movement only until the control switch is returned to the off
position between successive operation.
All electrical control switches and the local operating gear shall be
clearly labeled in a suitable manner to indicate the direction of tap
Transfer of source in the event of failure of one AC supply shall
not affect the tap changer.
OLTC Control of Three Phase Transformers
Each three phase transformer shall be suitable for local and remote
control. The control feature shall provide the following:(a)
‘Local-remote’ selector switch mounted in the local OLTC control
cabinet shall switch control of all load tap changers as followings:
When the selector switch is in ‘local’ position that local
OLTC control cabinet mounted ‘raise-lower’ switch specified in clause (b) below. Remote control of the raise/lower function shall be
possible prevented.
When the selector switch is in ‘remote’ position that local
OLTC control cabinet mounted ‘raise-lower’ switch specified in clause (b) below shall be in-operative. Remote control of the raise/lower
function shall be possible from the remote control panel. The ‘localremote’ selector switch shall have at least two spare contacts per position
which are closed in that position but open in the other position.
A ‘raise-lower’ control switch/push button shall be provided in the
local OLTC control cabinet. This switch shall be operative only when
‘local remote’ selector switch is in ‘local’ position.
An OFF-ON tap changer control switch shall be provided in the
local OLTC control cabinet of the transformer. The tap change shall be inoperative in the OFF position. Also the OFF-ON switch shall have atleast
- 20 -
one spare contact per position which is closed in that position but open in
the other position.
The cranking device for manual operation of the OLTC gear shall be
removable and suitable for operation by a man standing at ground level.
The mechanism shall be complete with the following:a) Mechanical tap position indicator which shall be clearly visible from
near the transformer.
b) A mechanical operation counter.
c) Mechanical stops to prevent over-cranking of the mechanism beyond
the extreme tap positions.
d) The manual control considered as back up to the motor operated
load tap changer shall be interlocked with the motor to block motor
start-up during manual operation. The manual operating mechanism
shall be labeled to show the direction of operation for raising the HV
terminal voltage and vice-versa.
Remote Electrical Group Control
The OLTC control scheme offered shall have provision of remote
electrical group control during the parallel operation of transformer. This
is in addition to independent control of OLTC:
A four position selector switch having Master, Follower,
Independent and Off position shall be provided in the remote
OLTC control panel for each transformer. This shall be wired to
enable operator to select operation of OLTC in either Master,
Follower or Independent mode.
Out of step relays with time contacts shall also be provided to give
alarm and indication in case tap position in all the transformers
under group control are not in same position.
Master Position
If the selector switch is in Master position, it shall be possible to
control the OLTC units in the follower mode by operating the
controls of the master unit. Independent operation of the units
under Follower mode shall have to be prevented. However the
units under independent mode will be controlled independently.
Follower Position
If the selector switch is in Follower mode, control of OLTC shall
be possible only from panel of the Master unit.
- 21 -
Independent Position
In this position of Selector Switch, Control of OLTC of individual
unit shall only be possible.
OLTC Control of single phase transformer
For three single phase units operating as three phase bank the OLTC gear
shall be suitable for local and remote control. The control features shall
provide the following:
Local Electrical Control
‘Local remote’ selector switch mounted in the local OLTC
control cabinet shall switch control of all on load tap
changers as under:
When the selector switch is in ‘local’ position, it shall be possible
to operate any of the ‘raise-lower’ control switches specified in
section (iii) and (iv) below.
Control of the raise-lower function from the control panel shall be
When the selector switch is ‘remote’ position the raise-lower
functions form switches in local OLTC control cabinets as well as
cooler control cabinet specified in Section (iii) and (iv) shall be
inoperative. Remote control of the raise-lower function of the three
phase transformer banks shall be possible from the control panel.
The remote local selector switch shall be have atleast two spare
contacts per position.
‘Independent –combined selector switch shall be provided
for single phase transformer only and shall be mounted in
the cooler control cabinet. This switch shall control the on
load tap changers as follows.
When the selector switch in combined position, it shall be possible
in raise-lower taps by operating the combined ‘raise-lower’ switch
in cooler control cabinet or the remote raise-lower switch.
Independent operation of each single phase transformer will be
When the selector switch is in independent position it shall be
possible to raise-lower taps by operating individually each single
phase transformer through their individual ‘raise-lower’ control
switch mounted in Local OLTC Control Cabinet. Combined
- 22 -
operation of all the three (3) single phase transformers will be
A combined raise-lower control switch shall be provided in
the cooler control cabinet f or a bank of three single phase
transformers only. This switch shall be functional only
when ‘local-remote’ selector switch is in local position and
‘Independent combined’ selector switch is in combined
An individual ‘raise-lower’ control cabinet for each single
phase transformer. This switch shall be functional only
when ‘local –remote’ selector switch is in local position
and ‘individual –combined’ selector switch is in individual
Local Manual Control
The cranking device for manual operation for OLTC gear shall be
removable and suitable for operation by a man standing at ground level.
The mechanism shall be complete with the following.
Mechanical tap position indicator which shall be clearly visible
from near the transformer
A mechanical operation counter.
Mechanical stops to prevent over-cranking of the mechanism
beyond the extreme tap position.
The manual control considered as back up to the motor operated on
load tap charger control shall be interlocked with the motor to
block motor start-up during manual operation. The manual
operating mechanism shall be labelled to show the direction of
operation for raising the voltage and vice-versa.
Remote Electrical Group Control
The remove OLTC scheme offered shall have provision to raise or lower
taps only for the complete bank of three single phase transformers.
Individual single phase OLTC operation shall not be possible from the
remote control panel.
The OLTC control scheme shall have provision of remote electrical group
control when the three phase bank is in parallel operation with three phase
transformers/three phase banks of single phase transformers.
A four position selector switch having Master, Follower,
Independent and Off position shall be provided in the
remote OLTC control panel for three (3) phase bank of
single phase transformers. This shall be wired to enable the
operator to select of OLTC in either Master, Follower or
Independent mode.
Out of step relays with timer contacts shall also be provided
to give alarm and indication in case taps of all the
transformers under control are not in same position.
- 23 -
Master Position
If the selector switch is in master position, it shall be
possible to control the OLTC units of other transformer
banks/three phase transformer in the follower made by
operation the master unit.
Follower Position
If the selector switch is in Follower position control of
OLTC shall be possible only from panel where master
mode is selected.
Independent Position
In independent position of selector switch, control of
OLTC shall be possible only from the panel where
independent mode is selected.
The control circuits shall comply with following conditions:
An interlock to cut off electrical control automatically upon recourse being
taken to the manual control in emergency.
Reinforcement of the initiating impulse for a tap change, ensuring a
positive completion once initiated to the next (higher or lower) tap.
“Step-by-Step” operation ensuring only one tap change from each tap
changing impulse and a lock-out of the mechanism if the control switch
(or push button) remains in the “operate” position.
An interlock to cut-out electrical control when it tends to operate the gear
beyond either of the extreme tap positions.
An electrical interlock to cut-off a counter impulse for reverse step change
being initiated during a progressing tap change and until the mechanism
comes to rest and resets circuits for a fresh position.
Tap change in progress indication shall be provided by means of an
indicating lamp at the Employer’s control panel. Necessary contacts for
this and for remote tap position indicator at Employer’s control panel shall
be provided by the Bidder.
Protective apparatus, considered essential by the Bidder according to
specialties of the gear.
Local OLTC Control Cabinet, Cooler Control Cabinet and Remote Tap
Changer Control Panel.
Each single/three phase transformer unit shall be provided with local
OLTC control cabinet, cooler control cabinet and RTCC panel.
The sheet steel used for cooler control cabinet and local OLTC control
cabinet shall be at least 2.5mm thick. The degree of protection shall be
IP:55 in accordance with IS:13947. The gaskets used shall be of neoprene
- 24 -
rubber. All the separately mounted cabinets and panels shall be free
standing floor mounted type and have domed or sloping roof. All the
control cabinets shall be provided with suitable lifting arrangement.
A space heater, and cubicle lighting with ON-OFF switch shall be
provided in each panel.
Necessary shorting of terminals shall be done at the cooler control cabinet,
local OLTC cabinet and remote OLTC panel. All the CT secondary
terminals in the cooler control cabinet shall have provision for short
circuiting to avoid CT open circuit while it is not in use.
Cooler Control Cabinet
The cooler control cabinet shall have all necessary devices meant for
cooler control and local temp. indicators. All the contacts of various
protective devices mounted on the transformer and all the secondary
terminals of the bushing CTs shall also be wired upto the terminal board
in the cooler control cabinet. All the necessary terminals for remote
connection to Employer’s panel shall be wired upto the cooler control
The cooler control cabinet shall have two (2) sections. One sections hall
have the control equipment exclusively meant for cooler control. The
other section shall house the temperature indicators, aux. CTs and the
terminal boards meant for termination of various alarm and trip contacts as
well as various bushing CT secondary. Alternatively the two sections may
be provided as two separate panels depending on the standard practice of
the Bidder.
The temperature indicators shall be so mounted that the dials are about
1200mm from ground level. Glazed door of suitable size shall be provided
for convenience of reading.
Local OLTC Control Cabinet
The local OLTC control cabinet shall house all necessary devices meant
for OLTC control and indication. It shall be complete with the following:
A circuit breaker/contactor with thermal overload devices for
controlling the AC Auxiliary supply to the OLTC motor.
Cubicle light with door switch
Space heaters to prevent condensation of moisture
Padlocking arrangement for hinged door of cabinet
Cable terminal glands for power and control cables to the OLTC
- 25 -
Remote Tap Changer Control Panel
The Contractor shall supply a Remote Tap Changer Control (RTCC) panel
suitable for remote operation of On load tap changing gear.
The RTCC panel shall house actuating switch for electrical raise/lower
control, tap position indicator, signal lamps for “Tap Change in progress”
and “Tap Changer out of step” and all other auxiliary devices for remote
electrical control of the OLTC. For tap position indicators, the dual output
type OLTC transducer shall be provided in the RTCC panel. One of the
output of this transducer shall be used for local indication of tap position
in RTCC panel and other output (0-10mA or 4-20mA) shall be used for
The RTCC panel shall be located in Employer’s control room. The size &
colour of the RTCC panel shall match with Employer’s control panel. The
matching details shall be furnished at the time of award.
Auxiliary Power Supply of OLTC, Cooler Control and Power Circuit
Two auxiliary power supplies, 415 Volt, three phase four (4) wire shall be
provided by the Employer at cooler control cabinet for OLTC and cooler
control and power circuit.
All loads shall be fed by one of the two feeders through an electrically
interlocked automatic transfer switch housed in the cooler control cabinet
for on load tap changer control and cooler circuits.
Design features of the transfer switch shall include the following:
Provision for the selection of one of the feeder as normal
source and other as standby.
Upon failure of the normal source, the loads shall be
automatically transferred after an adjustable time delay to
standby sources.
Indication to be provided at cooler control cabinet for
failure of normal source and for transfer to standby source
and also for failure to transfer.
Automatic re-transfer to normal source without any
intentional time delay for following re-energization of the
normal source.
Both the transfer and the re-transfers shall be dead transfers
and AC feeders shall not be paralleled at any time.
Power Supply for OLTC Circuits
AC Feeder shall be brought to the local OLTC control cabinet by
the Contractor after suitable selection at cooler control cabinet for
- 26 -
which description sis given in 3.11.2 above, for control power
circuit of OLTC.
The Contractor shall derive AC power for OLTC control circuitry
from the AC feeder as mentioned above by using appropriately
rated dry type transformers. If the control circuit is operated by DC
supply, then suitable main and standby converters shall be
provided by the Contractor to be operated from AC power source.
Power Supply for Cooler Circuits
Control and power supplies are to be given for cooler circuits after the
selection as mentioned in 3.11.2 above.
The Contractor shall derive AC power for Cooler Control Circuitry from
the AC feeder as mentioned above by using appropriately rated dry type
transformer. If the control circuit is operated by DC supply then suitable
main and standby convertors shall be provided by the Contractor, to be
operated from AC power source.
Necessary isolating switches and HRC fuses shall be provided at suitable
points as per Employer’s approved scheme.
The following fittings shall be provided with each three phase transformer covered in
this specification:
Conservator for main tank with oil filling hole and cap, isolating valves, drain valve,
magnetic oil level gauge with low level alarm contacts and dehydrating breather.
Conservator for OLTC with drain vale, oil surge relay, filling hole with cap, prismatic
oil level gauge and silicagel breather.
Oil Preservation equipment
Pressure relief devices with alarm/trip contacts.
- 27 -
Buchholz relay double float/reed type with isolating valves on both sides, bleeding
pipe with pet cock at the end to collect gases and alarm and rip contacts.
Air release plug
Inspection openings and covers
Bushing with metal parts and gaskets to suit the termination arrangement
Winding temperature indicators for local and remote mounting. One remote winding
temperature indicator with a four point selector switch shall be provided for the three
windings for three phase unit to have selection of any of the three windings.
Cover lifting eyes, transformer lifting lugs, jacking pads, towing holes and core and
winding lifting lugs.
Protected type mercury or alcohol in glass thermometer.
Bottom and top filter valves with threaded male adaptors, bottom sampling valve and
drain valve.
Rating and diagram plates on transformers and auxiliary apparatus.
Flanged bi-directional wheels/trolley for movement
Cooler control cabinet
On load tap changing gear
Cooling equipment
Bushing current transformers
Oil flow indicator
Drain valves/plugs shall be provided in order that each section of pipe work can be
drained independently
Terminal marking plates
Valves schedule plates
The fittings listed above are only indicative and other fittings which generally are
required for satisfactory operation of the transformer are deemed to be included.
Inspection and testing
The contractor shall carry out a comprehensive inspection and testing
programme during manufacturer of the equipment. An indication of
inspection envisaged by the Employer is given under Clause 501. This is
however not intended to form a comprehensive programme as it is
Contractor’s responsibility to draw up and carry out such a programme in
the form of detailed quality plan duly approved by Employer for necessary
- 28 -
Tank and Conservator
Certification of chemical analysis and material tests of plates
Check for flatness
Electrical interconnection of top and bottom by braided tinned copper
Welder’s qualifications and weld procedure
Testing of electrodes for quality of base materials and coatings
Inspection of major weld preparation
Crack detection of major strength weld seams by dy penetration test.
Measurement of film thickness of:
Oil insoluble varnish
Zinc chromate paint
Finished coat
Check correct dimensions between wheels, demonstrate turning of
wheels through 900C and further dimensional check.
Check for physical properties of materials for lifting lugs, jacking pads,
etc. All load bearing welds including lifting lug welds shall be subjected
to NDT.
Leakage test of the conservator
Certification of all test results
Sample testing of core materials for checking specific loss, bend
properties, magnetization characteristics and thickness.
Check on the quality of varnish if used on the stampings:
Measurement of thickness and hardness of varnish on stampings
Solvent resistance test to check that varnish does not react in hot
- 29 -
Check over all quality of varnish by sampling to ensure uniform
shining colour, no bare spots, no over burnt varnish layer and no
bubbles on varnished surface.
Check on the amount of burrs
Bow check on stampings
Check for the overlapping of stampings. Corners of the sheet are to be
Visual and dimensional check during assembly stage.
Check for interlaminar insulation between core sectors before and after
Check on completed core for measurement of iron loss and check for
any hot spot by exciting the core so as to induce the designed value of
flux density in the core.
Visual and dimensional checks for straightness and roundness of core,
thickness of limbs and suitability of clamps
High voltage test (2 kV for one minute) between core and clamps
Certification of all test results.
Insulating Material
Sample check for physical properties of materials
Check for dielectric strength
Visual and dimensional checks
Check for the reaction of hot oil on insulating materials
Dimension stability test at high temperature for insulating material.
Tracking resistance test on insulating material
Certification of all test results
Sample check on winding conductor for mechanical properties and
electrical conductivity.
Visual and dimensional checks on conductor for scratches, dent marks
- 30 -
Sample check on insulating paper for pH value, bursting strength and
electric strength.
Check for the reaction of hot oil on insulating paper.
Check for the bonding of the insulating paper with conductor.
Check and ensure the physical condition of all materials taken for
windings is satisfactory and free of dust.
Check for absence of short circuit between parallel strands
Check for brazed joints wherever applicable
Measurement of voltage ratio to be carried out when core/yoke is
completely restacked and all connections are ready.
Conductor enamel test for checking of cracks, leakage and pin holes.
Conductor flexibility test
Heat shrink test for enameled wire
Certification of all test results
Checks before drying process
Check condition of insulation on the conductor and between the
Check insulation distance between high voltage connections, cables
and earth and other live parts.
Check insulating distances between low voltage connections and earth
and other parts.
Insulation of core shall be tested at 2 kV /minute between core to bolts
and core to clamp plates.
Check for proper cleanliness and absence of dust etc.
Certification of all test results
Checks during drying process
Measurement and recording of temperatures, vacuum and drying time
during vacuum treatment
Check for completeness of drying by periodic monitoring of IR and
Tan delta.
- 31 -
Certication of all test results
Assembled Transformer
Check completed transformer against approved outline drawings,
provision for all fittings, finish level etc.
Test to check effective shielding of the tank.
Jacking test with oil on all the assembled transformers
Dye penetration test shall be carried out after the jacking test.
Bought Out Items
The makes of all major bought out items shall be subject to Employer’s
The Contractor shall also prepare a comprehensive inspection and
testing programme for all bought out/sub-contracted items and shall
submit the same to the Employer for approval. Such programme shall
include the following components:a) Buchholz relay
Axles and wheels
Winding temperature indicators for local and remote
Oil temperature indicators
Bushing current transformers
Cooler control cabinet
Cooling equipment
Oil Pumps
Fans/Air Blowers
Tap Change gear
Terminal connectors
- 32 -
The above list is not exhaustive and the contractor shall also
include other bought out items in his programme.
Factory Tests
Routine Tests
All standard routine tests in accordance with IS:2026 with dielectric tests
corresponding to method 2 shall be carried out on each transformer.
Operation and dielectric testing of OLTC shall also be carried out as per
Following additional routine tests shall also be carried out on each
Magnetic Circuit Test
After assembly each core shall be tested for 1 minute at 2000Volts
between all bolts, side plates and structural steel work.
Oil leakage test on transformer tank as per Clause below.
Magnetic balance test
Measurement of no-load current with 45V, 50Hz ac supply on LV side.
Frequency response analysis (FRA)
High voltage withstand test shall be performed on auxiliary equipment
and wiring after complete assembly.
Type tests
Following type tests shall be conducted on one transformer of each rating:
Temperature Rise Test as per IS:2026 (Part-II)
Gas chromatographic analysis on oil shall also be conducted before and
after this test and values shall be recorded in the test report. The sampling
shall be in accordance with IEC 567. For the evaluation of the gas analysis
in temperature rise test the procedure shall be as per IS:9434 (based on
IEC:567) and results will be interpreted as per IS:10593 (based on IEC599).
Tank vacuum test as per Cl.No. (i) below.
Tank pressure test as per Cl. No. (ii) below.
- 33 -
Measurement of capacitance and tan delta to determine capacitance
between winding and earth.
Lightning impulse withstand test in all phases as per IS:2026 (As type
test, only for 220 KV class & below)
Additional type tests
Following additional type tests other than type and routine tests shall also
be carried out on one unit of each type:
Measurement of zero Seq. reactance (As per IS:2026, for 3-Phase
transformer only)
Measurement of acoustic noise level.
Measurement of power taken by fans and oil pumps.
Measurement of harmonic level in no load current
Measurement of transferred surge on LV (tertiary) winding due to HV
lightning impulse and IV lightning impulse.
Routine tests on bushings
The following tests shall be conducted on bushings
Test for leakage on internal fillings.
Measurement of creepage distance; dielectric dissipation factor and
Dry power frequency test on terminal and tapping
Partial discharge test followed by dielectric dissipation factor and
capacitance measurement.
Type tests on fittings:
All the following fittings shall conform to type tests and the type test
reports shall be furnished by the contractor along with the drawings of
equipment/fittings as per the clause no. 9.2 of the Section-GTR. The list of
fittings and type test requirement is:
- 34 -
1. Bushing ( Type test as per IS:2099/IEC:137)
2. Buchholz relay (Type test as per IS:3637 and IP-55 Test on terminal
3. OLTC (Temperature rise of contact, Short Circuit test, Mechanical test
and Dielectric test as per IEC:214 and IP-55 test on driving mechanism
4. Cooler control cabinet (IP-55 test)
5. Pressure relief device test
The pressure relief device of each size shall be subjected to increase in
oil pressure. It shall operate before reaching the test pressure specified in
transformer tank pressure test at Cl.No. (ii) below. The operating
pressure shall be recorded. The device shall seal off after excess pressure
has been released
The terminal box/boxes of PRD should conform to degree of protection
as per IP-55 of IS:13947.
6. Magnetic oil level gauge & terminal box for IP-55 degree of protection.
7. Air Cell (Flexible air separator)-oil side coating, Air side under coating.
Air side outer coating and coated fabric as per IS:3400/BS:903/IS:7016.
8. OTI & WTI- Switch setting & operation, switch differential, switch
9. Oil pump-Vacuum test at 250 torr maximum, oil pressure test at 1
kg/cm2 for 24 hrs. Temperature rie test by resistance method, IP-55
degree of protection for terminal box.
10. Cooling fan and motor assembly-Free air delivery, Temperature rise,
sound level, running at reduced voltage, IP-55 degree of protection for
terminal box.
Tank Tests
Routine Test
Oil Leakage Test
All tanks and oil filled compartments shall be tested for oil tightness by
being completely filled with air or oil a viscosity not greater than that of
insulating oil conforming to IS:335 at the ambient temperature and
applying a pressure equal to the normal pressure plus 35 KN/Sq.m (5 psi)
measured at the base of the tank. The pressure shall be maintained for a
period of not less than 12 hours for oil and one hour for air during which
time no leak shall occur.
Type Tests
Vacuum Test
One Transformer tank of each size shall be subjected to the specified
vacuum. The tank designed for full vacuum shall be tested at an internal
pressure of 3.33KN/Sq.m absolute (25 torr) for one hour. The permanent
- 35 -
deflection of flat plate after the vacuum has been released shall not exceed
the values specified below:
Horizontal length
of flat plate (in mm)
(in mm)
Upto and including 750
751 to 1250
1251 to 1750
1751 to 2000
2001 to 2250
2251 to 2500
2501 to 3000
Above 3000
Pressure Test
One transformer tank of each size, its radiator, conservator vessel and
other fittings together or separately shall be subjected to a pressure
corresponding to twice the normal head of oil or to the normal pressure
plus 25KN/m2 whichever is lower measured at the base of the tank and
maintained for one hour. The permanent deflection of flat plates after the
excess pressure has been released shall not exceed the figure specified
above for vacuum test.
Pre-Shipmet Checks at Manufacturer’s works
Check for interchangeability of components of similar transformers for
mounting dimensions.
Check for proper packing and preservation of accessories like radiators,
bushings, dehydrating breather, rollers, buchholz relay, fans, control
cubicle, connecting pipes, conservator etc.
Check for proper provision for bracing to arrest the movement of core and
winding assembly inside the tank.
Gas tightness test to confirm tightness.
Derivation of leakage rate and ensure the adequate reserve gas capacity.
Inspection and Testing at Site
The Contractor shall carry out a detailed inspection and testing programme
for field activities covering areas right from the receipt of material stage
upto commissioning stage. An indicative programme of inspection as
- 36 -
envisaged by the Employer is given below and in the document No.
OS/T&C/Bay/95 (Pre commissioning procedures and formats for
substation bay equipment), which will be available in the respective sites
and shall be referred by the contractor. However, it is contractor’s
responsibility to draw up and carry out such a programme duly approved
by the Employer. Testing of oil sample at site shall be carried out as per
Cl.3.4 above.
Receipt and storage checks
Check and record condition of each package, visible parts of the
transformer etc. for any damage.
Check and record the gas pressure in the transformer tank as well as in the
gas cylinder.
Visual check for wedging of core and coils before filling up with oil and
also check conditions of core and winding in general.
Check and record reading of impact recorder at receipt and verify the
allowable limits as per manufacturer’s recommendations.
Installation Checks
Inspection and performance testing of accessories like tap changers,
cooling fans, oil pumps etc.
Check the direction of rotation of fans and pumps
Check the bearing lubrication.
Check whole assembly for tightness, general appearance etc.
Oil leakage test
Capacitance and tan delta measurement of bushing before
fixing/connecting to the winding, contractor shall furnish these values for
site reference.
Leakage test on bushing before erection.
Measure and record the dew point of nitrogen in the main tank before
Commissioning Checks
Check the colour of silicagel in silicagel breather.
Check the oil level in the breather housing, conservator tanks, cooling
system, condenser bushing etc.
Check the bushing for conformity of connection to the lines etc.
Check for correct operation of all protection devices and alarms:
- 37 -
Buchholz relay
Excessive winding temperature
Excessive oil temperature
Low oil flow
Low oil level indication
Fan and pump failure protection.
Check for the adequate protection on the electric circuit supplying the
Check resistance of all windings on all steps of the tap changer, Insulation
resistance measurement for the following:
Control wiring
Cooling system motor and control
Main windings
Tap changer motor and control
Check for cleanliness of the transformer and the surroundings.
Continuously observe the transformer operation at no load for 24 hours.
Gradually put the transformer on load, check and measure increase in
temperature in relation to the load and check the operation with respect to
temperature rise and noise level etc.
Phase out and vector group test
Ratio test on all taps.
Magnetizing current test
Capacitance and Tan Delta measurement of winding and bushing
DGA of oil just before commissioning and after 24 hours energisation at
Frequency response analysis (FRA)
Contractor shall prepare a comprehensive commissioning report including
all commissioning test results and forward to Employer for future record.
- 38 -
Technical Parameters
Auto ii) 105 MVA
Rating –H.V./I.V.
i) 315 MVA/ 315 MVA
LV (Teritary)
105 MVA reactive and 5 ii) 35 MVA reactive &
MVA active
2 MVA active
Rating of different 60%/80%/100%
Type of transformer : Constant ohmic Impedance
type (Refer note I)
Voltage ratio
400/200 Kv
50 Hz
(Refer Note I)
i) HV-IV impedance
at 750C
- Max. Voltage Tap : 10.3%
- Principal Tap
- Min. Voltage Tap : 15.4%
ii) HV/LV impedance: 60%*
iii) IV/LV impedance:
(* impedance are minimum indicative only).
- 39 -
ii) 105 MVA/105 MVA
Overload capacity
As per IS: 6600-1972/IEC-354
Temperature rise over
500C Ambient Temp.
Of top oil measured by
Of winding measured by
resistance method
Max. design ambient temp
i) System fault level (kA)
ii) 1.2/50 micro sec.
impulse withstand voltage
iii) 20/200/500 micro
second switching
surge withstand voltage
iv) One minute power
frequency withstand
voltage kV (rms)
Winding connection :
Solidly grounded
Vector Group
Tap Changer
i)Tap Changer
ii)Tap Control
: Tap changer shall be located on the 220 KV side
of the series winding for achieving + 10% of HV
variation in the step of 1.25%.It shall be of constant
flux voltage variation type as be of constant flux
voltage variation type as per CI.3.2 of IS:2026 PartIV-1977.
: Full capacity –on load tap changer suitable for
group/independent, remote/local electrical and local
manual operation and bi-directional power flow.
- 40 -
iv) 250/2500 micro sec.switching:
impulse withstand vol.(kVp)
v) One minute power frequency :
withstand voltage kV (rms)
vi) Minimum. total creepage
distances (mm)
vii) Mounting
Tank Tank
cover cover
Partial discharge level
500 pico-columb
Noise Level
As per NEMA-TR-I.
i) Rated Voltage (kV)
ii) Rated Current (Amp)
iii) 1.2/50 micro sec. lightning
impulse withstand vol.(kVp)
Note: 1 In case of parallel operation with existing transformer, the impedance, OLTC
connection and range and the winding configuration (if necessary) is to be
The losses at principal tap of each transformer shall be stated in the tender. The
manufacture shall state separately no load, load and Auxiliary losses at rated
frequency & voltage and load losses corresponding to OFAF rating (at full load at
750C) for fair comparison of difference of losses. It is essential that tenderers
quote firm losses.
For the purpose of arriving at comparable prices, the iron, copper and auxiliary
losses shall be capitalized as per formula given below:
=3,86,700 (Wi + 0.45 Wc + 0.45Wp)
Iron (No. Load ) Losses
Wc =
Copper (Load) Losses
Wp =
Auxilary Losses
The maximum sealing of component and weightage average losses shall be as per
clause 8.0 and no positive tolerance shall be allowed on these values
- 41 -
In case losses of the T/F are found to be higher then the specified values at any
stage within three years from the date of commissioning the capitalized value of
excess losses, as compare to the specified value shall be recovered from the
performance bank guarantee as per following capitalization formula:Amount recoverable =
Rs. 3,86,700 (Wi + 0.45 Wc + 0.45Wp).
Losses:The losses of each transformer shall be stated in the tender for OFAF at principal
tap rating (at full load at 75 deg. C) which shall be less than:Sr. Type of Losses
315 MVA 400/220KV T/F
No load losses
86 KW (Max)
Load Losses
482 KW (Max)
Auxiliary Losses
16KW (Max)
overall average wt. losses (based on
279KW (Max.)
capitalisation formula of Wi+0.45
Bushing Current Transformer
Current Transformers shall comply with IS:2705/IEC-185.
It shall be possible to remove the turret mounted current transformers from the
reactor tank without removing the tank cover. Necessary precautions shall be
taken to minimize eddy currents and local heat generated in the turret.
Current transformer secondary leads shall be brought out to a weatherproof
terminal box near each bushing. These terminals shall be wired out to cooler
control cabinet/marshalling box using separate cables for each core.
Bushing current transformer parameters indicated in this specification are
tentative and liable to change within reasonable limits. The contractor shall obtain
Employer’s approval before proceeding with the design of bushing current
Technical Parameters
9.5.1 Current transformer parameters for 3-ph 315 MVA 400/220 KV
On each phase connection
Minimum knee point voltage and accuracy class:
Class PS
Class PS
Class PS
0.2 Class
0.2 Class
30 VA ISF< 5
30 VA ISF< 5
- 42 -
Maximum CT Resistance
(Near the wdg)
earth fault
earth fault
earth fault
e) Maximum magnetisation Core-1 60mA
current (at knee point voltage)Core-2
9.6.1 Accuracy class PS as per IS: 2705
9.6.2 Class (for the relevant protection and duties) as per IEC 185.
Parameters of WTI CT for each winding shall be provided by the contractor.
Oil Storage Tank
This specification covers supply of oil storage tank of 15 cubic meter capacity
along with complete accessories.
The oil storage tank shall be designed and fabricated as per relevant Indian
Standards e.g. IS:803 or other internationally acceptable standards.
Transformer oil storage tanks shall be towards & rested on pneumatic tyres of
adequate quantity & size. The tank shall be to cylindrical shape & mounted
horizontally and made of mild steel plate of adequate thickness. Size of the
storage tank shall be as follows:
2.5 meter
15 cubic metre
The tank shall be designed for storage of oil at a temperature of 1000C.
10.3.1 The Bidder may further note that maximum height of any part of the complete
assembly of the storage tank shall not exceed 4.0 metres above road top.
10.3.2 The tank shall have adequate number of jacking pad so that it can be kept on jack
while completely filled with oil. The tank shall be provided with suitable saddles
so that tank can be rested on ground after removing the pneumatic tyres.
- 43 -
10.3.3 The tank shall also fitted with manhole, outside & inside access ladder, silicagel
breather assembly, inlet & outlet valve, oil sampling valve with suitable adopter,
oil drainage valve, air vent etc. Pulling hook on both ends of the tank shall be
provided so that the tank can be pulled from either end while completely filled
with oil. Bidder shall indicate the engine capacity in horse power to pull one tank
completely fitted with oil. Oil level indicator shall be provided with calibration in
terms of litre so that at any time operator can have an idea of oil in the tank.
Suitable arrangement shall also be provided to prevent overflow in the tank. Four
nos. suitable rubber hoses with couplers and unions each not less than 10 metre
long shall also be provided.
10.3.4 The internal & external surfaces to be painted shall be shot or sand blasted to
remove all rust and scale of foreign adhering matter or grease. All steel surfaces
in contact with insulating oil shall have painted with two coats of heat & oil
resistant anti-corrosive paint.
All steel surfaces exposed to weather shall be given a primary coat of zinc
chromate, second coat of oil & weather resistant paint of a colour distinct from
primary and final two coats of glossy oil & weather resistant light grey paint in
accordance with shade no. 631 of IS:5. All paints shall be carefully selected to
withstand heat & extremes of weather. The paint shall not scale off or crinkle or
be removed by abrasion due to normal handling. The minimum thickness of
outside painting of tank shall be 20 microns per coat the total thickness shall be
within 70 to 100 microns.
10.3.5 The tank shall contain a self mounted centrifugal oil pump with inlet and outlet
valves, with couplers suitable for flexible rubber hoses and necessary switchgear
for its control. There shall be no rigid connection to the pump. The pump shall be
electric motor driven, and shall have a discharge of not less than 6.0kl/hr. with a
discharge head of 8.0m. The pump motor and the control cabinet shall be
enclosed in a cubical with IP-55 enclosure.
11.1 Oil sampling bottles shall be suitable for collecting oil samples from transformers
and shunt reactors, for Dissolved Gas Analysis. Bottles shall be robust enough, so
that no damage occurs during frequent transportation of samples from site to
Oil sampling bottles shall be made of stainless steel having a capacity of 1 litre.
Oil Sampling bottles shall be capable of being sealed gas-tight and shall be fitted
with cocks on both ends.
The design of bottle & seal shall be such that loss of hydrogen shall not exceed
5% per week.
- 44 -
An impermeable oil-proof, transparent plastic or rubber tube of about 5mm
diameter, and of sufficient length shall also be provided with each bottle along
with suitable connectors to fit the tube on to the oil sampling valve of the
equipment and the oil collecting bottles respectively.
- 45 -