Detail Considerations for HV/EHV power transformers design and assembly

DESIGN:

• The power transformer shall be used for bi-directional flow of rated power. The transformer and accessories shall be designed to facilitate inspection, cleaning and repairs and for operation where continuity of supply is the primary consideration. All apparatus shall be designed to ensure satisfactory operation under sudden variations of load and voltage as may be met with under working conditions of the system including those due to short circuits.

• All materials used shall be of the best quality and of the class most suitable for working under the conditions specified and shall withstand the variations of temperatures and atmospheric conditions arising under working conditions without inner distortion or deterioration or setting up of undue stresses in any part & also without affecting the strength and suitability of the various parts for the work which they have to perform.

• All outdoor apparatus, including bushing insulators with their mountings, shall be so designed as to avoid pockets in which water can collect.

• All connections and contacts shall be of ample cross-sections and surfaces for carrying continuously the specified current without undue heating and fixed connections shall be secured by bolts or set screws of ample size, adequately locked. Lock nuts shall be used on stud connection carrying current.

• The transformer 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 circuits.

• 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-I.

• The transformer shall be capable of being loading in accordance with IS:6600/IEC-354. There shall be no limitation imposed by bushings, tap changers etc. or any other associated equipments.

• The transformer and all its accessories including CTs etc shall be designed to withstand without any 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 and LV system to which the subject transformer will be connected

• Transformer shall be capable of withstanding thermal and mechanical stresses caused by symmetrical or asymmetrical faults on any winding.

TANK:

• The transformer tank and cover or BELL type tank shall be fabricated from good commercial grade low carbon steel suitable for welding and of adequate thickness. The thickness of each side plate shall be indicated in GTP.

• The tank and the cover shall be of welded construction. All seams shall be welded and where practicable they shall be double welded.

• The tank wall shall be reinforced by stiffener of structural steel for general rigidity.

• The tank shall have sufficient strength to withstand without permanent distortion (i)filling by vacuum, (ii) continuous internal gas pressure of 0.35 atmospheres with oil at operating level and (iii) mechanical shock during transportation.

• The tank cover shall be bolted to the tank and the transformer design shall be such that the tank will not be split between the lower and upper cooler connection for untanking. The tank covers shall be fitted with pockets at the position of maximum oil temperature corresponding to MCR (Maximum Continuous Rating) for RTD sensors and bulbs of oil and winding  temperature indicators. It shall be possible to remove these sensors bulbs without lowering the oil in the tank. The tank wall penetrations shall be leak proof, suitably marked with respective sensor identification.

• A man-hole with a welded flange and a bolted cover shall be provided on the tank cover. The man-hole shall be of a sufficient size to ease access to the lower ends of the bushings, terminals etc.

• All bolted connections to tank shall be fitted with suitable oil-tight gasket, which shall give satisfactory service under the operating conditions. Special attention shall be given to the methods of making the hot oil-tight joints between the tank and cover as also between the cover and the bushings and all other to ensure that the joints can be remade satisfactorily and with ease, with the help of semi-skilled labours. Where compressible gaskets are used, steps shall be provided to prevent over compression. Bushings, turrets, cover of accessories, holes and other devices shall be designed to prevent any leakage of water into or oil from the tank. There should not be any leakage at least for three years and this should be guaranteed. All the gaskets to be provided shall be of RC70C or RC80C grade. Necessary tests certificates from manufacturer shall be submitted along with acceptance test report. The gasket to be used shall not be older than One year.

• Suitable guides shall be provided for positioning the various parts during assembly or dismantling. Adequate space shall be provided between the covers and windings and the bottom of the tank for collection of any sediment.

• Lifting eyes or lugs shall be provided on all parts of the transformer requiring independent handling during assembly or dismantling. In addition, the transformer tank shall be provided with lifting lugs and bosses properly secured to the sides of the tank for lifting the transformers either by crane or by jacks.

• The design of the tank, the lifting lugs and bosses shall be such that the complete transformer assembly filled with oil can be lifted with the use of those lugs without any damage or distortions.

• The tank shall be provided with two suitable copper alloy or any other suitable material lugs for the purpose of grounding.

• The tank shall be equipped with the following valves with standard screw connection for external piping. All valves up to and including 100 mm shall be of GM and larger valves shall be of Cast Iron bodies with GM fittings. They shall be of full way type with internal screw and shall open when turned counter clock wise when facing the hand wheel, along with suitable locking in open and close positions.

• One drain valve of adequate size with eccentric reducer and flange, located on the low voltage side of the transformer. This valve shall be equipped with a small sampling cock. The draining valve must be at bottommost location of the tank.

• One filter valve of adequate size with eccentric reducer and flange, located at the top of tank on the high voltage side. The opening of this valve shall be baffled to prevent airation of oil.

• One filter valve of adequate size with eccentric reducer and flange, located on the high voltage side of the transformer above the bottom of the tank.

• Suitable valves shall be provided to take sample of oil from the OLTC chamber during operation of transformer.

• A valve of other suitable means shall be providing to fix the on line dissolved Gas monitoring system to facilitate continuous dissolved gas analysis. Location and size of the same shall be finalized during detailed engineering.

• Pressure relief valve of adequate size & number/s shall be provided on main tank as well as for OLTC.

• All hardware used shall be cadmium plated / electro galvanised.

• Necessary provision for installation of On Line monitoring system,shall be made for satisfactory performance through out the life of transformer. Location and size of the same shall be finalized during detailed engineering.

UNDER CARRIAGE:

• The transformer tank shall be supported on a structural steel base equipped with forged steel single flanged wheels suitable for moving the transformer completely with oil.

• Jacking pads shall be provided. It shall be possible to change the direction of the wheels through 900 when the transformer is lifted on jacks to permit movement of the transformer both in longitudinal and transverse direction. A standard track gauge (preferably 1676 mm) in both longitudinal and transverse directional shall be chosen.

• Pulling eyes shall be provided to facilitate movement of transformer and they shall be suitably brazed in a vertical direction so that bonding does not occur when the pull has a vertical component.

CORE:

• The transformer may be of core or shell type. The core shall be built up with high-grade non-ageing cold rolled grain oriented silicon steel laminations having high permeability and low hysterisis loss. The core material shall be procured directly from manufacturer or through accredited marketing organization of reputation.

• The thickness of lamination shall be 0.27 mm or less. Surface insulation of laminations shall be rust resistant and have high inter laminar resistance. Insulation shall withstand annealing temperature as high as 850 0C. Insulation shall be resistant to hot cooling medium. Laminations are not to be punched.

• The manufacturer should have in house core cutting facility for proper monitoring & control on quality & also to avoid any possibility of mixing of prime material with defective/second grade material. This should be indicated invariably in the QAP. The purchaser may witness the core-cutting process. In case the in-house core cutting facility is not available

• The manufacturer will offer the core for stage inspection and get approval from purchaser during manufacturing stage. The Manufacturer has to produce following documents at the time of stage inspection for confirmation of use of prime core materials.

1. Invoice of supplier

1. Mills of approved test certificates

1. Packing list

1. Bill of lading

1. Bill of entry certificate by custom.

1. Second grade/ defective material

1. Only after the inspection and approval from purchaser, the core material will be cut in-house OR sent to external agency for cutting individual laminations. In case the core is sent to external agency for cutting, manufacturer representative will have full access to visit such agency for the inspection of the cutting of core.

• After being sheared, the laminations shall be treated to remove all burrs and shall be re-annealed to remove all residual stresses. The insulation of the lamination shall be insert to the action of hot transformer oil. Paper and varnish insulation will not be accepted. The nature of insulation should be specified in the tender.

• The core shall be rightly clamped to ensure adequate mechanical strength and to prevent vibration during operation. The clamping structure shall be so constructed that eddy currents will be minimum.

• The core shall be provided with lugs suitable for lifting the complete core and coil assembly of the transformer.

• The core and the coil assembly shall be so fixed in the tank that shifting will not occur when the transformer is moved or during a short circuit.

• The transformer shall be designed in such a way that the flux density in the steel core corresponding to the Rated voltage and the rated frequency shall be not exceeding 1.727 tesla.

• Core and Frame terminals should be brought out on transformer top so as to enable meggaring.

• The core and the coil assembly shall be so fixed in the tank that shifting will not occur and cause any damage when the transformer is moved shifted, or during a short circuit. The maximum flux density in any part of core or yoke at 10% continuous over voltage condition shall not exceed 1.9 tesla.

• The complete core and core coil assembly of bolt less core type transformer shall be so assembled that the axis and the plate of outer surface of the coil stack shall not deviate from the vertical plane by more than 25 mm.

• In case transformer with variable flux, the voltage variation which would affect flux density at every tap shall be kept in view while designing the transformer.

• Transformers shall be designed to withstand the following over fluxing conditions:

1. 110 % of maximum flux density corresponding to rated voltage Continuous for all transformers

1. 125 % & 140 % of max. flux density corresponding to rated voltage for 1 minute and 5 sec. respectively

1. Air core reactance of HV winding shall not be less than 20% and minimum knee point voltage shall not be less than 1.1 p.u.

WINDING:

• The conductor for winding shall be of electrolytic grade copper. The winding shall be so designed that all coil assemblies of identical voltage ratings shall be interchangeable and field repairs can be readily done, without special equipment. The coils shall be supported between adjacent sections by insulating spacers and the barriers, bracings and other insulation used in the assembly of the windings shall be arranged to ensure a free circulation of the oil and to reduce hot spots in the windings.

• The insulation paper shall be of high quality and the value of degree of polymerization shall not be less than 1200 Pv and the necessary test certificate shall be submitted along with the stage inspection report. Provision shall be made  in the tank, for taking sample, in future, of paper for testing purpose and location shall be easily accessible and indicated on the transformer tank by affixing special caution plate.

• The insulation of the coils shall be such as to develop the full electrical strength of the windings. All materials used in the insulation and assembly of the windings shall be insoluble, non-catalytic and chemically inactive in the hot transformer oil and shall not soften or otherwise be adversely affected under the operating conditions.

•  All threaded connections shall be provided with locking facilities. All leads from the winding to the terminal board and bushings shall be rigidly supported to prevent injury from vibration. Guide tubes shall be used where practicable.

• The windings shall be clamped securely in place so that they will not be displaced or deformed during short circuits. The assembled core and windings shall be vacuum dried and suitably impregnated before removal from the treating tank.

• The copper conductors used in the coil structure shall be best suited to the requirements and all permanent current carrying joints in the windings and the locks shall be welded or brazed.

• Windings shall be subjected to a shrinkage treatment before final assembly, so that no further shrinkage occurs during service. Adjustable device shall be provided for taking up any possible shrinkage of coils in service if required.

• The conductor shall be transposed at sufficient intervals in order to minimize eddy currents and equalize the distribution of currents and temperature along the windings.

• The tapping winding shall be provided separately from main winding to minimize the out of balance forces in the transformer at all voltage ratios.

• Transformer shall be designed and constructed to withstand, without damage, the thermal effects on external short circuits (SC) for 3 seconds under conditions

• Manufacturer shall invariably indicate, the cross sectional area of all windings with respect to the current density adopted.

• Manufacturer shall have to submit the calculations for thermal & dynamic ability to withstand short circuits.

• The cooling calculations shall be submitted with technical bid.

• Fiber optic sensors shall be embedded in each phase of the winding located where the temperature is highest. The location and details shall be indicated in the respective drawings.

• Tertiary Windings:

• 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.

• The Tertiary winding shall be designed to withstand mechanical and thermal stresses due to dead short circuit on its terminals.

• The tertiary winding shall be suitable for connection to LT Transformer for auxiliary supply.

• INSULATING OIL:

• The oil for first filling together with 10% extra shall be supplied with each transformer. Particular attention shall be paid to deliver the oil free from moisture having uniform quality throughout. The oil may be supplied either in sealed tanker, or in non-returnable sealed steel drums, which will be opened at site

• The supplier of transformer shall furnish test certificates of the insulating power oil supplied against their acceptance norms, prior to dispatch.

• Subsequently oil samples shall be drawn At manufacturer’s works before and after heat run test and shall be tested for following:

1. BDV in kVrms

1. Moisture content

1. Dissolved Gas Analysis – samples for DGA shall be taken from sampling device within 24 hrs prior to commencement of heat run test and immediately after this test. The acceptance norms shall be as per IS:10593 (based on IEC-599)

• prior to filling in main tank at site and shall be tested for BDV and moisture content and Corrosive sulphur detection test as per ASTM D1275 subjecting oil for 150 0C for 48 hrs for acceptance norms

• prior to energisation at site and shall be tested for the following:

• BDV in kVrms

• Moisture content

• Tan Delta at 90 deg cen.

• Resistivity at 90 deg cen.

• On Line Moisture and Gas In Oil Analyser For New Transformer With Model Analysis Software And Remote Data Transfer/Communications through internet shall be provided as per Technical Specifications.

INSULATION:

• The dielectric strength of winding in insulation & of the bushings shall conform to the values given by manufacturer.

• The partial discharges in the transformer at the time of dispatch shall not be more than 100 pC at 1.5 p.u.

• The Maximum Limit of value of tan delta at 20 0C shall be 0.5% for windings, 0.4% for bushings and 0.2 % for oil.

• The HV/MV winding of the transformer shall have graded insulation. The LV winding of transformer shall have full insulation. The insulation class of the neutral end of the windings shall be graded to 95 kV (Impulse) and 38 kV(Power frequency) withstand.

TEMPERATURE RISE:

• The transformer shall be installed out-door without any protection from sun and rain. The maximum hot spot temperature rise shall be limited to 105° C with Class - A insulation. Each transformer shall be capable of operating continuously at its normal rating without exceeding the temperature rise limits specified as under:

• Winding (measured by resistance) Temp. rise in 0 C ONAN 55

• ONAF / ODAF 55

• Top oil (measured by thermometer). Temp. rise in 0 C ONAN 50

• ONAF / ODAF 50

• Cores Not to exceed that permitted for the adjacent part of the winding.

• NOTE: The reference subject temperature for the purpose of temp. rise shall be 50 0 C. The gradient in temperature between phases shall not be more than 10 oC. Heat flow diagram shall be submitted by successful Manufacturer.

• The transformer will deliver rated current without exceeding temperature rise when operating on 105% of the rated voltages.

• The transformer shall be capable of being operated without danger on any tapping at the rated MVA with voltage of ±10% corresponding to the voltage of that tapping.

FREQUENCY:

• The transformer shall be suitable for continuous operation with a frequency variation of + 3 % from normal of 50 Hz without exceeding the specified temperature rise.

PARALLEL OPERATION:

• The similar ratio transformers shall operate satisfactorily in parallel with each other if connected between high voltage and low voltage bus-bars. Also, wherever specified, the transformers shall be suitable for parallel operation with existing transformers. The details of existing transformers will be provided.

IMPEDANCES:

• Supplier shall indicate the guaranteed impedance and tolerances and also the upper and lower limits of impedances, which can be offered. Impedance shall include positive and zero sequence and shall be expressed in terms of the branches of the star connected equivalent diagrams, all on the same KVA base and the range shall be for each branch of the equivalent circuit in turn. The transformer impedances shall be as specified by the manufacturer for approval.

TAP CHANGING MECHANISM:

ON LOAD TAP CHANGER:

• Each transformer shall be provided with on load tap charging mechanism.This shall be designed for remote control operation from switchboard in the control room. In addition the tap changer shall include the followings:

1. An oil immersed tap selector and arcing switch for arc suppressing tap selector, provided with reactor of resistor for reduction of make & break arcing voltage and short circuits.

1. Motor driven mechanism.

1. Control and protection devices.

1. Local tap changer position indicator.

1. Manual operating device.

1. Pressure relief device

• The on load tap changer shall be so designed that the contacts do not interrupt arc within the main tank of the transformer. The tap selector and arcing switch or arc suppressing selector switch shall be located in one or more oil filled compartments. The compartment shall be provided with a means of releasing the gas produced by the arcing. It shall be designed so as to prevent oil in the tap selector compartment from mixing with the oil in the transformer tank. A Buchholtz relay shall be provided to indicate accumulation of gas and alarm thereof.

• The tap changer shall be capable of permitting parallel operation with other transformer of the same type.

• The transformer shall give full load output on all taps. The manual operating device shall be so located on the transformer that it can be operated by an operator standing at the level of the transformer track. It shall be strong and robust in construction.

• The control scheme for the tap changer shall be provided for independent control of the tap changers when the transformers are in independent service. In addition, provision shall be made to enable parallel control also at times so that the tap changers will be operated simultaneously, when one unit is in parallel with another so that under normal condition the tap charger will not become out of step and this will eliminate circulating currents.

• Additional features like master, followers and visual indication during the operation of motor shall also be incorporated.

• Necessary interlock blocking independent control when the units arc in parallel shall be provided.

• Under abnormal conditions such as may occur, if the contactor controlling one tap changer sticks, the arrangement must be such as to switch off supply, to the motor so that an out of step condition is limited to one tap difference between the units. Details of out of step protection provided for the taps should be furnished in the tender.

• The contactors and associated gear for the tap change driving motors shall be housed in a local kiosk mounted adjacent to or on the transformer.

• In addition to the above equipment, the supplier shall supply a separate panel for installation in purchaser’s control room for remote operation with the following accessories.

1. Raise and lower push Button switch.

1. Remote tap position Indicator of digital type, device for indicating ‘ON’ & ‘OFF’ of Fan / Motor / Pump of cooler control.

1. Microprocessor based Annunciation

1. Out of step relay and indication.

1. Name-plate for each component.

1. An alarm indication lamps showing tap changing in progress.

• RTCC panel shall be compatible to SCADA operation.

• Any other accessory required for satisfactory operation or required during detail engineering.

• RTCC panel shall be either front or rear door opening. The requirement shall be informed during detailed engineering.

• Complete particulars of the tap changing gear including the capacity of the motor shall be stated in the tender.

• Tap changer shall be suitable for bidirectional power flow. The tap changer rating shall be more than maximum rated current of transformer.

Manual control

• 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 following:

1. Mechanical tap position indicator which shall be clearly visible

1. A mechanical operation counter

1. Mechanical stops to prevent over-cranking of the mechanism beyond the extreme tap position

1. The manual control considered as back up to the motor operated load tap changer control shall be interlocked with the motor to block motor start-up during manual operation.

1. The manual operation mechanism shall be labeled to show the direction of operation for raising the HV terminal voltage and vice-versa

Automatic Voltage Regulating Relays :

The AVR relay shall be provided, if asked in particular bid

• 
   The scheme shall detect the following:

• failure of auxiliary supply,

•  failure of PT supply and (iii) failure of mechanism to complete the tap changing operation.

• The relay shall have necessary contacts to be connected to the alarm & / or to the Annunciator available in the panel for visual and audible indication of the failure of trip circuit.

• The AVR relay shall be compatible to SCADA operation of any make.

• All the necessary wiring shall be carried out in RTCC panel and schematic drawings shall be submitted with the technical bid and during detailed engineering for approval in duplicate.

OIL PRESERVING EQUIPMENT:

• Air cell type conservator tank is to be provided for oil conservator system.

• Manufacturer shall offer diaphragm type oil sealing in the conservator to prevent oxidation and contamination of oil due to contact with water. In this type of oil preservation system, conservator shall be fitted with a dehydrating filter breather.

• In this system, using a flexible Diaphragm shall prohibit contact of oil with atmosphere or nitryle rubber reinforced nylon cloth air cell.

• Diaphragm used shall be suitable for continuous operation in an atmosphere of 100 0C to which transformer oil is likely to rise.

• The connection of the air cell to the top of the reservoir shall be by an air proof seal permitting entrance of air into the cell only.

• The diaphragm of the conservator shall withstand the vacuum during installation and maintenance. Otherwise provision shall be made to isolate the conservator from main tank during vacuum by providing vacuum sealing valve in the pipe connecting the main tank with the conservator.

BUSHINGS:

• The bushings shall have high factor of safety against leakage to ground and shall be so located as to provide adequate electrical clearances between bushings and grounded parts. Bushings of identical voltage rating shall be interchangeable.

• All bushings shall be equipped with suitable terminals of approved type and size and shall be suitable for bimetallic connection.

•  The insulation class of the high voltage neutral bushing shall be properly coordinated with the insulation class of the bushings of the high voltage winding.

• Each bushing shall be so coordinated with the transformer insulation that all flash over will occur outside the tank.

• All main winding and neutral leads shall be brought out through out door type bushings which shall be so located that the full flashover strength will be utilized and the adequate phase clearance shall realized.

• All porcelain used in bushings shall be of the wet process, homogeneous and free from cavities or other flaws. The glazing shall be uniform in colour and free from blisters, burrs and other defects.

• The bushings for 66 kV and above shall be of the oil filled condenser type (hermetically sealed)

• The characteristics of the oil used in the bushings shall be the same as that of the oil in the transformer.

• All bushings shall have puncture strength greater than the dry flashover value.

• Main terminals shall be solder-less terminals and shall be suitable for ACSR ”Moose” Conductor. The spacing between the bushings must be adequate to prevent flashover between phases under all conditions of operation.

• Special adjustable arcing horns may also be provided for the bushings

• The Manufacturer shall give the guaranteed withstand voltages for the above and also furnish a calibration curve with different settings of the co-ordination gap to the purchaser to decide the actual gap setting. Manufacturer’s recommendations are also invited in this respect.

• Bushing CTs should be provided for REF protection

• The tan delta and capacitor measurement tap shall be provided.

COOLING:

AIR BLAST, FORCED COOLED OIL TRANSFORMERS:

• Unit cooler arrangement for transformer:

• Design of cooling system shall satisfy the performance requirements.

• Each Unit Cooler shall have its own cooling fans, oil pumps, oil flow indicator, shut off valves at the top and bottom of at least 80 mm 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.

• 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 conform to IS: 325/IEC34. Each cooling fan 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/IEC:34-5

• The cooler and its accessories shall preferably be hot dip galvanised or corrosion resistant paint should be applied to it.

• Expansion joint shall be provided on top and bottom cooler pipe connections as per requirement.

• 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.

• Cooling Equipment Control (OFAF or ODAF) Cooling Suitable manual control facility for unit cooler shall be provided.

• The changeover to standby unit cooler bank oil pump in case of failure of any service unit cooler 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 unit cooler manually.

• Cooler fans & oil pumps of all unit coolers (except standby cooler) shall operate continuously.

• The starting of unit cooler shall be done as soon the Circuit Breaker of HV/IV/LV side is switched on.

• Once started the cooling shall remain in operation as long as the transformer is in service. When the transformer is switched off the cooling shall continue to run for a further duration of 30 minutes .This timer shall be at least adjustable from 15 to 60 minutes.

• Starting the pumps on load shall provide the cooling system a lead on the temperature that is about to follow during high loading conditions. Spurious operation should however be avoided by appropriate settings.

• Adequate warning/ safety labels are required to indicate that the fans may start at any time.

• All settings shall be adjustable.

• If any one group(s) is out of service and isolated, this shall not affect the automatic starting of the other unit cooler.

• Indicating Devices

• Following lamp indications shall be provided in cooler control cabinet:

• Control Supply failure.

• Cooling fan failure for each bank.

• Cooling pump failure for each pump.

• Common thermal overload trip

• Cooler supply changeover.

• Cooler Supply failure (standby).

• Cooler unit failure for each unit cooler

• No oil flow/reverse oil flow for pumps.

• Thermal overload trip for each fan / pump.

• One potential free initiating contact for all the above conditions shall be wired independently to

• the terminal blocks of cooler control cabinet.

Cooling Equipment Control:

• 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 changeover of cooler control over entire cooling option. 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 manually.

• The fan circuit shall be operated at different Temperature for group-I fans and group-II fan from WTI.

• The pump circuit shall be also operated from same WTI contacts at diff. Temperature and Fiber optic temperature sensors.

• The connection shall be with stud type terminal blocks and CT connector shall be of link type so that it can be shorted for testing or checking circuit.

• Materials for cooler unit:

• Tubes : Copper

• Fins : Copper

• Tube plate : Copper

• O-ring gasket NBR 70°

• Header Steel, coated with RILSAN (polyamide 11)

• Flange connection DIN 2633 or ANSI B16.5 150 Lb

• Drain/venting plug Acc. to DIN 933

• Washer Copper

• Casing and framework Hot dipped galvanized steel

• Cellular rubber gasket EPDM

• Fan casing Aluminum

• Impeller blade Aluminum

• Impeller hub Steel, epoxy-polyester powder painted

• Safety switch Plastic housing IP 54 / IEC 947-3

• Electric motor IP 55 / IEC 34-1 with drain holes

• Fan guard Steel wire, zinc-manganese phosphated and epoxy painted

• The unit cooler shall be attached to and mounted on the transformer tank.

• ONAN / ONAF / OFAF cooled transformers shall be designed to operate at no load for 4 hours without any cooler unit in service. ONAN / ONAF cooled transformers shall also be capable of delivering its rated MVA for 20 minutes with the loss of oil cooling equipment while the transformer is carrying full load.

• The cooling fan shall be operated at 2 sets of temperatures for fan GR I & II.

• The oil pump shall be also operated by WTI and Fiber optic temperature sensors.

• The Manufacturer shall specify the loading of the transformers in case of failure of one or more set of fans or pumps.

• In case of ONAN / OFAF cooled transformers, provision of automatic changeover from main supply to stand by auxiliary supply should be available in case of failure of main supply. Necessary alarm etc. for this may also be included.

CENTRE OF GRAVITY:

The centre of gravity of the assembled transformer shall be low and as near the vertical centerline as possible. The transformer shall be stable with or without oil.

ACCESSORIES:

Each transformer shall be provided with the following accessories. 


• Dial Image sensing type mercury free thermometers for ONAN / ONAF and ONAN / OFAF Transformers

• In case of ONAN / ONAF or ONAN / OFAF type transformers, it shall automatically actuate the fans / pump also.

• Accuracy class of WTI shall be +/- 1.5% or better.
• The dial type indicating thermometers of robust baton mounted on the side of the transformer at a convenient height to read temperature in the hottest part of the oil and fitted with alarm and trip contacts and contacts for switching in and switching out the cooling system at pre-determined temperatures.
• A dial Image sensing type mercury free thermometer for indicating oil temperature fitted with maximum pointer and adjustable alarm and trip contacts.
• The OTI shall be compatible for remote SCADA operation.
• On winding hot spot thermometer detector in one winding of each phase

• Any special cable required for shielding purpose, for connection between cooler control cabinet and remote WTI control circuit, shall be in the scope of supplier. Only one RWTI with a four point selector switch shall be provided for all the windings.

• The WTI shall be compatible for remote SCADA operation.

• One magnetic type oil level gauge with low alarm contacts and dial showing minimum, maximum and normal oil levels. The gauge shall readable from the transformer base level. A low gas pressure electric alarm device shall also be provided, if the transformer is equipped with inert gas pressure equipment.

• One oil-filling valve (inlet).

• One oil drain valve.

• One filter valve located at the top of tank on the HV side.

• One filter valve located near the bottom of tank of the HV side of the transformer.

• Oil sampling devices.

• Pressure relief device: Sudden/Rapid pressure rise release relay

• A safety valve of the chimney type with an equalizer pipe interconnecting the top of the conservator and upper most part of the safety valve should be provided to prevent rise of oil in the safety valve, pipe.

• A stopcock should be provided in the inter-connecting pipe. An air release cock shall also be fitted in convenient position. The safety valve pipe shall preferably take off from the side of the transformer tank near to the tank cover and not from top of tank cover.

• A Buchholz relay with alarm and tripping contacts to detect accumulation of gas and sudden changes of oil pressures, complete with two shut-off valves and flange coupling to permit easy removal without lowering oil level in the main tank, a bleed valve for gas venting and test valve. The relay shall be provided with a test cock suitable for a flexible pipe connection for checking its operation & taking gas sample. A copper or Stainless Steel tube shall be connected from the gas collector to a valve located at 1200 mm above ground level.

• Grounding terminals; one on each side of transformer.

• Diagram and rating plate.

• One set of equipment for control, protection, indication and annunciation for each transformer comprising motor contactors, detecting elements or devices, indicating apparatus, instruments relays, annunciations etc.

• Suitable weatherproof cubicle for housing the control equipment, terminals blocks etc, (one for each transformer) and one indoor cubicle for each transformer for remote control of unit coolers, on load tap changer alarm and indicating devices.

• Dehydrating Filter Breather for OLTC conservator. Silica gel breather to be fitted with conservator shall be designed such that:

1. Condition controlled maintenance free dehydrating breather for main conservator:
2. Each Silica gel breather shall be equipped with a humidity sensor, a condition based microprocessor control unit and LED status indication. The function shall be tested via a test button. 
3. A stainless steel filter at the bottom shall protect the silica gel chamber against external environment influences.
4. Dehydrating breathers work according to the following principle. When the oil conservator suctions in air (e.g., due to the reduced load), the air flows through a filter made of high-grade steel wire mesh to the inside of the device. This filter & the dust cap, filters the dust, sand and other dirt particles from the air. The filtered air flows through the desiccant chamber filled with colorless, moisture adsorbing pellets and are dehydrated. The dehydrated air rises further via the pipe in the oil conservator. The dehydrating breather is mounted on the pipe to the oil conservator. A suitable counter-flange must be installed on the pipe to mount the dehydrating breather. The desiccant contained in the drying assembly is dehydrated using sensor which is controlled by the built-in heating unit, thus obviating the need for periodic desiccant replacement.
5. It is of clear view type design so that moisture absorption indication by change in color of silica gel is visible from a distance
6. Passage of air is possible through silica gel only
7. Height of breather mounting shall not be less than 1200 mm from rail top level
8. Size of breather shall be sufficient 
9. The nos. of breathers shall be Three or more as required for main conservator and shall be Two for OLTC conservator
10. Silica gel is isolated from atmosphere by an oil seal.
11. The main Transformer tank conservator shall be fitted with a silica gel Breather of the Maintenance-Free type at a height of 1200 mm from rail top level. 

The Maintenance Free Type of Breather shall fulfill the objectives like reduced site inspections, no storage or replacement of the desiccant no pollution and disposal problem of the used up desiccant.

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TERMINAL MARKING:

Each terminal including the neutral shall be clearly marked on both the primary and secondary side in accordance with the diagram of connection supplied with the transformers.

CLEANING AND PAINTING:

Before painting or filling with oil or compound oil, un-galvanized parts shall be completely cleaned and free from dust, sealed and grosses and all external rough surfaces on casting shall be filled by metal deposition. The interior of oil transformer tanks and other filled chambers and internal structural steel work shall be cleaned of all sealed and rust by sand blasting or other approved method. These surfaces shall be painted with an oil resisting varnish or paint.

PACKING AND TRANSPORT:

Transportation:

The Manufacturer shall dispatch the transformer filled with oil or in an atmosphere of Dry Air or Nitrogen. The Manufacturer shall take care of the weight limitation on transport and handling facility at site. Necessary arrangement shall be

ensured by the Manufacturer to take care of pressure drop of Dry Air/Nitrogen

during transit and storage till completion of oil filling during erection. A gas

pressure testing valve with necessary pressure gauge and adaptor valve

shall be provided. The transformer shall be fitted with sufficient number of

impact recorders during transportation to measure the movement due to

impact in all three directions. The impact recorder shall be provided with

suitable communication port (USB port) to down load data at any

time. The impact recorder shall be Return after submission of all the

data in hard and soft copy.

1.27.2 All parts shall be adequately marked to facilitate field erection.

1.27.3 In case of synthetic resin bonded paper type bushing is offered; special

attention shall be paid in packing so as to avoid moisture ingress. The details

of the bushing and the method of packing shall be stated in the bid.

1.27.4 Loose Material e.g. bolts nuts etc. shall be packed in gunny bags and sealed

in polyethylene bags with proper tagging. Component containing glass shall

be carefully covered with shock absorbing protective material. All flanges etc.

which are prone to scratches shall be provided with wooden caps bolted in

place. Fragile Material shall be securely braced within the containers or

otherwise amply fastened and packed to prevent shifting or rattling. Soft nonhydroscopic

packing materials shall be placed between hard packing

Materials and fragile equipment. Article which do not completely filled the

selected container must be cushioned, braced, fastened or blocked to prevent

damage to the article it self of destruction of container. Inner bracing or

blocking must be such that content’s weight is distributed over entire interior

surface rather than concentrate on one or two critical points. all opening in the

equipment / accessories shall be tightly covered, plugged or capped to

prevent foreign material to enter in.

1.27.5 Any material found short / damaged inside the intact packing shall be supplied

  at no extra cost to the purchaser.

ABB all protection relay manuals

Substation grounding details

SPONSOR

Substation Grounding

The grounding connections provided to substation equipment and structures fall under two categories, namely

a. Safety Grounds

b. System Grounds

System ground is normally for neutral grounding and safety ground is for equipment grounding. Minimum conductor size for equipment safety grounding.

All safety ground termination shall be made directly on to the ground grid. All system ground shall be terminated on to a ground rod interconnected to the grounding grid.

SOIL RESISTIVITY MEASUREMENT

Measurement

·      A number of measuring techniques are described in detail in ANSI/IEEE 8. The Wenner's four-pin method as described in ANSI/IEEE 8shall be used for measurement of soil resistivity. As many readings as required for various spacing and depth, in all the eight directions, sufficient to model the soil shall be carried out.

·      Soil resistivity readings shall normally be taken under dry conditions, during Summer months, if possible, However the same shall not affect the project’s schedule.

·      Fill up soil resistivity shall be carried by soil modeling in laboratories on samples dried to 2% moisture content after compaction.

·      Soil resistivity measurement shall also be carried out before and after fill up and compaction of soil at site.

Interpretation of Test Results

For soils with resistivity value less than 500 Ohm, if the difference between the highest and the lowest readings are within 30 % then the soil can be considered as uniform soil. For soils with resistivity value greater than 500 Ohm, if the difference between the highest and the lowest readings are within 20 % then the soil can be considered as uniform soil. For uniform soils, the mean value shall be considered as soil resistivity value. In case of wide variations in field readings, computer software alone shall be used to simulate two-layer model or multi-layer soil model. Two-layer soil models are good approximation of many soil structures, while multi-layer soil models may be used for more complex soil conditions. Software shall be based on IEEE-80.

 

Backfill Material

Backfill material shall have possibly the same soil resistivity or better than that of the original soil. In case of considerable backfill the soil resistivity shall be taken after completion of the backfill compaction. The same shall be used for grounding calculations. In case of delay of backfill activity at site the estimated value of resistivity of the backfill material or that of the existing soil whichever is higher shall be used for grounding calculations.

SELECTION OF GROUNDING CONDUCTOR MATERIAL, SIZE AND JOINTS

Basic Requirements

• ·           Copper material shall be used for grounding.
• ·           Soft drawn, stranded copper shall be used for the ground grid conductors.
• ·           The conductor shall be round shaped for maximum cross-sectional contact with the ground. In coastal zone with low soil resistivity, tinned copper conductor shall be used. Copper-clad steel shall be used for ground rods.
• ·    Each element of the ground system (including grid proper, connecting ground leads, and electrodes) shall be so designed that it shall :
• ·       Resist fusing and deterioration of electric joints under the most adverse combination of fault-current magnitude and fault duration to which it might be subjected.
• ·           Be mechanically rugged to a high degree, especially in locations exposed to physical damage.
• ·     Have sufficient conductivity so that it will not contribute substantially to dangerous local potential differences.

Equipment grounding details

Steel Structures and Switch Racks

Switch racks and every steel structure that supports insulators or electrical equipment shall be grounded by means of bolted connections at two (2) diagonally opposite legs. Equipment mounted on steel supporting structures shall have separate grounding conductors. The pigtail ground conductor shall be supported on the structure at 1.0 meter intervals by clamps. Casting pigtail conductor inside the steel structure concrete foundation is not acceptable.

Fences / Gates

If space permits a perimeter ground conductor shall be laid which follows the

fence line and the gate in any position (open or close) and the fence then shall be bonded electrically at corner posts, gate posts and every alternate line post. The gates shall be bonded to the gateposts with a flexible copper cable or braid.

The barbed wire on the top of the SSD (Safety and Security Directive) type

fence/boundary wall, if applicable, shall be bonded to the grounding grid.

Cables

Metallic cable sheaths shall be effectively grounded by connecting a flexible braid to the sheath to eliminate dangerous induced voltages to ground.

Control Cables

Metallic sheath of control cables shall be grounded at both ends to the grounding grid via ground busbar in the cubicle.

Power Cables

a. Sheath of Power cables rated 69kV to 380kV shall be grounded directly from one end and through SVL(Sheath Voltage Limiters) from the other

b. Grounding of sheath of single core cables rated for 34.5kV and 13.8kV shall be from One end only.

c. Sheath of three core cables rated for 13.8kV shall be grounded at both ends.

 If ring type CTs are installed on power cables, the grounding of sheath shall be done such that the sheath current to ground will not influence CT secondary current.

Instrument Cables

Instrument cables carrying analog or digital signals shall have their metallic screening grounded at one point by means of PVC insulated grounding wire connected to separate instrument ground bar which is insulated from cubicle ground.

Signal Cables

All signal cables used in telemetering and communications shall have their shield grounded at one end only to reduce interference from stray sources.

Cable Tray System

Cable tray system shall be grounded with bare copper conductor of 50mm² size at

both ends and shall be bonded across gaps including expansion gaps

Substation Buildings

Substation buildings shall be encircled by a grounding conductor.

Reinforcement bars of the substation buildings and equipment foundation in the yard shall be connected to the main grounding grid at least at two diagonally opposite points.

For grounding of the electrical apparatus installed inside substation

buildings two separate exposed copper conductors/strips of size per manufacturer recommendation, each connected to the grounding grid at two (2) different points shall be laid. The grounding grid shall be laid inside the substation buildings and

it shall be connected to the main grid outside the buildings, at minimum two points.

Metal building(s)

·         Metal buildings shall be grounded at each substructure column with a minimum size of l20mm² bare copper conductor.

·         Angle irons installed on indoor trenches to support the metallic covers shall also be grounded at both ends.

·         Metallic doors in substation buildings shall be grounded with a flexible copper cable or braid.

HVAC

All air conditioning ducts inside the control building(s) shall be grounded at both ends and cross bonded at all joints and across the non-metallic duct connecting Air Handling Unit (AHU).

Grounding of control panels, Operating Mechanism Housing, Box, etc and other equipments associated with HVAC shall be done

Metallic Conduits

All metallic conduits shall be connected to the grounding grid at each manhole or at terminating points by using a conductor size of 50 mm². Conduits terminating in metal junction boxes shall be grounded by means of grounding studs or brazed connections. Where several conduits or junction boxes are located adjacent to each other, an adequately sized solid wire shall be used to interconnect the boxes. It shall be connected to grounding system at one single point.

Circuit Breakers and Disconnect Switches

·         All circuit breakers and disconnect switches shall be grounded at two diagonally opposite corners from two separate points of the grounding grid. Further  grounding switch blades of Disconnect Switch shall be directly grounded to grounding grid.

·         Good electrical connection shall be maintained between the steel structure and any bolted accessories mounted on it.

·         Operating Handles for Outdoor Switches as a large percentage of fatal accidents from voltage gradients are associated with manual operating handles of disconnect switches, etc.

·         A metal grounding plate or mat (operating platform), shall be placed where the operator must stand on it to operate the device. The operating handles shall be grounded by connecting a ground conductor (preferably flexible wire, braid strap) from the vertical operating pipe to the supporting structure, then continuing another stranded ground conductor to the switch operating platform. It is reiterated that the operating handle and the platform shall not be directly connected to the grounding grid but instead both connected to the support structure which in turn shall be connected to the grounding grid at least at two diagonally opposite points.

Terminal Transmission Tower Grounding

Terminal transmission towers located adjacent to the substation shall be connected to

the substation grounding grid at two diagonally opposite points. The shield wire shall

be connected to the tower structure, which in turn is connected to the grounding grid.

 Lightning Masts

Metal lightning masts shall have one safety ground.

Ring Main Unit (RMU)

The RMU inside the substation, if applicable, shall have two safety ground connections.

Oil Tanks and Oil /Water Pipings

All oil tanks shall be grounded at two points with bolted cable connections to two

different points of the grounding grid. Oil piping shall be grounded at intervals of

12m. Runs shorter than l2m shall be grounded at least at two points. Water piping

shall be connected to the grounding system at all service points. In addition, two

copper conductors of adequate size, shall be connected to the main water pipe from two separate points of the grounding grid.

Metal Clad Switchgear

Metal Clad switchgear shall have two safety grounds connected to the switchgear grounding bus. Withdrawable circuit breakers and PTs shall be provided with reliable connection to the ground bus. Grounding via the roller wheels and the rail is not acceptable.

Grounding of Lighting Equipment

Grounding of the lighting fixtures, lamp holders, lamps, receptacles and metal poles supporting lighting fixtures shall be per Article 250 and 410 of NEC (NFPA 70).

Portable Equipment

Portable electrical equipment shall be grounded in accordance with the

applicable requirements of Articles 250 of the NEC (NFPA 70).

Temporary Grounding

All the components used for temporary protective system shall meet IEEE Std. 1246,

“Guide for Temporary Protective Grounding System Used in Substations”.

Instruments, Relays and Meters

Instruments, meters and relays shall be grounded in accordance with the requirements of the NEC, Articles 250-120 to 126 and Articles 170 to 178.

Equipment Requiring both Safety and System Grounds

All operating grounds shall have their connections made to the grounding rods, which in turn shall be connected to the grounding grid.

Power Transformers

·         Power transformer tanks shall be safety grounded at two points diagonally opposite to each other. These connections shall be made from two different points of the grounding grid.

·         A separate system ground shall be provided for the neutral of the transformer by means of two (2) stranded copper wires. The neutral copper wire shall be sized for the system fault level.

·         The neutral grounding wires shall be insulated from the transformer tank by support insulators mounted on the tank wall and shall be connected to the grounding grid directly.

·         Independently mounted radiator bank and LPOF/XLPE cable termination

·         boxes shall be separately grounded at two diagonally opposite locations.

·         Tertiary windings and stabilizing windings shall be grounded per IEC60076-3, Annexure B.

Instrument Transformers

·         Potential and current transformers shall have their metal cases grounded.

·         The grounding terminal of the potential transformers shall be connected to the grounding grid.

·         The neutral point of the secondary connections of potential and current transformers shall be grounded to the ground grid in the control/relay room instead of switchyard to reduce the transient overvoltages.

·         Other requirements of instruments transformer grounding shall be per IEEE C57.13.3, “Guide for Grounding of Instrument Transformer Secondary Circuits and Cases”.

Surge Arresters

·         Where surge counter and/leakage current indicating meters are installed, a 5 kV insulated cable shall be used between arrester ground terminal and surge counter. The surge monitor's ground terminal shall be connected to the ground grid via two (2) 240 mm² stranded copper conductors.

·         The system ground conductor shall be as short as possible, free of sharp bends, and shall not be installed in metallic conduit. In addition, ground rods shall be driven adjacent to the arrester connection to the grounding grid to provide the lowest ground grid resistance at this point.

Station Auxiliary Transformer

Station auxiliary transformer shall be safety grounded at two locations diagonally opposite. One system ground shall be directly connected to the neutral wye connected windings that are to be solidly grounded.

Shunt Capacitors

Shunt capacitors are considered safety grounded when mounted on a metal structure

that is connected to the grounding grid. One system ground conductor shall be connected to the grounding grid when the capacitors are to be connected in a grounded star configuration.

Coupling Capacitor Voltage Transformers (CCVTs)

The grounding terminal and neutral point of secondary connections of CCVT shall be connected to the grounding grid similar to potential transformer as