Guide to the Installation of Overhead Transmission Line Conductors

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Contents
1. Overview..............................................................................................................................................
1.1 Scope............................................................................................................................................ 1
1.2 Purpose......................................................................................................................................... 1
1.3 Application................................................................................................................................... 1
2. References............................................................................................................................................
3. Definitions and cross reference of terminology...................................................................................
3.1 Definitions and terminology for conductor stringing equipment ................................................ 2
3.2 Acronyms................................................................................................................................... 21
4. Conductor stringing methods.............................................................................................................
4.1 Slack or layout method .............................................................................................................. 22
4.2 Tension method.......................................................................................................................... 22
5. Grounding equipment and methods...................................................................................................
5.1 Protective grounding principles ................................................................................................. 23
5.2 Protection of personnel .............................................................................................................. 23
5.3 Hazards and electrical concepts ................................................................................................. 24
5.4Grounding equipment, methods, and te.s.t..i.n..g...................................................................... 24
5.5 Transmission line construction grounding systems ................................................................... 34
6. Communications ................................................................................................................................
7. Conductor reels ..................................................................................................................................
7.1 Reel types................................................................................................................................... 40
7.2 Reel handling ............................................................................................................................. 40
8. Special requirements for mobile equipment ......................................................................................
8.1 Reel stand................................................................................................................................... 40
8.2 Helicopter................................................................................................................................... 41
8.3 Tensioner bullwheel characteristics........................................................................................... 41
8.4Puller and tensioner operating character.i.s..t.i.c..s.................................................................... 43
8.5 Pilot line winder operating characteristics................................................................................. 44
9. Travelers ............................................................................................................................................
9.1 Sheave diameter ......................................................................................................................... 44
9.2 Configuration of groove............................................................................................................. 45
9.3 Bearings ..................................................................................................................................... 46
9.4Material and construction.......................................................................................................... 46
9.5 Lining......................................................................................................................................... 47
9.6 Electrical characteristics ............................................................................................................ 47
9.7 Bundled configurations.............................................................................................................. 47
9.8 Helicopter travelers.................................................................................................................... 48
9.9 Uplift rollers and hold-down blocks .......................................................................................... 4 8
9.10 Traveler suspension ................................................................................................................... 49
10. Typical procedures for stringing operations ......................................................................................
10.1 Pull, tension, anchor, and splicing sites ..................................................................................... 49
10.2 Section between snub structures ................................................................................................ 51
10.3 Conductor splicing ..................................................................................................................... 52
10.4Stringing procedures .................................................................................................................. 54
10.5 Sagging procedures.................................................................................................................... 60
10.6 Deadending precautions............................................................................................................. 65
10.7 Clipping-in ................................................................................................................................. 65
10.8 Damper installation.................................................................................................................... 66
10.9 Spacer and spacer damper installation....................................................................................... 66
11. Special conductors .............................................................................................................................
11.1 ACSS—steel supported aluminum conductor ......................................................................... 67
11.2 T-2 Conductor—twisted bare conductors................................................................................ 67
11.3 Self-damping conductor (SDC) ................................................................................................. 69
11.4Composite overhead groundwire with optical fibers (OPGW....).............................................. 70
11.5 All-dielectric self supporting fiber cable (ADSS) ..................................................................... 71

After the Storm: Gulf Power Rebuilds Electrical System and Restores Power

Less than two weeks after Hurricane Michael left the Florida Panhandle--and 30 hours ahead of the estimated timeline--Gulf Power restored power to more than 95 percent of all of its customers who were safely able to take power, according to the company. 

By working alongside storm crews from across the country, Gulf Power was able to rebuild a significant portion of the electrical system in Bay County, which serves about 103,000 customers. Stan Connally, chairman, president and CEO of Gulf Power, says it was a major milestone.

“It was important for us to quickly mobilize the resources needed to rebuild our energy system and restore power for our Gulf Power customers for this critical first step in recovery for the entire community,” Connally says on a story on Gulf Power's Web site. “However, the work is just beginning for many, with thousands of Gulf Power customers unable to safely reconnect.”

While more than 95 percent of the customers who could take power is restored, Gulf Power estimates about 15,000 to 20,000 customers still cannot safely take power due to hurricane damage to their home or business.

“The entire storm restoration team has felt the appreciation and encouragement from the community, and we want to thank each one of our customers for their patience and support as we worked around the clock to restore power,” Connally said. “And that commitment won’t stop as we transition our work to customer reconnects and completing the rebuild of the smart-grid infrastructure.”

Support crews from across the country traveled to Florida to help Gulf Power to restore power. Although many of these linemen have already returned home, more than 1,000 crew members including transmission, distribution, tree trimmers and damage assessment team members will remain as boots on the ground, according to Gulf Power. This large restoration team will be focused on customer reconnects, electrical equipment clean up and physically and electrically repairing the system to pre-storm condition and reliability.

“We are keeping a team of approximately 10 times the number of crew members than normally work the four-county area affected by Hurricane Michael to ensure we can serve our customers as quickly and safely as possible. It is our mission to assist in accelerating the recovery of these communities because this is our home and we are here to stay,” Connally says. “This was an unprecedented storm, and our unprecedented response will continue.”

 View the following photo gallery to see Colin Hackley's photos of the storm restoration, damage from the hurricane and the rebuilding process by the mutual aid crews. Also, for more information, visit Gulf Power's Web site. 

Source:

https://www.tdworld.com/overhead-transmission/after-storm-gulf-power-rebuilds-electrical-system-and-restores-power

Optical Current Transformers the new trend in the power sector

Optical Current Transformers are based on the Faraday effect that causes a change in the polarization angle of the light when it crosses a magnetic field. Optical Current transformers stand-out in terms of accuracy range (which is virtually unlimited), safety (no risk of explosive failures), environmental protection (no SF6) and they are fully passive. The only active element in the system is the IEC61850 Merging Unit.

A modular optical current transformer that can be used for any voltage level. The device is based on three elements: an optical current transducer (SDO-ICT), the insulator and the merging unit (SDO MU).
the source is : https://www.arteche.com

Cable testing

MV

The details of Site Tests required for newly installed and old in service cable after repair, alteration are explained here.

L.V. POWER CABLE:

a) New LV Power Cables will be subjected at Manufacturers work to all routine tests in

accordance to IEC 245.2, 540, 332-1.

b) After laying underground or in cable trenches, the new cables should be tested by 1000 Volts

Megger each phase separately as IEC 540 and 227.

c) New LV Cables are tested by 2kV A.C. 60/50Hz for one minute each core separately while the

other cores grounded along with their sheather armour. If no collapse of voltage occur during

the test period then cables are considered passed the test.

d) L.V. Cable insulation resistance test by 2kV megger is also useful after A.C. H.V. Test.

The insulation resistance more than 100 M Ohms is acceptable.

However, actual insulation resistance cab be estimated approximately by following formula:

IR = (36.7/L) * (Log 10D) at 20oC

IR = Insulation Resistance in Megga Ohms

L = Length of Cable in Meters

D = Nominal Outer. Diameter of insulation

e) For old control and Power Cables 500V Megger Tet is enough and IR Value > 10 Megga

Ohms is acceptable.

f) Note the Insulation Resistance Value depends upon many factors like Temperature, Humidity,

Length of Cable,Type ofinsulating material and thickness of insulation. So, it varies from

cable to cable and time to time.

PILOT CABLE:

These cables are used for unit Protection of 33kV and 132kV underground Power Cables and

For Telecommunication and data transfer purpose for Substations to SCADA SYSTEM.

These cables are layed alongwith the respective power cables in the same trench. These cables are

required to be tested after jointing is completed and termination into the end boxes. So, following

tests are necessary to perform for newly layed Pilot Cables:

a) LOOP RESISTANCE TEST AND SHEATH CONTINUITY TEST:

Loop resistance must be checked for all pairs (usually 12 pairs).

Normally two sizes are used , one with resistance 19 Ohm/Km Loop and the

other with 58 Ohm/Km Loop at 20oC ambient temperature.

However, 5% variation in Loop R is acceptable. Actual Resistance/Km Loop should be referred

to Manufacturers data Sheet.

Sheath armour continuity check. Should also be made at same time.

b) CROSS TALK/COUPLING TEST:

Each pair in the Pilot Cable should be continued with its two wires coupled together by twisting

all along the route. If one of these two wires split in a joint by jointing mistake and go into other

pair. Then the two mixed pairs would get interference from each other as undesirable

phenomenon. So, cross talk or coupling test is performed as following:

This test is made by telephone set installed at both ends of same pair. If the voice from both end

alternatively is very celar and shift at one end the telephone set to other pairs, no voices are

listened from any otherpair except the right ones, then the pair has no split anywhere and no cross

talk.

If any weak talk comes in other pairs, there is split in pairs of jointing places. It should be located

and remove. The test on other pairs should be continued same way.

c) MEGGER TEST:

For newly installed Pilot Cables 5kV Megger should be used as under:

I - All core to ground with other cores grounded.

II - Between cores of pairs.

III - By 1kV megger check sheath armour to gound after disconnecting it from ground.

IV - The Insulation Resistance value in case I and II should be more than 1000 Mega Ohms

and in case of III it should be more than 2 Mega Ohms

V - For old repaired Pilot Cables 1kV Megger should be used and the IR value more than 2 to

10 Megga Ohms acceptable for putting into service.

VI - D.C. high voltage test is not recommended at Site for new or old in any case. However,

H.V. Tests are required to be performed at manufacturers work place in accordance to

IEC 189-1.

MEDIUM VOLTAGEPOWER CABLES:

a) New cables will be subjected to Manufacturers works all Routine Testsin accordance to IEC

502 – for XLPE and IEC 55 – 1 for paper insulated power cables and other relevant

Specifications.

b) D.C. High Voltage Tests on new and old Power Cable are not preferred. However, in case

of absence of other sets reduced D.C. H.V. test can be performed for shorter time.

c) Instead of D.C. High Voltage test it is recommended to perform H.V. Test at Site for new

and old cables with (VLF) very low frequency (0.1 Hz) approximately sine wave. Zero to

Peak of the voltage wave is considered as test voltage level.

d) VLF (0.1 Hz) should be used specially for XLPE insulated cables at Site. Following are the

tests to be performed on new and old Cables.

CONTINUITY TEST:

Continuity on all phases not including sheath armoring case of newly installed cable should be

checked after laying Underground and Old Cables after repair.

PHASE SEQUENCE CHECK:

At both ends of termination points R, Y, B phases should match to the R, Y, B phase sequence of

Switchgear. For information when standing at back side of Switchgear panel your right hand is

Red phase, Middle Y phase and your left hand is the Blue phase. The cable both end phases

should match the Switchgear sequence at S/S ends.

MEGGER TEST:

a) 5kV megger should be used for each Phase to grund and Phase to Phase also.

b) Insulation Resistance Value on megger should be noted carefully after certain time when

the value of IR becomes constant usually 5 to 10 minutes for longer cables are sufficient.

The cable should be earthed same time at least before and after megger test.

c) Megger test should be performed before and after each High Voltage test, the value of IR

should be recorded carefully.

HIGH VOLTAGE TEST:

a) Perform High Voltage test with Very Low frequency 0.1 Hz Test set on each phase

separately with other twophase grounded along with armour

b) Peak voltage level of 0.1 Hz wave is considered as test voltage level and it should be for

new cables 3U.0 for 15 Minutes

c) For old repaired cales same VLF (0.1 Hz) Test voltage is used at Ö3 U.o for 10 Minutes.

d) Safety rules should be followed before any H.V. Test start and cable should be

discharged and ground before and after each high voltage test.

e) For judgement to a good cable the Megger value should be > 1000 M Ohms and no

collapse of voltage due to insulation failure inside cable should occur during the test

period.

f) Some new cable are jointed with old cables, the test voltage level to be used should be the

same as old cable and the IR value > 300 M is acceptable.

g) V. Ts, power transformers and other unwanted equipment should be disconnected from

the cable circuit before starting of VLF H.V. Test.

HIGH VOLTAGE POWER CABLES & ACCESSORIES 132KV AND ABOVE

RATINGS:

Following test are required to be performed after laying and jointing.

OIL FILLED CABLES:

a) Oil flow test.

b) Impregnation test.

c) Conductor Resistance test and phasing check.

d) Capacitance test.

e) Sheath Insulation Resistance (IR) Test.

f) Varification of Cross bonding System.

g) Sheath contact resistance for joint boxes (JB).

h) Test on sheath voltage limiter (SVL)

i) High voltage test.

XLPE CABLES:

a) Conductor resistance test and phasing check.

b) Capacitance test.

c) Sheath IR Test.

d) Variation of cross bonding System.

e) Contact resistance of Links in JB.

f) SVL. IR Test

g) High Voltage test.

SITE TESTS ON OLD/REPAIRED OIL FITTED/XLPE CABLES:

a) Sheath IR Test and phasing checks.

b) Contact resistance of links in JB.

c) Oil Flow Test in case of Oil Filled only.

d) Reduced High Voltage test alongwith Megger Test.

3.4-4 BRIEF DESCRITION OF:

a) Oil Flow Test:

This test is performed on each oil section afte jointings have been completed. Oil is fed at

one end of oil section through oil tanks and the other end is keep open to flow oil out. At

feeding end a constant pressure is developed by oil tank and cable itself. A stabilized oil

guage valis noted at feeding and same time oil flow meter installed at feeding end shows

the actual flow rate liters/sec.

The theoretical value of oil flow rate Q can be calculated as under:

Q = Pr /25.5 *n*  L    Liters/Second

Q= Rate of flow Liters/Second

P= Total pressure difference on the Section (bar).

r= Internal radius of cable pipe (mm) (ALUMINUM SHEATH)

n= Viscosity of oil at the Test Temperature (Centipoise).

L= Oil Cable section length (meters).

Compare the observed value and calculated value of Q, there should be no extra ordinary

difference for good cable without obstruction in oil flow path.

IMPREGNATION TEST:

Purpose of this test is to know that paper insulation mentioned inside pipe is fully impregnated by

oil or not. For this purpose a measured quantity of oil is drawn from the Cable System (with feed

tank shut off) and consequent drop in pressure noted.

The impregnation co-efficient (K) is calculated as under:

K = dv/v * 1/dp

K = Impregnation Co-efficient.

dv = The volume of oil withdraw (Liter)

dp = drop in pressure (bar)

v = Volume of oil in Cable Section (Liter)

The calculated value of K should not be more than 4.5 x 10^-4 for a good judgement.

CONDUCTOR, RESISTANCE, CAPACITANCE AND PHASING TEST:

Prior to put loop for end in two phases phasing should be confirmed by simple contnuity check by

grounding each phase in turn. Then thick loop is inserted between Ph-Ph at one end and

resistance Measured by RLC Bridge at other end each Phase to Phase in turn.

For capacitance loop should be taken out and capacitance of each phase to sheath measured by

RLC Bridge.

SHEATH ‘IR’ TEST:

a) For new cables apply 4kV D.C. for 1 minute to cable ALUMIUM SHEATH with ground after

opening all grounding links in link boxes.

No collapse of voltage should occur during the test period. If test is repeated then 5kV should

used for one minute.

b) For old cables the test should be done by 5kV megger and IR Value more than 2 Megga

Ohms is acceptable

TO BE CONTINUED SOON

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.