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MANUAL General The purpose of this manual is to explain how the transformer should be installed, commissioned, operated and maintained in general. In addition to the instructions given in the manual, reference should be made to : IEC-60076
Power Transformer
IEC-60296
Specification for unused mineral insulating oils for Transformer & Switchgear
IEC- 60137
Insulating Bushing for alternating voltage above 1000V.
IEC-60354
Loading Guide for oil immersed power transformer.
IEC-60364
Specification for Gas operated relays
IEC-156
Method for the determination of the electric strength of insulating oils.
IS-10028
Code of Practice for Installation of Transformers.
IS-335
Code of Practice for Maintenance of Transformers
IS-1866
Code of Practice for maintenance of Transformer oil
IS-1646
Code of Practice for Electrical Installation
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TRANSFORMERS GENERAL Transformer is one of the most vital and important electrical machinery. The development of the present day power system is very much attributable to the large number and types of transformer that are in operation in the system, such as, generator transformers, step-up transformers, step-down transformers, interlinking transformers, power transformers & distribution transformers etc. Being a static machine, it is inherently reliable compared to other machines,. Distribution transformers are a important link between the power system and millions of electricity consumers. Any failure of this important equipment, apart from adversely affecting the consumers, will also mean considerable financial loss to the electricity undertaking. It is therefore of important that utmost care is taken in the design, manufacture, testing, installation, and maintenance of transformers. A transformer consists of a magnetic core made out of insulated silicon steel laminations. Two distinct sets of windings, one called primary and other called secondary winding, are wound on such core. The transformer helps in converting low voltage into high voltage or visa-versa and accordingly the transformer is termed step-up or step-down. The winding to which the voltage is applied is called primary winding, where as the winding to which the load is connected is called secondary winding. The transformer works on the principle of electro-magnetic induction. Such phenomena can take place in a static device, only, if the magnetic flux is continually varying. It is therefore clear that static transformers can only be used with alternating currents only. When an alternating EMF is applied to the primary winding of a transformer with the secondary winding open circuited, a small current flows in the primary winding which serves to magnetize the core and to feed the iron losses of the transformer. As primary and secondary windings are wound on the same core, the magnetizing flux is the same for both the windings. The magnetizing flux corresponds to the magnetizing current in the primary and the number of turns of the primary winding. Primary and secondary windings are wound on the same core, hence the induced voltage per turn is the same for both primary and secondary winding. Also the absolute value of induced voltage in the primary and secondary windings is proportional to the number of turns in the respective windings. TRANSFORMER CONSTRUCTION DETAILS The main parts of a transformer are; 1.
Transformer core
2.
Transformer Windings.
3.
Transformer Tank and Radiators.
Transformer core Every transformer has a core, which is surrounded by windings. The core is made out of special cold rolled grain oriented silicon sheet steel laminations. The special silicon steel ensures low hysteretisis losses. The silicon steel laminations also ensure high resistively of core material which result in low eddy currents. In order to reduce eddy current losses, the laminations are
Page 3 kept as thin as possible. The thickness of the laminations is usually around 0.27 to 0.35 mm. The transformer cores construction are of two types, viz, core type and shell type. In core type transformers, the windings are wound around the core, while in shell type transformers, the core is constructed around the windings. The shell type transformers provide a low reactance path for the magnetic flux, while the core type transformer has a high leakage flux and hence higher reactance. The ideal shape for the section of the core is a circle, as this would mean no wastage of space between the core and windings, except the space taken by the insulation between laminations. A perfectly circular section of core would mean varying dimensions for each successive lamination, which may not be economical. A compromise is therefore struck and a stepped core (four or six steps) construction is normally preferred. The net sectional area is calculated from the dimensions of the various sections and giving due allowance for the insulation thickness. The yoke section is arranged similar to the limb section. To make the best use of the grain oriented silicon steel it is necessary that the flux run parallel to the direction of the rolling for as much of the magnetic path as possible. This is achieved by selecting identical cross-section and shape for core and yoke sections and having mitered corners. The materials used are such as to give low hysteretic losses, for a particular flux density. These are dependant on weight of material used and design flux density. In case a low flux density is employed, the weight of material increases, which in turn also leads to increase in length of mean turn of transformer coil. Both these aspects result in increase in losses. Similarly, the eddy current loss depends on the quality of material thickness of laminations and the flux density employed. The limb laminations in small transformers are held together by stout webbing tape or by suitably spaced glass fiber bends. The use of insulated bolts passing through the limb laminations has been discontinued due to number of instances of core bolt failures. The top and bottom mitered yokes are interleaved with the limbs and are clamped by steel sections held together by insulated yoke bolts. The steel frames clamping the top and bottom yokes are held together by vertical tie bolts. TRANSFORMER WINDINGS The primary and secondary windings in a core type transformer are of the concentric type only, while in case of shell type transformer these could be of sand-witched type as well. The concentric windings are normally constructed in any of the following types depending on the size and application of the transformer 1.
Cross over Type.
2.
Helical Type.
3.
Continuous Disc Type.
Cross-over type winding is normally employed where rated currents are up-to about 20 Amperes or so. In this type of winding, each coil consists of number of layers having number of turns per layer. The conductor being a round wire or strip insulated with a paper covering. It is normal practice to provide one or two extra lavers of paper insulation between lavers. Further, the
Page 4 insulation between lavers is wrapped round the end turns of the lavers there by assisting to keep the whole coil compact. The complete windings consists of a number of coils connected in series. The inside end of a coil is connected to the outside end of adjacent coil. Insulation blocks are provided between adjacent coils to ensure free circulation of oil. In helical winding, the coil consists of a number of rectangular strips wound in parallel racially such that each separate turn occupies the total radial depth of the winding. Each turn is wound on a number of key spacers which form the vertical oil duct and each turn or group of turns is spaced by radial keys sectors. This ensures free circulation of oil in horizontal and vertical direction. This type of coil construction is normally adopted for low voltage windings where the magnitude of current is comparatively large. The continuous disc type of windings consists of number of Discs wound from a single wire or number of strips in parallel. Each disc consists of number of turns, wound radically, over one another. The conductor passing uninterruptedly from one disc to another. With multiple-strip conductor. Transpositions are made at regular intervals to ensure uniform resistance and length of conductor. The discs are wound on an insulating cylinder spaced from it by strips running the whole length of the cylinder and separated from one another by hard pressboard sectors keyed to the vertical strips. This ensures free circulation of oil in horizontal and vertical direction and provides efficient heat dissipation from windings to the oil. The whole coil structure is mechanically sound and capable of resisting the most enormous short circuit forces. The windings coils after manufacture are subjected to drying out in an oven by circulation of hot air at around 80 degree centigrade. The pre drying and shrinking of coils is achieved in this process. The coils are further dried un-till the required insulation resistance is achieved. In case of larger distribution and power transformers, the assembled core and windings are further subjected to drying out at about 100C and 730mm absolute pressure to drive out water vapor and gas from the windings. Appropriate clamping arrangements in the form of rings are provided on the windings to adjust for any shrinkage of insulation. The clamping rings could be either metallic with suitable earthing arrangements or of insulating material. The insulation of the windings comprises of insulating cylinders between LV windings and core and between HV winding. Also insulating barriers are provided where necessary, between adjacent limbs, in some cases and between core yoke and coils. The leads from top and bottom end of windings and from such tapings, as may be provided, are brought out to a few centimeters length only. The electrical connection from these leads to the terminals or bushings consist of either copper rod or strips depending on the current to be carried. Copper rods are insulated with bakelite tubes and supported by cleats. Which in turn are supported from the vertical tie rods passing through the top and bottom yoke clamps. When copper strips are used for low voltage leads no insulation need to be provided, except the cleats, which hold the strip in position. The strips are however wrapped with linen or varnish cloth at the point where it passes through the leads. Leads from tapings are brought out to a point just below the top oil and so arranged that tapings may be readily changed by means of off load Tap changer.
Page 5 TRANSFORMER TANK Transformer tanks commonly used are of the following types; 1.
Plain sheet steel tank.
2.
Sheet steel tank with external cooling tubes.
3.
Radiator tanks.
4.
Tanks with corrugated wall panels.
Plain sheet steel tanks are used where the size of the tank provides adequate cooling surface to dissipate the heat generated on account of losses inside the transformer. Normally transformers up-to 50KVA could be manufactured without external cooling tubes. For transformers of higher rating, tanks are constructed with external cooling tubes to provide additional surface for heat dissipation. The cooling tubes could be circular or elliptical. Elliptical tubes with smaller width are employed where one of the sides of the transformer is fully occupied by on load tap changer. This ensures more tubes on the given surface thereby providing more area for heat dissipation. In larger tanks, stiffeners are also provided on the sides of the tank to prevent bulging of the tank under oil pressure. The tubes are welded on the inside of the tank, while all other joints are welded both, inside and outside. Large size transformers, above 5 MVA rating are normally provided with detachable Radiator banks to provide required cooling surface. The radiator bank consists of series of elliptical tubes or a pressed steel plate assembly welded into top and bottom headers. The radiator bank is bolted on to the tank wall and two isolating valves are fitted into the oil inlet and outlet. In case of very large transformers, even detachable radiator banks mounted onto the tank walls do not provide adequate cooling surface. IN such cases, separate self supporting coolers are provided which are connected to the main transformer through large detachable pipes. This type of arrangement is good for naturally cooled transformers, as well as, for forced cooled transformers. Forced air cooling could be provided by means of suitable fans located below the cooler banks. Similarly, forced oil cooling could be provided by installing an oil pump in the return cold oil pipe connecting the main transformer tank to the cooler bank. For outdoor transformers, the transformer has to be water-tight. For this purpose, the cover bolts are closely spaced and a substantial tank flange of ample width is provided. Further a Neoprene bonded cork gasket is provided between the tank flange and the cover. The bushing insulators are selected considering the maximum system voltages encountered in the system and pollution conditions prevailing at site. The joints are made water-tight by use of Neoprene bonded cork gaskets. Transformers of rating 1 MVA or more are also normally provided with a conservator tank connected to the main tank. The conservator tank has a capacity of about 10% of the oil content of the main tank.
Page 6 TRANSFORMERS FITTINGS & ACCESSORIES Standard Fittings Standard Fittings listed below are normally provided on the transformers for the correct and safe operation of the unit. These fittings conform to the requirement of International Standard IEC 60076. 1. Rating and terminal marking plate. 2. Tap Changing arrangement (a) Off – circuit tap changing switch (b) Off – circuit tap changing link (c) On Load tap changer 3. Two earthing terminals 4. Lifting Lugs 5. Drain – cum filter valve 6. Pressure Relief Device 7. Silicagel dehydrating breather. 8. Oil Level Indicator. 9. Thermometer Pocket. 10. Conservator with drain plug and filling hole. 11. Air Release plug. 12. Jacking lugs (above 1600 KVA) 13. Filter valve (top tank) 14. Under base unidirectional flat rollers. TERMINAL ARRANGEMENT a)
Bare Bushings or
b)
Cable box. i)
Compound filled for PLC cables (upto 33000 Volts) or
ii)
Air filled for PVC cable s (Upto 11000 Volts) or
c)
Bus Duct (Bare bushing enclosed in housing upto 600 Volts)
d)
Disconnection chamber between cable box and transformer tank.
e)
Additional bare neutral terminal.
Page 7 OPTIONAL FITTINGS These are optional fittings provided at an extra cost, if customer specifically orders them. a) Winding temperature indicator b) Oil temperature indicator c) Gas and oil actuated (Buchholz) relay d) Conservator drain valve e) Shut off valve between conservator and tank. f) Magnetic oil level gauge g) Explosion vent h) Filter valve (Bottom of tank) i) Skid under base with haulage holes j) Junction box.
GENERAL DESCRIPTION OF FITTINGS & ACCESSORIES : STANDARD FITTINGS (a) Rating and Terminal marking plate :The transformer is supplied with rating and terminal marking plate of a non corrosive metal or metal with protective covering on which all information concerning the rating. Voltage ratio, weights, oil quality etc. along with the serial number of the unit is engraved. (b) Tap changing arrangement :i) Off - circuit tap changing switch :-The transformer is fitted with an off-circuit tap changing switch to obtain required tap voltage. It can be hand operated by a switch handle mounted on the tank. Locking device is fitted to the handle to padlock it on any tap position and also to prevent any unauthorized operation of switch. The switch mechanism is such that it can be locked only when it is bridging two contacts on any particulars tapping position and cannot be locked in any intermediate position. It is important that the transformer should be isolated from the live lines, before moving the switch. Operating the switch when transformer is energized, will damage the switch contacts due to severe arcing between the contacts, and may damage windings also. II) Off – circuit tap changing links : Contact bridging links are provided inside the transformer tank, to obtain required tap voltage. Links are required to be unbolted and are fixed in any required position of the tap.
Page 8 Links are approachable from inspection holes in tank cover. In case of conservator units, oil level has to be dropped below the inspection opening before unbolting inspection covers. III) On – Load tap changer : On load tap changer is normally mounted on the tank is a separate housing and connected to winding leads through copper studs fixed on a insulated terminal board Terminal board is on leak proof. Oil in the tank need not be lowered down for a attending to OLTC gear. Please see OLTC leaflet for the operation and maintenance instructions. c) Earthing Terminals : Core laminations assembly is connected to core clamping frame by a cu. strip which is in turn connected to the tank. Two earthing terminals are provided on the tank which should be connected to the earthing system of supporting structure of transformer or the station. d) Lifting Lugs : Two / Four lifting lugs of adequate capacity are provided on tanks to lift completely assembled transformer filled with oil. All lugs are designed for simultaneous use and should be used simultaneously to lift the transformer. Two/Four lifting lugs are provided on core clamps for un tanking the core and windings. All heavy fittings are also provided with individual lifting lugs. e) Valves and drain plugs:
I) Valves Transformer is equipped with Drain cum filter valve at bottom of tank. Filter valve at top of tank. Valves are fitted with plugs / blanking plates to stop the dirt or moisture entering inside the valve and avoid the contamination of the transformer oil. II) Drain Plugs Drain Plug is provided on conservator to drain out oil. f) Silicagel dehydrating breather : Silica gel breather is fitted with silica gel which absorbs moisture from the air entering the transformer, thus preventing deterioration of oil and insulation due to moisture condensation.
Page 9 The breather contains oil unit at the bottom which prevents the entry of dust solid particles present in the air. The colour of silica gel is blue when dry and turns pink when it has absorb a certain percentage of moisture by weight. The change in colour of gel can be observed through window on a container. Breather when fitted should examined to ascertain, that the silica gel is dry (blue in colour) The frequency of inspection of gel depends upon local climate and operating conditions. This dehydrating breather is used in conventional type of transformer where breather in transformer is applicable. In case of hermetically sealed transformer, silica gel breather are not required, there is no breathing in this transformer. g) Oil Level indicator : Plain Oil level gauge Indicates oil level in tank or conservator, window opening is fitted with grooved Perspex sheet and metal frame to give clear indication of oil level. h) Thermometer Pockets : This pocket is provided to measure temperature of the top oil in tank with a mercury in glass type thermometer. It is essential to fill the pocket with transformer oil before inserting the thermometer, to have uniform and correct reading. One additional pocket is provided for dial type thermometer (OTI) with contacts. I) Conservator with drain plug and filling hole : Conservator is normally provided on all ratings of transformers which provides the space for the expansion / contraction of oil on account of the variation of oil temperature during service. It prevents the oil in the tank from coming in direct contact with the atmosphere and protects it from deterioration. Conservator is provided with silicagel breather, oil level gauge, oil filling hole with blanking plate and drain plug for draining/ sampling of oil contaminated by moisture and sludge. J) Air release plug Air release plug is normally provided on the tank cover for transformer with conservator. Space is provided in the plug which allows air to be escaped without removing the plug fully from the seat. Plug should be unscrewed till air comes out from cross hole and as soon as oil flows out it should be closed. Air release plugs are also provided on radiator headers and outdoor bushings. K) Jacking lugs: Four jacking lugs are normally provided for transformers above 1600 KVA. All lugs should be used simultaneously to avoid damage to jacking pads and tank. Suitable hydraulic or mechanical jacks may be used to jack up the transformer.
Page 10 L) Rollers Four rollers, plain or flanged type, are provided on e4very transformer. Suitable arrangement of track for the rollers should be made at site, to facilitate movement of the transformer. The track provided should be leveled properly so that all wheels rest on the track. Rollers are normally detached from the tank base at the time of transport.
TERMINAL ARRANGEMENT
a) Bare bushing : i) Draw through bushing (oil flood type) :-Winding lead is soldered to the stem of bushings which is drawn through the hole in porcelain and is fixed outside the porcelain with leak proof gasket. Oil is flooded through the hole of the porcelain which acts as an insulation between the lead and earth in addition to the porcelain insulation. ii) Solid Bushing (through stem type) Through stem protruding out of porcelain on either side is provided with nuts and washers to take windings lead on one side and supply cable on the other side. Bushing is completely sealed on one end for oil tightness. b) Outdoor through stem type porcelain bushings. c) Cable box type porcelain bushings. d) Cable box type epoxy bushings.
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a) Outdoor through stem type porcelain bushings.
b) Cable box type porcelain bushings.
c) Cable box type epoxy bushings.
Page 12 B) Cable sealing boxes
i) Cable \sealing boxes for PILC cable compound filled :--
Bushing mounted on cable box flange, are integral part of cable box shell and protrude inside the shell to receive supply cable. Supply cable is inserted through the wiping gland and individual cores are connected to respective bushings inside the box. Lead joint is made between the wiping gland and lead sheathing of the cable to stop the ingress of moisture inside the shell. Armouring of cables is clamped inside or outside gland joint and earthed suitably. Cable box is sealed with gasketed joints and insulating compound is filled upto filling level. ii) Cable sealing box for PVC cable – air filled :Bushing mounted on the tank are integral part of tank. Detachable cable sealing box shell covers the bushings protruding inside the shell. Supply cable passes through the compression gland and is sealed with a rubberring between the PVC cable sheath and gland. Armouring of cable is clamped inside or outside the gland and earthed suitably.
Page 13 C) Bus Duct Bushing mounted on the tank wall are covered with a small protective metal housing with a flange suitable to take customer’s bus duct. Flexible copper links should be provided to connect bushings to customer’s bus.
D) Disconnecting Chamber This arrangement is provided to disconnect the transformer from the cable fox. It is provided for cable box with PILC cables, which facilitates disconnection of cable box from transformer without lowering the oil in the tank. It has two sets of bushings one on the side of transformer and the other on the cable box. Disconnecting links are provided for disconnecting bushings on transformer from the bushings on cable box. Lower oil in chamber if it is connected to conservator. Disconnecting chamber facilities cable testing, disconnection of cable box for replacement or for removing the transformer for repairs without decompounding the cable. E) C.T. Terminals CT secondary leads are brought out to the CT terminal boxes located at different places as shown on outline drawing. These boxes are supplied with detachable un drilled gland mounting plate. Drilling of gland plate is to be done at site to suit CT Secondary cable glands. F) Bare Connection In case of overhead lines the bare conductors are terminated to the transformer bushings. Care should be taken to keep proper clearances between the connections and the earth. In case, aluminum ACSR conductors are used for connection to transformer terminals, suitable bimetallic type connectors should be used to prevent electrolytic corrosion at the contact surfaces of the dissimilar metals.
Page 14 OPTIONAL FITTINGS
a) Winding temperature Indicator The windings temperature indicator indicates ‘’ Hot spot’’ temperature of the winding. This is a ‘’Thermal Image type’’ indicator. This is basically an oil temperature indicator with a heater responsible to raise the temperature equal to the ‘’Hot spot’’ gradient between winding and oil over the oil temperature. Thus, this instrument indicates the ‘’Hot Spot’’ temperature of the windings. Heater coil is fed with a current proportional to the windings current through a current transformer mounted on the winding under measurement. Heater coil is either placed on the heater bulb enveloping the sensing element of the winding temperature indicator immersed in oil or in the instrument. The value of the current fed to the heater is such that it raises the temperature by an amount equal to the hot spot gradient of the winding, as described above. Thus temperature of winding is simulated on the dial of the instrument. Pointer is connected thought a mechanism to indicate the hot spot temperature on dial. WTI is provided with a temperature recording dial main pointer. Maximum pointer and re setting device and two sets of contacts for alarm and trip. b) Oil Temperature Indicator
Page 15 Oil temperature indicator provides local temperature of top oil. Instruments are provided with temperature sensing bulb, temperature recording dial with the pointer and maximum reading pointer and resetting device. Electrical contacts are provided to give alarm or trip at a required setting (on capillary tube type thermometer). c) Gas and oil actuated Buchholz relay
In the event of fault in an oil filled transformer gas is generated, due to which buchholz relay gives warning of developing fault. Buchholz relay is provided with two elements one for minor faults (gives alarm) and other for major faults (tripping). The alarm elements operates after a specific volume gets accumulated in the relay. Examples of incipient faults which will generate gas in oil are:Buchholz Relay i)
Failure of core bolt insulation.
ii)
Shorting of lamination and core clamp.
iii)
Bad Electrical contact or connections.
iv)
Excessive hot spots in winding.
The alarm element will also operated in the event of oil leakage. The trip element operates due to sudden oil surge in the event of more serious fault such as: i)
Earth fault due to insulation failure from winding to earth.
ii)
Winding short circuit inter turn, interlayer, inter coil etc.
iii)
Short circuit between phases.
iv)
Puncture of bushing.
The trip element will also operate if rapid loss of oil occurs. During the operation of transformer, if there is an alarm transformer should be isolated from lines and possible reasons, listed above for the operation of relay should be checked starting
Page 16 with simple reason such as loss of oil due to leaks, air accumulation in relay chamber which may be the absorbed air released by oil due to change in temperature etc. Rating of contacts :0.5 Amps. At 230 Volts AC or 220 Volts. DC. Magnetic Oil Gauge
This is a dial type gauge, mounted directly on the conservator to indicate oil level. This is supplied with low level alarm contacts (if asked for) and the electrical connections of which are brought out to a terminal box of the oil gauge. Suitable alarm circuit may be connected to these terminals. This oil gauge is not dispatched separately, but is mounted on the conservator, with float arm adjusted to correct length. Explosion Vent Explosion vent is provided to give protection against the excessive pressure that may developed inside the transformer due to internal fault. On specific requirement the explosion vent is provided with two diaphragms one at the bottom (near tank) and the other at the top. If excessive pressure is developed in the tank, both diaphragms will rupture and oil in tanks will be thrown out through the vent. One pressure equalizer pipe is provided between explosion vent and the conservator to maintain equal pressure in the empty spaces of vent and conservator. In this case oil level indicator is provided on the explosion vent to indicate rupture of bottom diaphragm.
Page 17 Skid under base Skid under base with haulage holes is provided at the bottom of tank. The holes provided in the transformer.
under base arrangement are suitable for towing the complete
Junction Box Terminal blocks are provided in the box to take the incoming and outgoing cable leads from various instruments fitted on transformers e.g. Buchholz relay, Winding temperature indicator, Oil temperature indicator, Magnetic oil gauge etc. Box is provided with Blank gland mounting plate, which should be drilled suitably to receive glands. Temperature indicators with capillary tubes and electrical contacts are mounted inside the box, when asked for.
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PACKING AND DESPATCH Packing Transformer are generally dispatched oil filled and ready for the service. Occasionally, in case of large units. It is necessary to dismantle certain fittings and pack them separately to meet the transport limitations and avoid any transit damage. In such case oil is lowered below the top tank cover and space between the oil and tank cover is filled with dry air at normal atmospheric pressure. The oil required for topping up conservator, radiators etc. is supplied separately in sealed drums. Parts generally detached for transport are as follows :a) Conservator with oil gauge. b) Detachable cooling radiators, when provided. In case of corrugation, they are fixed type. c) Other accessories such as Buchholz relay, Explosion vent, silica gel breather, Magnetic oil gauge, Pipe connection. INSPECTION The transformer may get damaged during transit, hence on the receipt of unit at site, it is necessary to examine main unit and components very carefully with packing list sent with each transformer. Damages and shortages, if any, should be reported to the Kotsons Private Limited, Agra. With the photograph and other useful evidence wherever possible. Open delivery certificate should be obtained from the carriers. Particular attention should be given to the damages of : a) Main tank b) Protruded fittings on the tank, such as valves conservator, brackets, welded / bolts, radiators if applicable, corrugations etc. c) Porcelain bushings : Porcelain parts and current carrying stems. d) Accessories : i)
Magnetic oil gauge
Dial glass, float and mercury switches
ii)
Dial type thermometer
Dial glass, pointers, capillary tube, stem, protection caps of operating keys for maximum indication pointer and contacts.
iii) Buchholz relay
Winding glasses, operating floats and mercury switches.
iv) Explosion vent
Glass and bakelite diaphragm for cracks and puncture
v)
Marshalling / Junction Window glasses, inside instruments (if any) and boxes cable glands.
e) Corrugated fins.
Page 19 f) Oil Oil leakage through the tank cover, bushing / valves etc. if any A sample of oil should be taken from bottom of the tank and tested for the Electric strength (break down voltage) which should not be below 50 KV (RMS) when tested on a standard test gap as specified in IEC 600296. UNLOADING Transformer and all packing cases containing the accessories should be handled in right position indicated, and while handling should not drop down to avoid damages to the delicate instruments packed inside. a) Lifting : The transformer should be unloaded by means of crane or lifting device of sufficient capacity. (Please refer the rating plate for details of weight). All lifting lugs provided for the purpose must be used for the lifting of transformer to avoid unbalance and undue stresses on the lug. Parts other than lifting lugs should not be used for lifting. b) Jacking : Four jacking lugs are provided on transformer, each designed to share quarter load. While using jacking pads, utmost care should be exercised in the proper application of jacks, as any improper application will not only damage the jacking pads but also the transformer. c) Towing : Rollers / wheels should be used for shifting the transformer from one place to other roller axles or radiators should not be slinged for the purpose of towing. Haulage holes provided on the tank should be used. d) Packing cases : Cases containing various accessories should be handled carefully so that the contents are not damaged. STORAGE a) Main transformer : On receipt of the transformer at site, it is desirable to erect and commission with minimum delay. If the transformer is not required to be commissioned immediately, it should be properly stored on wooden base & case to avoid deterioration of external finish. Transformer dispatched with dismantled conservator, may absorb moisture when stored for long periods due to entire tanks not filled, with oil, therefore best way to avoid this, is to mount conservator temporarily and fill with oil to ‘’ filling level’’ mark. Silicagel breather must be fitted in position and checked periodically for the condition of gel to ensure that it remains blue. The gel should be reactivated or replaced once it turns pink. It is advisable to check the condition of gel every week. Oil should be circulated and filtered periodically, to ensure that megger values do not drop. Past experience shows that, if megger values become too low, it is very difficult to improve them. It is desirable to check Electric Strength of oil at least once in a month. b) Accessories : Accessories like bushings, explosion vent, buchholz relay, dial type temperature indicator, terminal box, magnetic oil gauge etc. should be repacked in packing case and stored indoors till such time they are required. They are likely to get damaged if they are stored outdoors.
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INSTALLATION: Indian code of Practice for Instattation and a Maintenance of Transformer IS: 10028 generally covers all the relevant aspects of the special attention required at the time of erection of transformer, should be referred before commissioning installation works. Precautions a) Instruction To Erection Personnel : Persons having access to the interior of the transformer while inspection, fitting bushing or any other fittings, should empty their pockets and all loose articles. Spanners and other tools in use should be sccurely tied with a cotton tape ta a suitable point so that they can be recovered if accidentally fropped inside. b) Cleaning : fibrous cleaning materials should bot be used. The presence of loose fibres in suspension with transformer oil can reduce its insulation property. If cleaning or wiping is necessary, this should be done with clean dry oil using soft non-fluffy cloth. c) Fire risk : Transformer oil is inflammable and under certain circumstances in a confined place become explosive. Naked light and flame should never be used near the transformer. d) Condensation : Transformer oil and other solid insulation normally used in transformerconstruction absorb moisture easily, particularly when cold. Condensation can be caused if the interior of a tank is exposed to atmosphere. But taking adequate care this can be minimised. The transformer can be protected from “damp” hazards by circulating warm and dry oil through it until temperature is 50 to 100 C above ambient. This should be done before allowing access to the interior of tank. Warm oil should be circulated all the time while transformer is open to atnosphere. e) Painting : Corroded components should be cleaned thoroughly to remove the rust, dirt and grease and repaint with one coat of red oxide primer and two coats of enamel paint. Retouching of paint may be required where paint is damaged during transit. Location and Foundation : A leveled and firm foundation strong enough to support complete weight of transformer and prevent accumulation of water should be provided. Continluous external vibrations may set up physical movements in core and windings and in turn lead to the breakdown. While locating transformer care should be taken to provide adequate clearnces from the live parts of transformer to earth parts. Easy access should be provided all round and to diagram plate, thermometer, valves, oil gauges etc. so that they can be easily reached or read. Suitable rails or tracks should be used for rollers and wheels should be locked to prevent accidental movement.. where transformer is enclosed in chamber it should be ensured that it gets goods ventilation.
Page 21 Wherever transformers are mounted on poles the structure should be strong enough to withstand weight of the transformer, wind pressure and other atmospheric conditions. Erection of transformer : Generally small transformers upto 1000 KVA are despatched oil filled fitted with all fittings. Fittings dismantled to facilitate transport, may be reassembled in the following sequence depending on the site conditions. Sequence of erection : a) Fitting of rollers b) Oil check c) Conservator d) Buchholz relay and pipe work e) Silicagel dehydrating f) Explosion vent g) Terminals HV – LV bushings or cable boxes, CT terminals etc. h) Topping up of oil i) Radiators j) Temperature indicators k) Marshalling Boxes Details of erection : All fittings and accessories should be assembled as shown in outline drawing. a)
Rollers : i)
Before mounting of rollers they should be checked for soundness of casting and that bearing surmace is not damaged.
ii) Bearing surface should be greased or oiled before mounting. iii) Assembly of rollers become simple if the transformer is lifted by overhead crane or pully block. Transformer may be jacked up with mechanical or hydraulic jacks.(Jacking pads are provided for transformers 1600 KVA and above). Height of jacking pads from ground, without wheels in positions, is not less than 300 mm. Suitable wooden or steel packing may be given under jacks to raise the jack height. b)
Oil check : A sample of oil should be taken out from bottom of tank and tested for the Electric strength, ( break – down voltage ) which should not be below 30 KV (RMS) when tested on a standard test as specified in IEC – 600296 Table 1-9. (Electrode dia. 120.5 mm
Page 22 polished, electrode gap 2.5 mm # 0.1). The electric strength shall be arithmetic mean of the six results on same sample of oil. c)
Conservator : when dismantled for transport ( Ref.3.1) i)
Before assembly of conservator, make sure that inside of vessel and pipe work is free from rust, dust, grease and moisture. If found corroded, clean the rust and repaint with oil resistant paint from inside. Enamel paint given is on the outside on a coat of red oxide-zinc chromate primer.
ii)
Mount conservator on the corresponding supports in tank cover. Pipe work should be assembled in the following scwuence. Conservator through Buchholz relay to tank. Conservator to silicagel breather.
iii) Magnetic oil gauge should be assembled on position. iv) Plain oil gauge window glass should be checked for the cracks which may lead to oil leaks and entry of moisture in the conservator. Gasket should be tightened uniformly to make joints leak proof. Deteriorated and leaky gaskets should be replaced. d)
Buchholz relay Assembly precautions (if dismantled) i)
Relay pipe work should be slopping 3 deg. To 7 deg. Upwards towards conservator.
ii)
Arrow heads on the body should be pointing towards the conservator.
iii) Floats which operate elements should be free. iv) Check that mercury switches are intact. v)
Test alarm and trip mechanism.
In order to test alarm and trip mechanism, the relay is provided with two separate petcocks which enable air to be injected into the body of the relay. For testing inject air using a cycle pump or any other suitable device until switch operates (check continuity of contacts), To test trip elements, the valve controlling the bottle is opened quickly so that air or oil rushes in, impinges on the flap and depresses it operating the switch. On each relay petcock is provided for taking out samples and releasing gas from the top of relay housing. Testing of relay may be carried out after fitting in the conservator pipe, and filling of oil in conservator. It is recommended that the relay should not be taken apart and under no circumstances should any alteration be made to operating floats or flaps.
Page 23 e) Silicagel Dehydrating breather : Breather is connected to the conservator through a pipe work. Breather assembly should be suitably clamped to prevent vibrations. i)
Fill up transformer oil through filling hole, in the bottom compartment of breather upto the filling level mark. Air entry hole is sealed with the adhesive tape, which should be removed for free passage of air.
ii)
Check that sili9cagel is blue in colour, if not, reactivate or replace with fresh gel.
iii)
Remove cork / blanking tape from top screwed hole and fix breather on the pipe work.
f) Explosion vent i)
Explosion vent is provided with a bakelite diaphragm. Normally transformer is dispatched fitted with explosion vent. If dismantled for the transport, remove blanking plate from bottom of vent before assembly on tank.
ii)
Check whether inside of vent is corroded. Rust should be completely removed and inside painted with oil resistant paint.
iii)
Before assembly make sure that top and bottom diaphragms if provided are intact.
iv)
Diaphragm should be held between two gaskets and care should be taken to clamp the flanges uniformly to prevent cracking of diaphragm.
v)
Pressure equalizer pipe may be provided on against specific requirement which connects top of explosion vent and vents it to the atmosphere through the breather.
TERMINAL ARRANGEMENT Bare Bushing (Draw through type) Erection of bushing :-(i)
Oil level should be lowered below the bushing mounting hole on tank.
(ii)
Remove blanking plate carefully from the tank lid and bring out the soldered stem and winding lead without exerting undue pressure while pulling which may damage the soldered joint or insulation on the lead. If the joint / insulation is damaged re-solder the joint and re-insulate the lead.
(iii)
Lay the sealing washer on hole.
(iv)
Lay the clamping ring on scaling washer.
(v)
Insert the separator or stem.
(vi)
Lower the insulator in hole and at the same time insert the soldered stem through porcelain.
(vii)
Assemble stem sealing washer (Neoprene ring) and cap at the top of stem and screw the top nuts to hold the soldered stem loosely.
Page 24 (viii)
Lift the clamping ring and place the clamping members on welded studs underneath the clamping ring. Adjust the bushing in position and clamp with nuts and washers.
(ix)
Fit the arcing horns in appropriate position.
(x)
Two nuts are provided at the top of stem for connecting the supply lines.
(xi)
Care must be taken that no loose part falls into transformer tank.
(xii)
Release air by loosening the stem and slightly pushing it down wards.
Changing of insulator in case of failure (i)
Disconnect the transformer from supply lines.
(ii)
Drop the oil level below the mounting hole on tank.
(iii)
Remove the arcing horns.
(iv)
Remove the top cap by unscrewing the nuts at the top. Hold the soldered stem with lead securely so that it does not slip in tank or get damaged while removing the porcelain.
(v)
Remove the defective porcelain by removing the fixing nuts on the studs and clamping members. While removing the porcelain care must be taken not to damage the inside soldered joint and the threaded part of stem.
(vi)
Assemble the new porcelain as given in Erection of Bushing.
Bushing (through stem type) Through stem bushing :(a)
Outdoor through stem type porcelain bushing.
(b)
Cable box type type porcelain bushing.
(c )
Cable box type epoxy bushings.
Erection of bushing Outdoor through stem type porcelain bushing : (i)
Lower the oil level below the bushing mounting hole on tank.
(ii)
Assemble sealing washer, bottom porcelain (in case of two part bushing) and fix winding lead to stem and clamp nut with stem.
(iii)
Insert the assembled stem through the bushing hole from inside opening provided on cover may be used.
(iv)
Lay sealing washer on bushing mounting hole from outside.
(v)
Assembly the top insulator over the protruded stem.
(vi)
Assembly brass washer, sealing washer and nuts on the top stem.
(vii)
Connect the winding line lead at the end of stem.
of tank. Inspection
Page 25 (viii)
Care must be taken that no loose part falls into transformer tank.
(ix)
For the replacement of outside part of porcelain, complete bushing need not be disconnected. Outside porcelain can be removed by removing the outside nuts and sealing washers, keeping the stem in position.
Cable Box Terminal (i)
Drop oil level below the bushing fixing hole on the tank.
(ii)
Lay sealing washers on tank hole.
(iii)
Insert the bushing through the hole in the tank. Assemble resilient washer and bushing clamp over fixing stud of bushing.
(iv)
Clamp the bushing adequately by the screwing nut over the clamp.
(v)
Connect the winding line lead at the end of the stem.
(vi)
Care must be taken that no loose part falls into the transformer tank.
Topping up of transformer oil Topping up of oil is required for transformers which have been dispatched with lowered oil level. (i)
Oil should be topped up only after fitting conservator pipe work, explosion vent, HV, LV terminal arrangement and CT terminals, Oil level is lowered 20 mm below the tank cover, and under no circumstances it should be reduced further which might expose the switch and windings to the moisture in atmosphere.
(ii)
Oil used for topping up is delivered in tins, drums should be tested for dielectric strength before topping up. Clean and dry oil must be used for topping up.
(iii)
While filling oil, it is preferable to pass oil through filter plant. Ensure that filter papers are new and clean.
(iv)
Conservator may be filled up until oil level reaches ‘’ Filling Level’’ mark for prevailing temperature (30 deg.C).
Detachable Radiators Normally radiators are attached to the tank, if they are dismantled for the transport, they should be reassembled on the tank as follows : (i)
Radiators entries are blanked inspect inside of radiators.
with blanking plates. Remove
blanking plate and
(ii)
If the inside portion is corroded, clean with sand paper and remove the rust, dust etc.
(iii)
Flush radiators with clean transformer oil.
(iv)
Remove blanking plates from top and bottom radiator valves fitted on tank. Care should be taken to keep radiator valves in closed position while mounting the
Page 26 radiators. It is likely that valves may leak slightly clean containers may be kept under valve to collect oil. (v)
Dry and clean oil should be filled in each radiator immediately as soon as it is mounted, keeping both top and bottom valves closed while filling oil. Open bottom valve between radiator and tank keeping top air release vent open to drive away locked air inside the radiator. Top valve should be open till oil with cut air bubbles flows out.
(vi)
After few hours of filling of all the radiators, air release vents at top should be opened again to drive away accumulated air due to suspended air bubbles.
(vii)
Finally top valve should be opened.
(viii)
Care should be taken while mounting the radiators and filling with oil, there is always reserve oil in the conservator and topped up continuously, if necessary. Oil level should never drop below, ‘’ Filling level’’ mark. Air should be allowed to enter in the conservator through silicagel breather only to exclude moisture from the air entering the conservator. Filling hole of conservator should be always kept closed after use.
(ix)
After opening the radiator valves should be locked in position by fitting cap in proper position. Arrow indication should be towards ‘’OPEN’’.
Temperature Indicator (i)
Oil temperature indicators are generally of two types, one with rigid stem and the other type with capillary tube. They are fitted with temperature sensing elements at the end of rigid stem or capillary tube. Winding temperature Indicator is provided with capillary tube with sensing element (bulb) at the end of tube. Sensing elements are enclosed in metal bulb, which are fitted in pockets provided on tank over in the hottest oil region.
(ii)
Before inserting temperature sensing bulb inside the pocket, transformer oil or heat conducting grease should be filled in pocket. The union coupling on the bulb should be screwed tightly on the pocket so that water does not penetrate inside the pocket.
(iii)
Capillary tube of instrument must be routed and fixed such a way that it suffers less risk of being pinched or bent or cut off. Plastic straps are provided with each instrument for fixing the tube. Surplus length of tube should not be cut off since the pressure balanced system will be destroyed. Tube may be made into loop more than 150 mm diameter and tied to tank in suitable position. Utmost care should be taken while fitting sensing bulb in the pocket since it is likely that capillary tube may form sharp bends and damage the instrument.
(iv)
Instruments are calibrated and under no circumstances indicator pointer should be moved by hand or bent, as it might suffer permanent damage. If the instrument is not giving correct temperature indication as a result of improper handling or any other cause, it may be calibrated as given in instrument’s pamphlet.
Page 27 (v)
Winding temperature indicator (with a separate heater pocket). Temperature sensing bulb provided at the end of capillary should be fitted in the heater pocket in housing fitted on tank cover. Two terminals provided in housing are connected to the heating coil of heater pocket inside the housing (outside tank) and to current transformer secondary terminals from the inside of tank. (These are normally connected before dispatch of transformer). Housing is air filled. Instrument is housed in the marshalling box.
Marshalling Box Box should be fitted on transformer and leads from various instruments should be connected to terminal blocks in respective positions. Outgoing leads from customers instruments and control panels should be also connected in position. Gland plates are normally provided undrilled, which are required to be drilled at the site to take the cables. Off Circuit Switch (i)
Switch should be operated to inspect that it moves freely and locks in every tap position.
(ii)
Locking arrangement should be checked and padlock provided.
Page 28
COMMISSIONING OF TRANSFORMER After finalisation of the erection work the following tests should be conducted :PRE TEST INSPECTION (a)
Sample of oil taken from the transformer and subjected to electric test (break down value) of 50KV (RMS) as specified in IS : 335.
(b)
Release trapped air through air release plugs and valve fitted for the purpose on various fittings like radiators, bushing caps, tank cover, Bushing turrets etc.
(c)
The float lever of the magnetic oil level indicator (if provided) should be moved up and down between the end position to check that the mechanism does not stick at any point. If the indicator has signaling contact they should be checked at the same time for correct operation. Checking the gauge by draining oil is a more positive test.
(d)
Check whether gas operated really (if provided) is mounted at angle by placing a spirit level on the top of the relay. See that the conservator is filled upto the filling oil level marked on plain oil gauge side and corresponding to the pointer reading in MOG side. Check the operation of the alarm and trip contacts of the relay independently by injecting air through the top cocks using a dry air bottle. The air should be released after the tests. Make sure that transformer oil runs through pert cock of Buchholz relay.
(e)
Check alarm and trip contacts of WTIs, Dial type thermometer, magnetic oil gauge etc. (if provided).
(f)
Ensure that off circuit switch handle is locked at the desired tap position with padlock.
(g)
Make sure that all valves except drain, filter and sampling valves are opened (such as radiator valves, valves on the buchholz relay pipe line if Provided).
(h)
Check the condition of silicagel in the breather to ensure that silicagel in the breather is active and colour is blue. Also check that the transformer oil is filled in the silicagel breather upto the level indicated.
(i)
Check tightness of external electrical connections to bushings.
(j)
Give a physical check on all bushing for any crack or any breakage of porcelain. Bushing with cracks or any other defects should be immediately replaced.
(k)
Check the neutral earthing if specified.
(l)
Make sure that neutrals of HV / LV are effectively earthed.
(m)
Tank should be effectively earthed at two points.
(n)
Check that the thermometer pockets on tank cover are filled with oil.
(o)
If the oil temperature indicator is not working satisfactorily, loosen and remove the thermometer bulb from the pocket on the cover and place it with a standard
Page 29 thermometer in a suitable vessel filled with transformer oil. Warm the oil slowly while string it and take reading of the thermometers if an adjustment of the transformer thermometer is necessary the same many be done. Also check signaling contacts and set for the desired temperature. For transformer furnished with thermometers for both oil and winding temperature, the signaling contacts are recommended to be set to operate at the following temperatures. Alarm
Trip
Oil Temperature
85 deg.C +/- 2deg.c
95 Deg.C +/- 2deg.C
Winding Temp.
100Deg.C +/- 2 Deg.C
110Deg.C +/- 2 deg.C
Higher winding temperature manufacture.
settings can be adopted in consultation with
(p)
CT secondary terminals must be shorted and earthed if not in use.
(q)
Check relief vent diaphragm for breakage. See that the bakelite diaphragm at bottom and glass diaphragm at top are not ruptured.
(r)
Check all the gasketed joints to ensure that there is no leakage of transformer oil at any point.
(s)
Clear off extraneous material like tools earthing rods, pieces of clothes, waste etc.
(t)
Lock the rollers for accidental movement on rails.
(u)
Touching of paint may be done after erection.
TEST Ratio, Polarity and phase relationship The ratio should be checked at all taps and between all the windings. Ratio is usually checked by applying a single phase 230V (Approx) supply on the high voltage side and measuring the voltage on the low voltage side at all tap positions. The measured voltage on LV side shall conform to declining trend between tap (min) to tap (max) position for all phases. Polarity and interphone connections are checked while measuring the ratio. This can be checked by the Avometer method. The primary and secondary windings are connected together at points indicated in sketch shown below. A low voltage three phase supply is then applied to the HV terminals. Voltage measurements are then taken between various pairs of terminals as indicated in the diagram. Readings obtained should be the vectorial sum of the separate voltages of each winding under consideration.
Page 30 Insulation Resistance The insulation resistance between winding and earth should be measured with a good 2500/1000 Volts megger and values should be compared to the test report values. It is preferable to have a motor operated megger and the readings taken after one minute from starting. Before measuring the insulation resistance, it should be made sure that the bushings are cleaned thoroughly with clean cotton cloth. The lead wires of the meaggers should not have joints. They should also give reading of more than maximum value readable on the megger. Magnetizing Current A three phase 415 Volts supply is given to the HV winding of the three phase transformer and simultaneous current readings of the three phases are taken using low range AC ammeters of the same accuracy class. Values should be recorded for the future reference. Magnetic balance test A single phase supply of approximately 230 Volts is applied to one phase of HV winding and the induced voltage on other two HV phases measured separately. Test voltage should be applied to HV winding only. Applying voltage LV winding may induce abnormal high voltage in HV winding which may prove hazardous. Tests should be carried out on all three phases. When single phase supply is given to middle phase the induced voltage measured on two extreme phases should be approximately equal. If supply is given to one extreme phase, induced voltage on middle phase is expected to be substantially high than the other extreme phase. In each test the sum of the induced voltages on other two phases would be within +/-5% of the applied voltage. If abnormal voltage difference is observed on extreme phase, when voltage is applied to middle phase, matter should be referred to Kotsons Pvt. Ltd., Agra works for advice, supported with the full test observations. High impedance volt meter like ‘’AVOMETER’’ should be used for the test. Tests should be carried out by connecting a series lamp (approx 25 watts capacity) at supply side to restrict higher current , if any. A fault in winding may be suspected if the series lamp glows brightly or the induced voltage readings on different phases indicate zero or very low value. 230 Volts single phase supply be given between lines, if windings are delta connected and induced voltages measured between the other two phases. Whereas supply is given to line and neutral of one phase in case of star connected windings, induced voltages should be measured between the line and neutral of other two phases. Alarm Circuits Tests a) Buchholz relay – air injection test Alarm & trip circuits b) Oil temperature indicator – alarm & trip circuits c) Magnetic oil level gauge – alarm circuits d) Winding temperature indicator – alarm & trip circuits.
Page 31 Oil Sampling Test Electric strength (break down voltage) should be 50 kv (RMS) when tested to IS 335-1992 please see section 3 for other details. COMMISSIONING After tests are completed satisfactorily, the transformer may be energized from the incoming side on no load and operated for one or two hours. The transformer hum should be observed for any abnormality. After two hours transformer may be switched off and the gas operated relay checked for gas collection. Sometimes dissolved air or air bubbles comes out due to energizing which is misunderstood for gas. So in case of gas collection it should be energized again. Other instruments may also be checked for any abnormal indications. If everything is found satisfactory, the transformer may be energized again and loaded gradually. The following details are to be furnished to the manufacturer after commissioning :a) Details of transformer including its serial number. b) Date of Commissioning c) Substation / generating station where commissioned. d) Protection given to the transformer such as lightning arrester, differential protection, circuit breaker on HV / LV etc. The nature of the test conducted and the results of test should be recorded and made available in the station as a permanent record for future reference. Tools Required (a) Screw drivers of appropriate size. (b) Spanner of appropriate size. Transformer Oil Oil is used as coolant and dielectric in the transformer and keeping it in good condition will assist in preventing deterioration of the insulation, which is immersed in oil. Transformer oil is always exposed to the air to some extent therefore in the course of time it may oxidize and form sludge if the breather is defective, oil may also absorb moisture from air thus reducing dielectric strength. Oil supplied with transformer, generally complies with IS : 335.
Page 32 Precautions : (a) Oil is easily contaminated, it is very important while sampling the oil and filling the tank to keep the oil free from contamination. (b) All equipment used when handling the oil must be washed with clean transformer oil before use. (The oil used for washing must be discarded). Particular attention shall be paid to the cleanliness of bends, valves and other points where the dirt and moistures tends to collect. (c) For sampling the oil, glass containers with glass stoppers are preferred over the metal type, which are susceptible to contamination by dirt etc. Cleanliness is essential as small amount of dirt and water greatly affect the accuracy of test results. Was shall not be used for sealing the sample oil bottles. However, the stopper can be covered by a pack of silicagel tied in a piece cloth. (d) Flexible steel hose is recommended for handling insulation oil, some kinds of synthetic rubber hoses are also suitable, but only those known to be satisfactory should be used. Ordinary rubber \ hose should not be used for this purpose as oil dissolves the sulphur from the rubber and is there by contaminated. Hose used for handling oil should be clean and free from loose dust, rust or scale. (e) Transformer must always be disconnected from the electricity supply system before the oil level in tanks is lowered. (f) Oil must not be emptied near naked lights as vapour released is inflammable. (g) Minute quantities of moisture (Particularly in the presence of fiber or dust)_ lower the dielectric strength of the oil. Therefore to reduce the risk of condensation of the moisture entering the oil containers should be taken into a warm room and should not be opened until the entire body has attained the same temperature as the room. (h) The oil level should be che3cked at frequent intervals and any excessive leakage of oil investigated. There may be slight loss of oil by evaporation, this need not cause concern if the tank is topped up at regular intervals. (i)
All minor leaks or sweating should be repaired as quickly as possible.
(j)
Oil topping up should, generally comply with IS 335-1972 and preferably be from the same source as the original oil. The oils from different crudes may not be completely mixable and may separate into layers, however if they are made from same feed stock, they can be mixed. New oil from a different source may be added, as made up only, but not exceeding about 10%.
(k) Condition of oil can be checked by ascertaining its acidity and electric strength. Both properties 1972.
should be measured at regular intervals by method given in IS 335-
The acidity is normally between 0.3 to 0.5 mg KOH per g of oil, the cover should be removed to ascertain the condition of the interior of the tank and core and windings, oil should be treated or discarded if sludge or corrosion is evident. It may be noted the reconditioning by centrifugal separation filtration does not remove sludge, dust, dirt etc and will tend to retard the process of deterioration. Filters with fullers earth will help to reduce acidity in the used oils and in addition improve the resistively.
Page 33 Oil with excessive acidity should be reconditioned. Reconditioning can under taken only at the factory. (l)
Sample of the oil should be taken at regular intervals and tested Recommended intervals for tests are mentioned in maintenance schedule.
generally be to IS 335.
Testing Oil after filtration should meet the requirements as laid down under the clause specification. However, to know the conditions the following preliminary tests can be carried out. (a) Crackle test A rough test can be made by closing one end of a piece of steel tube 2.54 cm dia (1 inch B.S. pipe is suitable), heating the closed end to just under dull red heat and plunging it into the oil sample with the ear close to the open end. If the oil contains large moisture a sharp crackle will be heard. Dry oil will only sizzle. (b) Electric Strength When tested in accordance with IIEC – 600296 and IS 6792 electric strength (Break down voltage should not be less than 50 KVA RMS) test should be carried out six times on the same sample and the result obtained will be arithmetic means of six result.
Page 34
MAINTENANCE OF TRANSFORMER NOTE : This activity should be performed by qualified person having required knowledge about maintenance of transformer/electrical equipments generating high voltages. 1.
General : If a transformer is to give long and trouble free service, it should receive a reasonable amount of attention and maintenance. Maintenance consists of regular inspection, testing and reconditioning where necessary. Records should be kept giving details of any usual occurrence and also of any previous results taken. The principal object of maintenance is to maintain the insulation in good condition. Moisture dirt and excessive heat in contact with oxygen are the main causes of insulation deterioration and avoidance of these will in general, keep the insulation in good condition. Factors lead to aging of insulation during aging process, are due to chemical and physical effects. The decay of the insulation follows the chemical reaction rate. In case of oil immersed transformer, if the sustained operating temperature of 750C is higher by 60C to 100C it will reduce the life of the transformer. Following are the factors affecting the life of transformer. (a) Moisture : Due to higher affinity of water, the transformer oil and the insulation paper absorb moisture from the air, which result in decrease of dielectric strength. Hence preventive steps should be taken to guard against moisture penetration to the transformer. This will include blocking of all openings for free access of air n storage and frequent reactivation of breathers in service. (b) Oxygen : Oxygen may be present inside the transformer oil due to air pockets trapped in the windings, etc. The oxygen reacts on the cellulose of insulation and decomposes it, which will result in sludge formation, blocking free circulation of oil. Sometimes, the oxygen may act as catalytic agent and increase the operating between hot oil and bare copper. (c) Solid impurities : Dielectric strength of oil diminishes appreciable by minute quantities of solid impurities. It is therefore a good practice to filter the oil after it has been in service for reasonable time. (d) Varnishes : Some varnishes particularly of the oxidizing type enter readily in reaction with transformer oil and precipitate sludge on the windings. Synthetic varnishes having acid inhibiting properties generally delay the natural formation of acid and sludge in the oil. This should be done in mind by the maintenance engineer when rewinding and replacing the coils during repairs of transformers. (e) Effects of slackness of windings: Coils are fully clamped before the transformer leaves the factory. Natural setting of coils may take place during first few months of operation. Slackness of winding may cause a
Page 35 failure due to repeated movement of coils which may wear the conductor insulation at some places and lead to an interterm failure. The coils may also get displaced under the load conditions or momentary short circuit which may cause electric and magnetic unbalance. It is therefore a good practice to lift the core and winding of a transformer and remove any slackness by tightening the rods of pressure screws. 2. Maintenance Procedure : (a) No work should be done on the transformer unless it is disconnected from all external electrical circuits and all windings have been solidity earthed. General notes on information given under storage handling and inspection are applicable for routine maintenance. Oil level shall be maintained to keep the insulation under oil and lowered to reasonable before slackness and removing bolts, nuts and conservators, radiators, etc. No fire shall be near the transformer while work is going on. Precautions are to be taken to secure tools with tape outside the tank, to prevent them from dropping inside the tank. (b) Core and Winding : It is recommended that the core and windings be removed from the tank for visual inspection as per the maintenance schedule at the end. The windings should be examined to ensure the no sludge has deposited blocking the oil ducts. Any loose nut and bolt should be tightened. Adjusting rods / coil clamping screws provided should be tightened to bear evenly and firmly on the coil clamping block/rings, if there is any slacking of windings. Before lifting the core and windings from the tanks it is necessary to disconnect the windings from the bushings inside the tank, to disconnect the off circuit tap switch handle, to remove earthing strip between the core and the tank, and remove fixing bars between core clamps and tanks guide bars. The core and windings must be removed with great care under cover and in dry lace. If this is not possible a visual inspection of as much of the transformer as can be seen within its tanks should be carried out. Refer untaking instruction also. (c) Off circuit tap switch : i) This is an integral part of the transformer. Care should be taken to switch off the transformer from line before operating the switch handle. To operate the switch, unlock the operating handle by removing the locking strip/pin. Move the handle to required position and relock. Tapping position number can be seen through the windows of handle alternatively on the handle when window not provided. ii)
Off Circuit tap changing links : Tap changing links are provided inside the tank. They are located under oil, and are accessible from the inspection opening to operate the links. Links should be unbolted by loosening nuts on studs and fixed in desired tap position.
Page 36 iii)
On Load tap changer : On load tap changer is normally mounted on the tank is a separate housing and connected to winding leads through copper studs fixed on a insulated terminal board Terminal board is on leak proof. Oil in the tank need not be lowered down for a attending to OLTC gear. Please see OLTC leaflet for the operation and maintenance instructions.
(d) Conservator : Conservator is so designed that the lower part acts as sump in which any impurities entering the conservator will collect. A drain plug is fitted at the lowest point of the conservator for draining and sampling oil. While sampling, care must be taken to run off any contaminated oil before taking a sample for testing. The inside of the conservator should be cleaned every 2 to 3 years by flushing clean transformer oil to remove sludge and other impurities. Oil level should be maintained at “Filling level” mark (+300C). (e) Oil gauges : Plain oil gauge : Oil gauge glass should be kept clean so that the gauge can fulfill their purpose. Broken glasses should be replaced immediately. Magnetic oil gauge : When the conservator is stripped for cleaning, the mechanism of the float type oil gauge should be inspected and cleaned. The function of alarm and trip contacts should be checked. (f) Silicagel dehydrating breather : Breather should be examined frequently to ascertain if the silica gel requires changing. The frequency of inspection depends on local climate and operating conditions. More frequent inspection are needed when the climate is humid and when transformer is subject to fluctuating load. The crystals of silica gel in the rather act as an indicator and changed from blue to pink, on becoming saturated with moisture. When majority of crystals have turned pink, the silica gel should b heated in or over a shallow pan at a temperature of 1500C to 2000C until the original blue colour is regained. This usually takes 2 to 3 hours. Oil should be cleaned to remove dust and dirty oil. The level in oil seal should be maintained with fresh oil. Check that inside of connecting pipe between breather and conservator is clean and not rusted. If necessary clean and paint inside with oil resistant paint. (g) Buchhloz relay : Routine operation and mechanism inspection tests should be carried our at one or two yearly intervals respectively. Te operation is tested by injecting air through lower level petcock of double float buchhloz relay. (refer installation manual of Buchhloz relay for testing). After inspection, any air, which has accumulated in the gas chamber, must be released at the upper level petcock leaving the chamber full of oil. To carry out mechanical inspection, the oil level should first be brought below the level of relay or the shut off valve between conservator and tanks closed and the mechanism removed. The float should be tightly clamped and be in good order. If a mercury switch is defective owing to the glass being cracked. There will b clouding of the glass. In such cases the switch must be replaced.
Page 37 During operation if there is an alarm and gas is found to be collected, the transformer should be isolated from lines and the gas should be tested and analysed to find out the nature of fault. Sometimes if may be noticed that the gas collected is only air. The reason for this may be that the oil is releasing any absorbed air due to change in temperature. Internal faults can be identified to a great extent by a chemical analysis of gas.
i) ii) iii) iv)
By examining the gas it is possible to detect the nature of fault. Colorless and odorless gas with faint odor of oil is air trapped in the oil or insulation. Grayish white, non inflammable gas with sharp penetrating odor may by due to over heating or faulty insulation. Yellowish and inflammable gas may be due to surface leakage on material like wood. Dark gray and inflammable gas may be due to flashover in oil or due to excessive overheating of oil caused by a fault in the winding or core.
(h) Explosion vent : The diaphragm, which is fitted at the exposed end of the vent should be inspected at frequently intervals and replaced if damaged. Failure to replace the diaphragm quickly may allow the ingress of moisture in the transformer. If the bottom diaphragm has broken because of a faulty in the transformer, inspection should be carried our to determine the natural and cause of the fault. (i) Gasket : Gasket sometimes shrink during service. It is therefore, necessary to check the tightness of all bolts / fasteners of gasketed joints. The bolts should be tightened evenly around the joints to avoid uneven pressure. Leaky gaskets should be replaced as soon as the circumstances permit. (j) Pipe Work The pipe work should be inspected at least once a year. Leak may be due to badly sealed joints caused by misalignment, in which case the pipes should be aligned and joints remade. (k) Temperature Indicators At each yearly maintenance inspecting, the level of oil in the pockets holding thermometer bulbs should be checked and the oil replenished if required. The capillary tubing should be fastened down again if it has become lose. Dial glasses should be kept clean. Temperature indicators, if found to be reading incorrectly, should be calibrated with standard thermometer immersed in hot oil bath. (L) Paint work : During storage and service, the paint work should be inspected once a year and necessary painting or retouching carried out. If the metal surface is exposed and becomes dirty rusty or greasy because of delay in repairing the paint work, the surface must be thoroughly cleaned before repainting to ensure a good bond between metal and paint. If recommended paints are not available, any good quality synthetic enamel paint may be used.
Page 38 (M) Unmaking and tanking details : General : For transformers assembled with bushings, conservator, pipe work etc. Before removing the top cover from the tanks following care should be taken: (i)
(ii) (iii) (iv) (v) (vi) (vii) (viii) (ix) (x)
All the bushing/cable boxes/pockets mounted on cover (connected to core and winding ) should be mechanically disconnected off the transformer and removed carefully. External cable connections laid over the cover connected to terminal box, buchholz relay, OTI etc. Should be disconnected and removed. Thermometer bulbs along with capillary tubes to be removed. Pipe work from the cover to the conservator to be removed, if conservator is separately supported on tank brackets. Oil should be drained out below the tank cover level. All isolating valves should be closed. Disconnect CT secondary leads connection when CTs are mounted on the live part. Disconnect core and winding (live part) from all external connections, to avoid damages while removing cover from the tank. Detach the fixing bolted to the square angular guide bars on the tank. Before lifting core and windings cheek that the lifters are rigidly fixed to core clamps. (A)
(i) (ii)
Drain out oil below switch handle level from transformer tank. It is important that the switch handle assembly should be dismantled before untanking the core and windings. (B)
(I) (II) (III) (IV)
(V)
Untanking the core and windings of transformer with off circuit switch:
Untanking of core and windings of transformer with OLTC
Drain out oil below the cover level of transformer and OLTC. Detach the winding leads from the terminal board between transformer and OLTC. Ensure that the lead number tags are in position on the leads. If not, tag the leads correctly to reconnect the leads in correct position on terminal board. Give temporary support under the OLTC housing before lifting the core and windings from the tank. It is likely that the tank may become unstable due to weight of OLTC without core and windings in main tank. Refer OLTC leaflet for further information.
DRYING OF TRANSFORMER : If as a result of tests carried out in section 6.4, presence of moisture is indicated or the oil does not withstand dielectric strength or the insulation resistance readings are not satisfactory is shall be necessary to dry out the transformer.
Page 39 Method of drying out Normally hot oil circulation method should be used for drying out the transformer. In special circumstances where the above method does not give satisfactory results, short circuit method with hot oil circulation as described in 8.1.2 should be used. In this method both oil and the core and winding inside the tank are simultaneously dried out with stream line or other filter with heater and vacuum pump. The moisture is driven out from the windings into the oil and removed from the oil by evaporation and filtering. Constant circulation of hot oil through a filter will lead to gradual deterioration of the oil and probably a partial breakdown of the constituents of the oil due to the continuous mechanical action and filtering under high temperature. Further this tends to increase the acidity of oil. Great care should. Therefore, be exercised in circulating the oil through the filter during the drying out period. The tank sides and top cover should be covered with some covering like tarpaulin. The LV winding should be short circuited and a three phase supply of 415 volts be applied to HV side of the transformer. It should be ensured that the short circuit current does not exceed the rated current of transformer. The temperature of the top oil should be measured by a thermometer. The oil temperature should not exceed the limit of 80 degree C. During the drying out process temperature of the top oil and the insulation resistance of the winding should be taken every two hours. As the temperature of the oil rises, the insulation resistance may fall and will eventually reach a steady value. The temperature should be kept constant until the insulation resistance shows steady increasing upward trend. When this poking is reached the drying out process is complete and the application of heat may be discontinued. As during the cooling down process, the transformer reaches 60dgree C the insulation resistance should again be measured. The insulation resistance in mega ohms varies inversely with the temperature and for a 10dgree C change of temperature, mega ohms change by ration generally in the range 2.1 to 1.4 :1. Precautions during drying out : Drying out is a delicate operation and great care and judgment are necessary in carrying it out correctly to avoid damage to the transformer winding or oil. Care should be taken to see that maximum limits of temperature specified are never exceed i.e.80 degree c. It is essential that the insulation tester and the thermometer used for the purpose are reliable. Only spirit type thermometer are to be used for temperature measurement. Mercury thermometer shall not be used everywhere except in the pocket provided for this purpose. In no case transformer should be left unattended during any part of the drying out period. The transformer should be observed continuously throughout the drying out process and all observations shall be recorded. Insulation resistance should be measured at least with a 100 volts meggar. The life of the insulation decreased with every drying out process. Therefore, it would be desirable to dry out the transformer at 50dgree c less that maximum temperature stipulated i.e. 80dgree c.
Page 40
TESTING OF TRANSFORMERS Transformer is a static machine with no moving parts. It is meant for transmitting power from one voltage level to another at the same frequency without exceeding the temperature rise limits. The temperature rise has to be kept within permissible limits as with higher temperature the life of the insulation decreases considerably. The studies conducted have revealed that with 6* increase in insulation temperature over permissible value the life of transformer gets reduced by about 50%. The efficient working of the transformer and its reliability is very important for the power system performance. This can only be achieved by using quality materials, ensuring high standards of workmanship during manufacture, and various testing of components and finished products. Various tests carried out at different stages of manufacture assure quality and confirmation of design calculations. The final tests on fully assembled transformer also assure suitability of the equipment for satisfactory performance. The raw materials used in the manufacture of transformers are properly inspected and tested where necessary. The fully assembled transformer is then thoroughly checked for, Quality •
Long life
•
Reliability
•
Manufacturing Tolerance
•
Design Verification
•
R&D
For this purpose various tests are conducted as specified in IS2026/IEC726/BS171. The basic testing requirements and the testing methods are set out in the national/international standards referred ti above. It is important that these tests are carried out thoroughly to ensure reliable and efficient performance of the transformer during its lifetime. Some of the tests specified in Indian Standards are listed below; A. Routine tests. 1.
Measurement of winding resistance.
2.
Verification of voltage ratio.
3.
Verification of vector Group.
4.
Measurement of no load current and losses.
5.
Measurement of impedance and load losses.
6.
Measurement of insulation resistance.
7.
Power frequency withstand Voltage.
8.
Induced over voltage test.
B. Type Tests. 1.
Impulse Voltage withstand test.
2.
Temperature rise test.
