Transformer Vector Technic

© ABB India, BMTG - 1 Apr ’02 , Author: Nag Ramesh

Electrical Installation Techniques

Medium sized Distribution Transformers

Nag Ramesh 31st December 2007

Contents

© ABB Inida, July 14-15/‘03 - 2, Author: Nag Ramesh

Sl. No.

Topic

Slide No.

1

Caution

7

2

Types of Transformers

8 - 12

3

Transformers – Standard Accessories

13 - 15

4

Transformers – Standard Accessories - Wheels

16 -17

5

Transformers – Standard Accessories - Bushings

18 - 19

6

Transformers – Standard Accessories – Arcing Horns

20

7

Transformers – Standard Accessories - Conservator

21

8

Transformers – Standard Accessories - Oil level gauge

22

9

Transformers – Standard Accessories - Silica Gel Breather

23 - 24

10

Transformers – Standard Accessories - Buchholtz Relay

25 - 26

11

Transformers – Standard Accessories - Gas drawing device

27

12

Transformers – Standard Accessories - Pressure relief valve/ Explosion vent

28 - 29

Contents


Sl. No.

Topic

Slide No. 30 -32

13

Transformers – Standard Accessories – Radiators, Radiators assembly and disassembly

14

Transformers – Standard Accessories - Butterfly Valve for Radiator

33

15

Transformers – Standard Accessories Oil Temperature Indicator (OTI)

34

16

Transformers – Standard Accessories Winding Temperature Indicator (WTI)

35 - 36

17

Transformers – Standard Accessories Protection Junction Box (Cable Termination Box)

37 - 38

18

Transformers – Gaskets

19

A completely assembled Transformer (typical 1600 kVA)

40 -43

20

Transportation of Transformers

44 - 45

21

Transfer of responsibility, Handling and lifting

46

22

Lifting of Transformer by Lugs and Jacks

47

39

Contents


Sl. No.

Topic

Slide No.

23

Lifting of Transformer by Fork Lift and transferring

48

24

Lifting of Transformer – Wire Ropes

49

25

Receiving the transformer at site

50 -51

26

Storage of Transformer prior to energizing

52 - 53

27

Indoor Installation

54 - 57

28

Outdoor Installation

58

29

Foundation Construction – rails and shims

59

30

Foundation Construction – oil pit and oil sump

60

31

Shifting to and placement on foundation

61

32

Minimizing noise emission

62

33

Oil sampling from a Transformer

63

34

Oil sampling from an oil drum or tanker

64

35

Oil sampling kit and test cup for dielectric strength

65

Contents


Sl. No.

Topic

Slide No.

36

Erection of Transformers transported fully assembled and filled with Oil

66

37

Erection of Transformers transported with loose parts and filled with Oil

67 - 69

38

Erection of Transformers transported with out Oil

70 -73

39

Additional Supports

74

40

Oil Filtration

75

41

Oil Filtration Machine

76

42

Oil Filtration Process

77 - 80

43

Oil Filtration Process - Oil filtration performance analysis

44

Power Circuit Connections

82 - 87

45

Pre-energisation Checks

88 - 103

46

Check-list to be filled in before energisation

47

Energisation

81

104 – 105 106 107

Contents


Sl. No.

Topic

Slide No.

48

Energisation – Check List

108

49

Operation – Temperature rise and loading capacity

109 - 113

50

Parallel Operation of Transformers

114 - 116

51

Load distribution between parallel running Transformers

52

Protection of Transformers

118 - 120

53

Maintenance and supervision during operation

121 - 123

54

Investigation of Transformer disturbances

124 - 128

55

Repairs and Spare Parts

56

Safety Precautions

57

Installation Record for Transformers

58

Applicable Standards

133 - 134

59

Acknowledgement and references

135 - 136

60

Disclaimer

137

61

Power and Productivity for a better world

138

117

129 130 - 131 132

Medium sized Distribution Transformers Caution 1. Installation of Transformer should be done by skilled personnel or contractors who are trained and qualified in installation of Transformer and similar electrical equipment. All on-site regulations must be carefully observed (installation regulations for electrical systems, regulations for Transformers and environmental safety). 2. In order to eliminate danger to anyone, the Transformer must be installed in such a way that touching the Transformer is impossible after it has been energized.


3. Precautions must be taken to ensure that access to the Transformer is possible only after it has been isolated from the mains (e.g., by forced interlock between the switchgear and the Transformer access doors), exceptions are customer requirements and Transformers with cable boxes for both primary and secondary and Transformers mounted on pedestals with porcelain to ground clearance > 2,74 m. 4. To prevent any risk, suitable measures must be taken during erection and maintenance work to ensure that the Transformer cannot be reconnected by any unauthorized persons.

Types of Transformers Transmission of energy is generally divided in two parts; first is transmission over long distances at high voltages, which is supported by Power Transformers. The second part is distribution of the energy from substations to the various users; this is supported by Distribution Transformers in various hierarchies.

1. LARGE DISTRIBUTION TRANSFORMERS, LDT Power range

5000 kVA and above

Primary voltage

Up to 72,5 kV

Available fluids

Mineral oil, dimethyl silicone,


esters and synthetic hydrocarbons. Transformers of this type are used for receiving the energy from higher voltage levels and to transform and distribute the energy to lower voltage level substations (11 or 33 kV) or directly to large industrial consumers.

Types of Transformers 2. MEDIUM DISTRIBUTION TRANSFORMERS, MDT Power range

315 - 5000 kVA

Primary voltage

Up to 36kV

Available fluids

Mineral oil, dimethyl silicone, esters and synthetic hydrocarbons.


Transformers of this type are used to step down three phase high voltage to low voltage for energy distribution, mainly in metropolitan areas and for industrial applications.

These transformers are available in three phase dry type and oil filled (hermetically sealed or with Oil Conservator and Breather). In hermetically sealed type, flexible corrugated tank walls enable sufficient cooling of the transformer and compensate for changes in the oil volume due to temperature variations during operation.

Types of Transformers SMALL DISTRIBUTION TRANSFORMERS, SDT Power range

Up to 315 kVA

Primary voltage

Up to 36kV (generally 11 kV)

Available fluids

Mineral oil, dimethyl silicone, esters and synthetic hydrocarbons.


Transformers of this type are used to step down three-phase high voltage to low voltage for energy distribution, mainly in the countryside or low-density populated areas.

Types of Transformers DRY-TYPE DISTRIBUTION TRANSFORMERS Dry-type Transformers are used to minimize fire hazard and other environmental contamination on surroundings and people, like in large office buildings, hospitals, shopping centers and warehouses, sea going vessels, oil and gas production facilities and other sites where a fire has potential for catastrophic consequences. They are available as resin impregnated, vacuum cast resin and Resibloc versions. Resin impregnated Transformers are rarely used in distribution applications.


Vacuum cast resin dry-type Transformers Power range

50 kVA up to 30 MVA

Primary voltage

Up to 52 kV (generally 33 kV)

Climate class

C2

Vacuum cast means that the high voltage windings are cast-in in epoxy and cured in vacuum. The high voltage windings are typically disk winding.

Types of Transformers Resibloc® dry-type Transformers Power range

30 kVA up to 40 MVA

Primary voltage

Up to 52 kV

Climate class

C2


Resibloc® is an ABB patented process for the high voltage winding. The high voltage winding is multi layer type with a cross wound glass fiber insulation soaked in epoxy, cured in open atmosphere.







Transformers – Standard Accessories Wheels They are always bi-directional, flat-type or flanged. They are usually supplied disassembled from the transformer. Their assembly can be executed lifting the transformer with a crane or with some jacks placed on special brackets in one of the three ways shown in the enc. dwg. 1. Trolley 3. Wheel 5. Pivot 7. Spacers 9. Flask

2. Flask 4. Trolley 6. Wheel 8. Fastener plaque pins with screw


Type 1 Medium sized Distribution Transformers: fix the flask unit (with the wheels) to the trolley by means of the two special screws in the suitable direction. Type 2 Small sized Transformers: insert the pin into the wheel and into the trolley holes in the suitable direction, together with the spacers. Fasten the pin with the help of the special plaque and screw.

Type 1

Type 2

Transformers – Standard Accessories Wheels


Type 3 Medium transformers: fix the flask (with the wheels) to the back flask with the four special screws in the suitable direction.

Transformers – Standard Accessories Bushings


1. Upper porcelain 2. Lower porcelain 3. Stem 4. Cap 5. Washer 7. Gasket 8. Gasket 9. Gasket 10. Gasket 11. Gasket 12. Washer 13. Nuts 14. C- Washer

The components of the bushings are: - metallic stem coupling internal and external live parts of the transformer; - insulating porcelain; - oil-tight gaskets; - coupling and clamping nuts; - Arcing horns.

Instructions for Bushing replacement: Unscrew the nuts (13) without letting the stem turn (3). Remove the cap (4), the gasket (7), the porcelain (1) and the gasket (9). After replacing the damaged part reassemble the bushing operating in the opposite direction taking care not to let the stem turn (3). Breathe where necessary (Buchholz relay, bushings) loosening the nuts (13).

Transformers – Standard Accessories Bushings Bushings are usually already assembled on the transformer: in some particular cases, they are shipped separately and must be assembled upon arrival, following manufcaturer’s instruction, after removing the closing caps on the transformer and with the oil level below the transformer cover. Bushings must be stored in their original package, until their assembly. The porcelain part is particularly fragile, therefore maximum care must be taken when handling or fixing the external connection, avoiding dangerous stresses. No particular maintenance is required, except the periodical cleaning (once every 6 months) of the external porcelain surface.


In case of oil leakage, check: - the pressure on the gasket; - the porcelain conditions; - the gaskets conditions. When the replacement of one of the bushing components is required, follow the instruction supplied with the part. In any case, it is advisable to keep one or more spare bushing and relative gaskets as emergency spares.

Transformers – Standard Accessories Arcing Horns on Bushings They are provided on the bushings as a protection to the transformer against transient over-voltages across windings and earth due to lightning and switching surges. They are generally provided on request. The lower horn is fastened on to flange of the bushing with one of the fixing screws and the upper horn is fixed to top end nut of the bushing by means of a bracket. Adjust the gaps ‘K” and “C” as per manufacturer's recommendation.


System Voltage

BIL Level

Arcing Horn Distance K (mm)

kV

kV

ABB

IEC

12

75

60

85

17.5

95

70

155

24

125

100

155

36

170

140

220

Transformers – Standard Accessories Conservator


8

1) air inlet pipe (transformer side) 2) conservator filling plug (transformer side) 3) conservator supporting brackets 4) bracket fixing plates 5) lifting lugs 6) conservator drain plug (transformer side) 7) oil level gauge

8) pipes and fittings for connection to the transformer tank 9) pipes and fittings for connection to the tap changer 10) oil level gauge of the tap changer 11) conservator. drain plug (tap changer side) 12) conservator filling plug (tap changer side)

Transformers – Standard Accessories Oil level gauge

1 - Signalling switch 2 - Float 3 - Rod 4 - Gland

The level gauge is made up of the following components: - main body with Plexiglas graded window - float and drive - magnet with external signalling disk - minimum oil level signalling switch


Usually the oil level gauge is supplied already assembled on the conservator.If not, it has to be fixed to the proper stud bolts, inserting the relevant gaskets. During transformer filling utmost care has to be taken to check that the gauge index is corresponding to the oil temperature (taken by the thermometer). The switch contact is normally closed. Perform the electrical connections according to the Wiring Diagram (being the terminal numbered on the board), or find the terminals through a simple continuity test. The level gauge grounding is performed by a suitable little connecting terminal. Before the transformer start-up, it is advisable to check the gauge and the switch for perfect operation, taking into consideration that the switch trips slightly in advance in respect to the minimum level signal. No particular maintenance is required for the level gauge. Eventual oil leakage can be eliminated by replacing the tight gasket, after having lowered the oil level in the conservator.

Transformers – Standard Accessories Silica Gel Breather 1 - internal thread 2 - inox cylinder 3 - transparent hose 4 - labyrinth 5 – oil cup

Silicagel braether is a transparent tank of salts of Silica chemically pure, with Cobalt indicator. The air sucked into the transformer (due to thermic contractions of oil mass) passes through this. Silicagel absorbs humidity in the air signalling the reached degree of saturation by change of colour: Orange or Blue = completely dry Purple or Pink or White = completely saturated.


The salts contained in the breather can be taken off and regenerated by heating them at 120-150°C until they get their original blue colour again. The upper and lower parts of the breather consist of compact casting in aluminium alloy. The transparent hose, which contains the salts, is of Plexiglas or glass (for chlorinated oil transformer) and it is protected by a stainless steel cylinder drilled in such a way as to allow the visual inspection of salts. In the lower part there is a closing system which prevents the contact between air and salts: this closing system can be mechanical or hydraulic, and allows the air passage in the two directions (inlet and outlet) only when there is a pressure difference between the inside and the outside of the transformer.

Transformers – Standard Accessories Silica Gel Breather (contd.) Silicagel breather is normally shipped disassembled to avoid alteration due to oil leakage caused by transports jolts. It must be assembled to the suitable pipes coming from the conservator of the transformer. The oil cup must be attached to the breather after removing the cork plug on the breather. Fill the oil cup with oil up to the mark on the cup before fitting it on to the breather. Silicagel breather needs no particular maintenance, apart from periodical regeneration (or replacement) of salts, and periodical check of oil level in the cup.


Silicagel breather, before assembling, has to be stored in its original package. The breather should be removed during oil filtration to avoid oil getting mixed with Silica gel during oil filtartion, and the pipe from the conservator must be plugged. When 2/3rd of the Silicalgel has turned Purple, Pink or White, it has to be replaced or regenerated.

Transformers – Standard Accessories Buchholtz Relay

Buchholtz relay usually has two contacts (tripping and alarm) And can operate closed - open - or one closed and one open. It Is made of: - an external case complete with its parts as shown on the drawing; - an internal metal frame float, mercury switches or magnetic controlled switches; a metal fan calibrated for signal at strong oil back flows (from the transformer to the conservator). Usually Buchholz relay is supplied already mounted on small & medium rating transformers and disassembled on high rating ones.


Eventually it must be assembled during erection as per manufacturer’s instruction, generally observing the following: 1. Before assembly, check that the Buchholz is not damaged and it is free from humidity. If not so, it has to be dried in a furnace at a temperature not higher than 70°C; 1. Terminal board 2. Breather cock 3. Mechanical test button 4. Gland 5. Drain plug 6. Inspection holes 7. Pneumatic test valves

2. Assemble the Buchholz and related gaskets between the flanges of the pipes placed between the cover of the transformer and the conservator; an arrow on the relay shows the direction of assembly from the transformer to the conservator.



Buchholtz Relay 3.

Connect the earth, alarm & tripp terminals: their identification is made with the diagram on the relay.

4.

Make sure that eventual shut-off valves are open; then fill the Buchholz relays with oil through the conservator.

5.

Breath the Buchholz with the breather cock.

6.

Test operation of Buchholz relay as follows:

6.1

Mechanical test: Remove the protective cap screwed on the button (item 3); push the button checking the contact closing (or opening) by a continuity test;

6.2

Pneumatic test: Blow-in air through the suitable valve (item 7) checking the closing (or opening) contact as above; Alternatively, close the shut-off valves (if there are any) at the top and at the bottom of the Buchholz: drain the relay through the drain plug checking the contact closing (or opening) as above. Then fill the Buchholz again, and breath. In any case, both contacts have to be handled with care in order to avoid internal damages and decalibration. Such relays must be stored in their original packages during storage. However, it is advisable to keep at least one spare part complete of gaskets because model change and it is not always possible to interchange them.

Transformers – Standard Accessories Gas drawing device

The gas drawing device must be connected to the breather pipe of Buchholz relay by means of the suitable tube foreseen for this purpose, and must be fixed to proper support according to the overall dimension drg. After filling it with oil, breathe from the cock (item 3). During the normal working the cock (item 2) is open, while 3 and 4 are closed. This way the box and the tube are filled with oil.


If you want to draw the eventual gas collected in the Buchholz relay, open cock 4 taking care to let cock 2 always open: thus the oil can come out from the box and the gas contained in the relay can enter the box. 1. Connection to Buchholz relay 2. cock 3. Breathing and gas drawing cock 4. Oil drawing and drain cock 5. Gas inlet valve for relays circuit test

When gas has moved into the box (check through the graduated hole) close cocks 2 and 4. To drain gas open cock 3. The gas drawing device is also equipped with a gas inlet valve for the relays circuit test. This test has to be carried out before starting the transformer up, operating the same way as stated for the Buchholz relays. Before assembling, keep the device stored in its original package.



Pressure relief valve/ Explosion vent

1. Switch for opening signal 2. Breathing screw 3. Spacer 4. Gasket 5. Holes for screw stud 6. Visual Indicator for valve opening 7. Orienting cover 8. Fixing screws for the protecting cover 9. Valve body 10. Gasket seat 11. Calibrating nut

Transformers – Standard Accessories Pressure relief valve/ Explosion vent Spring loaded explosion valve is usually supplied already mounted on the transformer. Its assembly must be carried out placing the valve case and its gaskets on the proper hole on the transformer, as indicated in manufcaturer’s drawing. After filling the transformer with oil, trapped air must be let out by loosening the breathing screws and then tightening them back. At this point the protecting cap must be so positioned by orienting it as to direct the hot oil sprays in the least dangerous direction, in case of eventual opening. These vents are equipped with the following devices: - a calibrating nut placed on the central stem, which blocks the springs. If the nut is screwed up the pressure rises, and viceversa. The calibration ranges normally from about 0,2 to 0,7 atmosphere;


- a visual indicator for valve opening: this is a pin projecting from the central part of the protective cover when the valve opens. To make it go back to its right position it is necessary to push it in side wards; - a signalling switch: this is a quick-trip limit switch enclosed into a tight box. It operates simultaneously with the visual indicator; the contact can be normally closed or open. Signalling is continuous until the visual indiactor is put again in its normal position. Pressure relief valve/ Explosion vent has to be stored in its original packing until assembly. Before assembling, it has to be made are free from dirt and moisture. No particular maintenance is required.

Transformers – Standard Accessories Radiators

A - Lifting hook C – Gasket E - Fixing plaque for radiators G - Oil drain valve

B - Upper shut-off valve D - Upper breathing valve F - Lower shut-off valve H - Shut-off valve Lever

On distribution and small rating transformers, radiators are welded to the tank and need no particular maintenance, except for usual and periodical cleanings which are necessary in the long run. On higher rating transformers, radiators are usually of detachable type.


When radiators are supplied disassembled, and the storage is foreseen to be long, it is advisable to keep them stored indoor, in order to prevent deterioration of surfaces due to atmospheric condensing. The flange closing the caps must not be removed: if accidentally removed, make sure that radiators are free from humidity, rust or dirt before assembling. It is better to keep a complete set of gasket as emergency spares and to keep the closing caps of radiators and flanges for an eventual reuse.

Transformers – Standard Accessories Radiators Assembly -

Make sure that the shut-off valve (B&F) are safely closed: remove the caps from valves and radiators;

-

Place the gasket (C) in the proper seats;

-

Assemble one radiator at a time, fill it with oil, proceeding as follows: a) open the breathing valve (D) of the upper header; b) open the lower shut-off valve (F) making the oil flow into the radiator very slowly, in order to avoid air bubbles; c) when the radiator is filled with oil, close the upper breathing valve;


d) open the upper shut-off valve (B) to permit the oil circulation. Proceeding as above, the oil level in the conservator decreases: therefore it is necessary to restore the oil level in the conservator as gradually as radiators are assembled and filled. Assemble the radiator fixing plaques by means of the proper flanges (E).



Radiators disassembly -

Close both the shut-off valves (F&B);

-

Open the upper breathing valve (D);

-

Open the drain valve (G); empty the radiator pouring oil into a dry clean receptacle;

-

Remove the radiator, place the closing caps on the shut-off valves (B&F) without removing the gasket (C) from their seats;

-

Place the rubber blanking caps on the radiators headers in order to avoid internal dirt and moisture;

-

Close the breathing and drain valves (D&G).

Transformers – Standard Accessories Butterfly Valve for Radiator

Opening and closing of radiators’ butterfly valves have to be performed as follows: - loosen the ring nut (2) in order to reduce the pressure on the gasket (3); - turn the lever (1) in order to open or close the valve; - tighten the ring nut (2).


To replace the gasket (3), remove the pin (4), remove the lever (1), unscrew and remove the ring nut (2), remove the block (5) and withdraw the gasket (3) by means of a nail. Then replace the gasket. After the replacement, fit again all parts in the reverse sequence.

Transformers – Standard Accessories Oil Temperature Indicator (OTI) 1. Bulb for temperature measurement 2. Rotating female plug 3. Capillary plated with lead and flexible pipe 4. Pipe union with spring for application of vibration damping suspension 5. Sealing threaded ring 6. Sealing box for connecting terminals 7. Pointers for fixed contacts of alarm and trip

It is generally a pneumatic tmeperature indicator and is used usually for top oil temperature measurement. It is supplied already assembled. Electrical signalling and tripping must be carried out according to the Wiring Diagram. The thermometer grounding is carried out from a suitable terminal. Dip the bulb into its pocket (thermo well filled with oil) and fit it with the rotating female plug. The trip contact pointer has to be calibrated as follows: - For OFAF: sum of the values of max. ambient temperature and max over-temperature allowed for the oil; - For OFWF: sum of the values of the cooling water temperature and max. over-temperature allowed for the oil.


The alarm contact pointer has to be calibrated at 5-10°C less than the trip one. Some thermometers are provided with a fourth pointer for showing the maximum temperature reached. They are usually supplied already calibrated: If not so, calibration must be carried out with its proper screw. All thermometers must be handled carefully to avoid damages of capillary. Before being assembled they have to be stored in their original packing. When a remote temperature indication is required, a pt100 sensor inserted into the thermo well is used.

Transformers – Standard Accessories Winding Temperature Indicator (WTI)


Measurement of winding temperature is carried out in an indirect way. The windings’ hot spot is assumed to be near top of the windings where they are surrounded by top oil. Further the temperature gradient between winding hot spot and top oil is dependent on losses in the winding. Winding losses are proportaional to square of the current. Hence winding hot spot temperature can be approximated by top oil temperature + a value proportional to the temperature diffrence i.e. a value proportional to square of the current. This is realised by sensor similar to OTI inside a resistor immersed in top oil, and current from secondary of a current transformer which reflects the winding current is made to flow through the resistor. A shunt is provided across the resistor for adjusting the indicated temperature to winding temperature calculated or measured during heat run test. Hence the temperature indiactor measures the top oil temperature to which is added the temperature difference between winding and top oil. The current transformer is mounted inside the transformer tank or in the cable termination box.

Transformers – Standard Accessories Winding Temperature Indicator (WTI) For remote indication, a pt100 element is used instead of pneumatic instrument. In some designs, winding top oil temperature gradient is added to top oil temperature in the dispaly unit using only an OTI and winding current transformer output. Like OTI, WTI also has alarm and trip contacts. On actuation of trip contact, upstream circuit breakers must be tripped with proper wiring from WTI/ marshalling box to HT Panel. Sometimes additional contacts are provided in WTI for control of additional forced cooling equipment. Normally a WTI does not need any calibration at site. Mounting instructions are similar to that of OTI.


In case of dry type type transformers, WTI will be 3 or 6 pt100 elements or thermistors embeded in the top of the winding and the sensors are connected to a temperature indicator-cum-scanner mounted on terminal box.



Protection Junction Box (Cable Termination Box)

1-2-3 - Fastening methods fixing the junction box to the transformer cover 4 - Junction box 5 - Air outlet 6 - Cover 7 - Air inlet 8 - Cable Gland 9 - Cable 10 - Transformer Bushing 11 - Direct coupling to bushing 12 - Cable termination busbar 13 - Cable termination 14 - Support insulator 15 - Front Cover with manholes or flange for panel (if required) 16 - Disconnecting chamber 17 - Copper Flexibles 18 - Gland Plate

Transformers – Standard Accessories Protection Junction Box (Cable Termination Box) - Assembly


1. 2. 3. 4. 5. 6.

Connect the Junction box to top cover of Transformer (or its termination box already assembled on to transformer). Remember to use the gasket (although not shown in the above diagram). Connect the cable termination busbars to the bushings. Assemble the glands and the gland plate to the box. Assemble the power cables to glands. Connet cable terminals to cable termination busbars Connect the earthing of glands to junction box.

Some accessories like Surge Arrestors and Current Transformers are also mounted inside the Cable Termination Box.

Transformers – Gaskets Gaskets of the cover, flanges and bushings are usually made of oil-resistant, vulcanized cork-sheets or oil resistant rubber. When the gaskets are leaking, the leaks can usually be stopped by tightening the bolts. When a gasket has to be detached or replaced, the sealing surfaces should be carefully cleaned and smeared with sealing varnish (oil resistant Shellac). Rubber gaskets are not to be smeared with varnish. When joining two gaskets, their ends should be bevelled and glued.


Oil-resistant rubber rings are used as gaskets for bushing bolts, shafts and spindles. Oil-resistant and heat-resisting plastic gaskets can also be used as shaft and spindle gaskets. All these gaskets can be tightened and replaced from outside the tank. When tightening the gaskets utmost care must be taken to prevent breaking of bolts or gaskets. In particular stud nuts must be tightened very carefully. Cork gaskets are not re-used.

A completely assembled Transformer (typical 1600 kVA) Elevation

1. 5. 6. 7. 8. 9. 10. 15.


18. 20. 21. 22. 24. 25. 29. 30. 33. 34. 39.

Transformer Tank OLTC Inspection cover for OLTC connection Flat Roller (bidirectional) Base Channel Earthing Terminal Rating & Diagram Plate Conservator with lifting lugs, oil filling cap and drain plug Silica Gel Breather Pressure equaliser pipe Jacking Pad Drain cum bottom filter valve with plug Buchholtz Relay Shut-off valve OTI WTI OLTC Surge Relay Surge Relay shut-off valve Anti earth-quake clamping

A completely assembled Transformer (typical 1600 kVA) Side View

16. 17. 19.

36.

37.


38.

Oil level gauge Conservator compartment for OLTC Explosion vent with oil sight glass with double diaphragm Silica Gel breather for OLTC conservator compartment Bottom Oil sampling valve with plug Dial type magnetic oil level gauge

A completely assembled Transformer (typical 1600 kVA) Plan

2. 3. 4. 11. 12. 13. 14. 23. 26.


27. 28. 31. 32. 35. 40.

HV Cable Box with disconnecting chamber LV Busduct LV additional neutral bushing Air release plug Thermometer Pocket for Glass bulb Thermometer Radiator with air release plug, lifting lug and drain valve Radiator valve Top filter valve with plug Lifting lug for complete transformer Lifting lug for top cover and active part Marshalling Box Pocket for OTI Pocket for WTI WTI CT Neutral CT

A completely assembled Transformer (typical 1600 kVA)


Other Details

Transportation of Transformers Preferably all construction work should have been carried out to the Transformer bay before the Transformer reaches site to prevent it from getting dirty or damaged. The transformer is supplied filled with oil and normally all accessories fitted, except for the largest units. The radiators, condenser bushings and conservator may be dismantled during transport.


The Transformer may be provided with dehydrating breather or sealed if provided with pressure relief device. During transportation the transformer is filled with oil so that active part is fully immersed in oil and oil level is below the lower cover surface; however, the oil level is left sufficiently low for bushing installation.

Transportation of Transformers During transport the following should be considered: Angle of tilting exceeding 10ºmust be specified in the contract; Prevention of damage to bushings, corrugated panels or radiators and accessories; Larger transformers should preferably be positioned with the longitudinal axis in the direction of movement; Secure against movement/ sliding by means of transport beams, wedges/ wooden blocks and fastening wire/ lashes to ensure safety during road traffic conditions;


Vehicle capacity/ suspension must be adequate for the transport weight of the transformer; Adapt vehicle speed to the road conditions; Any impact recorders to be specified in the contract; Any use of crates or containers.

Transfer of responsibility, Handling and lifting Transfer of responsibility This depends of the agreed terms and conditions of the contract.

Handling, lifting The Transformer must be kept in normal upright position, unless otherwise indicated by the documents. Only approved and suitable lifting equipment should be used. Forklift should be used only under transport pallets or transformer bottom. Load should not be applied to corrugated fins or radiators and their supports. While lifting the Transformer, only the fixtures provided should be used (transport eyebolts/ lifting lugs, bugles, lifting/ jack pads).


When hydraulic jacks are used, only provided jacking points should be used, and in such a way that twisting forces on the transformer tank are avoided. Bushings, Cable Boxes or Conservators should not be used for lifting, pulling or pushing the Transformer. When lifting a transformer with cable boxes on the cover, special care must be taken.


Lifting of Transformer by Lugs and Jacks


Lifting of Transformer by Fork Lift and transferring


Lifting of Transformer (Wire Ropes)

Receiving the transformer at site


1. Upon receiving the transformer at site, do the following:

-

Check the way in which the transformer has been secured on the trailer,

-

Check whether the delivery is complete according to order confirmation,

-

Compare the packing list with the goods received:

-

Check standard accessories:

- Packed together with transformer; - Separately packed.

2. Inspect the following: - The transformer nameplate; - Liquid level, when applicable. Any leakages?; - External damage, e.g. cracks in bushings; any damage to Radiator Fins; Dents or damages to Radiators; Colour matching of loose parts like conservator and radiator with colour of Transformer tank; Impact recorders indications when applicable; The receipt of the unit shall be signed for, and the result of the inspection shall be noted.

Receiving the transformer at site 3. Transformer shipments are normally insured. In case of damage revealed during the receiving inspection, do the following: • Make necessary arrangement to avoid further damage, • Contact the insurance company concerned and Supplier, • Make a report of the damage immediately,


• Do not start repairs until responsibilities are clarified and actions are agreed upon by all involved parties.



When storage of the transformer is required, the following recommendations should be noted: •

Store preferably in dry and clean locations, without any possibilities of mechanical damage and on a solid foundation.

•

If the transformer does not have a structural steel base, it should be placed upon supports to allow ventilation under the bottom of the transformer base.

•

If the Transformer is not filled with oil, fix the conservator and dehydrating Breather and fill it with oil up to the level corresponding to normal service temperature.

•

The Oil conservator and dehydrating breather must be checked to ensure that dry air is breathed (Conservator type only). Oil samples to be analyzed regarding moisture content prior to energizing.

•

Humidity/ condensation in control cubicles, driving mechanism for on-load tap changer, cable boxes and in cabinets of dry-type transformers etc. should be inspected/ removed.



When storage of the transformer is required, the following recommendations should be noted: •

Any drying and heating equipment of the cabinet (if any) must be connected to electric supply, to prevent the Transformer getting damp (especially in the case of dry type Transformer).

•

Storage temperature for dry-type transformer is in general: -25°C to 50oC, however for Resibloc: -60°C to 50oC. Oil filled Transformer: 5o to 50oC. Relative humidity should be less than 75%. If such a storage area is not available, then the Transformer must be kept under a sun-roof covered all around with thick plastic sheets with good ventilation.

•

Surface damage caused by transportation and installation must be repaired by touching up with paint of original color and shade. It is better to get a small can of the paint from the manufacturer of Transformer along with the Transformer.

•

Prior to energizing, perform a megger test between the different windings, and from the windings to earth. This applies to dry-type in particular.

•

Conservators must be stored into closed rooms without removing the gauges and the flange caps.

•

Bushings must be kept in a dry indoor space, in their original packing.

•

Radiators can be kept outdoor, but their mounting flanges must be protected against corrosion.

Indoor Installation 1. The Transformer room has to be dry, dust-free, suitably spacious and adequately ventilated. 2. Floor area must be adequate so that the Transformer is accessible for cleaning and inspection from all sides. Clearance between the wall and the transformer should be 0.75 m (wall on one side), 0.75 m (wall on two sides), 1.0M (wall on three sides), 1.25 m (wall on four sides, for transformers installed in cell/ collapsible front wire mesh). 3. Height of the room is defined by the space required for bushings, cables entrance and by air flow required for cooling. Minimum vertical clearance between the top of conservator tank and ceiling shall be 0.6 m.


4. When required it must be possible to lift the active part of the Transformer out of the tank while in the Transformer room for inspection. Otherwise, the Transformer will have to be moved to a place with necessary clearance height for lifting the active part. 5. The Transformer room has to meet the safety regulations regarding the construction and location. Pay attention to requirements as per Indian Electricity Code regarding civil engineering of Transformer rooms, safety regulations and fire protection regulations. The door of the room must open outwards and must be built on outer wall of the building. Doors leading to other areas inside the building must be avoided. If such a door is built, it must fulfill prescribed fire resistance class.

Indoor Installation


6. For naturally cooled Transformer (ONAN), satisfactory ventilation must be arranged to dissipate heat losses. The principle of cooling is shown in the adjoining figure. A fresh-air channel must be built under the Transformer so that the cooling air is divided as evenly as possible between the radiators.

Indoor Installation 7. A ventilation duct or opening leading out must be built on the ceiling or upper part of the wall. The area of the duct or opening has to be at least 10% more than the area of the inlet opening. If possible, air inlet and outlet openings should be located diagonally across the room, however both of them should lead out of the building and not into the building. Give customer or civil agency the calculation of ventilation openings for a normal Transformer room when ventilation is based on natural draught. Let them take help of a ventilation expert if required. Unless more complete air flow planning is available (which the expert can do), the free cross section of air inlet opening can be calculated from the formula given below: © ABB Inida, July 14-15/‘03 - 56, Author: Nag Ramesh

Where:

P = Total losses of the Transformer at full load; h = Effective height difference of air draught. This formula is valid for temperature difference of 150C between inlet and outlet air. If the space available does not allow big enough cooling openings, motorized fans will have to be used.

Indoor Installation


If adequate natural draught is not available, then force ventilate the transformer room with exhaust fans. As a guide each kilowatt of losses requires 4 cubic meters of air circulation per minute. Fresh air intake should be at floor level, and a ventilation duct leading to outer air must be built on the ceiling or upper part of the wall.

Outdoor Installation 1. The installation site be must such that there is as little as possible direct exposure to radiation from sun. A separate sun shade roof is recommended. 2. The installation site must also not neither gather excess snow in winter and water should not drip from the roof directly onto the Transformer. 3. The Transformer location has to meet Indian electricity code requirements regarding safety and Transformer installation. 4. The Transformer must not be located near combustible material.


5. Clearance between protective barrier and the Transformer should be minimum 2.13 m. 6. Transformers must not be placed closer than 10 m from each other unless a protective wall exists between them. Depending on the structure of the protective wall, they can be placed as close to each other as cooling and access conditions allow. For example, a steel concrete wall, which reaches to a height of 600 mm above tallest part of the Transformer and extends 1 m on both sides of the Transformers, can be used as a protective wall. Protective walls also help against vandalism and noise suppression.

Foundation Construction – rails and shims 1. Transformers are usually installed on a concrete foundation equipped with horizontal mounting rails. The Transformers are delivered with rollers installed or without rollers installed. Transformers on rollers must be fastened to foundation by wedges welded or bolted to rails or other means to prevent movement. 2. In order that gas, which may be generated in the Transformer, can freely move to the gas relay (Buchholtz relay), the Transformer has to be installed horizontally or slightly ascending towards the relay. The recommended ascent is 1-20. This can be accomplished by means of intermediate plates (shims) placed between the Transformer and foundation or between rollers and roller beams.


Shims may also be used to adjust any minor height difference between Transformer secondary terminations and bus ducts leading from secondary to Switchgear Panels.

Foundation Construction – oil pit and oil sump 3. An oil pit is recommended to be built under the Transformer location to prevent any oil fire spreading to other equipment. Its purpose, in case of oil leakage due to damage to Transformer, is to prevent and extinguish fire and collect the oil to prevent the fire from spreading.


The oil pit has to extend beyond the Transformer on all sides and is generally made of concrete. It must be big enough to contain the total oil quantity or alternatively allow the oil to be led via a discharge pipe into a separate oil sump. The oil pit must be so designed that the oil is extinguished when it runs into the sump. Wire netting is suitable for this purpose. The net must be installed over the mouth of the cavity built at the bottom of the pit and which leads to the oil discharge pipe. Coarse gravel is also suitable for this purpose. Then the pit must be covered with a dense steel grating at the bottom leaving 7.5-10 cm bottom clearance. Top of the grating must be covered with 25-50 cm deep layer of flatter stones and on the surface with 35-50 cm of gravel. If the installation has several Transformers, a common oil sump for collecting oil can be built outside the building. The oil pits of different Transformers must not be connected to each other, but they can be connected to a common pipe leading to the oil sump. The oil pits must be periodically maintained so that they meet the requirements all the time. The stone layers, net and grating must be kept clean. It also has to be seen that excessive amount of water is not collected in the pit.

Shifting to and placement on foundation 1. The height of the Transformer foundation, with respect road level, depends on how the

Transformer will be transported and shifted on to its foundation. If the Transformer is transported on a carriage or by deep-load truck, it is very important that height of the foundation is same as the loading height of the carriage or truck. Then the Transformer can be un-loaded sideways.

2. In case of a large Transformer, it is advisable for the rails of the foundation to reach the transport path and be at the same level. 3. If the transport truck is driven close to guide rails, the Transformer can be lowered onto rails by means of cranes.


4. Then two jacking points which are on one of the smaller sides are lifted up simultaneously and slowly by hydraulic jacks by 50 mm max and then other two jacking points are similarly lifted. This is repeated until the Transformer is lifted enough to fix the rollers/ wheels. Alternatively all the four jacking points may be lifted simultaneously. Then all the rollers/ wheels are fixed and the Transformer is lowered not more than 50 mm on each side until the Transformer rests firmly on its wheels/ rollers. Then jacks are removed and the Transformer is drawn to its final place along the rails. 5. If a crane is used to move the Transformer from its place of storage or unload from a truck on to its foundation, the foundation height is not of great importance, but space must be allowed for moving the crane boom & hook around and above the foundation. The foundation must be provided with locations for hydraulic jacks which have to be installed according to the corresponding lifting levels of the Transformer. The foundation must be provided with hauling hooks for fixing of pulling ropes.

Minimizing noise emission Depending on the location of the Transformer room, it may be necessary to provide sound insulation, e.g. anti-vibration pads to avoid transmission of vibrations to the structure.


Cooling fans, reflections from walls and resonance added to the noise emitted from the Transformer may increase the noise level considerably. Therefore, it is advisable to install the Transformer as far away as possible from walls.

Oil sampling from a Transformer Accurate sampling is extremely important to ensure that the sample is representative of the oil in the Transformer.


Use a 300cc stainless steel oil sample bottle with plastic tubing available from a laboratory or the manufacturer. Check that sample bottle is sealed. If the seal is broken, then clean the bottle with acetone, blow it out with dry air and dry it under vacuum. To take oil sample, connect the two sections of the plastic tubing to the sample bottle through gas cock connectors. Attach one tube to the sampling valve on the Transformer. The open end of the other tube should be placed in a scarp liquid container. Open both the valves of the sample bottle. The sample bottle should be held in vertical position with connection to the Transformer sampling valve below the sampling bottle. Very carefully open the sampling valve of the Transformer and fill the sample bottle until the oil overflows through the top and into the scrap container. Continue overflowing until no bubbles can be seen through the plastic tubing and until two quarts of oil have been collected in the scrap container. Close the top valve of the sample bottle, then bottom valve of the sample bottle and finally close the Transformer sampling valve and disconnect the plastic tubing. Tag the sample bottle showing the Transformer serial number, date of sampling, location and owner of the Transformer and send the bottle to a laboratory for analysis. Avoid shaking and jerks to the sample bottle.

Oil sampling from an oil drum or tanker


The sampling bottle should have a trap at the bottom. The bottle must be rinsed with solvent thoroughly dried prior to being used. Where large number of drums are involved, samples from up to five drums are mixed and if the dielectric strength of the composite sample is 30 kV min, then oil of all the drums may be added to the Transformer. If the dielectric strength is less than 30 kV, then samples from all the drums will have to be tested. It is also necessary to check the moisture content similarly (max 50 ppm).


Oil sampling kit and test cup for dielectric strength

Erection of Transformers transported fully assembled and filled with Oil 1. Inspect the Transformer carefully to find possible transport damages paying special attention to possible oil leaks, surface damages and bushings. 2. Check and tighten all gaskets and carry out cleaning operations. 3. Check oil level in the oil conservator and top up with oil if necessary.


4. The dielectric strength of the oil will be normally given Transformer’s operation and maintenance manual. Test the dielectric strength of the oil, if less than required arrange for oil filtration. The minimum value should be 40 kV/ 2.5 mm when tested with a device as per IEC 60156.


Erection of Transformers transported with loose parts and filled with Oil 1. The transformer must be inspected on receipt as already mentioned. 2. All parts removed for transportation must be re-fitted as per manufacturer's manual/ instructions. 3. The condition of the oil in the Transformer and in the separate drums must be checked and their dielectric strength tested. The minimum dielectric strength must be 40 kV/ 2.5 mm when tested with a device conforming to IEC 60516 (IS 335). 4. Bushings must be installed as per separate instructions given by the Transformer manufacturer. 5. The conservator must preferably dried in a furnace at a temperature of 80-900C. The inside of conservator must be cleaned. The cleaning can be performed by rinsing the conservator with hot and clean (vacuum treated) Transformer oil. The conservator must first be mounted on its brackets provided on the Transformer and then following accessories must be assembled: - any pipes for the tap changer oil and tap changer; - pipes to transformer flanges - Buchholz relay, - shut off valves, - silicagel breather; - level gauges, with manufcaturer supplied gaskets.

Erection of Transformers transported with loose parts and filled with Oil 6. During transportation, inlet and outlet of radiators (if any) are covered with solid flanges to prevent ingress of humidity and impurities into the radiators. These flanges must be removed then the radiators must be installed on the flanges provided on the Transformer tank. 7. For removal and installation of radiators, the fixing flanges of the Transformers are provided with shut-off valves which ensure that the Transformer is not emptied of oil. If the radiators are to be removed for a longer period, e.g. for transportation, then the mounting flanges of the transformers are also blocked with solid flanges. These flanges are also to be removed after ensuring that the shut-off valves are completely closed.


8. After all the parts are fitted, the oil required for the radiators and oil conservator must be added to the Transformer. While filling the oil, ensure that clean Transformer oil is carefully pumped into the oil conservator. If possible use a vacuum filter to process the oil in separate drums during filling. The lower shut-off valves must be open and upper shut-off valves must be first closed during oil filling. The air release screws on top of the radiators should also be open until the radiators are filled with oil. Keep them open until oil starts oozing out of the air release screws of the radiators. Then upper shut-off valves of the radiators must be opened. Top-up the conservator with oil until oil level indicator rises up to the height corresponding to temperature that will prevail in the normal operation.

Erection of Transformers transported with loose parts and filled with Oil Then trapped air must be released after oil filling in following order: - Transformer tank cover (lid); - Bushings; - Buchholtz relay.


Releasing air can be carried out by using air release screws fitted at suitable places. For bushings which do not have air release screws, upper gaskets must be loosened and retightened after the air is released.

Erection of Transformers transported with out Oil 1. The Transformer is filled at the factory either by dry air or Nitrogen at overpressure of 50 kPA (approx 0.5 bar) or at normal pressure depending on the conditions of the journey, transport method, distance, etc. 2. For supervising the over pressure, the Transformer is provided with a manometer. The Transformer should be filled with oil at site with in six weeks of dispatch from the factory. If oil is filled after this period, the pressure must be checked continuously and dry air or Nitrogen must be added when the overpressure falls below 10 kPA (0.1 bar).


3. Further, humidity of insulation used inside the Transformer must be inspected before filling the Transformer with oil. More details on inspection of insulation, humidity and filling of the oil are given subsequent slides. 4. Transformer provided with normal-pressure dry air is generally provided with a dehydrating breathers and sealed for transportation. The operating condition of the dehydrating breather must be checked until the Transformer is filled with oil. Further details on maintaining the breather are given in subsequent slides.

Erection of Transformers transported with out Oil 5. Fix all the accessories in the sequence and manner indicated in the installation manual supplied along with the Transformer by the manufacturer. Precautions to be taken while assembling conservator and radiators have already been explained. 6. Before filling, take samples of oil from each drum and test the dielectric strength of oil. The value should not be below 30 kV/ 2.5 mm. Maintaining Nitrogen pressure at 0.5 bar, force oil into the Transformer tank from bottom drain valve on the tank.


Then fix Radiators, Conservator and Bushings. Then fill radiators with oil as already explained and top up the conservator. Follow any recommendation or procedure given by manufacturer for oil filling. It may be necessary to filter the oil while filling using an oil filtration machine. It may also be required to fill the oil under vacuum with hot oil circulation for removal of trapped air in the active part and moisture in the windings. In case the active part has been exposed to outside atmosphere or in case external air ingress, consult the manufacturer. The active part may have to be dried in such cases, before oil filling.

Erection of Transformers transported with out Oil While filling oil under vacuum, bottom drain valve of the tank is connected to a vacuum machine and a vacuum of of o.5 bar (6.5 psi) is created. Then, the valve of the vacuum pump is closed and the oil is forced into the tank from bottom drain valve. Filling speed is correct if the vacuum does not considerably decrease during filling. Oil filling is stopped when the windings and insulation parts are below the oil level. When de-aerated oil is used, the vacuum must be maintained for several hours so that all the residual gas is removed from the oil. Then the oil level is made up by filling the oil through the conservator top valve. 7. If temperature of oil in the Transformer is different from 25oC, then oil level in the conservator must be corrected to correspond to actual temperature. If the oil level indicator has temperature scale, then oil is added or drained until the oil level is up to the prevailing temperature.


If the oil level indicator does not have temperature scale, then the conservator is first filled up to half the volume and then: - if the oil temperature is higher than 25oC, oil level must be made up to above the middle level; - if the oil temperature is less than 25oC, then oil must be drained from the tank below middle level.

Erection of Transformers transported with out Oil The volume of oil to be added or drained is calculated according to following formulae: V = 0.00088 x dT x M for mineral oil; V = 0.00104 x dT x M for silicone oil; Where V is the volume of oil to be added or drained in liters; M is the mass of oil in the Transformer at 25oC (given on the name plate) o dT is the deviation of actual oil temperature from 25 C.

8. After filling the oil, allow the oil to settle to settle for 24-72 hours as per manufacturer's recommendation for trapped air to come out and collect at the top.


9. Release any trapped air by opening air release vents provided on the Transformer.

Additional Supports It may be necessary to provide supports made out of MS angles or channels for following accessories: 1. 2. 3.

HT Cable Box; LT Cable Box; OLTC.


Refer Transformer manufacturer's manual and drawings for dimensional details.

Oil Filtration The Transformer Oil is processed with the help of an Oil Filtration Machine on three occasions: 1. When filling oil for the first time from oil drums to prevent contaminated oil being filled into the Transformer. 2. For purifying the oil during periodic maintenance as recommended by Transformer manufacturer 3. For regeneration of oil after several years of usage.


It is done to: • • • • • •

Extract moisture from the oil; Filter the particulates (solid particles and carbon residues); Degas the oil (removal of gases generated and dissolved in the oil during operation) especially light hydro-carbons; To remove acidity and restore oxidation stability; To rectify color; To remove sludge collected inside the Transformer.


Oil Filtration Machine

Capacity: 250 to 6000 Liter/ Hour


Oil Filtration Process


Oil Filtration Process 1.

Connect the Oil Filtration Machine to the Transformer by hoses in a closed loop arrangement (Oil should enter the Transformer through bottom valve of the tank and go out through its top valve of the tank) and fill the entire system including hoses with oil before starting the filtration process.

2.

Once the system is filled with oil, circulate the oil for 10 minutes to eliminate air from the system, air vents provided at various points on the Transformer will have to be slowly opened occasionally to let out the air and then closed.

3.

The Filtration machine may have two modes of operation: Purification and Regeneration. The choice of the mode depends on degree of contamination as determined by oil analysis done prior to filtration, application and manufacturer's recommendation.

4.

Purification: This is limited to moisture extraction, degassing and particulate filtration. The oil enters the machine where it is first heated by a set of electric heaters to maintain the oil temperature between 65 to 70oC.

Oil Filtration Process Then the oil is pumped through a pre-filter (5 microns) and a fine filter (0.5 microns). Then it enters a vacuum chamber and passes through fiberglass coalescing elements where it is exposed to high vacuum and is degassed and dehydrated. Then it is pumped back into the Transformer. The process is repeated until moisture content falls below desired level and the required dielectric strength is reached. The oil is tested at 4 to 12 hour interval (depending on the size of the Transformer and manufacturer's recommendation) for moisture content and dielectric strength


5.

Regeneration Oil is regenerated by forced percolation through columns of “adsorptive media” at around 65oC. The adsorptive media consists of Fuller Earth (a type of clay). Regeneration removes acids, sludge, restores oxidation stability and rectifies color. After regeneration, oil is pumped through a 0.5 micron filter and then it is degassed and pumped back into the Transformer. The process of regeneration takes around 8-12 hours depending on the quality of oil being treated and the size of the Transformer.

Oil Filtration Process After several hours of regeneration, the clay beds get saturated and will have to be reactivated. It is done by circulating the hot oil between the regeneration columns and reactivation columns. After reactivation, a small quantity of (0.1% of oil processed) oil scum is collected in a holding tank which drummed for resale and reprocessing or for disposal as hazardous waste. Clean oil trapped in the clay is recovered. Reactivation takes around 16 hours and upon its completion, oil regeneration can be resumed. 6.

De-sludging


Transformer may have to be de-sludged after several years of operation. The process is similar to regeneration except that oil temperature will be above its aniline point of about 75oC in order to dissolve the sludge inside the Transformer. It is done with freshly regenerated oil. Oil purification, regeneration and de-sludging can be done with the Transformer in either de-energized or energized condition. However for first filling of oil and purification at site, the Transformer has to be in de-energized condition. When done in energized condition, Oil flow rate has to be kept low, as per manufacturer's recommendation, to prevent turbulence inside the transformer.

Oil Filtration Process - Oil filtration performance analysis Oil filtration performance analysis Test Description

Method

Unit

Initial Oil Condition

Required Results after single pass

IEC 60296

mg KOH/ g

0.25

<0.03

---

Visual

Brown/ Cloudy

Clear Light Yellow

Breakdown Voltage

IEC 60156

kV

<30

>55

Moisture content

IEC 60814

ppm

< 2000

<20

Tan Delta (90oC)

IEC 60247

---

<0.01

<0.015

Interfacial Tension

ASTM/ ISO 6295

mN/ M

<15

>35

GC

% v/ v

8

0.01

IEC 61125

---

Depleted

Restored

Acidity


Color appearance

Gas content Oxidation Stability

164 Hours


Power Circuit Connections 1.

All electrical connections to the Transformer must be made by using appropriate terminals. Refer outline (general assembly) drawing given by the manufacturer.

2.

Cables and busbars must be of sufficient cross section in accordance with load current.

3.

Sharp bends in high voltage connections must be avoided.

4.

Connections must be made with out causing mechanical tension or torque on the connection terminals. All cables must be brought into termination box through cable glands or must be secured appropriately.

5.

For connections above 1000 A, flexible links must be used between busbars and the Transformer terminals to compensate for thermal expansion of busbars.


Following minimum clearances in air for phase-to-phase and phase-to earth parts must be maintained. This also applicable for distances between cables and windings. System Voltage (kV)

3.6

7.2

12

17.5

24

36

52

72.5

Minimum Clearance acc. To IEC 60076-3 phase-to-phase or phase-to-earth (mm)

60

90

110

170

210

280

450

630

Lightning Impulse withstand voltage (kV)

40

60

75

95

125

170

250

325

As most sites in India are lightning prone, it is advisable to consider above values.


7.

Bolted connections must be protected against corrosion by applying petroleum jelly on exposed metal surfaces and between joints.


Spring washers must be used to compensate for thermal expansions and for plastic deformations and maintain the necessary contact pressure. 1. 2. 3. 4. 5.

Bushing terminal Al. Busbar or cable lug Pressure Washer (SFS 3738) Conical Spring Washer (SF 3737) Galvanized bolt and Nut


Tightening should be stopped when the conical Washer just becomes flat.

Bolt Size Cl. 8.8

Tightening Torque (Nm)

M6

6 …9

M8

15…22

M10

30…44

M12

50…75

M16

120…190

Bolted Copper connections are made in the same way as Al joints. It is not however necessary to use pressure Washers. In Sulphurous atmospheres, the joint surfaces must be protected by Petroleum Jelly. All joints must be tight and secure. Torque Wrenches must be used for tightening. Aux. circuits of the Transformer and Tap changer must be connected as per wiring diagrams. All current transformers must be connected to measuring or instrumentation circuits. Secondary terminals of any CT not used must be shorted or earthed.


Earthing connections The earthing terminals of the Transformer must be connected to protective earthing system. The earthing terminals are usually provided diagonally on the base beams. Cross section of earthing conductors/ connections must be according to regulations and conditions at site.

10. Cable/ Busbar terminations


Cable terminations must be selected according to cables used and as per cable manufacturer’s recommendation. Normally indoor terminations are used inside enclosures. Cables must be fixed by clamps welded on to the enclosure or by glands on removable gland plates. With single core cables, it is better to use stainless steel gland plates to avoid gland plate heating. In case of busduct connection, flexible connectors are recommended.

Power Circuit Connections 10. Cable/ Busbar terminations (continued) High voltage connection is normally performed by copper cables and copper cable lugs. In some cases heat shrinkable connectors or elbow connectors are used. For aluminium-copper joints the copper is coated with tin, or bi-metal sheets (one side of copper and the other of aluminium) can be used between the joint. The aluminium surface must be larger than the copper surface. Aluminium parts shall always be placed above copper parts so that water cannot drain from the copper parts onto the aluminium (corrosion).


It must be remembered that good contact between joined aluminium surfaces can be achieved only if non-conducting oxide film is removed with a wire brush, file or similar immediately before joining, and renewed oxidation is prevented by applying a thin protective film of grease (neutral Vaseline). Jointing compound, which prevents the access of air and humidity into joints, must be used in the joint. The zinc crystals of the compound break down the layer of oxide on the aluminium.

Power Circuit Connections 11. Neutral Link Neutral link must be provided at a convenient location between Transformer Neutral terminal and Earth. If it is mounted on a wall, it must be at height of 1.8 to 2.6 m. Cables used for neutral earthing should be taken only through PVC pipes. 12. Tap Changer


Tap changer to be tried in all the positions for free operations and ensure equal resistances between 3 phases in all taps of tap changer.

Pre-energisation Checks The following checks have to the performed after erection and before start-up. 1.

Deaeration of HV and LV bushing Remove the eventual air within the HV and LV bushing as indicated in manufacturer’s instructions.

2.

Deaeration of radiators Open the upper breathing plug of radiators and remove the eventual air within.

3.

Deaeration of Buchholz relay


Open the lower drain plug of Buchholz relay for remove the eventual air within.

4.

Check nameplate data against data sheets Check whether details given on name plate are same as those in technical submittal approved by customer and our specification.

Pre-energisation Checks 5.

Check mechanical damage and paint Repair either of painted parts, or of parts damaged during the transport or during the placing of the transformer on its site. For repairing damage paint consider the following cases: A.

Scratches in the paint, of limited depth, the metal is still covered with a coat of paint, even of just some µ mm: Use a clean cloth imbued with non-oil solvents (for example thinner used for vinyl paints) to clean accurately the area, then proceed with the repairing of paint, with brush or roller respect to the range of damage paint.


B.

Deep cuts, up to the metal, with rust where the paint has been taken away: Using abrasive paper, remove the paint from the area till the metal is white and completely devoid of rust. Clean with a cloth imbued with non-oily solvents. Apply the paint cycle suitable for such transformer, following strictly the suggestions provided for this type of transformer


Check mechanical damage and paint (contd.) C.

Rusted metal-plates, devoid of paint; If possible, sand blast till the metal becomes white, or alternatively take off the rust with a metal brush; then rub down the border and, if necessary, retouch the brushed area. Apply the painting cycle scheduled for the transformer, following the suggestions indicated.

D.

Paint swellings, with disjunction of the paint from metal. With a metal spatula lift and take away the paint as far as possible, insisting on the borders, and then carry out the workings indicated in the previous point.


Before the application, check whether the the paint to be used for the retouches is of the same type as the paint applied originally by the Transformer manufacturer. It is better to get a can of the original paint from the Transformer manufcaturer. The above described suggestions are valid for all the metallic parts of the transformers. As far as radiators are concerned, in case C or D check either the possibility of restoring the paint or of replacing them. Do not use stucco to level the surface.


Check oil leakage.

7.

Check nuts and bolts tightness with torque wrench as per values given earlier, setting the torque on the wrench in the middle of the range given.

8.

Check alarm-trip contacts of Buchholz relay Check the alarm-tip contacts release: a) acting on the test buttons; b) lowering the relay oil level.

9.

Check trip contact of pressure relief device


Check the contact trip release by acting manually on microswicht pin.

Pre-energisation Checks 10. Check contact calibration of thermometer Oil temperature indicator: take out the bulb of the temperature indicator, dip it in hot water and check the alarm contact release and insert the bulb back into the thermpmeter pocket on the tank and fill the pocket with oil. Recommended settings for ONAN transformers

Alarm

Trip

Oil thermometer setting

85ºC

100ºC

Oil thermometer setting, when combined with a

90ºC

105ºC

105ºC

135ºC

winding temperature indicator Winding temperature indicator setting

11.

Check alarm contact of oil level indicator Check the alarm contact release through shunting the contact terminals.


12.

Check all valves Verify correct state of valve.

13. Check oil level Verify the oil level of conservator and add or drain oil if necessary.

Temperature settings for dry-type transformers should be according to the relevant temperature class of insulation.

Pre-energisation Checks 14. Check silicagel breather Verify the colour of salts of silicium and change or re-charge them if required. Check oil level in the liquid seal of the breather and make it up if necessary up to the mark.

15. Earthing correct Check that all metallic parts are correctly grounded.

16. Auxiliary circuits & Terminal blocks Check whether auxiliary circuits are wired up as per schematic diagram and whether wires coming to terminal blocks are properlu fixed and are in correct position. If not correct them after discussing with the manufacturer.

17. Check auxiliary (marshalling box) wiring


Verify that the connections are in according of the schematic diagram.

18. Check installation of emergency push button switch near the Transformer or on the door of its enclosure and wiring to upstream breaker trip circuit in HT Panel.

19. Instruments correctly installed Verify that the instruments are in according of the overall dimensions (GA) drawing.

Pre-energisation Checks 20. Insulation resistance (M Ohm) Resistance of Insulation between: - HV Terminals and LV Terminals+Tank/ Earth; - LV+HV Terminals and Tank/ Earth; - HV and LV Terminals; - Tank and Earth before making earth connections, in some special cases.


Test results should conform to Test Certificates/ reports submitted by manufacturer. Voltage of HV Wdg.

Voltage of Megger

Up to 11 kV

1000 V

22 to 33 kV

2500 V

Pre-energisation Checks 20. Insulation Resistance (Contd.)


The insulation resistance must be several thousand Mega-Ohm. However the value widely depends on measuring time and the temperature of the Transformer. Convert the measured values by multiplying with the conversion factors given in the following table: T (Ambient) oC

Correction Factor

T (Ambient) oC

Correction Factor

-10

0.13

35

2.8

-5

0.18

40

3.95

0

0.25

45

5.60

5

0.36

50

7.85

10

0.5

55

11.20

15

0.75

60

15.85

20

1.0

65

22.40

25

1.40

70

31.75

30

1.98

75

44.70

Two readings must be taken one at 15 s and another at 60 s. Minimum value of 60 s reading must be 1000 Mega-Ohm. Further, difference between the readings taken at 15 s and 60 s is used to determine dryness of the Transformer. R60/ R15 is normally between 1.2 – 3.0 in a dried Transformer. Value less than 1.2 indicates that the Transformer has moisture.

Pre-energisation Checks 21. Auxiliary circuit insulation Check the whole auxiliary circuits insulation by applying a 2000V-50 Hz for 6 s. Measured value must be greater than 2 Mega-Ohm.

22. Check Oil dielectric strength and moisture content, if already done, review test results. 22. Voltage ratio measurement


- If measurement is carried out by a Ratio Tester, all measurements have to comply with the report test; - If a voltmeter is used, then feed one winding of the transformer from HV side with a voltage of 415 V through a regulating Transformer and an Isolation Transformer and check whether corresponding LV side voltage is proportionally in compliance with that on the rating plate. Repeat the test for other two windings. Because of accuracy of voltmeters used, measured values may differ by test report values by a few percent. Refer diagram for making connections.

Pre-energisation Checks 23. Vector group measurement This test is important especially when two or more transformers are to be operated in parallel. It is always better to carry out this test at the same time as voltage ratio measurement and verify that the actual vector group corresponds to that indiacted on the rating plate. 1. 2. 3.

Discommect any and all neutral earthing (both primary an seconday side) Connect R phase terminal of primary to R phase terminal of secondary Apply 3ph, 415 V, 50 Hz supply to HT side (which is usually primary).

Measure various voltages given below and verify that measured values conform to realtions indiacted for concerned vector group. 23.2 Vector group Yy

23.1 Vector group Dd


Y

0

UYy < UYb UBb < UBy

y R, r

0

b

B

URY = URy + UyY

Y y R, r

n

UYy < UYb

N

b

B

UBb < UBy URN + URn = UnN

Pre-energisation Checks 23. Vector group measurement 23.3 Vector group Dz

23.4 Vector group Dy

0

5

Y

UYy < UYb U

Bb

By

b

23.5 Vector group Yd


n y

B

UYy = UYb

U

=U

U

>U

U

=U +U

Yy

B

>U

Yb

Bb bN

Yb

UBb > UBy

By br

rN

UYb > UBy

By

UnY = Unr + UrY

23.6 Vector group Yz

5

N

U

Bb

R, r

URB = URb + UbB

R, r b y

=U

Yy

Y

U

5

Pre-energisation Checks 23. Vector group measurement 23.7 Vector group Dd

23.8 Vector group Yy

6

Y

6

UYy > UYb

b

U

R, r

Bb


23.9 Vector group Dz

UYy > UYb UBb > UBy UYb = UBy

By

UyY = Uyr + UrY

B

y

>U

6

Y b R, r n y

UYy > UYb

N B

U

Bb

>U

By

UnN = Unr + UrN

Pre-energisation Checks 23. Vector group measurement 23.10 Vector group Dy

Y

U

b

R, r

Bb


23.12 Vector group Yz

UYy = UYb UBb < UBy UYb = UBy

11

R, r

UYy = UYb

N

y

By

URY = URn + UnY

B

11

Y

UYy = UYb

y n

23.11 Vector group Yd

11

b

U

B

Bb

By

URN = URb + UbN

Pre-energisation Checks


24.

No-load current measurement (required only in case of a suspected damage) A small voltage is fed between terminals of each phase and star point (in a star connected Transformer) or between phases (in a delta connected Transformer) via a regulating and an isolating Transformers and a series connected ammeter (0-100 mA range). The no-load current will be a few tens of mA depending on the impedance of the Transformer. Readings are taken for all phases with the same applied voltage. Currents for all the phases should be more or less same. In a damaged phase of the Transformer, the current can rise to tens of Ampere, hence the regulating Transformer must be protected by suitable MCB or a fuse.

HT

LT

Pre-energisation Checks 25. OLTC protective relay alarm contact Check the protective relay of OLTC as per Instruction manual supplied by Manufacturer. 26. OLTC oil level indicator contact Check the alarm contact of OLTC oil level indicator release through shunting the contact terminals.


27. Operation of Tap Changer (OCTC or OLTC) Check whether tap position changes or not on giving rise and lower commands and check tap position indication. Uaually On-Load Tap Changer is provided with a motor drive mechanism and a protective relay to monitor sudden flow of oil from diverter switch oil compartment to conservator. Check wiring and operation of protective relay and operation of motor drive. Refer manufcaturer’s instruction for operation and maintenance of OLTC. 28. If it is suspected that the Transformer has suffered a damage, the following additional measurements must also be carried out. 27 a. Winding Resistance 27 b. Winding capacitance.

Pre-energisation Checks An ordinary multi-meter may not be able to measure these values at all due to long time constant and weak dc source of the meter. A dc source of 24 Vdc or higher and an accurate resistance bridge and capacitance bridge will be required for these measurements. Measured results must be verified with test reports of the manufcaturer to arrive at any conclusion.


29. If the Transformer has been stored for a long time or has been inoperative for a long time, dielectric strength of and moisture content in the oil inside the Transfomer must be checked. The dielectric strength must be higher than 50 kV/ 2.5 mm and moisture content must be less than 20 ppm. If not, oil level must be made up and the oil must be filtered using an oil filteration machine. In cold Transformers the humidity is mostly absorbed in the the paper insulation which can not detected on the basis of oil study. In such cases monitor moisture content in the oil once in 48 hours after putting the Transformer on load. If there is a significant increase in the moisture content, take the Transfomer out of operation, the active part may have to dried in consultation with the manufcaturer. The Silica gel breather must also be checked and gel re-activated if required.



Sl. No.

Details

1

Deaeration of HV and LV bushing

2

Deaeration of radiators

3

Deaeration of Buchholz relay

4

Check nameplate data against data sheets

5

Check mechanical damage and paint

6

Check oil leakage

7

Check tightness of nuts and bolts

8

Check alarm - trip contacts of Buchholz relay

9

Check trip contact of pressure relief device

10

Check contact calibration of thermometer (OTI & WTI)

11

Check alarm contact of oil level indicator

12

Check all valves

13

Check oil level

14

Check silicagel breather

15

Earthing correct

Controlled

Remarks



Sl. No.

Details

16

Auxiliary Circuits & Terminal blocks correct

17

Check auxiliary wiring

18

Instruments correctly installed

19

Installation of Emergency PB Switch on Trafo Enclosure Door

20

HV to LV+E insulation resistance (MOHM)

21

HV+LV to E insulation resistance (MOHM)

22

HV to LV insulation resistance (MOHM)

23

Auxiliary circuit insulation 2kV - 50Hz, 1 min.

24

Check oil dielectric strength and moisture content

25

Voltage ratio measurement

26

Vector group

27

No-load Current

28

OLTC protective relay alarm contact

29

OLTC oil level indicator contact

30

Operation of Tap Changer (OCTC or OLTC)

Controlled

Remarks


Energisation 1.

Check all signalling in control room.

2.

Check HV and LV connection to bus-bar system.

3.

Put tap-changer in correct position prior to putting the Transformer in to operation (off-voltage conditions). Tap-changer usually has a handle, a position indicator and a locking device. After putting the tap-changer into correct position, secure the locking device. Normally smaller position number indicates higher voltage ratio and lower voltage on secondary side. Refer connection diagram provided by manufacturer.

4.

Put transformer in to operation in NO-LOAD condition for a duration recommended by the manufacturer. It is recommended to leave the Transformer connected to network and on no-load for 1 to 2 days so that any air bubbles in the oil have time to travel up and collect in the Buchholtz relay. When the Transformer is energised for the first time at site, you may hear a loud sound from the Transformer and the over-current relay may trip the HT breaker. It may not be because of any fault but because of magnetising in-rush current of the Transformer. The value of in-rush current depends on construction and size of the Transformer, magnetic remanace of the core and the moment of connection to the network.

Energisation 5.

Switch-off transformer from voltage supply and check the gas in Buchholz relay. During first few days after a new Transformer is connected to network, some gas may collect in the Buchholtz relay causing an alarm. This is normally due to air that remained under the cover during installation. When the Transformer warms up, this air starts moving up and accumulates in the Buchholtz relay. The quality of the gas can be seen from its colour and usually from smell. If it is air, then it will be colourless and will not have any smell. If it is otherwise, a gas sample will have to be taken and analysed in consulation with the manufcaturer.


If the gas collected is air, and the gas can be released and operation of the Transformer can be resumed. 6.

If all tests carried out are favourable, put transformer into operation and load.

Energisation – Check List


Sl. No.

Details

1

Check all signalling in control room.

2

Check HV and LV connection to bus-bar system.

3

Put tap-changer in correct position prior to putting the Transformer in to operation (off-voltage conditions).

4

Put transformer in to operation in NO-LOAD condition for 1 to 2 days.

5

Switch-off transformer from voltage supply and check the gas in Buchholz relay.

6

If all tests carried out are favourable, put transformer in operation and load COMMENTS

Controlled

Remarks

Operation – Temperature rise and loading capacity Aging of insulating materials is caused by changes in their chemical structure. Molecules of Oil and Paper are slowly decomposing already at room temperature and when heated the rate of decomposition increases. When Paper gets old, its cellulose molecule break and its mechanical strength decreases. When load on Transformer increases, temperature of winding and oil will rise and aging of insulating materials accelerates. The aging rate will almost double and corresponding life time of the Transformer will be halved when the temperature of hottest point inside the winding (hot spot temperature) rises by 6 to 8oC, with in the temperature range of 80 to 140oC.


In addition to temperature, other factors such as dissolved oxygen, acids and moisture also speed up the ageing of the insulation. Hence, ageing sets limits for loading capacity of oil insulated Transformers. As per IEC 60354 (Loading guide for oil immersed transformers), the normal operation of an air cooled Transformer is considered to be continuous operation at rated power at an ambient temperature of +20oC. The insulating material will age under these conditions at a speed which can be considered to be normal provided the Transformer is properly maintained.

Operation – Temperature rise and loading capacity


Continuous loading capacity at different ambient air temperatures is calculated based on a hot spot temperature of 98oC which is same as the temperature considered for normal operation. Following table gives loading capacity at different ambient air temperatures as per IEC 60354 for ONAN/ ONAF Trafos. Ambient air temperature oC

-20

-10

+/-0

+10

+20

+30

+40

Permissible continuous loading in %

130

123

116

108

100

91

82

Oil temperature in oC

67

69

71

73

75

78

80

In Practice, a Transformer is very seldom continuously loaded at the same load. If the Transformer loading is, part of the time, lower than the continuous load capacity at the relevant permissible ambient air temperature, it can be correspondingly loaded more at other times and still the aging can remain normal during the whole time (e.g. during 24 hours). IEC’s loading guide also gives short term loads on this basis. Next table gives the short term loading capacity as a function of the ambient air temperature and the loading time provided loading at other times of the day does not exceed 25% off 90% of the rated power. For short-term loading the temperature of the hottest spot in the winding is limited to 140oC and the maximum permitted power is 1.5 times the rated power.

Operation – Temperature rise and loading capacity Ambient Temperature oC

-20

-10

Loading time Hours/ Day

+/-0

+10

+20

+30

+40

Permissible loading/ Rated Power = 100%

0.5

150

150

150

150

150

150

150

150

150

150

150

126

150

118

1.0

150

150

150

150

150

150

150

150

150

150

150

110

150

104

2.0

150

150

150

150

150

150

150

147

150

132

149

99

137

95

4.0

150

150

150

148

150

140

142

130

134

118

124

95

115

88

6.0

149

144

144

139

138

132

131

123

123

112

114

93

98

84

8.0

144

140

138

134

132

128

124

119

116

109

108

93

98

84

12.0

139

137

133

131

125

123

117

115

110

105

101

92

92

80

Load during other time of the day is maximum

25

90

25

90

25

90

25

90

25

90

25

90

25

80


The following example explains the use of above two tables: Outdoor air temperature

= -10oC

Continuous load capacity

= 123/100 x rated power

Outdoor air temperature -10oC, loading time 4 hours and load during other times of the day, i.e. 20 hours is 90%, then, Short-term loading capacity for 4 hours = 148/100 x rated power, provided the load during other 20 hours is maximum = 90/100 x rated power.

Operation – Temperature rise and loading capacity The temperature of the transformer’s insulating material can be monitored with the winding temperature indicator, which is set to indicate approximately the temperature of the hottest point in the winding. According to IEC’s loading guide, the maximum short-term hot spot temperature can be +140oC. Due to practical difficulties appearing in the definition of the hottest point, it is recommended to use the values given in the following table to set the winding temperature's alarm and trip levels.


It must be noted that a Transformer’s short-term load capacity can not be judged on the basis of the oil temperature, because the oil temperature changes much slower with the load than the temperature rise of the winding. Depending on the volume, temperature of oil may rise 5 to 20 minutes slower than temperature of winding. Even at a continuous overload, temperature can not be used to estimate the overload, with out knowing the actual temperature rise, which takes place at the rated loading of the winding. For supervision of continuous loading, it is recommended to set the oil thermometer alarm and trip contacts according to top oil temperature value given in the following table. If the Transformer has been provided with winding temperature indicator, the setting of the oil thermometer contacts can be increased.

Operation – Temperature rise and loading capacity Overloading of a Transformer implies that its construction and the accessories used in it such as tap changer and bushing are correspondingly rated. In cases of doubt, contact the manufacturer of the Transformer for confirming the overload capacity. Overloading capacity of an old Transformer can be notably limited by aging of oil and insulation humidity.


Instrument

Alarm

Tripping

Oil Thermometer setting

85oC

100oC

Oil Thermometer setting when the Transformer is also provided with a winding temperature indicator with alarm & trip contacts

90oC

105oC

Winding temperature indicator setting

105oC

135oC

Above recommendation for alarm and trip settings are for ONAN Transformers.

Parallel Operation of Transformers When two or more Transformers are to be connected in parallel, it is important to remember that only the Transformers bearing identical indices (as given in following table) can be run in parallel. The index is stamped on the Transformer rating plate. Index Connection

Vector Diagram

Index

Dd0


0

5

Yy0

Connection Dd6

6

Yy6

Dz0

Dz6

Dy5

Dy11

Yd5 Yz5

11

Yd11 Yz11

Vector Diagram

Parallel Operation of Transformers Transformers with indices 5 and 11 can also be run in parallel if the conductors of both LV and HV side suitable crossed. Possible connections are given in following table. Following figure shows one of the possible alternatives. HV side

LV side

Main Busbar

L1

L2

L3

L1

L2

L3

Transformer having connection Dy11, Yd11, Yz5.

A

B

C

a

b

c

B

A

b

a

c

A

C

a

c

b

or C or


B

Parallel Operation of Transformers


Other conditions to be met for parallel operation are: 1.

Poles having same polarity on the HV side and LV side should be connected in parallel (except with Transformers with indices 5 and 11);

2.

Transformers should have the same transformation ratio;

3.

Their short circuit impedances should be same (with in 10% accuracy);

4.

The rated outputs of the Transformers should not deviate from each other more than 1:3.

Before connecting the Transformers in parallel for the first time, it is vital to check the voltages. For this purpose, only the HV sides should be connected in parallel and only the star points of the LV sides or two corresponding phases should be connected together. Then HV side should be energized and voltages between poles to be connected in parallel on LV should be checked. These voltages should be zero, where as voltages between phases should be normal. If this is not so, then the Transformers are not correctly connected. It is also essential to measure the currents in each Transformer after the Transformers are put into parallel operation. The currents should be distributed in proportion to their rated outputs.

Load distribution between parallel running Transformers Knowing rated output power crating SN and short circuit impedance Zk, the distribution of load between the Transformers can be calculated. During parallel operation, The Transformer with lowest short circuit impedance will be most heavily loaded. The maximum available total output Smax of the Transformer bank will be the total output when this Transformer (lowest Zk) is loaded up to its rated output. Following example shows the method of calculating Smax. Transformer 1 Transformer 2 Transformer 3

SN1 = 500 kVA SN2 = 800 kVA SN3 = 1000 kVA

Zk1 = 5.0% Zk2 = 5.4% Zk3 = 6.0%

Total rated output SN = 2300 kVA


Zkmin = 5.0% S1 = 500 kVA S2 = 800 kVA x 5.0/5.4 = 740 kVA S3 = 1000 kVA x 5.0/6.0 = 830 kVA Smax = S1 + S2 + S3 = 2070 kVA, i.e. 90% of total rated output of the bank.

Protection of Transformers A Transformer is a component in electrical distribution network which is most difficult to replace. If it is damaged, it will probably cause either a break in electricity distribution or require difficult emergency supply arrangements. Hence it has to be protected against over-current, short-circuit, over-voltage and any excessive gas and intrinsic faults. 1. Over-current protection Provided by over-current relays in switchgear panels on primary and secondary sides of the Transformer. It is advisable to use this protection as primary protection and protection provided by winding temperature indicator as back-up protection. The auxiliary supply used for these protections must be reliable and 48 Vdc or 110 Vdc from a well maintained battery bank is recommended.


2. Short Circuit protection Also provided by the over-current relay with an instantaneous over-current setting. Both the over-current and short circuit protections can also be provided by circuit breakers in the Switchgear panels on HV and LV side.

Protection of Transformers 3. Over-voltage protection Normally Transformers are designed to have adequate insulation level to withstand any over-voltages arising in operation due to any switching surges in the network and verified by subjecting them to impulse voltage withstand test (a type test). This may not guarantee protection against over-voltages of atmospheric origin. Hence spark gaps (arcing horns) in small and medium sized Transformers and surge arrestors in case of large Transformers are used to protect Transformers against such over-voltages.


When spark gaps are used Transformer manufacturer has to ensure that the Transformer is designed to withstand the stress of chopped wave caused by operation of the spark gaps. If star point of a Transformer is not brought out and not earthed, then any three pole over-voltage will cause voltage of star point to rise to a value higher than the voltages influencing the terminals (50 to 100%). Then manufacturer has to consider this additional voltage stress in the design of the Transformer. Same will be the case when the star point is brought out but not earthed, but such a Transformer can be protected by star point lightning arrestors (not by a spark gap) of rating 70% higher than the highest system voltage and the star point insulation should be adequate for the voltage stress.

Protection of Transformers 4. Excessive gas and intrinsic faults – Buchholtz Relay


The gas generated during such faults is collected in the Buchholtz Relay which is located on a pipe between oil conservator and main tank. The gas will displace oil in the relay and minor gas generation will cause functioning of alarm contact. If excessive gas is generated or when oil level falls, alarm contact will function first followed by tripping contact. Abrupt oil flow from the Transformer into the conservator will cause immediate functioning of tripping contact.

Maintenance and supervision during operation Although a Transformer needs minimal maintenance, it has to be carried out to ensure long life and good operating reliability. While carrying out any maintenance or supervision, instructions regarding safety distances, safety equipment, earthing and limitations to working on live conditions must be carefully followed. Maintenance and supervision activities to be carried out by operating personnel include:


1. 2. 3. 4. 5. 6.

Inspection of oil level; Inspection of oil and winding temperatures; Noise measurement/ observation; Operation of tap changer (if any). The tap changer must be turned from one extreme position to another extreme position and back to original position (under de-energized condition) at least once a year. Condition of dehydrating breather; Cleanliness of the Transformer including cleaning of insulators/ bushings during service interruptions. Use white spirit, Chlorothne VG or NU or easily evaporating cleaning agents for cleaning insulators/ bushings.

Maintenance and supervision during operation 7.

Inspection of external conductor and busbar joints with bushings for tightness of joints. Loose joints lead to overheated bushings and eventual damage to gaskets.

The frequency of the above inspection has to be decided by the operating personnel based on the size, location and importance of the Transformer in the complete installation. Apart from the above, functioning of protective, alarm, regulating and supervision instruments and equipment should be checked once a year. A recommended maintenance practice is given below.


Full maintenance may be required once in 15 to 20 service years, for which it is recommended to refer manufacturer’s instructions.

Maintenance and supervision during operation Recommended maintenance schedule Once a day Check Noise level Check Oil & Winding temperatures Check Oil level Condition of dehydrating Breather Clean Bushings Operation of Buchholtz Relay Check tightness of external connections and busbar joints with bushings


Dielectric strength and moisture content in insulating oil Operation of tap changer Operation of indicators and any other measuring instruments and operation of their contacts Check wiring and termination of auxiliary circuits

Once a Week

Once a Month

Once a Quarter

Once a Year

Investigation of Transformer disturbances If during operation, a Transformer protection gives an alarm or trips the Transformer from the network, the cause must be immediately investigated to ascertain whether its is a case of Transformer damage or some other disturbance in operation. 1.

Recording of disturbances


1.1 General - Date and time of occurrence; - Network data: Supply Voltage, Frequency and Primary Current, Secondary Voltage and Current; - Any other relevant actions made when the disturbance took place; - Weather data (thunderstorm, rain etc) and ambient temperature in the vicinity of the Transformer. 1.2 Function of protective equipment - List of relays and Breakers in the network that operated and their setting currents and other parameters recorded by them (for example earth fault relay); - Pick-up time and currents and readings of operative protective equipment like relays, Breakers, plotters relevant to the Transformer of interest.

Investigation of Transformer disturbances 1.3 Transformer (immediately after the fault) - Is the Buchholtz relay filled with gas, if so, its color and odor; - Is oil in Buchholtz relay sooty; - Thermometer readings; - Were coolers or tank damaged: - Are there any marks of arc on the bushings or on tank cover; - Preceding maximum load and mean value for last 3 hours. 2.

Function of Transformer protective equipment


Operation of some protective equipment such as Buchholtz relay, temperature protection or upstream over-current relay does not always mean that the Transformer is damaged. 2.1 Buchholtz relay can operate for instance when: - a trapped air bubble under the Transformer cover has become loose (an air bubble is colorless and odorless); - a short circuit current has passed through the Transformer (no gas bubbles); - a damage has taken place inside the Transformer (gas can be colored, have a bad smell and is generally combustible).

Investigation of Transformer disturbances Sequence of operations when Buchholtz relay operates is given below: B uchholtz Alarm O perates No C heck Alarm C ircuit

G as Present No

B uchholtz Trip O perates Yes

N o Is G as com bustible

Is gas com bustible

Yes G et G as sam ple analyzed

Yes

C heck O il D rain V alve No

Take the Transform er out of service and inform the m anufacturer


Leaky Yes Tighten D rain V alve

C ontinue the Transform er in S ervice

R elease gas, increase supervision, inform the m anufacturer if problem repeats

Investigation of Transformer disturbances 2.2 Sequence of operations when temperature protection operates is given below: Temperature protection operates Look for cause of over-load, ambient higher than rated ambient, cooling system not ok? Alarm only

Trip only

Alarm & Trip

Check the setting & operation of temperature indicators


Alarm contact correct Continue the Transformer in service, but increase supervision, do not allow oil temperature to exceed 100oC

Trip contact correct Continue the Transformer in service, but increase supervision, do not allow oil temperature to exceed 100oC. Inform the manufacturer if the problem repeats

Investigation of Transformer disturbances 2.3 An upstream over-current relay can operate, for example due to:


- passing short circuit; - inrush current impulse; - defective protective equipment; - winding damage inside the Transformer. 3.

If it can be ascertained that the disturbance is not due to any damage to windings or a damage inside the Transformer; the protective equipment can be re-set and the Transformer energized again.

4.

In case of damage to windings or any other damage inside the Transformer is suspected following tests must be carried out: - insulation resistance of windings; - transformation ratio (voltage ratio measurement); - no-load current measurement; - measurement of dc resistances of windings. Results of above tests must be studied or referred to manufacturer for ascertaining nature and extent of damage and necessary repairs.


Repairs and Spare Parts 1.

When performing repair work, all safety precautions mentioned earlier and mentioned under section on safety must be followed.

2.

It is the responsibility of the customer, after taking over of the Transformer or installation, to inspect, maintain and keep the Transformer in good condition.

3.

All failures during warranty period must be reported to ABB and in turn to the manufacturer with out any delay.

4.

All warranty repairs must be attended to by the manufacturer, if taken up by ABB or the customer, should be done only after obtaining written approval from the manufacturer with clear understanding on who should bear the repair costs.

6.

It is recommended that on-site repairs are limited to replacement of broken or damaged parts unless done in a well-trained repair shop. Many parts and accessories are usually less expensive to replace than to repair.

7.

For replacement deliveries of spare parts, inform manufacturer details of serial number, year of manufacture, rating and voltages of the Transformer and descriptions of the spare parts required.

Safety Precautions Following and any other safety advices during installation, commissioning and maintenance:


1. 2.

Never work on transformers or any installed electrical equipment alone. Do not move or lift a transformer without applying adequate equipment and safety precautions. 3. Do not make any connections which do not comply with the rating plate. 4. Do not apply abnormal mechanical strain on the terminals. 5. Do not reconnect when the transformer is energized. 6. Do not attempt to change tap setting while the transformer is energized. 7. Do not energize or perform maintenance on the transformer without proper earth connection. 8. Do not operate the transformer without alarm and monitoring systems connected. 9. Do not remove any enclosure panels while the transformer is energized. 10. Do not tamper with interlocks, alarms and control circuits. 11. Be aware of possible need for magnetic field protection.

Safety Precautions 12. Perform a final inspection prior to energizing:


-

All external connections have been made properly; All connections are tight and secure; All accessory circuits are operational; All tap connections are properly positioned; The neutral and earth connections have been properly made; Fans – if supplied – are operational; Proper clearance is maintained from high voltage bus to terminal equipment; The correct transformer ratio exists for units with internal terminal boards.

Installation Record for Transformers The attached installation record must be filled-in and signed by both the ABB’s representative and Customer’s representative. The record has provision for recording the inspection details and test results of five Transformers. If it is required to simplify the test record for a particular project, the non-applicable rows may be deleted.


Transformers\Installation Record for Transformers.xls


Applicable Standards 1.

IS : 2026 (Part 1) -1977 Specification for power transformers; part 1 General.

2.

IS : 2026 (Part 2) -1977 (Reaffirmed 2001) Edition 2.2 Specification for power transformers; part 2 Temperature rise.

3.

IS : 2026 (Part 3) -1981 (Reaffirmed 2001) Edition 3.1 Specification for power transformers; part 3 Insulation levels and dielectric tests.

4.

IS : 2026 (Part 4) -1977 (Reaffirmed 2001) Edition 2.2 Specification for power transformers; part 4 Terminal markings, tappings and connections.

5.

IS : 2026 (Part 5) -1994 (Reaffirmed 1999) Power transformers; part 5 Transformer/ Reactor bushings – minimum external clearance in air – specification.

6.

IS : 10028 (Part 2) – 1981 (Reaffirmed 2001) Code of practice for selection, installation and maintenance of transformers; part 2 Installation.

7.

IS : 10028 (Part 2) – 1981 (Reaffirmed 2003) Code of practice for selection, installation and maintenance of transformers; part 3 Maintenance..

Applicable Standards 8.

IEC 60076-1 Edition 2.1 Power transformers – part 1: General.

9.

IEC 60076-2 Edition 2 Power transformers – part 2:Temerature rise.

10. IEC 60076-3 Edition 2 Power transformers – part 3: Insulation levels, dielectric tests and external clearances in air. 11. IEC 60076-4 Edition 1 Power transformers – part 4: Guide to lightning impulse and switching impulse testing – Power transformers and reactors. 12. IEC 60076-5 Edition 2 Power transformers – part 5: Ability to withstand short circuit.


13. IEC 60076-8 Edition 1 Power transformers – part 8: Application guide

Acknowledgement and References This training module has been prepared using mostly the information available with in ABB but scattered. Some information has been borrowed from published literature of some manufacturers of oil filtration machines. The collected information has been filtered to make it applicable for installation of only medium sized distribution Transformers. Flow charts for taking decisions during Transformer disturbances such as operation of some protections have been added. A detailed installation record applicable for medium sized distribution Transformers has been prepared and added. The presentation is mostly practical and only on installation with almost nothing about Transformer theory, design or calculations. Mr. Sajith S Kumar has made a significant contribution in preparing this training module by converting some parts of ABB’s Distribution Transformer Handbook into preliminary slides.


Mr. R. Shanbhag has contributed in identifying the course contents and by giving existing installation check lists. My sincere thanks to Authors of several ABB documents mentioned in the reference list. I am also thankful to Mr. N. Narayana Rao, Mr. S. Prasanna, Mr. V. Srinivasu, Mr. R. Shanbhag and Mr. Sajith S Kumar for taking part in reviewing this module for adequacy and correctness of contents.


Acknowledgement and References 1.

Distribution Transformer Handbook ABB Ident # 1LAC000003.

2.

Manual for installation, start-up, operation & maintenence, Document 1LIT000004M02R0 Rev. 0 Date 20021129 of ABB T&D SpA Unità Operativa Trasformatori di Distribuzione.

3.

Oil Insulated Distribution, large distribution and special transformers – Instructions for transport, installation, operation and maintenance, 1LFI1136-en-rev. G of ABB, Vassa, Finland.

4.

Instructions for Transformer Insulating Oil, Instruction Leaflet 48-500-11F of ABB Inc., USA.

5.

Brochure and technical specification of Oil Filtration Equipment of Flitervac International Inc, Canada.

Disclaimer Author will not be directly or indirectly responsible for any damages caused to personnel, equipment or surroundings consequent to usage of this presentation or material or information or knowledge contained in this presentation. Author will not be directly or indirectly responsible for infringement of any intellectual property rights, patents, copy rights or any other similar rights caused due to usage of this presentation or material or information or knowledge contained in this presentation.


Persons or organizations using this presentation or material or information or knowledge contained in this presentation are requested to use their discretion before such usage.


Transformer Vector Technic

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