Parts of Power Transformer in Detail Understand

Core and magnetic circuit

The core of transformer is made up of thin laminations (0.27/0.23 mm) of special electrical grade.
The CRGO sheet steel (with 5 % silicon) is used for power transformers.
Each individual lamination is provided with Zinc Oxide (Carlite) coatings or varnish insulation on both sides.
Joints of the laminations are designed such that the electromagnetic flux is always in the direction of the grain orientation.
The core is built up by placing the lamination in an interleaved fashion.
Such a construction of core ensures low core losses.
The core is clamped and provided with rigid insulated bolts.
In new technique, core bolts are avoided and laminations are held together by fiber glass bands.
The core clamping structure is designed such that, it takes care of all the forces produced in the windings in the event of any short circuit.
The core may be 3 limbs or 5 limbs.
66/11 KV, 66/22 KV, 132/11 KV, 132/66 KV, 220/66 KV, 220/132 KV transformers are of having 3 limb core.
400/220/33 KV transformers are of 5 limb core.

Windings and insulation system

Windings of transformer are made from paper insulated copper conductor.
Parallel conductors are transposed at regular interval through the winding for ensuring equal flux linkages and current distribution between strands.
Disc construction is adopted for the high voltage winding to ensure uniform distribution of impulse voltage.
Insulation spacers in the windings are arranged such that, oil is directed through the entire winding for ensuring proper cooling.
The insulated windings are placed on the core limbs.
The windings are rigidly supported on the core by the insulation system.
The design of insulation system is based on voltage distribution during transient over voltages and the forces during the short circuit condition.
Types of winding depends upon voltage and current rating of winding.
Types of windings:

Helical winding

For LV side of 66/11KV, 66/22 KV & 132/11 KV transformers

For Tertiary of 400/220/33KV, 220/132/11 KV

Continuous disc type winding

For HV side of 66/11 KV, 66/22 KV transformers

Interleaved disc type winding

For HV & LV side of 132/66 KV, 220/66 KV, 220/132 KV

Shielded disc type winding

For HV & LV side of 400/220/33 KV

Types of vector group of winding:

Following vector groups are adopted in transformers

66/11 KV, 66/22 KV – Dyn11
220/66 KV,132/11 KV- YNyn0
132/66 KV,220/132 KV – Yyna0
220/132/11,400/220/132- Yyna0d11

The disposition of windings from core is as below.

66/11 KV, 66/22 KV- Core/LV/HV with tapping
132/11 KV- Core/LV/Tapping/HV
220/66 KV-Core/Tapping/LV/HV
132/66 KV- Core/Tapping/Common/Series
220/132 KV-Core/Tapping/Common/Series
220/132/11 KV- Core/Tertiary/Tapping/Common/Series
400/220/33 KV- Core/Tertiary/Tapping/Common/Series

Leads and terminal arrangement

HV winding leads are taken out from top plate of the transformer and terminals are named 1U, 1V, 1W.

LV winding leads are taken out from top plate of the transformer from opposite side of HV leads and are named 2U, 2V, 2W for 132/66 KV, 66/22 KV, 220/66 KV, 400/220 KV or from top plate on one side of HV leads for 66/11 KV & 132/11 KV where power cables are to be used for LV side.

Tertiary winding leads are taken out from top plate of the transformer from one side of HV leads and are named 3U, 3V, 3W for 400/220/33 KV.

Bushings

The winding terminals are taken out and passed from the windings to terminal bushings.
Types of bushings

Plain porcelain and oil filled type

Terminal bushings up to 36 KV class, 3150A are normally of plain porcelain and oil filled type.

Oil communicating type bushings must be filled with transformer oil in operation and this is achieved by keeping conservator above bushing level. So that bushing remains filled with oil.

Condenser type

Terminal bushing of 52 KV class and above are of OIP condenser type.

The oil inside the condenser bushing will not be communicating with the oil inside the transformer.

Oil level gauge is provided on the expansion chamber of condenser bushing.

It consists of an active part with oil impregnated paper core built up around a centre pipe/rod with aluminum foils at pre designed location for best possible internal & external dielectric strength.

The active part is dried under the heat and vacuum and impregnated with insulating oil.

The impregnated core is assembled inside porcelain and aluminum flange and conservator with oil resistive synthetic rubber seals are held together by a spring pack which also takes care of expansion & contraction of centre pipe/rod.

The annular space between core and porcelain is filled with the same oil.

Nitrogen gas cushion is provided in conservator to take care of expansion and contraction of oil.

An oil level prismatic/magnetic gauge is provided on the conservator.

At mounting flange, an air vent plug is provided for air release of the transformer turret.

The outermost condenser layers of core is taken out electrically through the test tap and grounded through a threaded cap.

Arcing horns are provided on bushings if LA is not provided.

Type of bushing connection.

Lead type: In 132/66 KV, 66/11 KV Tr., winding leads are directly passed through hollow bushing and on end, threaded copper thimble is provided with lock by means of pin and the terminal is screwed down fully with the gasket.

Rod type: In large 400/220 KV, 220/66 KV, 220/132 Tr., winding leads ends are provided with lugs. Bushing rod is passed through the bushing and on end, threaded rod is locked by means of pin and the terminal is screwed down fully with gasket. Connection of rod end and winding ends are connected through inspection covers.

Stem type: In stem type, the central conductor itself acts as the current carrying part and the transformer lead is to be connected at bottom terminal through the inspection windows.

Insulation oil

It is petroleum based mineral oil.
It provides high electric strength.
It permits good transfer of heat.
It is clear and transparent in appearance.
It has low specific gravity: Particles which have become suspended in oil, will settle down on the bottom of the tank more readily and at a faster rate, a property aiding the oil in retaining its homogeneity.
It has a low viscosity: Oil with low viscosity, has greater fluidity, will cool transformer at a much better rate.
It has low pour point: will cease to flow only at low temp.
High flash point: It has higher the flash point, lower the oil will tend to vaporize.
Not attack insulting materials and structural materials.
Has chemical stability and hence ensure long life service.

Tank

The complete core and coil assembly is put in the tank.
Tanks are manufactured by welded steel plates and are suitable for withstanding full vacuum and positive pressure of 1 atmosphere.

Types of tank

Bell type tank: Large transformers 220/66 KV, 220/132 KV, 400/220 KV transformers are having such type of tank construction.
Bottom tank with top plate: Small power transformers 66/11, 66/22 KV transformers are having such type of tank construction.

OLTC (On load tap changer)

Transformers are provided with on load tap changers.
It is used for variation of voltage on secondary side as required.

66/11 KV, 66/22 KV, 132/66 KV, 220/66 KV, 220/132 KV transformers are provided with voltage variation -5% to + 15% in step of 1.25% (16 steps- 17 taps)
400/220 KV transformers are provided with voltage variation -10% to +10 & in step of 1.25% (16 steps-17 taps)

OLTC may be placed on HV or LV side.

132/11 KV, 66/11KV, 66/22 KV

HV side/Normal tap-9/ With increase in tap no., secondary voltage increases.

132/66 KV, 220/66 KV, 220/132 KV, 400/220 KV

LV side/Normal tap-13/with decrease in tap no., secondary voltage increases.

It is designed for changing the taps in following modes.

1. Manually by crank handle
2. Manually from local motor drive
3. Manually from remote mode from RTCC
4. Automatically from RTCC by AVR (For 132 KV & above)

As per construction, OLTC are of two types.

OLTC with combined selector and diverter switch.(The pennant cycle single resistor tap changer)

-It is provided in 132/11KV, 66/11 KV, 66/22 KV class transformers.

-It is provided in separate OLTC tank having two separate chambers attached to the main tank.

-In one chamber, it consists of selector switch, driving shaft, phase board, selector switch arms, fixed and moving contacts, transition resistor, copper leads, terminal (barrier) board for mounting on transformer, connector between tap changer leads and terminal board, oil level indicator, filter valves- top and bottom and drain plug

-In another chamber, it consists of energy accumulator with spring, selector wheel, locking arm, mechanical limit barrier, slip clutch, necessary gears, mechanical tap position indicator, tap changer operation counter and handle for manual operation.

-Separate PRV and OSR are provided.

OLTC with selector switch in main tank and diverter switch in separate compartment.

-Provided in 400/220 KV, 220/132 KV, 220/66 KV, 132/66 KV class transformers.

-Selector switch is inside of the transformer tank comprising driving shaft, selector switch arms, fixed and moving contacts.

– In selector units, terminations are made.

-Diverter switch is in separate chamber consists of transition resistor, driving shaft, fixed and moving contacts, fiber glass cylinder

-Diverter switch chamber is filled with separate oil with separate conservator, breather, OSR and PRV.

Oil conservator tank

Oil conservator is provided to take care of the expansion and contraction of the transformer oil which takes place during normal operation of transformer.
Oil conservators are of two types
1. Conservator in which oil is in direct contact with atmosphere.
  - Old 220,132 & 66 KV class transformers and OLTC conservators are having this type of conservator.
2. Conservator with air cell:
  - Transformer oil is isolated from the external environment by flexible separator or air cell.
  - The air cell expands and contracts with the oil and isolates the oil from air.
  - This keeps transformer oil at atmosphere pressure.
  - The inside of air cell opens to atmosphere through silica gel breather.
  - This system eliminates oxidation of oil due to air contact.
  - Excellent resistance to oxygen, ozone & humidity.
  - Excellent impermeability to oil & gases.
  - Long service life without maintenance.

Dehydrating silica gel breather

It is connected to the pipe from top of the oil conservator.
It is transparent hollow cylinder tube which contains chemically pure silicium salt (Sio2) silica gel with cobalt indicator.
It is hard blue crystals which has considerable absorption power for moisture.
Oil cup is provided in the bottom of cylinder which acts as oil seal
When the transformer is loaded or unloaded, the oil temp. inside the transformer tank rises or fall and accordingly the air volume inside the tank changes by either sucking in or pushing out the air . This phenomena is called breathing of the transformer.
The air which is being sucked in contains either foreign impurities and/ or humidity which changes dielectric strength of transformer oil. Hence it is necessary that, the air entering into the transformer is free from moisture & foreign impurities.
When air from out side is breathed in, it passes through crystals and the moisture in the air is absorbed . Thus the air reaches the conservator is dry. Dust particles are partly trapped in oil seal and partly trapped by crystals of the silica gel. Silica gel crystals change color from dark blue to pink depending upon absorption of certain quantity of moisture.
For proper dehumidification of air, it is necessary that this change of silica gel is reconditioned from pinkish white to deep blue by heating it/replacing it.
It is provided both on main tank conservator and OLTC conservator.

Cooling system

Transformers are designed for following type of cooling system.
1. ONAN – Oil Natural Air Natural

– In this type of cooling, only radiators are provided which are attached to the tank in small transformers and in large transformers separate radiator bank is provided.

-Oil flows through the winding and radiators by thermosyphonic effect.

ONAF- Oil Natural Air Forced

– In this type of cooling, adequate nos. of fans are provided for cooling of the radiators and hence transformer oil.

-One switch contact of WTI is used for controlling of fans on auto mode.

-One fan is kept in stand by mode in each group.

-Setting to be adopted for fan control

Fan start- 65 deg

Fan stop- 55 deg

OFAF-Oil Forced Air Forced

– In this type of cooling, adequate no. of oil pumps are provided.

– It circulates the transformer oil between main tank and radiator and hence cooling of transformer oil.

-One switch contact of WTI is used for controlling of oil pumps on auto mode.

One pump is kept in standby node.

– Setting to be adopted for oil pump control

Oil pump start- 55 deg

Oil pump stop- 45 deg

Main Buchholz relay

Buchholz relay is also called gas relay/ sudden pressure relay.
It is fitted in the feed pipe between conservator and main tank at 5 to 7 deg tilt from main tank to conservator tank
It has two elements upper & lower.
Both elements consists of mercury switch attached to a float/bucket

Upper element

In case of incipient fault within transformer, the gas is produced which being light, rises up ward and tries to go into conservator.
Due to collection of these gases, the oil level in the relay falls.
When sufficient gas is accumulated and as the oil level goes low, switch is operated.
This contact is used for alarm circuit so the transformer can be dis connected before incipient fault grows into serious one.
Same switch will also operate on low oil level such as slow oil leak.

Lower element

The lower element contains a mercury switch mounted on a hinged type flap.

In case of heavy fault within the transformer, the gas generated is more violent and the oil displaced by the gas bubbles rushes through the connecting pipe to conservator tank.

The lower float/bucket rotates and mercury switch is operated which trips the transformer.

In some relay, bottom element is provided with flap attached to float/bucket in the direct path of oil from the transformer to the conservator.

The lower float is operated for the following serious faults

Short circuit between phases
Winding to earth fault
Winding short circuit
Puncture of bushing

OTI (Oil Temperature Indicator)

OTI indicates maximum temperature of transformer oil.
It consists of a sensor bulb, capillary tube and a direct reading dial.
The dial gauge can be fixed away from the point of temp measurement on the top of the transformer.
The sensor bulb is fixed in thermometer pocket filled with transformer oil at location of hottest oil (on top plate of transformer)
The liquid inside the bulb in contact with the hot oil in the thermometer pocket expands and the pressure of the vapor inside the capillary increases and results in movements of the below. With the help of a link and lever mechanism, there is movement to the disc carrying pointer and switches which indicates the temp on the dial of the thermometer.
One oil temp high alarm and one oil temp high trip switch is provided on the disc of meter.

WTI (Winding Temp. Indicator)

WTI works on the same principle except that WTI is provided with an additional below heating element.
As it is not possible to measure the winding temp directly, it is done indirectly by means of a thermal image process.
The heating element is fed by a CT with a current proportional to the load in the winding.
The temp increase of the heating element is thereby proportional to the temp increase of the winding over top oil temp
As bulb of the instrument is located in the hottest oil zone, it senses the max oil temp
The operating below thus gets additional movement, stimulating the increment of winding temp above max oil temp and thus indicates hottest spot temp of winding.
Maximum indicating pointer red in color is fixed to the instruments window and operated by a peg driven by the indicating pointer.
It can be reset with a screw driver after removing knurled weather proof knob.
A switch testing knob is fixed on the bottom of the lid to check switch without removing the lid.

Prismatic oil level gauge

A special prismatic glass oil level indicator is provided on one side of oil conservator
In conservator without air cell, it shows oil level in the transformer tank.
In conservator with air cell, it shows the condition of air cell. If the air cell is fully inflated, the prismatic oil gauge shows full oil level. If the oil level is not properly inflated/leakage in air cell/oil level is lowered due to leakage.
The glass is of special design. Normally the color of oil is plain transparent but when it is seen through the prismatic glass, it shows reddish brown color.

MOG (Magnetic oil level gauge)

It is fixed on one side of conservator.
It is having oil level gauge showing Empty/ Filling level/ ¼ level/ ½ level/ ¾ level/ Full level.
In conservator without air cell, oil in the conservator is filled up to ½ levels during commissioning.
In the conservator with air cell, oil in the conservator is filled up to filling level/ 30 deg level.
It is provided with one switch. When oil level falls to empty level, the switch closes and gives alarm.
It consists of gear assembly, magnetic couple, float arm, cam assembly with mercury switch & dial with pointer.
The float is hinged and swings up or down when oil level rises or falls. The rise or fall rotates the bevel gear and thus pinion of the gear assembly. The pinion in turn rotates the driving magnet inside the conservator. The follower magnet positioned outside carries a pointer and a cam. The pointer reads the oil level and the cam is set to operate the mercury switch at low level.

PRV (Pressure release valve)

PRV (Pressure release valve) is mounted on the top plat of transformer main tank.
It is provided with a pressure die cast alluminium flange with nitrile gasket for mounting on transformer.
The stainless steel diaphragm is loaded with two reverse wound spring and seals 75 mm and 150 mm port.
If there is major fault inside the transformer tank, and due to that, the pressure inside the tank rises above a preset limit, the diaphragm gets lifted instantaneously.
It gives way throwing gas and oil out side and hence excessive pressure drops. The diaphragm then restores to its original position.
The lift of the diaphragm is utilized to operate a flag indicator and micro switch
For larger power transformer, two nos. of PRV are provided.
If PRV does not operate properly and relieve pressure within few milliseconds, the tank may burst.
In large transformers, PRV is mounted on all 3 phase OLTC diverter switch compartment.
In 66 KV class transformers, PRV is mounted on OLTC compartment top plate.
The NO contact of micro switch is used for tripping of transformer.
Operating pressure set on the PRV is 0.42 kg/sqcm, 0.49 kg/ sqcm, 0.56 kg/ sqcm, 0.70 kg/sqcm

Double diaphragm explosion vent

It is provided on top of the main tank.
It consists of bent pipe of suitable diameter with aluminium diaphragm at both ends.
A protective wire mesh is fixed on the opening of the transformer to prevent the pieces of ruptured diaphragm from entering the tank.
Near the lower end of the vent, there is small oil level indicator.
When the lower diaphragm ruptures, transformer oil rises in the vent pipe and is visible through the indicator indicating the failure of lower diaphragm.
In case the pressure developed is not reduced to safe value, after bursting of lower diaphragm, upper diaphragm also gives way throwing the gas and oil outside and thus protecting the transformer tank from mechanical damage.
Conservator and explosion vent are connected with equalizing pipe.
It should be removed when vacuum is applied.

OSR (Oil surge relay)

It is connected in a pipe between OLTC and OLTC conservator.
In large transformer, 3 nos. are provided for 3 phases and in small transformers, one no. is provided.
OSR are of two types:
1. OSR with self reset float:

It is like Buchholz relay but with only one mercury switch. It is operated when there is sudden surge in the OLTC during operation of tap or low oil level.

It is self reset type

It is provided with Test/Service knob.

OSR without float:

It is operated when there is sudden oil surge in the OLTC during operation of tap.

Once operated, it is required to reset from top side reset switch on the relay.

It is not operated due to low oil level.

It is provided with Test/Reset switch on top cover of relay

RTCC panel (Remote tap changer control cubicle)

It is provided in control room
It consists of metallic panel with control circuit and annunciation
It houses control circuit for remote operation of OLTC, TPI (Tap Position Indicator),
It houses control circuit for remote operation of cooling fans
It houses control circuit for

Marshalling kiosk

It is metallic weather proof kiosk.
It houses OTI, WTI thermometers.
The transformer is protected by certain protection devices mounted on the transformer at different locations.
These protection devices have switches and contacts which are required to be connected to protection scheme in C & R panels.
In order to facilitate connection of all such devices to the protective scheme, the leads of all protection devices are wired to terminals of MK. Following protective devices are to MK.
1. Large transformers

-OTI alarm

-OTI trip

-WTI alarm R ph, Y Ph, B Ph

-WTI trip R Ph, Y Ph, B Ph

-Main Buchholz alarm

-Main Buchholz trip

-Main tank PRV 1

-Main tank PRV 2

-OLTC PRV R Ph, Y Ph, B Ph

-OLTC OSR R Ph, Y Ph, B Ph

-Main tank/ OLTC tank MOG alarm

-Bushing CT secondary

Small transformers

-OTI/WTI alarm

-OTI/WTI trip

-Main Buchholz alarm

-Main Buchholz trip

-Main tank PRV 1

-OLTC PRV

-OLTC OSR

-MOG alarm

It has hinged lockable door with a glass window from which OTI & WTI readings can be observed.

In small transformers, it is directly mounted on transformer tank.
In large transformers, it is directly mounted or separately fixed near transformer.

Cooler control kiosk

In large transformers separate cooler control kiosk is provided.
It is metallic weather proof kiosk fixed near transformer.
It is provided with cooling fans control & oil pump control circuit
The transformer is provided with cooling fans and oil pumps on transformers.
Control circuit of these devices is wired to terminals of cooler control box.
It has hinged lockable door with a glass window from which control supply lamps can be observed

Radiators

It is from pressed sheet steel.
of the radiator, nos. of fins in one radiator and size of the fins depends upon capacity of transformer
Inside surface of the radiator is painted with varnish.
In small transformer, radiators are directly mounted on the tank with Flap type (Butterfly type) radiator valves provided on tank.
In large transformer, separate radiator bank is provided.
Radiators are mounted with the help of flap valves on the headers provided on top and bottom of the tank and are supported of one frame.
It is provided with drain and air release plug

Cooling fans

Cooling fans are provided in the transformers designed for dual rating ONAN/ONAF.
They are mounted on brackets which are fixed directly on radiators or on separate frame fixed on ground near radiators.
The fans are controlled from cooler control box near transformer on local manual & auto mode.
The fans are also controlled from RTCC on remote manual and auto mode.
When fans are on auto mode, they are switched ON by using one of the mercury switches of WTI

For prevention of hunting, temp diff. should be min. 5 deg between switching ON and OFF of the fans

Oil pump

The oil pumps are provided in the transformer designed for triple rating ONAN/ONAF/OFAF.
The pumps and driving motors are assembled as a single unit on common shaft.
The oil pump is controlled from cooler control box near transformer on local manual & auto mode.
The fans are also controlled from RTCC on remote manual and auto mode.
When pumps are on auto mode, they are switched ON by using one of the mercury switches of WTI.
For prevention of hunting, temp. diff. should be min 5 deg between ON and OFF of the pumps.

Oil flow indicator

It is provided besides of oil pump.
It consists of vane type sensor. Its surface is kept at right angle to dir. Of flow.
When liquid starts flowing through pipe, the vane gets deflected along with vane shaft.
This deflection is used to indicate flow inside the pipe.
The pointer indicated rate of full flow.
It is provided with mercury switch. When flow drops near to 70%, of full flow, the switch operates.
One stand by oil flow indicator is provided for stand by oil pump.

Various valves

Valve is nothing but one type of switch
Types of valves

Gate valve

It is used for faster flow of oil

It is provided on both sides of Buchholz relay and both sides of ,OLTC surge relay

Glob valve

It is used for very slow flow of oil

It is provided for most of the application of transformer such as

Oil filling valve on main tank
Oil sample valves on main tank
Oil filter valves on main tank
Oil drain valve on main tank, conservator of main tank and OLTC tank

Plug for air release on main tank, header, conservator tank
Oil suction valve on OLTC
Valves provided for conservator and breather

Butterfly/Flap/Radiator/Shut off valve

It is used between radiator and main tank and between transformer main tank and radiator bank
It is operated by a special tool or spanner and provision is given for locking in open/close position.
OPEN/CLOSE markings are provided on the body with an arrow mark for position indication
To indicate the disc position in the cap removed condition, some suitable mark on the spindle head is provided.
It operates at 90 deg

NRV (Non Return Valve)

It permits fluid to flow in one direction
It is operated by pressure alone, having no extra means of control
It is provided in large transformer between main tank and radiator bank but before oil pump.

Ball valve

It is used on Buchholz relay as air release valve

Bushing CT

In small transformer, on no. CT for winding temp indicator is provided on winding of the transformer.
The secondary terminals are taken out and connected in MK box.
In large transformers, 3 nos. CT on all 3 phase for winding temp indicator, 6 nos. CT on all 3 HV & 3 LV windings and one no CT on neutral of each windings for protection purpose are mounted on bushing of the transformer.
The secondary terminals are taken out and connected in MK box.

Rollers

Transformers are provided with rollers on bottom of the main tank
In small transformers 4 nos. of single wheels are provided with lock arrangement on rail
In large transformers 4 nos. of twin wheels are provided with lock arrangement
It is used for slight movement of the main tank at the time of commissioning and dismantling
It provides space for cooling by natural air circulation at bottom of the transformer.

Air release plugs

It is provided for air release from various locations of transformer such as Top plate, Bushing turret, Radiators, Headers, OLTC tank
It is provided at suitable location such a way that no air pocket is left in side

Earthing system

All transformers are provided with two nos. of earthing pads of suitable size on main tank
Two earthing strips of suitable size are connected from earthing pads to two separate earthing pits of the substation/mat earthing
Neutral of the star winding is required to be earthed
Two earthing strips of suitable size are connected from earthing pads to two separate earthing pits