POWER TRANSFORMER
POWER TRANSFORMER
Specifications to be referred:
Sl. No.
Standards
Topic
1
IS-335-1972
Specification for New insulating
oil for transformer and switch gear
2
IS-2026 (Part-1)-1977
Specification for power transformers-General
3
IS-2026 (part-2)-1977
Specification for power transformers-
Temperature rise
4
IS-2026(part-3)-1977
Specification for power transformers-Insulation levels
and dielectric tests
5
IS-2026 (part-4)-1977
Specification for power transformers-Terminal markings &
Tapping connections
6
IS-2026 (part-5)-1977
Specification for power transformers- Bushings
7
IEC:60-1973
High voltage test techniques
8
IEC:71-1976
Part 1,Part 2
Insulation coordination Terms, definitions,
principles and rules.
Application guide.
9
IS-3639
Fitting & accessories for Transformers
Principles of Power Transformer:
|
Sl. No. |
Standards |
Topic |
|
1 |
IS-335-1972 |
Specification for New insulating oil for transformer and switch gear |
|
2 |
IS-2026 (Part-1)-1977 |
Specification for power transformers-General |
|
3 |
IS-2026 (part-2)-1977 |
Specification for power transformers-
Temperature rise |
|
4 |
IS-2026(part-3)-1977 |
Specification for power transformers-Insulation levels and dielectric tests |
|
5 |
IS-2026 (part-4)-1977 |
Specification for power transformers-Terminal markings & Tapping connections |
|
6 |
IS-2026 (part-5)-1977 |
Specification for power transformers- Bushings |
|
7 |
IEC:60-1973 |
High voltage test techniques |
|
8 |
IEC:71-1976 Part 1,Part 2 |
Insulation coordination Terms, definitions, principles and rules.
Application guide. |
|
9 |
IS-3639 |
Fitting & accessories for Transformers |
Principles of Power Transformer:
- Transformers are Static piece of Electrical equipment Transferring Power from one winding at one voltage to another winding at another voltage.
- The Transformer is one of the most efficient machines with 95% efficiency, generally in lower capacities while even 99% efficiency is also achievable in higher ranges.
- The fundamental principle on which the Transformer works is the Electromagnetic Induction (self and mutual).
- Physically, a winding called Secondary is wound around a Magnetic core and over that another winding called Primary is wound round.
- Application of an Alternating voltage V1 to the primary winding results in flow of a current and production of a self induced emf (or Counter emf) whose direction is in opposition to the one producing it.
- The flow of current in the Primary produces a magnetic flux which links the secondary winding and results in the production of a secondary emf called mutually induced emf.
- This emf is in phase opposition to the primary applied voltage V1 and its magnitude depends upon the rate of change of flux and the secondary number of turns.
- It can be seen that the more the primary and secondary circuits are linked together, the more direct is the exchange of energy between them.
- Thus when the Transformer is on No Load, the current drawn by the primary is expended to meet the Iron Losses in the core (Hysterisis and Eddy Current Losses).
- In fact, the No Load Current which is the resultant of the active current and the magnetizing current actually lags behind the applied Voltage at an angle less than 90 degrees.
- It can be seen that E1 = 4.44 * f * N1 * Bmax * A and E2 = 4.44 * f * N2 * Bmax * A from which it can be derived that V2 / V1 = N2 /N1 = E2 / E1 = K. If K > 1, the Transformer is a Step up Transformer and if K < 1 it is a Step down Transformer.
- It can also be shown when a Load is connected to the Transformer secondary, that on account of the Load, a separate loss of energy occurs and that the No Load Loss is independent of the Load Loss ( it is called the Fixed Loss) where as the Load Loss is a Variable Loss.
- The ideal condition for maximum efficiency of a Transformer is worked out to be when the No Load Losses equal the Load Losses
- Transformers are Static piece of Electrical equipment Transferring Power from one winding at one voltage to another winding at another voltage.
- The Transformer is one of the most efficient machines with 95% efficiency, generally in lower capacities while even 99% efficiency is also achievable in higher ranges.
- The fundamental principle on which the Transformer works is the Electromagnetic Induction (self and mutual).
- Physically, a winding called Secondary is wound around a Magnetic core and over that another winding called Primary is wound round.
- Application of an Alternating voltage V1 to the primary winding results in flow of a current and production of a self induced emf (or Counter emf) whose direction is in opposition to the one producing it.
- The flow of current in the Primary produces a magnetic flux which links the secondary winding and results in the production of a secondary emf called mutually induced emf.
- This emf is in phase opposition to the primary applied voltage V1 and its magnitude depends upon the rate of change of flux and the secondary number of turns.
- It can be seen that the more the primary and secondary circuits are linked together, the more direct is the exchange of energy between them.
- Thus when the Transformer is on No Load, the current drawn by the primary is expended to meet the Iron Losses in the core (Hysterisis and Eddy Current Losses).
- In fact, the No Load Current which is the resultant of the active current and the magnetizing current actually lags behind the applied Voltage at an angle less than 90 degrees.
- It can be seen that E1 = 4.44 * f * N1 * Bmax * A and E2 = 4.44 * f * N2 * Bmax * A from which it can be derived that V2 / V1 = N2 /N1 = E2 / E1 = K. If K > 1, the Transformer is a Step up Transformer and if K < 1 it is a Step down Transformer.
- It can also be shown when a Load is connected to the Transformer secondary, that on account of the Load, a separate loss of energy occurs and that the No Load Loss is independent of the Load Loss ( it is called the Fixed Loss) where as the Load Loss is a Variable Loss.
- The ideal condition for maximum efficiency of a Transformer is worked out to be when the No Load Losses equal the Load Losses
Types of transformers
Transformers are largely divided into Power and Distribution Transformers and Step up or step down Transformers. Depending upon utility, the following other types of classification is also in use:
- Generator Transformer
- Unit Auxiliary Transformer
- Station Transformer
- Transmission
Sub-Station Transformers
- Auto
Transformers
- HVDC Converter Transformer
- Arc Furnace Transformers
- Traction Transformers
- Earthing Transformer or the Zig-Zag
- Generator Neutral earthing
Transformer
- Series and Shunt Reactors (strictly not a transformer)
- Generator Transformer
- Unit Auxiliary Transformer
- Station Transformer
- Transmission Sub-Station Transformers
- Auto Transformers
- HVDC Converter Transformer
- Arc Furnace Transformers
- Traction Transformers
- Earthing Transformer or the Zig-Zag
- Generator Neutral earthing Transformer
- Series and Shunt Reactors (strictly not a transformer)
Constructional features of a transformer:
About 70% of the material used is
(1) Core (2) Winding (3) Insulating Oil Others are : (1) Structural Steel (2) Electrical grade paper such as kraft/crepe paper, (3) Press Board (4) Wood and laminated wood (5) Paper covered conductor / PVC cable (6) Insulating tapes such as cotton/Glass woven/ Polyester resin (7) Gaskets such as Neoprene/ Nitrile Rubber/Synthetic Rubber (8) Low Loss CRGO (H1B-0.23, H1B-0.27, Laser Grade ZDMH-0.23, Amorphous Metal core (9) Copper (high conductivity, sheet, strip, foil, rod, tube, tinned, flexible, braided, flat, and insulated conductors (10) Aluminum conductors-insulated, plates, alloys etc.
Core:
Core
Transformer Core
- Core provides a high permeability closed path for Flux. It supports the winding.
- It consists of laminated sheet steel (CRGO or Amorphous).
- The vertical member of the core is called the Limb /Leg and the Horizontal member of the core is called the Yoke.
- Core plates or laminations are very thin and coated with insulating varnish.
- There are two types of construction of the core viz; Core Type and Shell type. Generally in India, Core type of construction with Two/Three/Five limbed cores is used.
- Generally five limbed cores are used where the dimensions of the Transformer is to be limited due to Transportation difficulties.
- In three limbed core the cross section of the Limb and the Yoke are the same where as in five Limbed cores, the cross section of the Yoke and the Flux return path Limbs are very less (58% and 45% of the principal Limb).
- Shell type of construction of the core is widely used in USA. In the construction of the core the laminations of the Limb and Yoke are interleaved.
- The joint where these laminations meet could be Butt or Mitered. In CRGO, the Mitered Joint is preferred as it reduces the Reluctance of the Flux path and reduces the No Load Losses and the No Load current (by about 12% & 25% respectively).
- The Limb and the Yoke are made of a number of Laminations in Steps. Each step comprises of some number of laminations of equal width. The width of the central strip is Maximum and that at the circumference is Minimum.
- The cross section of the Yoke and the Limb are nearly circular. Mitered joint could be at 35/45/55 degrees but the 45 one reduces wastage.
- The assembled core has to be clamped tightly not only to provide a rigid mechanical structure but also required magnetic characteristic.
- Top and Bottom Yokes are clamped by steel sections using Yoke Studs. These studs do not pass through the core but held between steel sections.
- Of late Fiber Glass Band tapes are wound round the Limbs tightly up to the desired tension and heat treated.
- These laminations, due to
elongation and contraction lead to magnetostriction, generally called Humming
which can be reduced by using higher silicon content in steel but this makes
the laminations become very brittle.
Types of Windings:
 |
| Windings |
(2) Spiral: Used up to 33 kV for low currents using strip conductor. Wound closely on Bakelite or press board cylinders generally without cooling ducts. However, multi layer windings are provided with cooling ducts between layers. No Transposition is necessary.
(3) Helical: Used for Low Voltage and high currents .The turns comprising of a number of conductors are wound axially. Could be single, double or multi layer winding. Since each conductor is not of the same length, does not embrace the same flux and of different impedances, and hence circulating currents, the winding is transposed.
(4) Continuous Disc: Used for 33kv and 132 kv for medium currents. The coil comprises of a number of sections axially. Cooling ducts are provided between each section.
(5) Interleaved Disc: used for voltages above 145 kV . Interleaving enables the winding withstand higher impulse voltages.
(6) Shielded Layer: Used up to 132 kV in star connected windings with graded insulation. Comprises of a number of concentric spiral coils arranged in layers grading the layers. The longest at the Neutral and the shortest at the Line Terminal. The layers are separated by cooling ducts. This type of construction ensures uniform distributed voltages.
Distribution transformers:
Distribution Transformers are normally Three Phase.
These are generally rated at 11kv on
the HV side and 415/433volts on the LV side. Some Distribution Transformers are
rated at 33kv on HV side and 415/433 volts on LV side. The normal Ratings of
the Distribution Transformers are:
25, 50, 75,100, 250,315,500,630 & 1000kva.The Standard Frequency of
operation in India is 50Hz.
The Types of Distribution Transformers are:
(1)
Conventional
Non CSP
(2)
Conventional CSP
(3)
Sealed Non CSP
(4)
Sealed CSP
(5)
Hermetically
sealed.
All the above types of Transformers are Oil Filled. The
Core could be made of CRGO or
Amorphous. Amorphous core provides low No Load Losses, No Load Current, Low
Eddy Current Losses; the Core can be very thin, suitable up to Flux Density of
1.58T against 1.92T of CRGO.The Insulation materials used are Press Board,
Kraft Paper, and Perma wood and Mineral Oil. The Winding Material is Copper, or
Aluminum.
The cooling is generally natural. The cooling is done by one or more
layers of Tubes, or Pressed Steel Radiators, or Corrugated Fins. The
Transformers are suitable for Structural Mounting, Plinth Mounting (or Pad
Mounted).
Distribution Transformers are normally Three Phase.
These are generally rated at 11kv on
the HV side and 415/433volts on the LV side. Some Distribution Transformers are
rated at 33kv on HV side and 415/433 volts on LV side. The normal Ratings of
the Distribution Transformers are:
25, 50, 75,100, 250,315,500,630 & 1000kva.The Standard Frequency of
operation in India is 50Hz.
The Types of Distribution Transformers are:
(1)
Conventional
Non CSP
(2)
Conventional CSP
(3)
Sealed Non CSP
(4)
Sealed CSP
(5)
Hermetically
sealed.
All the above types of Transformers are Oil Filled. The
Core could be made of CRGO or
Amorphous. Amorphous core provides low No Load Losses, No Load Current, Low
Eddy Current Losses; the Core can be very thin, suitable up to Flux Density of
1.58T against 1.92T of CRGO.The Insulation materials used are Press Board,
Kraft Paper, and Perma wood and Mineral Oil. The Winding Material is Copper, or
Aluminum.
The cooling is generally natural. The cooling is done by one or more
layers of Tubes, or Pressed Steel Radiators, or Corrugated Fins. The
Transformers are suitable for Structural Mounting, Plinth Mounting (or Pad
Mounted).
High voltage single phase distribution transformers:
These are generally connected between Phases on HV side and between phase and Neutral on LV side. These have the advantage of: Reducing Isq.R Loss, and Lines, prevent Failures of Transformers, provide quality supply, and prevent unauthorized Energy usage, Low Maintenance, and Easy to mount. Normally Stacked, CORE type construction with Mitred joint is used. Shell type core can also be used. The Core can be AMORPHOUS Metal or CRGO.
The conductor of the winding could be DPC or SE Aluminum or Copper. The transformer can have a LT MCCB and a HV Fuse Link and LA on HV side for Protection. The Transformer Tank could be Round, Elliptical or Rectangular but generally round. The Transformer is suitable for Pole Mounting or Plinth Mounting.
After Extensive cleaning, the Tank is sprayed with Powder Coated Paint which is extremely Hard, Scratch Free and Glossy. These Transformers when used for Agricultural Pump sets in Rural areas can ensure Quality, Trouble Free, Reliable, Dedicated Power supply to the consumer and protect him from the Adverse Effects of unauthorized Tapping of supply.
Dry Type Resin Cast Transformers:
These are Transformers, in which the insulating medium is Gas or Dry compound without insulating oil. These can be single phase or three phase, ventilated or non- ventilated, sealed with primary voltage greater than 600 volts. These have special applications.
Amorphous metal transformers:
The use of Amorphous Metal Core instead of Silicone
sheet steel reduces the No Load Losses by about 80%. A proprietary molten alloy
of Iron, Boron, and Silicone is cooled rapidly at a rate of one million degrees
centigrade per second such that Crystals are not formed. The Metal can be drawn
very thin (0.025mm) and so exhibits very low eddy current loss. It is
non-crystalline and it has a random molecular structure.
When AC Magnetic field is applied, the random atomic structure causes
less friction and hence lesser Hysterisis
Loss. However, it has a Low Space
Factor of 80% as compared to
96% in respect of CRGO. Space Factor is defined as the Ratio of Core Cross
sectional Area to the Area available for the Core, which means the weight, and
cost (by about 30%) is more. The high
initial cost however is compensated by the Lower No Load Losses which is
advantageous when the Total Owning Cost over the life period of the Transformer
is considered. This advantage helps in efficient use of available generation,
better Demand side Management, Low Noise
level, Lesser Temperature rise, reduction
in Emissions at generating stations, etc.
The use of Amorphous Metal Core instead of Silicone
sheet steel reduces the No Load Losses by about 80%. A proprietary molten alloy
of Iron, Boron, and Silicone is cooled rapidly at a rate of one million degrees
centigrade per second such that Crystals are not formed. The Metal can be drawn
very thin (0.025mm) and so exhibits very low eddy current loss. It is
non-crystalline and it has a random molecular structure.
When AC Magnetic field is applied, the random atomic structure causes
less friction and hence lesser Hysterisis
Loss. However, it has a Low Space
Factor of 80% as compared to
96% in respect of CRGO. Space Factor is defined as the Ratio of Core Cross
sectional Area to the Area available for the Core, which means the weight, and
cost (by about 30%) is more. The high
initial cost however is compensated by the Lower No Load Losses which is
advantageous when the Total Owning Cost over the life period of the Transformer
is considered. This advantage helps in efficient use of available generation,
better Demand side Management, Low Noise
level, Lesser Temperature rise, reduction
in Emissions at generating stations, etc.
Completely Self Protected (CSP) Transformers:
These transformers have primary protective fuse mounted inside the HV
Bushings. A circuit breaker is provided immersed in the oil in the Tank for LV
protection. This LT Circuit breaker trips for over loads and alerts the
operator against over loads. The loads can be reduced and the CB taken back
into service. However, it allows the CB to be closed in emergency by means of the external control for emergency restoration.
It limits the temperature to 110 degrees Centigrade.
An external indication gives a warning of over load. A
surge arrestor provided on the HV
Bushings protects the Transformer against external surges. The Top cover of the
Transformer Tank is welded. This transformer requires less maintenance and does
not allow outsiders to meddle with.
These transformers have primary protective fuse mounted inside the HV
Bushings. A circuit breaker is provided immersed in the oil in the Tank for LV
protection. This LT Circuit breaker trips for over loads and alerts the
operator against over loads. The loads can be reduced and the CB taken back
into service. However, it allows the CB to be closed in emergency by means of the external control for emergency restoration.
It limits the temperature to 110 degrees Centigrade.
An external indication gives a warning of over load. A
surge arrestor provided on the HV
Bushings protects the Transformer against external surges. The Top cover of the
Transformer Tank is welded. This transformer requires less maintenance and does
not allow outsiders to meddle with.
Power Transformers:
The Basic Material used in the construction of the Transformer are:
(1)
Structural
Steel
(2)
Silicon Steel
(3)
Hard Drawn copper/Aluminum conductor
(4)
Solid Insulation
(5)
Insulating
Oil
The Limits of Temperature rise as per IS 2026 (Part2):
(1) Maximum Ambient
Temperature: --- 500C
(2) Maximum Daily
Average Air Temperature: --- 400C
(3) Max. Yearly
weighted average Air Temp: --- 320C
(4) Minimum Ambient
Air Temperature: --- 050C
For Water Cooled Transformers, the Maximum Temperature
of cooling water should not be more than 30C and the average
daily temperature should
not be more than 250C As long as the Hot Spot Temperature based
on Maximum yearly weighted average temperature is within 980C, the
Transformer can have an expected life of 25 years. The Normal Vector Groups
are: Yd1, Yd11, Yyo, Dy11, Dd0, Dy1, and Dzo. Depending on the application, the
following other vector groups are also being used: Dz10,Yz11,Dd4,Dz4,Yz1,Dy5,Dd2,Dz2,Yd5,Yz5,Yd7,Dz0,Dd6,Dd8,Dz6,Dd10,Dy7,Yy
6,Yz7.
The Limits of Temperature rise as per IS 2026 (Part2):
(1) Maximum Ambient
Temperature: --- 500C
(2) Maximum Daily
Average Air Temperature: --- 400C
(3) Max. Yearly
weighted average Air Temp: --- 320C
(4) Minimum Ambient
Air Temperature: --- 050C
For Water Cooled Transformers, the Maximum Temperature
of cooling water should not be more than 30C and the average
daily temperature should
not be more than 250C As long as the Hot Spot Temperature based
on Maximum yearly weighted average temperature is within 980C, the
Transformer can have an expected life of 25 years. The Normal Vector Groups
are: Yd1, Yd11, Yyo, Dy11, Dd0, Dy1, and Dzo. Depending on the application, the
following other vector groups are also being used: Dz10,Yz11,Dd4,Dz4,Yz1,Dy5,Dd2,Dz2,Yd5,Yz5,Yd7,Dz0,Dd6,Dd8,Dz6,Dd10,Dy7,Yy
6,Yz7.
Accessories & Fittings:
The Transformers have the following Accessories & Fittings:
(1) The HV &
LV Bushings
(2) Neutral and Body Earthing
Terminals
(3) Tank and its Lifting
Lugs
(4) Top Cover with its Lifting Lugs
(5) Drain, Sampling
and Filling Valves.
(6) Oil Level Gauge
(7) Explosion Vent Diaphragm
(8) Silica Gel Breather
(9) Off Circuit
Tap Changer
(10) Conservator Tank
(11) Thermometer Pocket
(12) Base Channels
for Mounting.
The Vector Group of the Three Phase Transformer is generally Dy11.
Bushing: A structure carrying
one or several conductors through a partition such as a wall or tank, and insulating it or them there from, incorporating the means of attachment (flange or
other fixing device) to the partition. The conductor may form an integral part of the bushing or be drawn through.
Liquid Filled Bushing — A bushing in which the space
between the inside surface of the insulating envelope and the solid
major insulation is filled with oil or another insulating liquid.
Liquid Insulated Bushing — A bushing in which the major
insulation consists of oil or another insulating liquid.
Gas Filled Bushing — A bushing in which the space between the inside
surface of the insulating envelope and the solid major insulation is filled
with gas (other than ambient air) at atmospheric or higher pressure.
The
definition includes bushings which are intended to form an integral part of gas
insulated equipment, the gas of the equipment being in communication with that
of the bushing.
Gas Insulated Bushing — A bushing in which the major
insulation consists of gas (other than ambient air) at atmospheric or higher
pressure.
The Transformers have the following Accessories & Fittings:
(1) The HV &
LV Bushings
(2) Neutral and Body Earthing
Terminals
(3) Tank and its Lifting
Lugs
(4) Top Cover with its Lifting Lugs
(5) Drain, Sampling
and Filling Valves.
(6) Oil Level Gauge
(7) Explosion Vent Diaphragm
(8) Silica Gel Breather
(9) Off Circuit
Tap Changer
(10) Conservator Tank
(11) Thermometer Pocket
(12) Base Channels
for Mounting.
The Vector Group of the Three Phase Transformer is generally Dy11.
Bushing: A structure carrying
one or several conductors through a partition such as a wall or tank, and insulating it or them there from, incorporating the means of attachment (flange or
other fixing device) to the partition. The conductor may form an integral part of the bushing or be drawn through.
Liquid Filled Bushing — A bushing in which the space
between the inside surface of the insulating envelope and the solid
major insulation is filled with oil or another insulating liquid.
Liquid Insulated Bushing — A bushing in which the major
insulation consists of oil or another insulating liquid.
Gas Filled Bushing — A bushing in which the space between the inside
surface of the insulating envelope and the solid major insulation is filled
with gas (other than ambient air) at atmospheric or higher pressure.
The
definition includes bushings which are intended to form an integral part of gas
insulated equipment, the gas of the equipment being in communication with that
of the bushing.
Gas Insulated Bushing — A bushing in which the major
insulation consists of gas (other than ambient air) at atmospheric or higher
pressure.
NOTE:
1. This definition includes bushings which are intended to form
an integral part of gas insulated equipment, the gas of the equipment
being in communication with that
of the bushing.
2. A bushing which contains solid insulating materials other
than the envelope containing the gas (for
example, support for conducting layers or insulating cylinder) is a composite
bushing.
Oil Impregnated Paper Bushing — A bushing in which the major
insulation consists of a core wound from untreated paper and subsequently
impregnated with an insulating liquid, generally the transformer oil. The core
is contained in an insulating envelope; the space between the core and the
insulating envelope being filled with the same insulating liquid as that used
for impregnation.
Resin Bonded Paper Bushing — A bushing in which the major
insulation consists of a core wound from resin coated paper. During the winding
process, each paper layer is bonded to the previous layer by its resin coating
and the bonding is achieved by curing the resin.
NOTE: A
resin bonded paper bushing may be provided with an insulating envelope, in
which case the intervening space may be filled with an insulating liquid or
another Insulating medium.
Resin Impregnated Paper Bushing — A bushing in which the major
insulation consists of a core wound from untreated paper and subsequently
impregnated with a curable resin.
NOTE: A
resin impregnated paper bushing may be provided with an insulating Envelope, in
which case the intervening space may be filled with an insulating liquid or
another insulating medium.
Ceramic, Glass or Analogous Inorganic
Material Bushing —A
bushing in which the major insulation consists of a ceramic, glass or
analogous inorganic material.
Cast Resin Insulated Bushing — A bushing in which the major
insulation consists of a cast organic material with or without an inorganic
filler.
Composite Bushing — A bushing in which the major insulation consists of a combination of different
insulating materials.
Capacitance Graded Bushing — A bushing in which metallic or
non- metallic conducting layers are arranged within the insulating material for
the purpose of controlling the distribution of the electric field of the
bushing.
1. This definition includes bushings which are intended to form
an integral part of gas insulated equipment, the gas of the equipment
being in communication with that
of the bushing.
2. A bushing which contains solid insulating materials other
than the envelope containing the gas (for
example, support for conducting layers or insulating cylinder) is a composite
bushing.
Oil Impregnated Paper Bushing — A bushing in which the major
insulation consists of a core wound from untreated paper and subsequently
impregnated with an insulating liquid, generally the transformer oil. The core
is contained in an insulating envelope; the space between the core and the
insulating envelope being filled with the same insulating liquid as that used
for impregnation.
Resin Bonded Paper Bushing — A bushing in which the major
insulation consists of a core wound from resin coated paper. During the winding
process, each paper layer is bonded to the previous layer by its resin coating
and the bonding is achieved by curing the resin.
NOTE: A
resin bonded paper bushing may be provided with an insulating envelope, in
which case the intervening space may be filled with an insulating liquid or
another Insulating medium.
Resin Impregnated Paper Bushing — A bushing in which the major
insulation consists of a core wound from untreated paper and subsequently
impregnated with a curable resin.
NOTE: A
resin impregnated paper bushing may be provided with an insulating Envelope, in
which case the intervening space may be filled with an insulating liquid or
another insulating medium.
Ceramic, Glass or Analogous Inorganic
Material Bushing —A
bushing in which the major insulation consists of a ceramic, glass or
analogous inorganic material.
Cast Resin Insulated Bushing — A bushing in which the major
insulation consists of a cast organic material with or without an inorganic
filler.
Composite Bushing — A bushing in which the major insulation consists of a combination of different
insulating materials.
Capacitance Graded Bushing — A bushing in which metallic or
non- metallic conducting layers are arranged within the insulating material for
the purpose of controlling the distribution of the electric field of the
bushing.
NOTE:
1.
Generally, the major insulation of a capacitance graded bushing is constituted of one of the following:
a) Oil impregnated paper,
b) Resin bonded
paper,
c) Resin impregnated paper,
d) Cast resin,
e) Gas or other insulating fluid, and
f) Composite.
2.
A capacitance graded bushing
may be provided with an insulating envelope, in which case the intervening
space may be Indoor Bushing — A bushing, both ends of which are intended to be in
ambient air but not exposed to external atmospheric conditions.
1.
Generally, the major insulation of a capacitance graded bushing is constituted of one of the following:
a) Oil impregnated paper,
b) Resin bonded
paper,
c) Resin impregnated paper,
d) Cast resin,
e) Gas or other insulating fluid, and
f) Composite.
2. A capacitance graded bushing may be provided with an insulating envelope, in which case the intervening space may be Indoor Bushing — A bushing, both ends of which are intended to be in ambient air but not exposed to external atmospheric conditions.
NOTE:
1.
In indoor installations,
moisture condensation on the surface of the
bushing is to be prevented, if necessary by ventilation or heating.
2.
This definition includes
bushings operating in air at temperatures above ambient, such as occurs with
air-insulated ducting.
Outdoor Bushing — A bushing, both ends of which are intended to be in ambient air and
exposed to external atmospheric conditions.
Outdoor-Indoor Bushing — A
bushing, both ends of which are
intended to be in ambient air. One end is intended to be exposed to external
atmospheric conditions and the other
end is intended not to be so exposed
Indoor-Immersed Bushing — A bushing, one end of which is
intended to be in ambient air but
not exposed to external atmospheric conditions and the other end to be immersed in an insulating medium other than
ambient air (for example, oil or
gas)
Outdoor-Immersed Bushing — A bushing, one end of which is intended to be in
ambient air and exposed to external atmospheric conditions and the other end to be immersed in an insulating medium other than air (for example, oil or gas).
Completely Immersed Bushing — A bushing both ends of which are
intended to be immersed in insulating media other than ambient air (for
example, oil or gas).
1.
In indoor installations,
moisture condensation on the surface of the
bushing is to be prevented, if necessary by ventilation or heating.
2.
This definition includes
bushings operating in air at temperatures above ambient, such as occurs with
air-insulated ducting.
Outdoor Bushing — A bushing, both ends of which are intended to be in ambient air and
exposed to external atmospheric conditions.
Outdoor-Indoor Bushing — A
bushing, both ends of which are
intended to be in ambient air. One end is intended to be exposed to external
atmospheric conditions and the other
end is intended not to be so exposed
Indoor-Immersed Bushing — A bushing, one end of which is
intended to be in ambient air but
not exposed to external atmospheric conditions and the other end to be immersed in an insulating medium other than
ambient air (for example, oil or
gas)
Outdoor-Immersed Bushing — A bushing, one end of which is intended to be in
ambient air and exposed to external atmospheric conditions and the other end to be immersed in an insulating medium other than air (for example, oil or gas).
Completely Immersed Bushing — A bushing both ends of which are
intended to be immersed in insulating media other than ambient air (for
example, oil or gas).
Earthing:
The Core of the Transformer is clamped to a Frame,
which is in turn connected to the Tank. The Transformer is provided with two separate Earthing Terminal connections. These must be connected to two distinct Earthing Electrodes in the Substation. The Earth Resistance of
these Electrodes must be less than 0.5 ohms. The Combined earth resistance shall be less than 0.1 ohm.
For Distribution Transformers, normally Dy11 vector
Group, the LT Neutral is earthed by a separate Conductor section of at least
half the section of the conductor used for phase wire and connected to a
Separate Earth whose Earth Resistance must be less than 1 ohm. The Body of the
Tank has two different earth connections, which should be connected to two
distinct earth electrodes by GI flat of suitable section.
For Large Power Transformers, Neutral and Body
Connections are made
separately but all the Earth Pits are connected in parallel so that the
combined Earth Resistance is always maintained below 0.1 ohm. The individual
and combined earth resistance is measured periodically and the Earth Pits
maintained regularly and electrodes replaced if required.
3.7.3 Lifting Lugs: Two or Four
Lifting Lugs are provided depending upon the size of the weight / size,
for lifting the Transformer. The core also similarly provided
with two or Four Lifting Lugs.
3.7.4 Drain, Sampling and Filling
Valves:-There are two different types of Valves. (1) Wheel Valve (2) Butterfly Valve. The Wheel Valves
are generally made of Cast Iron or gunmetal. These are used as Bottom
Drain/Filter Valve, Top Filter Valve, and Isolating Valve between the Main Tank
and the Conservator. The Butterfly Valves are used between the Main tank and
the Radiators.
3.7.5 Off Circuit Tap Changer: These are
provided on the side of the Transformer. These have to be operated only when
the Transformer is de-energized. This can be manually operated with a Cranking
Handle. A Lock is provided to lock the Tap Changer
The Core of the Transformer is clamped to a Frame,
which is in turn connected to the Tank. The Transformer is provided with two separate Earthing Terminal connections. These must be connected to two distinct Earthing Electrodes in the Substation. The Earth Resistance of
these Electrodes must be less than 0.5 ohms. The Combined earth resistance shall be less than 0.1 ohm.
For Distribution Transformers, normally Dy11 vector
Group, the LT Neutral is earthed by a separate Conductor section of at least
half the section of the conductor used for phase wire and connected to a
Separate Earth whose Earth Resistance must be less than 1 ohm. The Body of the
Tank has two different earth connections, which should be connected to two
distinct earth electrodes by GI flat of suitable section.
For Large Power Transformers, Neutral and Body
Connections are made
separately but all the Earth Pits are connected in parallel so that the
combined Earth Resistance is always maintained below 0.1 ohm. The individual
and combined earth resistance is measured periodically and the Earth Pits
maintained regularly and electrodes replaced if required.
3.7.3 Lifting Lugs: Two or Four
Lifting Lugs are provided depending upon the size of the weight / size,
for lifting the Transformer. The core also similarly provided
with two or Four Lifting Lugs.
3.7.4 Drain, Sampling and Filling
Valves:-There are two different types of Valves. (1) Wheel Valve (2) Butterfly Valve. The Wheel Valves
are generally made of Cast Iron or gunmetal. These are used as Bottom
Drain/Filter Valve, Top Filter Valve, and Isolating Valve between the Main Tank
and the Conservator. The Butterfly Valves are used between the Main tank and
the Radiators.
3.7.5 Off Circuit Tap Changer: These are
provided on the side of the Transformer. These have to be operated only when
the Transformer is de-energized. This can be manually operated with a Cranking
Handle. A Lock is provided to lock the Tap Changer
in any Tap Position but not at any intermediate position. Interlocks can
also be provided to trip off the Transformer if this mechanism is meddled when
the Transformer is energised.
3.7.6 Ratio Changing Links: Some times,
a Transformer will be required to function as a Common Standby for two
different Power Transformers with separate
LV voltages like 11 kV and 33kv. It is possible, for instance to manufacture
and use a Power Trasformer with two different Ratios like 132/33-11kv.This
Transformer can be used as 132/33 or 132/11 kv by changing the Ratio Changing
the Links.
For changing the Links, the Transformer has to be
de-enrgised; oil has to be drained
below the inspection cover for the Links. After changing the Ratio by the
Links, the Ratio has to be tested before taking back the unit into service.
On Load Tap Changer:
The Tap Changer has a Motor Drive Mechanism. The Motor
rotes in the clockwise or anti clock
wise direction when the taps raised or lowered either manually
or electrically. The Tap changer operation is Step by Step. Limit switches
provided cut off the OLTC at the End Taps. The Total operation by Motor takes
about 40 to 70 milli seconds. A Bank Of Energy Storing Springs are provided ,
whose stored energy will be released for a very fast completion of Tap changing operation even when the
drive motor supply is interrupted. In some cases the Transformer will be isolated if
the Tap is stuck in an
intermediate position.
The OLTC can be in One Compartment for 33kv
Transformers and in two compartments to provide the selector Switch inside the
Main tank and the Diverter outside the Main Tank. Make before Break switches,
and Transition Resistors are provided in Diverter. The OLTC can be of a three phase Type or three Single-phase
type. A separate Bucholtz
relay or surge
relays are provided
to take care of any faults inside
the OLTC gear. A separate Conservator Tank is provided for 33kv
Transformers where as the Main
Conservator Tank is partitioned for OLTC in large Power Transformers. The
Contacts, Oil in divertor have to be maintained periodically for good performance.
3.7.8 Conservator Tank: This
takes care of volumetric expansions of Oil in the Transformer. Its capacity is
about 5% of the Main Tank. The Pipe from the Main Tank projects about 3 cm above the bottom of the Conservator
Tank to collect sludge/moisture.
The Oil level in the Main Conservator is indicated by a Magnetic Oil
Level Indicator which has also provision for Low Oil Level alarm or Trip.
Generally the OLTC conservator has a prismatic oil level indicator.
A Silica Gel Breather is also provided for each of the Conservator Tanks. In case of large Power Transformers, a synthetic
rubber made expansion bellow Barrier or a Diaphragm Barrier is provided to
which the Breather is connected. This arrangement eliminates the contact of
Transformer oil with Air.
The Tap Changer has a Motor Drive Mechanism. The Motor
rotes in the clockwise or anti clock
wise direction when the taps raised or lowered either manually
or electrically. The Tap changer operation is Step by Step. Limit switches
provided cut off the OLTC at the End Taps. The Total operation by Motor takes
about 40 to 70 milli seconds. A Bank Of Energy Storing Springs are provided ,
whose stored energy will be released for a very fast completion of Tap changing operation even when the
drive motor supply is interrupted. In some cases the Transformer will be isolated if
the Tap is stuck in an
intermediate position.
The OLTC can be in One Compartment for 33kv
Transformers and in two compartments to provide the selector Switch inside the
Main tank and the Diverter outside the Main Tank. Make before Break switches,
and Transition Resistors are provided in Diverter. The OLTC can be of a three phase Type or three Single-phase
type. A separate Bucholtz
relay or surge
relays are provided
to take care of any faults inside
the OLTC gear. A separate Conservator Tank is provided for 33kv
Transformers where as the Main
Conservator Tank is partitioned for OLTC in large Power Transformers. The
Contacts, Oil in divertor have to be maintained periodically for good performance.
3.7.8 Conservator Tank: This
takes care of volumetric expansions of Oil in the Transformer. Its capacity is
about 5% of the Main Tank. The Pipe from the Main Tank projects about 3 cm above the bottom of the Conservator
Tank to collect sludge/moisture.
The Oil level in the Main Conservator is indicated by a Magnetic Oil
Level Indicator which has also provision for Low Oil Level alarm or Trip.
Generally the OLTC conservator has a prismatic oil level indicator.
A Silica Gel Breather is also provided for each of the Conservator Tanks. In case of large Power Transformers, a synthetic
rubber made expansion bellow Barrier or a Diaphragm Barrier is provided to
which the Breather is connected. This arrangement eliminates the contact of
Transformer oil with Air.
Parallel Operation:
The condition required
for paralleling two Transformers is:
(1)
Same Polarity
(2)
Same Voltage Ratio
(3)
Same Percentage Impedance.
(4)
Same Vector
Group
(5)
Same Phase sequence.
The condition required
for paralleling two Transformers is:
(1)
Same Polarity
(2)
Same Voltage Ratio
(3)
Same Percentage Impedance.
(4)
Same Vector
Group
(5)
Same Phase sequence.
Protection (Internal and External) of Transformer:
Internal protection:
a. Bucholtz Relay: This Gas operated relay is a
protection for minor and major faults that
may develop inside a Transformer and
produce Gases. This relay is located
in between the conservator tank and the Main Transformer tank in the pie line
which is mounted at an inclination of 3 to 7 degrees. A shut off valve is
located in between the Bucholtz relay and the Conservator. The relay comprises
of a cast housing which contains two pivoted Buckets counter balanced weights.
The relay also contains two mercury switches which will send alarm or trip signal to the breakers
controlling the Transformer.
In healthy condition, this relay will be full of oil
and the buckets will also be full of oil
and is counter balanced by the weights. In the event of a fault inside the transformer, the gases flow up to the
conservator via the relay and push the oil in the relay down. Once the oil
level falls below the bottom level of the buckets, the bucket due to the weight
of oil inside tilts and closes the mercury switch and causes the Alarm or trip
to be actuated and isolate the transformer from the system.
b. Oil Surge/ Bucholtz Relay
for OLTC: This relay operating on gas produced slowly or in a surge due
to faults inside the Diverter Switch of OLTC protects the Transformer and
isolates it from the system.
f.
Conservator Magnetic Oil Level Protection: Inside the conservator tank, a float is used to sense the levels of oil and move. This is transmitted to a switch
mechanism by means of magnetic coupling. The Float and the Magnetic mechanism
are totally sealed. The pointer connected to the magnetic mechanism moves
indicating the correct oil level and
also provision is made for Low oil level alarm by switch.
External protection:
(1)
Lightning Arrestors on HV & LV for Surge Protection
(2)
HV / LV Over Current
Protection(Instantaneous /IDMT- Back up)
(3)
Earth Fault Protection (Y connected side)
(4)
REF (HV & LV) ( For internal
fault protection)
(5)
Differential
Protection (for internal
fault protection)
(6)
Over Fluxing Protection (against system Kv & HZ variations)
(7)
HG Fuse Protection for Small Capacity
Transformers.
Normally Each Power Transformers will have a LV
Circuit Breaker. For a Group of Transformers up to 5 MVA in a substation, a
Group control Circuit Breaker is provided. Each Transformer of 8 MVA and above
will have a Circuit Breaker on the HV side.
(1)
Lightning Arrestors on HV & LV for Surge Protection
(2)
HV / LV Over Current
Protection(Instantaneous /IDMT- Back up)
(3)
Earth Fault Protection (Y connected side)
(4)
REF (HV & LV) ( For internal
fault protection)
(5)
Differential
Protection (for internal
fault protection)
(6)
Over Fluxing Protection (against system Kv & HZ variations)
(7)
HG Fuse Protection for Small Capacity
Transformers.
Normally Each Power Transformers will have a LV
Circuit Breaker. For a Group of Transformers up to 5 MVA in a substation, a
Group control Circuit Breaker is provided. Each Transformer of 8 MVA and above
will have a Circuit Breaker on the HV side.
Drying of Transformers:
Distribution Transformers: These are thoroughly dried at the Factory and filled with New Filtered and Tested Transformer Oil before dispatch. When
it is received at site, the IR values and the Oil Tests (Dielectric and
Acidity) are checked. If OK, the Transformer can be commissioned if all the
pre-commissioning Tests are satisfactory.
Power Transformers: These are
also thoroughly dried at the Factory before dispatch. However, large
Transformers are sent without oil with Conservator, Radiators, Bushings,
Protective devices etc packed separately and the Transformer filled with
Nitrogen. All the Manholes, etc is blanked off. Under such circumstances, the drying out in the field will take very
less time.
The real drying is not so much for the Oil but for the
winding, which might have absorbed moisture. Vacuuming, and oil Filtration is
necessary and the process may take more
than a month. The criteria
to determine that the drying out is completed are the
245 kV Power transformers:
(a) Two winding
Three phase rating
MVA
Voltage ratio
Impedance voltage
(percent)
Cooling
50
220/66 kV
12.5
ONAN/OFAF (or)
ONAN/ODAF
100
220/66 kV
12.5
ONAN/OFAF (or)
ONAN/ODAF
100
220/33 kV
15.0
ONAN/OFAF (or)
ONAN/ODAF
(b)Inter Connecting Auto Transformers
35,50
220/33
10
ONAN/OFAF
50
220/132
10
ONAN/OFAF
100
220/132
12.5
ONAN/ONAF/OFAF
(or) ONAN/ONAF/ODAF
160
220/132
12.5
ONAN/ONAF/OFAF
(or) ONAN/ONAF/ODAF
200
220/132
12.5
ONAN/ONAF/OFAF
(or) ONAN/ONAF/ODAF
Auto Transformers(420 Kv Level)Constant Percentage Impedance:
Three-phase
HV/IV/LV
Voltage ratio
Tapping range per
cent
Per cent
impedance voltage
Cooling
MVA
HV-IV
HV-LV
IV-LV
100/100/33.3
400/132/33
+10% to -
10% 16 steps
of 1.25%
12.5
27
12
ONAN/ONAF
200/200/66.7
400/132/33
+10% to -
10% 16 steps
of 1.25%
12.5
36
22
ONAN/ONAF
or ONAN/ONAF
2580/250/83.3
400/220/33
+10% to -
10% 16 steps
of 1.25%
12.5
45
30
ONAN/ONAF
or ONAN/ONAF
315/315/105
400/220/33
+10% to -
10% 16 steps
of 1.25%
12.5
45
30
ONAN/ONAF
or ONAN/ONAF
500/500/166.7
400/220/33
+10% to -
10% 16 steps
of 1.25%
12.5
45
30
ONAN/ONAF
or ONAN/ONAF
630/630/210
400/220/33
+10% to -
10% 16 steps
of 1.25%
12.5
45
30
ONAN/ONAF
or ONAN/ONAF
Auto transformers (800 kV level):-
Three phase rating HV/IV/LV MVA
Voltage ratio KV
Tapping
range(percent)
Percent impedance voltage
Cooling
HV-IV
HV-
LV-
IV-LV
315/315/105
765/220/33
+4.5%
-7.5%
24 steps
12.5
40
25
ONAN/ONAF
or ONAN/ODAF
or ODAF
630/630/210
765/400/33
-do-
12.5
60
40
-do-
750/750/250
-do-
-do-
-do-
-do-
-do-
-do-
1000/1000/333.3
-do-
-do-
14
65
45
-do-
1500/1500/500
-do-
-do-
-do- tolerance
-do-
+10%
-do-
+ 15%
+ 15%
SAMPLE CHECK
LIST IN TRANSFORMER ERECTION:
1. Testing of Oil samples
for BDV in Main Tank and OLTC.
2. Oil level in Main Conservator and OLTC Conservator
3. Capacitance and Tan Delta
values of all the HV Bushings
4. Oil level in HV Bushings
5. Cleanliness of Bushings and Tightness of connections
6. Tightness of Bushing Test tap plugs
7. Locking in of Main/ OLTC Bucholtz
relays released
8. Locking
in of MOG is released
9. Test switch of Bucholtz
relay kept in S (service) position
10. All the Radiator valves
(Top & Bottom)
Opening ensured
11. Release air from Bushings
and close the valve
12. Release air fro the Bucholtz relays
and close
13. Test Bucholtz
relay for proper operation by air injection
14. Shut off valve of Main/OLC Bucholtz/Surge relays are opened
15. Oil seal is ensured
for the Breather
16. Silica gel for the Breather is Blue in color
17. All Radiator
Top and Bottom
valves are opened
18. All Filter/
sampling/ drain valves
are properly closed
19. Main/OLTC Conservator Oil filling caps are tight.
20. Oil filling
in OTI and WTI pockets/
Calibration of WTI/OTI
21. No oil leaks any where
22. Explosion vent diaphragm top and bottom
are in place
23. Release air from top cover
24. Tank Double
point earth and connected to two different
earths
25. Earth Resistance of Earth pits
26. HV/ LV neutrals are earthed
27. Bi directional Rollers are locked
28. Alarm and indications
and trip connections ensured after operating the concerned relays
Distribution Transformers: These are thoroughly dried at the Factory and filled with New Filtered and Tested Transformer Oil before dispatch. When
it is received at site, the IR values and the Oil Tests (Dielectric and
Acidity) are checked. If OK, the Transformer can be commissioned if all the
pre-commissioning Tests are satisfactory.
Power Transformers: These are
also thoroughly dried at the Factory before dispatch. However, large
Transformers are sent without oil with Conservator, Radiators, Bushings,
Protective devices etc packed separately and the Transformer filled with
Nitrogen. All the Manholes, etc is blanked off. Under such circumstances, the drying out in the field will take very
less time.
The real drying is not so much for the Oil but for the
winding, which might have absorbed moisture. Vacuuming, and oil Filtration is
necessary and the process may take more
than a month. The criteria
to determine that the drying out is completed are the
245 kV Power transformers:
|
(a) Two winding |
|||
|
Three phase rating MVA |
Voltage ratio |
Impedance voltage (percent) |
Cooling |
|
50 |
220/66 kV |
12.5 |
ONAN/OFAF (or) ONAN/ODAF |
|
100 |
220/66 kV |
12.5 |
ONAN/OFAF (or) ONAN/ODAF |
|
100 |
220/33 kV |
15.0 |
ONAN/OFAF (or) ONAN/ODAF |
|
(b)Inter Connecting Auto Transformers |
|||
|
35,50 |
220/33 |
10 |
ONAN/OFAF |
|
50 |
220/132 |
10 |
ONAN/OFAF |
|
100 |
220/132 |
12.5 |
ONAN/ONAF/OFAF (or) ONAN/ONAF/ODAF |
|
160 |
220/132 |
12.5 |
ONAN/ONAF/OFAF (or) ONAN/ONAF/ODAF |
|
200 |
220/132 |
12.5 |
ONAN/ONAF/OFAF (or) ONAN/ONAF/ODAF |
Auto Transformers(420 Kv Level)Constant Percentage Impedance:
|
Three-phase
HV/IV/LV |
Voltage ratio |
Tapping range per cent |
Per cent
impedance voltage |
Cooling |
||
|
MVA |
|
|
HV-IV |
HV-LV |
IV-LV |
|
|
100/100/33.3 |
400/132/33 |
+10% to - 10% 16 steps of 1.25% |
12.5 |
27 |
12 |
ONAN/ONAF |
|
200/200/66.7 |
400/132/33 |
+10% to - 10% 16 steps of 1.25% |
12.5 |
36 |
22 |
ONAN/ONAF or ONAN/ONAF |
|
2580/250/83.3 |
400/220/33 |
+10% to - 10% 16 steps of 1.25% |
12.5 |
45 |
30 |
ONAN/ONAF or ONAN/ONAF |
|
315/315/105 |
400/220/33 |
+10% to - 10% 16 steps of 1.25% |
12.5 |
45 |
30 |
ONAN/ONAF or ONAN/ONAF |
|
500/500/166.7 |
400/220/33 |
+10% to - 10% 16 steps of 1.25% |
12.5 |
45 |
30 |
ONAN/ONAF or ONAN/ONAF |
|
630/630/210 |
400/220/33 |
+10% to - 10% 16 steps of 1.25% |
12.5 |
45 |
30 |
ONAN/ONAF or ONAN/ONAF |
Auto transformers (800 kV level):-
|
Three phase rating HV/IV/LV MVA |
Voltage ratio KV |
Tapping
range(percent) |
Percent impedance voltage |
Cooling |
||
|
HV-IV |
HV- LV-
IV-LV |
|
||||
|
315/315/105 |
765/220/33 |
+4.5% -7.5% 24 steps |
12.5 |
40 |
25 |
ONAN/ONAF or ONAN/ODAF or ODAF |
|
630/630/210 |
765/400/33 |
-do- |
12.5 |
60 |
40 |
-do- |
|
750/750/250 |
-do- |
-do- |
-do- |
-do- |
-do- |
-do- |
|
1000/1000/333.3 |
-do- |
-do- |
14 |
65 |
45 |
-do- |
|
1500/1500/500 |
-do- |
-do- |
-do- tolerance |
-do- +10% |
-do- + 15% |
+ 15% |
SAMPLE CHECK
LIST IN TRANSFORMER ERECTION:
1. Testing of Oil samples
for BDV in Main Tank and OLTC.
2. Oil level in Main Conservator and OLTC Conservator
3. Capacitance and Tan Delta
values of all the HV Bushings
4. Oil level in HV Bushings
5. Cleanliness of Bushings and Tightness of connections
6. Tightness of Bushing Test tap plugs
7. Locking in of Main/ OLTC Bucholtz
relays released
8. Locking
in of MOG is released
9. Test switch of Bucholtz
relay kept in S (service) position
10. All the Radiator valves
(Top & Bottom)
Opening ensured
11. Release air from Bushings
and close the valve
12. Release air fro the Bucholtz relays
and close
13. Test Bucholtz
relay for proper operation by air injection
14. Shut off valve of Main/OLC Bucholtz/Surge relays are opened
15. Oil seal is ensured
for the Breather
16. Silica gel for the Breather is Blue in color
17. All Radiator
Top and Bottom
valves are opened
18. All Filter/
sampling/ drain valves
are properly closed
19. Main/OLTC Conservator Oil filling caps are tight.
20. Oil filling
in OTI and WTI pockets/
Calibration of WTI/OTI
21. No oil leaks any where
22. Explosion vent diaphragm top and bottom
are in place
23. Release air from top cover
24. Tank Double
point earth and connected to two different
earths
25. Earth Resistance of Earth pits
26. HV/ LV neutrals are earthed
27. Bi directional Rollers are locked
28. Alarm and indications and trip connections ensured after operating the concerned relays
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