2. STANDARD PROTECTIONS USED FOR TRANSFORMERS
• Differential
• Over current
• Earth fault
• Restricted earth fault
• Over fluxing
• Mechanical protection
buchholtz
pressure relief device
• Temperature protection
WTI/OTI
3. • Differential Protection
Current differential relaying can be used to protect network transformers. The relays are connected to current
transformers on the high side and low side of the network transformer. The net operating current to the relays is
the difference between input and output currents to the network transformer zone of protection. Differential
relaying provides a clearly defined zone of protection.
• Biased type with adjustable bias setting of 10-50%
• Triple pole
• Harmonic restraint feature
• No of bias wdgs as applicable
• Operating current setting of 15% or less
• Over current protection
– Given on LV side as primary protection
– On HV side as back up protection
– IDMT or def time as applicable
– Need to be co-coordinated ( for time and current)with down stream protection
Directional if the power flow is bi-directional
4. • Back up earth fault protection
– Single pole
– Def time /IDMT As applicable
– Time delay 0.3 – 3 sec for def time relay
– Standard curves for IDMT
– Setting ranges to suit the application
• Over fluxing
– Operate on V/F principle
– Inverse characteristics to suit the transformer o/f withstand capability
– Shall have alarm and trip stages
Restricted earth fault Protection
Single pole
Generally high impedance type preferred
Setting rage of 5-20% 0r 10 – 40 %
Stable for through faults
Include stabilizing resistor for through fault
stability
7. How does a transformer work?
• A transformer is an electrical device used to convert AC power
at a certain voltage level to AC power at a different voltage, but
at the same frequency.
• The construction of a transformer includes a ferromagnetic core
around which multiple coils, or windings, of wire are wrapped.
The input line connects to the 'primary' coil, while the output
lines connect to 'secondary' coils. The alternating current in the
primary coil induces an alternating magnetic flux that 'flows'
around the ferromagnetic core, changing direction during each
electrical cycle. The alternating flux in the core in turn induces
an alternating current in each of the secondary coils. The
voltage at each of the secondary coils is directly related to the
primary voltage by the turns ratio, or the number of turns in the
primary coil divided by the number turns in the secondary coil.
8. Transformer Core
• Construction in which the iron circuit is surrounded by
windings and forms a low reluctance path for the
magnetic flux set up by the voltage impressed on the
primary.
10. • The core of shell type is shown, in which The winding is
surrounded by the iron Circuit Consisting of two or more paths
through which the flux divides. This arrangement affords somewhat
Better protection to coils under short circuit conditions.
• In actual construction there are Variations from This simple
construction but these can be designed
• With such proportions as to give similar electrical characteristics.
11.
12.
13. • For core material, high-grade, grain oriented silicon steel strip is
used. Connected by a core leg tie plate fore and hind clamps by
connecting bars. As a result, the core is so constructed that the
actual silicon strip is held in a sturdy frame consisting of clamps
and tie plates, which resists both mechanical force during
hoisting the core-and-coil assembly and short circuits, keeping
the silicon steel strip protected from such force.
• In large-capacity Transformers, which are likely to invite
increased leakage flux, nonmagnetic steel is used or slits are
provided in steel members to reduce the width for preventing
stray loss from increasing on metal parts used to clamp the core
and for preventing local overheat. The core interior is provided
with many cooling oil ducts parallel to the lamination to which a
part of the oil flow forced by an oil pump is introduced to achieve
forced cooling.
14. Tank.
• The tank has two main parts:
• a –The tank is manufactured by forming and welding steel plate to
be used as a container for holding the core and coil assembly
together with insulating oil.
• The base and the shroud, over which a cover is sometimes bolted.
These parts are manufactured in steel plates assembled together
via weld beads. The tank is provided internally with devices usually
made of wood for fixing the magnetic circuit and the windings. I
• In addition, the tank is designed to withstand a total vacuum during
the treatment process. Sealing between the base and shroud is
provided by weld beads. The other openings are sealed with oil-
resistant synthetic rubber joints, whose compression is limited by
steel stops.
• Finally the tank is designed to withstand the application of the
internal overpressure specified, without permanent deformation.
15. Conservator
• The tank is equipped with an expansion reservoir
(conservator) which allows for the expansion of the oil
during operation. The conservator is designed to hold a
total vacuum and may be equipped with a rubber
membrane preventing direct contact between the oil and
the air.
16.
17. • Various parts of the tank are provided with the following arrangements
for handling the Transformer.
• Four locations (under the base) intended to accommodate
bidirectional roller boxes for displacement on rails.
• Four pull rings (on two sides of the base)
• Four jacking pads (under the base)
• Tank Earthing terminals:
• The tank is provided with Earthing terminals for Earthing the various
metal parts of the Transformer at one point. The magnetic circuit is
earthed via a special external terminal.
18. Valves
• The Transformers are provided with sealed valves, sealing joints,
locking devices and position indicators.
• The Transformers usually include:
- Two isolating valves for the "Buchholz" relay.
- One drainage and filtering valve located below the tank.
- One isolating valve per radiator or per cooler.
- One conservator drainage and filtering valve.
• And when there is an on-load adjuster:
- Two isolating valves for the protection relay.
- One refilling valve for the on-load tap-changer.
- One drain plug for the tap-changer compartment.
19. Connection Systems
• Mostly Transformers have top-mounted HV and LV bushings according to DIN or IEC in their
standard version. Besides the open bushing arrangement for direct Connection of bare or insulated
wires, three basic insulated termination systems is available. Fully enclosed terminal box for cables
• Fig. (17&18) Available for either HV or LV side, or for both. Horizontally split design in degree of
protection IP 44 or IP 54. (Totally enclosed and fully protected against contact's With live parts, plus
protection against drip, splash, or spray water.)
• Cable installation through split cable glands and removable plates facing diagonally downwards.
• Optional conduit hubs suitable for single-core or three-phase cables with solid dielectric insulation,
with or without stress cones.
• Multiple cables per phase are terminated on auxiliary bus structures attached to the bushings
removal of Transformer by simply bending back the cables.
20. 7 - The dehydrating breather
• The dehydrating breather is provided at the entrance of the conservator of oil immersed
equipment such as Transformers and reactors.
• The conservator governs the breathing action of the oil system on forming to the temperature
change of the equipment, and the dehydrating breather removes the moisture and dust in the air
inhaled and prevents the deterioration of the Transformer oil due to moisture absorption.
• Construction and Operation
• See Fig. (20) The dehydrating breather uses silica - gel as the desiccating
• Agent and is provided with an oil pot at the bottom to filtrate the inhaled air. The specifications of
the dehydrating breather are shown in Table (1) and the operation of the component parts in
Table (2).
21.
22. • 1. Case
• 2. Peep window
• 3. Flange
• 4. Oil pot
• 5. Oil pot holder
• 6. Breathing pipe
• 7.Filter
• 8. silica-gel
• 9.Absorbent
• 10. Oil (Transformer oil)
• 11. Wing nut
• 12.Cover
• 13. Suppression screw
• 14. Set screw
• 15. Oil level line
23. Item Action
Silica -gel Removes moisture in the air inhaled by the Transformer.
Blue silica -gel In addition to the removal of moisture, indicates the Extent of moisture
absorption by discoloration.
(Dry condition) (Wet condition )
Blue ------ Light purple ----- Light pink
Oil pot Oil and
filter
Removes moisture and dust in the air inhaled by: the Transformer or
reactor. In addition, while it is not performing breathing action, it
seals the desiccating agent from the outer air to prevent unnecessary
moisture Absorption of the desiccating agent.
absorbent Absorbs dust and deteriorated matter in the oil pot, to Maintain the oil
pot in a good operating condition.
24. Bushing
• Having manufactured various types of bushings ranging from 6kV-class to 800kV-
class, Toshiba has accumulated many years of splendid actual results in their
operation.
• Plain-type Bushing
• Applicable to 24 kV-classes or below, this type of bushing is available in a standard
series up to 25,000A rated current. Consisting of a single porcelain tube through
which passes a central conductor, this bushing is of simplified construction and small
mounting dimensions; especially, this type proves to be advantageous when used as
an opening of equipment to be placed in a bus duct
25. • This bushing, of enclosed construction, offers the Following
features:
• • High reliability and easy maintenance.
• • Partial discharge free at test voltage.
• • Provided with test tapping for measuring electrostatic capacity and
tan δ.
• • Provided with voltage tapping for connecting an instrument
Transformer if required.
26. Buchholz Relays
• The following protective devices are used so that, upon a fault development inside a Transformer, an
alarm is set off or the Transformer is disconnected from the circuit. In the event of a fault, oil or
insulations decomposes by heat, producing gas or developing an impulse oil flow.
• To detect these phenomena, a Buchholz relay is installed.
• The Buchholz relay is installed at the middle of the connection pipe between the Transformer tank
and the conservator.
• There are a 1st stage contact and a 2nd stage contact as shown in Fig. (28). the 1st stage contact is
used to detect minor faults.
• When gas produced in the tank due to a minor fault surfaces to accumulate in the relay chamber
within a certain amount (0.3Q-0.35Q) or above, the float lowers and closes the contact, thereby
actuating the alarm device.
• The 2nd stage contact is used to detect major faults. In the event of a major fault, abrupt gas
production causes pressure in the tank to flow oil into the conservator. In this case, the float is
lowered to close the contact, thereby causing the Circuit Breaker to trip or actuating the alarm device.
27.
28. Temperature Measuring Device
• Liquid Temperature Indicator (like BM SERIES Type) is used to measure oil temperature
as a standard practice.
• With its temperature detector installed on the tank cover and with its indicating part
installed at any position easy to observe on the front of the Transformer, the dial
temperature detector is used to measure maximum oil temperature.
• The indicating part, provided with an alarm contact and a maximum temperature pointer,
is of airtight construction with moisture absorbent contained therein; thus, there is no
possibility of the glass interior collecting moisture whereby it would be difficult to observe
the indicator Fig. (30&31). Further, during remote measurement and recording of the oil
temperatures, on request a search coil can be installed which is fine copper wire wound
on a bobbin used to measure temperature through changes in its resistance.
29. Winding Temperature Indicator
• The winding temperature indicator relay is a
conventional oil temperature indicator
supplemented with an electrical heating element.
• The relay measures the temperature of the hottest
part of the Transformer winding. If specified, the
relay can be fitted with a precision potentiometer
with the same characteristics as the search coil for
remote indication.
30. • The temperature sensing system is filled with a liquid, which changes in volume with varying
temperature. The sensing bulb placed in a thermometer well in the Transformer tank cover senses
the maximum oil temperature. The heating elements with a matching resistance is fed with current
from the Transformer associated with the loaded winding of the Transformer and compensate the
indicator so that a temperature increase of the heating element is thereby proportional to a
temperature increase of the winding-over-the maximum- oil temperature.
• Therefore, the measuring bellows react to both the temperature increase of the winding-over-the-
maximum-oil temperature and maximum oil temperature. In this way the instrument indicates the
temperature in the hottest part of the Transformer winding.
• The matching resistance of the heating element is preset at the factory.
31. Pressure Relief Device
• When the gauge pressure in the tank reaches
abnormally
• To 0.35-0.7 kg/cm.sq. The pressure relief
device starts automatically to discharge the oil.
• When the pressure in the tank has dropped
beyond the limit through discharging, the device
is automatically reset to prevent more oil than
required from being discharged.
32. COOLING SYSTEM
• The kinds of cooling medium and their symbols adopted
• by I.S. 2026 (Part 11)-1977 are:
• (a) Mineral oil or equivalent flammable insulating liquid O
• (b) Non flammable synthetic insulating liquid L
• (c) Gas G
• (d) Water W
• (e) Air A
• The kids of circulation for the cooling medium
• and their symbols are:
• (a) Natural N
• (b) Forced (Oil not directed) F
• (c) Forced (Oil directed) D
• Each cooling method of Transformer is identified by four symbols.
• The first letter represents the kind of cooling medium in contact
• with winding, the second letter represents the kind of circulation for
• the cooling medium, the third letter represents the cooling medium
• that is in contact with the external cooling system and fourth symbol represents the
• kind of circulation for the external medium.
• Thus oil immersed Transformer with natural oil circulation and forced air external cooling is designated ONAF.
33. • For oil immersed Transformers the cooling
systems normally adopted are:
• 1- Oil Immersed Natural cooled – Type ONAN.
• In this case the core and winding assembly is
immersed in oil. Cooling is obtained by the
circulation of oil under natural thermal head only.
• In large Transformers the surface area of the
tank alone is not adequate for dissipation of the
heat produced by the losses.
• Additional surface is obtained with the provision
of radiators.
• 2. Oil Immersed Air Blast - Type ONAF
• In this case circulation of air is obtained by fans.
It becomes possible to reduce the size of the
Transformer for the same rating and
consequently save in cost.
34. • 4. Forced Oil Air Blast Cooled - Type OFAF
• In this system of cooling also circulation of oil is
forced by a pump. In addition fans are added to
radiators for forced blast of air.
• 5. Forced Oil Natural Air Cooled - Type OFAN
• In this method of cooling, pump is employed in
the oil circuit for better circulation of oil.
35. • 6. Forced Oil Water Cooled - Type OFWF
• In this type of cooling a pump is added in the oil circuit for forced circulation of oil, through a separate
heat exchanger in which water is allowed to flow.
• 7. Forced Directed Oil and Forced Air Cooling -ODAF.
• It should be remembered that Transformers cooling type OFAF and OFWF will not carry any load if
air and water supply respectively is removed. It is quite common to select Transformers with two
systems of Cooling e.g., ONAN/ONAF or ONAN/OFAF or sometimes three systems e.g.,
ONAN/ONAF/ OFAF.
• These determine the type of cooling upto certain loading.
• As soon as the load exceeds a preset value, the fans/pumps are Switched on. The rating of a
Transformer with ONAN/ONAF cooling may be written, say, as 45/60 MVA. This means that so long
as the load is below 45 MVA, the fans will not be working.
• These are Switched on automatically when the load on the Transformer exceeds 45 MVA. Type of
cooling has a bearing on the cost of the Transformer.
• It shall be appreciated that the ONAN cooling has the advantage of being the simplest with no fans or
pumps and hence no auxiliary motors.
36. INSULATING OIL
• In Transformers, the insulating oil provides an insulation medium as well as a heat transferring medium that carries away heat produced in the
windings and iron core. Since the electric strength and the life of a Transformer depend chiefly upon the quality of the insulating oil, it is very
important to use a high quality insulating oil.
• The insulating oil used for Transformers should generally meet the following requirements:
• (a) Provide a high electric strength.
• (b) Permit good transfer of heat.
• (c) Have low specific gravity-In oil of low specific gravity particles which have become suspended in the 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.
• (d) Have a low viscosity- Oil with low viscosity, i.e., having greater fluidity, will cool Transformers at a much better rate.
• (e) Have low pour point- Oil with low pour point will cease to flow only at low temperatures.
• (f) Have a high flash point. The flash point characterizes its tendency to evaporate. The lower the flash point
• the greater the oil will tend to vaporize. When oil vaporizes, it loses in volume, its viscosity rises, and an explosive mixture may be
formed with the air above the oil.
• (g) Not attack insulating materials and structural materials.
• (h) Have chemical stability to ensure life long service. Various national and international specifications have been issued on insulating oils
for Transformers to meet the above requirements.
37. • The specifications for insulating oil stipulated in Indian Standard 335: 1983 are given
below.
35 X
Ω / cm
1500 X
Characteristic Requirement
1 Appearance The oil shall be clear and transparent and
free from suspended matter or sediments.
2 Density at 29.5°C, Max 0.89 g/cm3
3 Interfacial tension at 270°C, Min. 0.04 N/m.
4 Flash point Min. 104 °C
5 Pour Point Max. - 9 °C
6 Corrosive Sulphur (in terms
of classification of copper strip).
Non-corrosive.
7 Electric strength (breakdown voltage) Min.
(a) New unfiltered oil
(b) After filtration
30 kV (rms)
60 kV (rms).
8 Dielectric dissipation factor
(tan δ) at 90 °C Max.
0.002
9 Specific resistance (resistivity):
(a) At 9 0 °C Min.
(b) at 2 7 0 °C Min.
Ω / cm
10 Oxidation stability.
(a) Neutralization value, after oxidation Max.
(b) Total sludge, after oxidation, Max.
0.4 mg KOH/g
0.10 percent by weight
11 Presence of oxidation inhibitor The oil shall not contain antioxidant
38. Gases analysis
• The analysis of gases dissolved in oil has proved to be a highly practical method for the
field monitoring of power Transformers.
• This method is very sensitive and gives an early warning of incipient faults. It is indeed
possible to determine from an oil sample of about one litre the presence of certain gases
down to a quantity of a few mm3 , i.e., a gas volume corresponding to about 1 millionth of
the volume of the liquid (ppm).
• The gases (with the exception of N2 and O2) dissolved in the oil are derived from the
degradation of oil and cellulose molecules that takes place under the influence of thermal
and electrical stresses. Different stress modes, e.g., normal operating temperatures, hot
spots with different high temperatures, partial discharges and flashovers, produce
different compositions of the gases dissolved in the oil.
39. • The relative distribution of the gases is therefore used to evaluate the origin of the gas production
and the rate at which the gases are formed to assess the intensity and propagation of the gassing.
Both these kinds of information together provide the necessary basis for the evaluation of any fault
and the necessary remedial action.
• This method of monitoring power Transformers has been studied intensively and work is going on in
international and national organizations such as CIGRE, IEC and IEEE.
• APPLICATION.
• The frequency with which oil samples are taken depends primarily on the size of the Transformer
and the impact of any Transformer failure on the network.
• Some typical cases where gas analysis is particularly desirable are listed in the following:
• 1 - When a defect is suspected (e.g., abnormal noise).
• 2 - When a Buchholz (gas-collecting) relay or pressure monitor gives a signal.
• 3 - Directly after and within a few weeks after a heavy short circuit
• 4 - In connection with the commissioning of Transformers that are of significant importance to the
network, followed by a further test some months later.
40. • Different routines for sampling intervals have been developed by different utilities and in different
countries.
• One sampling per year appears to be customary for large power Transformers (Rated >= 300 MVA
>= 220 kV).
• The routine that has been used over a long period of time of checking the state of the oil every other
year by measuring the breakdown strength, the tan value, the neutralization coefficient and other
physical quantities is not replaced by the gas analysis.
• Extraction and analysis
• To be able to carry out a gas analysis, the gases dissolved in the oil must be extracted and
accumulated.
• The oil sample to be degassed is sucked into a pre-evacuated degassing column. A low pressure is
maintained by a vacuum pump. To assure effective degassing (> 99 per cent), the oil is allowed to
run slowly over a series of rings which enlarge its surfaces.
• An oil pump provides the necessary circulation. The gas extracted by the vacuum pump is
accumulated in a vessel.
• Any water that may have been present in the oil is removed by freezing in a cooling trap to ensure
that the water will not disturb the vacuum pumping.
• The volumes of the gas and the oil sample are determined to permit calculation of the total gas
content in the oil. The accumulated gas is injected by means of a syringe into the gas
chromatograph, which analyses the gas sample.
41. • The result is plotted on a recorder in the form of a chromatogram.
• Using calibration gases it is possible to identify the different peaks on a chromatogram.
Recalculation of the height of a peak to the content of this gas is done by comparison
with chromatogram deflections from calibration gases.
• With the composition of the gas mixture and the total gas content in the oil sample
known; the content (in ppm) of the individual gases in the oil is obtained. The following
gases are analyzed:
• 1 - CARBON MONOXIDE CO
• 2 - CARBON DIOXIDE CO2
• 3 - HYDROGEN H2
• 4 - ETHANE C2H6
• 5 - ETHENE C2H4
• 6 - ACETYLENE C2H2
• 7 - METHANE CH4
• 8 - PROPANE C3H6
42. Type Of Gas Caused By
CARBON MONOXIDE, CO
CARBON DIOXIDE, CO2
AGEING
HYDROGEN, H2
ACETYLENE, C2H2
ELECTRIC ARCS
ETHANE, C2H6
ETHENE, C2H4
PROPANE, C3H6
LOCAL OVERHEATING
HYDROGEN, H2
METHANE, CH4 CORONA
44. PARALLEL OPERATION OF THREE-PHASE
TRANSFORMERS
• Ideal parallel operation between Transformers occurs when (1) there are no circulating currents
on open circuit, and (2) the load division between the Transformers is proportional to their kVA
ratings. These requirements necessitate that any - two or more three phase Transformers, which
are desired to be operated in parallel, should possess:
• 1) The same no load ratio of transformation;
• 2) The same percentage impedance;
• 3) The same resistance to reactance ratio;
• 4) The same polarity;
• 5) The same phase rotation;
• 6) The same inherent phase-angle displacement between primary and secondary terminals.
• 7) The same power ratio between the corresponding windings.
45. Connections of Phase Windings
• The star, delta or zigzag connection of a set of windings of a three phase Transformer or of
windings of the same voltage of single phase Transformers, forming a three phase bank are
indicated by letters Y, D or Z for the high voltage winding and y, d or z for the intermediate and
low voltage windings. If the neutral point of a star or zigzag connected winding is brought out, the
indications are Y N or Z N and y n and z n respectively.
Phase Displacement between Windings
• The vector for the high voltage winding is taken as the reference vector. Displacement of the
vectors of other windings from the reference vector, with anticlockwise rotation, is represented by
the use of clock hour figure.
• Some of the commonly used connections with phase displacement of 0, +30°, -180° and -330°
(clock-hour setting 0, 1, 6 and 11) are shown in below figure.
46.
47. Tap Changer
• The method to change the ratio of Transformers by means of taps on the winding is
as old as the Transformer itself. From a very early stage, Transformers with a turn
ratio changeable within certain limits have been used for electrical power
transmission, since this is the simplest method to control the voltage level as well as
the reactive and active power in electrical networks.
• Switching devices were needed which permitted the change of the turn ratio of
Transformers under load condition, i.e. Without interrupting the load current such
Switching devices - today called "on-load taps changers" (OLTC) – were introduced
to Transformers more than 70 years ago.
48. HIGH-SPEED RESISTOR TYPE OLTC
• The high-speed resistor type OLTC is designed either as a tap selector and
a diverter Switch, or as a selector Switch combining the functions of the tap
selector and diverter Switch into one device.
• The selector Switch principle is represented in Fig. The OLTC comprising a
tap selector and a diverter Switch lends itself for any application up to the
highest Transformer rating. Line-end applications with highest voltages for
equipment of 362 kV and rated through-currents of 4500 A have been
realized. Figure (4) shows an OLTC comprising a tap selector and diverter
Switch. With the tap selector-diverter Switch concept the tap-change is
affected in two steps. The tap adjacent to the one in service is pre-selected
load free by the tap selector.
Principle scheme of a
selector Switch type
OLTC
Principle scheme of a-tap selector
and diverter Switch type OLTC
49. • Switching sequence for tap-changer on
Switching from position 6 to position 5.
• a) Position 6. Selector contact V lies on tap 6
and selector contact H on tap 7. The main
contact x carries the load current.
• b) Selector contact H has moved in the no-
current state from tap 7 to tap 5.
c) The main contact X has opened. The load current passes through thec) The main contact X has opened. The load current passes through the
resistor Ry and the resistor contact y.resistor Ry and the resistor contact y.
d) The resistor contact u has closed. The load current is shared betweend) The resistor contact u has closed. The load current is shared between
Ry and Ru The circulating current is limited by the resistance of Ry + Ru.Ry and Ru The circulating current is limited by the resistance of Ry + Ru.
e) The resistor contact y has opened. The load current passes throughe) The resistor contact y has opened. The load current passes through
Ru and contact uRu and contact u
f) The main contact V has closed, resistor Ru. Has been short-circuitedf) The main contact V has closed, resistor Ru. Has been short-circuited
and the load current passes through the main contact V. The tap-and the load current passes through the main contact V. The tap-
changer is now in position 5.changer is now in position 5.
50.
51. • 132/11.5kV 30 MVA Operational Tests and Measurement of Audible Noise Level
• The following checks will be carried out.
• 1. Winding Insulation Level
• 2. Ratio Test
• 3. Vector Group Test
• 4. Cooling System Control Sequence Test
• 5. Local/Remote Tap Change Operations (Mech. and Elec.)
• 6. Operation of Protective Devices
• Load drop compensation and winding temperature CTs mounted in the trans-former should be proved to be in the correct phase and have the correct ratio
where practicable.
• Buchholz relays: the alarm and trip initiation shall be proved by means of the test button, if provided, or by shorting the appropriate terminals at the relay.
• Winding temperature tripping and alarms shall be proved by operating the appropriate initiating Switches. The cooler control ON and OFF shall be proved
when three phase 'A.C.' supplies are available by operating the ON/OFF Switch and the appropriate temperature indicating initiating Switches. The direction
of the fan rotation must be checked in accor¬dance with the mark.
• Tap-changer position indicator should be proved to indicate the correct tap position. The limit Switch must function properly to prevent the tap-changer from
further movement beyond the two extreme tap positions. Tap-changing shall be tested for every step ensuring stepping relay functions correctly.
• Measurement of audible noise level on site will be carried out under the following conditions using a sound level meter (IEC Pub 551 type 1 or equivalent):
• 1. The background noise level at all measuring points shall not exceed 45dBA in accordance with ANSI standard.
• 2. Only the Transformer under test shall be energized and shall have been on soak for at least 24 hours prior to measurement. The tests shall be carried out
at rated voltage with all normal fans running at no load conditions.
• 3. The audible sound level of each Transformer in turn will be measured at a number of points 30 meters from the substation.
• 4. The average value of the noise measurements for each Transformer shall be taken and this value checked to ensure it does not exceed 50dBA.
52.
53. • 1. Check the Transformer oil level.
• 2. Check all valves of the Transformer are in the service position.
• 3. Check the cooling fans operation.
• 4. Check the automatic tap changer operation.
• 5. Check for the Transformer protection functions,
• 6. Check that the service settings are adopted for oil temperature & winding
temperature instruments.
• 7. General inspection of all Transformers parts to ensure its healthy
condition.
