In order to provide long and trouble free service, it is important that a careful and
regular supervision and maintenance of the transformer and its components is carried out.
The frequency and extent of such a supervision and maintenance is dependent on the experience, climatic conditions, environment, service conditions, loading pattern etc. All work done on transformers should be recorded in history register for future reference.
Efforts have been made to cover all important maintenance practices for transformers and
reactors in this chapter with details of interpretation of test results.
I.General Supervision
(a)Dirt and Dust
The external transformer surfaces shall be inspected regularly; and when required
cleaned of dust, insects and other airborne dirt. Transformers/ reactors installed near
polluting industry/cement plants, etc., need special care and more frequent cleaning of the
bushings and other components. All Marshalling Boxes and OLTC cubicle are to be kept properly closed so that there will not be entry of dust inside, which is difficult to clean.
(b)Rust and Treatment
A regular inspection is to be carried out of the external surface treatment of the
transformer tank and radiators. Possible rust damages are removed and the surface
treatment restored to original state by means of the primer and finish-paints of the
transformer to minimize the risk of corrosion and its subsequent spreading. These checks
also include looking for signs of oil leaks on gasket areas and welded areas containing
oil. The touch-up paint as and when required as per site condition and re-painting is
recommended once in five years. However transformers in coastal areas and more
corrosive atmosphere may require more frequent painting
(c) Check for any Signs of Mechanical Damage
Checks must be carried out for mechanical damage to the fabrications and
associated equipment. Particular attention should be given to vulnerable areas such as
radiators. If damage is seen on the equipment, a decision must be taken as to its
seriousness. It may be necessary to take corrective actions such as the replacement of an item of equipment.
(d) Check on all Joints for Signs of Leakage
All joints, both welded and gasketed, must be checked for signs of oil leakage. If
there is any doubt of a leak, the area must be cleaned of oil, using a suitable solvent
(methyl alcohol) and sprayed with liquid chalk. This will promote the flow of the leak
and give a good indication as to the exact location of the leak, if in fact there is one. If a
leak is suspected on a gasket, the joint must be tightened until such time that it can be
changed with a new gasket. If a leak is apparent at a welded joint once again clean the area and apply liquid chalk and allow to dry. This will highlight the point exactly if in
fact there is a leak. It must be properly repaired with welding procedures when
convenient. Prior to leaving the leak, it must be highlighted with a marker, or something
similar, so it is not lost when permanent repair takes place. Other areas commonly associated with oil leaks are drain plugs in radiators, valves in the oil management and cooling system and the gas and oil actuated relay.
e) Check for Oil Level
It is good practice to check all oil levels associated with the equipment. This will
incorporate the expansion vessel and all oil filled bushings. Also the oil in the oil seal
should be maintained. Some bushings in transformers will be below the conservator oil
level and some above. If there is leakage in bushing at the oil end, the level will be low or
high depending upon the level of conservator. External leak on bushing will lead to
indicate low oil level. This is to be observed accordingly and if there is leak, action is to
be initiated immediately as bushing failure may lead to failure of entire transformer
OLTC oil conservators are always kept at lower level compared to the main
conservator tank so that OLTC oil will not mix with main tank oil. An increase in level of
oil in OLTC conservator tank indicates internal leakage and action is to be taken
accordingly. After energizing of the transforn1er, a certain settling may appear in sealing joints. This applies especially to sealing joints with plain gaskets that are not placed in grooves. These should therefore be re-tightened. For correct torque for tightening the bolt, the manufacturer's recommendations are to be followed.
(f)Check on the Surrounding Areas
Once all the checks are completed, a check should be made to ensure that all
materials or tools, used for maintenance work, have been removed. All clothes and other
debris must be disposed off. The transformer compound should be left in a clean and tidy
condition.
II.Checks on Breathers
(a)Checks on Silica Gel Breather
In open breathing transformer, the breather plays active role in maintaining the
transformer dry by admitting dry air when transformer breathes. In transformers having
air cell or diaphragm, the breather ensures dry air inside the air cell or above the
diaphragm. The silica gel inside the breather should become pink from bottom to top over
a period of time. Any de-colorization at top or sides indicates leakage in container and
need to be attended immediately. In order to prevent severe deterioration of the silica gel,
it is recommended that it is replaced when half to two thirds of the silica gel has become
saturated and turned pink in colour. Failure to do so will severely retard the drying
efficiency of the breather. The silica gel can be reactivated by heating it to 130°C-140°C
in a ventilated oven until it has achieved the bright blue colour. Check that the oil level is
correct in the oil cup at the breather base and fill oil if the level is found low
Note: Do not exceed the temperature stated above otherwise the colour impregnation will be destroyed and the silica gel will turn black.
Immediately after re-activation the loose silica gel must be placed in a sealed
container to prevent absorption of moisture on cooling. The silica gel should be stored in
sealed condition until required for use.
Self indicating (blue) silica gel contains the dye cobalt chloride which has been
classified carcinogenic by an European Commission directive and ia a banned substance because of its potential health hazards. In Europe the silica gel breathers are to be disposed in 'Class l' disposal locations for hazardous waste products or incinerated.
An alternative to the blue self-indicating silica gel is SILICA GEL ORANGE with
an organic indicator. The colour changes from orange to light yellow as it absorbs
moisture. The specifications of silica gel orange are as shown below
(b)Drycol Breather Check (If Available)
Drycol breathers are provided in some transformers where air cell is not provided.
It condenses the moisture inside the conservator and brings it out as water droplets. Silica gel breather will also be provided for these transformers. The following checks need to be carried out for drycol breathers:
Operation of counter reading: Check on a regular basis that the counter is
functioning. Record the figures each time a check is made so that a progressive check is
recorded.
Defrost current condition indicates that water is still being ejected from the
breather
Press the test button and check that a defrost current is being indicated. Check that
the two red neon lights are ON and the amber neon light is OFF. Release the test button and check that the counter has advanced one count and that freeze current is indicated
IIIChecks for Conservator
(a)Visual Check for Conservator Oil Level
The transformer oil conservator is provided with an oil-level indicator graduated
from 0 to 1 or min to 6 or "low" to "full" with grading depending on the manufacturer.
Normally the face of oil gauge or dial of Magnetic oil level Gauge (MOG) is marked at
the 35°C (or normal). These indications are relative to temperature of the operating
equipment. The oil level indicated should be recorded along with top oil temperature.
If corrected oil level is normal, no additional action is required, whereas if it is above or below the normal level, it may be necessary to remove or add some oil. The correct oil-filling level is specified on an information plate that is placed on the transformer Rating plate panel. At an oil temperature of + 45°C, the conservator should be half filled. If the level exceeds the "full" oil must be drained off. If the value is "low"or "min", oil must be filled in.
(b)Leakage Test for Air Cell
Normally leakage test for air cell fitted inside the conservator is carried out before
installing the conservator in its position or at the time of major overhaul. During service,
the leakage in the cell or in the sealing of the conservator can be detected by the oil level
in the prismatic oil level indicator, if provided, on the conservator. If there is no leakage,
the prismatic oil level indicator will show "Full" oil level. However, in case of leakage,
the oil level in the prismatic oil level gauge shall be lower than "Full" level.
For Releasing Air from Conservator Fitted with Air Cell
Pressurize the Air Cell up to the maximum pressure as specified by the manufacturer and open the air vent valves provided on the top of the conservator until oil starts coming out. Then close the valves. Release pressure from the Air Cell and refit
breather.
For Releasing Air from Conservators Fitted with Diaphragm Type Air Sealing
Open the Air Release Valve provided on the top of the diaphragm and start filling
oil into the conservator, preferably from the valve provided at the bottom of the
conservator. Filling of oil from the oil filling valve at the bottom of the transformer tank
is avoided because it may result in entry of air into the transformer which may get
trapped in the winding and result in unnecessary accumulation of air in the Buchholz Relay at some later stage.
Continue filling oil into the conservator until it is full and oil starts coming out of
the Air Release Valve. Close the Air Release Valve after ensuring that all the air has
come out from the oil portion below the diaphragm.
Slowly drain the oil from the conservator until the oil level as indicated on the oil
level gauge corresponds to the transfom1er oil temperature.
Before making the leakage test of air cell for the, transformer in service, oil should
be drained out to the lower level of conservator. Apply pressure as specified by the manufacturers to inflate the air cell. Adjust the pressure after 6 hrs, if required. Check
temperature and maintain the air cell at almost the same temperature for 24 hrs. If there is
no loss of pressure during 24 hrs, it means the air cell is not having leak.
(c)Caution
Any heating process like welding, grinding etc. are not allowed on the assembled
conservator fitted with air cell diaphragm as it is highly sensitive to heat.
IV.Check for Cubicle and Marshalling Kiosk and Valves
Marshalling Cubicle and Kiosk Check
The following need to be checked and ensured while inspecting and checking the
*Marshalling Boxes.
*Condition of paint
*Operation of door handles, Hinges
*Condition of door seal.
*Door switches
*Lights and heaters
*Thermostats
*Operation of heating and lighting switches
*Secure mounting of equipment
*Checking of tightness of cable terminations
*Checking of operation of contactors
*HRC fuses and their rating
*Operation of local alarm annunciation by pushing push buttons provided for lamp
test, acknowledge, reset, system test, mute etc. to cover all system function.
*Source change over test check by putting off power sources alternatively.
*Check for plugs for dummy holes, glass windows and replacement, if found
missing/ broken
V.Checks for Auxiliaries
(a)Cooling System
The cooling surfaces of radiators shall be inspected regularly and when required
cleaned of dust, insects, leaves or other air borne dirt. The cleaning is suitably carried out
by means of water flushing at high pressure. Precaution should be taken to cover the fanmotor so that water may not go inside. Alternatively cleaning can be done with cleaning solution and cloth.
The fan-motors are provided with permanent - lubricated bearings and double sealing rings. The motor bearings are axially clamped with spring-washers. If the sound level of the fan increases, first tighten all mounting supports and in case any abnormal sound is noticed in fan motor, then action should be taken for repair! replacement.
(b)Cooling System-Fans-Controls
Fan controls are designed to operate both manually and automatically with set
temperature. Manual, Control is to be turned 'ON' to operate cooling system for checking.
Oil pumps need to be checked by observing their flow gauges. Measurement of pump
current reveals any abnormality. Any significant imbalance of current between the
terminals greater than 15-20% is indicative of the problem with the pump motor.
Checking for correct rotation of fans and pumps to be ensured as reverse rotation may not provide desired result.
(c)Calibration of OTI / WTI
Temperature indicators in transformers are not only used for indication purpose
they are used as protective device also. The accuracy of these devises is to be ensured for
correct operation of alarm and tripping and also to prevent mal operation. The
temperature bulb is to be removed from its well on the side/ top of transformer. Using a
temperature controlled calibration instrument in oil bath the temperature of the bulb
should be slowly raised in steps of 5°C and observed for temperature reading. If the
temperature deviation is more than ± 5°C compared to the standard thermometer reading,the thermometers are to be replaced with healthy one.
(d)Checking of Cooler Control, Alarm and Trip Settings
Setting of temperature should be as per approved scheme. Access the local
winding/ oil temperature indicator and rotate the temperature indicator pointer slowly to
the first stage cooling value (say 65°C). Check that the fans of those coolers set to first
stage are operating. Continue rotating the pointer to the second stage cooling value (say
8O°C). Check that the fans of those coolers set to second stage are operating. Continue
rotating the pointer to the alarm value (say 110°C). Check with the control room to
ensure that the alarm signal has been received. Continue rotating the pointer to the trip value (say 125°C). Check with the control room to ensure that the trip signal has been
received.
(e)Gas Pressure Relay
There are two types of gas pressure relays. The most common type is mounted at
the transformer top body. Internal arcing in liquid filled electrical power equipment
generates excessive gas pressures that can severely damage equipment and present
extreme hazards to personnel. The gas pressure relay is intended to minimize the extent of damage by quickly operating and venting out the pressure. It will reset when the
pressure becomes normal. A pointer is provided to indicate the operation of this relay and the relay is connected for tripping the transformer on operation. There will be oil spillage whenever the relay operates. Smaller transformers are provided with explosion vent
where the diaphragm will rupture due to heavy internal pressure and releases the
pressure. The diaphragm needs to be replaced when it operates. There are some
transformers fitted with sensitive sudden pressure relay, which operates on rate of change of differential pressure and trips the equipment.
(f)Buchholz Relays
The use of gas-operated relay as protection for oil-immersed transformers is based
on the fact that faults as flashover, short-circuit and local overheating normally result in
gas-generation. The gas-bubbles gathering in the gas-operated relay affect a floatcontrolled contact that gives an alarm signal.
For testing of the contact functions, buchholz relays are provided with a test knob
on the cover. Unscrew the protective cap and press down the knob by hand. The spring
loaded knob with a pin inside the relay actuates first the alarm device and then the
tripping device. After testing, screw on the protective cap again.
Checking the operation of Buchholz relay in case of low oil level is carried out by
closing step valve in both sides of the relay and draining of oil through oil drain valve
provided in Buchholz relay. First alarm and then trip contact should operate to indicate
healthiness.
To check the relay for oil surge, manufacturers recommendations for particular
relays to be followed
(g) Bushings
Bushings are most failure prone in any transformer/ reactor. Failure of bushings
could lead to the fire in transformer and total damage. For uniform voltage distribution
across capacitance graded bushings, bushing porcelains shall be cleaned from dust and
dirt during shutdown maintenance. In areas where the air contains impurities as salt,
cement dust, smoke or chemical substances, shorter intervals are required.
VI.Operational Checks and Inspection / Maintenance of Tap Changer
EHV Transformers are provided with tap changer to have voltage control. To
enable operation of taps during service, On- Load Tap Changers (OLTC) are provided in
EHV transformers. OLTCs may be located in either the high voltage winding or the low
voltage winding, depending on the requirements of the user, the cost effectiveness of the application and tap changer availability. OLTC being a current interrupting device requires periodic inspection and maintenance. The frequency of inspection is based on time in service, range of use and number of operations.
(a)Precautions
This testing shall be carried out during shutdown period and all testing shall be
done under total de-energisation condition. Ensure the isolation of transformer for high
voltage and low voltage side with physical inspection of open condition of the concerned
isolators/ disconnectors. In case tertiary is also connected, ensure the isolation of the
same prior to commencement of testing
(b)Tap Changer Hand Operation
Check hand operation of the tap changer up and down the full range before
electrical operation is attempted and that the handle interlock switch will not allow
electrical operation while the handle is inserted. In addition where single phase tap
changers are employed check their tap positions agree and are reached simultaneously at motor drive unit head. Continuity check should be done for any discontinuity during tap changing operation by connecting an analogue multi meter across HV and IV bushing in case of auto transformers and relevant winding in case of two winding transformers and change the tap positions from maximum to minimum.
(c)Maintaining Circuit
Check the maintaining circuit for correct sequence by hand winding unit half way
through a tap and then remove the handle. Energize the drive motor and ensure that the
motor continues to drive the tap changer in the same direction
(d)Drive Motor
With the tap changer in mid position check the direction of rotation and measure
the start and running currents in both the raise and lower mode of operation and record
their values.Set the motor overload to 10% above running current
(e)Out of Step Relay
Move one tap changer is in the three-phase bank to be one position out of step
with other two. Check the tap changer faulty alarm is activated. Repeat for other two
phases.Hold the raise and lower push buttons in following a tap change to ensure it only
moves one tap at a time hence checking the step by step relay.
(f)Tap Change Incomplete Alarm
Check the operation of the tap changer incomplete alarm, including the flag relay,
by winding the unit by hand half way through a tap change and monitoring their correct
operation and time to operate.
(g)Remote Indication
Check the remote indication and control facility is proved to the outgoing
terminals of the marshalling kiosk.
(h)Tap Changer (Surge) Protective Relay
Check the tripping function of the relay. Open the cover and press button "Trip". Check
that all circuit breakers of transformer operate properly. Press push Button "Reset" close
the cover and tighten it.
(i)Inspection and Maintenance of OLTCs
Normally the temperature of the OLTC compartment may be few degrees Celsius
less than the main tank. Any temperature approaching or above that of the main tank
indicates an internal problem. Prior to opening the OLTC compartment, it should be
inspected for external symptoms of potential problems. Such things as integrity of paint,
weld leaks, oil seal integrity, pressure relief device and liquid level gauge are all items
which should be inspected prior to entering the OLTC.
Following de-energisation, close all valves between oil conservator, transformer
tank and tap-changer head, then lower the oil level in the diverter switch oil compartment
by draining of oil for internal inspection. Upon opening the OLTC compartment, the door
gasket should be inspected for signs of deterioration.
The compartment floor should be inspected for debris that might indicate abnormal wear and sliding surfaces should be
inspected for signs of excessive wear.
Finally, the tap selector compartment should be flushed with clean transformer oil
and all carbonization, which may have been deposited, should be removed. Min BDV
should be 50 kV and moisture content should be less than 20 PPM
Dissolved Gas Analysis (DGA)
Transformer undergoes electrical, chemical and thermal stresses during its service
life which may result in slow evolving incipient faults inside the transformer. The gases
generated under abnormal electrical or thermal stresses are hydrogen (H2),
methane(CH4), ethane(C2H6), ethylene (C2H4), acetylene (C2H2), carbon_monoxide
(CO), carbon dioxide (CO2), nitrogen (N2) and oxygen (02) which get dissolved in oil.
Collectively these gases are known as FAULT GASES, which are routinely detected and
quantified at extremely low level, typically in parts per million (ppm) in dissolved Gas
Analysis (DGA). Most commonly method used to determine the content of these gases in oil is using a vacuum Gas Extraction apparatus/ Head Space Sampler and gas
chromatograph.
DGA is a powerful diagnostic technique for detection of slow evolving faults
inside the transformer by analyzing the gases generated during the fault which gets
dissolved in the oil. For Dissolved Gas Analysis to be both useful and reliable, it is
essential that sample taken for DGA should be representative of lot, no dissolved gas be
lost during transportation and laboratory analysis be precise and accurate. Effective fault gas interpretation should basically tell us first of all, whether there is any incipient fault
present in the transformer. If there is any problem, what kind of fault it is. Whether the
fault is serious and the equipment needs to be taken out of service for further
investigation.
DGA can identify deterioration of insulation oil and hot spots, partial discharge,
and arcing. The health of oil is reflective of the health of the transformer itself. DGA
analysis helps the user to identify the reason for gas formation and materials involved and
indicate urgency of corrective action to be taken
The evolution of individual gas concentrations and total dissolved combustible gas
(TDCG) generation over time and the rate of change (based on IEC 60599 and IEEE
C 57-104 standards) are the key indicators of a developing problem. Some of the
recognized interpretation techniques are discussed below:
Individual Fault Gases Acceptable Limits
When no previous DGA history of Transformer is available, to ensure that a transformer
is healthy or not, the DGA results are compared with the gassing characteristics exhibited by the majority of similar transformer or normal population. As the transformer ages and
gases are generated, the normal levels for 90% of a typical transformer population can
determined. From these values and based on experience, acceptable limits or threshold
levels have been determined as given in table (as per IEC 60599) below:-
possible between the OLTC compartment and the main tank or between the respective
conservators. These gases may contaminate the oil in the main tank and affect the normal
values in these types of equipment. “NO OLTC” refers to transformers not equipped with
an OLTC, or equipped with an OLTC but not communicating with or leaking to the main
tank.
However it is improper to apply threshold level concept without considering the
rate of change of the gas concentration in Dissolved Gas Analysis. When an abnormal
situation is indicated by above table, a testing schedule is devised with increased
sampling frequency
It is to be understood that there is no definite interpretation method available, which can indicate the exact location and type of the! fault. The different interpretation methods only provide guidelines to make expert interpretation about the equipment. Apart from the DGA results various other factors are taken into consideration such as past history of the transformer, grid condition, loading patterns, voltage and frequency profile, etc.
It is to be understood that there is no definite interpretation method available, which can indicate the exact location and type of the! fault. The different interpretation methods only provide guidelines to make expert interpretation about the equipment. Apart from the DGA results various other factors are taken into consideration such as past history of the transformer, grid condition, loading patterns, voltage and frequency profile, etc.