relays must function properly when called upon to do so and this can only be assured by
regular inspection and testing.
2. Obtain a sample of any gas which may be present in the gas chamber of the Gas Detector and test it for inflammable gasses. The presence of an inflammable gas definitely establishes the fact that trouble has occurred or is developing inside the transformer.
Oil Level Gauges:
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 MOG is marked at the 35oC (of normal) point, High & Low. These letter indications are relative and have no specific relationship to any real value. The oil level, indicated by MOG on the side of conservator, should be recorded along with top oil temperature. The top oil temperature reading should be used to correct the oil level gauge reading.
The corrected oil level should be in the normal (35oC) range.If corrected oil level is normal, no additional action is required. If it is above or below the normal level, it may be necessary to add or remove some of the oil. The correct oil-filling level is specified on an information plate which is placed on the transformer plate panel. At an oil temperature of +45oC, the conservator should be half filled. If the level exceeds the value “full”, oil must be drained off. If the value is “low” or “min”, oil must be filled in.
In case of stuck MOG or broken lever of MOG and where there is no oil level gauge, the
prediction of oil level is difficult. In such cases, the actual oil level is to be ascertained by
attaching a PVC pipe to the drain valve of conservator tank and holding the pipe vertically.
In addition, level gauge (wherever fixed) may be observed and compared with oil level
reading of MOG. The cause of any incorrect level should be determined/rechecked and
corrective steps taken prior to taking any other action e.g. check for oil leakage.
Breathers:
Silica gel dehydrating breathers are filled with a sight glass so that the color of the crystals
may be seen. The color changes from blue to pink as the crystals absorb moisture. When the
crystals get saturated with moisture, they become predominantly pink and should, therefore, be reactivated. The body of the breather should be removed by loosening/ undoing the nuts. If the crystals have been kept in an inner container, the container should be removed, but if they are not, the crystals should be removed into a shallow tray. The crystals should be heated at temperatures of about 200oC until the whole mass is at this temperature and the blue colour has been restored. Clean the breather and replace the dry crystals and renew the oil in the cup at the bottom. Also ensure that air passage is free. Clean air passage, if blocked.
Bushings:
Bushings generally require little or no maintenance other than periodic checking of the oil level as indicated by the gauge and cleaning of the porcelain. Bushing exposed to salt spray, cement dust or other heavy deposits are subjected to a grater hazard and must be cleaned regularly to prevent flash over.
1. Examine bushings thoroughly for broken porcelain, fine hair line cracks, or any signs
of flashover or heating.
2. Inspect all connections and leads. All connection should be tight and examined for
broken strands.
3. Oil in oil-filled & non-hermetically sealed bushing-Check for dielectric strength
(minimum BDV should be 40 kV). If low, filter or replace oil.
4. Inspect oil level filled bushings and examine these thoroughly for loss of oil. If oil
level is low, fill the oil of 60kV BDV
5. Inspect cement which secures porcelain and collar for scaling or other damages.
6. Examine bushing for evidence of undue transverse stresses, which may be caused by
external connections.
7. It is desirable that periodic Tan Delta and Capacitance measurements be made on the
bushings and the values compared with those stamped on the name plate .The tan
delta bridge used should be preferably operate at 5 K.V. or higher for useful results,
but low voltage bridges are still capable of indicating the entrance of moisture into the
bushing.
Capacitance and Tan Delta of condenser type bushings - Measure capacitance & tan
delta. If capacitance has increased by 10% or more as compared with the original
values, replace bushings.
i. If the tan delta values exceed the limits given below for
different types of bushing, replace bushing.
a. BHEL OIP Bushing-0,007(0.7%)
b. BHEL SRBP Bushing-0.015(1.5%)
All other makes-0.01(1.0%)
CONSERVATOR TANK
Working of Air Cell type conservator.
Air cell performs two main functions. First, it separates the transformer insulating oil from
the atmosphere (thus preventing gas or moisture contamination of the oil). Second, it
maintains a constant atmospheric pressure on the transformer oil. The flexible air cell is
connected to top of conservator tank through a gasket joint. The airline from this connection
is connected to dehydrating breather which ensures dry atmosphere inside the air cell. Under normal operation, the air cell is completely surrounded by oil and floats, as high as it can, in the conservator. As the transformer oil volume changes, the air cell inflates or deflates by equivalent volume. The float of magnetic oil level gauge makes contacts with the underside of air cell and follows the motion up and down thereby giving an indication of the oil level inside the conservator.
In the event when the air cell becomes damaged and gets filled with oil, it will sink and activate the low level alarm on the oil level gauge. The system will then function as the
normal conservator and it will not affect the normal operation of the transformer.
Air in atmoseal conservators with glass- Oil level in oil sight glass should be full. If oil level
in oil sight glass is not full, it means there is air in contact with oil in the conservator.
Ascertain reasons. Then release air from conservator as per procedure given below.
FOR RELEASING AIR FROM CONSERVATOR FITTED WITH PRONAL:
Pressurise the Pronal up to a maximum of 0.1 kg. /cm2 and open the air vent valves until oil start coming out. Then close the valves. Release pressure from the pronal.
Oil level in transformer- Check oil level in conservator as shown by MOG with reference to oil temperature and then check oil level in OLTC Conservator also. Top up with fresh filtered oil, if low. For conservators fitted with pronal, the oil is to be filled from the bottom drain valve of conservator through filter machine and air is to be released from the conservator as per procedure given above. For conservator fitted with diaphragm, oil is to be filled from the bottom drain valve of conservator keeping the air release valve open till oil starts coming out of the air release valve. Stop oil filling and close the air release valve. Then lower the oil up to the MOG mark corresponding to the oil temperature.
PRESSURE RELIEF DEVICE (PRD)/ SUDDEN PRESSURE RELAY (SPR)
PURPOSE:
Pressure Relief Device / Sudden Pressure Relay plays a vital role in the
protection of power transformers from excessive pressure inside which may occur due to internal fault or any other reason.
If a short circuit occurs inside a transformer, the arc vapourises the transformer oil and a
heavy pressure is built up. If the pressure is not released immediately in a few milli second,
the transformer tank will get bulged and will rupture with oil spreading everywhere creating a fire hazard.
When pressure inside the transformer tank rises above pre determined safe limit, a spring
mounted diaphragm lifts from its seat for releasing the oil, vapour or gases to reduce the
pressure inside the transformer tank. The diaphragm regains its position as soon as the
pressure in the tank drops below set limit.
The lifting of the diaphragm also operates a flag indicator and a micro switch which is
connected for annunciation/tripping. The flag and the micro switch remain operated until
they are manually resetted.
ISOLATION REQUIRED:
The testing is carried out on complete shut down of the transformer / reactor and the DC supply to the relay is extended while carrying out the operational checks. Though DC supply is extended, the tripping is isolated so that the trip command is not extended to the breaker.
PRECAUTIONS:
i. The operating and resetting pressures are not site settable; no effect is made to set the
pressure setting at site.
ii. The flag unit is very delicate and care must be taken while handling the same.
iii. Gasket to be changed when cable terminal box is fitted back after the test is over.
iv. Read the manufacturer’s instructions before any operation.
PROCEDURE:
Only the operation of the micro switch and the associated cabling and correct operation of the relays are tested during routine maintenance.
The functioning of the switch operation is tested by actuating the flag by lifting the operating rod manually or as advised by the manufacturer and the annunciation in the control panel as well as the extension of the trip command to be monitored.
It is also necessary to check the condition of the gasket at the cable termination box. The
gaskets are to be changed every time the terminal box is opened.
Foundations:
1. Examine foundation for cracks in the surface, which would permit water to penetrate.
2. Inspect foundation for uneven settling.
3. Examine anchor bolts for signs of corrosion and loosening. In many cases anchor
bolts, which have worked free, may provide a path for water to penetrate into the
body of the concrete. This should be repaired.
4. Check steps for transformer track to see that transformer will not roll off.
Gaskets:
The material used for gaskets is often cork-bonded with an oil proof agent, such as
Neoprene, and it is recommended to stock a set of spare gaskets.
1. Examine all gaskets for leaks. These may be due to improper fitting or unequal
tightening of bolts or studs. Improperly installed studs may frequently be the cause of
trouble.
2. Inspect all exposed edges for decomposition and fatigue. The exposed portion should be given a protective coating.
3. Inspect studs and nuts. All nuts should be tight and studs should be threaded sufficient
to permit full travel of nut.
External Connections:
Earthing Connections:
a. All connections should be tight. If they appear blackened or corroded, undo the
connection and clean down to bright metal with emery paper. Remake the connections
and give it a heavy coating of grease. It is particularly important that heavy current
carrying connections should be properly maintained. If the metal has the characteristic
blush tinge (which indicates that it has been hot) then, in most cases, the connection
shall not be considered satisfactorily. Either it has become loose or dirty, or the
conductor is not suitable for carrying over the current.
b. The earth connections shall be properly maintained. A small cooper loop to bridge the
top cover of the transformer and the tank may also be provided to avoid earth fault-
current, passing through the fastening bolts, when there is a lightening of high voltage
surge or failure of bushings.
c. The importance of proper grounding has also its own place. Improperly grounded
apparatus constitutes hazard not only to the electrical equipment but to every person
who may come in contact with it.
d. All ground wires should be thoroughly examined and tested as follows:
i. Inspect ground wire to see that it is of proper size.
ii. Examine connections at both ends. These should be solid and making good
contact. A ground wire from the transformer tank should be solidly connected
to station ground. All connections should be welded properly.
WINDING TEMPERATURE INDICATOR TESTS:
The transformer and associated equipment must be taken out of service, isolated and earthed while checking the operation of the winding temperature indicators.
All results must be recorded in the log for comparison during future checks in service.
If the winding temperature in a transformer shows a tendency of rising without a
corresponding increase of the load, which may be due to reduction in the cooling ability of the cooling equipment (dirt, dust), the thermometer should be checked first.
Temperature Indication Calibration of the Winding Temperature Indicator
Remove the winding temperature indicator bulb from the transformer pocket positioned in the tank cover.
Insert the bulb into the calibrated temperature controlled bath.
Raise the temperature of the bath in 5oC steps and check the response of the winding temperature indicator after ten minutes. This procedure should be continued up to a
maximum temperature of 130oC. The tolerance for the temperature indication is + 3oC.
Lower the temperature of the bath in 5oC steps and check the response of the winding temperature indicator after ten minutes. At the same time, check the transducer output. The
tolerance for temperature indication is + 3
oC.
Check the alarm and trip switch settings by rotating the pointer slowly to the set
temperatures. These settings will be indicated using a multi-meter. Record the values at which the switches operated.
Once these checks are completed, return the bulb to the pocket in the transformer cover. Do not forget to bring the maximum level pointer to match the temperature indicator.
Oil Temperature Indicator Tests:
Remove the OTI bulb from the pocket on the transformer lid and insert it into the calibrated temperature controlled oil bath.
Increase the temperature of the oil bath in 20oC steps from 0oC up to a maximum temperature of 120oC. Check and record OTI readings against bath temperatures up the range (tolerance + 3 oC).