GENERAL
As is generally known, a transformer consists essentially of the magnetic core built-up of insulated silicon steel lamination upon which are wound two distinct sets of coils suitably located with respect to each other and termed as primary and secondary windings. Such a combination may be used to step up or step down the voltage. The techniques used in the design and construction of high voltage transformers vary from supplier to supplier. The active parts of a transformer consist of core and windings.
CORE
Core is made from lamination of cold rolled grain oriented silicon steel. The specific loss at operating flux densities in silicon steel is very low.
WINDINGS
Paper insulated copper conductor is used for windings. The conductors are transposed at regular intervals for ensuring equal flux linkage and current distribution.
COOLING
Core and windings are immersed in an oil filled tank. Normally, oil flows through winding and enter cooler or radiator by thermosyphonic effect.
Depending upon the rating, the transformer employs ONAN, ONAF, OFAF and OFWF types of cooling.
ONAN - Oil Natural Air Natural
ONAF - Oil Natural Air Forced
OFAF - Oil Forced Air Forced
OFWF - Oil Forced Water Forced
TANK AND COVER
Steel plates are used for fabricating transformer tanks and covers. They are designed to withstand full vacuum and a positive pressure of 0.3 kg/cm2 above the normal oil head.
CONSERVATOR
Conservator takes care of the expansion and contraction of transformer oil, which takes place due to loading and releasing of load. Modern transformers are provided with separate air shell in the conservator which prevents direct air contact with the transformer oil.
A separate conservator is provided for the on-load top changer diverter switch. Magnetic oil level gauges are provided in the conservator tanks which can give alarm to the operators and isolate the transformer in the event of oil level falling below a preset value.
PRESSURE RELIEF DEVICE
A pressure relief device is provided with an alarm and trip contacts. When excessive pressure is built inside the transformer in the event of severe fault, the pressure relief device releases the excess pressure.
For smaller transformers, an explosion vent is provided with a lighter diaphragm which breaks in the event of increasing internal pressure.
BUCHHOLZ RELAY
This gas and oil actuated relay is provided in the oil pipe which connects the conservator and the main tank. For any internal fault inside the transformer, this relay is actuated. This relay operates on the well-known fact that every type of electric fault in an oil-filled transformer gives rise to gas. This gas is collected in the relay to actuate the alarm and trip contacts.
SILICA GEL BREATHER
Expansion and contraction of oil due to loading causes breathing. External air gets in during the time of contraction. Silica gel absorbs the moisture in the air and prevents moisture entry into the oil.
TEMPERATURE INDICATORS
For continuous measurement of oil and winding temperatures, separate meters are used. These meters have alarm and trip contacts.
BUSHINGS
High voltage connections from the windings pass to the terminal bushings. These bushings are hermitically sealed and filled with oil for EHV transformers. This oil does not communicate with the main transformer oil. A separate oil level gauge is provided for monitoring the oil level in the bushings.
TAP CHANGER
There are two types of tap changers viz., on load and off load. In on load tap changer, tap position changes, when the transformer is energized either through manual mode or auto mode. The OLTC diverter switch has separate oil which needs periodical changing as some amount of arcing takes place during tap changing operations. This has a separate conservator and a Buchholz relay.
PROTECTIONS FOR TRANSFORMER
The following protections are provided normally for a transformer.
(i) Over current protection
(ii) Restricted Earth fault protection
(iii) Over voltage protection alarm
(iv) Over fluxing ( generator transformers )
(v) Surge protection
(vi) Differential protection ( above 5 MVA )
(vii) Oil temperature high protection
(viii) Winding temperature high protection
(ix) Oil level low protection
(x) Buchholz protection
(xi) Pressure relief device
The relays checking and calibration procedures are not covered in this document.
MAINTENANCE
It is essential to carry out regular and careful inspection on the transformer and associated components/equipment and carry out maintenance activities to provide long life to the equipment and achieve trouble-free service.
IN ORDER TO CARRY OUT THE NECESSARY INSPECTION AND MAINTENANCE WORKS, NECESSARY SAFETY PROCEDURES SUCH AS LINE CLEARANCE/EQUIPMENT SHUTDOWN ETC., WILL BE STRICTLY ADHERED TO, WHEREVER NECESSARY.
The frequency of inspection depends on climate, environment, load conditions and also the age of the transformer. The inspection cum maintenance schedule starts with every hour and continues as given below.
HOURLY
The following parameters are to be checked every hour and recorded. If the observed value exceeds the value given by the supplier, immediate remedial action should be taken.
(i) winding temperature
(ii) oil temperature
(iii) load current
(iv) terminal voltage
Normally, maximum allowed winding temperature is 55º C above ambient and oil temperature is 45º C above ambient (actual allowed value may vary from supplier to supplier).
DAILY
(i) Oil level in main conservator
(ii) Oil level in OLTC
(iii) Oil level in bushing
(iv) Leakage of water into cooler (OFWF)
(v) Water temperature (OFWF)
(vi) Water flow (OFWF)
(vii) Colour of silica gel
QUARTERLY CHECKING/ REPLACEMENT
Reconditioning of silica gel breather.
Checking of water cooler functioning
Checking of cooling fans functioning
Gear oil for tap changer mechanism
Checking of cooling pumps and motor functioning
HALF YEARLY
(i) Inspection of all gaskets and joints
ANNUALLY
(i) Protective relays, alarms, meters and circuits to be checked and calibrated
(ii) IR value and Polarisation Index
(iii) Tan delta and capacitance of bushings
(iv) BDV of transformer oil.
(v) Oil resistivity
(vi) Power factor of oil
(vii) Interfacial tension of oil
(viii) Acidity and sludge of oil
(ix) Flash point of oil
(x) Water content of oil
(xi) Dissolved gas analysis
(xii) Replacing of OLTC oil
(xiii) Thermo vision scanning
(xiv) Earthing measurements
(xv) Tan delta and capacitance of winding
ONCE IN FIVE YEARS
(i) Furan analysis (Once in a year after the first 5 years)
(ii) Overhauling of OLTC diverter switch (once in 5 years or after completion of 50,000 operations whichever is earlier)
ONCE IN TEN YEARS
Overhaul, inspection including lifting of core and winding.
