Below, given shows leading features to be considered from the construction point of view :-
(i) According to core,
1. Core type.
2. Shell type.
(a) Simple Shell,
(b) Distributed Shell.
(ii) According to winding.
1. Two winding (ordinary type).
2. Single winding (Auto type).
3. Three winding (Power transformer).
(iii) According to phase.
1. Single phase.
2. Three phase.
(iv) According to arrangement of coils.
1. Cylindrical type.
2. Disc type.
(v) According to use.
1. Power Transformers.
2. Distribution Transformers.
3. Instrument Transformers
(a) Current transformer,
(b) Potential transformer.
(vi) According to type of cooling
(a) Natural cooled.
(i) Natural air cooled.
(ii) Oil immersed natural cooled.
(iii) Oil immersed natural cooled with forced oil circulation.
(b) Forced air cooled.
(i) Oil immersed air blast cooled.
(ii) Oil immersed air blast cooled with forced air circulation.
(c) Water cooled.
(i) Oil immersed water cooled.
(ii) Oil immersed water cooled with forced oil circulation.
(1) Power Transformer is simple static equipment but does a very gigantic job. Modern A.C. Power system has vast net work of transmission lines. The power transformer is costly equipment in the substation and this requires approximately 50% of the capital outlay on the erection of substation.
Due to its vital role, it is necessary that the greatest care is taken in its design, proper selection, transport, erection and maintenance. The main components of any transformer can be listed as below:-
1. Magnetic circuit consisting of core and clamping structures.
2. Primary, Secondary and tertiary winding.
3. Tank (filled with transformer oil)
4. Insulation for the winding.
5. Oil conservator with oil level gauge.
6. H.T. and L.T. terminals.
8. Other accessories and fittings.
Protection taken in Power Transformer -
(A) Gas and Oil Surge Protection
(B) Emergency Vent.
(C) Over Current and Earth Fault Protection:
To protect the transformer against the effects of external short circuits and excess overloads, plain O/C and protection with IDMTL type relay is provided. The current setting of the Earth fault relays are set according to the permitted sustained overload of the transformer and is arranged to trip both the High Voltage and Low Voltage circuit breakers.
On small transformers where plain O/C and E/ F is the main protection and transformer is fed from one end only. A high system instanteous element is also incorporated in these relays to provide protection against terminal and internal winding faults.
While selecting the setting for instantaneous element magnetic in rush currents of the transformer under normal switching are kept in view.
In addition to the above protection, standby earth fault protection is also used to the transformer while the neutral is earthed through resistor which has short time ratings. Standby earth fault relay is operated from current transformer in the neutral circuit and its operating time is matched with the thermal rating of the resistor. This type of protection is sometimes set in two stages:-
In the first stage only low voltage side is cut off and if the fault still persist the high voltage breaker is tripped in second stage.
(d) Restricted Earth Fault Protection:
An earth fault in the winding is the most common type of transformer fault. It is best detected by using restricted type of earth fault protection. Differential protection covers almost all the internal faults in case of solid earthed transformers but restricted earth vault protection is used in conjunction with differential protection in case of important large size transformers where neutral is earthed through resistor.
This type of protection is also applied on smaller transformers where for differential protection is not justified. The Restricted earth fault scheme is a balanced system of protection and can be applied to either star or delta winding. In case of resistance earthed neutral, the amount of earthing resistor and the relay setting is selected accordingly.
(e) Differential Protection:
The modern differential scheme provides in a. single scheme the most complete coverage for winding and terminal faults. The system operates on the well known Merz each output terminal is balanced against each other. An internal fault creates an unbalance which operates the differential protection. The C.T. ratio and type of connections heed special type of transformer and its connection.
Modern differential scheme are very fast operating and stable for through faults in the C.T. circuits because of different magnetising characteristics and covers almost complete transformers winding for phase to earth faults in case of solidly earthed neutrals.
(f) Since transformer is the most important substation equipment, it becomes important to economies on its capital cost as insulation level of transformer, So as to affect cost reduction, lightening surges travelling through transmission lines are not allowed to reach the transformer by having proper setting on arching horns on transmission lines and other equipment protect against direct lightening strokes etc.
Maintenance kinds of maintenance of transformers are necessary:-
(I) Routined Maintenance:
Transformers need less attention than other electrical apparatus. Because of neglected maintenance insulation progressively deteriorates. Severe short circuit is resulted by it and renders total of the transformer. In normal condition transformer should last for 40-50 year in comparison to a rotating machine which lasts for 10-15 years.
Some transformer manufacturers has tendency to over rate within a particular size and shape, it at lower cost in the highly competitive market. This produces trouble to maintenance Engineer and they should be more vigilant.
In order to increase the life span of the transformer monthly or bimonthly inspections should be performed. It includes checks of temperature, oil level and leaks in the liquid filled transformers.
Inspector should check dirt accumulation on high voltage bushings, rusting, discoloured connections and the accumulation of refuge on the transformer lids, can cause a flash over at the high voltage terminals.
(2) Dry Type Transformers:
Dry type transformers normally have air as cooling medium. So, much attention should be paid to prevent entrance of water from windows, pipes etc. Transformer should be kept 30 cm away from the wall to allow free circulation of air. Settled dust on winding and core reduce heat dissipation.
After disconnecting from the main, windings may be clean with vacuum cleaner. All insulating surfaces should be cleaned with a dry cloth. If insulation resistance tests give indication of absorbing moisture by windings, they should be heated. To dry with internal heat, is a slower process.
It can be done by short circuiting one winding and applying reduced voltage to the other. Temperature should not exceed 100°C measured by spirit thermometer placed between ducts in the winding.
Mercury thermometers must not be used because induced current in the mercury will give erroneous result. However, drying with internal heat is not proper. If the insulation resistance is less than 50,000 ohms at 20°C external heating is done by directing heated air into the bottom of the ventilating ducts.
(3) Liquid Filled Transformers:
This type of transformer requires more care than dry one. Insulating medium liquid either, mineral oil or an inert synthetic liquid manufactured under trade name-chloroxtol, Inerteen, Pyranol etc. These liquid should be examined at least once a year for presence of sludge or moisture.
Oxidation on transformer coils and cooling ducts reduces heat transfer capacity and causes higher operating temperature. One part of water in million by volume part oil will reduce the dielectric strength of oil by half. If oxidation is there, liquid should be drained; winding should be washed down with clean liquid from a filter press.
The old liquid should be filled back to the tank through the filter. Liquid filled transformers should be inspected, under cover at least once in every 10 years. The transformer should be de-energized, liquid drained, to expose top of the core and coils. Coils should not be exposed to the air any more except it is necessary to make inspection.
Water cooled transformers have cooling coils in the oil. Oil immersed self cooled transformers depends on free circulation of oil through tubes. Seals in water coil are removed by blowing compressed air.
After that the coils are filled with acid solution sp. gr is 1.10. Acid is allowed to stand for one hour and then coils are flushed with clean water. Any leak in welded joint may be repaired by welding. Oil filled transformer may leave an explosive mixture of oil and air.
After neutralizing a transformer from its power source, careful inspection and tests according to manufacturer's instructions can be conducted.
The insulators and porcelain parts should be examined for cracks, checks, chips, breaks, and flash over streaks. Chipped glaze exceeding 12 mm in depth of over an area exceeding 625 square mm on an insulator should be examined by a qualified electrical engineer.
Terminal ends should be inspected for tightness, corrosion, and damage to cubic clamps. The main and ground terminals should be examined tor Colorations and corrosion indicated by blue, green, white, or grown corrosion
It is also important to determine whether oil-filled bushings contain adequate lubricant. Cable connections should be examined for broken, burned, corroded, or missing strands.
(1) Once the transformer is energized, the concrete foundation and support pad should be examined for settling, movement, and surface cracks exceeding 1.5mm in width. Inspect anchor bolts for tightness and missing parts.
(2) When an enclosure or case is opened for any reason, it should be examined for signs of internal moisture and, if present, examined for plugged breathers and leakage.
(3) Examine coils and cores for deficiencies, dirt, and sludge. If probing down the sides with a glass rod indicates dirt and sludge exceeding 12mm, the oil should be changed and the coils and cores cleaned.
(4) The automatic tap changers (load ratio control apparatus) should be inspected in accordance with manufacturer's instructions. Clean and lubricate all moving parts.
(5) Forced-air fans and fan controls should be inspected for defective bearings, inadequate lubrication, dirt, bent or broken fan blades or guards, lack of rigidity of mountings, and indications of corrosion or rust.
(6) Wooden mounting platforms should be examined for cracks, breaks, and rot, burning and charring at contact points indicates a grounding deficiency.
(7) Rust, corrosion, distortion looseness, missing parts, and broken parts are all problems that should be considered when examining hangers, brackets, braces, and connections of an energized transformer.
(8) Name plates and warning signs should be clean, readable, and properly placed.
(9) All gaskets should be examined for leakage, cracks, breaks, and brittleness. Polychlorinated biphenyls are the toxic waste often found in transformers, capacitors, and other electrical equipment.
In 1976 Congress passed the Toxic Substances Control Act and specifically directed the Environmental Protection Agency to regulate "PCBs, the Act prohibits the intentional manufacture of PCBs for other than research purposes.
PCBs are contained in electrical equipment, however, and are present in the environment as a result of the manufacture and use of PCBs in electrical equipment for over 50 years.
The risks inherent in having a PCB transformer on the premises include noncompliance with EPA regulations. Leaks, if not repaired in time, can result in fines and tremendous health problems and fire can contact the transformer and release its toxic into a building atmosphere.
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