OPERATION & MAINTENANCE

5.1 OPERATION

I.S.2026-1977 states that a Power Transformers loaded at its rated KVA will fulfill its normal life when its hottest spot temperature [ H.S.T.] is 98˙C for an air temperature of 32˙C As the ambient air temperature exceeds 32˙C the H.S.T. exceeds 98˙C The insulation ageing { which decides the life of transformer}increased with increase of H.S.T. increase of H.S.T. by 6˙C doubles the rate of ageing { i.e. reduces the life by 50%}. When the ambient air temperature is lower than 32˙C the H.S.T. is lower than 98˙C. The insulation ageing decreases thus increases the life of transformer.

It is considered {by the I.S. 6600-1973} that periods of accelerated ageing due to higher ambient temperature are compested by the period of reduced ageing due to lower ambient temperature.

Some times a transformer may be required to deliver load higher than its rating. Guidance on overloading it provided by I.S. 6600 Basis for the recommendation is an follows.

In a 24 hours loading. If ambient temperature is low and loaded for a substantial time duration is also low a short time over load may be permitted such that equivalent aging due to this varying load will be the same as that of loading over a period of 24 hours.

5.2 Maintenance

5.2.1 General

Compared to most electrical equipments transformer requires relatively less maintenance. However in order to obtain a long and trouble free service from the transformer. It must be properly maintained.

Maintenance consist of regular inspection testing and reconditioning When necessary. Principal object of maintenance is to maintain the insulation in good condition. Moisture dirt and high temperature causes insulation deterioration i.e. ageing. Hence they must be prevented.

No work should be done on the transformer unless It is them from falling inside the tank.

Maintenance hints on main constituents of the transformer are given in the following paragraph.

Department of Electrical Engineer A.I.E.T-Jaipur Recommended frequency of maintenance is given at the end of the chapter. Detailed guidance on Maintenance is given in I. S. 10028 { part-II} – 1981.

5.2.2 OIL

In the transformer, oil is used as an insulating liquid as well as for cooling, It is necessary to maintain required oil level and also to maintain this oil in good condition I.S. 1866. gives recommendations in details for the maintenance of oil. A few guide lines are given here.

i. Check if there is any leakage of oil. It noticed, corrective action must be immediately taken

ii. The oil level should be checked at frequent intervals and if necessary, topping up should be done with oil complying with I.S. 335.

iii. Samples of oil should be taken at regular intervals and tested.

iv. If the dielectric strength is below the value recommended in I.S. 1866, the oil should be duly filtered. It may be noted that the products of decompositions with, water acids etc. remain in the oil and accelerate the rate of aging. Thus, timely treatment of oil is a must. The filteration will remove moisture, dust sludge etc. Removal of sludge is important as otherwise it can stick to coils and hamper the heat dissipation from coils.

However if the results indicate that acidity needs to be improved. Them a simple filteration would not be sufficient and a Fullers Earth treatment would be required.

5.2.3 Care & Winding

It id recommended that the core and windings are removed from the tank for visual inspection as per the maintenance schedule given.

Depending upon the arrangement provided, tap switch handle, connections to bushings etc. may have to be disconnected before lifting up core and wings assembly

The windings should be examined and if sludge has been deposited it should be washed away with oil jet. Any loose nut and bolt should be tighterned.

Adjust the rods/coil clamping screws provided to remove any slackness of windings. This is important as slackness in winding will permit movement of coils under the action of Short Circuits forces and repeated coil movement can wear out insulation and give rise to a fault.

Department of Electrical Engineer A.I.E.T-Jaipur

5.2.4 On Load Tap Changer

On load tap changer is, normally, mounted on the tank in the separate housing and connected to winding leads through copper studs fixed on insulated terminal board. Terminal board is oil-tight. Hence oil in the tank need not be lowered for attending O. I. T. C.

In some other types O. I. T. C. is housed in the main tank by suitable mounting on the top cover Here again O.L.T.C. oil in which arcing tanks place is not allowed to mix with the man tank oil.

Please refer to O.L.T.C. menual for operation and maintenance instruction of O.L.T.C.

5.2.5 Conservator & Oil level Indicator

The Inside of conservator should be cleaned by flushing , clean Transformer oil to remove sluge and other impurities. In bigger transformers, detachable end plate is provided to facilitate cleaning of conservator.

Oil gauge glass should be cleaned if then glass is found broken, the same should be replaced.

Magnetic oil guage to be attended while cleaning the conservator.

The mechanism should be inspected and cleaned Float should be checked to see that there is no oil in the float. This operation of alarm and trip contacts should to be checked.

5.2.6 Silica Gel Dehydrating Breather

Breather should be examined to ascertain if the silica gel requires changing. More frequent inspection are needed when the climate is humid and when transformer is subjected to fluctuating load. The colour of silica Gel crystals in the breather acts as an indicator as an indicator as it changes from blue to pink when saturated with moisture. When majority of crystal have turned pink. The silica gel should be reactivated by beating in oven of a shallow pan at a temperature of 150‘C to 200‘C until the original colour is gained. This usually takes 2 or 3 hours. Oil cup should be cleaned to remove dust and dirty oil. Oil seal should be filled with fresh oil.

5.2.7 Buchholz Relay

Routine operation and mechanical inspection should be carried out as per the manufacturing instruction.

Department of Electrical Engineer A.I.E.T-Jaipur

5.2.8 Pipe work

The pipe work should be inspected for leakages, which may be due, to bad seated joints, joints should be re-made and leakage stopped.

5.2.9 Explosion Vent

The diaphragm at the exposed end of the exposion vent should be inspected and replaced if damaged Failure to replace the diaphragm quickly may allow the ingress of moisture in the transformer,

Whenever button diaphragm ruptures. Oil rises inside the explosion vent pipe and visible in the oil level indicator on explosion vent. It diaphragm is broken because of fault in the transformer. Inspection should be carried out to determine the nature and case of their fault.

5.2.10 Bushing

Porcelain insulators should be cleaned and minutely examined for any cracks, chipping off & other defects. All such bushings should be replaced. In case of any sign of oil leakage from the condenser bushings the matter should be referred to us.

5.2.11 External connections Include Earthing

Check all external electrical connections including earthing. They should be tight. If they appear blackened or corroded, unbolt the connection and clean with emery paper. Remake the connections and give a heavy coating of conducting grease. It is particularly important that heavy current carrying connections are properly made as any loose connections give rise to eat generation & temperature rise which could be detrimental to the connection.

Department of Electrical Engineer A.I.E.T-Jaipur

5.3 OTHER PROTECTIVE EQUIPMENT

5.3.1 CURRENTTRANSFORMER

Protective transformers include current transformer and voltage transformer.

The relays are connected in the secondary circuit of protective transformer. The stand and secondary voltage circuit of protective transformer. The standard voltage of current transformer is either transformer is 110 v. between phase and 63.5 between phase and neutral.

The burden of protective transformer is specified in volt amp at rated secondary current at specified the power factor.

The accuracy of protective transformer is specified by classification based on limits of ratio error and phase error.

5.3.2 POTENTIAL TRANSFORMER

There are too types of construction electromagnetic VT & CVT. CVTs are used for voltage above 66 KV.

Transient performance of VTs is important in protection. CVT should not given Ferro resonance & secondary over voltage. Electromagnetic is superior to CVT in respect

5.3.3 LIGHTENING ARRESTER (gap less zone)

Zinc oxide gapless arresters or metal oxide arrester. There are gapless

& consist of zinc-oxide dises in series zone arresters have. Superior V/I characteristic & higher energy absorption level and they are preferred for EHV & HVDC installations.

For protective converter equipment from DC side surges have the different criteria as compared with A.C application. The D.C arrester should be suitable for operation in inductive circuit & capable of discharging.

Relative long duration has to take place without oil of zero passages. To fulfill such requirement & to provide low protective levels special design of active gap is necessary zone arrester with active gaps are used. These have superior characteristic & high discharged capability.

Department of Electrical Engineer A.I.E.T-Jaipur

5.4 Prevention Maintenance of Transformer

The normal life of a well operated transformer may be taken as 25-35 years.

It may serve longer if careful installation rigid inspection & preventive maintenance is carried out the following points need consideration.

5.4.1 Over heating

Over heating damages the installation the amount of defrerioration depends on the degree of over heating & length of time duration, the damage also occurs due to mechanical means such as vibration or movement of transformer. In some case transformer may burst in to flames the oil when overheated, forms sluge, which is deposited on the winding & tends to clog the cooling ducts.

5.4.2 Impurities

Impurities in the reduce its insulating properties & speeds up slugging the moisture is absorbed by the paper from the atmosphere through the oil.

5.4.3 Mechanical damage

The connections may become loose or dirty. The porcelain of bushing may gate cracked or chipped.

5.4.4 Frequency of Maintenance

It is necessary to check the various parts at comparatively short intervals.

5.4.5 Breather

Change in weather or in load cycle may increase the intake of moisture considerably & therefore it becomes necessary to attend the breather move frequently per month.

Department of Electrical Engineer A.I.E.T-Jaipur

5.4.6 Oil

The oil in high voltage transformer needs testing twice in week.

5.4.7 Temperature

When the transformer is installed & put in service for the first time, the oil temperature must be taken at interval of about 2 hours for the first two days.

The temperature of oil may than be checked for further 2 or 3 days during these hours to see whether the same temperature is reached every day. If the temperature remains below 70`c the temperature may be checked one 9 month or so unless the load increases or weather gats considerable hot. In case of temperature exceeds 70`c more frequeat checking of temperature is required.

5.4.8 Complete over hall

From time to time the transformer should be taken out of service for a complete examination & over hall, which should include lifting or core &

winding from the tank for inspection.

Department of Electrical Engineer A.I.E.T-Jaipur

5.5 MAINTENANCE SCHEDULE

Recommended maintenance schedule is given below. However, depending on the local situation and importance of transformer, frequency my be increased.

Department of Electrical Engineer A.I.E.T-Jaipur

Department of Electrical Engineer A.I.E.T-Jaipur