Cast Resin Dry-Type VS. Oil-Type Transformers (Maintenance Schedules)

Feb 25, 2018 | Products | 0 comments

Transformers are cost intensive but essential components. In order to maintain high performance and long functional lives, various maintenance activities on transformers are recommended. Let’s take a look at how the maintenance schedules of cast resin dry-type VS. oil-type transformers compare:


Cleaning Of TransformerAnnually
Checking Temperature SensorsDaily
Visual / Control System
Fan MaintenanceAnnual Inspection


Physical inspection of transformerAnnually
Leakage of oil and reading the MOG (Magnetic Oil Gage) of tanksDailyIn case of unsatisfactory oil level in the MOG – oil to be filled in transformer and transformer tank to be checked for oil leakage. If oil leakage is found, take required action to plug the leakage. If silica gel becomes pinkish, it should be replaced.
Dielectric strength test (BDV) and moisture content (PPM)AnnuallyA dielectric strength is a measure of the voltage which is conducted by the transformer oil. At times it is the presence of contaminants which conduct electricity faster than the oil – which in turn lowers the dielectric breakdown voltage.
Neutralisation testAnnuallyThis test is carried out to ascertain the level of acid present in the oil. When oxidation occurs in the transformer oil, it leads to build-up of acid.
Dissolved Gas Analysis Test (DGA)Annually for 132KV transformer.
Once every 2 years for transformers below and above 132KV.
It helps analyse the gases which are present in the transformer oil.
Oil level check
WeeklyIf required, the oil in the bushing has to be filled up to correct level. Oil filling must be done under shutdown condition.
BushingsAnnuallyVisual inspection.
BushingsEvery 5 yearsVisual inspection and cleaning.
Interfacial tension testThis test detects the presence of polar compounds such as paint, varnish and paper.
Tan and delta testOnce every 2 yearsThe presence of water is measured, as water can decrease the dielectric breakdown voltage of the transformer oil.
Color testThis test indicates aging, quality and the presence of contaminants in the transformer oil.
Transformer excitation current testEvery 5 years
Insulating oil – DGA, physical, and chemical testsAnnually
Top oil and winding thermometer inspectionAnnually
Oil level indicator operation5 years
Sudden pressure relay5 years


Ambient temperatureHourly
Winding temperatureHourlyEnsure temperature rise is within specified limits.
Oil temperatureHourlyEnsure temperature rise is within specified limits.
Load (amps)HourlyCheck against rated figures given on the name plate.
Oil level in transformerDailyCheck oil level gauge.If low, top with dry oil – find whether there is any leak.
Oil level in bushingDaily
Dehydrating breatherDailyCheck that air passages are free. Check color of active agent.If silica gel is pink, change by new charge. The old charge may be reactivated for using again.
Oil level in OLTC conservatorDailyCheck oil sight window or oil level gauge.If low, top with new dry oil.
Relief diaphragm of OLTC explosion ventDailyReplace if cracked or broken.
Cooler fan, bearing motor & operating mechanismDailyCheck the bearings. Examine contacts, check manual control and interlock.Lubricate the bearing. Replace burnt or worn contacts.
BushingsQuarterlyExamine for cracks and dirt deposit.Clean the dirt. If cracked or broken replace the bushing.
Oil in transformer

Oil in the diverter switch of OLTC


Half yearly or at the end of 5000 operations

Check for dielectric strength and water content.

a. Dielectric strength

b. Water content

Take suitable action to restore quality of oil.

Filter or replace if BDV is less than specified value.

Measure the water content using KARL FISHER method. Replace / recondition if it exceeds the limits specified.

Oil in transformerYearlyCheck acidity resistivity, tan delta and sludge.
Oil filled condenser bushingYearlyRefer to the maintenance schedule for OIP condenser bushings.Filter or replace.
Gasket jointsYearlyTighten the bolts evenly to avoid uneven pressure.
Cable boxesYearlyCheck sealing arrangements and find out whether there is any leak.Replace gasket if leaking.
Relays alarm and other circuitsYearlyExamine relay and alarm contacts, their operation fuses etc. Check relay accuracy.Clean the components. Replace contacts and fuses if necessary.
PaintingYearlyRusting/colorTouch up to be done.
Earth resistanceYearlyTake suitable action if earth resistance is high.
Arcing contactsAfter 50000 operations of the OLTCLow oil levelReplace if necessary.
Lubricating oil in the gear box of driving mechanism5 yearly
1000kVA to 2000kVA

7–10 yearly
above 3000kVA

Low oil level

Overall inspection including core and coil.

Add or replace with lubricating oil.

Wash the core and coils with clean oil.


Please note that we have moved from Honeydew to Chloorkop, Edenvale. We are now fully operational from our new premises – 24 August 2020

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