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 Transformer Annually
Checking Temperature Sensors Daily
Visual / Control System
Fan Maintenance Annual Inspection


Physical inspection of transformer Annually
Leakage of oil and reading the MOG (Magnetic Oil Gage) of tanks Daily In 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) Annually A 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 test Annually This 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
Weekly If required, the oil in the bushing has to be filled up to correct level. Oil filling must be done under shutdown condition.
Bushings Annually Visual inspection.
Bushings Every 5 years Visual inspection and cleaning.
Interfacial tension test This test detects the presence of polar compounds such as paint, varnish and paper.
Tan and delta test Once every 2 years The presence of water is measured, as water can decrease the dielectric breakdown voltage of the transformer oil.
Color test This test indicates aging, quality and the presence of contaminants in the transformer oil.
Transformer excitation current test Every 5 years
Insulating oil – DGA, physical, and chemical tests Annually
Top oil and winding thermometer inspection Annually
Oil level indicator operation 5 years
Sudden pressure relay 5 years


Ambient temperature Hourly
Winding temperature Hourly Ensure temperature rise is within specified limits.
Oil temperature Hourly Ensure temperature rise is within specified limits.
Load (amps) Hourly Check against rated figures given on the name plate.
Voltage Hourly
Oil level in transformer Daily Check oil level gauge. If low, top with dry oil – find whether there is any leak.
Oil level in bushing Daily
Dehydrating breather Daily Check 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 conservator Daily Check oil sight window or oil level gauge. If low, top with new dry oil.
Relief diaphragm of OLTC explosion vent Daily Replace if cracked or broken.
Cooler fan, bearing motor & operating mechanism Daily Check the bearings. Examine contacts, check manual control and interlock. Lubricate the bearing. Replace burnt or worn contacts.
Bushings Quarterly Examine 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 transformer Yearly Check acidity resistivity, tan delta and sludge.
Oil filled condenser bushing Yearly Refer to the maintenance schedule for OIP condenser bushings. Filter or replace.
Gasket joints Yearly Tighten the bolts evenly to avoid uneven pressure.
Cable boxes Yearly Check sealing arrangements and find out whether there is any leak. Replace gasket if leaking.
Relays alarm and other circuits Yearly Examine relay and alarm contacts, their operation fuses etc. Check relay accuracy. Clean the components. Replace contacts and fuses if necessary.
Painting Yearly Rusting/color Touch up to be done.
Earth resistance Yearly Take suitable action if earth resistance is high.
Arcing contacts After 50000 operations of the OLTC Low oil level Replace if necessary.
Lubricating oil in the gear box of driving mechanism 5 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.