Cast Resin Dry-Type VS. Oil-Type Transformers (Maintenance Schedules)
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:
I. CAST RESIN DRY-TYPE TRANSFORMERS
| ITEM TO BE INSPECTED | RECOMMENDED FREQUENCY |
|---|---|
| Cleaning Of Transformer | Annually |
| Checking Temperature Sensors | Daily Visual / Control System |
| Fan Maintenance | Annual Inspection |
II. OIL DISTRIBUTION TRANSFORMERS
| ITEM TO BE INSPECTED | RECOMMENDED FREQUENCY | INSPECTION ACTIONS |
|---|---|---|
| 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. |
| Bushings 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 |
III. POWER TRANSFORMERS
| ITEM TO BE INSPECTED | RECOMMENDED FREQUENCY | TO BE CHECKED | INSPECTION ACTIONS |
|---|---|---|---|
| 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 |
Quarterly
Half yearly or at the end of 5000 operations |
Check for dielectric strength and water content.
a. Dielectric strength |
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 |
Low oil level
Overall inspection including core and coil. |
Add or replace with lubricating oil.
Wash the core and coils with clean oil. |