For EHV Transformer
1. Take oil sample for BDV Testing every year. BDV should be greater than 80 kV in 2.5 mm gap. (Oil sample should be taken in Dry season. probably form Nov. to Feb.)
2. If BDV is less than 80 kV then replace the oil of diverter switch and also clean n flush the diverter switch. (without racking out from Main Tank)
3. Even if BDV is greater than 80kV replace the oil biannually invariably. Also clean n flush the diverter switch. (without racking out from Main Tank)
4. If any abnormal test results found in winding resistance test then overhauling of Diverter switch by Racking out / Dismantling /reassembling to find out the detail root cause of variation in test result.
5. While carrying out work Gear Box of Horizontal Lever to be attended for oiling greasing.
6. Check for working of heaters in OLTC control Box
7. Check for vermin proofing of OLTC control Box.
8. Check for OLTC Operation Remote/Local Electrical & mechanical.
9. Check for simultaneous operation of all three phase. (By noise & by Display on Top cover of diverter switch)
10. Take operation of each tap (forward & reverse) for observing normal functioning.
11. Buchholz, PRV & OSR , SPRR to be attended for water proofing w.r.to terminal cover, micro switch, cable gland, cable shape etc….(As instructed in prev. letter)
12. Check for OLG of conservator tank & attend if not visible. (Apply white paint on back screen of OLG i.e on support body of OLG for better visibility of oil)
13. Carryout logic checks of all Transformer Protection. (As instructed in prev. letter)
B. For 66 kV Transformer
1. Take oil sample for BDV Testing every year. BDV should be greater than 60 kV in 2.5 mm gap. (Oil sample should be taken in Dry season. probably form Nov. to Feb.)
2. If BDV is less than 60 kV then carryout Overhauling of OLTC with replacement of oil.
3. Even if BDV is greater than 60 kV then carryout Overhauling of OLTC with replacement of oil biannually invariably. i.e carryout Overhauling of OLTC with replacement of oil biannually invariably irrespective of oil BDV(60<BDV<60)
While carrying out 66/11kV X’mer OLTC Overhauling, following check points to observed…
1. Check for working of heaters in OLTC control Box.
2. Check for vermin proofing of OLTC Control Box.
3. Check for oil leakage from Top plate (OLTC)/bottom plate, replace gasket if required.
4. Check for oil leakage from main shaft.
5. Check for OLTC Operation Remote/Local Electrical & mechanical.
6. Take operation of each tap (forward & reverse) for observing normal functioning.
7. After draining oil, flush the inside mechanism precisely.
8. Check all barrier board connection for tightness.
9. Check connection of resistors for tightness & measure resistance.
10. Check all fix contacts for healthiness & tightness.
11. Check for Moving roller contacts precisely & replace if required.
12. Take operation for all tap from 1 to 17 & reverse and check for alignment of all phase.
13. During operation of each tap check contact area of moving contact with fix contact.
14. Check tightness of moving Jaw & alignment of all phase jaw. (All Jaw should be align)
15. Lubricate all moving part of OLTC.
16. Clean all moving/Fix contacts if carbonised.
17. Check all flexi.Cu. Connections for tightness & healthiness.
18. Check clearance between all tubular pipe connection between Fix contact & barrier board terminals. (Proper clearance must be verified before filling oil, to avoid undue tripping due to surge voltage.)
19. Check for OLG of conservator tank & attend if not visible. (Apply white paint on back screen of OLG i.e on support body of OLG for better visibility of oil)
20. Buchholz, PRV & OSR to be attended for water proofing w.r.to terminal cover, micro switch, cable gland, cable shape etc…. (As instructed in prev. letter)
21. Carryout logic checks of all Transformer Protection. (As instructed in prev. letter)
Above strategy is to be adopted for Overhauling of OLTC and all above points are to be checked while carrying out overhauling of OLTC invariably, no lacuna regarding this activity is tolerable.