1. Transformer

2. Transformer Oil Coolant Insulating medium
Basic raw material is Low viscosity lube Transformer oil base stock (TOBS), obtained by fractional distillation and subsequent treatment of crude petroleum
TOBS further refined by acid treatment process to yield transformer oil

3. Moisture content
the amount of free and dissolved water present in the oil
Expressed in ppm
Moisture affects electrical characteristics of oil
Accelerates deterioration of insulating paper

4. Flash point
Temperature at which oil gives so much vapour that this vapour, when mixed with air, forms an ignitable mixture and gives a momentary flash on application of flame under prescribed conditions

5. Viscosity
Measure of oil resistance to continuous flow without the effect of external forces
Oil must be mobile
Viscosity increases at low temperature

6. Pour point
The temperature at which oil will just flow under the prescribed conditions

7. Electric strength(breakdown voltage):
BDV is the voltage at which bd occurs between two electrodes when oil is subjected to an electric field under prescribed conditions
Indicates the presence of contaminating agents like moisture, fibrous materials, carbon particles, precipitable sludge and sediment

8. Resistivity (specific resistance)
Most sensitive property of oil
Low value indicates presence of moisture and conductive contaminants
Insulation resistance of a winding is also dependent upon the resistivity of oil

9. Dielectric dissipation factor(DDF)
Indicate the quality of an insulation
High value indicates the presence of contaminants or deterioration products

10. Chemical properties Neutralization value Oxidation stability
Sediment ad precipitable sludge
Corrosive sulphur

11. Drying-out of Power transformers
The transformer oil and insulation is hygroscopic
If the transformer is dispatched without oil or is left idle for a long period, the oil and insulation absorbs moisture
drying out may be necessary prior to commissioning
Purpose is to expel the moisture from the oil, the winding insulation and the other internal parts
Otherwise, transformer cannot withstand service voltage for a long duration and its insulation may fail prematurely

12. Procedure of drying out
The transformer oil/winding is heated by one of the following methods for a prolonged period( ten hours to four weeks)
Periodic readings of (1) oil and winding temperature (2) power input (3) insulation resistance
Temperature of oil is maintained at 80oC and that of windings at 90oC
The values of insulation resistance start falling in the beginning of drying out process

13. DRYING OUT CURVES OF A TRANSFORMER

14. This indicates that the moisture drops are getting distributed in the winding and oil in the form of vapour
After several hours, the insulation resistance becomes steady
This indicates that water vapours are distributed in the insulation and oil
On further continuation drying-out, the insulation resistance values start rising. This indicates that moisture is expelled from the windings and oil

15. The drying-out process is stopped when the insulation resistance value (hot) is more than the specified value, during the rising mode of the drying-out process and the polarisation index and dielectric strength of the oil are satisfactory(P.I – 1.3, BDV – 45kV for 4mm gap)

16. (1)Drying of core and coils using oven
Where a suitable oven is available, the core and coils can be effectively dried in it by raising the temperature to a level not exceeding 80oC
A large volume of air should pass through the oven to remove moisture and vapours
Insulation resistance check will indicate when the coils are dry
Core and coils can also be dried in its own tank in an oven
Transformer tank should be suitable for full vacuum
Full vacuum is kept in the tank and a temperature of 75oC is maintained
Dry nitrogen is used for breathing the vacuum

17. (2) Drying by Short-circuit method
The transformer can also be dried by heating the coils by short-circuiting the low voltage winding and supplying a reduced voltage at high voltage terminals
Current should not exceed 70% of normal rated current and oil temperature should be of the order of 75oC
Winding temperature in no case should exceed 90oC
Winding temperature can be monitored by measuring winding resistance

18. (3)Drying out by Streamline Filter machine (BHEL)
Most practical method of drying out is by circulation of hot oil through streamline filter machine incorporating oil heater and vacuum chamber
The vacuum pump of the filter machine should have the capacity of creating vacuum as high as possible but not less than 710mm of mercury
Drying out process can be made faster by creating vacuum in the transformer tank by lagging the transformer tank to prevent loss of heat

19. The oil temperature in transformer should be of the order of 75oC
It should be seen that the oil temperature at the filter machine in no case exceeds 85oC
Drying process can be terminated when transformer oil characteristics are achieved within permissible limits and insulation resistance of winding shows a constant or rising trend

20. Preparations of drying-out
Lagging the tank with fire resistance mat such as asbestos-cloth, glass sheet provide external shields to prevent drought of cold air
Connecting thermocouples, placing thermometer, calibration, arranging measuring instruments
To bring-out the well-insulated leads from the windings from the terminal bushings for the measurement of insulation resistance
To bring-out thermocouple leads through one of the opening in the tank
To prepare a log book

21. Precautions while drying-out
Never leave the transformer unattended during any part of the process. The transformer should be watched and observed
Transformer to oil temperature should never exceed 85oC. The maximum temperature of anything in contact with the oil should never exceed 90oC
Maintain log sheet
Use lagging to prevent loss of heat through the tank walls and effect of cold draughts
Use proper ventilation to remove the moisture given off by transformer oil

22. Duration of drying-out
1 to 6 days for 11kV transformer
10 days to 30 days for 220kV transformer
15 days to 40 days for 400kV transformer

23. Details about method of drying
In the first stage of drying-out, the insulation resistance reduces.
This indicates release of moisture within insulation and oil.
In the second stage, the insulation resistance is steady.
In the third stage, insulation resistance starts increasing indicating that moisture is being expelled
Drying out process is stopped when sufficient insulation resistance and Polarization index is reached during third phase
BDV of oil samples is measured after every four hours

24. Explain the steps in drying out of a Power Transformer
Preliminary preparation of the machine, source of heat, measurement etc.
Arrange the set-up
Apply heat by one of the suitable means gradually
Take periodic reading of
- Clock time
- Temperature of windings, body and air, ambient
- Insulation resistance values of 15second Megger
reading and 60second Megger reading
- Winding resistance (At the beginning and at the
end)

25. Maintain steady temperature or specified value (winding temperature not to exceed 60oC or 70oC depending upon the insulation class). Measure periodically the insulation resistance values.
Initially, during the first few hours, the values of insulation resistance reduces eventhough the heat is being applied for drying-out.
During initial heating period, the moisture trapped in the insulation in the form of small globules gets released within the insulation. Hence the insulation resistance value starts reducing.

26. Intermediate stage. After a span of a few hours or a few days, the insulation resistance reaches a steady value. This indicates that moisture has spread all over the insulation.
The input power is reduced to reduce the temperature rise
Rising stage. After a few hours of steady value, the insulation resistance starts rising
This indicates that the moisture has vaporised and is being expelled from the winding. The input power is reduced further.
The drying out process is stopped when the desired value if insulation resistance(hot) and polarization index is reached. The input power is switched off.

27. Transformer Oil Chemical composition:
Consists of four major generic classes of organic compounds, namely, paraffins, napthenes, aromatics and olefines
Oil is usually termed paraffinic base or Napthenic base

28. Physical properties Density: significant in low temperature zone
Interfacial Tension (IFT)
Measure of molecular attractive force between oil and water molecules at their interface
Gives an indication of the degree of sludging of oil