Drying Out Procedure For Transformer


Drying Out Procedure For Transformer

Drying Out Procedure For Transformer

Hello All Welcome back to our blog Electrical Online 360°, today we discuss one of the most important topic regarding the Transformer and the topic name is Drying out Procedure for Transformer. So Let's start the learning in simple language with simple explanation.

1. Preparations Of Drying Out

1)Bring out thermometer / thermocouple leads, leads through one of the opening from transformer tank.

2)Bring out insulated leads form the windings from the terminals bushing. This needed for measurement of insulation resistance.

3)Prevent the drought of cold air by lagging the tank wire fire resistance mat/asbestos, cloths / Glass sheet. 

4)Arrange measuring instruments, connection thermocouples placing thermometer, calibration equipment etc.

5)To prepare log -book.

2. Purpose Of Drying Out: 

Surrounding Atmosphere Contain Inherent Moisture

Transformer oil and insulation of winding and other insulation hygroscopic and immediately absorb moisture from surroundings. If the machinery is kept idle for a period then this absorbed moisture must be driven out by applying drying out process.

If the transformer is not dried out then it will not withstand the service voltage for a longer span of time and insulation may fail to work resulting in short circuit too.

3. Procedure Of Drying Out

Generally drying out is for (i) oil and (ii) windings.

Oil /windings are heated up for drying by following method which is modern method : 

a) Drying out by circulation of transformer oil through purifier (filter plant)

the filtering plant consists of

  1. Vacuum tank
  2. Vacuum pump
  3. Spray chamber
  4. Centrifugal blower
  5. Heating chamber 
  6. Heaters
  7. Strainer
  8. Pumps for circulation of oil

√ To remove moisture, sludge dirt and any solid impurities the oil from transformer tank is circulated through filtering plant.

√ Thermostat is set for the outlet temperature of 90°,

√ A pipe is dipped in the transformer oil tank up to the bottom level of tank and the oil is Drawn from the tank by pump.

√ The outlet of the filtering plant is delivered back to the top of transformer tank.

√ The process continues operation of filtering plant throughout a day.

√ In a day 1 hours shunt down of plant is taken which is required for cleaning purposes.

√ During this one hour centrifugal filters cones pads of vacuum filter are cleaned thoroughly.

√ It is observed that insulation resistance falls at the beginning due to moisture drops are getting distributed in the winding and oil in the form a vapour.

√ After a length of period (many hours) insulation resistance becomes steady due to distribution of water vapour is distributed in the oil /windings.

√ After further time in hours the complete moisture is expelled out and insulation resistance value increases.

√ The process is then stopped when the insulation (hot) resistance value is more than the specified value.

√ See that polarisation index (PI) is = 1.3

√ See that dielectric strength (BDV) is = 45 kV from mm gap.

4. Precautions During the Drying Out Process

1) See that transformer to oil temperature not to exceed 85°C. 

2) Maximum temperature of anything in contact with oil not to exceed 90°.

3) All the while keep the transformer attended by a person. Don't leave it alone. 

4) Use lagging sheets to avoid of cold draughts and to prevent loss of heat through tank walls.

5)Let the moisture removed to pass out easily from transformer oil though proper ventilator.

5. Time Period

Different in different situations.

Take the samples of oil from bottom and top to check it up or polarization index and Dielectric strength. Generally time required is:

11 kv Transformer : upto 6 days

220 kV Transformer : 10 to 30 days

400 kv Transformer: 15 to 40 days.

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