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ness of transformer is achieved. If not, then the cycle INTRODUCTION OF DRY AIR
is repeated again and again until recommended Dew GENERATOR PLANT
Point is achieved. Final Relative Humidity of tank inside
should be less than 1%. Initially one or two Nitrogen or A Dry Air Generator (DAG) plant with a capacity of around
Dry-Air cycles may be kept for 24 hrs. After that it may 1.45 Cu M/min was taken on rent for this installation.
be kept for 48 hours depending upon the condition.
Being hygroscopic in nature, Dry-Air tends to absorb Connections
moisture from winding insulation.
DAG plant is connected to main tank through a 25
Table 1: CBIP recommendations on Dew Point with NB valve of Nitrogen Injection. A pressure gauge is
Insulation Temperature (Source: Courtesy BHEL, Bhopal) connected to the 25 NB Top Sampling valve to check
the positive air-pressure inside main tank. Vacuum pump
Temp. of Max. Temp. of Max. is connected to the 15 NB valve of main tank top PRV
Insulation permissible Insulation permissible air release. A vacuum gauge is connected to the 25 NB
Dew Point Dew Point valve of main tank top air-release. It is recommended
(oF) (oC) to maintain the Dry-Air Injection and Evacuation point
(oF) (oC) at diagonally opposite sides of the main tank for better
circulation of the Dry-Air. It will help to draw out the
0 – 78 – 17.77 – 61.11 trapped moisture from the winding. Suitable pressure
gauges are used to monitor pressure of main tank filled
5 – 74 – 15.0 – 58.88 with dry air.
10 – 70 – 12.22 – 56.66 Figure 3: Vacuum pump and Dry-Air Generator plant
connected to Main Tank
15 – 66 – 9.44 – 54.44
Process
20 – 62 – 6.66 – 52.22
After completion of inspection and necessary repairing,
25 – 58 – 3.33 – 49.99 the main tank was connected with a double stage
vacuum pump of 2000 LPH capacity and subjected to
30 – 53 – 1.11 – 47.22 vacuuming of continuous 24 hours followed by Dry-
Air purging. The result found was not in order to the
35 – 48 + 1.66 – 44.44 recommended value.
40 – 44 + 4.44 – 42.22 Again, evacuation of the transformer carried out for
another 60 hours as recommended by manufacturer
45 – 40 + 7.44 – 39.39 followed by Dry-Air purging. The results are as follows:
50 – 35 + 9.99 – 37.22
55 – 31 12.77 – 34.99
60 – 27 15.55 – 32.77
65 – 22 18.33 – 29.99
70 – 18 23.11 – 27.77
75 – 14 23.88 – 25.55
80 – 10 26.66 –23.33
85 –6 29.44 –21.11
90 –1 32.22 – 18.33
95 +3 34.99 – 16.11
100 +7 37.75 – 13.88
110 + 16 43.33 –v8.88
120 + 25 48.88 – 3.88
130 + 33 54.44 + 0.55
140 + 44 59.99 + 5.55
42
is repeated again and again until recommended Dew GENERATOR PLANT
Point is achieved. Final Relative Humidity of tank inside
should be less than 1%. Initially one or two Nitrogen or A Dry Air Generator (DAG) plant with a capacity of around
Dry-Air cycles may be kept for 24 hrs. After that it may 1.45 Cu M/min was taken on rent for this installation.
be kept for 48 hours depending upon the condition.
Being hygroscopic in nature, Dry-Air tends to absorb Connections
moisture from winding insulation.
DAG plant is connected to main tank through a 25
Table 1: CBIP recommendations on Dew Point with NB valve of Nitrogen Injection. A pressure gauge is
Insulation Temperature (Source: Courtesy BHEL, Bhopal) connected to the 25 NB Top Sampling valve to check
the positive air-pressure inside main tank. Vacuum pump
Temp. of Max. Temp. of Max. is connected to the 15 NB valve of main tank top PRV
Insulation permissible Insulation permissible air release. A vacuum gauge is connected to the 25 NB
Dew Point Dew Point valve of main tank top air-release. It is recommended
(oF) (oC) to maintain the Dry-Air Injection and Evacuation point
(oF) (oC) at diagonally opposite sides of the main tank for better
circulation of the Dry-Air. It will help to draw out the
0 – 78 – 17.77 – 61.11 trapped moisture from the winding. Suitable pressure
gauges are used to monitor pressure of main tank filled
5 – 74 – 15.0 – 58.88 with dry air.
10 – 70 – 12.22 – 56.66 Figure 3: Vacuum pump and Dry-Air Generator plant
connected to Main Tank
15 – 66 – 9.44 – 54.44
Process
20 – 62 – 6.66 – 52.22
After completion of inspection and necessary repairing,
25 – 58 – 3.33 – 49.99 the main tank was connected with a double stage
vacuum pump of 2000 LPH capacity and subjected to
30 – 53 – 1.11 – 47.22 vacuuming of continuous 24 hours followed by Dry-
Air purging. The result found was not in order to the
35 – 48 + 1.66 – 44.44 recommended value.
40 – 44 + 4.44 – 42.22 Again, evacuation of the transformer carried out for
another 60 hours as recommended by manufacturer
45 – 40 + 7.44 – 39.39 followed by Dry-Air purging. The results are as follows:
50 – 35 + 9.99 – 37.22
55 – 31 12.77 – 34.99
60 – 27 15.55 – 32.77
65 – 22 18.33 – 29.99
70 – 18 23.11 – 27.77
75 – 14 23.88 – 25.55
80 – 10 26.66 –23.33
85 –6 29.44 –21.11
90 –1 32.22 – 18.33
95 +3 34.99 – 16.11
100 +7 37.75 – 13.88
110 + 16 43.33 –v8.88
120 + 25 48.88 – 3.88
130 + 33 54.44 + 0.55
140 + 44 59.99 + 5.55
42
