Page 41 - ECLECTIC MARCH-2022 INSIDE PAGES_p2.indd
P. 41
lement weather condition throughout the month of in order to manage installation downtime. Installation
August, September’ 2021. There might be a possibility of the aforesaid accessories was carried out by opening
of external moisture ingression inside main tank respective covers which again exposed the winding to
causing contamination of the transport gas (N2) during the atmosphere. Exposer times are as follows:
transportation.
Day 4: Installation of HV Turret CTs & Bushings: 8 Hrs.
Weather Condition
Day 5: Installation of IV Turret CTs & Bushings: 6 Hrs.
Kolkata recorded the highest rainfall this year (2021)
following incessant downpour in the metropolis Day 6: Installation of TV CTs & Bushings: 4 Hrs.
throughout that month. Average rainfall in the month
of September is around 318 mm in Kolkata whereas this Considering all, we were compelled to keep the CCA
year September has witnessed a rainfall of nearly 613 exposed to this extreme humid weather almost for 30
mm which is around 92% higher than the average. In hours in six different working-days.
this inclement scenario, CESC was ready to adapt new
technologies in EHV transformer installation to maximize As per manufacturer’s recommendation, voltage class
the life of a transformer. greater than 132 kV & up to 220 kV, if the exposed hours
in air is up to 16 hours, then the cold vacuuming hours in
The Relative Humidity also remained more than 80% tank after achieving vacuum of 10 mbar is 48 hours. The
which is not advisable for a EHV transformer installation. same recommends 60 hours of cold vacuuming if the
As recommended by IEEE C 57.93™-2007, the transformer exposed hours are between 16 to 32 hours.
should not be opened under circumstances that permit
the entrance of moisture, such as on days of high relative RECOMMENDED METHODOLOGY OF
humidity (60% or higher), without precautions to limit DEHYDRATION
the entrance of moisture. Unfortunately, this forbidden
act also had to be done. Application of HighVacuum followed by Dry Air circulation
is a modern and proven method of in situ drying out of
Exceptionally elongated exposure to CCA recommended by reputed manufacturers, relevant
Atmosphere autonomous bodies and standards.
The Insulation Resistance value between Core & Frame The vacuum should be drawn from the top of the tank
was found ‘zero’ which implies that Core & Frame are connecting suitable Vacuum pump to any valve fitted
accidentally connected somewhere inside the tank. To at the top of the tank. After ascertaining that there is
pin point the problem area & attend the issue we had to no leakage in the transformer from gaskets/ valves
keep the main tank open for the following time period: etc., pull out vacuum and keep the transformer under
near absolute vacuum (1 torr or less) for about 48 to
Day 1: Inspection at the upper half of the main tank: 96 hours running the vacuum pump continuously.
4 Hrs. Then the vacuum is broken with dry nitrogen or air.
The dew point of the inlet of Nitrogen or Dry-Air is
Day 2: Inspection at the bottom half of the main tank & to be measured and should be in the order of - 50 °C
attending fault: 6 Hrs. or below. When the Nitrogen or Dry-Air comes to the
positive pressure of 0.15 kg/cm2, it is stopped and
Day 3: Re-inspection at the rear side of the frame & core: kept for 24 to 48 hours. Then the nitrogen or Dry-Air
2 Hrs pressure is released and the outlet Nitrogen or Dry-
Air Dew Point is measured. If the Dew Point matches
Due to continuous rainfall throughout the day, oil the recommended value (Table-I) or below then the
handling became impossible. Therefore, it was decided
to carry on with installation of Turret CTs, Bushings etc.
41
August, September’ 2021. There might be a possibility of the aforesaid accessories was carried out by opening
of external moisture ingression inside main tank respective covers which again exposed the winding to
causing contamination of the transport gas (N2) during the atmosphere. Exposer times are as follows:
transportation.
Day 4: Installation of HV Turret CTs & Bushings: 8 Hrs.
Weather Condition
Day 5: Installation of IV Turret CTs & Bushings: 6 Hrs.
Kolkata recorded the highest rainfall this year (2021)
following incessant downpour in the metropolis Day 6: Installation of TV CTs & Bushings: 4 Hrs.
throughout that month. Average rainfall in the month
of September is around 318 mm in Kolkata whereas this Considering all, we were compelled to keep the CCA
year September has witnessed a rainfall of nearly 613 exposed to this extreme humid weather almost for 30
mm which is around 92% higher than the average. In hours in six different working-days.
this inclement scenario, CESC was ready to adapt new
technologies in EHV transformer installation to maximize As per manufacturer’s recommendation, voltage class
the life of a transformer. greater than 132 kV & up to 220 kV, if the exposed hours
in air is up to 16 hours, then the cold vacuuming hours in
The Relative Humidity also remained more than 80% tank after achieving vacuum of 10 mbar is 48 hours. The
which is not advisable for a EHV transformer installation. same recommends 60 hours of cold vacuuming if the
As recommended by IEEE C 57.93™-2007, the transformer exposed hours are between 16 to 32 hours.
should not be opened under circumstances that permit
the entrance of moisture, such as on days of high relative RECOMMENDED METHODOLOGY OF
humidity (60% or higher), without precautions to limit DEHYDRATION
the entrance of moisture. Unfortunately, this forbidden
act also had to be done. Application of HighVacuum followed by Dry Air circulation
is a modern and proven method of in situ drying out of
Exceptionally elongated exposure to CCA recommended by reputed manufacturers, relevant
Atmosphere autonomous bodies and standards.
The Insulation Resistance value between Core & Frame The vacuum should be drawn from the top of the tank
was found ‘zero’ which implies that Core & Frame are connecting suitable Vacuum pump to any valve fitted
accidentally connected somewhere inside the tank. To at the top of the tank. After ascertaining that there is
pin point the problem area & attend the issue we had to no leakage in the transformer from gaskets/ valves
keep the main tank open for the following time period: etc., pull out vacuum and keep the transformer under
near absolute vacuum (1 torr or less) for about 48 to
Day 1: Inspection at the upper half of the main tank: 96 hours running the vacuum pump continuously.
4 Hrs. Then the vacuum is broken with dry nitrogen or air.
The dew point of the inlet of Nitrogen or Dry-Air is
Day 2: Inspection at the bottom half of the main tank & to be measured and should be in the order of - 50 °C
attending fault: 6 Hrs. or below. When the Nitrogen or Dry-Air comes to the
positive pressure of 0.15 kg/cm2, it is stopped and
Day 3: Re-inspection at the rear side of the frame & core: kept for 24 to 48 hours. Then the nitrogen or Dry-Air
2 Hrs pressure is released and the outlet Nitrogen or Dry-
Air Dew Point is measured. If the Dew Point matches
Due to continuous rainfall throughout the day, oil the recommended value (Table-I) or below then the
handling became impossible. Therefore, it was decided
to carry on with installation of Turret CTs, Bushings etc.
41
