The tightness test of a transformer is applicable only for newly supplied transformers before filling oil.
A Power Transformer is generally transported in an unassembled condition. Some parts of a transformer include the Main tank, Conservator tank, Radiator, Bushings, Buchholz Relay, etc. All these parts are assembled at the site after placing the Main tank at the Transformer foundation.
So, there may be the possibility of leakage because of improper tightness of nuts and bolts, damaged gaskets, damaged O rings, etc., during assembling.
Therefore, Tightness test of the Transformer is necessary before vacuuming and filling oil to the transformer.
What Happens When a Transformer is Vacuumed Without Attending Leakage?
Vacuuming of a Transformer is generally done in the main tank part where the core and windings are placed.
The purpose of vacuuming is to create a negative pressure inside the main tank so that any moisture present in the transformer core or windings is removed through the vacuum pump.
Suppose vacuuming of the transformer is started without attending to the leakage. In that case, the transformer core or windings will absorb moisture or other foreign materials from the surrounding air to the core or windings through the leakage path as there is negative pressure inside the transformer.
This may lead to insulation puncture, sparking in the windings, and may also lead to short circuits.
Therefore, it is necessary to seal the transformer properly before vacuuming.
What Happens When Oil is Filled Without Attending Leakage?
If insulating oil is filled to the transformer when there is a leakage even if it is minor then of course there will be oil leakage during O&M, and the transformer needs to be filled with insulating oil from time to time to maintain a proper level.
Also, the insulating oil may absorb moisture from the air through the leakage path. And, during rainy seasons, water particles may be mixed with oil.
This may deteriorate the insulating property of the oil and may lead to unnecessary sparking in the core and windings, short circuits inside the transformer, etc.
Therefore, a proper tightness check of the transformer is mandatory before filling the oil.
Transformer Tightness Test With Dry Air or Nitrogen
After all the parts are assembled, the tightness test of the Transformer is done by purging Dry Air or Nitrogen and creating positive pressure (more than the atmospheric pressure) inside the Transformer.
Requirements
- Dry Air Generator or Nitrogen Cylinder with Dew Point less than -50 deg. C.
- Pressure Gauge
- Dew Point Meter
Procedure
- Shut all the valves of the transformer or seal any other openings.
- Connect the Dry Air Generator Hose / Nitrogen Cylinder to any of the filtration valves of the transformer.
- Connect the Pressure Gauge to any other valves for monitoring pressure.
- Test the dew point of Dry Air or Nitrogen. If it is less than -50 deg. C proceed with the next step.
- If the dew point is found ok, fill the Transformer with Nitrogen or Dry Air until the pressure of 4-5 psi or 0.3kg/cm2 (approx.) is achieved.
- Now shut the valve through which nitrogen or dry air has been filled.
- Check the Nitrogen or Dry Air pressure inside the Transformer on the Pressure Gauge.
- Keep the transformer for 24 hours.
- After 24 hours check the pressure again. If it is found to be the same, then there is no leakage in the Transformer.
- Now, check the dew point of the Dry Air or Nitrogen inside the Transformer Tank. The dew point should be less than -25 deg. C. at an ambient temperature of 20 to 30 deg. C.
If the pressure and Dew Point of Nitrogen or Dry Air found to be OK, then the final tightness test with vacuum i.e., vacuum drop test can be done.
Photos
#1 Dew Point of Dry Air at the Dry Air Generator end: -69.7 deg. C.
#2 Pressure Inside Transformer Before Filling Dry Air
#3 Pressure Inside Transformer After Filling Dry Air: Approx. 6 PSI.
#4 Dew Point of Dry Air Inside the Transformer after 24 hours: -40 deg. C.
What If Pressure Drops After 24 Hours?
The drop in pressure inside the Transformer may be due to two reasons.
First, due to the change in weather conditions or ambient temperature and the second is due to leakage in the transformer.
If there is a minor change in pressure inside the transformer due to weather then it may be neglected and can be proceeded with further process of vacuum drop test.
For example, if the Dry Air or Nitrogen was purged inside the transformer on a sunny day with an ambient temperature of approx. 30 deg. C, and the next day is rainy with an ambient temperature of approx. 20 deg. C, then there will be a minor drop in Nitrogen or Dry Air Pressure inside the transformer.
This change is pressure due to the change in weather may be neglected.
However, if the pressure dropped significantly from the previous day, then, there may be leakage in the transformer.
The leakage can be from anywhere. But in general, the leakage is from the joints because of improper tightness, improper placement of gaskets, or wrong gasket sizes. The leakage may also be from the bushings.
This leakage can be detected using various methods. One method that I prefer to detect leakage is using soap water.
There was one instance while I was engaged in the commissioning work of a 20 MVA,132/33kV Transformer in Jiribam, Manipur. There was leakage in the transformer due to which pressure was dropping every time. It took us more than 10 days to detect the leakage.
We tried tightening all the nuts and bolts again, checked all the gaskets and O rings, all the valves but could not find the leakage.
At last, we found that the leakage was from one of the LV Bushings. The LV bushings were oil-filled type and the leakage was from the top part of the bushing from which air is released.
Therefore, a thorough check is necessary for detecting transformer leakage. Otherwise, you will end up losing too much time like me.
What If Dew Point Is Not Found in Permissible Limit?
If the dew point is more than -25 deg. C for the ambient temperature of 20 to 30 deg. C, and there is no drop in pressure inside the transformer, then there may be moisture absorbed by the insulation of the core and windings.
Removal of moisture from Transformer windings and core is a very hard job and it is almost impossible to get the desired value at site.
Generally, vacuuming is done from 12 to 48 hours depending upon the voltage rating and capacity of the transformer.