field processing methods fo transformer dry out
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Methods to remove and to dry out transformer oils in fieldTRANSCRIPT
Field Processing Field Processing Methods for Methods for Transformer Transformer
Dry outDry outby Derek Baranowski by Derek Baranowski
1. If water is removed from the oil only:A large percentage of the total water in the systemwill still remain.After the transformer is returned to service, water in the insulation will migrate to the drier oil until equilibrium is met and the transformer will test wet.
2. The rate of aging of the cellulose materials is directly proportional to the water content. That is if water content is doubled from 1 to 2 percent, the life is halved.
3. Low water content of the solid insulation permits greater extremes in temperature cycling without impairing the dielectric integrity of the system.
Importance of Removing Water from the Insulation
Factors Affecting Water Content of the Insulation
Water may be adsorbed in the solid insulation from:• Exposure to atmosphere during installation and
inspections.• From aging of the paper.• From leaks.• Ingress through the oil expansion system.• Water remaining from the manufacturing of the
transformer may also vary.
Water may be adsorbed in the solid insulation from:• Exposure to atmosphere during installation and
inspections.• From aging of the paper.• From leaks.• Ingress through the oil expansion system.• Water remaining from the manufacturing of the
transformer may also vary.
Importance of Removing Water from the
Insulation• Water in the transformer will
seek equilibrium throughout the entire insulation system including the oil and the insulation.
• Data shows that there is a relationship between the amount of water in the oil and the amount in the paper insulation.
• Although this relationship exists, the paper has a higher tendency to hold water than the oil – so more of the water resides there.
Where is the water?
• Factors Affecting the Rate of Insulation Drying –Things We Can’t Change
• In a transformer there are generally different insulation elements, differing in thickness and nature (board, paper tape or sheet).
• Drying out times in the inside layers depend on the nature of the material and on the path along which the water must travel to reach the surface.
• Water is removed from the exterior surfaces first and sucked away by the vacuum pumps.
• Water in the inside layers migrates to the surface (called diffusion).
• In a transformer there are generally different insulation elements, differing in thickness and nature (board, paper tape or sheet).
• Drying out times in the inside layers depend on the nature of the material and on the path along which the water must travel to reach the surface.
• Water is removed from the exterior surfaces first and sucked away by the vacuum pumps.
• Water in the inside layers migrates to the surface (called diffusion).
• Factors Affecting the Rate of Insulation Drying –Things We Can Change
• Speed and efficiency at which heat is added to the system.
• Minimizing heat loss to the surroundings.
• The pumping speed of the vacuum pumping system.
• Size of the vacuum hose.• Vapor traps to trap moisture and oil
from entering the vacuum pumps.
• Speed and efficiency at which heat is added to the system.
• Minimizing heat loss to the surroundings.
• The pumping speed of the vacuum pumping system.
• Size of the vacuum hose.• Vapor traps to trap moisture and oil
from entering the vacuum pumps.
Methods of Drying Transformer Insulation
• Hot Air.• Oil Circulation . • Vacuum Only (with or without a cold trap).• Heat and Vacuum (with or without a cold
trap).• On Line Systems
• Hot Air.• Oil Circulation . • Vacuum Only (with or without a cold trap).• Heat and Vacuum (with or without a cold
trap).• On Line Systems
Why Add Heat?• For the water in
the transformer to turn into vapor (change phase) heat is required just to allow the phase change.
• The moisture leaving the transformer takes this heat with it.
Why Add Heat?• For the water in
the transformer to turn into vapor (change phase) heat is required just to allow the phase change.
• The moisture leaving the transformer takes this heat with it.
Temperature Temperature –– the key to success!the key to success!
Why We Add Heat• It takes 143.4 btu to turn 1 lb of ice
into water • It takes 970.3 btu to turn 1 lb of
water into steam • It takes 1220 btu to turn 1 lb of ice
directly into steam (sublimation)(All values for atmospheric pressure)
Why We Add Heat• It takes 143.4 btu to turn 1 lb of ice
into water • It takes 970.3 btu to turn 1 lb of
water into steam • It takes 1220 btu to turn 1 lb of ice
directly into steam (sublimation)(All values for atmospheric pressure)
Temperature Temperature –– the key to success!the key to success!
Why We Add Heat• The heat required for the phase
change of the water in the transformer comes from the surrounding oil and transformer parts.
• As heat moves from the oil into the water, the temperature of the oil and transformer drops.
• Therefore replacement heat is added by continuously reheating the oil and pumping it into the transformer.
• If the heat is not replaced, eventually freezing can occur.
Why We Add Heat• The heat required for the phase
change of the water in the transformer comes from the surrounding oil and transformer parts.
• As heat moves from the oil into the water, the temperature of the oil and transformer drops.
• Therefore replacement heat is added by continuously reheating the oil and pumping it into the transformer.
• If the heat is not replaced, eventually freezing can occur.
Temperature Temperature –– the key to success!the key to success!
Why We Add Heat• Once freezing occurs, water must
go from solid to vapor without becoming liquid (sublimation).
• Sublimation requires more heat than vaporization so temperature continues to drop until the process stalls.
• When this happens the vacuum will get very good since no water vapor is being pumped.
• When the transformer is returned to service, the ice will melt and the insulating system will test ‘wet’.
Why We Add Heat• Once freezing occurs, water must
go from solid to vapor without becoming liquid (sublimation).
• Sublimation requires more heat than vaporization so temperature continues to drop until the process stalls.
• When this happens the vacuum will get very good since no water vapor is being pumped.
• When the transformer is returned to service, the ice will melt and the insulating system will test ‘wet’.
Haley’s CometA ball of frozen ice flying through space for
thousands of years because in the absence of heat, sublimation is a very slow
process.
Temperature Temperature –– the key to success!the key to success!
Why We Add Heat• If some heat is necessary . . . then the
more we add the better . . . Right?
Why We Add Heat• If some heat is necessary . . . then the
more we add the better . . . Right?• No, if we try to run too hot, transformer oil will
vaporize with the water and end up in the vacuum pumps. Transformer oil is a blend consisting of lighter and heavier fractions. The ‘light ends’ have a lower boiling temperature and will boil off.
• Higher temperatures increase risk of thermal damage to oil and equipment.
Temperature Temperature –– the key to success!the key to success!
The Methods
Methods of Drying – Hot Air
The Hot Air Method involves:• Heating the inlet air to 80-100 Deg C and delivering it to the
bottom of the transformer• Using enough heating and insulation to achieve an exit air
temperature of within 20 Deg C of the inletBenefits:• Can be used on transformers that can’t take vacuum.Limitations:• Not highly efficient• Oxygen accelerates aging of the insulation• Does not do a good job of removing water from deep within the
insulation.
The Hot Air Method involves:• Heating the inlet air to 80-100 Deg C and delivering it to the
bottom of the transformer• Using enough heating and insulation to achieve an exit air
temperature of within 20 Deg C of the inletBenefits:• Can be used on transformers that can’t take vacuum.Limitations:• Not highly efficient• Oxygen accelerates aging of the insulation• Does not do a good job of removing water from deep within the
insulation.
Methods of Drying – Oil Circulation
Oil Circulation involves:• Continuously circulating the entire volume of the
oil in the transformer through the purifier then back into the transformer.
• Oil is dried in the purifier vacuum chamber, then put back into the transformer where it picks up moisture from the insulation.
• Main Limitation: very slow
Oil Circulation involves:• Continuously circulating the entire volume of the
oil in the transformer through the purifier then back into the transformer.
• Oil is dried in the purifier vacuum chamber, then put back into the transformer where it picks up moisture from the insulation.
• Main Limitation: very slow
Methods of Drying – Vacuum Only
The Vacuum Only Technique:• Is used primarily to remove surface moisture adsorbed after the original
dryout of a unit. This would include water from the atmosphere or from human perspiration and respiration during inspection or assembly.
Precautions:• **It is important not to apply high vacuum suddenly on wet transformers
to prevent damage to the insulation caused by rapidly boiling off of water.
• The lowest temperature recommended for vacuum drying without heat is usually 10 Deg C for less than EHV (<345 kV) units and 20 Deg C for EHV to prevent ice formation
• Very good vacuums must be achieved because water removal is lessefficient at low temperatures because the vapor pressure of the water is so low.
The Vacuum Only Technique:• Is used primarily to remove surface moisture adsorbed after the original
dryout of a unit. This would include water from the atmosphere or from human perspiration and respiration during inspection or assembly.
Precautions:• **It is important not to apply high vacuum suddenly on wet transformers
to prevent damage to the insulation caused by rapidly boiling off of water.
• The lowest temperature recommended for vacuum drying without heat is usually 10 Deg C for less than EHV (<345 kV) units and 20 Deg C for EHV to prevent ice formation
• Very good vacuums must be achieved because water removal is lessefficient at low temperatures because the vapor pressure of the water is so low.
Vacuum – Leak Test
What is a leak test:– It is a check of the
transformer main tank, radiators, bushings etc. to verify that they will hold vacuum.
What is a leak test:– It is a check of the
transformer main tank, radiators, bushings etc. to verify that they will hold vacuum.
Vacuum – Leak Test
When to Perform a Leak Check?:• Prior to vacuum filling a transformer.
The fill process will not proceed until the leak test is acceptable.
Why?• To assure that rain, humidity or other
contamination is not drawn into the tank by vacuum to contaminate the environment.
When to Perform a Leak Check?:• Prior to vacuum filling a transformer.
The fill process will not proceed until the leak test is acceptable.
Why?• To assure that rain, humidity or other
contamination is not drawn into the tank by vacuum to contaminate the environment.
Typical Leak Test
Typical Procedure:• Apply vacuum and pull down to 1000 microns (1 Torr).• Valve off vacuum hose at the transformer.• Monitor vacuum level for 30 minutes and log reading
every 5 minutes.• After 30 minutes examine the total vacuum lost during
the test.• Acceptable loss is ≤ 700 microns for a new
transformer.• Acceptable loss is ≤ 1300 microns for a used or rebuilt
transformer.
Typical Procedure:• Apply vacuum and pull down to 1000 microns (1 Torr).• Valve off vacuum hose at the transformer.• Monitor vacuum level for 30 minutes and log reading
every 5 minutes.• After 30 minutes examine the total vacuum lost during
the test.• Acceptable loss is ≤ 700 microns for a new
transformer.• Acceptable loss is ≤ 1300 microns for a used or rebuilt
transformer.
Typical Automated Transformer Leak test
Methods of Drying – Heat and Vacuum
There are 3 common methods used to heat the transformer insulation:– Running current through the transformer windings while
under vacuum while pulling vacuum– Heating the oil in the transformer and then removing it
quickly to expose the insulation to vacuum– Lowering the oil level in the transformer and spraying a
heated portion of the oil over the insulation while pulling vacuum. This is the most widely used technique.
There are 3 common methods used to heat the transformer insulation:– Running current through the transformer windings while
under vacuum while pulling vacuum– Heating the oil in the transformer and then removing it
quickly to expose the insulation to vacuum– Lowering the oil level in the transformer and spraying a
heated portion of the oil over the insulation while pulling vacuum. This is the most widely used technique.
• Typical Manhole adapter with spray nozzle
• Typical vacuum connection using sudden pressure relief connection
(Vacuum only)
Use of Cold Traps in Drying Processes
Dry Ice and Acetone / Liquid Nitrogen
• Usually mounted on top of transformer• Uses either liquid nitrogen or a slurry or
dry ice and acetone to cool the thimble.• Water freezes to the thimble and
therefore can be collected for measurement.
• Protects vacuum pumps from water contamination and makes them more efficient since water is removed from the air stream before going through the pump.
Use of Cold Traps in Drying Processes
Cascade Cold Trap• Usually mounted in the trailer, in a
separate trailer or sits on the ground.
• Uses electric power and refrigeration to chill freeze plates.
• Water freezes to the plates providing the same benefits as the previous trap but without need for liquid N2, dry ice or acetone.
• Heated defrosting allows rapid ice thawing and speedy measurement.
Cold Traps - They’re not just for measuring moisture!
• One pound of water becomes 100 cubic feet of water vapor at 100 torr.
• One pound of water becomes 10,000 cubic feet of water vapor at 1 torr
• A cold trap condenses that vapor into a solid so that the vapor no longer has to be removed by the vacuum system.
Automated Moisture Monitor
The Automated Moisture Monitor is similar to the cold trap method except that the water is not collected. Instead, the moisture content of the air being pumped from the transformer, as well as the pumping speed and pressure, are used to calculate the number of pounds of water removed per hour plus a running total. If the amount of insulation and its initial moisture content are known, the amount of water removed can provide a sense of the insulation dryness.
Pros and Cons• Sometimes initial dryness and mass of insulation is difficult to
ascertain.• Equipment cost.• Benefit of having electronic monitoring and data gathering.
The Automated Moisture Monitor is similar to the cold trap method except that the water is not collected. Instead, the moisture content of the air being pumped from the transformer, as well as the pumping speed and pressure, are used to calculate the number of pounds of water removed per hour plus a running total. If the amount of insulation and its initial moisture content are known, the amount of water removed can provide a sense of the insulation dryness.
Pros and Cons• Sometimes initial dryness and mass of insulation is difficult to
ascertain.• Equipment cost.• Benefit of having electronic monitoring and data gathering.
Automated Moisture Monitor
On Line Systems
• Operate unattended while transformer in operation
• Take advantage of heat created by operation
• Can be used on non vacuum rated transformers
• Ideally suited for transformers where time is not of the essence.
On Line Systems
How wet is it still?
There are several methods of determining the estimated moisture content of a transformer that has undergone a drying process.
The following is a brief discussion of them most commonly used methods.
Vapor Pressure
Vapor Pressure measurement consists of isolating the transformer while under vacuum and monitoring vacuum decay over time (often 30 minutes) then using a Piper Chart to estimate water content.
Pros and Cons• Useful because can be made while a transformer is being
processed using vacuum gauges.• The measurement is an average for the entire system which
may not be a good indicator if the system is not in equilibrium.• Not always a good indicator of the dryness of the most
inaccessible locations in the insulation.
Vapor Pressure measurement consists of isolating the transformer while under vacuum and monitoring vacuum decay over time (often 30 minutes) then using a Piper Chart to estimate water content.
Pros and Cons• Useful because can be made while a transformer is being
processed using vacuum gauges.• The measurement is an average for the entire system which
may not be a good indicator if the system is not in equilibrium.• Not always a good indicator of the dryness of the most
inaccessible locations in the insulation.
Vapor Pressure
Typical Piper Chart
Rate of Water Collection in a Cold Trap
The Cold Trap method involves measuring the amount of water collected in a trap over a four-to-six hour period and the amount of water collected per hour is calculated. If the amount of insulation and its initial moisture content are known, the amount of water removed can provide a sense of the insulation dryness.
Pros and Cons• Sometimes initial dryness and mass of insulation is difficult to
ascertain.• Must have a cold trap available that efficiently traps the majority
of the water so it can be measured.• Collecting the water in a trap prevents it from collecting in the
vacuum pump.
The Cold Trap method involves measuring the amount of water collected in a trap over a four-to-six hour period and the amount of water collected per hour is calculated. If the amount of insulation and its initial moisture content are known, the amount of water removed can provide a sense of the insulation dryness.
Pros and Cons• Sometimes initial dryness and mass of insulation is difficult to
ascertain.• Must have a cold trap available that efficiently traps the majority
of the water so it can be measured.• Collecting the water in a trap prevents it from collecting in the
vacuum pump.
Power Factor
Power Factor has been used for many decades to assess the drying process of transformers using low voltage test or by the use of probes and models to assess the condition of the solid insulation.
Pros and Cons• Can be used for acceptance testing and at completion of field
drying after oil filling to ensure a dry unit.• Can be used to detect pockets of water that may be deeply
imbedded that may be missed by other methods that rely on equilibrium conditions to be accurate.
Power Factor has been used for many decades to assess the drying process of transformers using low voltage test or by the use of probes and models to assess the condition of the solid insulation.
Pros and Cons• Can be used for acceptance testing and at completion of field
drying after oil filling to ensure a dry unit.• Can be used to detect pockets of water that may be deeply
imbedded that may be missed by other methods that rely on equilibrium conditions to be accurate.
Vacuum Endpoint and Time
The Use of Vacuum Endpoint and Time consists of pulling vacuum until a decided vacuum endpoint is achieved and then maintaining that vacuum for a certain time period.
Pros and Cons• Low cost.• Must be sure transformer core temperature is
sufficient to prevent freezing otherwise a faulty indication will be given.
• Not very precise, especially from one transformer geometry to another.
The Use of Vacuum Endpoint and Time consists of pulling vacuum until a decided vacuum endpoint is achieved and then maintaining that vacuum for a certain time period.
Pros and Cons• Low cost.• Must be sure transformer core temperature is
sufficient to prevent freezing otherwise a faulty indication will be given.
• Not very precise, especially from one transformer geometry to another.
Water Content of the Oil
Water Content of the Oil is typically determined using a Karl Fischer tester or Panametrics hygrometer.
Pros and Cons• Requires equilibrium conditions.• The operating temperature has a tremendous effect
on the water content of the oil and therefore PPM values may be inadequate to determine the overall dryness of the insulation system.
Water Content of the Oil is typically determined using a Karl Fischer tester or Panametrics hygrometer.
Pros and Cons• Requires equilibrium conditions.• The operating temperature has a tremendous effect
on the water content of the oil and therefore PPM values may be inadequate to determine the overall dryness of the insulation system.
Dew Point
Dew Point measurement of the gas inside the transformer is used to provide an estimate of the average surface water content of the insulation after reaching steady state (equilibrium) conditions.
Pros and Cons• Requires equilibrium conditions.• Is done without oil in the transformer so must be
done on a new unit or after vacuum drying a unit.
Dew Point measurement of the gas inside the transformer is used to provide an estimate of the average surface water content of the insulation after reaching steady state (equilibrium) conditions.
Pros and Cons• Requires equilibrium conditions.• Is done without oil in the transformer so must be
done on a new unit or after vacuum drying a unit.
For additional information please contact Baron USA, Inc.
931 528 8476
www.baronusa.com