Field Processing Methods for Transformer Dry out by Derek Baranowski

Transcription

1 Field Processing Methods for Transformer Dry out by Derek Baranowski

2 Importance of Removing Water from the Insulation 1. If water is removed from the oil only: A large percentage of the total water in the system will 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.

3 Factors Affecting Water Content of the Insulation Water may be be adsorbed in in the solid insulation from: Exposure to to atmosphere during installation and inspections. From aging of of the paper. From leaks. Ingress through the oil oil expansion system. Water remaining from the manufacturing of of the transformer may also vary.

4 Where is the water? 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.

5 Factors Affecting the Rate of Insulation Drying Things We Can t Change In In a transformer there are are generally different insulation elements, differing in in thickness and and nature (board, paper tape tape or or sheet). Drying out out times in in the the inside layers depend on on the the nature of of the the material and and on on the the path path along which the the water must travel to to reach the the surface. Water is is removed from from the the exterior surfaces first first and and sucked away by by the the vacuum pumps. Water in in the the inside layers migrates to to the the surface (called diffusion).

6 Factors Affecting the Rate of Insulation Drying Things We Can Change Speed and efficiency at at which heat is is added to to the system. Minimizing heat loss to to the surroundings. The pumping speed of of the vacuum pumping system. Size of of the vacuum hose. Vapor traps to to trap moisture and oil oil from entering the vacuum pumps.

7 Methods of Drying Transformer Insulation Hot Air. Oil Circulation.. Vacuum Only (with or or without a cold trap). Heat and Vacuum (with or or without a cold trap). On Line Systems

8 Temperature the key to success! Why Add Heat? For the water in in the transformer to to turn into vapor (change phase) heat is is required just to to allow the phase change. The moisture leaving the transformer takes this heat with it. it.

9 Temperature the key to success! Why We Add Heat It It takes btu to to turn 1 lb lb of of ice into water It It takes btu to to turn 1 lb lb of of water into steam It It takes 1220 btu to to turn 1 lb lb of of ice directly into steam (sublimation) (All (All values values for for atmospheric atmospheric pressure) pressure)

10 Temperature the key to success! Why We Add Heat The The heat heat required for for the the phase change of of the the water in in the the transformer comes from from the the surrounding oil oil and and transformer parts. As As heat heat moves from from the the oil oil into into the the water, the the temperature of of the the oil oil and and transformer drops. Therefore replacement heat heat is is added by by continuously reheating the the oil oil and and pumping it it into into the the transformer. If If the the heat heat is is not not replaced, eventually freezing can can occur.

11 Temperature the key to success! Why We Add Heat Once freezing occurs, water must go go from from solid solid to to vapor without becoming liquid (sublimation). Sublimation requires more heat heat than than vaporization so so temperature continues to to drop drop until until the the process stalls. When this this happens the the vacuum will will get get very very good since no no water vapor is is being pumped. When the the transformer is is returned to to service, the the ice ice will will melt melt and and the the insulating system will will test test wet. Haley s Comet A ball of frozen ice flying through space for thousands of years because in the absence of heat, sublimation is a very slow process.

12 Temperature the key to success! Why We Add Heat If If some heat is 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.

13 The Methods

14 Methods of Drying Hot Air The The Hot Hot Air Air Method involves: Heating the the inlet inlet air air to to Deg Deg C and and delivering it it to to the the bottom of of the the transformer Using enough heating and and insulation to to achieve an an exit exit air air temperature of of within Deg Deg C of of the the inlet inlet Benefits: Can Can be be used on on transformers that that can t take take vacuum. Limitations: Not Not highly efficient Oxygen accelerates aging of of the the insulation Does not not do do a good job job of of removing water from from deep within the the insulation.

15

16 Methods of Drying Oil Circulation Oil Circulation involves: Continuously circulating the entire volume of of the oil oil in in the transformer through the purifier then back into the transformer. Oil Oil is is dried in in the purifier vacuum chamber, then put back into the transformer where it it picks up up moisture from the insulation. Main Limitation: very slow

17

18 Methods of Drying Vacuum Only The The Vacuum Only Technique: Is Is used primarily to to remove surface moisture adsorbed after after the the original dryout of of a unit. unit. This This would include water from from the the atmosphere or or from from human perspiration and and respiration during inspection or or assembly. Precautions: **It **It is is important not not to to apply high high vacuum suddenly on on wet wet transformers to to prevent damage to to the the insulation caused by by rapidly boiling off off of of water. The The lowest temperature recommended for for vacuum drying without heat heat is is usually Deg Deg C for for less less than than EHV EHV (<345 kv) kv) units and and Deg Deg C for for EHV EHV to to prevent ice ice formation Very Very good vacuums must be be achieved because water removal is is less less efficient at at low low temperatures because the the vapor pressure of of the the water is is so so low. low.

19

20 Vacuum Leak Test What is is a leak test: It It is is a check of of the transformer main tank, radiators, bushings etc. to to verify that they will hold vacuum.

21 Vacuum Leak Test When to Perform a Leak Check?: Prior to to vacuum filling a transformer. The fill process will not proceed until the leak test is is acceptable. Why? To assure that rain, humidity or or other contamination is is not drawn into the tank by vacuum to to contaminate the environment.

22 Typical Leak Test Typical Procedure: Apply vacuum and pull down to to 1000 microns (1 (1 Torr). Valve off off vacuum hose at at the transformer. Monitor vacuum level for for minutes and log reading every 5 minutes. After minutes examine the total vacuum lost during the test. Acceptable loss is is 700 microns for for a new transformer. Acceptable loss is is 1300 microns for for a used or or rebuilt transformer.

23 Typical Automated Transformer Leak test

24 Methods of Drying Heat and Vacuum There are 3 common methods used to to heat the transformer insulation: Running current through the the transformer windings while under vacuum while pulling vacuum Heating the the oil oil in in the the transformer and and then then removing it it quickly to to expose the the insulation to to vacuum Lowering the the oil oil level level in in the the transformer and and spraying a heated portion of of the the oil oil over over the the insulation while pulling vacuum. This This is is the the most widely used technique.

25

26 Typical vacuum connection using sudden pressure relief connection (Vacuum only) Typical Manhole adapter with spray nozzle

27 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.

28 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.

29 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.

30 Automated Moisture Monitor The The Automated Moisture Monitor is is similar to to the the cold cold trap trap method except that that the the water is is not not collected. Instead, the the moisture content of of the the air air being pumped from from the the transformer, as as well well as as the the pumping speed and and pressure, are are used to to calculate the the number of of pounds of of water removed per per hour hour plus plus a running total. If If the the amount of of insulation and and its its initial moisture content are are known, the the amount of of water removed can can provide a sense of of the the insulation dryness. Pros Pros and and Cons Sometimes initial dryness and and mass of of insulation is is difficult to to ascertain. Equipment cost. Benefit of of having electronic monitoring and and data data gathering.

31 Automated Moisture Monitor

32 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.

33 On Line Systems

34 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.

35 Vapor Pressure Vapor Pressure measurement consists of of isolating the the transformer while under vacuum and and monitoring vacuum decay over over time time (often minutes) then then using a Piper Chart to to estimate water content. Pros Pros and and Cons Useful because can can be be made while a transformer is is being processed using vacuum gauges. The The measurement is is an an average for for the the entire system which may may not not be be a good indicator if if the the system is is not not in in equilibrium. Not Not always a good indicator of of the the dryness of of the the most inaccessible locations in in the the insulation.

36 Vapor Pressure Typical Piper Chart

37 Rate of Water Collection in a Cold Trap The The Cold Trap method involves measuring the the amount of of water collected in in a trap trap over over a four-to-six hour hour period and and the the amount of of water collected per per hour hour is is calculated. If If the the amount of of insulation and and its its initial moisture content are are known, the the amount of of water removed can can provide a sense of of the the insulation dryness. Pros Pros and and Cons Sometimes initial dryness and and mass of of insulation is is difficult to to ascertain. Must have a cold cold trap trap available that that efficiently traps the the majority of of the the water so so it it can can be be measured. Collecting the the water in in a trap trap prevents it it from from collecting in in the the vacuum pump.

38 Power Factor Power Factor has has been used for for many decades to to assess the the drying process of of transformers using low low voltage test test or or by by the the use use of of probes and and models to to assess the the condition of of the the solid solid insulation. Pros Pros and and Cons Can Can be be used for for acceptance testing and and at at completion of of field field drying after after oil oil filling to to ensure a dry dry unit. unit. Can Can be be used to to detect pockets of of water that that may may be be deeply imbedded that that may may be be missed by by other methods that that rely rely on on equilibrium conditions to to be be accurate.

39 Vacuum Endpoint and Time The Use of of Vacuum Endpoint and Time consists of of pulling vacuum until a decided vacuum endpoint is is achieved and then maintaining that vacuum for for a certain time period. Pros and Cons Low cost. Must be be sure transformer core temperature is is sufficient to to prevent freezing otherwise a faulty indication will be be given. Not very precise, especially from one transformer geometry to to another.

40 Water Content of the Oil Water Content of of the Oil is is typically determined using a Karl Fischer tester or or Panametrics hygrometer. Pros and Cons Requires equilibrium conditions. The operating temperature has a tremendous effect on on the water content of of the oil oil and therefore PPM values may be be inadequate to to determine the overall dryness of of the insulation system.

41 Dew Point Dew Point measurement of of the gas inside the transformer is is used to to provide an an estimate of of the average surface water content of of the insulation after reaching steady state (equilibrium) conditions. Pros and Cons Requires equilibrium conditions. Is Is done without oil oil in in the transformer so so must be be done on on a new unit or or after vacuum drying a unit.

42 For additional information please contact Baron USA, Inc