TRANSFORMER MAINTENANCE

1. PURPOSE

This document is to provide guidance to Bureau of Reclamation (Reclamation) powerplant personnel in maintenance, diagnostics, and testing of transformers and associated equipment. 

2. INTRODUCTION TO RECLAMATION TRANSFORMERS 

Transformers rated 500 kilovoltamperes (kVA) and above are considered power transformers. Reclamation has hundreds of power transformers with voltages as low as 480 volts (V) and as high as 550 kilovolts (kV). 

All generator step-up (GSU) transformers, and many station service, and excitation transformers are considered power transformers because they are rated 500 kVA or larger. 

Standards organizations such as American National Standards Institute/Institute of Electrical and Electronic Engineers (ANSI/IEEE) consider average GSU transformer life to be 20 to 25 years. This estimate is based on continuous operation at rated load and service conditions with an average ambient temperature of 40 °C (104 °F) and a temperature rise of 65 °C. This estimate is also based on the assumption that transformers receive adequate maintenance over their service life [24]. Reclamation, Bonneville Power Administration, and Western Area Power Administration conduct regular studies to determine statistical equipment life. These studies show that average life of a Reclamation transformer is 40 years. Reclamation gets longer service than IEEE estimates because we operate at lower ambient temperatures and with lower loads. A significant number of transformers were purchased in the 1940s, 1950s, and into the 1970s. Several have been replaced, but we have many that are nearing, or are already well past, their anticipated service life. We should expect transformer replacement and failures to increase due to this age factor. 

Current minimum replacement time is around 14 months; a more realistic time may be 18 months to 2 years. In the future, lead times may extend well beyond what they are today. Therefore, high quality maintenance and accurate diagnostics are important for all transformers, but absolutely essential for older ones—especially for critical transformers that would cause loss of generation. It is also very important to consider providing spares for critical transformers. 

3. TRANSFORMER COOLING METHODS 

Heat is one of the most common destroyers of transformers. Operation at only 10 °C above the transformer rating will cut transformer life by 50%. Heat is caused by internal losses due to loading, high ambient temperature, and solar radiation. It is important to under­ stand how your particular transformers are cooled and how to detect problems in the cooling systems. ANSI and IEEE require the cooling class of each transformer to appear on its nameplate. Cooling classifications, with short explanations, appear in sections 3.1 and 3.2. The letters of the class designate inside atmosphere and type or types of cooling. In some transformers, more than one class of cooling and load rating are indicated. At each 

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DRY TYPE TRANSFORMER MAINTENANCE SUMMARY 

See Section 3.1 

When new after energizing and allowing temperature and loading to stabilize 

After 1 week of operation at normal loading 

Annually 

(Note:  The time between these periodic inspections may be increased if the first internal inspection of windings and connections are found clean and in good condition and if loading is at or below nameplate rating.) 

Do an infrared scan and compare with temperature gage, if any.

If transformer is gas filled (nitrogen [N2]), check pressure gage against data

sheets; never allow gas pressure to fall below 1 pound per square inch

(psi).

Check loading and compare with nameplate rating.

Functionally test fans and controls for proper operation.

Functionally test temperature alarms and annunciator points.

Check area around transformer clear of debris and parts storage.

Check transformer room for proper ventilation.

Perform infrared scan and compare with temperature gage, if any. 

Check temperature gage, if any, and compare with nameplate rating. 

Check loading and compare with nameplate rating. 

Perform an infrared scan before de-energizing.

De-energize and remove panels for internal inspection.

Use vacuum to remove as much dirt as possible.

After vacuuming, use low pressure dry air (20 to 25 psi) to blow off

remaining dirt.  Caution:  Make sure air is dry.

Check for discolored copper and discolored insulation.

Check for corroded and loose connections.

Check for carbon tracking on insulation and insulators.

Check for cracked, chipped, and loose insulators.

If windings are found dirty, add filter material to air intake ports.

Check fan blades for cleanliness; remove dirt and dust.

Check fans, controls, alarms and annunciator points.

Check pressure gage on N2 filled transformers; compare with weekly data

sheets; never allow pressure to fall below 1 psi. 

Repair all problems found in above inspections.

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step of additional cooling, the rating increases to correspond with increased cooling. Note that the letter “A” indicates air, “FA” indicates forced air (fans), “O” indicates oil, “FO” indicates forced oil (pumps), “G” indicates some type of gas, and “W” indicates there is a water/oil heat exchanger. 

3.1 Dry Type Transformers 

Cooling classes of dry type transformers are covered by ANSI/IEEE standard C57.12.01 Section 5.1 [1]. A short explanation of each class is given below. 

1. Class AA are ventilated, self-cooled transformers. This means that there are ventilation ports located in outside walls of the transformer enclosure. There are no fans to force air into and out of the enclosure with typically no external fins or radiators. Cooler air enters the lower ports, is heated as it rises past windings, and exits the upper ventilation ports. (It will not be repeated below; but it is obvious that in every cooling class, some heat is also removed by natural circulation of air around the outside of the enclosure.) 

2. Class AFA transformers are self-cooled (A) and additionally cooled by forced circulation of air (FA). This means that there are ventilation ports for fan inlets and outlets only. (Inlets are usually filtered.) Normally, there are no additional ventilation ports for natural air circulation. 

3. Class AA/FA transformers are ventilated, self-cooled (same as Class AA in item 1). In addition, they have a fan or fans providing additional forced-air cooling. Fans may be wired to start automatically when the temperature reaches a pre-set value. These transformers generally have a dual load rating, one for AA (self-cooling natural air flow) and a larger load rating for FA (forced air flow). 

4. Class ANV transformers are self-cooled (A), non-ventilated (NV) units. The enclosure has no ventilation ports or fans and is not sealed to exclude migration of outside air, but there are no provisions to intentionally allow outside air to enter and exit. Cooling is by natural circulation of air around the enclosure. This transformer may have some type of fins attached outside the enclosure to increase surface area for additional cooling. 

5. Class GA transformers are sealed with a gas inside (G) and are self-cooled (A). The enclosure is hermetically sealed to prevent leakage. These transformers typically have a gas, such as nitrogen or freon, to provide high dielectric and good heat removal. Cooling occurs by natural circulation of air around the outside of the enclosure. There are no fans to circulate cooling air; however, there may be fins attached to the outside to aid in cooling. 

3.1.1 Potential Problems and Remedial Actions for Dry Type Transformer Cooling Systems [14]. It is important to keep transformer enclosures reasonably clean. It is also important to keep the area around them clear. Any items near or against the transformer impede heat transfer to cooling air around 

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the enclosure. As dirt accumulates on cooling surfaces, it becomes more and more difficult for air around the transformer to remove heat. As a result, over time, the transformer temperature slowly rises unnoticed, reducing service life. 

Transformer rooms and vaults should be ventilated. Portable fans (never water) may be used for additional cooling if necessary. A fan rated at about 100 cubic feet per minute (cfm) per kilowatt (kW) of transformer loss [5], located near the top of the room to remove hot air, will suffice. These rooms/vaults should not be used as storage. 

When the transformer is new, check the fans and all controls for proper operation. After it has been energized and the loading and temperature are stable, check the temperature with an infrared (IR) camera and compare loading with the nameplate. Repeat the temperature checks after 1 week of operation. 

Once each year under normal load, check transformer temperatures with an IR camera [4,7]. If the temperature rise (above ambient) is near or above nameplate rating, check for overloading. Check the temperature alarm for proper operation. Check enclosures and vaults/rooms for dirt accumulation on transformer surfaces and debris near or against enclosures. Remove all items near enough to affect air circulation. To avoid dust clouds, a vacuum should first be used to remove excess dirt. Low pressure (20 to 25 pounds per square inch [psi]) dry compressed air may be used for cleaning after most dirt has been removed by vacuum. The transformer must be de-energized before this procedure unless it is totally enclosed and there are no exposed energized conductors. Portable generators may be used for lighting. 

After de-energizing the transformer, remove access panels and inspect windings for dirt- and heat-discolored insulation and structure problems [14]. It is important that dirt not be allowed to accumulate on windings because it impedes heat removal and reduces winding life. A vacuum should be used for the initial winding cleaning, followed by compressed air [7]. Care must be taken to ensure the compressed air is dry to avoid blowing moisture into windings. Air pressure should not be greater than 20 to 25 psi to avoid imbedding small particles into insulation. After cleaning, look for discolored copper and insulation, which indicates overheating. If discoloration is found, check for loose connections. If there are no loose connections, check the cooling paths very carefully and check for overloading after the transformer has been re-energized. Look for carbon tracking and cracked, chipped, or loose insulators. Look for and repair loose clamps, coil spacers, deteriorated barriers, and corroded or loose connections. 

Check fans for proper operation including controls, temperature switches, and alarms. Clean fan blades and filters if needed. A dirty fan blade or filter reduces cooling air flow over the windings and reduces service life. If ventilation ports do not have filters, they may be fabricated from home-furnace filter material. Adding filters is only necessary if the windings are dirty upon yearly inspections. 

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OIL-FILLED TRANSFORMER MAINTENANCE SUMMARY 

Task After 1 Month of Service Annually 3 to 5 Years 

Before energizing, inspect and test all controls, wiring, fans alarms, and gages. 

Indepth inspection of transformer and cooling system, check for leaks and 

Oil pumps load current, oil flow indicators, fans, etc. See 3.2.5, 3.2.6, and 4.1. Thermometers  4.1.2 and 3. 

Oil pumps load current, oil flow indicators, fans etc, see 3.2.5, 3.2.6 and 4.1 Thermometers  4.1.2 and 3, 

Check diaphragm or bladder for leaks if you have conservator.  See 4.2.2. 

proper operation. Heat exchangers. Transformer tank 4.1.1. 

heat exchangers Transformer tank 4.1.1 

Do a DGA. Oil level gages 4.1.4. Pressure relief 4.1.5. 

Oil level gages 4.1.4 Pressure relief 4.1.5 Do a DGA. Do a DGA 

IR scan of See 3.2.5 and 4.1.8. See 3.2.5 and 4.1.8. transformer cooling system, bushings and all wiring. 

Test all controls, See 3.2.5, 4.1.4, 4.1.5. See 3.2.5 Thermometers.  See 4.1.3. relays, gages; test alarms and annunciator points. Inspect pressure relief for leaks and indication for operation (rod extension) see 4.1.5 Oil level gages  4.1.4. Inspect pressure relief 4.1.5. Sudden pressure relay 4.1.6. Buchholz relay 4.1.7. Test alarms, fan and pump controls, etc. See 3.2.6. 

Inspect transformer bushings. 

Check with binoculars for cracks and chips; look for oil leaks and check oil 

check with binoculars for cracks and chips, look carefully for oil leaks and check oil levels. levels IR Scan See 4.1.8 IR scan.  See 4.1.8. 

Indepth inspection of bushings, cleaning waxing if needed. 

Close physical inspection, cleaning/ waxing, and Doble testing, plus checks in boxes above left.  See 4.1.8. 

Doble test Doble test transformer and See 4.1.8 and 4.7. transformer and bushings before bushings. energizing. See 4.1.8, 4.7. 

Inspect pressure See 4.2.2. See 4.2.2. controls if you have a Also see 4.2.1 to test pressure nitrogen over oil gage if trans has N2  over oil transformer. with no means to automatically Inspect pressure add N2. gage. 

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Check pressure gages by looking at the weekly data sheets; if pressure never varies with temperature changes, the gage is defective. Never allow the pressure to go below about 1 psi during cold weather. Add nitrogen to bring the pressure up to 2½ to 3 psi to insure that moist air will not be pulled in. 

3.2 Liquid-Immersed Transformers 

Cooling classes of liquid-immersed transformers are covered by IEEE C57.12.00 Section 5.1 [2]. A short explanation of each class follows: 

3.2.1 Liquid-Immersed, Air-Cooled. There are three classes in this category. 

1. Class OA: Oil-immersed, self-cooled. Transformer windings and core are immersed in some type of oil and are self-cooled by natural circulation of air around the outside enclosure. Fins or radiators may be attached to the enclosure to aid in cooling. 

2. Class OA/FA: Liquid-immersed, self-cooled/forced air-cooled. Same as OA above, with the addition of fans. Fans are usually mounted on radiators. The transformer typically has two load ratings, one with the fans off (OA) and a larger rating with fans operating (FA). Fans may be wired to start automatically at a pre-set temperature. 

3. Class OA/FA/FA: Liquid-immersed, self-cooled/forced air-cooled/forced air-cooled. Same as OA/FA above with an additional set of fans. There typically will be three load ratings corresponding to each increment of cooling. Increased ratings are obtained by increasing cooling air over portions of the cooling surfaces. Typically, there are radiators attached to the tank to aid in cooling. The two groups of fans may be wired to start automatically at pre-set levels as temperature increases. There are no oil pumps. Oil flow through the transformer windings is by the natural principle of Figure 1.—Typical Oil Flow. convection (heat rising). 

3.2.2 Liquid-Immersed, Air-Cooled/Forced Liquid-Cooled. There are two classes in this group. 

1. Class OA/FA/FOA: Liquid-immersed, self-cooled/forced air-cooled/forced liquid, and forced air-cooled. Windings and core are immersed in some type of oil. This transformer typically has radiators attached to the enclosure. The transformer has self-cooling (OA) natural ventilation, forced air-cooling FA (fans), and forced oil-cooling (pumps) with additional forced air-cooling (FOA) (more fans). The transformer has three load ratings corresponding to each cooling step. Fans and pumps may be wired to start automatically at pre-set levels as temperature increases. 

2. Class OA/FOA/FOA: Liquid-immersed, self-cooled/forced oil, and forced air- cooled/forced oil, and forced air-cooled. Cooling controls are arranged to start only part of the oil pumps and part of the fans for the first load rating/temperature increase, and the remaining pumps and fans for the second load rating increase. The nameplate will show at least three load ratings. 

3.2.3 Liquid-Immersed, Water-Cooled. This category has two classes. 

1. Class OW: Transformer coil and core are immersed in oil. Typically a oil/water heat exchanger (radiator) is attached to the outside of the tank. Cooling water is pumped through the heat exchanger, but the oil flows only by natural circulation. As oil is heated by the windings, it rises to the top and exits through piping to the radiator. As it is cooled, the oil descends through the radiator and re-enters the transformer tank at the bottom. 

2. Class OW/A: Transformer coil and core are immersed in oil. This transformer has two ratings. Cooling for one rating (OW) is obtained as in 1 above. The self- cooled rating (A) is obtained by natural circulation of air over the tank and cooling surfaces. 

3.2.4 Liquid-Immersed, Forced Liquid-Cooled. This category has two classes. 

1. Class FOA: Liquid-immersed, forced liquid-cooled with forced air-cooled. This transformer normally has only one rating. The transformer is cooled by pumping oil (forced oil) through a radiator normally attached to the outside of the tank. Also, air is forced by fans over the cooling surface. 

2. Class FOW: Liquid-immersed, forced liquid-cooled, water cooled. This transformer is cooled by an oil/water heat exchanger normally mounted separately from the tank. Both the transformer oil and the cooling water are pumped (forced) through the heat exchanger to accomplish cooling. 

3.2.5 Potential Problems and Remedial Actions for Liquid Filled Transformer Cooling Systems. 

Leaks. Tanks and radiators may develop oil leaks, especially at connections. To repair a leak in a radiator core, you must remove the radiator. Small leaks may also develop in headers or individual pipes. These small leaks possibly may be stopped by peening with a ball peen hammer. Some manufacturer’s field personnel try to stop leaks by using a two-part epoxy while the transformer is under vacuum. Do not try this unless the transformer has been drained, because a vacuum may cause bubbles to form in the oil that can lodge in the winding and cause arcing. When all else fails, the leak may be welded with oil still in the radiator, if proper precautions are carefully observed [3, 4]. Welding with oil inside will cause gases to form in the oil. Take an oil sample for a dissolved gas analysis (DGA) before welding and 24 hours after re-energizing to identify gas increases due to welding. If the leak is bad enough, the tank may have to be drained so the leak can be repaired. Treat leaks carefully; do not ignore them. Oil leaks are serious maintenance and environmental issues and should be corrected. Radiators may need to be cleaned in areas where deposits appear on pipes and headers. Dirt and deposits hamper heat transfer to the cooling air. Finned radiators must be cleaned with compressed air when they become dirty. 

Plugs. After 1 month of service and yearly, perform an IR scan and physical inspection of radiators and transformer tanks [4,7]. Partially plugged radiators will be cooler than those performing normally. You may also feel the radiator pipes by hand. Plugged radiator sections or individual pipes/plenums will be noticeably cooler; however, you will not be able to reach all of them. Radiators may become plugged with sludge or foreign debris; this usually occurs in water tubes on the oil/water heat exchanger. Do not forget to check the bleed line for two-walled heat exchangers. 

If plugged radiators are discovered, they need to be corrected as soon as possible. Some radiators are attached to the main tank with flanges and have isolating valves. These may be removed for cleaning and/or leak repair without draining oil from the transformer. If radiators are attached directly to the main tank, oil must be drained before cleaning them. If radiators are plugged with sludge, chances are the transformer is sludged up also. In this case, the oil should be reprocessed and the transformer cleaned internally. Competent contractors should be obtained if this is necessary. 

Sludge. If temperature seems to be slowly increasing while the transformer is operating under the same load, check the DGA for moisture, oxygen, and the interfacial tension (IFT). The combination of oxygen and moisture causes sludging, which may be revealed by a low IFT number. Sludge will slowly build up on windings and core, and the temperature will increase over time. 

Valve Problems. If your transformer has isolating valves for radiators, check to make sure they are fully open on both top and bottom of the radiators. A broken valve stem may cause the valve to be fully or partially closed, but it will appear that the valve is open. 

Mineral Deposits. Don’t even think about spraying water on the radiators or tank to increase cooling except in the most dire emergency. Minerals in the water will deposit on radiators as water evaporates and are almost impossible to remove. These minerals will reduce the efficiency of cooling still further. Additional fans blowing on radiators and/or transformer tank is a better alternative [4]. 

One IR scan performed on a transformer running at higher than normal temperature revealed that the oil level was below the upper radiator inlet pipe, which prevented oil circulation. The oil level indicator was defective and stuck on normal. These indicators must be tested as mentioned below. 

3.2.6 Cooling System Inspections. After 1 month of service and yearly, inspect and test the fans. Look at the fans anytime you are around transformers in the switchyard or in the powerplant. If it is a hot day and transformers are loaded, all the fans should be running. If a fan is stopped and the rest of the group is running, the inactive fan should be repaired. During an inspection, the temperature controller should be adjusted to start all the fans. Listen for unusual noises from fan bearings and loose blades and repair or replace faulty fans. Bad bearings can also be detected with an IR scan if the fans are running. 

After 1 month of service and yearly, inspect and test the oil pumps. Inspect piping and connections for leaks. Override the temperature controller so that the pump starts. Check the oil pump motor current on all three phases with an accurate ammeter; this will give an indication if oil flow is correct and if unusual wear is causing additional motor loading. Record this information for later comparison, especially if there is no oil flow indicator. If the motor load current is low, something is causing low oil flow. Carefully inspect all valves to make sure they are fully open. A valve stem may break and leave the valve partially or fully closed, even though the valve handle indicates the valve is fully open. Pump impellers have been found loose on the shaft, reducing oil flow. Sludge buildup or debris in lines can also cause low oil flow. If motor load current is high, this may indicate impeded pump rotation. Listen for unusual noises. Thrust bearing wear results in the impeller advancing on the housing. An impeller touching the housing makes a rubbing sound which is different from the sound of a failing motor bearing. If this is heard, remove the pump motor from the housing and check impeller clearance. Replace the thrust bearing if needed, and replace the motor bearings if the shaft has too much play or if noise is unusual. 

Three phase pumps will run and pump some oil even when they are running backwards. Vane type oil-flow meters will indicate flow on this low amount. The best indication of this is that sometimes the pump will be very noisy. The motor load current may also be lower than for full load. If this is suspected due to the extra noise and higher transformer temperature, the pump should be checked for proper rotation. Reverse two phase leads if this is encountered.[4] 

After 1 month of service and yearly, check the oil flow indicator. It has a small paddle which extends into the oil stream and may be either on the suction or discharge side of the pump. A low flow of only about 5 feet per second velocity causes the flag to rotate. Flow can be too low, and the indicator will still show flow. If there is no flow, a spring returns the flag to the off position and a switch provides an alarm. With control power on the switch, open the pump circuit at the motor starter and make sure the correct alarm point activates when the pump stops. Check that the pointer is in the right position when the pump is off and when it is running. Pointers can stick and fail to provide an alarm when needed. Oil flow may also be checked with an ultrasonic flow meter. Ultrasonic listening devices can detect worn bearings, rubbing impellers, and other unusual noises from oil pumps. 

Pumps can pull air in through gaskets on the suction side of the pumps. The suction (vacuum) on the intake side of the pump can pull air through gaskets that are not tight. Pump suction has also been known to pull air through packing around valve stems, in the suction side piping. This can result in dangerous bubbles in the transformer oil and may cause the gas detector or Buchholz relay to operate. Dissolved gas analysis will show a big increase in oxygen and nitrogen content [4]. High oxygen and nitrogen content can also be caused by gasket leaks elsewhere. 

After 1 month of service and yearly, inspect water-oil heat exchangers. Test and inspect the pumps as mentioned above. Look for and repair leaks in piping and heat exchanger body. Examine the latest dissolved gas analysis results for dissolved moisture and free water. If free water is present and there are no gasket leaks, the water portion of the water-oil heat exchanger must be pressure tested. A leak may have developed, allowing water to migrate into the transformer oil, which can destroy the transformer. If the heat exchanges piping is double-walled, check the drain for water or oil; check manufacturer’s instruction manual.