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
DRY TYPE TRANSFORMER MAINTENANCE SUMMARY
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.
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.
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 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.
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
DRY TYPE TRANSFORMER MAINTENANCE SUMMARY
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.
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.
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 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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