4.2 Oil Preservation Sealing Systems 

The purpose of sealing systems is to prevent air and moisture from contaminating oil and cellulose insulation. Sealing systems are designed to prevent oil inside the transformer from coming into contact with air. Air contains moisture, which causes sludging and an abundant supply of oxygen. Oxygen in combination with moisture causes greatly accelerated deterioration of the cellulose. This oxygen-moisture combination will greatly reduce service life of the transformer. 

Sealing systems on many existing Reclamation power transformers are of the inert gas (nitrogen) pressure design; however, we have many other designs. Current practice is to buy only conservator designs with bladders for transformer voltages 115 kV and above and capacities above 10 mega-volt- amps (mva). Below these values, we buy only inert gas pressure system transformers, Some of the sealing systems are explained below. There may be variations of each design, and not every design is covered. The order below is roughly from earlier to more modern. 

4.2.1 Sealing Systems Types. 

Free Breathing. Sealing systems have progressed from early designs of “free breathing” tanks, in which Figure 6.—Free Breathing an air space on top of the oil is Transformer. vented to atmosphere through a breather pipe. The pipe typically is screened to keep out insects and rodents and turned down to prevent rain from entering. Breathing is caused by expansion and contraction of the oil as temperature changes. These earlier designs did not use an air dryer, and condensation from moisture formed on inside walls and tank top. Moisture, oxygen, and nitrogen would also dissolve directly into oil from the air. This was not the best design. As mentioned before, a combination of oxygen and moisture accelerates deterioration of cellulose insulation. Moisture also decreases dielectric strength, destroying insulating quality of the oil, and causes formation of sludge. If you have one or more of these earlier design transformers, it is recommended that a desiccant type air dryer be added to the breather pipe. Sealed or Pressurized Breathing. This design is similar to the free breathing one with addition of a pressure/vacuum bleeder valve. When the transformer was installed, pressurized dry air or nitrogen was placed on top of the oil. The bleeder valve is designed to hold pressure inside to approximately plus or minus 5 psi . The Pressurized Breathing same problems with Transformer. moisture and oxygen occur as previously described. Problems are not as severe because “breathing” is limited by the bleeder valve. Air or N2 is exhausted to the outside atmosphere when a positive pressure more than 5 psi occurs inside the tank. This process does not add moisture and oxygen to the tank. However, when cooling, the oil contracts and, if pressure falls 5 psi below the outside atmosphere, the valve allows outside air into the tank, which pulls in moisture and oxygen. 

Once each year, check the pressure gage against the weekly data sheets; if the pressure never varies with seasonal temperature changes, the gage is defective. Add nitrogen if the pressure falls below 1 psi to keep moisture laden air from being pulled in. Add enough N2 to bring the pressure to 2 to 3 psi. 

Pressurized Inert Gas Sealed System. This system keeps space above the oil pressurized with a dry inert gas, normally nitrogen (figure 8). This design prevents air and moisture from coming into contact with insulating oil. Pressure is maintained by a nitrogen gas bottle with the pressure regulated normally between 0.5 and 5 psi. Pressure gages are provided in the nitrogen cubicle for both high and low pressures (figure 9). A pressure/ vacuum gage is normally connected to read low pressure gas inside the tank.  This gage may be located on the transformer and normally has high and low pressure alarm contacts.  See section 4.2.2 which follows.

Caution:  When replacing nitrogen cylinders, do not just order a “nitrogen cylinder” from the local welding supplier.  Nitrogen for transformers should meet ASTM D-1933 Type III with - 59 °C dew point as specified in IEEE C-57.12.00-1993, paragraph 6.6.3  

4.2.2  Gas Pressure Control Components.  After 1 month of service and yearly, inspect the gas pressure control components.  There is normally an adjustable, three-element pressure control system for inert gas, which maintains a pressure range of 0.5 to 5 psi in the transformer tank.  There is also a bleeder valve that  exhausts gas to atmosphere when pressure exceeds relief pressure of the valve, normally 5 to 8 psi. 

Caution:  The component part descriptions below are for the typical three- stage pressure regulating equipment supplying inert gas to the transformer.  Your particular unit may be different, so check your transformer instruction manual.  

High Pressure Gage. The high pressure gage is attached between the nitrogen cylinder and high pressure regulator that indicates cylinder pressure.  When the cylinder is full, the gage will read approximately

2,400 psi. Normally, the gage will be equipped with a low pressure alarm that activates when the cylinder is getting low (around 500 psi). However, gas will still be supplied, and the regulating equipment will continue to function until the cylinder is empty. Refer to figure 9 for the following descriptions. 

High Pressure Regulator. The high pressure regulator has two stages. The input of the first stage is connected to the cylinder, and the output of the first stage is connected internally to the input of the second stage. This holds output pressure of the second stage constant. The first stage output is adjustable by a hand-operated lever and can deliver a maximum of whatever pressure is in the cylinder (2,400 psi when full) down to zero. The second stage output is varied by turning the adjusting screw, normally adjusted to supply approximately 10 psi to the input of the low pressure regulator. 

Low Pressure Regulator. The low pressure regulator is the third stage and controls pressure and flow to the gas space of the transformer. The input of this regulator is connected to the output of the second stage (approximately 10 psi). This regulator is typically set at the factory to supply gas to the transformer at a pressure of approximately 0.5 psi and needs no adjustment. If a different pressure is required, the regulator can be adjusted by varying spring tension on the valve diaphragm. Pressure is set at this low value because major pressure changes inside the transformer come from expansion and contraction of oil. The purpose of this gas feed is to make up for small leaks in the tank gaskets and elsewhere so that air cannot enter. Typically, a spring-loaded bleeder for high pressure relief is built into the regulator and is set at the factory to relieve pressures in excess of 8 psi. The valve will close when pressure drops below the setting, preventing further loss of gas. 

Bypass Valve Assembly. The bypass valve assembly opens a bypass line around the low pressure regulator and allows the second stage of the high pressure regulator to furnish gas directly to the transformer. The purpose of this assembly is to allow much faster filling/purging of the gas space during initial installation or if the transformer tank has to be refilled after being opened for inspection. 

Caution: During normal operation, the bypass valve must be closed, or pressure in the tank will be too high. 

Oil Sump. The oil sump is located at the bottom of the pressure regulating system between the low pressure regulator and shutoff valve C. The sump collects oil and/or moisture that may have condensed in the low pressure fill line. The drain plug at the bottom of the sump should be removed before the system is put into operation and also removed once each year during operation to drain any residual oil in the line. This sump and line will be at the same pressure as the gas space in the top of the transformer. The sump should always be at a safe pressure (less than 10 psi) so the plug can be removed to allow the line to purge a few seconds and blow out the oil. However, always look at the gas space pressure gauge on the transformer or the low pressure gauge in the nitrogen cabinet, just to be sure, before removing the drain plug. 

Shutoff Valves. The shutoff valves are located near the top of the cabinet for the purpose of isolating the transformer tank for shipping or maintenance. These valves are normally of double-seat construction and should be fully opened against the stop to prevent gas leakage around the stem. A shutoff valve is also provided for the purpose of shutting off the nitrogen flow to the transformer tank. This shutoff valve must be closed prior to changing cylinders to keep the gas in the transformer tank from bleeding off. 

Sampling and Purge Valve. The sampling and purge valve is normally located in the upper right of the nitrogen cabinet. This valve is typically equipped with a hose fitting; the other side is connected directly to the transformer gas space by copper tubing. This valve is opened while purging the gas space during a new installation or maintenance refill and provides a path to exhaust air as the gas space is filled with nitrogen. This valve is also opened when a gas sample is taken from the gas space for analysis. When taking gas samples, the line must be sufficiently purged so that the sample will be from gas above the transformer oil and not just gas in the line. This valve must be tightly closed during normal operation to prevent gas leakage. 

Free Breathing Conservator. This design adds an expansion tank (conservator) above the transformer so that the main tank may be completely filled with oil. Oil expansion and air exchange with the atmosphere (breathing) occurs away from the oil in the transformer. This design reduces oxygen and moisture contamination because only a small portion of oil is exchanged between the main tank and conservator. An oil/air interface still exists in the conservator, exposing the oil to air. Eventually, oil in Figure 10.—Free Breathing the conservator is exchanged with oil in Conservator. the main tank, and oxygen and other contaminates gain access to the insulation. 

If you have transformers of this design, it is recommended that a bladder or diaphragm-type conservator be installed (described below) or retrofitted to the original conservator. In addition, a desiccant-type air dryer should also be installed. 

Conservator with Bladder or Diaphragm Design. A conservator with bladder or diaphragm is similar to the design above with an added air bladder (balloon) or flat diaphragm in the conservator. The bladder or diaphragm expands and contracts with the oil and isolates it from the atmosphere. The inside of the bladder or top of the diaphragm is open to atmospheric pressure through a desiccant air dryer. As oil expands and contracts and as atmospheric pressure changes, the bladder or diaphragm “breathes” air in and out. This keeps air and transformer oil essentially at atmospheric pressure. The oil level gage on the conservator typically is Figure 11.—Conservator magnetic, like those mentioned earlier, with Bladder. except the float is positioned near the center of the underside of the bladder. With a diaphragm, the level indicator arm rides on top of the diaphragm. Examine the air dryer periodically and change the desiccant when approximately one-third of the material changes color. 

Note: A vacuum will appear in the transformer if piping between the air dryer and conservator is too small, if the air intake to the dryer is too small, or if the piping is partially blocked. The bladder cannot take in air fast enough when the oil level is decreasing due to rapidly falling temperature. Minium ¾- to 1-inch piping is recommended. This problem is especially prevalent with transformers that are frequently in and out of service and located in geographic areas of large temperature variations. This situation may allow bubbles to form in the oil and may even activate gas detector relays such as the Buchholz and/or bladder failure relay. The vacuum may also pull in air around gaskets that are not tight enough or that have deteriorated (which may also cause bubbles) . 

Bladder Failure (Gas Accumulator) Relay. The bladder failure relay (not on diaphragm-type conservators) is mounted on top the conservator for the purpose of detecting air bubbles in the oil. Shown at right (figure 12) is a modern relay. Check your transformer instruction manual for specifics because designs vary with manufacturers. No bladder is totally impermeable, and a little air will migrate into the oil. In addition, if a hole forms in the bladder, allowing air to migrate into the oil, the relay will detect it. As air rises and enters the relay, oil is displaced and the float drops, activating the alarm. It is similar to the top chamber of a Buchholz relay, since it is filled with oil and contains a float switch. TO TRANSFORMER TANK BLADDER CONSERVATOR TANK TO DESICCANT AIR DRYER VENTS VALVE FLOAT ELECTRICAL CONNECTION 

Caution: Never open the vent of the bladder failure relay unless you have vacuum or pressure equipment available. The oil will fall inside the relay and conservator and pull in air from the outside. You will have to recommission the relay by valving off the conservator and pressurizing the bladder or by placing a vacuum on the relay. See your specific transformer instruction manual for details. 

Caution: When the transformer, relay, and bladder are new, some air or gas is normally entrapped in the transformer and piping and takes a while to rise and activate the relay. Do not assume the bladder has failed if the alarm activates within 2 to 3 months after it is put into operation. If this occurs, you will have to recommision the relay with pressure or vacuum. See your specific transformer instruction manual for details. If no more alarms occur, the bladder is intact. If alarms continue, look carefully for oil leaks in the conservator and transformer. An oil leak is usually also an air leak. This may be checked by looking at the nitrogen and oxygen in the dissolved gas analysis. If these gases are increasing, there is probably a leak; with a sealed conservator, there should be little of these gasses in the oil. Nitrogen may be high if the transformer was shipped new filled with nitrogen. 

Every 3 to 5 years, (if the conservator has a diaphragm) remove the conservator inspection flange and look inside with a flashlight. If there is a leak, oil will be on top of the diaphragm, and it must be replaced. The new diaphragm material should be nitrile. If the conservator has a bladder and a bladder failure relay, the relay will alarm if the bladder develops a leak. If the conservator has a bladder and does not have a bladder failure relay, inspect the bladder by removing the mounting flange and look inside with a flashlight. If there is oil in the bottom of the bladder, a failure has definitely occurred, and the bladder must be replaced. Follow procedures in the specific transformer instruction manual for draining the conservator and replacement; designs and procedures vary and will not be covered here. 

Auxiliary Tank Sealing System. The auxiliary tank sealing system incorporates an extra tank between the main transformer tank and the conservator tank. Inert gas (normally nitrogen) is placed above oil in both the main and middle tanks. Only oil in the top conservator tank is exposed to air. A desiccant air dryer may or may not be included on the breather. As oil in the main tank expands and contracts with temperature, gas pressure varies above the oil in both (figure 13). Figure 13.—Auxiliary Sealing System. Changes in gas pressure causes oil to go back and forth between the middle tank and the conservator. Air containing oxygen and moisture is not in contact with oil in the main transformer tank. Oxygen and moisture are absorbed by oil in the conservator tank and interchanged with oil in the middle one. However, since gas in the middle tank interchanges with gas in the main tank, small amounts of oxygen and moisture carried by gas still make their way into the transformer. 

With this arrangement, the conservator does not have to be located above the main tank, which reduces the overall height. If you have one or more of these type transformers without desiccant air dryers, they should be installed.