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1.1 Background

Visual inspection of the transformer exterior reveals important condition information. For example, valves positioned incorrectly, plugged radiators, stuck temperature indicators and level gauges, and noisy oil pumps or fans. Oil leaks can often be seen which indicate a potential for oil contamination, loss of insulation, or environmental problems. Physical inspection requires staff experienced in these techniques.

1.2 Temperature Indicators Online
Winding temperature indicator

Winding temperature indicator

Check all temperature indicators while the transformer is online. The winding temperature indicator should be reading approximately 15 degrees above the top oil temperature. If this is not the case, one or both temperature indicators are malfunctioning. Check the top oil temperature next to the top oil indicator’s thermowell with an infrared camera.

Compare the readings with the top oil indicator. Reset all maximum indicator hands on the temperatures indicating devices after recording the old maximum temperature readings.

High temperature may mean overloading, cooling problems, or problems with windings, core, or connections.

1.3 Temperature Indicators Offline

When the transformer is offline and has cooled to ambient temperature, check the top oil and winding temperature indicators; both should be reading the same. If not, one or both temperature indicators are malfunctioning. Check the calibration according to the proper procedure. Also compare these readings with the indicated temperature on the conservator oil level indicator; all three should agree.

1.4 Conservator
Figure 1.—Conservator Oil Level

Figure 1.—Conservator Oil Level

Check the oil level gauge on the conservator. See figure 1 at right. This gauge indicates oil level by displaying a temperature. Compare the indicated temperature on the conservator level gauge with the top oil temperature indicator. They should be approximately the same.

Calibrate or replace the conservator oil level indicator if needed, but only after checking the top oil temperature indicator as shown in the above section. Reference also IEEE 62-1995™ [11], section 6.6.2. If atmospheric gases (nitrogen, oxygen, carbon dioxide) and perhaps moisture increase suddenly in the DGA, a leak may have developed in the conservator diaphragm or bladder. With the transformer offline and under clearance, open the inspection port on top of the conservator and look inside with a flashlight. If there is a leak, oil will be visible on top of the diaphragm or inside the bladder. Reclose the conservator and replace the bladder or diaphragm at the first opportunity by scheduling an outage. If there is no gas inside the Buchholz Relay, the transformer may be re-energized after bleeding the air out of the bladder failure relay.

A DGA should be taken immediately to check for O2, N2, and moisture. However, the transformer may be operated until a new bladder is installed, keeping a close eye on the DGAs. It is recommended that DGAs be performed every 3 months until the new bladder is installed. After the bladder installation, the oil may need to be de-gassed if O2 exceeds 10,000 ppm. Also, carefully check the moisture level in the DGAs to ensure it is below recommended levels for the particular transformer voltage. Check the desiccant in the breather often; never let more than two-thirds become discolored before renewing the desiccant. All efforts should be made to keep the oxygen level below 2,000 ppm and moisture as low as possible.

1.5 Conservator Breather
Figure 2.—Conservator Breather

Figure 2.—Conservator Breather

Check the dehydrating (desiccant) breather for proper oil level if it is an oil type unit. Check the color of the desiccant and replace it when approximately one-third remains with the proper color. See figure 2 for a modern oil type desiccant breather. Notice the pink desiccant at the bottom of the blue indicating that this portion is water saturated. Notice also that oil is visible in the very bottom 1-inch or so of the unit.

Many times, the oil is clear, and the oil level will not be readily apparent. Normally, there is a thin line around the breather near the bottom of the glass; this indicates where the oil level should be.

Compare the oil level with the level indicator line and refill, if necessary. Note the 1¼-inch pipe going from the breather to the conservator. Small tubing (½ inch or so) is not large enough to admit air quickly when the transformer is de-energized in winter. A transformer can cool so quickly that a vacuum can be created from oil shrinkage with enough force to puncture a bladder. When this happens, the bladder is destroyed; and air is pulled into the conservator making a large bubble.

1.6 Nitrogen

If the transformer has a nitrogen blanket, check the pressure gauge for proper pressure. Look at the operators recording of pressures from the pressure gauge. If this does not change, the gauge is probably defective. Check the nitrogen bottle to insure the nitrogen is the proper quality (see PEB No. 5 [20]). Check for any increased usage of nitrogen which indicates a leak. Smaller transformers such as station service or smaller generator-step-up transformers may not have nitrogen bottles attached to replace lost nitrogen. Be especially watchful of the pressure gauge and the operator’s records of pressures with these.

The pressure gauge can be defective for years, and no one will notice. The gauge will read nearly the same and will not vary much over winter and summer or night and day. Meanwhile, a nitrogen leak can develop; and all the N2 will be lost. This allows air with oxygen and moisture to enter and deteriorate the oil and insulation. Watch for increased oxygen and moisture in the DGA. An ultrasonic and sonic leak detection instrument (P-2000) is used for locating N2 leaks.

1.7 Oil Leaks
Oil Leaks

Oil Leaks

Check the entire transformer for oil leaks. Leaks develop due to gaskets wearing out, ultraviolet exposure, taking a “set,” or from expansion and contraction, especially after transformers have cooled, due to thermal shrinkage of gaskets and flanges. Many leaks can be repaired by applying an epoxy or other patch.

Flange leaks may be stopped with these methods using rubberized epoxy forced into the flange under pressure. Very small leaks in welds and tanks may be stopped by peening with a ball-peen hammer, cleaning with the proper solvent, and applying a “patch” of the correct epoxy.

Experienced leak mitigation contractors whose work is guaranteed may also be employed. Some leaks may have to be welded. Welding may be done with oil in the transformer if an experienced, qualified, and knowledgeable welder is available. If welding with oil in the tank is the method chosen, oil samples must be taken for DGA both before and after welding. Welding may cause gases to appear in the DGA and it must be determined what gases are attributed to welding and which ones to transformer operation.

1.8 Pressure Relief Device

With the transformer under clearance, check the pressure relief device indicating arm on top of the Figure 3.— Pressure Relief Device. transformer to see if it has operated. If it has operated, the arm will be in the up (vertical) position, and alarm and shutdown relays should have activated.

Figure 3.— Pressure Relief Device

Figure 3.— Pressure Relief Device

CAUTION:
Do not re-energize a transformer after this device has operated and relays have de-energized the transformer, until extensive testing to determine and correct the cause has been undertaken. Explosive, catastrophic failure could be the result of energization after this device has operated.

1.9 Oil Pumps

If the transformer has oil pumps, check flow indicators and pump isolation valves to ensure oil is circulating properly. Pump motor(s) may also have reversed rotation, and flow indicators may still show that oil is flowing. To ensure motors are turning in the proper direction, use an ammeter to check the motor current. Compare results with the full-load-current indicated on the motor nameplate. If the motor is reversed, the current will be much less than the nameplate full-load-current.  Check oil pumps with a vibration analyzer if they develop unusual noises.

Have the DGA lab check for dissolved metals in the oil and run a metal particle count for metals if the bearings are suspect. This should be done immediately, as soon as a bearing becomes suspect; bad oil-pump bearings can put enough metal particles into the oil to threaten transformer insulation and cause flashover inside the tank. An explosive catastrophic failure of the transformer tank could be the result.

1.10 Fans and Radiators

Inspect all isolation valves at the tops and bottoms of radiators to ensure they are open. Inspect cooling fans and radiators for cleanliness and fans for proper rotation. Check for dirty or damaged fan blades or partially blocked radiators. Fans are much more efficient if the blades are clean and rotating in cool air. Normally, fans blow cool air through the radiators; they should not be pulling air through. Check to see if fans are reversed electrically (i.e., pulling air first through the radiators and then through the fan blades). This means the blades are rotating in warm air after it passes through the radiator which is much less efficient. Place a hand on the radiator opposite the fans; air should be coming out of the radiator against your hand.

Watch the blades as they rotate slowly when they are starting or stopping to determine which way they should be rotating and correct the rotation if necessary.

1.11 Buchholz Relay
Figure 4.—Buchholz Relay

Figure 4.—Buchholz Relay

Inspect the isolation valve on the Buchholz relay to ensure it is open. With the transformer offline and under clearance, examine the Buchholz relay by lifting the window cover (center in figure 4 at right) and looking inside. If there is gas inside, the oil will be displaced, and the gas will be evident as a space on top the oil. If sufficient gas is found to displace the upper float, the alarm should be activated. The small valve at the top left is to bleed the gas off and reset the relay. If a small amount of gas is found in this relay when the transformer is new (a few months after startup), it is probably just air that has been trapped in the transformer structure and is now escaping; there is little cause for concern.

If the transformer has been on line for some time (service aged), and gas is found in the Buchholz, oil samples must be sent to the lab for DGA and extensive testing. Consult with the manufacturer and other transformer experts. A definite cause of the gas bubbles must be determined and corrected before re-energization of the transformer.

1.12 Sudden Pressure Relay
Figure 5.—Sudden Pressure Relay

Figure 5.—Sudden Pressure Relay

An example relay is shown in figure 5 at the left. The purpose of this relay is to alarm if there is a sudden pressure rise inside the tank. This relay is very sensitive and will operate if the pressure rises only a little. If a very small pressure change occurs caused by a small electrical fault inside the tank, this relay will alarm. In contrast, the pressure relief device (shown above in figure 5) operates if a large pressure builds inside the tank caused by heavy arcing and heating causing the oil to boil and bubble. Inspect the isolation valve to ensure it is open.

With the transformer offline and under clearance, functionally test the sudden pressure relay by slowly closing the isolating valve. Leave it closed for a few seconds and reopen the valve very suddenly; this should activate the alarm. If the alarm does not activate, test the relay, and replace it with a new one if it fails to function.

1.13 Bladder Failure Relay

On newer transformers, a bladder failure relay may be found on or near the conservator top on the oil side of the bladder. This relay is near the highest point of the transformer. Its purpose is to alarm if the bladder fails and admits air bubbles into the oil.

The relay will also serve as a backup to the Buchholz relay. If the Buchholz relay overfills with gas and fails to activate an alarm or shutdown, gas will bypass the Buchholz and migrate up into the conservator, eventually to the bladder failure relay. See figure 6. Of course, these gases should also show up in the DGA. However, DGAs are normally taken only once per year, and a problem may not be discovered before these alarms are activated.

Figure 6.—Bladder Failure Relay

Figure 6.—Bladder Failure Relay

If the bladder failure alarm is activated, place the transformer under clearance and check the Buchholz for gas as mentioned in section 1.10. Open the conservator inspection port and look inside with a flashlight to check for oil inside the bladder. Bleed the air/gas from the conservator using the bleed valve on top of the conservator. If the transformer is new and has been in service for only a few months, the problem most likely is air escaping from the structure as mentioned in section 1.11.

With the transformer under clearance, open the inspection port on top of the conservator and look inside the bladder with a flashlight. If oil is found inside the bladder, it has developed a leak; a new one must be ordered and installed.

SOURCE: Transformer Diagnostics, Fascilities Instructions, Standards And Techniques

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Transformer Oil Diagnostics

Transformer Oil Diagnostics

In addition to dissipating heat due to losses in a transformer, insulating oil provides a medium with high dielectric strength in which the coils and core are submerged. This allows the transformers to be more compact, which reduces costs. Insulating oil in good condition will withstand far more voltage across connections inside the transformer tank than will air. An arc would jump across the same spacing of internal energized components at a much lower voltage if the tank had only air. In addition, oil conducts heat away from energized components much better than air.

Over time, oil degrades from normal operations, due to heat and contaminants. Oil cannot retain high dielectric strength when exposed to air or moisture. Dielectric strength declines with absorption of moisture and oxygen. These contaminants also deteriorate the paper insulation. For this reason, efforts are made to prevent insulating oil from contacting air, especially on larger power transformers. Using a tightly sealed transformer tank is impractical, due to pressure variations resulting from thermal expansion and contraction of insulating oil. Common systems of sealing oil-filled transformers are the conservator with a flexible diaphragm or bladder or a positivepressure inert-gas (nitrogen) system. Reclamation GSU transformers are generally purchased with conservators, while smaller station service transformers have a pressurized nitrogen blanket on top of oil. Some station service transformers are dry-type, self-cooled or forcedair cooled.

Conservator System

A conservator is connected by piping to the main transformer tank that is completely filled with oil. The conservator also is filled with oil and contains an expandable bladder or diaphragm between the oil and air to prevent air from contacting the oil. Figure 1 is a schematic representation of a conservator system (figure 1 is an actual photo of a conservator).

Figure 1: Conservator with Bladder

Figure 1: Conservator with Bladder

Air enters and exits the space above the bladder/diaphragm as the oil level in the main tank goes up and down with temperature. Air typically enters and exits through a desiccant-type air dryer that must have the desiccant replaced periodically. The main parts of the system are the expansion tank, bladder or diaphragm, breather, vent valves, liquid-level gauge and alarm switch. Vent valves are used to vent air from the system when filling the unit with oil. A liquid-level gauge indicates the need for adding or removing transformer oil to maintain the proper oil level and permit flexing of the diaphragm.

Oil-Filled, Inert-Gas System

A positive seal of the transformer oil may be provided by an inert-gas system. Here, the tank is slightly pressurized by an inert gas such as nitrogen. The main tank gas space above the oil is provided with a pressure gauge (figure 12. Since the entire system is designed to exclude air, it must operate with a positive pressure in the gas space above the oil; otherwise, air will be admitted in the event of a leak. Smaller station service units do not have nitrogen tanks attached to automatically add gas, and it is common practice to add nitrogen yearly each fall as the tank starts to draw partial vacuum, due to cooler weather. The excess gas is expelled each summer as loads and temperatures increase. Some systems are designed to add nitrogen automatically (figure 2) from pressurized tanks when the pressure drops below a set level. A positive pressure of approximately 0.5 to 5 pounds per square inch (psi) is maintained in the gas space above the oil to prevent ingress of air. This system includes a nitrogen gas cylinder; three-stage, pressure-reducing valve; high-and low-pressure gauges; high-and low-pressure alarm switch; an oil/condensate sump drain valve; an automatic pressure-relief valve; and necessary piping.

Figure 2: Typical Transformer Nitrogen System

Figure 2: Typical Transformer Nitrogen System

The function of the three-stage, automatic pressure-reducing valves is to reduce the pressure of the nitrogen cylinder to supply the space above the oil at a maintained pressure of 0.5 to 5 psi. The high-pressure gauge normally has a range of 0 to 4,000 psi and indicates nitrogen cylinder pressure. The low-pressure gauge normally has a range of about -5 to +10 psi and indicates nitrogen pressure above the transformer oil. In some systems, the gauge is equipped with high- and low-pressure alarm switches to alarm when gas pressure reaches an abnormal value; the high-pressure gauge may be equipped with a pressure switch to sound an alarm when the supply cylinder pressure is running low. A sump and drain valve provide a means for collecting and removing condensate and oil from the gas. A pressure-relief valve opens and closes to release the gas from the transformer and, thus, limit the pressure in the transformer to a safe maximum value.

As temperature of a transformer rises, oil expands, and internal pressure increases, which may have to be relieved. When temperature drops, pressure drops, and nitrogen may have to be added, depending on the extent of the temperature change and pressure limits of the system.

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