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. 

4. OIL-FILLED TRANSFORMER INSPECTIONS

A transformer maintenance program must be based on thorough routine inspections. These inspections must be in addition to normal daily/weekly data gathering trips to check oil levels and temperatures. Some monitoring may be done remotely using supervisory control and data acquisition (SCADA) systems, but this can never substitute for thorough inspections by competent maintenance or operations people. 

4.1 Oil-Filled Transformers 

After 1 month of service and once each year, make an indepth inspection of oil-filled transformers. Before beginning, look carefully at temperature and oil level data sheets. If temperature, pressure, or oil level gages never change, even with seasonal temperature and loading changes, something is wrong. The gage may be stuck or data sheets may have been filled in incorrectly. Examine the DGA’s for evidence of leaks, etc. 

4.1.1 Transformer Tank. Check for excessive corrosion and oil leaks. Pay special attention to flanges and gaskets (bushings, valves, and radiators) and lower section of the main tank. Report oil leaks to maintenance, and pay special attention to the oil level indicator if leaks are found. Severely corroded spots should be wire brushed and painted with a rust inhibitor. 

4.1.2 Top Oil Thermometers. These are typically sealed spiral-bourdon-tube dial indicators with liquid-filled bulb sensors. The bulb is normally inside a thermometer well, which penetrates the tank wall into oil near the top of the tank. As oil temperature increases in the bulb, liquid expands, which expands the spiral tube. The tube is attached to a pointer that indicates temperature. These pointers may also have electrical contacts to trigger alarms and start cooling fans as temperature increases. An extra pointer, normally red, indicates maximum temperature since the last time the indicator was reset. This red pointer rises with the main pointer but will not decrease unless manually reset; thus, it always indicates the highest temperature reached since being set. See the instruction manual on your specific transformer for details. 

4.1.3 Winding Temperature Thermometers. These devices are supposed to indicate hottest spot in the winding based on the manufacturers heat run tests. At best, this device is only accurate at top nameplate rated load and then only if it is not out of calibration [17]. They are not what their name implies and can be misleading. They are only winding hottest-spot simulators and not very accurate. There is no temperature sensor imbedded in the winding hot spot. At best, they provide only a rough approximation of hot spot winding temperature and should not be relied on for accuracy. They can be used to turn on additional cooling or activate alarms as the top oil thermometers do. 

Winding temperature thermometers work the same as the top oil thermometer (4.1.2) above, except that the bulb is in a separate thermometer well near the top of the tank. A wire-type heater coil is either inserted into or wrapped around the thermometer well which surrounds the temperature sensitive bulb. In some transformers, a current transformer (CT) is around one of the three winding leads and provides current directly to the heater coil in proportion to winding current. In other transformers, the CT supplies current to an auto-transformer that supplies current to the heater coil. The heater warms the bulb and the dial indicates a temperature, but it is not the true hottest-spot temperature. 

These devices are calibrated at the factory by changing taps either on the CT or on the autotransformer, or by adjusting the calibration resistors in the control cabinet. They normally cannot be field calibrated or tested, other than testing the thermometer, as mentioned. The calibration resistors can be adjusted in the field if the manufacturer provides calibration curves for the transformer. In practice, most winding temperature indicators are out of calibration, and their readings are meaningless. These temperature indications should not be relied upon for loading operations or maintenance decisions. 

Fiber optic temperature sensors can be imbedded directly into the winding as the transformer is being built and are much more accurate. This system is available as an option on new transformers at an increased cost, which may be worth it since the true winding “hottest-spot” temperature is critical when higher loading is required. 

Thermometers can be removed without lowering the transformer oil if they are in a thermometer well. Check your transformer instruction manual. Look carefully at the capillary tubing between the thermometer well and dial indicator. If the tubing has been pinched or accidently struck, it may be restricted. This is not an obvious defect, and it can cause the dial pointer to lock in one position. If this defect is found, the whole gage must be returned to the factory for repair or replacement; it cannot be repaired in the field. Look for a leak in the tubing system; the gage will be reading very low and must be replaced if a leak is discovered. Thermometers should be removed and tested every 3 to 5 years as described below. 

Thermometer Testing. Every 3 to 5 years, and if trouble is suspected, do a thermometer testing. Suspend the indicator bulb and an accurate mercury thermometer in an oil bath. Do not allow either to touch the side or bottom of the container. Heat the oil on a hotplate while stirring and compare the two thermometers while the temperature increases. If a magnetic stirring/heating plate is available, it is more effective than hand stirring. Pay particular attention to the upper temperature range at which your transformers normally operate (50 °C to 80 °C). An ohmmeter should also be used to check switch operations. If either dial indicator is more than 5 °C different than the mercury thermometer, it should be replaced with a spare. A number of spares should be kept, based on the quantity of transformers at the plant. Oil bath test kits are available from the Qualitrol Company. After calling for Qualitrol authorization at 716-586-1515, you can ship defective dial thermometers for repair and calibration to: Qualitrol Co., 1387 Fairport Rd., Fairport, NY 14450. 

The alarms and other functions should also be tested to see if the correct annunciator points activate, pumps/fans operate, etc. 

If it is not possible to replace the temperature gage or send it to the factory for repair, place a temperature correction factor on your data form to add to the dial reading so the correct temperature will be recorded. Also lower the alarm and 

pump-turn-on settings by this same correction factor. Since these are pressure- filled systems, the indicator will typically read low if it is out of calibration. Field testing has shown some of these gages reading 15 °C to 20 °C lower than actual temperature. This is hazardous for transformers because it will allow them to continuously run hotter than intended, due to delayed alarms and cooling activation. If thermometers are not tested and errors corrected, transformer service life may be shortened or premature failure may occur. 

4.1.4 Oil Level Indicators. After 1 month of service, inspect and every 3 to 5 years, check the tank oil level indicators. These are float operated, with the float mechanism magnetically coupled through the tank wall to the dial indicator. As level increases, the float rotates a magnet inside the tank. Outside the tank, another magnet follows (rotates), which moves the pointer. The center of the dial is normally marked with a temperature 25 °C (77 °F). High and low level points are also marked to follow level changes as the oil expands and contracts with temperature changes. The proper Figure 2.—Oil Level Indicator. way to determine accurate oil level is to first look at the top oil temperature indicator. After determining the temperature, look at the level gage. The pointer should be at a reasonable level corresponding to the top oil temperature. If the transformer is fully loaded, the top oil temperature will be high, and the level indicator should be near the high mark. If the transformer is de-energized and the top oil temperature is near 25 °C, the oil level pointer should be at or near 25 °C. 

To check the level indicator, you can remove the outside mechanism for testing without lowering transformer oil. After removing the gage, hold a magnet on the back of the dial and rotate the magnet; the dial indicator should also rotate. If it fails to respond or if it drags or sticks, replace it. As mentioned above, defective units can be sent to the factory for repair. 

There may also be electrical switches for alarms and possibly tripping off the transformer on falling tank level. These should be checked with an ohmmeter for proper operation. The alarm/tripping circuits should also be tested to see if the correct annunciator points and relays respond. See the transformer instruction book for information on your specific indicator. 

PROTECTIVE COVER TRIPPED POSITION COMPRESSION SPRINGS ALARM SWITCH

DIAPHRAGM GASKETS TRANSFORMER TANK SWITCH RESET LEVER COLORED ROD SHOWS

If oil has had to be lowered in the transformer or conservator for other reasons (e.g., inspections), check the oil level float mechanism.  Rotate the float mechanism by hand to check for free movement.  Check the float visually to make sure it is secure to the arm and that the arm is in the proper shape.  Some arms are formed (not straight).  

4.1.5  Pressure Relief Devices.  These devices are the transformers’ last line of defense against excessive internal pressure.  In case of a fault or short circuit, the resultant arc instantly vaporizes surrounding oil, causing a rapid buildup of gaseous pressure.  If the pressure relief device does not operate properly and pressure is not sufficiently relieved within a few milliseconds, a catastrophic tank rupture can result, spreading flaming oil over a wide area.  Two types of these devices are discussed below.  The instruction manual for your transformer must be consulted for specifics. 

Caution:  Never paint pressure-relief devices because paint can cause the plunger or rotating shaft to stick.  Then the device might not relieve pressure, which could lead to catastrophic tank failure during a fault.  Look at the top of the device; on newer units, a yellow or blue button should be visible.  If these have been painted, the button will be the same color as the tank.  On older units, a red flag should be visible; if it has been painted, it will be the same color as the tank. If they have been painted, they should be replaced.  It is virtually impossible to remove all paint from the mechanism and be certain the device will work when needed.

Newer Pressure Relief Devices.  Newer pressure relief devices are spring-loaded valves that automatically reclose following a pressure release.  The springs are held in compression by the cover and press on a disc which seals an opening in the tank top.  If pressure in the tank exceeds operating pressure, the disk moves upward and relieves pressure.  As pressure decreases, the springs reclose the valve.  After operating, this device leaves a brightly colored rod (bright yellow for oil, blue for silicone) exposed approximately 2 inches above the top.  This rod is easily seen upon inspection, although it is not always visible from floor level.  The rod may be reset by pressing on the top until it is again recessed into the device. The switch must also be manually reset.  A relief device is shown in the open position in figure 3 above.  

Caution: Bolts that hold the device to the tank may be loosened safely, but never loosen screws which hold the cover to the flange without referring to the instruction manual and using great care. Springs that oppose tank pressure are held in compression by these screws, and their stored energy could be hazardous. 

Once each year, and as soon as possible after a known through-fault or internal fault, inspect pressure devices to see if they have operated. This must be done from a high-lift bucket if the transformer is energized. Look at each pressure relief device to see if the yellow (or blue) button is visible. If the device has operated, about 2 inches of the colored rod will be visible. Each year, test the alarm circuits by operating the switch by hand and making sure the correct annunciator point is activated. If the relief device operates during operation, do not re-energize the transformer; Doble and other testing may be required before re-energizing, and an oil sample should be sent for analysis 

Every 3 to 5 years, when doing other maintenance or testing, if the transformer has a conservator, examine the top of the transformer tank around the pressure relief device. If oil is visible, the device is leaking, either around the tank gasket or relief diaphragm. If the device is 30 years old, replace the whole unit. A nitrogen blanketed transformer will use a lot more nitrogen if the relief device is leaking; they should be tested as described below. 

A test stand with a pressure gage may be fabricated to test the pressure relief function. Current cost of a pressure relief device is about $600, so testing instead of replacement may be prudent. Have a spare on hand so that the tank will not have to be left open. If the tank top or pressure relief device has gasket limiting grooves, always use a nitrile replacement gasket; if there are no grooves, use a cork-nitrile gasket. Relief devices themselves do not leak often; the gasket usually leaks. 

Older Pressure Relief Devices. Older pressure relief devices have a diaphragm and a relief pin that is destroyed each time the device operates and must be replaced. 

Caution: These parts must be replaced with exact replacement parts, or the operating relief-pressure of the device will be wrong. 

The relief pin determines operating pressure; a number, which is the operating pressure, normally appears on top of the pin. Check your specific transformer instruction manual for proper catalog numbers. Do not assume you have the right parts, or that correct parts have been previously installed—look it up. If the operating pressure is too high, a catastrophic tank failure could result. 

On older units, a shaft rotates, operates alarm/trip switches, and raises a small red flag when the unit releases pressure. If units have been painted or are more than 30 years old, they should be replaced with the new model as soon as it is possible to have a transformer outage. 

TRANSFORMER TO SENSING BELLOWS SILICONE FLUID BLEED VALVE BIMETAL TEMPERATURE COMPENSATOR CONTROL ORIFICE CONTROL BELLOWS PRESSURE BALANCE BEAM ACTUATOR ELECTRICAL SWITCH ELECTRICAL CONNECTOR TRANSFORMER OIL 1/8" DIAMETER DRAIN HOLE HOUSINGSILICONE SENSING FLUID MANIFOLD CONTROL BELLOWS

Once each year and as soon as possible after a through-fault or internal fault, examine the indicator flag to see if the device has operated.  They must be examined from a high-lift bucket if the transformer is energized.  A clearance must be obtained to test, repair, or reset the device.  See the instruction manual for your specific transformer.  Test alarm/trip circuits by operating the switch byhand.  Check to make sure the correct annunciator point activates.  

Every 3 to 5 years, when doing other maintenance or testing, examine the top of the transformer tank around the pressure relief device.  If the transformer has a conservator and oil is visible, the device is leaking, either around the tank gasket or relief diaphragm.  The gasket and/or device must be replaced.  Take care that the new device will fit the same tank opening prior to ordering.  Most of them are made by the Qualitrol Company; contact the manufacturer to obtain a correct replacement. 

4.1.6  Sudden Pressure Relay.  Internal arcing in an oil-filled power transformer can instantly vaporize surrounding oil, generating gas pressures that can cause catastrophic failure, rupture the tank, and spread flaming oil over a large area.  This can damage or destroy other equipment in addition to the transformer and presents extreme hazards to workers.

The relay is designed to detect a sudden pressure increase caused by arcing.  It is set to operate before the pressure relief device.  The control circuit should de- energize the transformer and provide an alarm.  The relay will ignore normal pressure changes such as oil-pump surges, temperature changes, etc.  

Modern sudden pressure relays consist of three bellows (see figure 4) with silicone sealed inside.  Changes in pressure in the transformer deflect the main sensing bellows.  Silicone inside acts on two control bellows arranged like a balance beam, one on each side.  One bellows senses pressure changes through a small orifice. The opening is automatically changed by a bimetallic strip to adjust for normal temperature changes of the oil. The orifice delays pressure changes in this bellows.  The other bellows responds to immediate pressure changes and is affected much more quickly.  Pressure difference tilts the balance beam and activates the switch. This type relay automatically resets

when the two bellows again reach pressure equilibrium. If this relay operates, do not re-energize the transformer until you have determined the exact cause and corrected the problem. 

Old style sudden pressure relays have only one bellows. A sudden excessive pressure within the transformer tank exerts pressure directly on the bellows, which moves a spring-loaded operating pin. The pin operates a switch which provides alarm and breaker trip. After the relay has operated, the cap must be removed and the switch reset to normal by depressing the reset button. 

Once every 3 to 5 years, the sudden pressure relay should be tested according to manufacturer’s instructions. Generally, only a squeeze-bulb and pressure gage (5 psi) are required. Disconnect the tripping circuit and use an ohmmeter to test for relay operation. Test the alarm circuit and verify that the correct alarm point is activated. Use an ohmmeter to verify the trip signal is activated or, if possible, apply only control voltage to the breaker and make sure the tripping function operates. Consult the manufacturer’s manual for your specific transformer for detailed instructions.