Knowledge sharing
MARCH-2019
ADDRESS: A/215, V.M. Balakrishnan Street, Sathyamoorthy Block, Ashok Nagar, Chennai - 600083. Phone: 044-23718587: 9840938585 / 9444314624 Email: [email protected] / Web: www.potentialsystem.com
Exterior inspection and Tests for Transformer
Periodic transformer exterior inspection (visual) reveals important condition information. For example,
valves positioned incorrectly, plugged radiators, stuck temperature indicator 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.
Transformer exterior inspection and tests
Physical inspection requires staff experienced in these techniques.
1. To check:
a. Oil leaks
b. Oil pumps
c. Fans and radiators
d. Age
2. To perform:
a. Infrared temperature analysis
Infrared for transformer tanks
Infrared for surge arresters
Infrared for bushings
Infrared for radiators and cooling systems
b. Corona scope scan
3. Other also important inspections
Knowledge sharing
MARCH-2019
ADDRESS: A/215, V.M. Balakrishnan Street, Sathyamoorthy Block, Ashok Nagar, Chennai - 600083. Phone: 044-23718587: 9840938585 / 9444314624 Email: [email protected] / Web: www.potentialsystem.com
TO CHECK:
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 performed 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.
Oil pumps
If the transformer has oil pumps, check the flow indicators and pump isolation valves to ensure that 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 analyser 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 performed 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, resulting in an explosive catastrophic failure of the transformer tank.
Knowledge sharing
MARCH-2019
ADDRESS: A/215, V.M. Balakrishnan Street, Sathyamoorthy Block, Ashok Nagar, Chennai - 600083. Phone: 044-23718587: 9840938585 / 9444314624 Email: [email protected] / Web: www.potentialsystem.com
Ultrasonic inspection of oil pump bearings
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 and 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 that 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.
Age
Transformer age is an important factor to consider when identifying candidates for replacement or
rehabilitation. Age is one indicator of remaining life and upgrade potential to current state-of-the art
materials. During transformer life, structural strength and insulating properties of materials used for suppo rt
and electrical insulation (especially paper) deteriorate.
Although actual service life varies widely depending on the manufacturer, design, quality of assembly,
materials used, maintenance, and operating conditions, the expected life of a transformer is about 40
years.
Knowledge sharing
MARCH-2019
ADDRESS: A/215, V.M. Balakrishnan Street, Sathyamoorthy Block, Ashok Nagar, Chennai - 600083. Phone: 044-23718587: 9840938585 / 9444314624 Email: [email protected] / Web: www.potentialsystem.com
Westar Energy, Inc. 69-kV (unknown capacity MVA) single phase transformers, play an important role in this Wichita-area
switchyard. The transformers are equipped with Conservator Tanks, which was a means to allow for oil expansion and
drainage under high temperature conditions. These units are not forced air, nor forced oil, however do contain cooling tubes
and substantial height. Their age may be around the late 1930s. They are 69-kV/40-kV phase-to-ground. They appear to
have NLTC, and do not have LTC configurations.
To perform:
Infrared temperature analysis
Infrared analysis should be conducted annually while equipment is energized and under full load, if
possible. IR analysis should also be conducted after any maintenance or testing to see if connections that
were broken were re-made properly. Also, if IR is performed during factory heat run, the results can be
used as a baseline for later comparison.
Infrared for transformer tanks
Unusually high external temperatures or unusual thermal patterns of transformer tanks indicate problems
inside the transformer, such as low oil level, circulating stray currents, blocked cooling, loose shields, tap
changer problems, etc. Infrared scanning and analysisis required annually for trending purposes by
National Fire Protection Association 70B,
Abnormally high temperatures can damage or destroy transformer insulation and, thus, reduce life
expectancy. Thermal patterns of transformer tanks and radiators should be cooler at the bottom and
gradually warmer ascending to the top.
Knowledge sharing
MARCH-2019
ADDRESS: A/215, V.M. Balakrishnan Street, Sathyamoorthy Block, Ashok Nagar, Chennai - 600083. Phone: 044-23718587: 9840938585 / 9444314624 Email: [email protected] / Web: www.potentialsystem.com
For a normal pattern; the red spot at the top is normal showing a “hot spot” top of B phase, about 110
degrees Fahrenheit (°F). Any departure from this pattern means a probable problem which must be
investigated. An IR inspection can find over-heating conditions or incorrect thermal patterns. IR scanning and
analysis requires trained staff, experienced in these techniques.
Normal transformer infrared pattern
Infrared for surge arresters
Surge arresters should be included when infrared scanning energized transformers. Look for unusual thermal
patterns on the surface of lightning arresters. The yellow in the top right of the image is a reflection not
associated with the arrester. A temperature profile of the arrester is shown as black lines. Note the hot spot
(yellow) about a third of the way down from the top. This indicates that immediate de-energization and
replacement must be undertaken! Catastrophic failure is imminent which can destroy nearby equipment
and be hazardous to workers. Also compare thermal patterns to sister units or earlier scans of the same
arrester. Scan all high voltage connections and compare them to nearby connections for unusual
temperatures.
Infrared image of defective arrester
Knowledge sharing
MARCH-2019
ADDRESS: A/215, V.M. Balakrishnan Street, Sathyamoorthy Block, Ashok Nagar, Chennai - 600083. Phone: 044-23718587: 9840938585 / 9444314624 Email: [email protected] / Web: www.potentialsystem.com
Infrared for bushings
IR scans of bushings can show low oil levels which would call for immediate de-energization and
replacement. This generally means that the seal in the bushing bottom has failed, leaking oil into the
transformer. The top seal has probably also failed, allowing air and up the bushing.
Another reason a bushing can exhibit a high oil level is that the top seal is leaking – allowing water to enter.
The water migrates to the bushing bottom, displacing the oil upward.
Remember, over 90% of bushing failures are attributed to water entrance through the top seal. Bushings
commonly fail catastrophically, many times destroying the host transformer or breaker and nearby equipment
and causing hazards to workers.
Infrared image of defective bushing
Low-oil level in a high voltage transformer bushing. Compare previous IR scans of the same bushing with the
current scan.
Doble hot-collar testing possibly may show this problem. However, Doble tests are run infrequently, and
the transformer has to be out of service, under clearance, and both primary and secondary conductors
removed, while an IR scan easily can be performed at any time.
Infrared for radiators and cooling systems
Examine radiators with an IR camera and compare them with each other. A cool radiator or segment
indicates that a valve is closed or the radiator or segment is plugged. The IR image (figure 4) shows that the
cold left radiator section is valves off or plugged.
Knowledge sharing
MARCH-2019
ADDRESS: A/215, V.M. Balakrishnan Street, Sathyamoorthy Block, Ashok Nagar, Chennai - 600083. Phone: 044-23718587: 9840938585 / 9444314624 Email: [email protected] / Web: www.potentialsystem.com
If visual inspection shows the valves are open, the radiator or segment must be isolated, drained, removed,
and the blockage cleared. Do not allow a transformer to operate with reduced cooling which drastically
shortens transformer life.
Remember, an increased operating temperature of only 8 to 10 °C will reduce transformer life by one-half. IR
scan all cooling systems, including heat exchangers, fans, pumps, motors, etc. Check inside control panels
for overloaded wiring, loose connections, and overheated relays. Look for unusual thermal patterns and
compare similar equipment.
Infrared image showing blocked radiators
Corona Scope Scan With the transformer energized, scan the bushings and surge arresters and all high voltage connections for
unusual corona patterns. Corona should be visible only at the top of bushings and arresters, and corona at
connections should be similar to sister connections. As a bushing deteriorates due to physical defects, the
corona pattern will grow progressively larger.
When the corona pattern reaches a grounded surface (i.e., the tank or structure), a flashover will occur,
destroying the bushing or arrester and, perhaps, the transformer. The corona scope will reveal this
problem long before a flashover.
Knowledge sharing
MARCH-2019
ADDRESS: A/215, V.M. Balakrishnan Street, Sathyamoorthy Block, Ashok Nagar, Chennai - 600083. Phone: 044-23718587: 9840938585 / 9444314624 Email: [email protected] / Web: www.potentialsystem.com
Other also important inspections
There are several required physical inspections that will not be covered here because they were addressed
previously. Be sure to inspect the following:
a. Winding temperature indicators
b. Pressure relief devices
c. Sudden pressure relay
d. Conservator bladder
e. Conservator breather and
f. Bladder failure relay
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