If the necessary oil level is not maintained, the windings will begin to overheat, eventually leading to transformer failure. Preventative maintenance of this issue should involve regular oil filtration. Aging transformers may display signs of electrical or mechanical failure. Mechanical failures involve physical displacement or damage of parts, and happen for a number of reasons, including but not limited to seismic activity and through faults.
Preventative and early onset maintenance of mechanical failure typically involve frequent Sweet Frequency Response Testing and induced voltage testing. Partial discharge present during voltage testing often indicates some sort of mechanical failure. Electrical failure typically involves line surges, which is a very common cause of transformer failure. Voltage spikes, switching surges and line faults are a few common culprits of electrical failure.
The primary method of preventing this is high quality surge protection and regular maintenance of the existing surge protection system. The regular checks and balances of surge protection should involve annually checking the ground connection of the surge arrester. Connection should be tight. Preventative maintenance of this issue should include making sure the electrical load settings are appropriate for the specific type of transformer being used.
Also, both mechanical and electrical damage may be prevented or detected early through use of the built-in devices that most transformers come with. These include the Buchholz and pressure rise relays and pressure relief device for mechanical purposes, and the differential, overcurrent and ground current relays for electrical. Regardless of the cause, the result can be remarkable. Transformers contain mineral oil keeping the transformer cool. When it becomes overcharged, the wiring can create heat and a spark.
This massive overpressure may eventually cause the transformer to rupture with a loud boom, flash and possibly a fireball that can create a large plume of smoke that can be seen from a long distance. Failed transformers can create a variety of hazards, so it is very important that you do not approach it. Inadequate maintenance has to bear the blame for not discovering incipient troubles when there was ample time to correct it.
Sabotage and Malicious Mischief This category is usually assigned when willful damage was evident. Surprisingly, there were no reports of transformer damage in the last 10 years due to this cause. Loose Connections Loose Connections could be included in the Maintenance category, but there was a sufficient number of reports to list it separately.
This is another departure from previous studies. This category includes workmanship and maintenance in making electrical connections. One problem is the improper mating of dissimilar metals, although this has decreased somewhat in recent years. Another problem is improper torquing of bolted connections. All Others This category encompasses all that could not be attributed to the above categories, including "Cause Undetermined.
From a cost standpoint, not only has the cost of repair increased dramatically, so has the cost of downtime. Rewinding or rebuilding a large power transformer can take six to 12 months.
A good maintenance program should include the following recommendations to help achieve maximum service life.
Installation and Operation Keep the electrical loading within the design range of the transformer. In liquid-cooled transformers, carefully monitor the top oil temperature. Install transformers in locations that are compatible with their design and construction. If placed outdoors, make sure the unit is rated for outdoor operation. Protect transformers from surges and other external hazards.
Test the Oil "The dielectric strength of transformer oil decreases rapidly with the absorption of moisture. Gas-in-oil analysis should be performed annually to measure the dissolved gases in the oil that are created by developing faults in the transformer.
The specific gas and the amount of gas can identify the type of fault. These tests include dielectric breakdown, acidity, interfacial tension, etc. Check the contacts for tightness, burning, pitting, freedom of movement, and alignment. The connection should be tight, and the lead should be as short as possible. The earth resistance should be checked during the dry season, and should not exceed 5 ohms.
There are a number of on-line systems currently on the market. The system vendors assemble a variety of probes and sensors, connect them to a data acquisition unit [DAU] and provide for remote telecommunications through a modem. The systems also incorporate an "expert system" to diagnose the problem and distinguish between events that are harmless and events that are dangerous.
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