Power and distribution transformers change ac voltage from one level to another, and/or change alternating current to direct current or direct current to alternating current using magnetic coupling.
These transformers can be either dry or liquid filled. In liquid-filled transformers, the windings and core are totally immersed in the liquid, often oil, which functions as the cooling/heat transfer medium and the insulator to control stray current. The lifespan of an oil-filled transformer depends largely on regular maintenance, but on average they remain in service 30 years or more. Like all electrical devices, faults can occur in transformers, which then lead to failures.
Thermal imagers can be used to monitor the external and internal condition of oil-filled transformers, and can even help prevent common failures. At a minimum, thermal imagers can examine external connections, cooling tubes, cooling fans and pumps as well as surfaces of critical transformers to detect problems before damage or failure occurs.
Ways to prevent common failures in:
High- and low-voltage bushing connections
Running at 50% normal load, scan bushings with the infrared camera. Look for overheating in a connection which could indicate high resistance, a loose or dirty connection or seal beaks. Also, compare phases, looking for temperate differences which could indicate unbalance or overloading. Compare phase connections to ambient air temperature. A temperature difference greater than 40° C (72° F) may indicate a serious condition requiring immediate attention.
Use infrared inspection to detect oil levels and radiator convection cooling problems that can reduce the life of the transformer. On oil-cooled transformers, cooling tubes will normally appear warm. If one or more tubes are comparatively cool, oil flow is being restricted and the root cause of the problem needs to be determined.
Inspect fans and pumps while they are running. A fan or pump operating normally will be warm. A fan or pump with failing bearings will be hot. A fan or pump that is not functioning at all will be cold.
Load tap changers (LTC)
LTCs are the seconding leading cause of transformer failures. A LTC main tank is generally warmer than an isolated LTC tank. Temperatures that are similar indicate a potential leak between the two tanks or severe “coking” or arc debris buildup on the LTC contacts.
Regular inspection and maintenance routes should include transformers on all main electrical circuits. Save thermal images of each transformer and track temperature measurements over time, using the software that comes with the IR camera. Having baseline images to compare later on will help you determine if temperature levels are unusual and, following corrective action, determine if maintenance was successful.
Perceived internal problems in oil-cooled transformers can often be verified by a gas-in oil analysis. The presence of methane in the oil indicates overheating. Acetylene indicates arcing. This test can also be used to help trend the severity of a problem in a transformer that simply cannot be taken down for repairs. Warning: Never draw liquid samples from an energized transformer except via an external sampling valve. Also, regular gauge and load monitoring and visual inspections for leaks, corrosion, et cetera will help guide further maintenance activities.
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