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There are all kinds of faults in electrical equipment and most are accompanied by heating. They are usually divided into external faults and internal faults from infrared diagnosis. As is known to all, in the operation of power system, current-carrying conductor generates resistance loss due to current effect, and there are a large number of connectors, joints and prods in the whole circuit of electric energy transmission.

Ideally, if contact resistance of various connectors, joints and prods in the transmitting loop is lower than that of bonding conductor, the loss and heat of the connection parts will not be higher than that of adjacent current-carrying conductor. But once some connectors, joints, or prods are poorly connected, increasing the contact resistance, the area will have more resistance loss and higher temperature rise, leading to local overheating. This is usually a kind of external failure.

The feature of external failure is local temperature rises and is easy to be discovered by infrared thermograph. If not handled in time, the situation will deteriorate quickly and easily cause accidents and losses. External failures account for a large proportion.

Internal failure of high-voltage apparatus mainly refers to electric circuit failure in enclosed solid insulation and casing as well as numerous faults caused by degradation of insulating medium. The reflected temperature rise on the outside is quite small, usually only a few kelvins because this failure appears inside the electrical equipment. It's highly expected the sensitivity of its inspection device.

Internal failure is characterized with small proportion and temperature rise, great damage and demanding inspection equipment.

According to the general statistics of long-term measured data and cases provided by related units, external thermal defects generally account for 90% ~ 93% of the total power equipment defect index while internal thermal defects account for only 7% ~ 10%.

In power industry, thermal infrared imager is very early applied to safety maintenance on equipment by detecting its thermal defects of electrical equipment and lines, such as adapter, bushing, circuit breaker, switch, transformer, power capacitor, arrester, power cable, bus bars, wires, GIS, low-voltage electrical appliances, insulator string and secondary circuit with electric and voltage thermal effect or others, which is crucial and effective to prompt discovery, prevention and handling of big accidents.

Electrical equipment thermal defects are usually heating phenomena which can be tested by certain way and caused by internal or external reasons.

It can be concluded as two kinds of defects according to the reasons. One is bad connection of long-term exposed elements influenced by temperature and humidity or surfaces scaling. Or the damage from the outside which reduces cross section of conduction and produces heating. For example, bad connection of joints, unpressed bolts and washers; corrosion oxidation after long running; Corrosion from reactive gas and dust in the air; conductor damage by component material fault and poor installation; reduction of cross section caused by various aspects such as vibration; unstable and exceeding over load.

The other one is inner defect such as too large resistance caused by bad connection of inner connection parts; aging, cracking and shedding of insulating materials; damp internal components with higher loss; blocked cooling medium pipelines.