Substation Changed CT Technology to Resolve Problems of Oil Leaks

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Oil leaking from current transformers presents a problem not only from the obvious maintenance perspective but also in terms of environmental impact. One Chinese power utility faced with exactly such a situation at one of its important substations has chosen to respond by changing over all ageing oil-filled CTs to new dry types.


Longtang Substation is one of the important nodes in the 220/110 kV network supplying power to the bustling commercial city of Shenzhen in southeast China. Built in 1992, the Longtang is exposed to heavy contamination, mostly in the form of dust build up from large nearby construction projects. This requires the entire station to be washed at least once a year, typically right after the dry winter season.

Only three oil-filled CTs remain at the 220 kV side of Longtang, two of which continue to leak oil. Old 110 kV units, recently removed from 110 kV side also show evidence of oil coating porcelain housings. oil leak Substation Changed CT Technology to Resolve Problems of Oil Leaks Screen Shot 2018 10 01 at 09
Only three oil-filled CTs remain at the 220 kV side of Longtang, two of which continue to leak oil. Old 110 kV units, recently removed from 110 kV side also show evidence of oil coating porcelain housings.
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oil leak Substation Changed CT Technology to Resolve Problems of Oil Leaks Screen Shot 2018 10 01 at 09
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For years now, a number of the current transformers installed at Longtang have been leaking. Apart from the need to regularly monitor oil levels in these ageing transformers and clean up after the leaks there has been another issue to contend with. The oil leaking out from the units has coated the surface of the equipment’s porcelain housings in a sticky layer of grease that traps yet more pollution and has proven very difficult to remove.

oil leak Substation Changed CT Technology to Resolve Problems of Oil Leaks Screen Shot 2018 10 01 at 09
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oil leak Substation Changed CT Technology to Resolve Problems of Oil Leaks Screen Shot 2018 10 01 at 09
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Shen Hong, an engineer with the Guangdong Power Grid that operates Longtang, explains that starting in 2000, a new design of dry type CT was purchased from a supplier near Beijing and a program started to replace all the old CTs. Today, only a handful of the original oil-filled units remain in operation and these are also scheduled to be replaced as soon as an opportunity allows.

oil leak Substation Changed CT Technology to Resolve Problems of Oil Leaks Screen Shot 2018 10 01 at 10
Most overhead lines entering and exiting Longtang substation are equipped with line surge arresters for improved reliability.
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Apart from dry-type CTs, a new generation of switches has also recently been installed at Longtang as part of the retrofit program. oil leak Substation Changed CT Technology to Resolve Problems of Oil Leaks Screen Shot 2018 10 01 at 10
Apart from dry-type CTs, a new generation of switches has also recently been installed at Longtang as part of the retrofit program.
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According to Wang Min, the sales agent representing the manufacturer of the new generation of CTs, environment due to bushings, which feature silicone weathersheds.

Shen explains that the changeout has been timed to cause minimal disruption and requires seven workers about 3 days to make the conversion for each affected circuit. He also notes that the new designs have been performing well since being put into operation.

oil leak Substation Changed CT Technology to Resolve Problems of Oil Leaks Screen Shot 2018 10 01 at 10
(Right) New dry type CTs (foreground) ready to be put into service to replace aged oilfilled CTs (Left) at the 110 kV side of Longtang.
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Longtang handles 15 incoming 220 kV circuits and because of the importance of this substation most of the lines entering and exiting are equipped with line surge arresters.