Bushings in oil and SF6

The “SF6 in oil – Bushing seal defects” criticality does not concern transformers directly, but rather bushings, accessories that allow conductors to pass from inside the transformer to the outside without coming into contact with the casing.

In generation step-up transformers (GSU), 14% of the failures are due to bushings (>100KV). CIGRE WG A2. 37 – Transformer reliability survey, Dec. 2015

Bushings are normally placed at the upper part of the transformer and are of different types depending on the voltages/currents in play and the type of external conductor to which they are to be connected. The bushings typically consist of two compartments; the lower one is inserted into the transformer, the upper one is anchored on the transformer casing. The upper compartment can be free or inserted inside metal enclosures filled with, for example, SF6 (sulfur hexafluoride).

For high currents (up to 30 KA), oil-air bushings with a high diameter central conductor are used; for high voltages oil-oil, oil-SF6 or condenser bushings are used, in which the central conductor is wound with layers of insulating paper alternating with layers of conductive material. In high and very high voltage, SF6 gas is commonly used as an insulating medium because it is not subject to ageing, is not toxic or flammable, and has good dielectric properties (nearly 3 times greater than air or nitrogen), and arc-interrupter and thermal properties.

Features of SF6

Sulphur hexafluoride (SF6) is a colourless, odourless, non-toxic, non-flammable, chemically inert gas with high dielectric properties, almost 3 times greater than air or nitrogen.
For over 50 years, SF6 has been used successfully in various industrial applications, especially on switches and disconnectors on electricity transmission and distribution lines. In other fields, it is used, for example, in particle accelerators, radar and the electronics industry or in the medical field, and particularly in magnetic resonance diagnostic machines and in eye surgery.
In the past it was also used in tyres, tennis balls, some types of gym shoes and as insulation in double glazing but its use in these applications has been banned since 2007. 7.

In medium and high voltage electrical equipment, SF6 is used for its high electric arc extinguishing properties due to its high dielectric strength and its capacity for recombination. Thanks to these properties, which are superior to other fluids such as air or nitrogen, electrical substations can be constructed with much more compact dimensions.

SF6 and the environment

Of the gases that cause the well-known “greenhouse effect”, SF6 is the one that requires the highest priority for action in terms of preventing/mitigating global climate change. Along with five other gases – [carbon dioxide (CO2), methane (CH4), nitrogen oxide (N2O), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs)] – SF6 was listed in the Kyoto Protocol (1997). Its climate impact is 22,800 times greater than carbon dioxide (CO2) and its residence time in the atmosphere is around 3,200 years (source:Federal Statistical Office:Survey of particular climate-active materials “sulphur hexafluoride” (SF6), Wiesbaden, 2015)

To prevent its dangerous effects on climate, use of the gas has been banned since July 4, 2007 for “civil/domestic” applications, however, in most industrial applications, SF6 gas continues to be used because alternatives with equivalent performance have not yet been identified.

Severe rules exist around the world to reduce SF6 gas emissions into the atmosphere. In the European Union, the F-Gas regulation, (CE) No. 517/2014, on the limitation of greenhouse gas emissions, came into force in 2014. The regulation sets out the general requirements for the inventory, management and treatment of SF6 gas and other fluorinated gases.

Supplementary information

Diagnostics of HV bushings through oil sampling and analysis

Date: November 2017
From: Transformers Magazine Special Edition on Bushings
Language: English
Author: Sea Marconi

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