Problems of oxidation and the formation of sludge in insulating oil have occurred in oil filled transformers over many years, but such problems have been rare in oil filled switchgear. This higher occurrence of sludged oil in transformers is generally accepted as being partly due to the higher operating temperatures compared to switchgear.

During the early part of 1996 a number of oil filled 11kV switchgear failures were investigated and were found to contain severely degraded oil which fitted the general description of 'sludged oil'. Subsequently, further switchgear units were discovered to contain sludge oil. This problem
of sludged oils became a national problem.

It appears that certain units are more prone to developing sludging of oil, particularly if free breathing. In addition, the ratio of copper to oil is an important factor. Copper is a catalyst for the oxidation of oil and a higher copper content will increase the probability of a unit becoming effected.

The products of the oxidation of oil are increased levels of acids and water, and in extreme cases the formation of solids and sludge. The low molecular weight acids which are generated are both corrosive and volatile, and account for the corrosion of metals at and above the oil level. When the top covers of sludged units are removed, a very unpleasant and distinctive smell emanates from the headspace of the tank. This indicates that there is a build up of volatile acids and gases in the headspace above the sludged oil.

The solid particulate that forms in the oil tends to accumulate on horizontal surfaces. Tracking of insulation has occurred which in some cases has led to catastrophic failure. Tracking occurs due to the presence of wet contaminants on the surface forming a conducting film. The leakage current through this film is interrupted at a narrow point, which results in a small spark or discharge. This causes localised heating and a small spot of charred material. With further contamination, the process repeats and the spots become extended into lines in the direction of the electric field.

Eventually these lines can cause a flashover when they extend from high potential to earth, or phase to phase. It is difficult to predict when such failures will occur.

Problem Investigation

To help provide a solution to the determination of sludged oils and the corresponding
physical hazards posed by the failure of the switchgear , EA Technology developed a method
to sample the head space gasses from , free breathing systems. The system is a portable
briefcase sized analysis kit, which includes a quality metering pump, 100 drager tubes
and all of the pipes and adaptors required for connection to the air vents in most of the common
types of switchgear. Once attached the pump withdraws a metered amount of head space gasses
for a set period through the dragger tube, a distinctive colour change will be noticed along a length of the tube should sludged oil be present , a colour table allows the user to determine the level of sludging. The Head Space Gas Testing Kit, is available from EA Technology at a price of £1990 . Please contact :- neil.hayden@eatechnology.com for details.