Background:
After running under elevated gas levels the
transformer was taken out of service for an on-site internal inspection on 9
May 2008. The internal inspection revealed discolouration on the top core clamp
and blocking with burn marks on the A phase tapping leads [Chauke1]
LEDT:
Figure 6-1 is the
Low Energy Degradation Triangle for this case study where the
first trigger of defective state was received on 18 June 1996. On the LEDT due
to the θ value this was
within the adjusted boundary but the R-value was above the 0.173 limit. This
trigger did not sustain the defective state where subsequent samples for the next
four years were all in the normal state. The next defective state trigger was
on the 7 November 2002 (due to the θ compensation) and then on the 13 November 2002. This was a true trigger
as subsequent samples were in the defective region. The absolute ppm values for
the three gases were H2 1 ppm, CH4 1ppm and CO 8 ppm. The
sample on the 18 November 2003 indicated further deterioration with ppm values
being; H2 35 ppm, CH4 15 ppm and CO 47 ppm. The
progression of the trend was along the CO axis indicating an increasing
percentage of H2 with constant percentage of CH4 and a
decreasing percentage of CO. The distribution of the samples was more widespread
in 2006 where the distribution started to contract in 2007 and further in 2008.
Combustible Gas Trend:
From figure 6-2
the combustible gas trend it is observed that initially the levels of carbon
monoxide was high with elevated levels of methane. The transformer oil was then
reconditioned after 22 October 2001. Evidence of this was seen in the next oil
sample taken on 6 March 2002 where the gases all lowered in values. The most
significant change was in carbon monoxide, methane and ethane. From the 13
November 2002 which was a trigger point in the LEDT the level of hydrogen
started to increase with an up down trend until the 8 May 2005. During this
period no conclusive diagnosis could be made just looking at the gas trends,
however with the LEDT the trend was well into the defective region. On the 22
March 2005 the next step change in gas increase was noticed before the hydrogen
levels started to take off above the 100 ppm levels. The methane levels
continued at a level between 30-40 ppm until 17 April 2008 when the next change
in gas levels occurred. This was the final change before failure occurred.
R-Value
The R-value
remained below the 0.173 limit from the period 1988 until the beginning of
1996. The first trigger out of this limit was 18 June 1996. This position
however did not sustain until the next trigger on 7 November 2002 (due to
compensation by θ
on the LEDT) and 13 November 2003. From this point all R-values was out of the
limit. This was a clear indication of a developing fault condition. The trigger
on the 7 November 2002 was received at least 5 years and 4 months prior to
being taken out of service.
Case Study A: Summary of Results
This case study A provides results indicating the
early detection of a defective state. The trigger was received at least 5½ years before it was finally taken out of service.
The initial result of normal state does correlate with the LEDT normal region
where all results were within the normal boundary. When a defective trigger was
received further results of the defective state was consistent with that of the
LEDT. There was also an increasing trend of the R-value with time. Another
positive aspect observed is that even though the combustible gas trends were
erratically increasing and decreasing cyclically the LEDT was relatively insensitive
to these changes such as to provide an increasing trend.
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