Background [2]
On the 15 May 2005
this 390 MVA GSU transformer failed. During the previous unit outage the
transformer oil was purified to remove moisture. The transformer was initially
commissioned in October 1971. In 1996 the transformer was involved in a
coupling transformer incident with another unit and was sent to the repair
facility for refurbishment in April 1997. It was reinstalled in February 2000
where it started to gas and was removed for an internal inspection revealing
loose flexible connections. This was repaired on-site and returned to service.
The gassing continued
slowly and the transformer was removed from service for an internal inspection
at the transformer repair facility which revealed a fault on the HV winding
crossover. This was repaired. The impulse test although acceptable revealed the
absolute value of the C-phase (300 pico-coulombs). PD test results being higher than that for the A and B phases (200
pico-coulombs).
The transformer was
then installed on 14 June 2004. Between September-December 2004 the on-line DGA
indicated an upward trend. The unit was then shut down for the generator
replacement project and the last sample was taken on 22 December 2004. The
transformer was returned to service on 25 March 2005 before undergoing oil
purification where it failed on 15 May 2005. After the failure the internal
inspection revealed damage to the C-phase HV winding as depicted in figure 5.
Analysis
From Figure 6, the
LEDT for the period 1991 to 1994 the oil samples remained in the Normal region until the first trigger on
the 21 October 1994 with hydrogen being 12 ppm, methane 56 ppm and carbon
monoxide 245 ppm. After the coupling transformer incident in 1996 the sample
taken on the 11 September 1996, moved into the T2 region with hydrogen being 59 ppm, methane 197 ppm and carbon
monoxide 322 ppm. Subsequent samples were all in the defective region T1/T2 and the trend was along the %CH4
axis. The transformer was then taken out of service in April 1997 for repairs.
The LEDT in figure 7 represents
samples for the period 2000 to 2005. The trend recorded at the top of the
triangle was in the second period (4 January 2000 to 13 March 2000) after the
transformer was repaired. As can be seen from this LEDT the transformer was
already in a bad state in the T3
fault region. The sample values at commissioning were 225 ppm hydrogen, 434 ppm
methane and 112 ppm carbon monoxide. On-site internal inspection revealed loose
flexible connections, which were repaired on-site and returned to service. The
trend still progressed within the T3
fault region and finally the transformer was taken out of service on 13 March
2000 where the recorded values for hydrogen, methane and carbon monoxide were
166 ppm, 530 ppm and 27 ppm respectively. The transformer was sent to the transformer
repair shop for repairs.
The transformer was
then installed on the 14 June 2004 with the oil sample results in the Normal region but soon after as observed
in figure 7 the trend started to move into the defective region. The oil was
then purified in an outage in December 2004 where the sample taken before this
on the 22 December 2004 revealed hydrogen, methane and carbon monoxide to be 14
ppm, 79 ppm and 226 ppm respectively. After the oil purification these values
were 2 ppm, 2 ppm and 38 ppm accordingly moving the transformer state back into
the Normal region. The next trigger
was received with the oil sample taken on the 13 May 2004, a day before the
failure.
Figure 8 indicates
the R-value trends with the initial samples below the 0.13 limit. The first
trigger out of this limit was 21 October 1994. The next trigger was on the on
11 September 1996 after the coupling transformer incident. The R-values
thereafter remained in the defective region (T2), even after the transformer was reinstalled in January 2000
after the on-site repair. When the transformer was sent to the repair shop in
2004 the samples started in the Normal
region but soon immediately moved into the defective region (T1). This was once again noted after the
purification of the oil where it moved into the defective region almost
immediately thereafter.
Summary
In this case study
the initial oil results recorded were in the Normal region and then progressed to T1 and T2 of the LEDT shortly
after the coupling transformer incident. The LEDT was consistent with capturing
these changes. An interesting observation is made in the period when the
transformer was installed in early 2000 where the oil sample results were
focused on the top of the triangle. These were for high levels of methane and
moderate levels of carbon monoxide. However after the on-site repair and with
the reconditioning of the oil the actual ppm values of the gases decreased but
the oil samples still remained in the top region of the triangle indicating
that the ratio of these gases was still the same but in lower ppm values. This
interesting observation provides some indication that the LEDT maybe
insensitive to fluctuations in the actual ppm values.
