POWER January 2011 - 15

not involved. The plot of the gas levels
is shown in Figure 3.
After the Dresden unit was monitored
overnight and the data showed continuously
increasing gases, maintenance
personnel tried decreasing the load but
saw no change in gassing rate. The unit
was subsequently taken off-line. Inspection
revealed that several low-voltage
cable conductors had varying degrees
of damage and one was burned through
(Figure 4). Apparently, it was the burn3.
Continuous monitoring. The concentrations of important dissolved gases are continuously
monitored in Exelon's Dresden 2 transformer. The event on August 28, 2005, is clearly
visible at the right of the plot. Courtesy: Exelon Corp.
Hydrogen (H2)
Carbon monoxide (CO)
Ethylene (C2H4)
10,000.00
Oxygen (O2)
Carbon dioxide (CO2)
Acelylene (C2H2)
Methane (OH4)
Ethane (C2H6)
Total dissolved combustible gas
1,000.00
100.00
10.00
1.00
180.00
160.00
140.00
120.00
100.00
80.00
60.00
40.00
20.00
0.00
0.10
8/7/2005
8/14/2005
8/21/2005
8/28/2005
4. Detecting damage. Damaged cable connectors at the Exelon Dresden 2 plant resulted
in increased levels of flammable gas in the transformer oil. Courtesy: Exelon Corp.
-20.00
-40.00
through event on August 28 that caused
the high rate of gassing.
Ultimately, the Dresden unit was replaced
by a new 952 MVA transformer,
which itself had a problem that was
caught by online monitoring. As the
weather changed from mid-winter conditions
to 75F to 80F in the spring of
2006, the gas monitor detected increases
in several of the flammable gases.
Upon close inspection, it was determined
in this case that the problem was
being caused by insufficient cooling of
the transformer oil. The situation was
rectified as soon as Exelon turned on
additional coolers in the transformer.
Positive Online Monitoring
Results
Subsequent to the events at Dresden,
an event occurred in January 2009 at
Exelon's Peach Bottom nuclear power
plant in Pennsylvania that resulted in
an increase in the total dissolved combustible
gases in one of the plant's main
power transformers from 1,800 ppm to
above 2,600 ppm in a 24-hour period,
including an increase in acetylene that
had never been recorded before. A 75%
load reduction resulted in no significant
change to the elevated levels of combustible
gas.
This event was preceded by a slow
upward trend in the concentration of
hot metal gas (on average 10 ppm/day,
primarily ethylene) in the 36-year-old
single-phase 390 MVA unit that had
caused the utility to schedule a replacement
of the unit in fall 2009. Because
of the January event, the transformer
was taken offline and replaced with an
on-site spare, avoiding what could have
been a catastrophic transformer failure
plus an associated reactor scram and
many weeks of lost generation.
Online dissolved gas monitoring is
proving its value in other Exelon plants
as well. Besides Dresden and Peach Bottom,
Exelon has used online monitoring
at its Limerick, Pennsylvania, reactor
plant. Recently, an eight-gas monitor
was set up at the plant to take hourly
measurements in order to monitor an
aging generation step-up (GSU) transformer.
Plant personnel can now monitor
the load and gassing response of their
GSU in order to ensure that it remains
within specified limits.
The Power Industry Is on the
Bandwagon
In all of these cases, frequent oil analysis
monitoring and preestablished action
14
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POWER | January 2011
Gas concentration (ppm)
http://www.powermag.com
POWER January 2011

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