POWER March 2020 - 35

GAS POWER
power price and previous de-rates during
such conditions, the project is expected
to pay for itself within about two and a
half years.
Technical Excellence
A generator cooling gas change from
hydrogen to helium won CTOTF's Innovation
Excellence award in the technical
category. The project was completed at
Oglethorpe Power Corp.'s (OPC's) Doyle
Energy Facility in Walton County, Georgia.
As most power plant personnel know,
hydrogen is regularly used as the cooling
medium in large electric generators
because it is better than air at transferring
heat. In fact, it is seven times more
effective, and it has one-fourteenth the
density of air, which means there are
less friction losses in the generator
when hydrogen cooling is employed.
Doyle Unit 1 is a GE Frame 7B gas
turbine with a 64-MVA/14.4-kV, 30 psi
hydrogen-cooled generator, which was
manufactured in Lynn, Massachusetts, in
1968. In 2017, OPC engineers conducted
a thorough assessment of the generator.
The inspection and testing indicated that
the stator asphalt insulation had deteriorated.
Evaluators estimated the unit had less
than two years of remaining life. Additionally,
several operational performance and
equipment condition gaps were found on
the unit's hydrogen cooling system.
The cost of getting the hydrogen cooling
system up to industry standards was
not insignificant. The estimated investment
required to upgrade the hydrogen
control panel, add auto-purging logic, install
hydrogen hazardous gas detectors,
make general system repairs, and refurbish
the seal oil system regulator and
float trap was approximately $400,000
to $500,000. The combination of issues
and the short expected remaining life led
OPC's team to look for alternatives to hydrogen
cooling in order to continue operating
the unit as long as possible without
investing a great deal of additional capital.
Air was considered as a cooling medium
alternative, but it would have resulted
in greater than a 50% de-rate of
the machine. So, with support from industry
subject matter experts, OPC personnel
researched using helium instead.
Compared to hydrogen, helium has very
similar thermal conductivity properties,
but it is about twice as dense. This could
be compensated for by operating the
helium pressure at half of the hydrogen
design pressure.
There are several additional benefits
of using helium. They include lower gas
March 2020 | POWER
consumption because it would leak less
easily; it's an inert/non-flammable gas; it
doesn't react with oxygen, making the
purity very stable; and it doesn't require
scavenging to maintain purity throughout
operations. One potential disadvantage
was the cost of helium, which was
about three times that of hydrogen at the
time of testing in 2018. Still, the team decided
it would be worthwhile to test the
helium cooling option given the short remaining
life expectancy of the generator.
Testing was completed in April 2018
with successful results. The unit operated
across its full range of loads with
minimal impact on generator operating
temperature. Generator gas and stator
temperatures were only elevated by 20F
to 25F at 175F.
During the 2018 peak summer season,
Doyle Unit 1 was dispatched 12 times for
peaking purposes, operating 57.2 hours.
As predicted, a generator fault caused by
age and insulation deterioration did occur
as the summer was winding down; however,
an inspection found no connection
between helium cooling and the failure.
In summary, the helium cooling alternative
eliminated highly flammable hydrogen
gas from the unit, and the helium
usage rate proved to be meaningfully
less than hydrogen, which meant the
cost differential between the two gases
was insignificant. This operational testing
of helium cooling verified the viability of
the gas as a cooling alternative for power
generators-knowledge that could be
useful in the future.
Maintenance Excellence
CTOTF recognized Exelon Generation
with its Innovation Excellence award in
the maintenance category. The awardwinning
project came about because
Exelon's Renewables division was finding
high gas content in some of its padmounted
transformers (PMTs). Engineers
determined that frequent changes in power
levels, caused by wind turbines ramping
up and down regularly with changing
wind speeds, were inducing immense
stress on PMTs, resulting in gases being
produced in the transformer oil.
Hydrogen in transformer oil, above all
other gases, is a key indicator of potential
problems. Therefore, Exelon's engineers
decided part of the solution would
be to install new monitoring systems to
better track hydrogen levels. They identified
Fairport, New York-based Qualitrol
Co. as a supplier that could help. Qualitrol's
DGA LT1 (Figure 2) is a wireless,
self-powered dissolved gas analyzer that
www.powermag.com
2. The DGA LT1 is a dissolved gas analyzer
designed as a wireless sensor to measure
dissolved hydrogen in transformer oil as well
as transformer oil temperature and moisture
concentration. It collects data without the
need for additional power or communications
infrastructure. Courtesy: Qualitrol
monitors hydrogen, as well as moisture
concentrations and temperature in the
dielectric insulating oil.
In the past, it's been impractical to install
dissolved gas analyzers on smaller
electrical assets due to high capital costs,
and even higher installation costs when
power sources and communications networks
are not readily available. With Qualitrol's
wireless, self-powered equipment,
however, the installation could be accomplished
at a much lower cost, and it could
be done in a fraction of the time.
With the use of this new solution, Exelon
can now trend hydrogen levels in
real time to aid engineers in understanding
how the PMTs operate under these
unique conditions. It also enables closer
monitoring of PMTs with extremely
high hydrogen, so operations can be
managed to prevent transformers from
reaching catastrophic failure thresholds.
Although this particular installation pertained
specifically to wind turbines, it is
a shared application that can be used at
CT sites as well.
Environmental Stewardship
Excellence
Las Vegas, Nevada-based NV Energy
won CTOTF's environmental
stewardship
award for its power plant air permitting
approach. Historically, the company's
generating station air permits did not allow
excursions beyond normal emission
limits, and the Clark County Department
of Air Quality (DAQ) would not issue variances.
Therefore, when an operational
problem resulted in high emissions, the
typical response was to shut the unit
down to avoid exceeding a permit limit.
Workers would fix the problem while offline
and then restart the plant, but the
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POWER March 2020

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