POWER April 2016 - 49

OPERATIONS & MAINTENANCE
1. To replace or not to replace, that is the question. In the graph shown here,
the remaining economic value of the exciter is the difference between its risk-cost curve and
the cost of a new one up to the point at which it would be replaced (shaded region). Early replacement,
as part of an upgrade program, must produce savings at least this large to be justified.
Source: BIS Consulting
$45,000
$40,000
$35,000
$30,000
$25,000
$20,000
$15,000
$10,000
$5,000
$0
Economic end of life
Leveled life-cycle cost of a new
exciter
2. Grand Coulee step-up transformers.
With a total installed capacity of
6,809 MW, Grand Coulee has enough spare
transformers (and generation, as it rarely runs
at full capacity) to weather a transformer failure.
Courtesy: BIS Consulting
Annual risk of exciter
failure, increasing with age
Current age
10
20
30
Age of exciter
understanding the variables. The experts who
do that work typically use large databases,
failure probabilities (which often follow
bathtub curves where failure rates decrease,
level out, and then increase as equipment
ages), and then adjust the numbers based on
operations and maintenance history. Every
situation is unique, and many require a rigorous
data-gathering process.
" All 47-year-old stator windings are
not alike, " said Johnson. " Some have been
abused or just had bad luck, and some are
still in pretty good shape. "
Upgrade? Not So Fast!
Many people jump to the conclusion that if
a unit is going to be offline for an extended
period of time, they should do every possible
upgrade or maintenance task that they can
think of to take advantage of the opportunity.
What many fail to take into account is that
cash is valuable. If the benefits of the work
are not realistically assessed and evaluated, a
lot of money can be wasted.
" My working theory about why that happens
is that, oftentimes, the decisions about
what's going to get done get made by engineers
who face the downside of a failure, "
Johnson said. " They look bad if things fail,
but they don't really look all that good if they
save a little bit of money. "
For example, if a project has a $50 million
budget and removing a job from the scope
saves $500,000, the work group may get a pat
on the back for being frugal. But, if the component
later fails as a result of the decision,
the person who made the final ruling could
face heavy scrutiny.
" I think where we've been able to help is
to get down on paper what would be the benefit
of not doing something, " Johnson said.
" So it's really a communication tool as much
as anything else. "
April 2016 | POWER
Aggregating information in a way that allows
decision-makers to see, for example,
that there is a 4.3% chance that something
will fail next year, a 4.37% chance that it will
fail the following year, and escalating percentages
thereafter, is very valuable (Figure
1). The consequences of a failure, such as one
month of lost energy production or collateral
damage to other equipment, must also be factored
into the equation.
Johnson offered one example from a project
he was involved in at the Grand Coulee
Dam (shown in the opening photo). His
group was brought in to evaluate the step-up
transformers (Figure 2) for a couple of the
older powerhouses. There were 18 units total.
He said the U.S. Bureau of Reclamation was
really worried about them failing.
" Our estimate agreed with them, " Johnson
said. " They actually were pretty likely to
fail, but because there were so many of them,
losing one really wasn't such a catastrophic
thing. The cost wasn't really all that high.
So, in our estimation, it was better at the time
to keep multiple spares on hand and tolerate
failure rather than spend a whole bunch of
money replacing all of the equipment. "
Although the Bureau of Reclamation has
since decided to move forward with a more
extensive transformer replacement project at
Grand Coulee, the decision to delay the work
extended the total useful life of the equipment
and saved money in the long run.
New Design Solves Gear Unit
Problems
In some cases, repeated failure makes the decision
to upgrade an easy one. That was the
case for Düsseldorf, Germany-based Uniper.
Its Hemsjo Nedre hydropower plant in
southern Sweden had experienced multiple
problems with an existing horizontal turbine
gear unit.
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The original single-stage gear unit was
supplied in 1981 by ASEA-a legacy brand
of Santasalo. It had been repaired several
times by third-party gearbox service companies,
but it continued to be plagued by high
operating temperatures, and some cracks had
formed on internal components. That was
when Santasalo got involved.
" We went onsite and checked the installation, "
said Henrik Cederberg, area sales manager
for Santasalo. " We made a whole new
calculation for this application. "
After assessing the situation, three options
were proposed to resolve the problems.
The first option was to repair the old ASEA
UAAF 170 unit by installing new internal
components, new bearings, and an upgraded
lubrication system. The second option was
to construct and install a new ASEA UAAF
unit, built in accordance with the old design
documentation. The third option was to create
a completely new design. This option would
allow a modern single-stage helical gear unit
to be installed, including a new lubrication
system and offline filter. Because it would
be built with the same dimensions as the old
ASEA unit, shaft positions, foundations, and
couplings would remain unchanged. Uniper
selected option three.
" A lot of things have changed since ASEA
made the original unit for the client, " Cederberg
said.
He noted that the profile of modern gears
is better than it used to be. These days, gear
teeth are more accurately designed for the
expected or calculated load. Compensation
grinding ensures full tooth contact and helps
reduce noise. Materials are better understood,
and the hardening process is more precise.
Bearings have also improved, with new types
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Annual cost

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POWER April 2016

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