POWER October 2016 - 46

OPERATIONS & MAINTENANCE
3. Reduced vibration. High-speed balancing allows for effective testing to ensure minimal
vibrations in any manufacturer's rotor. Courtesy: Sam Fentress Photography
porting it to a maintenance facility. If the rotor
is scheduled for a full rewind, the cycle
can last from 20 to 30 days, depending on the
generator size. A high-speed balance takes
place at the end of the repair cycle. If it's just
a matter of testing and inspecting the unit,
the cycle can be shortened to under 10 days,
including transportation, in-shop inspection,
high-speed balance, and return transport to
the site.
Shaving days off of an outage can be especially
important when regulatory review
is considered. One utility reported that if it
was able to take its unit offline and return it
to full load within a certain number of days,
then the regulatory review of the outage
would be much more favorable. So though
the unit spent an additional two days in the
shop for high-speed balancing, the operator
was still able to return it to service a full
week ahead of schedule and meet the regulatory
incentive.
4. Heat run. A full high-speed thermal test, or heat run, excites generator fields at speed to
assess the quality of the windings. Courtesy: Sam Fentress Photography
Additional Advantages
A high-speed balance offers at least two
other advantages. First, for both turbine
rotors and generator fields, all the major
components can be tested under rigorous
conditions. That means applying the greatest
centrifugal force that the components
are likely to experience during their service
lives, which allows the machine to be precision-balanced
to a fixed base point (Figure
3). By contrast, if the operator were to
complete only a low-speed balance and then
take the unit to speed, some change in the
balance is likely to occur, introducing vibration
and greater wear and tear.
at high speed. These features allow the testing
and balancing of steam turbines, gas turbines,
and generator rotors.
For example, at MD&A's high-speed
balance facility in St. Louis, Mo. (Figure
2), rotors can be turned in a high-tech
vacuum chamber at speeds of up to 4,320
rpm. Through a speed increaser, the facility
can also handle other rotating equipment at
speeds of up to 12,000 rpm.
Using this technology can minimize impact
on a generating unit's outage cycle. In
most cases, only two or three days are re46
quired
to fully test and balance a unit. From
a cost standpoint, completing the high-speed
balance when the machine is in the shop,
versus after a separate transportation cycle,
can be attractive. As a result, performing both
the repair and balancing of a turbine rotor or
generator field at one location means that the
machine will be able to come back online
faster.
Fast Turnaround
A typical repair cycle for a generator rotor
involves pulling it from the stator and transwww.powermag.com
Second,
full electrical testing can be completed
on the generator field as it runs at
speed. One problem that operators typically
face with their generator rotors is shorted
turns, which can lead to changes in vibration
with load due to thermal action as the parts
heat up. Performing a high-speed balance on
a generator rotor provides the opportunity
to perform a flux probe test. This extremely
beneficial test helps ensure that there are no
shorts at speed once the unit returns to service.
Doing a flux test is an advantage, because
any number of short turns can escape
detection in the wind shop simply because
they aren't detectable when the unit is at
standstill. This makes it essential for testing
to be done at speed, and the only way to do
that is with a high-speed balance.
A second type of test in addition to flux
probe is thermal testing, also referred to as a
heat run (Figure 4). In this test, technicians
bring the unit up to full speed and then apply
current to the windings. This allows the technician
to assess the quality of the windings in
terms of their ability to grow symmetrically
POWER | October 2016

http://www.powermag.com

POWER October 2016

Table of Contents for the Digital Edition of POWER October 2016