POWER August 2021 - 28

CONNECTED PLANT
Unique Monitoring Strategy for
the Unique Huntly Power Station
Monitoring steam turbo-generators in a typical thermal power station is normally
straightforward without any particular issues, but Huntly presented special challenges.
When the time came to replace the legacy monitoring system, very demanding
monitoring requirements had to be fulfilled.
Simon Hurricks and Mike Hastings
H
untly Power Station, the largest
thermal power station in New Zealand,
is owned and operated by
Genesis Power Ltd., one of four stateowned
enterprises that made up the
former New Zealand Electricity Department.
Huntly was originally built to operate
four 250-MW units, which were commissioned
from 1981 to 1985 (Figure 1).
Since 2004, a 385-MW combined cycle
plant, Unit 5, and a 50-MW gas turbine
for peaking, Unit 6, were added to the
power station.
The original units, Units 1 to 4, each
consists of a 250-MW turbo-generator, a
steam turbine (single reheat cycle), and
steam-driven main boiler feed pump. A
unique feature of these coal and gasfired
units is that they are mounted on
a steel support structure (Figure 2) to
provide protection in the event of an
earthquake.
Unit 3 has since been retired from
service, and Units 1 and 4 have been
operating to system requirements with
Unit 2 normally dry-stored. This has just
changed recently, however, because the
New Zealand hydropower storage capacity
is well below average for this time of
year and wind power is down. Therefore,
all three units (1, 2, and 4) are currently
2. One of the flexible steel beam support
structures designed for earthquake protection
is shown here from a distance (left) and up
close (right). Courtesy: B&K Vibro
running to conserve water for the predicted
dry winter this year.
One Problem Solved,
Another Created
The steel foundations serve an important
function from a safety point of view, but
proved to be a headache because of their
structural flexibility. The inboard bearing
of the generators (bearing #7) has historically
been subjected to high vibration
at running speed due to a 100 Hz resonance
in the flexible cross beam to which
the bearing pedestal was mounted, and
a lack of horizontal stiffness in the bearing
pedestal. This was compounded by
another resonance just below running
speed during run-up, caused by the
second critical speed of the two-pole
generator shaft. If there was a couple
imbalance, the critical speed vibration
would be excessive during run-up.
Several solutions were considered
to stiffen the structure, but the only option
available at reasonable cost was to
fit dynamic vibration absorbers to the
bearing pedestals. A dynamic vibration
absorber is a relatively small auxiliary
weight attached to the structure by a
flexible steel shaft that acts as a spring,
as illustrated in Figure 3. The combination
of the length of the shaft and the
mass of the weight is tuned so that the
auxiliary weight resonates at the same
frequency as that which is creating the
original problem, which in this case
was 100 Hz.
The effect of adding the absorber was
to split the original resonance into two,
one at a slightly higher frequency and
another at a slightly lower frequency.
As the operational speed is constant,
moving the resonance away from the
rotational speed reduced the vibration at
operational speed, however, it added a
resonance into the machine run-up. The
upper frequency resonance did not present
a problem because it was above the
operating speed.
Even with a reduced 100 Hz horizontal
resonance and a reasonably good
state of balance, the overall vibration
was still high at 24 millimeters/second
(mm/s) at 2,750 rpm during run-up. This
was because the overall vibration is the
vector sum of the horizontal resonance
plus the 1X running speed vibration that
results from being so close to the second
critical speed of the generator rotor
at 2,800 rpm, which cannot be reduced
(Figure 4). The overall vibrations during
run-up at machine speeds from 2,750 to
2,850 rpm can be significantly higher if
unbalance is present.
1. Huntly Power Station is New Zealand's
largest power station by capacity. It is located
about 90 kilometers (56 miles) southeast
of Auckland, New Zealand's largest city.
Courtesy: B&K Vibro
28
3. Dynamic vibration absorbers as seen from bearing #7. (These were mounted on all four
units.) Courtesy: B&K Vibro
www.powermag.com
POWER | August 2021
http://www.powermag.com
POWER August 2021

Table of Contents for the Digital Edition of POWER August 2021
