POWER November 2010 - 73

NUCLEAR O&M
tors, although the stress on the capacitors
is believed to be minimal. The expected
life of these capacitors is about 25 years.
Earlier generations of this drive exhibited a
capacitor lifetime of about 10 years.
The bus voltage on the supply bus of the
drives was relatively high because the plant
was shut down and many loads that were
usually running were off. The bus voltage
was near 4,450 VAC. The input transformer
tap setting was at zero, which should support
4,160 VAC ± 416 VAC. The initial
thought was to change the tap setting to
+5%, but this may have resulted in reduced
performance once the plant was back to
normal operating conditions.
The resolution was to desensitize the
excessive drive loss calculation during the
energizing process. Once the drive is running
a loaded motor, the original sensitivity
is restored. The start-up team quickly
resolved this fault with minimal impact on
the start-up schedule, and there has been no
recurrence of the problem.
Medium-Voltage Low-Speed Hold.
If the plant medium-voltage supply is degraded,
a power cell or motor toque limit
may be approached. To prevent a trip, or
damage to the drive or motor, a mediumLoss.
The closed cooling system for the
drive includes two pumps, heat exchangers,
a surge or expansion tank, and a deionizer.
The pump suction pressure should run
between 5 and 10 psi. The QC drives have
exhibited a loss of suction pressure over a
few weeks' time and require repressurization
with air.
The surge tank is an assembly of two
tanks paralleled together with metal to
composite piping fittings. Unfortunately,
the fitting were found to leak the air volume
required to maintain the desired suction
pressure, as the tank walls are relatively
thin and don't provide significant thread
engagement. Sealants have been used to
repair some of the leaks, including sealing
a threaded cap at the top of the tanks.
Pump flow, noise, and vibration are
routinely monitored, and no abnormalities
have been discovered during the low-
suction pressure conditions. The plant is
closely monitoring and repressurizing until
a new tank design can be provided by Siemens.
A final improved design is expected
to be available by December 2010.
Surge Tank Level Anomaly. The surge
tanks are translucent, so water level can be
observed through the tank walls, although
The Perfect Harmony drive uses a redundant
control system.
voltage low-speed hold was programmed
for the drive.
A speed hold is initiated whenever supply
bus voltage drops below 90%, which also
correlated with the first alarm for mediumvoltage
low. During start-up of the recirculation
pumps at minimum speed, a speed
hold was initiated when a reactor feedwater
pump was started, as both the reactor feedwater
pumps and the RRPs are on the same
bus. The bus voltage dipped below 90% of
4,160 VAC for about 6 seconds. The operators
were not used to seeing this behavior
before and identified it as a nuisance.
The team determined that the appropriate
way to implement this feature was
to relate it to a motor torque limit rather
than to the raw bus voltage. Therefore, a
linear relationship was developed relating
input voltage to motor output current. This
change was added to the system software,
and its effectiveness was successfully
tested during the Unit 1 commissioning in
spring 2010.
Cooling Water Pump Suction Pressure
72
a level gauge has been installed. The gauge
is connected to the tank via flexible rubber
hose. The indicated level can be affected if
a loop seal is created at the upper connection
of the gauge. The routing and length
of the upper hose on one of the surge tanks
has created a loop seal after the top hose
was used to fill the tank, which prevented
the water from draining. The hose was rerouted
to prevent the possibility of a loop
seal. Indicated tank water level now correlates
with actual level.
One-Shot Redundancy. As described
in the earlier article, the Perfect Harmony
drive uses a redundant control system.
The 480 VAC sources, 120 VAC sources,
supervisory programmable logic controllers
(PLCs), speed control microprocessors,
and communication network are all
redundant. However, many of the redundant
components are not accessible during
operation due to their proximity to active
medium-voltage components. If an A-side
component fails or power is lost, the drive
will continue to run on the B-side compowww.powermag.com
nent
or source. However, if the component
is not accessible, it cannot be restored with
the drive online. Other components are accessible
and are hot-swappable while the
unit is online.
Even though the speed control microprocessors
are accessible and can be replaced,
they cannot be rebooted and resynchronized
to the master processor during operation.
These processors are controlling the firing
of all the transistors in the power cells, and
a hot resynchronization of that process is
not recommended by Siemens. Exelon has
requested that Siemens explore that process
in hope of Siemens providing a future
enhancement for online restoration.
QC has experienced several backup
PLC failures. No PLC transfers occurred,
because only the backup was affected.
Alarms properly notified the operators of
the failures.
In one case, Siemens suggested that the
PLC be replaced, but doing so would have
required a shutdown. This assumption was
challenged, and an online resynchronization
process was developed. Additional
online software changes were required to
ensure that successful online replacement
processes were proven. The PLC was replaced
and resynchronized with the drive
online. Much testing was completed at the
factory and on a site stand-alone drive simulator
prior to performing this test in the
plant. PLC redundancy was successfully
restored. At this writing the root cause of
the failure is being investigated.
The good news is that these single PLC
failures did not cause trips, and the redundancy
feature of the control system was
successfully demonstrated.
Next, Exelon wants to make the inaccessible
components online-accessible so that
redundancy restoration can be maximized.
However, even with the current design, a
single failure of any control system component
will not cause a drive/pump trip.
Repair and restoration of redundancy can
be planned and then implemented during
a forced or a planned outage or during a
planned single-loop operation. ■
-James W. Morgan (james.morgan@
exeloncorp.com) is a principal engineer
for instrumentation and control with ILD
Inc. (www.ildpower.com). On assignment
to Exelon Nuclear's corporate
engineering department, he is the
lead engineer responsible for Exelon's
fleetwide upgrade of reactor recirculation
pump flow control systems. Timothy
Gode (timothy.gode@exeloncorp.com)
is the reactor recirculating system manager
for Exelon Nuclear's Quad Cities
Generating Station.
POWER| November 2010
http://www.ildpower.com http://www.powermag.com
POWER November 2010

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