POWER June 2010 - 18

Variable-Frequency Drives
Upgrade Reactor
Circulating Pumps
A recent trend in nuclear power plant upgrades
has been the replacement of the
motor-generator (MG) sets that drive the
reactor circulating pumps with variable-frequency
drives (VFD). Siemens' first application
of VFDs in this industry began in 2000
with an installation of six VFDs at the Browns
Ferry Nuclear Plant. The use of the VFD continues
to expand, and upgrades were recently
completed at several U.S. plants, including
the Hatch Nuclear Plant in Georgia.
MG sets were originally used to drive the
reactor circulating pumps (Figure 1). Their
purpose was to control reactor reactivity
by changing pump speed and therefore the
rate of fluid flow through a boiling water
reactor. Today, pumps retrofitted with VFDs
permit much more precise speed control
and have the potential for achieving speeds
(and fluid flow) above the speed set by the
line frequency of 60 Hz. An added benefit
is the combination of an AC induction motor
powered by a VFD, which results in a
much more efficient combination than an
MG set. The VFD can provide very slow acceleration
rates when low-power operation
is required. Because raising pump speeds
raises reactor power, plants are very cautious
about speed control.
Replacement of the MG sets addresses
the obsolescence and maintenance issues
associated with this older technology but
also yields several advantages once VFDs
are installed and operating.
Considerable Efficiency Savings
Typical MG sets are less than 70% efficient
compared to the current technology, Siemens'
Perfect Harmony VFDs, which have efficiencies
of greater than 95%. In fact, the
actual measured efficiencies at full load for
eight new VFDs installed in nuclear plants in
the U.S. ranged from 95.7% up to 96.6% for
various loads and speeds. At full load and
full speed (usually 60 Hz), the maximum efficiency
of about 96.5% is achieved.
When considering a VFD upgrade, make
sure that an integral transformer and input
and output filter components are part
of the package. Not all suppliers include
these components as part of the VFD system;
if that is the case, they must be separately
added. These components can add
anywhere from 0.5% to 1% of additional
losses, so a purchaser must also ensure that
18
1. Three-ring circus. The tertiary loop of the pressurized water reactor uses a circulating
pump to condense the turbine exhaust steam. By using a variable-frequency drive to control
the circulating pump, significant reductions in auxiliary power are possible, as well much more
precise control of the reactor. Source: Siemens LDA
Turbine
Secondary
loop
Pressurizer
Steam generator
Coolant
pump
Reactor
Primary loop
Containment wall
quoted efficiency includes the losses of all
the system components, not just those that
are part of the quoted package.
With the large-horsepower motors currently
used on the circulation pumps, the
savings due to efficiency improvement is
significant. One plant that replaced MG
sets with VFDs has reported a reduction
in house load of 2.7 MW when two 5,600hp
VFDs were installed. Another plant reported
a reduction in house load of 4 MW
when two VFDs were installed in a 7,300hp
application. The investment payback
for those two projects was approximately
one year based on house load reductions
alone. Other VFD advantages, such as increased
load, will further shorten the investment
payback (Figure 2).
Precise Speed Control
MG sets are usually difficult to control,
especially when trying to maintain power
production at the maximum allowable
level. The Siemens Perfect Harmony VFDs
can predictably and accurately control the
circulating pump speed to within 1 rpm of
the desired speed command. This allows
the plant to operate closer to the allowed
thermal power limit of the reactor.
Operating closer to the reactor thermal
power limit allows the plant to generate
additional electricity. The amount of additional
energy is plant-dependent but usually
results in an extra 1 MW to 4 MW when
using VFDs. If a plant can currently operate
within 3 MW or 4 MW of the plant's licensed
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2. Inspired investment. In addition to
more precise load control, a VFD is more efficient
than an MG set. Expect to enjoy between
a 1 MW and 4 MW reduction in auxiliary loads
using a VFD and a payback of the investment
within one year. Courtesy: Siemens LDA
Condensate
pump
Tertiary wall
Circulating
pump
Moisture separator
and reheater
Generator
limit with MG set controls, then typically
an extra 1 MW is available when using the
Siemens VFD. In some cases, plants can
only reliably maintain MG set control within
12 MW to 15 MW of their thermal limit.
In those situations, a 4-MW increase can
be achieved with the addition of the VFD
control. This is a significant cost advantage
to a plant, and the return on investment is
often as great as the house load reduction
from improved VFD efficiencies.
Precise Power Maneuvering
As a safeguard to prevent damage to nuclear
fuel and fuel rods, laminar circulating
water flow is desired, as well as slow
rates of power increase. These rates of
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POWER June 2010

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