POWER February 2012 - 42

INSTRUMENTATION & CONTROL
Specifying Nuclear DCS
Power Supplies
The consideration of power supplies has become critical to the success of converting
analog instrumentation and control systems to digital control systems
(DCSs). Careful planning is particularly necessary for nuclear power
plants, where instrumentation systems are required for safely shutting
down a reactor, mitigating the consequences of an accident, and performing
post-accident analysis.
By Paul E. Stanley, PE; Darrell W. Cooksey, PE; and Tom H. Crawford III, PE, Hurst Technologies Corp.
The duration of a voltage sag affects
ith the conversion of existing analog
instrumentation and control
(I&C) systems to digital control
systems (DCSs), careful consideration and
planning of power supply systems has become
critical. This article addresses the
most important issues that must be considered
in the design, operation, and maintenance
of power supply systems for nuclear
facilities, specifically: power quality, redundancy
and diversity of supply, good
planning prior to installation, and maintenance
of the equipment over its lifespan.
Pursue Power Quality
Most DCSs are provided with power conditioning;
however, the equipment selected
to provide power for these systems should
meet an established set of minimum requirements
for voltage regulation and
noise suppression to ensure that the output
of the uninterruptible power supply (UPS)
meets the manufacturer's requirements for
the DCS.
According to the Electric Power Research
Institute's (EPRI) Report TR1016731,
the most commonly reported
causes of failure for nonsafety-related
DCSs are electromagnetic interference
(EMI)-induced disturbances and power
supply transients and failures. Power supply
transients can include supply voltage
surges or spikes, frequency deviations, and
noise induced by harmonic content.
Voltage and Frequency Stability. Results
of testing documented in the EPRI
Report TR-1001072 reveal that programmable
logic controllers (PLCs) are most
susceptible to the depth, duration, and
likelihood of voltage sags. Disturbances
to the power system voltage and frequency
can result from lighting, short circuits, and
switching of loads such as capacitor banks.
All of these disturbances can cause transient
voltage surges and spikes.
42
PLC performance and can result in premature
system shutdown in some cases. Even
momentary voltage surges can result in
malfunction of input and output circuits,
especially if the PLC power supply is also
used to provide sensing voltage for the
inputs. For example, a UPS inverter produced
a square wave output that was incompatible
with three of five PLCs tested.
In contrast, a UPS inverter that produced a
true sine wave output effectively mitigated
voltage sags.
Regulation, Monitoring. The addition
of a constant voltage transformer to the
PLC's power supply was found to be very
effective in improving the ability of the
PLC to ride through such voltage surges
and transients. UPS systems that supply
equipment that includes a DCS should be
designed to produce a true sine wave output
to reduce the impact of harmonics.
Battery chargers are not designed to
operate as a stand-alone rectifier for an
inverter and, therefore, must be always
connected in parallel with a set of batteries
for the UPS inverter to properly compensate
for any transients.
Electromagnetic Interference. EMI and
power surges can affect the performance of
safety-related equipment in nuclear power
plants. This is especially true for DCSs,
where noise from EMI can be misinterpreted
as legitimate logic signals. The U.S. Nuclear
Regulatory Commission's Regulatory Guide
(RG) 1.180 addresses this issue. DCS equipment
must be tested in accordance with the
standards recommended in this guideline.
UPS equipment should meet the minimum
standards for radiated and conducted emissions
to ensure electromagnetic compatibility
with the control systems to which they supply
power.
EPRI TR-102323 provides guidelines
for data collection, testing, and practices
www.powermag.com
W
to ensure electromagnetic compatibility
in DCSs in nuclear power plants and
also to meet the requirements specified in
RG 1.180. In addition, surge suppression
should be installed on all power and control
circuits surrounding these systems to
eliminate unwanted disturbances or noise.
Consider Redundancy and Diversity
All DCSs should have more than one power
supply, at least one of which will allow for
an orderly shutdown in the event of a loss
of offsite power. All DCSs should be fed
from an AC power source that allows orderly
shutdown and data retention during
the loss of normal AC power. DCSs that
provide redundant design functions should
be set up such that failure of a single power
supply will not result in multiple failures
and/or a loss of more than one system.
The need for UPS-backed power is primarily
dependent on the criticality of the
control system and the availability of sufficient
emergency power until normal AC
power can be restored. Control systems
with a single AC power feeder should
always be powered from a UPS-backed
source. Those with multiple redundant
power supplies should be powered from at
least one UPS-backed AC power source.
Consider supplying the UPS for critical
control systems from a diesel generator-backed
source, especially for those
systems needed for safe shutdown and
subsequent monitoring following a loss of
normal AC power.
Power supplies to multi-train systems
should be powered from at least one UPSbacked
power supply. Where operation is
needed during a loss of normal AC power-such
as in shutdown systems, data
loggers, and vital computers and alarm
systems-consider a second UPS-backed
source to maximize reliability and eliminate
single-point failure.
POWER | February 2012

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POWER February 2012

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