Solution
for Open
Phase
Faults
By Power System Analysis Team,
Tennessee Valley Authority.
The TVA Power System Analysis Team
(Mark Bowman, Tamatha Womack,
Preston Cooper, and Tim Fallesen)
works out of the corporate office for TVA
Nuclear. TVA formed
this in-house group
in an effort to
preserve design
basis knowledge,
allowing for
significant
improvement in
engineering quality
and cost savings.
They are primarily
responsible for
performing system-
wide electrical
analysis for TVA’s
entire nuclear fleet. Together, the team
has over 70 years of experience in power
plant electrical design and analysis
and provides leadership by active
involvement in industry working groups
such as IEEE and EPRI.
Nuclear Energy Institute’s Top Industry
Practice (TIP) Awards highlight the
nuclear industry’s most innovative
techniques and ideas.
This innovation won the 2015 Equipment
Reliability Award.
The team members who participated
included: Mark Bowman, Sr. Prog
Mgr, Power System Analysis; Tamatha
Womack, Sr. Prog Mgr, Power System
Analysis; Preston Cooper, Prog Mgr,
Power System Analysis; Timothy
Fallesen, Engineer, Power System
Analysis.
Summary
Since the identification of open-
phase fault vulnerability at nuclear plants
worldwide, the nuclear industry has rec-
ognized that no relevant automatic detec-
tion/protection scheme existed for this
new failure mode. TVA’s Power System
Analysis team has developed a multi-
faceted solution that has led to dramatic
cost savings compared to other industry
approaches. Significant advancements
were made on several fronts. These in-
clude enhancements to analytical mod-
eling of key power system components;
the development of new academically-
sound power system analysis method-
ologies; incorporation of those method-
ologies into industry standard validated
analysis software; and verification of key
elements in a first-
of-a-kind field test.
These efforts have
enabled TVA to de-
velop an innovative
and
comprehen-
sive solution: apply
proven protective
relaying at the Class
1E power system
boundary along with
open-phase
detec-
tion technology for
standby transformers.
This solution, which
is transferable to other nuclear plants,
provides direct assessment of the key
parameter needed to ensure safety-relat-
ed equipment protection and increases
nuclear safety by allowing the safety-re-
lated power system to remain connected
to preferred offsite power whenever pos-
sible.
The timely development of this
innovative and cost-saving approach has
allowed virtually seamless alignment
with existing industry standards, NRC
expectations, and the NEI industry
initiative on open-phase. It is projected
to save TVA greater than $17M when
compared to other industry solutions.
Individual elements of this effort
have been nationally and internationally
recognized as key contributors by various
entities, including NEI, NRC, INPO,
WANO, and EPRI.
Safety Response
The main goal of the effort was to
provide the best protection scheme for
the safety-related power system without
increasing the risk of an unnecessary loss
of offsite power (LOOP)*.
This goal has been fully realized in the
TVA solution. The use of proven Class 1E
qualified protective relay hardware in a
Class 1E design configuration provides
complete protection of safety system
functionality during any open-phase
condition while eliminating any common
mode failure mechanism (NRC preferred
design). In addition, the analytical
techniques pioneered by the team
have led to a relay scheme design that
provides this protection while allowing
the maximum margin to ride through
anticipated operational occurrences,
including certain design basis events.
The resulting design solution ensures
that a given offsite power circuit will
be disconnected only if a voltage
unbalance affects the functionality of
safety equipment. Overall nuclear safety
is enhanced by preserving defense-in-
depth; not arbitrarily transferring away
from an offsite power source based solely
on the existence of an open-phase.
Cost Savings
The ability to perform the open-
phase fault analysis and conceptual
design with an in-house power system
analysis team has allowed for millions
of dollars in cost savings for design
and installation when compared to the
expected costs of other industry solutions.
Relying on Class 1E hardware to provide
100% protection of the safety-system
drastically reduces the cost of the overall
design scheme. Not only is the Class 1E
hardware less expensive, but a less costly
commercial-grade version of the EPRI
Open Phase Detection (OPD) system will
only be required on selected (standby)
offsite power transformers. The design
scheme allows the EPRI OPD system to
function as a detection (only) system and
eliminates costly requirements needed
for a safety-system protective function
(redundancy,
qualification
testing,
automatic transfer of safety-buses, etc.).
L-R: Tim Fallesen, Preston Cooper,
Mark Bowman and Tamatha
Womack.
Nuclear Plant Journal, March-April 2016 NuclearPlantJournal.com
45
*Increasing LOOP frequency can
dramatically increase reactor core
damage frequency (CDF).
