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STP 3&4 Presentation on NRC Bulletin 2012-01 February 18, 2014 1
South Texas Project Units 3 & 4 Presentation on Open Phase Condition NRC Bulletin 2012-01
STP 3&4 Presentation on NRC Bulletin 2012-01 February 18, 2014 2
Attendees Scott Head, Regulatory Affairs Manager, STP 3&4
Steve Thomas, Engineering Manager, STP 3&4
Al Gutterman, Morgan, Lewis & Bockius LLP
Jim Tomkins, Licensing, STP 3&4
Dale Wuokko, Licensing, TANE
STP 3&4 Presentation on NRC Bulletin 2012-01 February 18, 2014 3
Desired Outcomes
STP 3&4 Electrical Distribution System and Switchyard
Open Phase Condition (OPC) Strategy
Agenda
STP 3&4 Presentation on NRC Bulletin 2012-01 February 18, 2014 4
Review the response of the STP 3 & 4 design to an open phase condition.
Understand the rationale for NINA responses to NRC Bulletin 2012-01.
Establish a framework for the development of an acceptable pathway toward timely resolution.
Desired Outcomes:
STP 3&4 Presentation on NRC Bulletin 2012-01 February 18, 2014 5
DG DG DG
CTG
UAT A UAT B UAT C RAT A RAT B
PG
PIP
Class 1E DIV. I DIV. II DIV. III
Main Gen
Main Transformer 3 Single Phase
Breaker
Normal Alignment
Other Unit
345kV 26kV 13.8kV 4.16kV
CTG 1 CTG 2
CTG 3
STP 3&4 Presentation on NRC Bulletin 2012-01 February 18, 2014 6
STP 3&4 Design Characteristics
1. Generator circuit breaker which allows for immediate backfeed without a fast or slow bus transfer.
2. Three, 4.16 kV Class 1E Divisional buses, two normally powered from the main transformer/UAT’s and one normally powered from RAT.
3. Two RAT’s vs. one 4. Three independent undervoltage/degraded voltage
relays connected phase-to-phase in a delta configuration with 2-out-of-3 coincidence logic. Loss of single phase will cause two of the three relays to trip.
5. Wye/delta MPT configuration 6. Class 1E bus is normally loaded
STP 3&4 Presentation on NRC Bulletin 2012-01 February 18, 2014 7
STP 3&4 345kV Switchyard
Unit 4 Main
Transformer
Unit 4 RAT A
Unit 4 RAT B
Unit 3 Main
Transformer Unit 3 RAT A
Unit 3 RAT B
345kV Line
345kV Line
345kV Line
345kV Line
345kV Line
345kV Line
STP 3&4 Presentation on NRC Bulletin 2012-01 8
STP 3 & 4 Electrical Distribution System cont’d
Main Transformer backfeed The wye/delta connection recreates the lost phase in
the delta connected secondary The transformer MVA rating is about 15 times greater
than the load needed Negative sequence currents are relatively small
~1.5% under design load conditions STP Unit 2 experienced a single phase event on high
side of Main Transformers in 2001 Generator tripped, safety loads operated without issue
STP 3&4 Presentation on NRC Bulletin 2012-01 9
STP 3 & 4 Electrical Distribution System cont’d
Motors are rated 15% greater than driven equipment maximum brake horsepower.
NEMA “derating” to 90% (0.9 x 1.15 = 1.035) would accommodate a voltage unbalance of 3% and resulting negative sequence currents.
IEEE suggest voltage unbalance protective setpoints (if provided) be set at 3%.
Voltage unbalance of 1.5% would not degrade motors. An alarm provides adequate time for operators to realign
available power feeds to ESF buses without the sudden loss of power resulting from an automatic disconnect.
STP 3&4 Presentation on NRC Bulletin 2012-01 10
Undervoltage and Degraded detection Uses 3 independent undervoltage relays Actuation of undervoltage and degraded voltage isolates the
ESF bus and starts the EDG. The primary side of each of the instrument potential
transformers (PTs) is connected phase-to-phase (i.e., a “delta” configuration) such that a loss of a single phase will cause two of the three undervoltage relays to trip, thus satisfying the two-out-of-three logic. (DCD 8.3-1)
STP 3 & 4 Electrical Distribution System cont’d
STP 3&4 Presentation on NRC Bulletin 2012-01 11
STP 3 & 4 Electrical Distribution System cont’d
DCD 8.3.1 states: The loss of voltage condition and the degraded voltage condition are sensed by independent sets of three undervoltage relays (one on each phase of the 6.9 kV bus) which are configured such that two-out-of-three trip states will initiate circuitry for transferring power from offsite power to the onsite diesel generator (after a time delay for the degraded voltage condition). The primary side of each of the instrument potential transformers (PTs) is connected phase-to-phase (i.e., a “delta” configuration) such that a loss of a single phase will cause two of the three undervoltage relays to trip, thus satisfying the two-out-of-three logic.
TS Bases 3.3.1.4 states: The signals for this Function originate in undervoltage relays connected to each phase of the 4.16 kV bus. The phases are connected so that the loss of a single phase will cause two of the undervoltage relays to trip. The three undervoltage relays are combined in 2/3 logic so that a loss of any phase will cause starting of the associated DG while a failure in one of the relays will not cause a spurious start. A time delay is provided to prevent starting the DG due to transient conditions on the bus.
STP 3&4 Presentation on NRC Bulletin 2012-01 February 18, 2014 12
OPC Strategy
NEI Open Phase Strategy
Safety functions remain available given an open
phase condition
Detect within a reasonably short
period of time
Safety functions do not remain operable
given an open phase condition
Install detection and automatic separation
Modify the plant such that safety functions remain
available
STP 3&4 Presentation on NRC Bulletin 2012-01 February 18, 2014 13
OPC Strategy Why is STP 3&4 different?
Proposed industry schedule – 2017 timeframe Susceptibility to open phase events is design
specific. STP 3&4 has a robust design. Safety functions remain operable.
STP 3&4 design is not complete; STP 3&4 equipment has not been procured
ITAAC will confirm the adequacy of the as-built design
STP 3&4 Presentation on NRC Bulletin 2012-01 February 18, 2014 14
Questions and Comments