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