05000373/LER-2017-003

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LER-2017-003, Automatic Reactor Scram due to Main Generator Trip on Differential Current During Back-Feed Operations
Lasalle County Station, Unit 1
Event date: 02-13-2017
Report date: 04-14-2017
Reporting criterion: 10 CFR 50.73(a)(2)(iv)(A), System Actuation
3732017003R00 - NRC Website
LER 17-003-00 for LaSalle County Station, Unit 1, Regarding Automatic Reactor Scram due to Main Generator Trip on Differential Current During Back-Feed Operations
ML17104A085
Person / Time
Site: LaSalle, Lasalle Exelon icon.png
Issue date: 04/14/2017
From: Trafton W J
Exelon Generation Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
RA17-035
Download: ML17104A085 (4)


comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

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PLANT AND SYSTEM IDENTIFICATION

LaSalle County Station Unit 1 is a General Electric Boiling Water Reactor with 3546 Megawatts Thermal Rated Core Power.

The affected system is the 345 kV switchyard. The switchyard consists of a ring-bus arrangement for each unit and a cross-tie bus that interconnects the units' ring buses. Each unit has a main power transformer (MPT), system auxiliary transformer (SAT), and a unit auxiliary transformer (UAT). Each of the two main generators is connected to two half-size main transformers and a UAT through isolated phase electrical bus ducts. The main power transformers step up the voltage, where power is delivered to the switchyard. The main generator and associated isolated phase bus duct is protected from electrical faults by several relaying features that will trip the generator to protect it from damage.

CONDITION PRIOR TO EVENT

Unit(s): 1 Reactor Mode(s): 1 Event Date: February 13, 2017 Event Time: 2309 CST Mode(s) Name: Power Operation Power Level: 100 percent

DESCRIPTION OF EVENT

On February 13, 2017, LaSalle County Station Unit 1 was in Mode 1 at 100 percent power, while Unit 2 was in a planned refueling outage. Unit 2 switchyard switching activities were in progress necessary to establish electrical back-feed power supply to the Unit 2 MPT. Switching activities included closing a main generator output breaker on Unit 2.

At

2309 hours
0.0267 days
0.641 hours
0.00382 weeks
8.785745e-4 months

CST, the Unit Imain generator received a trip on the 'B' phase differential current relay and subsequent main generator lock-out, which resulted in a Unit 1 automatic main turbine trip and subsequent Unit 1 automatic reactor scram due to fast closure of the turbine stop valves. In response to the event, one of the safety relief valves actuated in the relief mode upon the turbine trip and subsequently reset with tailpipe temperature returning to normal. The plant was in a stable condition with reactor pressure maintained by the turbine bypass valves. Reactor water level was controlled with feedwater, and all control rods fully inserted for the scram.

CAUSE OF EVENT

The root cause of the Unit 1 trip on differential current was a marginal generator differential relay design prone to responding to faults outside its zone of protection. Since both units' 345 kV ring buses are connected together through a cross-tie bus, this allowed the Unit 1 generator, which is the closest and strongest source, to supply some of the current and resulted in its differential circuit creating an unbalanced current that actuated the differential relay.

Contributing to this event was the current switchyard alignment methodology used for MPT back-feed operations. The current methodology challenges the margin of electro-mechanical differential current protective relays. An additional contributing cause was the use of larger transformers in place of the existing MPT, which changed the nature of the in-rush transient and introduced different current transformers into the differential relay scheme.

There has been operating experience regarding similar consequences related to similar differential relay scheme designs and related to transformer in-rush transients. Although some of this experience was recognized by the station and the corporate organization, the opportunity to change the back-feed switching scheme to mitigate the in-rush transient was missed.

0500073 2017

3. LER NUMBER

003 comments regarcing burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not dsplay a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

3. LER NUMBER 2. DOCKET NUMBER 0500073 - 00

REPORTABILITY AND SAFETY ANALYSIS

This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event or condition that resulted in manual or automatic actuation of the reactor protection system (RPS), including reactor scram. There were no safety consequences associated with the event since the RPS and other emergency safety systems functioned as designed.

CORRECTIVE ACTIONS

The immediate corrective action was to revise future switching orders to break the tie between the Unit 1 and Unit 2 345 kV ring buses when performing a back-feed to the MPT. The station also revised switching orders to isolate the Unit 1 and Unit 2 345 kV ring buses for energizing the SAT for each unit and performing generator synchronization operations.

The corrective action to prevent recurrence of a unit scram during back-feed operations is to install a more robust generator differential relay protection scheme. In addition, the switching scheme will be revised to break the tie between the Unit 1 and Unit 2 ring buses when the MPTs are energized.

PREVIOUS OCCURRENCES

A review of station Licensee Event Reports for the past three years related to reactor trips identified instances where a reactor manual or automatic scram was inserted. However, there were no previous occurrences related directly to main generator operation at the station. One event was identified related to turbine-generator run-back as follows.

  • LER 374-2017-001-00, Manual Reactor Scram due to Turbine-Generator Run-Back Caused by Stem-Disc Separation in Stator Water Cooling Heat Exchanger Inlet Valve On January 23, 2017, operators initiated a manual scram of the Unit 2 reactor as a result of observing a generator run-back due to a generator stator winding cooling (GC) system malfunction. Inspections of the GC components were performed while the unit was shut down for a planned refueling outage. The cause of the GC system failure was stem-disc separation in the 'A' GC heat exchanger inlet valve. This event was reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event or condition that resulted in manual or automatic actuation of the RPS. There were no safety consequences associated with the event since there was no loss of safety function, and the RPS functioned as designed.

COMPONENT FAILURE DATA

Manufacturer: General Electric Company [G080] Device: Main Generator Differential Current (CFD) Relay Trip Component ID: GE Model CFD Differential Relay 1687-MP010B 2017 003