05000331/LER-2007-011

LER-2007-001, . Undervoltage ConditiOn Resulted in the Actuation of the Emergency Diesel Generators

Docket Number
Event date:	12-01-2007
Report date:	01-29-2008
Initial Reporting
ENS 43817	10 CFR 50.72(b)(3)(iv)(A), System Actuation
3312007001R00 - NRC Website

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Duane Arnold Energy Center 05000331 YEAR SEQUENTIAL I REVISION NUMBER � NUMBER

I. Description of Event:

On December 1, 2007 at 1435, while operating at 100% power with the Standby Transformer (EIIS: EB) inoperable due to latent design issues, a fault developed on the 161 KV electrical transmission line (EIIS: FK) to the Fairfax substation causing Duane Arnold Energy Center (DAEC) switchyard voltage to dip to approximately 62%. Switchyard Circuit Breakers (CB) 3110 and CB 8090 opened to clear the fault in approximately 3 cycles. At that time, the essential buses (1A3 and 1A4) (EIIS: EB) were supplied from the Startup Transformer (1X003) (EIIS:

EB). Since the Startup Transformer is supplied directly from the switchyard, the essential buses experienced a voltage drop similar to the switchyard voltage and dropped approximately 62% for 3 cycles, resulting in the sequential loading relays sending a start signal to the Emergency Diesel Generators (EDGs) (EIIS: EK). The sequential loading relays have a safety function of initiating sequential loading of the Core Spray and Residual Heat Removal pumps onto essential busses following a Loss of Offsite Power/Loss of Coolant Accident (LOOP/LOCA). Their secondary function, which is not a safety function, is to send a start signal to the EDGs on degraded voltage. These relays are considered backups since start signals to the EDGs are also provided by Startup and Standby Transformer under voltage and bus degraded voltage relays. It should be noted that their secondary function only requires an under voltage condition to exist for Neither EDG connected to its respective bus and loaded.

Cause of the fault , The fault was caused by ice and high winds.

Analysis of EDG Actuation The voltage transient caused both EDGs to start but not load onto their respective buses, 1A3 and 1A4. The supply breakers from the Startup Transformer (1A302 and 1A402) remained closed and bus voltage recovered within 3 cycles of dipping to approximately 62%.

The system is designed such that any one of the following three signals can automatically start the EDG:

• Loss of Coolant Accident (LOCA) signals of 2 psig high drywell pressure or 64.0" reactor vessel low-low-low level,

• Loss of Offsite Power (LOOP) signals of both the Startup and Standby Transformers or degraded essential bus voltage of rated voltage for 8-8.5 seconds.

• Essential bus under-voltage signal of FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Duane Arnold Energy Center 05000331 YEAR NUMBER NUMBER Since bus voltage dropped below 65% of rated, the EDG started per their design.

Analysis of EDGs Not Connecting to Their Respective Buses The EDG output breaker is designed to close automatically when the EDG receives an automatic start signal and the following conditions are met:

• The engine is up to 90% of rated speed (810 rpm).

• Essential bus supply breakers from the Startup and Standby Transformers are open (1A301, 1A401, 1A302 and 1A402).

• Under-voltage (

• Time delay relay 102-311, set at 15 cycles, has energized (starts timing when the 20% under-voltage condition exists and both input breakers from the supply transformers are open).

• EDG output is at 90% of rated voltage.

Since the supply breakers from the startup transformer 1A302 and 1A402 never opened coincident with the bus voltage never dropping below 20%, the EDGs did not load. Therefore the EDGs operated per their design during this event.

II. Assessment of Safety Consequences:

The automatic starting of the EDGs in response to a short-duration under-voltage condition had no personnel or radiological safety significance. Since the EDGs were capable of fulfilling their safety function throughout the event and the Startup Transformer was powering buses 1A3 and 1A4, there is no nuclear safety significance.

This event did not result in a Safety System Functional Failure.

III. Cause of Event:

The apparent cause for DAEC being susceptible to EDG auto starts due to electrical transmission line faults is that the sequential loading relays operate simultaneously with the switchyard protective relaying. Therefore, any fault affecting the local grid has the potential to start the EDGs.

IV. Corrective Actions:

The 'A' EDG was secured at 1507 on December 1, 2007. The 'B' EDG was secured at 1514 on December 1, 2007.

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Duane Arnold Energy Center 05000331 YEAR NUMBER NUMBER Actions have been initiated to assess the benefit of a modification to slow down the sequential loading relay logic to prevent spurious starts of the EDGs due to momentary grid disturbances.

V. Additional Information:

Previous Similar Occurrences:

A review of LERs over the previous 5 years revealed the following similar occurrence:

• LER 2007-004, "Severe Weather Causes Grid Disturbance Resulting In Loss of Shutdown Cooling.

EMS System and Component Codes:

EK — Emergency Onsite Power System EB — Low Voltage Power System — Class 1 E FK — Switchyard System Reporting Requirements:

This event was reported under 10CFR50.72(b)(3)(iv)(A), 'Any event or condition that results in valid actuation of any of the systems listed in paragraph (b)(3)(iv)(B) of this section' due to the Auto Start of the 'A' and 'B' Standby Diesel Generators. Reference EN 43817.