05000323/LER-2005-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | 11-19-2005 |
|---|---|
| Report date: | 01-18-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 42157 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3232005002R00 - NRC Website | |
I. Plant Conditions
Unit 2 was in Mode 1 (power operation) at approximately 100 percent power.
Unit 1 was in Mode 6 (refueling) as scheduled for the thirteenth refueling outage with core reload in progress and startup power cleared for scheduled maintenance.
II. Description of Problem
A. Background
Unit 1 startup transformer (SUT) 1-1 is an ABB 75 MVA power transformer with a Reinhausen load tap changer (LTC). The transformer utilizes two independent oil systems: The first is the main oil system which houses the transformer core and main windings. The second oil system houses the load tap changer contacts and other moving parts. When the LTC responds to varying voltage levels, the LTC will open and close numerous contacts under load that will generate heat and combustible gases. An oil pump will then be started to filter any moisture or arc products that are generated. If the LTC generates excessive heat and gases, sudden pressure relay 80MRST11 will initiate a signal to unit lockout relay 86SU, which trips 230kV power circuit breaker 212 isolating the plant from startup power.
Each Diablo Canyon Power Plant unit has three emergency diesel generators (EDGs) [EK][DG] which supply power to the 4.16kV vital AC buses [EA][BU] whenever power is either unavailable, or voltage degrades below the point at which required loads could become inoperable. EDGs automatically start on a safety injection signal, degraded or loss of voltage on the associated vital bus, or undervoltage on the 230kV startup power system.
During normal operation, the 4.16kV vital buses are powered from the auxiliary power system. The 230kV system provides an immediately available source of offsite power to the 4.16kV system. The 230kV system provides power to the SUTs [EA][XFMR] 1-1 and 2-1 (230kV to 12kV), which feeds the SUT 1-2 and 2-2 (12 kV to 4,160V), respectively.
SUTs 1-2 and 2-2 then supply power to each vital bus.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Diablo Canyon Unit 2 0 5 0 0 0 3 2 3 2005 0 0 2 0 0 3 OF
B. Event Description
On November 19, 2005, maintenance relay technicians were completing final relay testing and functional tests to support SUT 1-1 return to service.
One of the tests included the functional test of relay 80MRST11, the sudden pressure relay for SUT 1-1.
The work order directed checking LTC relay 80MRST11 in accordance with the vendor manual. The function test consisted of pushing a test button to verify contacts change state. The task was completed recently on a similar relay in Unit 2 without incident.
At 15:51 PST, the technician proceeded with the functional test of the relay and actuated relay 80MRST11, causing 230kV breaker 212 to trip open, and deenergizing 12kV site power. The three Unit 2 EDGs started on the loss of 230kV as designed.
At 16:15 PST, after determining the immediate cause for the loss of 230kV power, operators restored 230kV power and shutdown the EDGs.
C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event No inoperable plant structures, systems or components were involved in, or contributed to, this event. The startup bus protection and EDG starting scheme operated as designed.
D. Other Systems or Secondary Functions Affected None. Both Unit 1 and 2 were otherwise unaffected with power continuing to be supplied from the auxiliary transformers. The site lost the 12kV underground loop which powers administrative loads and offices.
E. Method of Discovery The event was self-revealing to licensed control room operators by annunciators indicating the startup bus degraded voltage condition and the Unit 2 EDG auto-starts.
F. Operator Actions On November 19, 2005, at 16:15 PST, operators cut out the sudden pressure relay on SUT 1-1, reset SUT 1-1 sudden pressure relay LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) Diablo Canyon Unit 2 0 5 0 0 0 3 2 3 2005 0 0 2 0 0 4 OF (device 63YUT11) reset relay 86SU (SUT 1-1 and 2-1 lockout), and restored 230kV offsite power. At 1721 PST, operators made a nonemergency event notification (EN 42157) in accordance with 10 CFR 50.72(b)(3)(iv)(A).
G.�Safety System Responses Prior to this event, Unit 1 had startup power cleared for planned maintenance. All three vital buses were powered from auxiliary power with their associated EDGs in standby. Following the event, auxiliary power remained as the Unit 1 power source and the EDGs remained in standby.
Prior to this event, Unit 2 was in Mode 1 (Power Operation) at full power.
The vital buses were powered from auxiliary power and EDGs were in standby. With the loss of 230kV startup power, the emergency EDGs received a start signal and started as designed.
III.�Cause of the Problem
A. Immediate Cause
The functional test of the sudden pressure relay on SUT 1-1 caused 230kV breaker 212 to trip, causing a loss of startup power for both units.
The relay trip cut-outs were not used to prevent breaker 212 from opening.
B. Root Cause
This was a human performance error. Utility personnel failed to review applicable drawings and take appropriate actions prior to relay actuation, which resulted in an unexpected loss of 230kV power. The technician was aware of the requirement to review drawings prior to testing and was aware that the Unit 1 startup system was under an active clearance. The technician had walked down the clearance but failed to recognize the clearance was not adequate for the scope of the test. It was assumed that a print review was not necessary since a previous functional test of a similar relay in Unit 2 during the last outage had no adverse consequences.
During the prejob brief, the supervisor did not specifically direct the technician to review all prints prior to performing the functional test. The supervisor was aware that it was a standing expectation, based upon the training received in "Basic Relay" class, that drawings always be reviewed LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Diablo Canyon Unit 2 0 5 0 0 0 3 2 3 2005 0 0 2 0 0 5 OF and the characteristics of the relay understood prior to any relay actuation.
Interviews with various technicians following this event confirmed the understanding of this expectation.
C. Contributory Cause Inconsistent relay work package quality:
- Work order clearance notes did not include a request for opening relay Trip Cut-Outs (TCOs) as part of the clearance, leaving it up to the experience of the technician to determine what prerequisites were required. The work orders were written with the "experienced" technician in mind and did not include prerequisites for isolating equipment.
Procedural guidance:
- DCPP procedure MP E-60.10, Generic Function Test, was not referenced in the work order; this procedure requires a circuit isolation plan.
- No specific functional testing procedural guidance exists for relay 80MRST11.
IV. Assessment of Safety Consequences
startup was cleared prior to the event and equipment necessary for decay heat removal was powered by auxiliary power. Unit 2 remained at full power and all vital buses remained powered by auxiliary power.
Therefore, the event is not considered risk significant and it did not adversely affect the health and safety of the public.
V. Corrective Actions
A. Immediate Corrective Actions
1. Utilized the Human Performance Investigation Tool with individuals involved to determine the root cause of this event.
B. Corrective Actions to Prevent Recurrence
1. Clarify testing standards by presenting tailboard or section meeting LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Diablo Canyon Unit 2 0 5 0 0 0 3 2 3 2005 0 0 2 0 0 6 OF discussions and have supervisors reinforce clearance and drawings reviews and implementation of circuit isolation plans. Develop and schedule a relay refresher drawing class.
2. Review and clarify recurring task work orders associated with relay testing:
- Emphasize use of the circuit isolation plan
- Perform a focused review of common systems
- Enhance instructions or MP E-60.10 references 3. Expand the MP E-60.1()"Generic Function Test" instructions.
4. Clarify the circuit isolation plan process.
5. Develop functional testing guidance for relay 80MRST11.
VI.�Additional Information
A. Failed Components
None
B. Previous Similar Events
Sequence Check Due to Personnel Error in Confirguring Test Equipment Setup," reported a similar event. Corrective action required supplemental (nonsite permanent) personnel to have a prejob brief in human performance error reduction techniques prior to work on sensitive plant equipment.