05000266/LER-1993-001, Revised LER 93-001-00:on 930107,all 4,160 Volt Degraded Grid Voltage Protection Channels Declared Inoperable Due to Discovery That Settings for Relays on Subj Channels Being Too Low.Plant Mods Will Be Made
| ML20134B302 | |
| Person / Time | |
|---|---|
| Site: | Point Beach  |
| Issue date: | 02/16/1993 |
| From: | Katers P WISCONSIN ELECTRIC POWER CO. |
| To: | |
| Shared Package | |
| ML20134B296 | List: |
| References | |
| LER-93-001-01, LER-93-1-1, NUDOCS 9302230212 | |
| Download: ML20134B302 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| 2662093001R00 - NRC Website | |
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l ADSTRACT On January 7, 1993, an evaluation was completed that showed the settings for the degraded grid voltage relays on the 4160V safety-related buses could be too low to provide adequate protection for all safety-related equipment. Therefore, all 4160V degraded grid voltage protection channels were declared inoperable at 1515 CST, invoking the requirements of Technical Specification Table 15.3.5-3, " Emergency Cooling," Item 4.a.
This invoked Technical specification 15.3.0, which requires that the i
affccted units be placed in hot shutdown within 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> and in cold shutdown within 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> if the condition is not corrected. An NRR Waiver l of Compliance was requested and granted to allow time to determine new l setpoints, calibrate the relays, submit a Technical Specification Change i
Request, and receive a license amendment for this setpoint change. The new I
setpoint determinations and setpoint changes were performed. These actions were completed January 15, 1993. A Technical Specification Change Request was submitted to the NRC in a letter dated January 18, 1993.
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0 10 0b 0' nln rtre n . - w anc e mu.mn EVENT DESCRIPTION Analysis of ABB Impell Corporation Calculation 0870-150-007, Revision 0, which determined the steady state and transient voltage levels on the PBNP clectrical distribution system at various plant operating conditions, began on approximately December 2, 1992. The results of this calculation indicated that there are several safety-related electrical loads which could have voltage levels lower than the minimum value required to assure continued operation following a trip of one unit or if one unit was at 100%
power coincident with a LOCA on the second unit and the off-site grid voltage was lower than normal.
Operation of electrical equipment at voltages lower than its electrical rating is not desirable because such operation may result in the equipment not properly performing its intended safety function or may result in damage to the equipment. For this reason, existing plant design includes protection against sustained operation of cafety-related loads at lower than recommended voltages.
On January 7, 1993, with both units operating at full power, analysis of the ABB Impell calculation was completed. The analysis concluded that the settings for the degraded grid voltage relays installed on the 4160V safety-related buses were too low to provide adequate protection for all safety-related equipment. Therefore, all 4160V degraded grid voltage protection channels were declared inoperable at 1515 CST, invoking the requirements of Technical Specification Table 15.3.5-3, " Emergency Cooling," Item 4.a, which allows continued power operation for up to 7 days provided the affected buses are being supplied by the associated emergency
. diesel generators.
Upon further review, it was determined that the requirements of Technical Specification Section 15.3.7, "nuxiliary Electrical Systems," Specifica-tion B.1.d would not allow this cicctrical system configuration. Technical Specification 15.3.7.B.1.d permits operation of the affected unit for up to 7 days with only one supply bus, A03 or A04 (the normal supply to Buses A05 and A06 respectively) out of service. Continued operation of a unit is prohibited with power supplied to both trains of safeguards from the associated emergency diesel generator.
The Point Beach Manager's Supervisory staff (MSS) determined that operation for an extended period of time with all four of the safety-related 4160V buses supplied from the diesel generators was not consistent with main-taining the optimum level of plant safety given the fact that grid voltage could best be maintained at a high level with both Point Beach units on line. This is a condition prohibited by the Technical Specifications.
Therefore, Technical Specification 15.3.0.A was entered at 1515 CST on January 7, 1993.
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su,a , . < ,oo a w m on The action statement for Technical Specification 15.3.0.A requires the affected unit (s) to be placed in hot shutdown within three hours. The power reduction on both units began at approximately 1620 CST. At i approximately 1640 CST, the NRC Office of Nuclear Reactor Regulation (NRR) granted a temporary hold of the 3-hour hot shutdown requirement pending formal request for a waiver of compliance. A verbal 14-day Temporary Waiver of Compliance from Technical Specification 15.3.0.A was requested and granted by NRR at 1920 CST on January 7, 1993. Both units were subsequently returned to full power at approximately 2000 CST.
The request for waiver was documented in a letter to the NRC dated January 8, 1993. The NRC provided confirmation of the waiver in a letter dated January 14, 1993. This letter changed the duration of the waiver to extend until a Technical Specification change for the degraded voltage relay setpoint is issued. New degraded voltage setpoints were determined and setpoint changes completed on January 15, 1993.
COMPONENT AND SYSTEM DESCRIPTION Degraded grid voltage relays are installed on each of safety-related 4160V '
Buses 1A05, 1A06, 2A05, and 2A06. The purpose of these relays is to detect lower than acceptable voltage levels on these buses. These relays generate the signal to disconnect the safety-related 4160V buses from the preferred off-site source. This results in an undervoltage condition on the safety-related buses, which causes the emergency diesel generators to start and energize these buses.
The degraded grid voltage relays were installed in 1981 and 1982 in response to an NRC generic letter dated June 2, 1977. At that time, it was determined that the minimum allowable voltage at which all 480V bus safety-related loads could operate was 414V (90% of the nominal 460V rating for the containment ventilation fan motors, which are considered to be the most limiting components). It was then determined that a minimum 3797V was necessary on the 4160V safety-related buses to assure 414V is available at the fan motors. The results of these evaluations were provided to the NRC as the basis for the degraded grid voltage relay setting of >/=3875V +/-2%,
which appears in Technical Specification Table 15.3.5-1, Item 9.
REPORTABILITY i This Licensee Event Report is provided pursuant to the requirements of 10 CFR 50.73 (a) (2) (ii) (B) , "Any event or condition that resulted in the condition of the nuclear plant, including its principal safety barriers, being seriously degraded, or that resulted in the nuclear power plant being in a condition that was outside the design basis of the plant." This report is also required by 10CFR50.73 (a) (2) (1) (B), "Any event or condition
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em . , * - ac - .mn prohibited by the plant's Technical Specifications." Additionally, a 1-hour NRC notification was made at 1642 CST on January 7, 1993 in accordance with 10 CFR 50.72 (b) (1) (i) ( A) . The NRC Resident Inspector was also informed about this event.
SAFETY ASSEDSMENT Operation with 4160V bus degraded voltage relay settings of >/=3875 +/-2%
will not ensure adequate protection of all safety-related equipment under degraded electrical system conditions. Analysis indicates that with the 345KV bus voltage at or below 351MV, a LOCA would cause the 480V bus voltage to be below the minimum required to ensure proper operation of safety-related equipment. Operation of equipment at lower than its electrical rating is not desirable because such operation may result in equipment not properly performing its intended function or may result in damage to the equipment itself. Without compensatory actions, this is unacceptable due to the need to prevent loss of safeguards performance.
CORRECTIVE ACTIONS The immediate corrective action for this situation was to issue guidance to the operators for reducing the chances of and handling a degraded voltage situation at PUNP. PBNP Operations Special Order 93-01 was issued on January 7, 1993. This special order specified the following compensatory action = for this situation:
- 1. The Unit 2 control operator was assigned to monitor and record hourly voltage readings on 4160V Buses A05 and A06 associated with Units 1 and 2.
- 2. System Control (located in Powaukee, WI) continued normal monitoring of the 345KV voltage of Bus Sections 1, 2, 3, 4 and 5. If System Control receives a voltage alarm indicating that bus section voltage has decreased to less than 354KV, PBNP would be .totified in accordance with normal practice, System control was informed about the increased importance of maintaining stable system voltage at PDNP.
- 3. Upon notification that system voltage had decreased below 354KV, a dedicated licensed operator would immediately report to the control room and continuously monitor voltage on the A05 and A0ti buses associated with Units 1 and 2.
The dedicated licensed operator would continue his duties until system voltage remains stable and greater than 354KV.
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- 4. The following actions would be taken by the Unit 2 control operator or the dedicated licensed operator when required:
- If the voltage on either Unit 1 or Unit 2 A05 and/or A06 buses drops below 4100V, System Control would be immediately requested to take action to increase system voltage until all vital bus voltages exceed 4100V.
- If the voltage on Buses A05 and/or A06 ascociated with Units 1 and/or 2 drops below 4050V, the on-site combustion turbine generator would be started but not loaded. The combustion turbine will serve as a back-up power supply (20 MWe capacity) to the diesel generators (5.7 MWe total capacity) if needed.
- If the voltage on Buses A05 and/or A06 associated with Units 1 and/or 2 drops below 4000V, the normal feeder breaker associated with the bus or buses below 4000V would be inimediately opened (1A52-57 for 3A05, 1A52-63 for 1A06, 2A52-70 for 2A05, and/or 2A52-76 for 2A06).
This would result in the starting of the appropriate emergency diesel generator (s) and the resupply of the bus from the emergency diosol generator (s). Normal voltage from the emergency diesel generator (s) would be verified on any bus which had its normal feeder breaker opened.
- 5. Following any action taken above, the Duty Shift Superintendent (DSS) would place Units 1 and 2 in the appropriate operating condition for the resulting electrical configuration as directed by Technical Specification 15.3.7.
- 6. The emergency diesel generators would not be removed from service for maintenance.
The Temporary Waiver of Compliance confirmation letter from the NRC, dated i
January 14, 1993, also required the following additional compensatory actions:
- 1. The on-site combustion turbine will be tested biweekly (twice per week) until the issuance of a permanent Technical Specification change.
l 2. Plant modifications will be made to change the undervoltage.
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- 3. The licensee will include documentation of the plans to verify the adequacy of the new undervoltage setpoint in the request for license amendment.
The degraded voltage relay setpoints at PBNP have been changed from the existing Technical Specification setpoint of >/=3875 volts +/-2% to the proposed >/=3959 volts +/-1/2%. This change was completed on January 15, 1993. A proposed Technical Specification change was submitted to the NRC on January 19, 1993. Plans to verify the new undervoltage setpoint are described in the Technical Specification change request letter.
The combustion turbine is being tested on a biweekly (twice per week) frequency until the Technical Specification amendments are issued. The other compensatory actions are no longer required because the actpoint change has made them unnecessary.
SIMILAR OCCURRENCES Other Licensee Event Reports that describe problema discovered in the electrical distribution system are:
. Unit 1 LER 90-004 B03/B04 Tio Breaker Single Fajlure Potential LER 90-012 B04/B02 and B03/B01 Bus Tie Breaker Single Failure Potential LER 89-009 DC Breaker Fault Current Capability LER 88-001 Single Failure Potential in 4160 V Safeguards Switchgear LER 88-003 Surveillance of 4160 V Safeguards Power Supply Undervoltage Relay Not in Accordance with Technical Specifications LER 82-025 480 V Relays Outside Technical Specification Limits LER 72-020 480 V Breaker Overcurrent Timing Possible Generic Defect Mnit 2 LER 82-003 Undervoltage Relay Time Delay Out of Specification l
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