05000305/FIN-2007006-06
Finding | |
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| Title | Non-Conservative Voltage Calculations for MCC Control Circuits |
| Description | The inspectors identified an Unresolved Item concerning assurance that MCC control circuits had adequate voltage to operate when required. Description: The inspectors reviewed calculation C-039-001 and other preliminary calculations used to support operability determinations for MCC control circuit voltage and identified several concerns including reliance on non-conservative test data and calculational errors, resulting in voltage potentially below acceptable levels for operability for several circuits. Specifically: Non-conservative Data from Control Power Transformer Tests: MCC control circuits are powered by a separate 480V/120V single phase control power transformer (CPT) in each starter bucket. The tests on CPTs were intended to determine the actual voltage ratio for use as inputs to the voltage drop calculations. The inspectors identified the following specific issues:
Separate measurements were performed with contactor inrush current and with the contactor picked up in steady state. Although the CPT secondary voltage was considerably lower with inrush current, calculation C-039-001 incorrectly used data from the steady state tests The tests did not include preloads such as auxiliary relays or indicating lights so all of the CPT data shows higher secondary voltage than would be realized in the actual field installed circuits containing these devices. The tests were performed using only one or two specimens for each size CPT. This provided very low confidence that the test results were bounding for all CPTs. The tests were performed under shop conditions and the calculation did not include margin for lower secondary voltages or higher pickup voltages that could result from higher temperatures in energized MCCs. Non-conservative Data From Contactor Tests: The calculation acceptance criteria used data from tests in lieu of the manufacturers guaranteed pick up voltage of 85 percent of rated voltage (102 Vac). The test values were considerably below the manufacturers guarantees, but no margin was included in the calculation to account for known differences between service and test conditions or for inherent uncertainties in test methods. The inspectors identified the following specific issues: The tests were performed using only one or two specimens for each size contactor. This provided very low confidence that the test results were bounding for all contactors. Calculation C-039-001 provided no margin for specimen variability.20 Attachmen The tests were performed under shop conditions and the calculation did not include margin for higher pickup voltages that could result from higher coil resistance due to higher temperatures in energized MCCs. No allowance was made for aging, and there has been no periodic voltage testing to demonstrate that the original contactor test data remains valid. Non-conservative Calculation Methodology: In addition to the concerns with test data described above, Calculation C-039-001 contained errors and omissions that contributed to the non-conservative results, as follows In some cases, average data was used in lieu of worst case data so that calculations were known to be non-conservative for some devices. The calculation only considered voltage drop in cable due to contactor current and did not include current from additional loads such as relays and indicating lights. No allowance was made for increased cable resistance due to effect of accident temperatures The calculation did not account for fuse resistance. The calculation used cable lengths based on lengths cut off reels and did not account for greater circuit length due to twisted construction of cables The licensee initiated CAPs 041709, 041801, and 041840 to address operability concerns for specific circuits and for the general concern for the non-conservative voltage drop calculation. The preliminary calculations used to support the operability evaluations in these CAPs were also non-conservative, and underwent several revisions over the course of the inspection to correct errors and omissions. The inspectors determined, based on licensees preliminary calculations for the circuits that approximately 41 circuits would have had less than 5V margin for operability based on voltage afforded by undervoltage relays. Toward the end of the inspection, the licensee performed modifications to several starter circuits, revised procedures to reduce MCC loads and revised the operability evaluation for the remaining circuits. The preliminary calculations supporting the revised operability justification showed a minimum margin of approximately 5V to account for uncertainties in test data, aging and uncertainties in other calculation inputs. Key inputs to the preliminary calculation included an unverified preliminary load flow calculation performed with the electrical transient analyzer computer program. The preliminary calculations also took credit for the minimum expected grid voltage of 140 kV which is controlled by non-safety related computer programs under the control of the transmission system operator. This voltage input was used in lieu of the design basis voltage afforded by the automatic safety related undervoltage relays. The inspectors concerns with the lack of assurance that MCC control circuits had adequate voltage to operate when required is considered an Unresolved Item (URI 05000305/2007006-06 (DRS)) pending the licensees completion of a past operability assessment of the several circuits and further inspector review. |
| Site: | Kewaunee  |
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| Report | IR 05000305/2007006 Section 1R21 |
| Date counted | Jun 30, 2007 (2007Q2) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | G Skinner M Yeminy R Langstaff S Burgess Z Falevits A Dahbur A Stone C Brown F Trapm Munirj Cassidy T Bilik M Kunowski S Burton K Barclay |
| INPO aspect | |
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Finding - Kewaunee - IR 05000305/2007006 | |||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Kewaunee) @ 2007Q2
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