ML102810257
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| ML102810257 | |
| Person / Time | |
|---|---|
| Site: | Kewaunee  |
| Issue date: | 10/05/2010 |
| From: | Feintuch K Plant Licensing Branch III |
| To: | Gadzala J, Sly C Dominion Energy Kewaunee |
| Feintuch K, NRR/DORL/LPL3-1, 415-3079 | |
| Shared Package | |
| ML102810276 | List: |
| References | |
| TAC ME4011 | |
| Download: ML102810257 (2) | |
Text
Enclosure REQUEST FOR ADDITIONAL INFORMATION KEWAUNEE POWER STATION LICENSE AMENDMENT REQUEST 236, AUTOMATIC OPERATION OF TRANSFORMER LOAD TAP CHANGERS (TAC NO. ME4011)
- 1. The License Amendment Request (LAR) states that the auxiliary power required during plant startup, shutdown and after a reactor trip is supplied from the American Transmission Company's 138 kilovolt (kV) and 345kV transmission systems, through the 13.8 kV/138 kV/345 kV Kewaunee Power Station (KPS) substation, and via the reserve auxiliary transformer (RAT) and tertiary auxiliary transformers (TAT). The RAT and the TAT can both be powered from either transmission system through the interconnecting auto-transformer. Either offsite power source (reserve or tertiary auxiliary transformer) can be manually aligned to either or both of the engineered safety features 4.16 kV buses. The two buses can be tied together via two bus-tie breakers in series, one on each bus. The design and licensing basis of the plant states that upon loss of the normal source of offsite power to the 4.16 kV safety-related bus, a fast bus transfer to the second qualified source of offsite power will occur. Provide a detailed evaluation of the safety bus voltages when a loss of one offsite source results in transfer of safety bus to the second source during a design basis event and automatic tap changer(s) on RAT and TAT respond to voltage perturbations.
- 2. Bus 1-5 is normally supplied from the TAT and Bus 1-6 is normally supplied-from the RAT. Thus, no bus transfer is required for the engineered safety features in the event of a unit trip. The plant was designed with an automatic bus transfer feature. Verify that a single failure of the bus transfer scheme will not result in transfer of the Busses 1-5 and 1-6 to a single source of offsite source during a design basis event.
- 3. The LAR also states that Buses 1-1 and 1-2 are connected via bus main breakers to the main auxiliary transformer (MAT) and RAT. Buses 1-3 and 1-4 are also connected via bus main breakers to the MAT and RAT. These buses supply power to the normal balance-of-plant auxiliaries. The Updated Final Safety Analysis Report figures in Chapter 8 indicate that the breakers connecting these busses to the RAT are normally open. The safety-related Bus 1-6 is normally connected to the X winding of the same RAT and the Bus 1-5 is normally connected to TAT. The licensee has verbally indicated that the grid is not stable if the 138kV system degrades by 10 percent, the load tap changer (LTC) limit for voltage correction, and it is therefore unlikely that the plant will be operating with the LTCs at the upper limit. In the event of a safety injection signal resulting in plant trip and transfer of Busses 1-1 and 1-2 to the Y winding and 1-3 and 1-4 to the X winding of the RAT, provide a detailed analysis of the interaction of the automatic LTCs and the safety bus voltages with accident loads sequencing for the following conditions:
a)
The 138kV system voltage at normal.
b)
The high voltage side of the RAT supply transformer (RST) operating at grid stability limit with appropriately loaded RAT.
c)
The high voltage side of the tertiary supply transformer (TST) operating at grid stability limit with appropriately loaded TAT.
+/- 0.9 percent of nominal bus voltage. Confirm that this setpoint ensures satisfactory operation of all safety related equipment and affords protection during prolonged operation under degraded voltage conditions independent of the operation of the automatic LTC.
- 5. The licensee states that the time delay of DVR is long enough to prevent inadvertent actuation of the relays from voltage dips due to large motor starts except when a reactor coolant pump motor starts with an engineered safety feature (ESF) bus below 3980V and that such starts are controlled administratively. In automatic LTC mode, the primary microcontroller monitors load and source voltages to create a signal based on the sensed secondary voltage of its associated TAT or RAT. The "X" winding of the RAT provides power to the ESF buses; therefore, the microcontroller senses the "X" winding voltage. During grid perturbations lasting a few seconds, the nonsafety large motor loads on the Y winding will slow down and require inrush current during simultaneous reacceleration. Provide a summary of the analyses performed to demonstrate that the voltage drop on the plant buses will not result in an inadvertent trip of the offsite power to the ESF buses for bounding case grid voltage perturbation that results in a plant trip.
- 6. The voltage decay that occurs following a generator trip was evaluated for response time of the LTCs. Provide a summary of the analyses performed for the bounding case with details on the maximum reactive power capability of the main generator and actual voltage values at the safety busses used in the analyses.
- 7. Failure of a tap changer during high summer load with a high tap setting could lead to high voltage but is considered to be limited due to relatively slow change in voltage during evolving grid conditions. Provide details on the consequences of tripping the largest load on the grid resulting in sudden change in grid voltage during peak summer conditions with failure of the LTC motor at high setting.