ML11188A072
| ML11188A072 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 07/06/2011 |
| From: | Leake H Nuclear Energy Institute |
| To: | Office of Nuclear Reactor Regulation |
| Basturescu S | |
| Shared Package | |
| ML11187A009 | List: |
| References | |
| Download: ML11188A072 (13) | |
Text
A Systematic Engineering Approach to Degraded Voltage Harvey Leake Palo Verde Nuclear Generating Station g
IEEE Subcommittee 4 IEEE 741 Working Group Degraded Grid Voltage Protection Workshop June 28-29, 2011
General Principles Our regulatory system must protect public health, welfare, safety, and our environment. It must be based on the best available science (Executive Order best available science (Executive Order 13563)
Regulations should be based on the best available knowledge from research and operational experience. (NRC Principles of Good Regulation) 2
Definitions
Degraded Voltage: Voltage < minimum needed to ensure essential load equipment operates without damage or malfunction
DVR Dropout Setting: Nominal setpoint at which, during a decrease in voltage, the degraded voltage relay begins timing out
DVR Reset Setting: Nominal setpoint at which, during an increase in voltage, the degraded voltage relay ceases timing out
Interrupted Flow Event: Design basis event in which injection is established from offsite power supply, then interrupted 3
LOCA with Degraded Voltage Draft RIS Approach y Bus Questions to consider:
- 1. Is flat voltage profile credible?
B di
?
DVR dropout voltage curs 1
2 Voltage at Safet Bounding?
- 2. Is assumed timing of DVR actuation (accident analysis time) credible?
- 3. Injection flow established prior to DVR timeout will be interrupted.
Accident analysis for this event?
Will resequencing be successful?
Time LOCA / degraded voltage occ DVR timeout Assume loads resequence onto diesel generator 3
LOCA with Degraded Voltage Components of Voltage Profile 0.8 1
0 40 Voltage Time (seconds)
Switchyard Voltage Degradation due to Generator Trip (Millstone effect) 5 0.9 1
0 40 Voltage Energization of Safety Loads Time (seconds)
Time (seconds) 0.9 1
0 40 Voltage Fast Bus Transfer Time (seconds)
LOCA with Degraded Voltage Considering Best Available Science y Bus Insights:
1 V lt fil i t i
t t fl t DVR dropout voltage 1
2 DVR reset voltage Voltage at Safet
- 1. Voltage profile is transient, not flat
- 2. DVR timeout unlikely to start at T=0, so actuation time > time delay setting
- 3. How to analyze interrupted flow effect?
- Accident analysis is for simultaneous LOOP/LOCA, not interrupted flow scenario
- Hydraulic effects (GSI-171)?
- Will safety equipment successfully resequence onto EDG?(GSI-171)?
Time LOCA occurs DVR timeout 3
6
LOCA with Degraded Voltage Conclusions DVRs provide unsatisfactory protection against this scenario
- Actuation would result in an unanalyzed interrupted flow event interrupted flow event Preventative strategy:
- Interrupted flow events
- Degraded voltage events 7
Prevention Strategy Considering Best Available Science y Bus DVR dropout voltage DVR reset voltage
- Criteria Voltage at Safet Time LOCA occurs
- Criteria Minimum credible steady-state voltage following automatic load actuation DVR reset voltage setting Longest transient voltage excursions below reset voltage setting < DVR time delay setting
- Effectively prevents interrupted flow scenario 8
DVR Voltage Setting Analysis Draft RIS Guidance calculation of voltages at terminals or contacts of all safety related equipment with the voltage at the DVR monitored bus at the DVR dropout setting providing the at the DVR dropout setting, providing the necessary design basis for the DVR voltage settings. In this manner, the DVR ensures adequate operational (starting and running) voltage to all safety related equipment.
9
DVR Voltage Setting Analysis Bottom Up Approach y Bus DVR dropout voltage Does not demonstrate that the DVR ensures adequate Voltage at Safet Time LOCA occurs operational (starting and running) voltage to all safety related equipment?
DVR assumed not to actuate Flat voltage profile provides no insight regarding protection during transient conditions
Motor starting
LOCA/generator trip 10
DVR Voltage Setting Analysis Consequence if DVR Does Not Reset y Bus DVR dropout voltage DVR reset voltage Voltage at Safet
- Interrupted flow event will occur if steady-state voltage following automatic load actuation is < DVR reset voltage setting
- Draft RIS suggests LOCA with degraded voltage should be analyzed, but does not discuss transient voltage characteristic nor interrupted flow effect Time LOCA occurs DVR timeout 11
Systematic Engineering Approach
- 1. Begin with clear descriptions of postulated initiating events and conditions
- Consider operating experience Generator trip was the initiating event for the 1976 Millstone event 2
Identify design requirements and objectives
- 2. Identify design requirements and objectives
- 3. Perform necessary research to obtain pertinent information
- Develop credible and bounding voltage profiles for each postulated scenario
- 4. Establish viability of proposed solutions by analyzing effect of voltage profile 12
Probabilistic Considerations Example Figure 100 105
%)
studies conducted during resolution of the issue showed that the contribution to CDF from the sequence of events was far less than originally anticipated Thus, the issue was RESOLVED with no new requirements. (GSI-171) studies conducted during resolution of the issue showed that the contribution to CDF from the sequence of events was far less than originally anticipated Thus, the issue was RESOLVED with no new requirements. (GSI-171)
Probability of being at given steady-state voltage following sequencing 85 90 95 0
0.2 0.4 0.6 0.8 1
Voltage (%
Probability (given a postulated accident)
DVR Tolerance Band Probability of consequential LOOP & interrupted flow not high enough to be a concern per GSI-171 Should consequential degraded voltage region be a concern?
13