Defense in Depth of Electrical Systems and Electrical Grid Interactions

Transcription

1 Defense in Depth of Electrical Systems and Electrical Grid Interactions Preliminary Findings and Recommendations to OECD Nuclear Energy Agency Committee for Safety of Nuclear Installations December 2008 Dr. John H. Bickel, Chairman DIDELSYS Working Group llc.com

2 This presentation summarizes preliminary DIDELSYS findings and recommendations OECD Nuclear Energy Agency created DIDELSYS working group in January 2008 Purpose: evaluate current defense in depth of nuclear power plant electrical systems to cope with plant and grid caused transients DIDELSYS working group consists of: John H. Bickel, ESRT, LLC (Sweden) - Chairman Alejandro Huerta, OECD/NEA Secretary Per Bystedt, SSM (Sweden) Tage Eriksson, SSM (Sweden) Andre Vandewalle, Nuclear Safety Support Services (Belgium) Franz Altkind, HSK (Switzerland) Thomas Koshy, USNRC (United States) David M. Ward, Magnox Electric Co. (United Kingdom) Brigitte Soubies, IRSN (France) Kim Walhstrom, STUK, (Finland) Alexander Duchac, EC Joint Research Centre Petten (European Commission) Robert Grinzinger, GRS (Germany) Ken Kawaguchi, JNES (Japan)

3 Simplified NPP Electrical System Internationally Recognized design standards: IAEA NS-G-1.8 IEEE Std. 308 KTA-3701 IEC Common to all standards is: Concept of interfacing safety related control and motive power to non-safety AC sources via Aux. and Startup Transformers.

4 All countries recognize need for lightning surge protection Unprotected: Lightning surge can pass back through Aux. Transformers to onsite equipment Design standards assure: Lightning surge diverted to ground Ground is not floating Example standards: IEC KTA-2206 IEEE Std. C62.23

5 Other Power/Voltage Surges Require Protection Standards recognize: Overvoltage vs. Duration spectrum and have inherent assumptions about Withstand Voltage capability Circuit breakers can t address lightning surges because they operate too slowly Surge arrestors can divert short duration Overvoltage Problem is sizing for different areas of concern

6 Experience indicates gap in protection NPP electrical systems nominally designed for operation with +/-10% Voltage Voltage above 120% but below lightning protection features operate is a terra incognita 2006 Forsmark-1 1 and 2008 Olkiluoto-1 1 events indicate: Previously assumed Withstand Voltage may be as low as: ~130%

7 Preliminary identification of events causing voltage surges in this range: Courtesy of the German Reactor Operators Association, VGB

8 Surge Stress Tests per IEC

9 Surge Arrestors are Very Complex Nonlinear Devices Surge arrestors have highly non-linear Voltage-Current relationships Surge arrestors also have significant dynamic characteristics as well

10 Role of Testing and Simulation Codes Testing has been performed for lightning arrestors, medium, and lower voltage surge arrestors Above photos from actual destructive tests by Siemens Key issue: What is design basis voltage surge for qualification tests?

11 Role of Testing and Simulation Codes Simulation tools exist for modeling 3-phase 3 in plant response to symmetric, asymmetric faults, generator excitation failures MATLAB/Simulink Power System Toolbox has been used by Helsinki Univ. of Technology (Preliminary 2007 results) To use in supporting single failure analyses Simulink models would require V&V commensurate with T-H T H analysis codes

12 Role of Testing and Simulation Codes Dynamic simulation of effect of local surges input to local equipment can also be done using SPICE models SPICE models are well established technology for telecommunications industry Similar to MATLAB/Simulink, SPICE models would require significant V&V for use in NPP single failure analyses

13 DRAFT Report on DIDELSYS Project Issued in DRAFT working group report for comment in December 2008 External Peer Review comments sought from EdF, Vattenfall AB Intent is to solicit further Peer Review comments from IEEE, IEC Convene technical information exchange meeting at OECD in May 2009 Focus of future safety improvements will be responsibility of industry standards working groups and national regulators.

14 To improve Defense in Depth: Combined actions are required to improve Robustness at each Defense in Depth level,, including: Preventing electrical grid - NPP interaction challenges to NPP electrical power systems (Preventing Grid Challenges) Improving Robustness of NPP electrical power systems to cope with electrical grid, and internal NPP electrical faults should they occur (Electrical System Coping) Improving NPP training, procedures, information capabilities to deal with degraded electrical systems (Procedures) Improving Coping Capability of NPP to deal with NPP electrical power system failures (NPP Coping) Improving capability to recover offsite grid to support NPP electrical power systems (Electrical System Recovery)

15 Preventing Grid Challenges Recommendations: WANO SOER and 2004 Addendum offer practical approaches to reduce electrical grid challenges, including: Have Binding Agreements for communication, coordination of planned activities Jointly planning, coordinating electrical circuit test & maintenance activities Grid operators: provide NPPs early warning of on-going grid problems NPP operators: provide grid early warning of operational limitations that might impact NPP power output Grid procedures must recognize NPP as priority load center requiring efforts to avoid shedding circuits to NPP

16 Electrical System Coping Recommendations: Identify range of possible voltage surge transients between nominal operation and existing lightning surge protection. Include consideration of combinations of events, such as: Large load rejection attempted runback to house load AND failure of main generator excitation system Conduct equipment review to determine current Voltage Withstand capability for power frequency over-voltage voltage transients (including: asymmetric cases) Give special emphasis to recently upgraded solid state equipment that may have least Voltage Withstand capability This includes: UPS units, rectifier circuits, chargers, I&C cabinet power supplies

17 Procedure Recommendations: WANO SOER and 2004 Addendum recommend NPP to have procedures for addressing : Degraded voltage Degraded grid frequency How well these recommendations have been implemented, information systems to monitor such events, thoroughness of procedures should be evaluated in each country

18 NPP Coping Recommendations : Recognizing that defense in depth requires improving ability to cope with losses of uninterruptible in-plant electrical buses: Review RPS and ESFAS logic circuits to identify any undesirable effects given loss of uninterruptible in-plant electrical buses Examples would include: generation of ADS signal in BWRs or AUTO Switchover to Recirculation in PWRs USNRC (1993) issued Information Notice describing concern and requiring evaluations & modifications for US NPPs Concern is not unique to US

19 NPP Coping Recommendations : For BWRs with All-electric Core Cooling: Evaluate providing a diverse means for promptly supplying power to core cooling systems This could include: Direct diesel driven pump Dedicated fast starting gas turbine

20 Electrical System Recovery Recommendations: WANO SOER and 2004 Addendum offer practical approaches to improve electrical system recovery: Grid procedures must recognize NPP as priority load center requiring highest priority for restoration