The Summer Meeting of IEEE Nuclear Power Engineering Committee Denver, CO, USA, July 12-14, 2016
IAEA GUIDANCE ON ENHANCING THE SAFETY OF ELECTRICAL POWER SYSTEMS
Alexander DuchacInternational Atomic Energy AgencyNSNI/SASPO Box 100, A-1400 Vienna, Austria
Magnus KnutssonRinghals AB
SE-432 85 Väröbacka, Sweden
Summer NPEC Meeting, Denver, CO, USA
Contents
• Evolution of specific safety requirements and recommendations on design of electrical power systems
• Motivation to update guidance on electrical power systems
• The IAEA Safety Report on Impact of Open Phase Conditions on the plant electrical systems– Objectives– Content– Summary conclusions
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Motivation to update guidance on electrical systems
• Operating experience over past 25 years– Effects of grid disturbances or human errors on plant safety systems– at least 10 SBO events– at least 16 OPC events
• Fukushima accident– A design vulnerability, CCF of electrical power systems due to
extreme external event
• Results of European Union Stress Tests (2012)– A design vulnerability to SBO event for many NPP designs
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4
SBO EVENTS
Year Nuclear Power Plant / Country
1986 Kori 4 (Republic of Korea)
1990 Vogtle 1 (United States of America)
1993 Narora 1 Event (Republic of India)
1993 Kola Event (Russian Federation)
2006 Forsmark 1 (Sweden)*
2007 Dampierre-3 (France)
2011 Fukushima Daiichi (Japan)
2012 Byron event (United States of America)**
2012 Kori 1 (Republic of Korea)
2013 Forsmark 3** (Sweden)
OPC EVENTS
Year Nuclear Power Plant / Country
1994 Kalinin Unit 1 (Russian Federation)
1997 Balakovo Unit 1, 3 (Russian Federation)
2000 Heysham 2 (United Kingdom)
2005 James A. FitzPatrick NPP and Nine Mile Point, Unit 1 (United States of America)
2005 Koeberg (South Africa)
2006 Vandellos Unit 2 (Spain)
2007 Beaver Valley Unit 1 (United States of America)
2007 Dungeness B (United Kingdom)
2011 Ringhals Unit 2 (Sweden)
2012 Byron Unit 1 (United States of America)
2012 Byron Unit 2 (United States of America)
2012 Bruce A Unit 1 (Canada)
2013 Forsmark Unit 3 (Sweden)
2014 Dungeness B (United Kingdom)
2015 Oconee (United States of America)
2016 Hinkley Point B (United Kingdom)
*Near SBO event**Degraded voltage resulted in inability to use on-site and off-site AC sources
SBO and OPC events continue to occur...
14 July 2016
Summer NPEC Meeting, Denver, CO, USA
Guidance evolution on electrical power systems
• NS-R-1 Requirements for Design (2004-2016)
– LOOP (Design of standby AC power sources)
• SSR 2/1 Requirements for Design (2016) and SSG-34 Design of
electrical power systems (2016)
– LOOP + SBO (Design of alternate AC power)
Complementary technical reports (2016)
– Design provisions for withstanding SBO
– Impact of Open Phase Conditions (in preparation)
– Grid Stability and Off-Site Power Reliability (in preparation)
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Guidance on electrical power supply
6
OECD/NEA DiDELSYSFukushima LessonsIEC, IEEE std.OECD/NEA WGELEC
Scope involves the whole electrical power systems
Before 2016
New Req. 68 on SBO
In publication
Since 2016
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Summer NPEC Meeting, Denver, CO, USA
New IAEA Safety Report on OPC – Why we need it?
• At least 16 OPC events occurred so far
• Resulted in equipment damages
• Degraded performance of the safety buses
• Currently installed instrumentation and protective schemes have not been adequate to detect an open phase condition and take appropriate action
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What is the safety concerns?
• A design vulnerability for many NPP ‘electrical’ designs
• A potential for severe voltage unbalance resulting in degradation or failure of electrical equipment
• The inability to detect and disconnect the degraded power source by current protection schemes
• A safety buss transfer to a standby off-site or on-site power supply may be prevented
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Objectives of the Safety Report
• Provide a common technical basis for OPC
• Document OPC aspects relevant for safety functions
• Outline critical issues which reflect the lessons learnt
• Outline technical guidelines to analyze plant design
protective measures
• Provide a description of existing practices and design
provisions
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What is included in the scope
• Relevant aspects of OPC in transmission systems or plant electrical systems
• Details on the methods used to identify vulnerability to OPC in existing protective schemes
– Preventive actions within the testing, surveillance and maintenance
– Protective measures such as fault detection and disconnection
– Design provisions for improvement of existing plant electrical design
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Failure modes and consequences
• Short duration voltage unbalances– less than a few seconds, occur during short circuits with or without
earth connection and switching operations
• Long duration of unbalanced voltage conditions– occur due to unbalanced loading of a three phase system
• Voltage unbalances of concern– lasting for extended durations due to various faults, e.g. breaker
poles fail to close/open, failure of transformer bushings or line insulator, etc.
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Effects of open phase conditions
• OPC on the high voltage side of a transformer
– Voltages can be present at all three phases downstream of the OPC at transformers and three-phase loads
– Voltages may be balanced on low voltage side under no load or lightly loaded conditions
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Effects of open phase conditions
• The voltage regenerated through the systems depends on:
– Location of the open phase
– Transformer winding, core configuration, and rated power
– System earthing arrangements
– Transformer loading, size and type of loads (e.g. inductive or
resistive)
– Properties of cables and overhead lines (capacitance, inductance)
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Unit Transformer
Auxiliary Transformers
Standby Transformer
Alternate AC Power Source
Standby AC Power Source
Main Generator
Switchyard
Safety Bus Safety Bus
Standby AC Power Source
On-site power system
Off-site power system
Section 3.2.2Section 3.2.1
Typical electrical diagram to illustrate the configuration used in sections safety report
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Effects of OPC on electrical equipment
• Induction motors• Convertors and battery chargers• Transformers• Main generator• Protection relays• Panel meters and metering schemes• Standby AC power source
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Evaluation of design vulnerability
• The magnitude of the unbalanced voltage and current• The electrical equipment ability to withstand and perform its
intended function during the unbalanced condition• The response of existing protection relays and
instrumentation to the unbalanced conditions and consequences
• The plant behaviour and the safety significance of OPC
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Summer NPEC Meeting, Denver, CO, USA
Calculations and simulations
• Typically, we do not test OPC during the commissioning• Calculation and simulations the only methods to study
effects of OPC• The aim is to determine the magnitude of unbalance U, I
– Evaluation needs to cover OPC under all electrical system configurations and loading conditions
– System response to an OPC depends on several parameters e.g. the transformer connection and core configuration, loading of the transformer, type of consumer and earthing principle
– Software– Modelling of transformers and loads
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0.2
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An example of simulated safety bus voltages, single OPC in the 400kV line to the unit transformer
0. 0.15 0. 0.25 0. 0.35 0.-6 -4 -2 0 2 4 6
t[s]
[kV
]
OPC at 0.2sec
Vector figure prior to OPC Vector figure after OPC
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0.1 0.15 0.2 0.25 0.3 0.35 0.4-6
-4
-2
0
2
4
6
t[s]
[kV
]
OPC at 0.2 sec
0.2
0.4
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An example of simulated safety bus voltages, double OPC in the 400kV line to the unit transformer
Vector figure prior to OPC Vector figure after OPC
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Phase to earth voltages on the high voltage side of an unloaded Yy0 transformer (before and after an OPC)
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Phase to earth voltages on the low voltage side of an unloaded Yy0 transformer before and after an OPC
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Summer NPEC Meeting, Denver, CO, USA
Electrical equipment withstand capability
• Is the equipment capable of withstanding the unbalanced conditions?
– Energized safety equipment (e.g. motors, battery chargers/ rectifiers, transformers and current transformers)
– Duration or the magnitude of unbalanced conditions has the potential to degrade safety system performance capabilities
• The aim is to either disconnect the equipment by protective schemes prior to damage or,
• Upgrade it to withstand the maximum unbalanced conditions
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Do we know behaviour of existing protection schemes during OPC?
• The sensitivity of existing protection relays in OPC
(overload, negative sequence current, under voltage)
• The measuring principle e.g. phase to phase, phase to
earth, symmetrical components
• The coincident logic e.g. 2 out of 2 or 2 out of 3
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Unit Transformer
Auxiliary Transformers
Standby Transformer
Alternate AC Power Source
Standby AC Power Source
Main Generator
Switchyard
Safety Bus Safety Bus
Standby AC Power Source
On-site power system
Off-site power system
Generator negative sequence
(U, I)
Safety bus (three phases U, I),
classified
HV side (three phases
U, I)
HV side (three phases U, I)
Zero sequence (U, I)
Zero sequence
(U, I)
LV side (three phases U, I)
Safety bus (three phases U, I),
classified
Where to detect OPC in electrical system?
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What action the protection does?
• Actuation logic design may depend upon the protected area and plant load
– if transformer is in standby mode without load, an alarm is activated; the operating personnel has some time to cope with OPC
– if transformer is in service mode with load an alarm is activated indicating OPC in the offsite power system; the time to respond to an OPC needs to be evaluated
– Automatic disconnection may be necessary when manual actions are slow to prevent impairments of equipment important to safety
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Design principles to be considered
• Reliability of OPC detection
• Spurious actuation concerns
• Prevention of losing all redundant safety features due to a
failure of OPC detection system
• Safety classification (e.g. Class 1E, SSG-30, IEC61226)
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If the plant is vulnerable to OPC - Interim measures
• A prompt diagnoses and response to OPC until permanent corrective actions are completed e.g.– Walkdowns, inspections, configuration of power supply, aggregation
of information from multiple equipment alarms, trips, vibration, temperature etc.
– Monitoring voltage at all phases– Improving procedures for verification of the voltages and clear
directions to operators– Training and briefing of operators on recognition of OPC– Procedures for manual bus transfer
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If the plant is vulnerable to OPC - Design solutions on the high or low voltage side of the transformer
• The measurement of one or more of the following parameters– Negative sequence voltage– Negative sequence current– Magnetization current– Zero sequence current– Zero sequence voltage– Current injection– Phase to phase or phase to earth voltage properly set for
unbalanced conditions
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Conclusions on OPC
• A design vulnerability for many NPP ‘electrical’ designs• A potential for degradation or failure of ‘essential’ electrical
equipment (non-safety as well as safety)• A protection scheme inability to detect and disconnect the
degraded power source• An ideal solution ‘one-fit-to-all’ may not be possible
– Where to detect, what to measure, how to measure, what action– Use of digital protective relays on safety buses– Classification and qualification of (digital) protective means
• An optimal solution - safety vs cost• Need for a consensus between industry and regulator
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Annexes – provide examples
• Calculations to evaluate the unbalanced voltage and current conditions
• Permanent corrective solutions
• A summary of open phase events that have occurred
• Some examples of the flux paths of different transformer configuration during open phase conditions
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The International Expert Team
• Geissler, W. AREVA, Germany• Giannelli, I–A. ENEL, Engineering and Research Division – ATN, Italy• Goberna, M. Slovenské elektrárne, a. s., Enel Group Company,
Slovakia• Kawaguchi, K. Nuclear Regulation Authority, Japan• Kawanago, S. MHI, Nuclear Engineering Company Ltd., Japan• Karlsson, M. Radiation Safety Authority, Sweden• Knutsson, M. Vattenfall AB, Ringhals NPP, Sweden• Lundbäck, M. Radiation Safety Authority, Sweden• Matharu, G. Nuclear Regulatory Commission, USA• Pepper, K. Office for Nuclear Regulation, United Kingdom• Richard, T–V. EDF, France• Zander, R–M. KGG, Gundremmingen NPP, Germany
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Thank you!
