Download Motivation to update guidance on electrical systems

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 Duchac
International Atomic Energy Agency
NSNI/SAS
PO Box 100, A-1400 Vienna, Austria
Magnus Knutsson
Ringhals AB
SE-432 85 Väröbacka, Sweden
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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
2
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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
3
SBO and OPC events continue to occur...
OPC EVENTS
SBO EVENTS
Year
Nuclear Power Plant / Country
Year
Nuclear Power Plant / Country
1994
Kalinin Unit 1 (Russian Federation)
1986
Kori 4 (Republic of Korea)
1997
Balakovo Unit 1, 3 (Russian Federation)
2000
Heysham 2 (United Kingdom)
1990
Vogtle 1 (United States of America)
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)
1993
1993
Narora 1 Event (Republic of India)
Kola Event (Russian Federation)
2006
Forsmark 1 (Sweden)*
2007
Dampierre-3 (France)
2007
Dungeness B (United Kingdom)
2011
Fukushima Daiichi (Japan)
2011
Ringhals Unit 2 (Sweden)
2012
Byron event (United States of America)**
2012
Byron Unit 1 (United States of America)
2012
Byron Unit 2 (United States of America)
2012
Kori 1 (Republic of Korea)
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)
2013
Forsmark 3** (Sweden)
*Near SBO event
**Degraded voltage resulted in inability to use on-site
and off-site AC sources
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
4
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)
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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Guidance on electrical power supply
Before 2016
Since 2016
Scope involves the whole electrical
power systems
New Req. 68 on SBO
OECD/NEA DiDELSYS
Fukushima Lessons
IEC, IEEE std.
OECD/NEA WGELEC
In publication
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
6
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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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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.
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
11
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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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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)
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
13
Typical electrical diagram to illustrate the configuration
used in sections safety report
Off-site power system
Switchyard
Section 3.2.1
Section 3.2.2
Unit
Transformer
On-site power system
Standby
Transformer
Auxiliary
Transformers
Main
Generator
Safety Bus
Safety Bus
Standby AC
Power Source
Standby AC
Power Source
Alternate AC Power Source
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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[kV]
An example of simulated safety bus voltages, single OPC
in the 400kV line to the unit transformer
OPC at 0.2sec
6
4
2
0
-2
-4
-6
0.
0.15
0.
0.25
t[s]
Vector figure prior to OPC
90
0.
0.35
Vector figure after OPC
90
1
120
0.
60
1
120
60
0.8
0.8
0.6
0.6
150
30
150
0.4
30
0.4
0.2
0.2
180
0
210
180
0
330
210
240
330
300
270
240
300
270
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
18
An example of simulated safety bus voltages, double OPC
in the 400kV line to the unit transformer
[kV]
OPC at 0.2 sec
6
4
2
0
-2
-4
-6
0.1
0.15
0.2
0.25
0.3
0.35
0.4
t[s]
Vector figure prior to OPC
Vector figure after OPC
90
90
120
60
0.6
0.6
150
150
30
30
0.4
0.4
0.2
0.2
180
0
210
330
300
270
60
0.8
0.8
240
1
1
120
180
0
210
330
240
300
Summer 270
NPEC Meeting,
Denver, CO, USA
14 July 2016
19
Phase to earth voltages on the high voltage side of an
unloaded Yy0 transformer (before and after an OPC)
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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Phase to earth voltages on the low voltage side of an
unloaded Yy0 transformer before and after an OPC
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
22
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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
23
Where to detect OPC in electrical system?
Off-site power system
HV side
(three phases
U, I)
Zero
sequence
(U, I)
Switchyard
Unit
Transformer
HV side (three
phases U, I)
On-site power system
Auxiliary
Transformers
Generator
negative
sequence
(U, I)
Zero sequence
(U, I)
Standby
Transformer
LV side (three
phases U, I)
Main
Generator
Safety bus (three
phases U, I),
classified
Safety Bus
Safety Bus
Standby AC
Power Source
Standby AC
Power Source
Alternate AC Power Source
Safety bus (three
phases U, I),
classified
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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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
Summer NPEC Meeting, Denver, CO, USA
14 July 2016
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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)
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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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
Summer NPEC Meeting,
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14 July 2016
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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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
28
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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
30
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
Summer NPEC Meeting,
Denver, CO, USA
14 July 2016
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Thank you!