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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 5 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 7 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 8 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 9 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 10 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 12 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 14 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 15 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 16 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 17 [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 20 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 21 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 24 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 25 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 26 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, Denver, CO, USA 14 July 2016 27 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 29 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 31 Thank you!