The overall electrical power supply system of Fennovoima Hanhikivi 1 plant (FH1) is subdivided into the onsite and offsite network, based on well-established principles from IAEA SSG-34.
The offsite power supply system consists of two independent grid connections to Fingrid’s high-voltage transmission system, the 400kV main grid connection and the 110kV standby grid connection, which supply the auxiliary power system. The onsite (auxiliary) power supply system supplies all loads of the reactor unit. To keep transient fluctuation and operating limits acceptable, rated power of auxiliary and standby transformer is designed to adequate capacity level.
The onsite electrical power systems and equipment’s (EPSE) that supply power to systems important to safety1 are essential to the safety of FH1 NPP and provides a graded protection with levels of defense in depth1 (DiD) against a wide variety of internal and external hazards, anticipated operational occurrences1 and accidents. FH1 project paves the way of licensing phase with 14 detailed electrical concepts explaining comprehensive approach of onsite and offsite EPSE including interfaces, hence assures adequate level of traceability for further basic and detail design development.
Various type of equipment’s have different standards applicability, having the right standards in place when a product is designed and qualified for life is a key factor in the success of Safety. Technical requirements are graded for reliability, redundancy and diversity based on European nuclear and IEC/CENELEC industrial standards. Finnish industrial national codes SFS used solely in Finland have priority in this project. If special nuclear equipment or technical requirement are not screen out by European code, ANSI accredited organization as IEEE is used. Preference of nuclear design code is favorable in German KTA standardization which have in some extent reference in the Finnish YVL Guides.
Qualification testing according to IEC/IEEE 60780-323 and used proven technology provides a high degree of confidence in the reliability performance under the normal and accident environmental conditions. The concept of aging is highlighted and implementation methods of qualification on intended use are subsequently developed. Specific treatment of equipment seismic qualification will be address for design basis earthquake, airplane crash and extreme earthquake. Non-classified normal operation equipment are fully in compliance with CE (”European Conformity”) according respective EU directive.
The analytical studies based on IEC 62855 for electrical calculations and analyses validate the robustness and adequacy of design margins and demonstrate the capability of electrical power systems, equipment and cables.
The guidance provided by regulations and guides from IAEA, YVL and WENRA is the evolutionary root of requirements, recommended methods and safety measures well considered in the design bases.
FH1 design use proven solutions from Leningrad nuclear power plant 2 but inevitable adaptation are done based on strict contractual EPC requirements covered with safety assessments. Physically separated 10 safety and safety related trains elevates FH1 EPSE on first of a kind robustness in the European Union among other NPPs already in operation.
Among many others, significant umbrella of electrical lines of defense consist from electrical protection units (A) and automatics (B).
A) The unit auxiliary power supply system take the form of a radial network and is protected with electronic and thermal trip units as part of circuit breakers, digital protection relays and fuse elements. In order to simplify the settings and the maintenance of the protection relays, for the calculation of the setting values always envelope values from the faults data are used, e.g. the lowest and / or highest short circuit current determines the resulting setting value.
As a result, the calculated relay setting value is valid for all identical feeder for all redundancies. The individual system equipment’s are singly fed and disconnected selectively in the shortest time possible in the event of faults. Discrimination by current relies on the fact that the fault current varies with the fault location, because of the difference in impedance values between source and fault. Faulty system items are to be identified by the protective equipment nearest to the fault location by time grading in the shortest possible time and are to be switched off selectively.
B) The power distribution main features of electrical automatics consist of voltage stabilization and control, interlocking and synchronization. The use of interlocks allows personal safety and the protection of equipment to be automatically ensured. They help prevent accidental operations in production and allow devices to be locked in order to isolate an installation. Switchover functions between distributions and coupling of various power sources is realized with synchronization units. Generators use automatic and manual voltage regulators. Voltage stabilization can be use in form of regulating transformers, modular electronic stabilizers and thyristor controllers for special loads.
The 10kV medium voltage emergency power systems composed with 4 emergency diesel generators that supply different loads are of utmost importance for the capability of the NPP to withstand a wide range of initiating events1. In the postulated case of “common cause failure” for all EDG systems, one emergency busbar can be supplied from the one off-site 10kV diesel generator supply connection, also cope with events e.g. Fukushima NPP.
Despite the rigorous design of passive systems shared in the VVER family, FH1 NPP have classified safety-related 400V station blackout diesel generators to supply loads required for design basis and beyond design basis accidents. In addition, consumers important for the safety, reliability and operability of the NPP are connected to the Uninterrupted Power Supply Systems. These systems are battery buffered (time-wise dependent on DiD level: 2h, 8h, 24h); in case that an interruption of the power supply from the grids and from the Diesel Generator occurs, the connected consumers will be supplied for a defined time from these batteries. FH1 NPP is the first project in European Union equipped with 20 safety and safety related battery systems achieving highest safety standards.
In Finland, nuclear market is already established based on operating nuclear power plants and engineering experiences gained from the modernization and new-build nuclear projects. Finnish companies can design and manufacture motors, generators and cables, including extensive installation of electrical equipment and building technologies. For FH1 NPP, Finnish electrical companies are already participating in the installation and engineering phases of the project.
Lukas Molitoris, text
The writer is Head of Electrical System Design at Rosatom.