Electrical Power System of a Nuclear Power Plant

RESEARCH WORK IN

ELECTRICAL POWER SYSTEM OF

NUCLEAR POWER PLANT

JOHN JEROME G. DIZON BSEE 5-3

INTRODUCTION There are two electric power sources, the offsite power system and the onsite power system, that can deliver power to the safety equipment in a nuclear power plant. The offsite power system, which consist of the power grid, the main generator, and the equipment that connects them to the nuclear

power plant distribution busses, is defined in the industry standards1 and NRC regulatory guides2 as %he preferred power system. The onsite power system, which consists of diesel generators, batteries, and associated equipment, is a backup power source to the preferred, offsite power system. In a published document,3 the NRC stated its concern about the reliability of the offsite power system as the preferred emergency source and about the possible damage to a pressurized water reactor (PWR) that could result from a rapid decay of power grid frequency. ORNL contracted with NRC to provide technical assistance to establish criteria that can be used to evaluate the offsite power system for the licensing of a nuclear power plant. The results of many of the studies for this contract are recommendations to assess and control the power grid during operation. This is because most of the NRC regulations pertaining to the offsite power system are related to the design of the power grid, and we believe that additional emphasis on monitoring the power grid operation will improve the reliability of the nuclear plant offsite power

supply.

TABLE OF CONTENTS 1

OFFSITE POWER SYSTEM 1.1 1.2 1.3 1.4

INTRODUCTION TRANSMISSION SYSTEM SWITCHYARD EXAMPLE OF OFFSITE POWER SYSTEM

2 ONSITE POWER SYSTEM 2.1 2.2 2.3 2.4

3

GENERAL DESIGN GUIDELINES 3.1 3.2

4

5

INTRODUCTION DISTRIBUTION SYSTEM POWER SUPPLIES EXAMPLE OF ONSITE POWER SYSTEM

GENERAL POSSIBLE ONSITE EVENTS

DESIGN BASES OF A NUCLEAR POWER SYSTEM 4.1 4.2 4.3 4.4 4.5 4.6 4.7

GENERAL OFFSITE GRID CONNECTION NORMAL POWER SUPPLY SYSTEM

4.8

UNIT TO UNIT POWER SUPPLY

SECURED ALTERNATING CURRENT POWER SYSTEM TOTAL LOSS OF ALTERNATING CURRENT POWER

DIRECT CURRENT POWER SYSTEMS

MAIN CONTROL ROOM,EMERGENCY CONTORL POST AND LOCAL CONTROL CENTRES

EXAMPLE OF ELECTRICAL POWER SYSTEM

INTRODUCTION The off-site power system is composed of the transmission system (grid) and switchyard connecting the plant with the grid. The off-site power system will ideally provide AC power to the plant during all modes of operation. It also provides transmission lines for out-going power. The border between on-site and off-site power systems is normally in the high voltage breakers closest to the plant. The offsite power system performs an essential role in terms of safety in order to supply the onsite power systems with reliable power from multiple off-site generators. The off-site power system is part of the preferred power supply. An intrinsically robust grid system provides a highly reliable offsite

power source as it rapidly dampens the effects of grid perturbations during normal co

ELECTRICAL POWER SYSTEM OF NUCLEAR POWER PLANT : OFFSITE POWER SYSTEM

TRANSMISSION SYSTEM The central station system of power generation and distribution of a nuclear power plant enables power to be produced at one location for immediate use at another location many miles or kilometres’ away. Transmitting large amounts of electric energy over long distances is accomplished most efficiently by using high voltage transformers. Without transformer the distribution of electric power would be difficult or impractical. Transformers are electrical circuit to another by magnetic coupling. Their purpose in a power distribution system is to convert AC power at one voltage level to AC power of the same frequency at another voltage level. High voltages are used in transmission lines to reduce the amount of current flow. The power transmitted in a system is proportional to the voltage multiplied by the current. If the voltage is raised, the current can be reduced to a smaller value, while still transmitting the same amount of power. Because of the reduction of current flow at high voltage, the size and cost of wiring are greatly reduced. Reducing the current also minimizes voltage drop (IR) and amount of power lost (I2R) in the lines. There are certain limitations to the use of high voltage in power transmission and distribution systems. The higher the voltage, the more difficult and expensive it becomes to safely insulate between the line wires, as well as from line wires to ground. The use of transformer in power systems allows this voltage to be changed to a higher and more economical voltage for transmission. At the load centres transformers allows the voltage to be lowered to a safer voltage and more suitable for a particular load. Power Grid Transformers, used to step up or step down voltage, make possible the conversation between high and low voltages and accordingly between low and high currents . By use of transformers, each stage of the system can be operated at an appropriate voltage level. Single-phase three-wire power is normally supplied to residential customers, while three-phase power is supplied to commercial and industrial customers.

SWITCHYARD A switching substation, or switchyard, is a substation without transformers that operates only at a single voltage level. Switchyards, used mainly for connections and interconnections, are essential for transmission, distribution, collection, and controlling the flow of electricity. The switchyard delivers power generated at the power plant to the electrical grid. Switchyards are generally classified by voltage level, circuit breaker and bus arrangements. Switchyards are often located directly adjacent to or near a power station.


TRANSMISSION SYSTEM The central station system of power generation and distribution of a nuclear power plant enables power to be produced at one location for immediate use at another location many miles or kilometres’ away. Transmitting large amounts of electric energy over long distances is accomplished most efficiently by using high voltage transformers. Without transformer the distribution of electric power would be difficult or impractical. Transformers are electrical circuit to another by magnetic coupling. Their purpose in a power distribution system is to convert AC power at one voltage level to AC power of the same frequency at another voltage level. High voltages are used in transmission lines to reduce the amount of current flow. The power transmitted in a system is proportional to the voltage multiplied by the current. If the voltage is raised, the current can be reduced to a smaller value, while still transmitting the same amount of power. Because of the reduction of current flow at high voltage, the size and cost of wiring are greatly reduced. Reducing the current also minimizes voltage drop (IR) and amount of power lost (I2R) in the lines. There are certain limitations to the use of high voltage in power transmission and distribution systems. The higher the voltage, the more difficult and expensive it becomes to safely insulate between the line wires, as well as from line wires to ground. The use of transformer in power systems allows this voltage to be changed to a higher and more economical voltage for transmission. At the load centres transformers allows the voltage to be lowered to a safer voltage and more suitable for a particular load. Power Grid Transformers, used to step up or step down voltage, make possible the conversation between high and low voltages and accordingly between low and high currents . By use of transformers, each stage of the system can be operated at an appropriate voltage level. Single-phase three-wire power is normally supplied to residential customers, while three-phase power is supplied to commercial and industrial customers.



INTRODUCTION The on-site power system is composed of distribution systems and power supplies within the plant. It includes the AC and DC power supplies needed to bring a the plant to a controlled state following anticipated operational occurrences or accident conditions and to maintain it in a controlled state or safe state until offsite sources can be restored. Stand-alone power supplies, for example separate power for security systems, are not included. The on-site power systems are separated into three different categories according to their safety significance. The major components of the On-site Power System include the plant generator, plant transformer, auxiliary transformer, standby transformer and the distribution system feeding unit auxiliaries, service auxiliaries, batteries, rectifiers, inverters/UPSs, cables and standby AC power sources. Portions of the On-site Power System are part of the preferred power supply. The on-site Electrical Power Systems are generally divided into three types of electrical systems according to the different power requirements of the loads: An Alternating Current (AC) power system. The functions of the assigned AC loads will tolerate a certain interruption in the power supply. Usually the AC power system includes a standby AC power source. The loss of the preferred AC power supply to the Electrical Power Systems triggers the startup of a standby electrical power source. In most cases plant safety analyses assume that the standby AC power source will be available for response to design basis accidents.



A direct current (DC) power system. This supplies DC loads, without interruption, from batteries. The DC system includes battery chargers that are connected to the AC system of the Electrical Power Systems. Often separate DC power systems will be provided to support loads of different safety classification. 

 A uninterruptible AC power system which supplies power from inverters or motor-generator sets that are in turn supplied from a DC source such as the DC power system or dedicated batteries with rectifiers, and include a bypass circuit to allow feeding safety loads directly from safety class AC power systems.

ELECTRICAL POWER SYSTEM OF NUCLEAR POWER PLANT : ONSITE POWER SYSTEM

DISTRIBUTION SYSTEM Distribution systems used to distribute power throughout large commercial and industrial facilities can be complex. Power must be distributed through various switchboards transformers, and panel boards without any component overheating or unacceptable voltage drops. This power is used for such applications as lighting, heating, cooling, and motor driven machinery. Typically the distribution system is divided into the following sections:

SERVICE ENTRANCE – This section includes conductors for delivering from the electricity supply system to the premises being served. 

FEEDERS – A feeder is a set of conductors that originates at a main distribution center and supplies one or more secondary or branch circuit distribution center. This section includes conductors for delivering the energy from the service equipment location to the final branch circuit over current device; this protects each piece of utilization equipment. Main feeder originate at the service equipment location, and sub feeders originate at panel boards or distribution centres’ at locations other than the service equipment location. 



BRANCH CIRCUITS – This section includes conductors for delivering the

energy from the point of the final over current device to the utilization equipment. Each feeder, sub feeder, and branch circuit conductor in turn needs its own properly coordinated over current protection in the form of a circuit breaker or fused switch.

POWER SUPPLIES The AC and DC power supplies needed to bring a the plant to a controlled state following anticipated operational occurrences or accident conditions and to maintain it in a controlled state or safe state until off-site sources can be restored. Stand-alone power supplies, for example separate power for security systems, are not included. The preferred power supplies are the normal supplies for all plant systems important to safety. They are, if available always the first and best choice of power supply to the safety electrical power systems. The preferred power supply includes portions of both the on-site and off-site systems.



GENERAL DESIGN GUIDELINES Electrical systems important to safety should fully implement the requirements of their design bases. The design of the power systems in the plant should consider all possible events that could occur in the electrical systems associated with the plant. These events can cause symmetrical and asymmetrical perturbations in the plant and can be initiated: a. In the transmission system with the plant on line, off line and shutdown, or as a consequence of the plant separating from the grid due to anticipated faults or voltage and frequency variations beyond an acceptable level. b. By the main generator tripping leaving the on-site power systems connected to the off-site or on-site power sources. c. In the on-site power systems as a result of an electrical event such as motor starting, phase to ground fault or switching surges. The impact of such events on all the onsite electrical power systems (AC and DC) should be evaluated and confirmed that the allowable voltage and frequency requirements are not exceeded and the protection system is adequate.

POSSIBLE ONSITE EVENTS Events on the onsite power systems to be considered include, but not limited to,

 Switching and lightning surges Voltage swells caused by loss-of-load scenarios  Voltage sags caused by motor starts and electrical faults off-site and on-site  Voltage interruptions caused by electrical faults off-site during shut-down operation;  Voltage interruptions caused by on-site faults;  Frequency deviations caused by turbine speed variations;  Deviating grid voltage and frequency;  Faults in the on-site power system (all voltage levels) cleared by first step or backup protection; ELECTRICAL POWER SYSTEM OF NUCLEAR POWER PLANT : GENERAL DESIGN GUIDELINES

POSSIBLE ONSITE EVENTS  Voltage sags caused by motor starts and electrical faults off-site and on-site  Voltage interruptions caused by electrical faults off-site during shut-down operation;  Voltage interruptions caused by on-site faults;  Frequency deviations caused by turbine speed variations;  Transmission system faults cleared by first step or backup protection;

 Deviating grid voltage and frequency;  Faults in the on-site power system (all voltage levels) cleared by first step or backup protection;  Main generator excitation malfunctions (high and low excitation);  Open conductors; and  Solar activity and geomagnetic induced currents.

ELECTRICAL POWER SYSTEM OF NUCLEAR POWER PLANT : GENERAL DESIGN GUIDELINES

EXAMPLE: POWER SYSTEM DESIGN OF A NUCLEAR POWER PLANT The typical system has a net output of 1000 MW. In achieving this station relies on auxiliary plant and services whose electrical power is supplied from the AC power systems. The total connected load is in excess of 100 MW. The electrical system is the main source of power to the reactor cooling auxiliaries during normal operation, for the protection system and the safety systems during normal and fault conditions. This paper will consider the designs of both the Emergency power supply systems and the Normal power supply systems . Both offsite (grid) and onsite (Diesel generators) electrical power supply systems are provided and the reliability of both these systems has direct effect on the safety of the plant. The electrical power supply systems design is as follow:  Off-site power system(Grid connection)  Turbo-Generator and its auxiliaries.  Class IV-Normal AC power supply systems  Class1E-Emergency power supply systems. (Class III –AC Emergency power supply system, Class II-AC supply from battery & Inverter and Class -1 DC supply from batteries.) NORMAL POWER SUPPLY SYSTEM The Normal Power supply System connects the power station to the National Transmission System (Grid) and distributes power throughout the power station at system voltages postulated by engineering considerations. This power is used both for auxiliaries required to operate the power station and to supply the Emergency power supply Systems with its preferred source of power. This power is also called as Off-site power. The designed system has bus voltage of 11kV. Safety Design Bases  The MPS provides power to process plant not forming a part of the essential systems but which may make a significant contribution to nuclear safety ,their availability is desirable for nuclear operation and Safety  To supply power to auxiliary loads with a high reliability and integrity so as to limit the number of challenges to the EPS  All supplies are designed to be continuously available during routine operation s including those associated with a reactor trip with the grid. reactor trip with the grid.

11 kV System The connection of the NPS to the grid is in the powerstation's 400kV substation. The substation is designed to operate with both bus-coupler breakers and both bus-section breakers closed. Individual circuits supplying the NPS will be selected to either the main busbar or the reserve busbar. The system has two unit buses and two station buses. This system has generator transformer, unit auxiliary transformers, station transformer as shown in figure 1.

ELECTRICAL POWER SYSTEM OF NUCLEAR POWER PLANT : EXAMPLE

EXAMPLE: POWER SYSTEM DESIGN OF A NUCLEAR POWER PLANT 3.3kV System The 3.3kV NPS derives its power supply only from the 11 kV NPS. The system consists of buses termed unit bus and station bus. The unit bus generally supply auxiliaries and loads directly associated with the turbo-generator operation and the station bus supply auxiliaries and loads associated with station services. 415V System The 415V NPS supplies power to non-safety related equipment which is either associated with the turbo-generator or required for station services. Generally all 415V switchboards associated with power generation or station services will be radially fed and their power sources derived from the 3.3kV NPS. The system consists of two tiers of switchgear types ie. Load centres and motor control centres. Load centres incorporate a 3.3kVl433V Class 'C' AN transformer fed by a motor switching device/circuit breaker on a 3.3kV unit EMERGENCY POWER SUPPLY SYSTEM The Emergency power supply systems (EPS) are divided into four separation groups or trains and provide power to all equipment required to ensure the safety of the Reactor. To ensure that these systems meet their safety functional requirements detailed safety criteria must be satisfied. The following Safety Design Bases include both general and those specific to the EPS. Safety Design Bases  The EPS is designed and qualified to adequately survive external hazards such as lightning, earthquakes, high winds and floods with consideration given to extremes of ambient temperature.  The postulated internal hazard such as fire, internal missiles or pipe break coincident with loss of grid sufficient equipment in the EES will remain.  Functional to allow a safe shutdown of the Reactor. The separation segregation and isolation criteria for plant and cabling are applied to preserve the independence of redundant essential equipment.  The EPS is divided into four similar trains. Each train is capable of supplying its connected loads independently of the other three trains. All auxiliaries and Services required for continued operation of a train are derived from that same train of the EES.  The EES will operate when power supplies are available from either onsite or off-site (grid) power sources. Each EES train has its own associated and independent essential diesel generator. Connections from the Main Power Systems are made through qualified devices designed to isolate these connections when necessary.


EXAMPLE: POWER SYSTEM DESIGN OF A NUCLEAR POWER PLANT 110V DC Electrical System The function of the llOV DC Electrical System is to provide battery backed power, control and switching supplies to the essential switchgear and control equipments. This includes supplies to DC driven Heating, Ventilating and Cooling (HVAC) plant for the essential switchgear, reactor protection equipment and control gear rooms located in the control building. The 11OV DC comprises four trains of equipment, each having a battery, charger, switch board and associated distribution fuse boards for each of the primary and secondary systems within a train. The 110V DC system is normally energised at all times from the 415V system. 11OV AC Essential Uninterruptible Power Supply System The function of the 11OV AC Essential Uninterruptible Power Supply (UPS) System is to provide battery backed single phase instrumentation supplies to control cubicles and other essential equipment. The 11OV AC Essential UPS System comprises four trains of equipment each having a single phase inverter, charger, static switch unit, battery, switchboard and associated distribution fuseboards for each of the primary and secondary systems within a train. Essential UPS System is normally energised at all times from the 415V system. Generally, all electrical plant is located in dedicated rooms with controlled environmental conditions. The Main Electrical Power System..The function of the llOV AC Uninterruptible Power Supply (UPS) System is to provide battery backed single phase instrumentation supplies to the control cubicles associated with the data processing system and all other equipment requiring a non-essential UPS .The 11OV AC UPS System comprises two trains of equipment each having a single phase inverter, charger, static switch unit, battery, switchboard and associated distribution fuse boards.. 48V DC Power System The function of the 48V DC Power System is to provide battery backed power for remote control relays for the main station switchgear. The system is significant to safety in that it is used to support restoration of off-site supplies to the system 250V DC System The principal function of the 250V DC Main is to provide battery backed power for the main turbo-generators DC driven plant. These are installed to prevent mechanical damage to the main turbine in the event of loss of normal AC supplies coincident with a turbine trip.. The 250V DC system comprises two trains of equipment each having two batteries, two chargers, two switchboards and associated distribution fuse boards. The system is normally energised at all times from the 415V plant protection.


EXAMPLE: POWER SYSTEM DESIGN OF A NUCLEAR POWER PLANT


EXAMPLE: POWER SYSTEM DESIGN OF A NUCLEAR POWER PLANT In nuclear plants there are several levels of electrical power distribution (other than from the generator to the generator transformer):  High Voltage (e.g. 4160, 6900, or 13,800 Volts AC, may also be abbreviated VAC)  Medium Voltage (e.g. 480 VAC)  Low Voltage (e.g. 120, 240 VAC, 260 VAC) High voltage systems are used to supply equipment that have motors with high horsepower ratings. Examples of these are:  Feedwater pumps  Recirculating or Reactor Coolant Pumps  Circulating Water Pumps  Condensate Pumps  Cooling Tower Pumps and Fans  High Pressure Emergency Makeup Pumps  Containment Spray Pumps Medium voltage systems are used to supply equipment that have motors with moderate horsepower ratings. Examples of these are:  Auxiliary or Emergency Feedwater pumps  Heating Ventilating Air Conditioning (HVAC) Fans and Chiller Units  Control Rod Drive Motor-Generator Sets  Motor operated valves Low voltage systems are used to supply equipment that have motors with low horsepower ratings. Examples of these are:  Lights  Small pumps


EXAMPLE: POWER SYSTEM DESIGN OF A NUCLEAR POWER PLANT


GENERAL The electrical power systems and components of the nuclear power plant on the one hand generate electrical power and supply it to the external grid and on the other hand supply electrical power to the plant’s systems from external and internal power supplies. The reliable operation of these systems is important for ensuring plant safety, accident management and the mitigation of the consequences of accidents. The Government Resolution (395/1991) presents general safety requirements for nuclear power plants. This resolution contains both general provisions for all safety systems and provisions for the electrical power systems of nuclear power plants. These are stated in more detail in Guide YVL 1.0, which sets forth the safety principles to be applied in nuclear power plant design. Guide YVL 1.1 describes how STUK controls the design, construction and operation of nuclear power plants. Guide YVL 5.2 gives the detailed design bases and safety requirements pertaining to electrical systems and components at nuclear facilities. Chapter 6 describes STUK’s regulatory control of a nuclear facility’s electrical systems and components. Section 5 of the Government Resolution (395/1991) prescribes that advanced quality assurance programmes shall be employed in all activities which affect safety and relate to the design, construction and operation of a nuclear power plant. Guide YVL 1.4 presents general requirements for quality management systems and Guide YVL 1.9 for quality management during operation. Guide YVL 2.0 applies generally to the design and regulatory control of nuclear power plant systems – specifically those assigned to a safety class – and specifies in more detail the general design requirements presented in Guide YVL 1.0. The safety importance of the function a system performs is essential in the focusing of STUK’s control activities. The safety classification of systems, structures and components affects their control. Paragraph one of section 21 of the Government Resolution (395/1991) prescribes that the functions important to the safety of the system, structures and components of a nuclear power plant shall be defined and the systems, structures and components safety-classified according to their safety significance. Detailed instructions for safety classification are given in Guide YVL 2.1. In addition, several other YVL guides apply to electrical power systems and components. Guide YVL 1.8 describes how STUK controls the modification, repair and preventive maintenance of systems, components and structures at nuclear facilities during operation. The guide also presents the obligations imposed upon licensees regarding this work. Guides YVL 2.2 and YVL 2.8 set forth the requirements for safety goals and their demonstration. The requirements for failure criteria are given in Guide YVL 2.7. Diesel generators and their auxiliary systems are dealt with in Guide YVL 5.1; valves and valve actuators in Guide YVL 5.3; instrumentation and control (I&C) systems in Guide YVL 5.5; air conditioning systems and equipment in Guide YVL 5.6; pumps in Guide YVL 5.7; and hoisting and transfer appliances in Guide YVL 5.8. Provision against earthquakes is addressed in Guide YVL 2.6; and fire protection in Guide YVL 4.3. A nuclear power plant’s radiation monitoring systems and equipment are dealt with in Guide YVL 7.11; and those radiation protection aspects to be considered in the design and layout of nuclear power plant systems and components in Guide YVL 7.18.

ELECTRICAL POWER SYSTEM OF NUCLEAR POWER PLANT : DESSIGN BASES OF ELECTRICAL POWER SYSTEM

GENERAL This guide sets forth licensee obligations regarding the design, implementation and operation of electrical power systems and components at nuclear power plants as well as STUK’s procedures pertaining to their control and inspection. In addition to this Guide, the Finnish Electrical Safety Act (410/1996) and Decree apply to nuclear facilities. Electrical safety regulations and other corresponding rules are based on the act and decree. Compliance with the electrical safety legislation is controlled by the competent authorities. The quality glossary used in this Guide complies with SFS-EN ISO 9000 . In accordance with the fourth paragraph of section 18 of the Government Resolution (395/1991), a nuclear power plant shall have on-site and off-site electrical power supply systems. The execution of the most important safety functions shall be possible by using either of the two electrical power supply systems In accordance with Guide YVL 1.0, the plant shall be provided with systems, which enable power supply from the main generator to the plant’s safety significant systems in case the connection to the external transmission gird is lost. The plant’s electrical power supply units shall be designed such that the loss of a single electrical power supply unit followed by the loss of the plant’s other power supply units, or their loss due to the same cause, is highly unlikely. The plant’s off-site and on-site electrical power supplies shall be designed such that each can alone ensure reactor decay heat removal, primary circuit integrity and reactor sub-criticality. The electrical power supplies of measuring systems for accident monitoring and management shall be designed in accordance with the accident instrumentation requirements of section 2.5 of Guide YVL 5.5. For severe accident management and monitoring, the nuclear power plant shall be equipped with monitoring devices, as described in section 3.6 of Guide YVL 1.0, whose electrical power supplies are to be independent of the plant unit’s other electrical power supply units. General design requirements for the electrical power systems of nuclear power plants are set forth in IEEE 308 [1], IEEE 765 [4], KTA 3701 [5] and IAEA DS303 [6], among others, which are referred to in this Guide.

ELECTRICAL POWER SYSTEM OF NUCLEAR POWER PLANT : DESSIGN BASES OF ELECTRICAL POWER SYSTEM

OFF-SITE GRID CONNECTION In accordance with Guide YVL 1.0, for electrical power supply, there shall be two separate, independent grid connections from the external grid to each parallel section of the on-site electricity distribution system. These grid connections shall be so designed that during operational conditions and postulated accidents, the simultaneous loss of both is unlikely. It must be possible to start operation of both grid connections quickly enough after the plant main generator has been separated from the grid. Plant design shall consider variations of voltage and frequency that occur in the external power transmission grid and affect the electrical systems and components of the nuclear power plant. The external grid connections and their auxiliary systems shall be electrotechnically dimensioned as well as physically and functionally separated from other electrical power systems in such a way that design-basis disturbances in the external power transmission grid do not jeopardise the operation of safety-classified components during plant operational transients and accidents. The plant unit’s off-site grid connections shall be electrotechnically dimensioned such that each connection alone has sufficient capacity to ensure the removal of decay heat, to assure primary circuit integrity and to maintain reactor subcriticality. Several units at the plant may share connections to the off-site power transmission grid. That being the case, each connection alone must have sufficient capacity to simultaneously carry out the aforementioned safety functions at all plant units. The design of off-site grid connections shall make unlikely the simultaneous failure of both of them from the same cause in consequence of operational transients, postulated accidents, weather phenomena or other external events. Plant design shall also consider all component failures and fires that could be caused by short circuits in the grid connections. In addition, auxiliary systems important for the operability of the connections, e.g. auxiliary voltage supplies and automatic switching devices, shall be designed in a way making the connections as independent as possible. The plant unit shall be provided with reliable, automatically starting changeover equipment for change-over switching between off-site grid connections. Change -over switching shall be designed to not unnecessarily start the plant’s safety systems. Manual change-over must be possible from the main control room or, in case of the loss of the main control room, from outside the main control room.

ELECTRICAL POWER SYSTEM OF NUCLEAR POWER PLANT : EVALUATION OF A NUCLEAR POWER SYSTEM

NORMAL POWER SUPPLY SYSTEM The plant’s normal power supply systems supply the necessary electrical power to the plant units’ electrical equipment and I&C systems, either from their own electrical power supplies or from the off-site power transmission grid. Normal power supply systems refer here to electrical power systems whose operation is not secured by auxiliary power supply systems. The design of normal power supply systems shall ensure that the disturbance or failure of a Safety Class 4 or Class EYT (non-nuclear) normal power supply system does not endanger the designed operation of a Safety Class 2 or 3 electrical power or I&C system. The functional separation of Safety Class 4 normal power supply systems shall be designed to render unlikely the deterioration, or failure, of the operating capability of its redundant subsystems due to the same electrical disturbance. The capability of the plant unit’s normal power supply systems shall be electrotechnically dimensioned to supply sufficient electrical power for the fulfilment of essential safety functions.

SECURED ALTERNATING CURRENT POWER SYSTEMS The operation of Safety Class 2 and 3 alternating current components shall be assured by supplying electric power from onsite emergency power supply systems. Those emergency power supply systems that carry out a safety function only shall be physically separated from plant sections for normal operation. Systems performing the same safety function, and their subsystems - whether they are similar to or different from one another - shall also be separated. The functional separation of safety-classified alternating current power systems shall be designed such that the deterioration, or failure, of their redundant subsystems due to the same electrical disturbance is unlikely. The systems are to automatically engage to ensure uninterrupted power supply, or power supply if a voltage break of permissible duration has occurred, in case normal power supply is disrupted in a way endangering the operability of components. The on-site emergency power supply systems shall be designed to assure the availability of Safety Class 2 and 3 secured alternating current power systems according to the operating time requirements set to them. It shall be possible to reliably take the emergency power supply systems into service even from the main control room and from local control centres. The design of the emergency power supply systems shall make them capable of reliably starting, engaging, receiving loads and feeding electrical power even during the most demanding loading situations (e.g. start-up situations) and operating conditions. The quality of the alternating current supply shall be kept such that the operating capability of the supplied components is not compromised. Requirements that apply to the diesel generators of nuclear facilities are discussed in more detail in Guide YVL 5.1. The emergency power supply systems shall be provided with sufficiently comprehensive, alarming condition monitoring systems to promptly detect and locate failures causing unavailability of the systems. For the duration of their functional testing, maintenance and repair, it shall be possible to safely disconnect from other electrical power systems those units that belong to the emergency power supply systems. If necessary, it shall be possible to reliably replace the power supply units of battery-backed alternating current systems with stand-by power supply connections facilitating the safe fulfilment of measures relating to the power supply units. ELECTRICAL POWER SYSTEM OF NUCLEAR POWER PLANT : EVALUATION OF A NUCLEAR POWER SYSTEM

TOTAL LOSS OF AC POWER In accordance with Guide YVL 1.0, in nuclear power plant design, the possibility of the on-site and off-site power supply units being simultaneously lost shall be considered. As provision against such a situation, the plant shall have available a power supply unit which is independent of the electrical power supply units designed for operational conditions and postulated accidents. It must be possible to introduce this power supply unit into operation quickly enough and its capability shall be sufficient to remove reactor decay heat, to ensure primary circuit integrity and to maintain reactor sub-criticality. Plant-unit specific, independent alternating current power supply units shall be dimensioned according to the above capacity requirement. An alternating power supply unit may be shared by several plant units. The capacity of the unit in question shall then be sufficient for the simultaneous removal of reactor decay heat, ensuring of primary circuit integrity and maintenance of reactor sub-criticality for all the nuclear facilities on the site. The design of an independent alternating current power supply unit shall be such that its failure simultaneously with the external power transmission grid connections, and due to the same cause, in consequence of weather phenomena or other external events is unlikely. In addition, auxiliary systems important for the operability of the supply unit and external grid connections, e.g. auxiliary power supplies and automatic switching systems, shall be designed such that the independent supply unit and external grid connections are as independent of each other as possible. It shall be possible to quickly and reliably take an independent power supply unit into service, if necessary. The design of the connections shall reliably prevent plant-to-plant spreading of electrical disturbances via them and their unplanned taking into service or engaging; also, their design shall reduce the likeliness of human errors during their planned taking into service and operation.

DIRECT CURRENT POWER SYSTEM To assure the operation of Safety Class 2 and 3 direct current equipment, their electrical power supplies shall be ensured by reliable and sufficiently efficient batteries to ensure an uninterrupted supply of direct current power in case of a disturbance in the supply of alternating current power, which endangers their operability. The batteries and their charging devices shall be dimensioned to reliably assure the operating capability of Safety Class 2 and 3 direct current power systems in accordance with system-specific operating time requirements. Guide YVL 1.0 prescribes that batteries backing up the operation of electrical systems important to safety shall maintain their capability to operate at least for two hours under any circumstances. The design bases of start-up batteries for combustion engines and of other special-purpose batteries shall be given case-by-case. Charging devices shall be capable of simultaneously feeding direct current to the loads and of charging storage batteries. A charging device shall be dimensioned such that its performance is not endangered even during the most demanding loading situations (e.g. start-up) and operating conditions. It shall be capable of feeding the necessary direct current to the loads even if the storage battery has been disconnected. Even then, the quality of the direct current supplied must not cause malfunctioning of the loads. Charging devices shall be designed to reliably prevent the passing of potential disturbances from alternating current power systems to a direct current power system via them. ELECTRICAL POWER SYSTEM OF NUCLEAR POWER PLANT : EVALUATION OF A NUCLEAR POWER SYSTEM

DIRECT CURRENT POWER SYSTEM Direct current systems performing a safety function only shall be physically separated from plant sections for normal operation. Systems and subsystems for the same safety function, whether they are similar or not, shall be separated from each other. The functional separation of safety-classified alternating current power systems shall make unlikely the operational weakening or malfunctioning of redundant subsystems due to the same electrical disturbance. Safety Class 2 and 3 direct current power systems shall be designed to be as independent of other systems as possible. Automatic features essential for the operation of redundant subsystems (e.g. protection and possible automatic switching systems), and auxiliary systems (e.g. auxiliary voltage and air conditioning) shall be designed according to the same principles as the subsystems proper. The auxiliary systems shall be dimensioned such that they are, in accordance with the set operating time requirements, capable of assuring the operating capability of Safety Class 2 and 3 secured alternating current systems in all plant operational conditions and postulated accidents. The design of direct current power supply systems shall ensure that the disturbance or failure of a Safety Class 4 or Class EYT (non-nuclear) direct current power supply system does not endanger the designed operation of a Safety Class 2 or 3 electrical or I&C system. Safety-classified direct current power systems shall be equipped with extensive enough alarming condition monitoring devices by which the operability of the systems can be continuously reliably monitored and failures causing their unavailability immediately detected and located.

MAIN CONTROL ROOM EMERGENCY, CONTROL POST AND LOCAL CENTRES The main control room of a nuclear power plant shall be equipped with devices providing information about the operational state, and deviations from it, of the plant’s electrical systems and the offsite power transmission grids; as well as with systems monitoring the operation of the plant’s electrical systems during operational transients and accidents. The need for emergency control operations from outside the main control room for normal and emergency power systems shall be analysed. The design bases for a nuclear facility’s main control room and emergency control posts are given in sections 2.3 and 2.4 of Guide YVL 5.5. Power supplies for the I&C systems of the main control room, the emergency control post and local control centres, which are needed to manage the nuclear power plant unit during operational conditions and accidents, shall be ensured by internal emergency power supply systems. In the main control room, the power supplies of the various subsystems of safety systems shall be reliably functionally separated to make unlikely their simultaneous failure from the same electrical disturbance. The power supplies for an emergency control post outside the main control room shall be separated from those for the main control room such that the total destruction in a fire of components contained in one fire compartment does not damage both power supplies so much as to prevent the fulfilment of safety functions. Guide YVL 4.3 prescribes that cables from the safety-related redundant subsystems to the main control room shall be routed through separate fire compartments. In case the cables from different redundant systems must exceptionally be situated in the same fire compartment, they shall be separated inside the compartment by means of distance, fire-resistant materials and fire insulation. The cable space below the main control room is an example of such a compartment. ELECTRICAL POWER SYSTEM OF NUCLEAR POWER PLANT : EVALUATION OF A NUCLEAR POWER SYSTEM

UNIT TO UNIT POWER SUPPLY The design of the alternating current power supply systems of nuclear power plant units shall enable unit-to-unit supply of electrical power within the site such that, where necessary, one unit can be maintained in a safe state in case of the loss of the off-site grid. The design of the power supply connection shall make unlikely the unit-to-unit propagation of an electrical disturbance via it and also the connection’s unplanned taking into service and engaging. The connection shall be available promptly and reliably enough where necessary. The control and switching actions of the connection shall be designed to minimise the probability of human error.

ELECTRICAL POWER SYSTEM OF NUCLEAR POWER PLANT : EVALUATION OF A NUCLEAR POWER SYSTEM

Electrical Power System of a Nuclear Power Plant

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