BS 7674:1993 download.Nuclear power plants — Instrumentation and control systems important for safety — Requirements for electrical supplies.
BS 7674 specifies the performance and the functional characteristics of the electrical supply systems required for the instrumentation and control (I&C) systems important to safety of a nuclear power plant. Guidance is also given on the possible use of these supplies for other I&C systems.
These supplies are required to be fed from primary sources of suitable redundancy and reliability, such that the safety and functional objective of the I&C system can be adequately achieved.
BS 7674 defines the methods of application of IAEA Safety Guides 50-SG-D3, 50-SG-D7 and 50-SG-D8.
The specific design requirements for the components of the I&C power supply system are outside the scope of this standard.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of BS 7674. At the time of publication, the editions indicated were valid. All normative documents are subject to revision, and parties to agreements based on BS 7674 are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. Members of IEC and ISO maintain registers of currently valid International Standards.
IEC 38:1983. IEC standard voltages. IEC 146-2:1974, Semiconductor convertors — Part 2: Semiconductor self-commutated con vertors. IEC 146-4:1986, Semiconductor convertors — Part 4: Method of specifying the performance and test requirements of uninterruptible power systems. IEC 293:1968, Supply voltages for transistorized nuclear instruments.
IEC 478, Stabilized power supplies, d.c. output. IEC 557:1982, IEC terminology in the nuclear reactor field.
IEC 639:1979, Nuclear reactors — Use of the protection system for non -safety purposes. IEC 686:1980, Stabilized power supplies, a.c. output.
IEC 709:1981, Separation within the reactor protection system.
IEC 780:1984, Qualification of electrical items of the safety system for nuclear power generating stations. IEC 801, Electromagnetic compatibility for industrial-process measurement and control equipment.
IEC 896, Stationary lead acid batteries — General requirements and methods of test.
IEC 980:1989, Recommended practices for seismic qualification of electrical equipment of the safety system for nuclear generating stations.
TAEA Safety guide 50-SG-D3:1980, Protection system and related featuring in nuclear power plants.
IAEA Safety guide 50-SG-D7 Rev. 1:1991, Emergency power system of nuclear power plants.
IAEA Safety guide 50-SG-D8:1984, Safety related instrumentation and control svste,n for nuclear power plants.
3 Definitions
For the purpose of this International Standard, the following definitions apply:
I&C systems important to safety see Guide of IAEA 50-SG-D8 safety systems
see IEC 557
I&C safety related systems see Guide of IAEA 50-SG-D8 separation within the reactor protection system
see IEC 709
safety system support features (power supplies)
see IEC 557
I&C non-interruptible electrical power supply
systems
systems which will not be lost solely as a result of loss of off-site and on-site power sources
(see Figure 1 and Figure 3) stationary lead acid batteries see IEC 896
other terms not defined above are defined in IAEA Safety Guide 50-SG-D7
4 System requirements
4.1 Function and description
I&C electrical supplies generally consist of the following non-interruptible systems (see Figure 1):
— a d.c. power system with batteries, supplying d.c. loads;
— an a.c. power system with internal batteries, supplying a.c. loads.
Taking into account factors such as design margins, temperature effects and deterioration with age, the capability of the float charged batteries shall be such as to meet all required load demands and conditions (including duty cycles, electrical transients during operational states and accident conditions) until such time as the standby generator can meet the load demands.
Batteries and chargers shall be operated in parallel so that the battery is maintained in the charged condition and supplies current to the loads only when a supply via the charger is not available.
Local batteries, e.g. batteries supplying a single instrument or cubicle, shall meet the applicable clauses of this standard.
6.3 Inverters and converters
The inverter design shall consider the following conditions in addition to summation of the total load capacity:
— the commutation failure limit shall be greater than the summation of peak currents generated under overload conditions by the power conversion units installed as part of the loads;
— the voltage control during loss of primary power supply shall maintain the output voltage within specified limits;
— the inverter shall be suitable for non-linear loads such as switched mode power supplies (SMPS) (see example 1 of Annex A);
— the frequency regulation; to facilitate rapid change-over, inverters should run synchronously with the dedicated back-up system when the system frequency is normal;
— the permissible output voltage and waveform quality (see clause 9).
To supply electronic devices, intermediate circuit d.c./d.c. converters may be used. These d.c./d.c. converters will be connected on the input side at the voltage of the battery equipment and will feed, on the output side, loads or groups of loads with a controlled voltage which may have a nominal value different from the battery voltage.
7 Requirements for distribution system design
The following requirements shall be considered in relation to each system.
7.1 System aspects
The supply system shall operate to maintain supplies important for safety under normal and relevant accident conditions.
The peak voltage has a major effect on the load performance and the current and voltage waveforms may have to be specified to be within certain limits. Annex A. example 1, shows typical values for an a.c. power supply. Other values depending on the nominal distribution voltage level are also acceptable if in accordance with IEC 38, IEC 293 and IEC 686.
8.4 Loads important to safety and loads not important to safety
The following design criteria should be followed to enhance the reliability of the I&C power supply systems:
— loads not important to safety shall be fed from the safety busbars only when connected through devices that prevent faults on these non-safety loads from adversely influencing the safety supplies (e.g. isolation devices);
— the capacity of the supply system shall be adequate to support both the safety and the relevant non-safety loads for the time required by the safety criteria.
9 Characteristics of supplies given to
individual loads
9.1 DC supplies
The characteristics of the d.c. supplies required by individual loads shall have a margin on the values specified for the output of the power supply system to allow for deterioration in service, and for the impedance of connections between the load and the supply. Where d.c.Ia.c. inverters or d.c./d.c. converters are provided, the variation of the voltage at the equipment can be designed to be less than at the battery (see Annex A, example 3).
Reference should be made to IEC 478.
9.2 AC supplies
The characteristics of a.c. supplies required by individual loads shall have a margin on the values specified for the output of the power supply system to allow for deterioration in service and for the impedance of the connections between the load and the supply.
Close control of the mean frequency may be required
when the frequency is used as a measure of time
(e.g. on synchronous recorder charts).
Examples of limits are given in Annex A.
In case of three-phase supplies, the loads should be appropriately balanced across the three phases. Reference should be made to IEC 686.
The designer shall review the merits of on-line and off-line tests for each part of the system, and the need to minimize manual reconfiguration of the system to carry out the tests.
Provision should be made to enable battery discharge testing to be carried out at design emergency-load current.
For test purposes the chargers may be operated in a test mode with an output voltage
(e.g. 2,05 V per cell) less than the float charging voltage of the battery. In this mode, the diodes of decoupled loads can be tested for the specified function.
13 Maintenance
A maintenance programme shall be provided for the equipment covered by this standard.
The design and arrangements of all components of the I&C power supply system shall provide a good layout for easy identification of components, easy maintenance and short repair times
(e.g. by accessibility and exchangeability). Particular attention shall be paid to testing and maintenance requirements for local batteries. The ageing of all batteries should be monitored.