reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

11Bibliography

11.1British Standards {BS)

Where an International Electrotechnical Commission (lECI number is given, the British Standard is identical with the IEC document,

BS4727: Part 1: Glossary of electrotechnical, power, telecommunication, electronics, lighting and colour terms: Part 2: Terms particular to power engineering Group 06. Switchgear and controlgear terminology (including fuse terminology): 1985

BS5311: Parts 1-7: Specification for a.c. circuit-breakers of rated voltage above 1 kV: 1976

8S5227: Specification for a.c. metal-enclosed switchgear and controlgear of rated voltages above 1 kV and up to including 72.5 kV: 1984

BS4752: Part 1: Specification for switchgear and controlgear for voltages up to and including 1000 V a.c. and 1200 V d.c. Part 1. Circuit-breakers: 1977 (IEC 157-1975: IEC 157-1A: 19761

88775: Part 2: Specification for contactors: Part 2, a.c. contactors for voltages above 1 kV and up to including 12 kV: 1974

BS3659: Specification for air-break circuit-breakers for alternating current systems above 1 kV and up to and including 11 kV: 1963 ( withdrawn and replaced by BS5311)

BS5424: Part 1: Specification for controlgear for voltages up to and including 1000 V a.c. and 1200 V d.c. Part 1, Contactors: 1977 1IEC158-1: 1970, IEC158-1A: 1975)

BS5419: Specification for air-break switches, air-break disconnectors, air-break switch disconnectors and fuse-combination units for voltages up to and including 1000 V a.c. and 1200 V d.c.: 1977 1IEC408: 19721

BS5486: Part 1: Low-voltage switchgear and controlgear assemblies. Part 1, Specification for type-tested and partially type-tested assemblies (general requirements): 1986 1IEC439-1: 1985)

BS5490: Specification for classification of degrees of protection provided by enclosures: 1977 (IEC529: 1976)

9S1432: Specification for copper for electrical purposes. Strip with drawn or rolled edges: 1970

B52898: Specification for wrought aluminium and aluminium alloys for electrical purposes. Bars, extruded round tube and sections: 1970

8S5253: Specification for a.c. disconnectors (isolators) and earthing switches of rated voltage above 1 kV: 1975

1352692: Part 1: Fuses for voltages exceeding 1000 V a.c. Part 1. Specification for current-limiting fuses: 1986 (IEC2821: 1985)

8S3938: Specification for current transformers: 1973 (19831

BS3941: Specification for voltage transformers: 1975 119821

BS162: Specification for electric power switchgear and associated apparatus: 1961

BS6581: Specification for common requirements for high-voltag e switchgear and controlgear standards: 1985 IIEC694: 1980)

854941: Part 1: Specification for motor starters for voltages up to and including 1000 V a.c. and 1200 V d.c.: 1979 1IEC292-1: 1969, IEC2921A: 19711

BS142: Electrical protection relays: 1982

BS158: Specification for the marking and arrangement of switchgear busbars, main connections and small wiring: 1961 (withdrawn)

BS3535: Specification for safety isolating transformers for industrial and domestic purposes: 1962

BS88: Parts 1 and 2: Specification for cartridge fuses for voltages up to and including 1000 V a.c. and 1500 V d.c: 1975 11982)

854343: Specification for industrial plugs, socket-outlets and couplers for a.c. and d.c. supplies: 1968

85196: Specification for protected-type non-reversible plugs, socket outlets, cable couplers and appliance-couplers with earthing contacts for single phase a.c. circuits up to 250 volts: 1961

11.2 Electricity supply industry (ESI) Standards

ESI Standard 37-1: 415 V a.c. switchgear, controlgear and fusegear

ESI Standard 37-3: A.C. metal-enclosed switchgear and controlgear of rated voltages 3.6 kV and 12 kV: Part 1: Circuit-breaker equipment (air-break): Part 2: Fused switching device equipment of 3.6 kV rated voltage

ESI Standard 50-18: Design and application of ancillary electrical equipment

11.3 Other relevant documents

Memorandum on the Electricity Regulations (SHW 928): UK Factories Act: 1961

DEF 59-100 Part 1: Fuseholders, carriers and bases electrical fuse (Block and extractor post types)

DEF 59-96 Part 1: Fuselinks electrical

Power Circuit Breaker Theory and Design: Edited by C.H. Flurscheim: Published by Peter Peregrinus Ltd

11 Earthing systems

11_1 Introduction

11.2 Differences in earth potential

11.2,1 Definiti ons

11.2.2 Acceptance criteria 11,3 Earthing systems design

11.3.1Systems having remote neutrals

11.3.2Faults on internal systems

11_3 3 Lightning protection

11.3.4Additional considerations

11.4Earth electrodes

11.4.1Sheet steel piles

11.4.2Cylindrical steel piles

11_4.3 Earth rods

11.4.4Earth strip

11.5Earth network construction and plant bonding

11.5.1Main earth network

11.5.2Instrument earth network

11.5.3Earth bond cable sizes

11.5.4Plant bonding arrangements

11.6Testing

11.6.1Earth resistivity measurement

11.6.2Earth electrode resistance measurement

11.6.3Commissioning tests

11.6.4Routine tests

12Lightning protection

12.1General requirements

12.2Lightning magnitudes and risks

12.3Application of requirements to power stations

12,4 Protection system design

12.4.1 Main and gas turbine chimneys 12,4,2 Main buildings

12.4.3Other buildings

12.4.4Buildings requiring special considerations 12,4.5 Fuel oil storage tanks

12.4.6Flammable gas production and storage plant

12.5Assessment of risks of sideflashing and interference

12.6Inspection, testing and records

13Lighting, heating and small power systems

13.1Introduction

13.2Lighting system design

13.2.1Objectives

13.2.2Specification

13.2.3General planning

13.2.4Detailed planning

13.2.5Appraisal

13.3Emergency lighting systems

13.4Lighting of special areas

13.4.1Battery rooms and chlorination plant rooms

13.4.2Hydrogen plant (Division 1 and Division 2 areas)

13.4.3Central control rooms

13.4.4Hazard warning lights

13.5Supplementary heating and minor power systems

13.6Distribution system

13.6.1General

13,6.2 Isolation and switching of individual fittings

13.6.3AC supplies

13.6.4DC supplies

13.6,5 Cabling

14Design and management techniques

14.1Introduction

14.2Planning

14.3Design

14.3.1Layout

14.3.2Cable support systems

14.3.3Information from plant contractors

14.3.4Cable systems and electrical circuit design

14.4Installation and contract management information

14.4.1Introduction

14.4.2The aims and functions of TPI cabling

14.4.3Designing

15References

Appendices

AValues of resistance and reactance for single-core elastomericinsulated cables I90° C maximum conductor temperature)

BValues of resistance and reactance for multicore PVC-insulated cables (70° C maximum conductor temperature)

CCurrent ratings for elastomeric-insulated cables

D Current ratings for PVC-insulated cables

ERating factors for variations in thermal parameters

FCross-sectional area of armour wire

G415 V motor parameters and selected fuse sizes

HMaximum cable route lengths

IMain protection for feeder and motor circuits

JAdvantages and disadvantages of various lamps used for lighting power station interiors

1 Introduction

The cabling system within a power station performs the essential function of connecting mechanical, electrical and control equipment together to form a total working entity. Cabling systems therefore form an interface between a variety of plant supplied under a large number of electrical and mechanical contracts, and information has to be drawn from each of these to complete the cable system design. During the power station construction period cabling systems are dependent on plant having been installed so that connections can be completed. It can therefore be seen that cabling is an important item in the organising and planning of design functions and site activities.

Furthermore, the number of cables has steadily increased with the size of boiler/turbine units, mainly due to the growth of control and instrumentation func-

tions. The number of cables installed on a power station varies with the type of plant, i.e., hydro, coal, oil or nuclear. Considering 660 MW units, the quantities of cables involved at the time of writing for recent projects are

(2 units, nuclear AGR)

The average cable route lengths for these projects varies from 51 m on Littlebrook D to 59 in on Heysham 2 and 74 m on Drax Completion. This means that

2 Cable systems and layout

2.1Segregation requirements

Before discussing layout in general, it is necessary to understand the segregation and separation requirements for various station types. The station layout, from the very beginning, has to take into account the disposition of ancillary plant and interconnecting cables to ensure that segregation requirements can be achieved. Segregation is provided to limit damage under accident conditions such as fire.

Segregation is defined as the physical division or isolation of one group of cables or plant from another by an enclosure or barrier of a certain specified fire rating. The barrier may be brick, concrete or special fireproof partitioning as described in Section 10 of this chapter, Separation is defined as the division of groups of cables by distance alone.

Segregation in fossil-fired and hydro plant is primarily provided to limit economic loss. However, in nuclear power stations it is provided for nuclear safety as well as economic reasons.

2.1.1 Segregation requirements for fossil-fired and hydro power stations

Segregation in fossil-fired and hydro power stations is provided to limit economic loss in the event of fire.

TABLE 6.1

Basic segregation requirements for conventional plant

It is a basic requirement that cabling of one unit be segregated from the cabling of all other units. This must be achieved on all major cable routes by the provision of suitable fireproof enclosures and barriers to prevent spread of fire from one unit's cabling to another, and also to contain combustion products. In the case of turbine halls and common boiler houses, clearly it is not practical to enclose all minor cable routes to achieve segregation and, because of the limited amount of cables, there is no need to contain the combustion products. Therefore in the case of these minor cable routes, segregation is achieved by isolating one from the other by distance.

Segregation requirements for station cables and plant will depend on the system design. When one station transformer is provided for each unit, electrical station services are provided on a unit basis. In this case, cables for these services will be segregated by fire barriers from the station services cables associated with other units. If, however, two station transformers are used to provide station services for the whole station, then a different solution is required. In this case, duplicated plant necessary for the operation of the station may be designated 'Station A' and 'Station B', and full segregation by fire barrier applied between the two designations. There is no need to segregate station cables from unit cables providing that in any one incident the total loss is not greater than one unit and/or half the station services. Main CW pumps are normally fed from a station system since these are installed hydraulically on a shared station basis. For CW pumps, cabling should be provided on a sufficient number of segregated routes such that no more than the output of one unit will be lost in a single incident. Where gas turbines are installed, these are normally on a unit basis and segregation should be provided such that the output of only one gas turbine is lost in a single incident. Another area where it is considered essential to provide segregation is where an emergency DC or guaranteed AC drive is provided for plant safety, as for example, the turbine lubricating oil supply. In these cases, full segregation by fire barrier should be provided between the main drive and the emergency drive. Where duplicate DC supplies for switchgear tripping are provided, these should be segregated from each other over their entire lengths. This requirement stems from instances where, under fire conditions, switchgear tripping supplies have been lost before main circuits have been tripped. Where segregation is not possible, it is permissible for one of the DC supplies to be cabled in short-time fireproof cable of the type described in Section 3.7 of this chapter.

We can now consider areas where segregation is not mandatory but which will result in better availability and security and will be employed if it can be incorporated without additional cost. The first area to consider is the II kV supplies to the unit which are derived from the unit transformer, station transformer or via interconnectors (see Chapter 1 System design).

Often segregation can be readily achieved between the interconnectors and the unit/station transformer feeds over the majority of their routes up to the cable race immediately below the switchgear. Similarly, where auxiliary transformers and feeders are duplicated within a unit, segregation can often be achieved for the majority of the cable routes without additional cost. Other circumstances where segregation should be applied if there is no cost penalty are main and standby plant, and also between boiler feed pumps where more than one is provided per unit.

Separation should be provided between control cables (containing analogue signals, digital signals or

plant protection signals) and power cables to minimise the effects of electrical interference. Control cables should be separated from single-core power cables by at least 600 mm and from muiticore power cables by at least 300 mm. This requirement does not apply to tail ends of routes where power and control cables are terminated in the same equipment, providing the length of run where separation distances are not met does not exceed 5 m. The basis for these electrical separation distances is discussed in Section 5 of this chapter.

2.1.2 Segregation requirements for nuclear power stations

The segregation requirements for conventional power stations to protect availability of plant are equally applicable to nuclear power stations.

However, in nuclear power stations additional segregation is necessary for the safety of personnel, the general public and plant. For these additional segregation requirements the occurrences considered are minor fire, safe shutdown earthquake (SSE), local flooding and a major incident within the station, i.e., turbine disintegration, major fire or hot gas release. The safety criterion normally applied is that any one of these incidents and its consequential effects shall not damage sufficient safety related cables to render the reactor trip and post-trip functions ineffective to a degree where an unacceptable probability of a district hazard would arise. In practice, this means that:

•The reactor must retain its ability to trip.

•A specified proportion of the post-trip cooling, monitoring and control systems must remain effective.

•Consequential faults must not degrade the effectiveness of the reactor trip system or post-trip cooling systems, e.g., gas circulator run-on.

The method of applying these criteria will depend on the type of reactor involved. However, to illustrate the principles, an advanced gas-cooled reactor (AGR) will be considered.

Firstly we must elaborate on the meaning of segregation and for this it is convenient to define two segregation classes. Segregation Class I is defined as 'cables, plant or equipment of different groups that must be separated by a barrier or enclosure having a minimum of four-hour fire rating and also be crash proof to the required standard for the safety hazard at the barrier/enclosure location'. Segregation Class II is defined as 'cables, plant or equipment of different groups that must be separated by a minimum of hour fire barrier or enclosure'. For cables installed direct-in-ground, cable groups should be separated by at least four metres for Class I and by at least one metre for Class II. The larger separation distance in-ground for Class I is in order to avoid accidental

Cables associated with plant protection, indication or alarms which in the event of a fire are essential to a n operator in the central control room, emergency indication centre or local control position are also Liesignated safety related. This includes fire fighting ,ervices and essential station communications. A fur- :her special function is heating and ventilation services

for contaminated areas.

The segregation of safety related cables is considered in the following section. However, the following rules are only written for areas outside the confines of the safety room. Cabling, conduits and trunking within the confines of the safety room are subject to special requirements, and these are considered too specialised o include in this volume.

Solely related cables excluding reactor safety trip s%cterns

This section deals with the segregation of safety re- laR;(1 cables with the exception of reactor safety trip

.stems which are a special case. Safety related cables

■■[II be discussed under three broad headings:

•Power cables The Heysham 2 power system ar-

rangement for one reactor is described in Chapter 1 and shown in Fig 6.1. The electrical trains forming the essential electrical system are broadly associated with the boiler circulator quadrants (A to D) of the reactor, and provide power supplies for two post-trip cooling systems. These two post-trip cooling systems are designated X and Y. Essential cooling system X is associated with forced gas circulation, e.g., gas circulators and forced-feed decay heat boilers. Essential cooling system Y is associated with natural

gas circulation and emergency feed to the main boilers.

Segregation must be employed to limit damage to the cables providing supplies to the plant and auxiliaries associated with the X and Y cooling systems. Two rules have been formulated to define the segregation to be employed, depending upon the type of incident to be protected against.

actor quadrants. Another option for this type of incident is that not more than all the X system supplies or, alternatively, all the Y system supplies fail. Thus, for a major incident, Class I segregation

A B and C D and their associated trains, or Class I segregation is required between complete sections of the X and Y systems.

In addition, separation shall be provided between the X system and Y system power cables associated with the same quadrant, and between power cables associated with different quadrants routed to and within system plant areas segregated on a half system basis. It is not mandatory to separate safety related power cables from other power cables except to ensure that the segregation principles have been maintained.

• Control cables The segregation specified for safety related power cables is also applied to the associated control cables where these are required in the performance of the safety related function. This applies to post-trip sequence control signals to essential plant.

There is no need to segregate control cables from associated power cables to the same equipment. However, it is prudent to separate them to reduce electromagnetic interference as defined for conventional plant. There is no special requirement to separate safety related control cables from other control cables associated with the same train. Typical post-trip cooling system and safety trip system cabling is shown in Fig 6.2.

• Cables for remote control and indications associated with safety related plant and equipment In the central control room (CCR), area segregation of control and indication cabling between trains/ quadrants is provided to Class II. This segregation is provided to limit economic damage and is not necessary for reactor safety for the following reasons:

(a)While the reactor is at power it is protected against rapidly developing faults by safety cir-

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