tends to cieate long runs of -4 35 V cables with a con­ sequent increase in cither cable size or volt drop. This has led to the consideration of alternatives. Of these, the type most favoured by the CEGB is the dry type air-cooled class C insulated (AN). These, as their description implies, contain no fluid al all and have the advantage of being considered totally fireproof and they can be made integral with the 415 V switchgear. They do, however, have the disadvantages of higher cost, heavier weight, and lower reliability than oil-filled transformers.

When dry type transformers are fitted in switchgear there are certain layout requirements to be considered. Firstly, they are relatively heavy and bulky so the switchgear increases in both weight and size necessitat­ ing bigger switchgear rooms with stronger floors. Also they dissipate heat so this must be considered when designing the heating and ventilation system. Being integral with the switchgear, the LV connections are busbars into the switchgear and the HV connections are cables.

14.6 Cables

The cabling on a power station performs the essen­ tial function of providing electrical interconnection between the many items of electrical and control equip­ ment. During station erection and commissioning, the completion of the cabling systems is dependent on the timely installation of plant items. It is evident therefore that station cabling is a very important consideration at the overall design and planning stages.

Layout of the main cable routes is to a large extent dictated by the location of plant, transformers, switch­ gear and the central control room. However, segrega­ tion of unit electrical services also helps to establish the layout, particularly for nuclear stations where segrega­ tion between quandrants has to be considered in more detail (see Fig 2.47).

It is a basic requirement that cabling for one particu­ lar unit be segregated from the cabling to other units, and cable tunnels are an ideal method of obtaining this segregation on the major cable routes. Also, because cable tunnels are located at basement level they give the added advantage of being completed early in the civil programme, hence delays in the cabling installa­ tion can be avoided (see Fig 2.48).

The importance of providing adequate accommo­ dation for cables cannot be over-emphasised, and a typical cable tunnel with a capacity of sixteen arms has on many occasions proved insufficient. Apart from installation difficulties, congestion of large quantities of cables creates many problems such as overheating, overloading cable supports, loss of separation para­ meters and high levels of combustible PVC insulation which increase the fire hazard. Hence at the design stage consideration must be given to the expected number and size of cables running in the network of tunnels.

When the basic cable tunnel design is established, arrangements should be made for ventilation, drainage, inserts for the cable supports and cable drum access facilities for cable pulling. From the safety point of view, personnel access points and emergency exits have to be located in conjunction with fire barriers and doors which divide the tunnels into sections and so restrict the spread of fire and dense smoke. Further, all major cable tunnels have a fixed waterspray fire protection system which is automatically initiated by linear heat detection cables running above and below each cable rack.

14.6.1Segregation

Cables are vital for the control and operational activi­ ties that take place in a modern power station. The failure of a data or power cable due to a small fire can have catastrophic effects on such activities. Therefore, where possible, it is important to design the cables/ system layout to limit the effects of such a situation and one such method is to have segregated cable routes.

For conventional stations, the basic principle for major cableways is that the cables for each unit shall be kept segregated, whilst on the minor routes segregation is achieved by routing the cables in different directions. Segregation is required to limit generation loss by preventing the spread of fire and damage to other units, hence not more than one unit should be lost. It is how­ ever possible to keep a fire-damaged unit on load by transferring to standby feeds which have been taken by a* different route to the main feeder; this segregation within the unit is generally referred to as the A and B routes. Segregation will depend on the system design and may affect cabling to such items as unit trans­ formers, station transformers, cooling water pumps, boiler feed pumps, gas turbines, etc. However, segre­ gated routes must be taken where duplicate DC sup­ plies for switchgear tripping are provided, also where main and emergency supplies are provided, e.g., tur­ bine lubricating oil pump. Where cables are installed direct in the ground, a distance of 1 m between segregated groups is considered adequate. For cables running parallel, in cable tunnels, etc., a 600 mm separation distance is necessary between control and single core power cables, this is to avoid inducing interference currents in the control cores, particularly under fault conditions. A separation distance of 300 mm between multi-core power and control cables, and also between single core and multi-core power cables is acceptable. On plant where control and power cables run side-by-side for a short distance, this length is limited to a maximum of 5 m for total run of cable. In cableways it is considered good practice to install the power cables on the uppermost racks to reduce unne­ cessary heating and hence thermal ageing of the control cables.

For nuclear stations, however, additional segregation is necessary for the safety of personnel, plant and

117

diesel house

ESSENTIAL

SUPPLIES BUILDING

CABLE FLAT

(BELOW INSTALLATION AND COMPUTER ROOMS)

KEY

TRAIN C

TRAIN 0

NON • ESSENTIAL ROUTES (GENERAL PURPOSE USE)

n

piant ycincai layout