14Electrical plant layout

14.1Introduction

The main objectives when establishing the layout of electrical plant for a power station are to:

•Achieve minimum construction costs.

•Minimise energy flow losses.

•Provide access for operation.

•Provide adequate space and facilities for ma tenance.

The transmission grid lines, grid substations, general transformers and station transformers, together w the boiler or reactor, turbine-generator, fuel supp waste removal and cooling water supplies are considered for their individual requirements befc arriving at a compromise station layout.

Design of the electrical auxiliaries system is deti mined by the size and number of generating units a

layout plant Electrical

Station desigrr'and layout

the required electrical drives. An outline of the station electrical system, in the form of a power system diagram must be produced before the layout of electri­ cal plant can proceed. This diagram will indicate the number and size of transformeis. switchgear voltage levels and allocation of main drives to switchboards. On a typical CEGB station this will result in a 3-voltage level system operating at 11 kV, 3.3 kV and 415 V. Having designed the electrical auxiliaries system, the following major equipment must be located:

•Transformers — generator, station, unit and auxiliary.

•Auxiliary switchgear — 11 kV, 3.3 kV and 415 V.

•Generator main connections.

•Power and control cables — allocate space for major routes.

•Control room equipment — including control and instrumentation (C and I) rooms housing computer and alarm equipment.

The layout of the power station electrical equipment must take account of the following design consider­ ations:

(a) All switchgear and electrical control equipment should be housed in clean and dry conditions.

•(b) The electrical switchgear and distribution equip­ ment should be located as near as possible to the

centre of the electrical loads.

(c)The location of the central control room (CCR) should be positioned to suit the best man­ movement arrangement, together with the best position for cabling facilities.

(d)Cable and electrical plant fire risk precautions should be taken in the form of fire protection systems and physical segregation of plant and cables. The principal considerations are that any incident on one unit should not affect any of the other units and, especially on nuclear power stations, sufficient alternative systems and plant, designed within the necessary segregation provi­ sions, should be provided to allow for the safe shutdown of the plant.

(e) Oil-filled transformers should be located externally to the main buildings.

Where cable routes extend outside of buildings, and therefore run under the ground, close co-ordination with other disciplines of the design team is essential to avoid clashes with other services such as surface and

(HV) cable connections between the generator trans­ formers, station transformers and grid substations, which may require to be routed in concrete troughs alongside roads and rail tracks, thus sterilising large areas of ground.

Chapter 2

It is of prime importance that all electrical plant layout design shall conform to the requirements laid down in relevant design codes and standards.

14.2 Auxiliary switchgear

Dedicated switchrooms are often used for the auxiliary switchgear; this gives unit-to-unit segregation and segregation within a unit if this is required lor reactor essential cooling. The switchgear is protected from any ^possible damage from external sources such as leaking pipes or vessels and any serious incident in the main plant areas. The switchrooms provide a degree of security and the correct environment for operation and maintenance of the switchgear.

There are several points to be considered regarding the location of the switchrooms and annexes. The lay­ out of the main plant dictates the locations of ancillary plant, and, in particular, the location and orientation of the turbine-generator determines the location of the generator transformer and unit transformer which in turn affects the location of the unit board. An economic and practical compromise has to be made on the positioning of the unit board to try and eliminate long and difficult cable runs from transformer to switchgear and then from switchgear to high voltage drives and feeds.

These considerations also apply to the station trans­ former and switchgear, except that the main factor determining the locations of these items is the entry of the grid feed to the station transformer.

The ideal location for, say, the 11 kV unit boards is a centrally located position, i.e.. between boiler house and turbine hall. This gives reasonably short and easy cable routes to boiler and turbine plant. An important parameter in the overall station cost is the specific cost per unit volume of the structure. A central position created specifically for electrical plant increases this cost considerably and possibly unnecessarily.

In a two-unit station if the central position is not selected, the auxiliary switchgear would be located in external switchgear annexes positioned as near as possible to suit the plant associated with switchgear. This allows an economic cable layout and unit segrega­ tion is achieved with the minimum of main cables cross­ ing the station.

Switchgear annexes external to the main buildings provide relatively easy access routes for initial installa­ tion and provide a logical and ready identification of various switchboards for operation and maintenance. Another consideration is the need for availability of the 11 kV station board and its lower voltage feeds for early commissioning and proving of plant. External annexes are conducive to this need for early completion. With the external annexe concept most of the auxiliary switchgear would be located in the annexes, i.e., 11 kV, 3.3 kV ahd 415 V with items such as coal plant .switch­ gear located near to its plant area.

If a central location is selected, this would typically house the 11 kV unit boards and feed the boiler and turbine auxiliaries, the 3.3 kV and 415 V switchgear would be in annexes on the wings of the main building.

Other factors to be considered in the location of auxiliary switchgear are:

(a)Access routes must be practicable — for initial installation and future possible removal, overhaul and replacement. All switchrooms must have at least one escape door ideally positioned diagonally opposite the main access doors; in large switch­ rooms more than one door is required.

(b)Existing facilities, such as cranes, may be utilised and if necessary hatchways may be provided in the roof of switchrooms.

(c)Adequate provision must be made for cable routes bearing in mind fire risks and separation or segre­ gation considerations.

(d)The location must be protected against water ingress from overhead water tanks, pipework, etc.

rhe unit transformers, station transformers mid 11 kV/3.3 kV unit and station auxiliary transformers are of the oil-cooled outdoor type and are located remote from the switchgear. The 3.3 kV/415 V trans­ formers are now of the air-cooled type and are integral with the 415 V switchboards.

14.2.111 kV and 3.3 kV switchgear

Once, the locations of the switchgear have been decided, the actual layout of the switchrooms must be considered.

The switchrooms must be large enough to accom­ modate any make of switchgear with sufficient space for possible future extensions to the switchboards. Adequate rear access is required to allow the cabling to be installed and terminated and to allow inspection and removal of rear mounted equipment. Sufficient space at the front of the switchboard is required for’ complete withdrawal of the circuit-breaker truck. The floor must be designed to include fixings, floor levelling and truck runners as specified by the switchgear manufacturer and also to take into account the effect of the truck wheels when a circuit-breaker is removed and manoeuvred.

Space should be allowed for maintenance activities in the switchroom. Wall space is required for control and alarm junction boxes and miscellaneous equipment such as lighting and small power distribution boards and transformers, telephone junction boxes and fire detection and protection panels.

The cabling is laid in dedicated cable races below the switchgear and passes through holes or slots in the floor onto cable supporting steelwork in the race. The posi­ tioning of the switchgear and the holes or slots in the floor must take into account the civil design of the building. Cate must be taken to avoid positioning the

Electrical plant layout

main cable entry over floor beams or any other major obstruction (see Figs 2.41 and 2.42).

The clear height under beams and services of the 11 kV and 3.3 kV switchrooms must be adequate to allow arc-shute removal, using the manufacturer’s lifting frame. The arc-shutes are removed for inspec­ tion and maintenance of the contacts and arc-shutes. A runway beam is required, positioned over the rear of the switchgear for installation and future removal of the voltage transformers.

Access doors of sufficient height and width are provided to allow an assembled circuit-breaker panel to be manhandled into the switchroom.

The switchroom lighting and small power installa­ tions should allow for safe operation and routine maintenance activities and testing. Specialist main­ tenance jobs will be carried out in the station work­ shops. The switchrooms must be conditioned to pre­ vent condensation and in areas of high ambient tem­ peratures sufficient ventilation provided to prevent any overheating problems.

14.2.2 415 V switchgear

The considerations applied to the high voltage switch­ gear are also applied to the lower voltage switchgear. However. 415 V switchgear locations are much more diverse, e.g.. water treatment plant, chlorination plant, etc., and to avoid any possible corrosion of the switch­ gear, dedicated switchrooms within these plant areas are necessary.

The 3.3 kV/415 V transformers are of the natural ailcooled type and are integral with the switchgear; this obviates the necessity of large low voltage (LV) cables from transformer to switchgear. Adequate space for installation, testing and maintenance of the transfor­ mers must be allowed for, hearing in mind that the transformers are bulkier than their associated switch­ gear. The floor has to be suitably designed for the weight of the transformer.

14.3 Turbine-generator auxiliaries

plant engineers and the turbine manufacturer, the electrical layout engineer must take a very detailed interest at an early stage to ensure that provision is made for personnel and cable access to all electrical items, and suitable locations are agreed for electrical equipment cubicles away from hot and potentially wet locations.

Some equipment imposes its own special restric­ tions; for example, the hydraulic control fluid used to operate the main steam valves must not be allowed to come into contact with the PVC insulation which is used on most cables. Chemical reaction causes the PVC to decompose. •

From the examples of station layout shown in this chapter it will be noted that the relative positions of many auxiliaries vary considerably from one station to another; the amount of equipment being dependent on the operating philosophy of the station. A base-load unit may possibly have one quick-start air pump and three maintaining air pumps, whilst a unit intended for flexible operation could have two quick-start and two maintaining air pumps.

The principal turbine-generator auxiliary plant which must be considered when preparing the electrical equipment layout for the turbine hall is as follows:

•Auxiliary oil.

•Seal oil.

•Jacking oil (normally one pump at each main bearing).

•Barring gear.

•Stator winding water.

•Distilled water.

•Hydrogen cooling.

•Condenser air extraction.

•Auxiliary cooling water.

•General service water.

•Hydraulic control fluid.

•Forced-air cooling compressors.

•Exhaust spray.

•Gland sealing steam.

•Feedheating.

•Electrical distribution cubicles.

•Instrumentation cubicles.

•Local equipment housings (possibly 20 per unit).

Most pumped systems have duplicate 100% duty units with instrumentation to measure pressure and tempera­ ture and give an automatic changeover should the running pump fail. Also, separate cable routes arc provided for each pump to meet the requirements for segregation — see Section 14.6.1 of this chapter.

In addition to the auxiliary equipment, the turbine­ generator will itself require extensive cable steelwork to cater for the many instrumentation and control devices along the length of the set. Most of these will be marshalled by the turbine manufacturer who will fit flexible metal harnesses, containing heat or oil resistant wiring as necessary, between the actual devices. The

devices may be located in a hostile environment and then junction boxes positioned in less hostile locations.

A large unit can have approximately 150 power operated valves, each of which need provision for power and control cables.

14.3.1Excitation equipment

All the latest generator excitation systems make use of an AC exciter with diode rectification and thyristor control. The diodes may be static and mounted in the automatic voltage regulator (AVR) and excitation cubicle,* in which case the cubicle must be mounted close to the generator to allow for the solid busbars necessitated by the heavy excitation current (5170 A for a 660 MW unit). Final connections are made to the pilot and main exciters by sliprings and brushgear.

Alternatively the diodes may be mounted between the exciter and the generator and rotate with the shaft, control and switching taking place between the main and pilot exciters. The lighter current permits the use of cable connections and allows greater flexibility in