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

floodlights 110 V tungsten or discharge-type flood- kghts are provided on the side of, or below, the bridge 'Li illuminate the crane handling area. A 415/110 V raiiNformer is provided on the crane for all lighting and heating services. The floodlights are controlled !tom the control station by a

`N.Ing separately fused.

Crane access ways and driver's cab fitted) are illuminated by 110 V

[uininaires controlled by a switch fuse.

3.10.2 Design requirements

The design requirements for Category B cranes are detailed in the preceding paragraphs of this section, whilst those for Category C are less onerous.

In view of the essential nature of Category A cranes and the stringent operational safety requirements necessitated by their location, the requirements applicable to Category 13 are supplemented by those briefly outlined below:

(a)Seismic qualification. The cranes, together with their control gear, are designed to ensure that a

(b)Control system security. All crane components and systems which are essential for the security of a load are designed to ensure that a single failure will not result in an incident. This is achieved either by built-in redundancy or by conservative design.

(c)Hoist motions. Preference is given to the provision of two independent systems, either of which is capable of handling the rated load in the event of failure of the other. If only one system is practicable, then a higher safety factor is applied for design purposes. When separate drive motors are employed, particular attention is paid to the need for them to be synchronised and equally loaded in operation.

Hoist overload systems are provided to ensure that the hoist rating is not exceeded. Such a system may consist of load cells operating into the crane control circuit.

In addition to the service and standby brakes described in Section 3.3.5 of this chapter, each hoist is equipped with an emergency brake capable of retarding and holding the rated load. The emergency brake is not deployed in normal operation, operating only in the event of a hoist malfunction, e.g., overspeed, incorrect rotation, load hang-up, or when the main supply contactor trips.

(d)Travel motions. Independent drive units are provided on each end carriage for long travel. Each unit is equipped with automatically operated service and standby brakes, those on either drive unit being capable of stopping a fully-loaded crane from full speed in the event of the other unit failing.

(e)Motors. 240 V DC drive motors may be used in preference to AC units when security of supply and

crane operation are of paramount importance.

(f)Cables and wiring. When cranes are exposed to nuclear radiation, the use of PVC insulated cable and wiring, which is prone to degradation in such environments, is avoided. Polymeric cables are used in preference.

4 Lifts

4.1 Types and general requirements

There are three categories of power station lift: passenger, goods and construction lifts. Lift design and construction, in general, is in accordance with the following British Standards: BS 2655 [13], BS5655 [14] and BSCP407 [15].

Construction lifts are installed early in the power station construction programme for use as passenger or goods lifts during the construction period. Control gear is provided suitable for the harsher environment existing during this period, with protection to 1P55 weatherproof of BS5490 [3] being specified.

Goods and passenger lifts are provided for use durin g the power station commissioning and operational Periods, by which time the environment is controlled. Control gear is therefore provided with less protectio n

— IP31 in control buildings, offices and workshops and IP54 in boiler houses and reactor buildings.

Some passenger lifts are designed to accept stretche r _ borne personnel.

4.2 Supplies and distribution

Three supplies are provided to each lift motor room:

•415 V three-phase 50 Hz three-wire.

•240 V single-phase 50 Hz.

•240 V DC.

The 415 V three-phase 50 Hz supply is connected into a triple-pole (TP) isolating switch which feeds the lift drive power circuits, and individually fuse-protected circuits for the lift car door drive-motor and control circuit transformer(s). A typical control circuit transformer has three secondary windings; 20 V for lift car and landing indicator lamps; 110 V centre-tapped to earth for miscellaneous requirements (such as trapdoor indicating lamps) and 130 V AC, rectified to 110 V DC, for control and interlock circuits.

The 240 V single-phase 50 Hz supply is connected into a single-pole and neutral (SP and N) isolating switch feeding an SP and N distribution fuseboard from which separate fused supplies are taken for lift car lighting, control cubicle heating, car-top transformers and lift shaft lighting.

The 240 V DC supply is connected into a doublepole (DP) isolating switch, feeding a DP distribution board from which supplies are taken for the audible alarm, lift shaft lighting, fire alarm and flood alarm (if appropriate).

4.3 Motor room equipment

Lift motor rooms are located at the top of the shaft for electrically-driven lifts and at the bottom for hydraulic types. They house the supply isolating switches and distribution boards; winding motor/generator; control, interlock and contactor equipment cubicles; lift car and shaft lighting switches; junction boxes for the trailing cables to the lift car, and cable junction boxes for telephone and station alarm circuits.

4.4 Lift drive systems

4.4.1 Electrical

Lift drives are either two-speed or variable-speed. The former uses motors, as detailed in BSCP 407 [151 , supplied at 415 V three-phase 50 Hz through a reversing contactor (raise/lower), a main contactor and a speed change contactor.

N.1otor circuits are provided with overcurrent, under- %oltage, single-phasing, phase failure and phase reversal protection.

4.4.4 Brakes

Lift car travel brakes are of fail-safe design, being

.ipplied mechanically immediately upon disconnection or failure of the supply to the release solenoid. To ,ater for a lift car stopped between floors, the brake fitted with a manual operating mechanism, which dliows the car to be moved to a convenient floor level by hand. Hand release of the brake causes all lift control circuits to be de-energised, allowing the lift to be nio+; ed by use of the manual brake release and winding

;._!ear only.

4.5 Lift car and landing equipment

4.51 Landing control facilities

\ complete set of pushbutton controls and indications is provided at each landing.

For goods lifts, this comprises a 'call' pushbutton, a 'call accepted' lamp and a series of 'lift position' lamps. Control is arranged so that personnel entering the lift car have complete control until they have disembarked and the doors have closed.

For passenger lifts, each landing is equipped with directional call buttons ('up' and `down'), a 'call ac-

Two 600 mm fluorescent fittings are installed in each lift car to provide a general illumination level of 100 lux. The lighting is supplied at 240 V single-phase 50 Hz from the distribution board detailed in Section 4.2 of this chapter, and is controlled by an isolating switch located in the motor room.

To safeguard against AC supply failure, each lift car is equipped with self-contained emergency lighting fittings capable of operating for a minimum of one hour following supply failure, in addition to the normal lighting.

4.5.4 Telephone

A wall-mounted telephone is provided in each lift car for emergency use.

4.5.5 Maintenance facilities

The TP isolating switch, connected in the main 415 V three-phase 50 Hz supply to the lift, has three positions; test/off/on. The test position allows control circuits to be tested and adjusted with the 415 V lift-drive power supply isolated. Control cubicle doors are interlocked to prevent access by personnel unless the isolating switch is in the 'off' or 'test' positions.

To facilitate maintenance of the lift car, a control selector switch is provided on the car top with three positions; test/normal/off. The test position inhibits control of the lift from any location other than the

5.2.4 Switchgear and contactor controlgear

Switchgear and contactor controlgear is located in non-hazardous areas and is provided to the same standards and specifications as that elsewhere in the power station (see Section 2.6 of this chapter).

5.2.5 Control and instrumentation equipment

Control and instrumentation equipment is located in non-hazardous areas and is provided to the same standards and specifications as that elsewhere in the power station, in accordance with CEGB specification US/76/10 [17].

5.2.6 Transformer/rectifier equipment

Transformer/rectifier equipment are located in nonhazardous areas and are provided to the same standards and specification as that elsewhere in the power station in accordance with BS417 [181, BS171 [19] and British Electricity Board Standard BEBST2 Part 5 [20].

An outline of the design requirements is given below. Figure 10.9 shows a typical transformer/rectifier equipment schematic diagram.

Transformers and chokes

Rectifier transformers and chokes are naturally aircooled: Class F to BS2757 [21] insulation is used but winding temperatures are limited to Class B BS2757 [21], in the interests of long life and reliability.

To allow compensation for distribution voltage variations, transformer primary windings are provided with off-load, bolted-link taps, giving ±5o on the nominal voltage.

An earthed screen is provided between primary and secondary windings to protect rectifier equipment in the event of a winding fault developing in the transformer.

Delta-connected transformer primary windings and double-star secondary rectifier connections are used to minimise ripple in the DC output and heating of electrolytic cells.

Rectifiers

Full-wave bridge rectifiers are used, employing either silicon junction diodes or thyristors. Because of the arduous operating conditions in a power station and the need for maximum reliability and life, individual diodes or thyristors are rated at twice the load current and the equipment designed for a cubicle ambient air temperature of 55 ° C. Spare diode capacity is built into each rectifier bridge arm.

Individual diode/thyristor fuses are fitted for protection in the event of diode/thyristor failure. To assist maintenance, indicating lights are provided to identify the rectifier arm in which failure has occurred.

Rectifiers are protected against voltage spikes by surge-suppression circuits. High speed semiconductor rectifier fuses are fitted to protect against overload

and short-circuit conditions. Rectifier equipme nt i s classified as electronic equipment and must meet the requirements of EES (1980) [6], referred to in Sectio n 2.5 of this chapter.

DC smoothing is provided to limit ripple content to 5% of the mean value under any of the supply varia, tions detailed in Section 2.1,2 of this chapter.

Protection

The basic protection requirements of transformer/ rectifier units are listed below and shown on Fig 10.9 ;

•A rectifier AC overload relay arranged to trip th e HV supply.

•A rectifier DC overload relay arranged to trip th e HV supply.

•Rectifier diode/thyristor fuses, with a monitoring system designed to trip the HV supply.

•Surge-suppression equipment fuses, with a monitoring system arranged to trip the HV supply.

5.2.7 Frost protection

All pipes and storage vessels which may be subjected to frost and may consequently jeopardise the safe and efficient operation of the plant are equipped with trace heating or immersion heaters, as appropriate.

Anti-frost equipment is controlled automatically by temperature sensors, either pocket thermostats or thermocouples, to maintain the temperature of the equipment around 5 ° C. Control circuits operate at 110 V AC. Thermostats located in hazardous Zone 1 areas are BASEEFA-certified and form part of an intrinsicallysafe circuit.

Trace heating in the form of tape is wrapped around pipework, each section or circuit being separately protected by fuses and supplied at 110 V AC from a 415/110 V transformer. The 110 V secondary winding is centre-tapped to earth to limit the voltage to earth to 55 V in the interest of personnel safety, and an earthed screen is provided between the two windings to prevent the transfer of excessive voltages from primary to secondary. Trace heating tapes are of two basic types, resistance and self-regulating, and have a uniform thermal rating per unit length.

Resistance-type trace heating tape comprises two insulated resistance elements running parallel to each other for the length of the tape and interconnected at one end to form a loop. A thermostatically controlled electrical supply to the loop regulates the heat output. The maximum length of a tape section is limited by its loop resistance and by the capacity of the supply.

Self-regulating trace heating tape comprises two conductors running parallel to each other for the length- of the tape and separated by a conductive 'core' ma terial, the conductance of which varies in response to- changes in temperature. The heat output is there fore self-regulating and thermostats are not necessarY.

I mmersion heaters are provided in storage vessels and are supplied at 240 V AC when their rating is low .enough to be controlled directly by thermostats. For higher ratings, a 415 V three-phase 50 Hz supply is used and controlled through a contactor by a temperature-sensing device. Each storage tank is fitted with a thermostat to control the temperature over the desired band. When overheating, such as might occur in the event of failure 01 a thermostat, could prove dangerous, an extra 'safety' thermostat set to operate at a higher temperature than the control thermostat is connected in series with the control thermostat. The safety thermostat is manually reset. Temperature sensors and immersion heaters are both designed to be removable for maintenance purposes, without the storage tank being drained.

Alarm thermostats are provided to warn the operator if the heating system fails to maintain the fluid above the minimum temperature.

5.2.8 Earthing and static protection

In hazardous areas, it is imperative that all equipment is bonded together and earthed in order to prevent the build-up of static or potential differences between items of plant or equipment, both of which create risk of a spark and hence ignition.

The 415 V switchgear earth-bar is connected to the station earth. All metal vessels, pipework and nonconducting metalwork, with the exception of electrolytic cells, are bonded together and connected to the s witchgear earth-bar with 70 mm 2 copper PVC-insu- lated and sheathed cable. The metal enclosures of all electrical equipment are treated similarly. Continuitybonding conductors are connected across all uninsulated pipework joints of bolted or screwed construction.

Precautions are taken to prevent the transfer of potentials along pipework into hazardous areas and to ensure that circulating currents cannot occur in pipework and supporting structures due to the high current levels in gas generating electrolytic cells. To this end, electrolytic cells are mounted on insulated supports and all metallic pipework connected to the cells has insulated flanged joints in the vicinity of the cell. Each low pressure delivery main into gas storage vessels is mounted on insulated supports and equipped with an insulated joint outside the storage area. The insulated joints and supports must have a minimum insulation resistance of 2 Mf2 at 500 V DC.

5.3 Hydrogen producing plant — electrolytic cell process

5.3.1 General description of plant

The electrolytic cell process for the production of hydrogen comprises banks of cells, using a solution of caustic soda and water, or potash and water, as an electrolyte. A low voltage DC supply from a trans-

former/rectifier unit to the cells causes hydrogen t o be produced at the cathode and oxygen at the anode. Since the rate of production is directly proportional to the DC current, hydrogen production is regulated by rectifier control. Electrolyte level is maintained by a demineralised water make-up system.

Hydrogen from the cells passes via a gasholder, at a maximum pressure of 12 mbar, to compressor s operating at high discharge pressure (25 to 30 bar) a n d

through driers and filters to the HP storage ve sse l . For distribution, hydrogen pressure is reduced to 10

bar. Oxygen is vented to atmosphere. A typical syst em diagram is shown in Fig 10.10.

The disposition of equipment within the hydrogen

plant compound is illustrated in Fig 10.11. In general terms, the plant layout consists of an outdoor storage

area, in which the HP and LP storage vessels are situated and a building containing the compressor, electrolytic cell and electrical equipment coon-is. Hydrogen compressors, filters and driers are housed in the compressor room; switchgear, transformer/rectifier equipment and the gas production control panel are housed in the electrical equipment room and he hydrogen producing cells in the cell room.

5.3.2 Classification of plant areas

The classification of the various areas, as defined in BS5345 [5], is as follows and electrical equipment is provided accordingly (see Section 5.2.1 of this chapter):

•Electrolytic cell room — Zone 1.

•Hydrogen compressor room — Zone 1.

•Storage area:

(a)Within 1 m of storage vessels or directly above them — Zone 1.

(b)Within 3 m of storage vessels excluding (a) — Zone 2.

(c)Remainder — non-hazardous.

•Electrical equipment room — non-hazardous.

5.3.3 Electrical, control and instrumentation equipment

DC connections and busbars

the electrolytic cells and the rectifier equipment consist of hard-drawn, high conductivity copper busbars complying with BS159 [22] and BSI58 [23] or PVC insulated and sheathed copper cables to BS6346 [24 1. Busbars within the hazardous environment of the cell room are insulated with anti-static PVC. From the cell room, the connections pass through wall bushings into the electrical equipment room where, if they are busbars, they are protected by a wire-mesh screen to prevent accidental contact. The wall bushings are gastight to ensure that hydrogen is prevented from entering the electrical equipment room under all circumstances.

----I

-

PPESSURE

GAS H.OL:,'ER

FILTAS

1-•.; gSTR S

HYDROGEN

VENT

HYDROGEN

HYDRAULLC

REL EF 'MOE

,-, DROGEN

CON7q0L

EXCESS FLOW

HvDROGEN TO

CONTROL PANEL

all entrances to hazardous areas and are permanently illuminated from a 110 V AC/DC supply.

Gas detectors, operated from the same 110 V AC/ DC supply as the warning notices, are installed in all rooms, including the non-hazardous electrical equipment room, and initiate audible and visual alarms at the entrance to the specific area where gas has been detected.

Back- up protection As mentioned in Section 5.2.1 of this chapter, the overall protection of the plant is assured by the provision of a back-up protection scheme. The scheme is arranged to trip the circuitbreaker of the incoming supply to the 415 V switchboard in the event of a contactor failing to open after initiation of a trip. Devices provided to initiate tripping of a contactor are either fitted with duplicate contacts or are duplicated to operate into the back-up protection scheme.

The plant control and back-up safety protection systems function independently to ensure that plant

safety is not jeopardised by control system faults. The back-up safety system is arranged so that, in the event of any plant parameter exceeding the normal control range such that it could give rise to a potential hazard, the circuit-breaker is tripped, rendering the plant safe.

Gas production control panel The gas production control panel is located in the electrical equipment room and forms the control centre of the plant. Control of

the hydrogen plant is normally automatic, but manual control and test facilities are provided. Incorporated in the control panel are the following facilities:

(a)A mimic diagram indicating the state of plant.

(b)Indicating and recording instruments for hydrogen pressure, temperature, purity, dryness and other data necessary for the operation and monitoring of the plant.

(c)Rectifier controls and indicators.