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

(ias recombination in lead acid cells

Hie negative plate in a lead acid cell consists of a lead alloy lattice or grid in which the spaces of the grid

ire filled with chemically-active lead sponge. If a fully ilarged negative plate is removed from the acid and exposed to the air it will heat up rapidly, produce

earn from the water on the surface and quickly form ,i deposit on the surface. During this chemical reaction,

orgen from the air reacts with the lead to produce oxide (Pb0) and heat.

The significance of this is that a fully charged ne- 2Luive plate is highly reactive with oxygen. This is the

INisis of lead acid gas recombination cells. When a J1312C current flows through a fully charged lead acid

electrolysis of water occurs to produce hydrogen

. or-a the negative electrode and oxygen from the posi-

, i‘e electrode. The evolution of the oxygen and hydro-

'.==r) gases does not occur simultaneously because the i: Nciency of the recharge of the positive electrode is :t ot as good as that of the negative. This causes oxygen

oke from the positive plate before hydrogen is olved from the negative plate.

\s the oxygen is evolved from the positive electrode, :here is a substantial amount of highly active lead

Tonge existing on the negative electrode before hydro- commences to evolve. Therefore, provided oxygen

On be transported to the negative electrode, conditions

I — IH

R— x100%

where R is the gas recombination efficiency

I is the average charging current which passes through the battery during test

IH is the average current equivalent, under Faraday's law, of hydrogen emitted during the test period

Since the hydrogen arises from the electrolysis of water an equivalent quantity of water is lost to the battery, giving rise to an equivalent loss of oxygen.

There are a number of theories of how the oxygen is removed from the battery. The most popular are corrosion of the positive electrode or sulphation of the negative electrode. The former would reduce conductivity of the positive electrode and the latter would reduce the capacity of the negative electrode. Loss of water would also affect the capacity of the cell. Each has a direct effect on the operational life of the battery.

Although referred to as sealed cells, a vent is provided to allow hydrogen to escape and prevent a dangerous build up of pressure inside the battery.

The recombination efficiencies obtained from the Harrison and Whitley technique have indicated that

the operational life of the batteries is of the order of 10 years.

The electrolyte is absorbed by a fibre-glass separation material between the plates. This allows free movement of gases between electrodes and with the vent removed \\ ould result in little or no electrolyte escaping because it would be held by the absorbent

separation material.

A ("vantages of recombination cell batteries

•A specially designed battery room is not necessary.

•Size, suitable for locating in equipment rooms.

•Ease of handling during installation.

•Lower level of maintenance.

•Low level of hydrogen liberated during charge.

Disadvantages of recombination cell batteries

•Lower operational life, 10 years compared to 25-30 years for Plante cell batteries.

•Difficult to determine the state of the battery, i.e., capacity available for an emergency.

•Lower level of maintenance required can lead to abuse of the battery by personnel assuming that the batteries are maintenance-free.

CEGB requirements for recombination cell batteries

Until the CEGB has accumulated a similar amount of operational experience to that for Plante cells the following requirements are specified. The power equipmeat racks (PERs) shall be provided with:

•Low volts alarm relay or equivalent across 48 V busbars (set at 43 V).

•High volts alarm relay or equivalent across 48 V busbars (set at 60 V).

•Low volts disconnect load relay or equivalent (set at 41 V).

•isolation facilities to enable disconnection of the rectifier output and load to each 48 V battery module.

•Facilities to enable open-circuit volts to be measured on each 48 V battery module in turn, allowing the remaining paralleled 48 V battery modules and rectifiers to supply load.

•Facilities to enable current flowing into each 48 V battery module to be measured.

•Facilities to enable current flowing out of each 48 V rectifier to be measured.

•A calibration graph for each 48 V battery module showing open-circuit volts against discharged ampere hour capacity, see Fig 8.5.

DISCHARGE AMPERE.HOURS

CAPACITY-I00

Fa. 8.5 Battery terminal voltage plotted against depth of discharge

These facilities will enable the recombination cell battery to be monitored and the available capacity to be determined.

For further details on batteries see Chapter 9.

2 Access to British Telecom national cable network

As British Telecommunications plc (BT) is the main telecommunication carrier in the United Kingdom, access to the BT national telephone cable network allows the use of the wide range of BT telecommunication services.

The main service is the public switched telephone network (PSTN), which provides telephone services to all United Kingdom and international telephone users and access to the sophisticated telecommunication services now available on the BT PSTN.

The CEGB also makes use of the BT national cable network for telecommunication circuits to locations external to the power station for telephony, data, grid system control, telecommunications and grid system protection.

BT provides telephone cables from the power station telecommunication rooms to two segregated points outside the boundary of the power station site and arranges for segregated routing of cables from the two off-site segregated points to the BT cable network. This is to reduce the probability of a common mode

2.1 On-site British Telecom cable req uirements

va segregated routes are provided on-site to the il oing locations:

•Station MTR.

•-tation administration building (PABX room).

•400/275 kV switching station telecommunications room (if on site).

UT cables are also routed on site between the above l, it ionS for interconnection of BT circuits. Typical jrrangemerits are shown on Fig 8.6.

The segregation of the two BT cable routes is continued between the segregated access points into the power station building and to the MTR.

At the design stage, two BT cable routes are planned to provide the required segregation and the cables are routed in unplasticised PVC cable trunking. This provides a degree of mechanical protection of the cables and reduces the fire risk to the BT external-type telephone cables, which often have a polythene sheath.

2.4 British Telecom cables

During the design stage of the station, the quantity and sizes, i.e., number of pairs, of the on-site BT cables

400 1V

SUB STATION

TEL E CO MMUN IC AT IONS

ROOM

ST OFF SITE CABLE ROUTE NO 1 TO ST CABLE NETWORK

STATION

ADMINISTRATION

BUILDING

BT OFF SITE CABLE ROUTE NO 2

TO ST CABLE NET WORK

U

MAIN ROAD

are specified by the CEGB, based on the known and estimated future telecommunication needs of the station, thus minimising the need for additional or replacement FIT cables during the life of the station.

BT regulations do not permit the jointing of on-site BT telephone cables directly connected to the off-site BT telephone cable network. This is to ensure that 13T personnel are not put at risk if the jointing of one of the cables coincides with a rise of earth potential at the power station site, during an electrical system fault in the power station, or the on-site 400/275 kV switching station.

2.5 Electrical isolation of British Telecom circuits

All of the above BT cable pairs are terminated by BT on isolation links (BT reference, Links Isolation Type 5A) in the locations detailed in Section 2.1 of this chapter, to permit easy electrical disconnection of the cable pairs from equipment installed in the station.

If the calculated rise of earth potential during electrical fault conditions exceeds 650 V AC RMS, then isolation equipment is provided on every working circuit to isolate the off-site BT circuits electrically from the power station telecommunication equipment. This prevents the transfer of the rise of earth potential but permits through-transfer of speech and signalling information, thus preventing damage to BT plant external

to the power station and also the transfer of electrical potential to BT personnel working on external Bi plant.

Figure 8.7 shows a circuit diagram of an exchange line isolating equipment and Fig 8.8 shows a circuit diagram for isolating equipment typically used for grid system control, telephony and protection of telecom. munication circuits.

3 British Telecom telephone services

A power station requires access to many BT servic es including the PSTN, telex, data and point to poi nt circuits.

3.1 Public switched telephone network (P SIN)

The majority of the direct exchange lines (DEL) for the station will be routed to the nearest BT telephon e exchange. However, if the station is in a rural area it is often prudent, in the interests of reliability of service and security of access to associated I3T services, for the DELs to by-pass the local BT telephone exchange and to be directly connected into a BT telephone exchange serving an urban area.

For additional security, a BT telephone on the supervisor's desk in the CCR has a DEL connected to an alternative BT telephone exchange to that serving the power station PABX. This ensures that access to the

5CaEEN

• 4,

0:RGUIT TO

>rEP:509NTE EXCHANGE

•Emergency speech circuits for fire and police services. Nuclear stations also require access to off-site nuclear emergency services.

•CEGB CTN circuits.

•Remote control of circuits for radio fixed stations located outside the power station.

Access to the private circuit network of NrierL:ury Communication Limited (MCL), the second largest British public telecommunications carrier, is also used (s4tere available) to provide an alternative to the BT private circuit network.

CA*

C

4

AA*

*BELAYS CA AA HAVE

HIGH INSULATION BETWEEN

BELAY COILS AND CONTACTS

FiG. 8.8 Transformer isolation unit for direct BT telephone lines or PABX exchange lines

BT PSTN is available to the supervisor should the BT telephone exchange serving the station fail. The telephone number of this DEL is not listed in the BT telephone directory and it is referred to as the 'exdirectory out-of-area' telephone. This DEL is routed in the BT cable network both on and off-site, to provide diversity with the direct telecommunications circuit to the remote grid control centre. This gives an alternative means of speech commuhication between the supervisor's desk and the grid control if the grid control communication link is faulty.

3.2 Telex

One telex machine is provided in the station connected to the BT telex network, its primary purpose being the transmission and reception of telexed information from locations not having access to the CEGB Corporate Data Network (CDN).

3.3 Data

Access to BT data circuits is required, such as the BT packet-switched service, for locations not having access to the CEGB CDN.

3.4 Private circuit network

Access is required to the BT private circuit network for:

•Grid system operation circuits.

•PAX and PABX exchange interconnection tie circuits.

4 On-site telecommunication cabling

4.1 General

Special factors related to the importance of a particular telecommunications system determine the type of cable used and its routing.

The cables for PAX main cable routes, either dedicated or via the station digital pair network, have fire-retardant sheathed cables as detailed in Chapter 6 of this volume, and are as used for control and instrumentation cabling within the power station. The cable for the personal ultra high frequency (UHF) radio systems, the grid control emergency and nuclear emergency very high frequency (VHF) radio systems, the direct wire telephone systems (DWTS) and the siren system is short-time fireproof cable, which will remain serviceable in a fire area for a minimum period of 20 minutes.

4.2 PAX telephone cabling

The power station digital pair network, which is detailed in Chapter 6, has multipair cable marshalling boxes in power station areas for the marshalling of digital circuits required by equipment located near to the boxes.

The marshalling boxes are cabled to central marshalling cubicles which are interconnected with multipair cables to enable digital circuits to be routed between many areas of the power station.

The PAX telephone circuits are routed by the digital pair network from station locations to the station MTR, where they terminate on the MDF.

The PAX telephone circuits use dedicated 20-pair modules in the digital pair network. All pairs of each dedicated module are through-connected from a 20pair terminal strip on the MTR MDF to 20-pair telephone cable marshalling boxes in the station areas. Figure 8.9 shows a typical telephone cable marshalling box, dedicated for telephone circuit use only.

For new power stations, at least 20% spare circuits (which are all through-connected to the MDF) are provided to each telephone cable marshalling box.

These may be used subsequent to the initial installation of the cable system to provide direct connections to the MTR MDF without having to interfere with the digital pair network interconnecting arrangements.

4.2.1 Main distribution frame and station PAX telephone cabling

The MDF in the MTR is the marshalling and terminat-

ing point for the majority of telecommunication cables entering the MTR.

The MDF is floor standing and has a line side (for termination of cables to the MTR), and an equipment side (for termination of cables to equipment in the MTR). The line and equipment sides are interconnected by single-pair jumpers as required. The line side is fitted with discrete 20-pair terminal strips having terminals for wire wrapping. Each block of terminal trips will

5.1.1 Strowger systems

The basic elements of a Strowger telephone exchange are the telephone extension line circuit, uniselector and selector. The arrangement of the elements used for a telephone call between two telephone extensions on a 100-line telephone exchange is shown on Fig 8.10 and is briefly described as follows:

•Handset of calling felephone is lifted and operates its discrete line circuit in the PAX which then operates its discrete uniseleetor.

•The wipers of the uniseleetor rotate to find the first unused selector circuit. Dialling tone is then returned to the calling telephone.

•The first digit is dialled and the wipers of the selector step vertically to a position determined by the digit dialled.

•The second digit is dialled and the selector wipers rotate into a horizontal bank of contacts coming to rest on contacts determined by the digit dialled. The contacts are associated with the called telephone.

•The selector then transmits ringing current to ring the bell of the called telephone via the wipers of the selector and also returns a ringing tone to the calling telephone.

•When the called telephone answers, the lines of the called telephone and calling telephone are connected via wipers of the uniselector and the selector, speech is then possible between the two telephones.

The Strowger system relies on the operation of complex electromechanical equipment which requires regular maintenance by skilled maintenance staff.

5.1.2 Crossbar systems

The crossbar system is more complex than the Strowge r system. The uniseleetor and selector functions of the Strowger system are performed by crossbars. Each crossbar unit comprises a matrix of contacts which are operated by levers (or fingers) to route circuits for speech and control through the telephone exchange.

The mechanical movement of the equipment parts is small compared with that of the Strowger system and there is a reduction in the amount of wear and readjustment, so the maintenance requirement of a crossbar system is much less than that of an equivalent Strowger system. However, the complexity of the crossbar system requires skilled staff to diagnose and repair faults.

5.1.3 Stored programme control (SPC) systems

The SPC telephone exchange is now specified for PAX and PABX requirements at new power stations, and is also specified when replacement of Strowger and crossbar PAXs and PABXs at existing power stations is necessary.

The SPC telephone exchange has few moving parts and a low maintenance requirement. Most faults can be rectified by relatively unskilled staff. Indeed, many problems are rectified by integral self-diagnostic equip-

Fig. 8.11 Elements of a basic stored programme control telephone exchange

The directory listed exchange lines have one common number listed In the BT local telephone directory, the

number normally dialled for the power station. On dialling the number, the BT exchange hunts for any one of the thirteen exchange lines to the station PABX which is not in use.

The ex-directory exchange lines enable priority incoming, calls to be received by the PABX at all times from CEGB personnel requiring access to the station PABX from the BT PSTN, including occasions when the directory listed exchange lines are all in use or Faulty.

Should the local BT telephone exchange fail, then a restricted service is available via the ex-directory out-of-area exchange line, which is connected to another BT telephone exchange, preferably in another town.

Outgoing calls, originated from PABX extensions having direct access to the BT PSTN, will first have access to the ex-directory lines and then to the directory listed exchange lines, the directory listed exchange lines being kept available for incoming calls from toe BT PSTN.

During normal conditions, the ex-directory nit-of- area line is only available for calls made via the station telephone operator,

The PABX will be powered from a battery-backed 48 V DC supply giving a minimum of seven hours standby should the mains supply fail to the associated 48 V DC charger.

The PABX will have a dual processor to ensure operation should one processor fail. Should the PABX fail completely, then the exchange lines are automatically, or manually, connected directly to nominated telephones to provide a minimum service for incoming and outgoing telephone calls via the BT PSTN.

A limited number of power station locations including the central control room (CCR), the station gatehouse (used by security personnel), the power station manager's office and the charge engineer's office are provided with telephones having direct access to the PSTN independent of the PABX. These telephones are designated direct exchange line (DEL) telephones.

6.1General facilities

The PABX will include present SPC telephone facilities as follows:

•Extension transfer of telephone calls.

•Conference facilities.

•Ring 'back when free.

•Operator recall.

•Group pick-up.

•Abbreviated dialling.

The PABX is chosen from a CEGB approved list which includes PABXs approved by the Department or Trade and Industry (DTI) for connection to the BT PSTN,

which have also satisfied the CEGB tests and criteria for use in power stations.

6.2 Night service facilities

Outside normal working hours, night service facilities for transferring incoming BT PSTN calls to the required PABX telephone extensions are provided from a PABX telephone extension on the CCR supervisor's desk. The night service facility is automatically transferred to a PABX telephone in the Gatehouse if the incoming call is unanswered at the supervisor's desk.

7 Paging systems

7.1 Lights and sounders

The paging of power station roving staff from the PABX, PAX and fixed locations has for a long time been recognised as a useful facility. Before the 1950s various methods of staff location were used, one method was the use of combinations of flashing and/or coloured lights, another method being the operation of sounders to a repeated morse code signal. These systems required high levels of maintenance and were ineffective in noisy areas.

7.2 Inductive loop paging systems

During the 1950s, an inductive loop paging system controlled from manual controllers was developed which operated by magnetic induction from a wire loop antenna. This antenna comprised a single-wire looped around a building or buildings with both ends of the loop being connected to the paging transmitter. For a large building or complex, the antenna sometimes comprised a number of loops to obtain the required cover. At a power station a number of antenna loops were necessary and a series/parallel arrangement of the individual loops was used to optimise the area of cover.

The signals transmitted by the system antenna were in the frequency range 200-4000 Hz. The pocket pagers carried by mobile staff received the signals and emitted an audible tone if the pager received its unique identity signal.

During 1961, a commercial inductive loop paging system manually-operated from a controller was installed at Uskmouth Power Station. The controller, which incorporated pager coding pushbuttons, was connected by cable to the main coding and transmitting equipment. An equipment was developed for the CEGB by one of the authors (F. Ashurst) to enable this paging system to be operated from any power station PAX telephone. At the time, this was a unique method of operation.

The method of operation from any PAX telephone required the dialling of a single-digit paging system