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

7.3 Radio paging systems

Radio paging systems have replaced all other types of paging system in CEGB power stations.

Most radio paging systems can be operated from manual controllers and from the power station PAX and PABX telephone systems, in a way similar to that of the inductive loop paging system. Present day radio paging systems provide an alerting signal and a simple numeric or alphanumeric information display.

The radio paging systems currently use a group of channels in the 26, 27 and 49 MHz band of VHF radio channels which are available at present for the specific use of radio paging. These channels are not exclusive to CEGB but are also available for other commercial users.

The licensing and allocation of the paging channels is controlled by the DTI who specify the conditions under which the system will operate. Each system it pplier is allocated a group of frequencies for use throughout the UK. The channel allocated by the supplier is selected so that mutual interference with other radio paging systems in the area of cover is minimised. The permitted maximum transmitted output and height of the antenna(s) are also specified to minimise mutual interference. However, the multi-use of the same paging channel in the area of cover will not necessarily cause interference as the coding of the signals transmitted on a common paging channel provides an additional safeguard to minimise the receipt of incorrect information by a paged person.

7.3.1 Component parts of a non-speech radio Paging system

It is CEGB policy to use non-speech radio paging

s stems in power stations, for reasons which will be explained later.

Three manual controllers are usually provided for each radio paging system, as follows:

•Test and programming controller fitted in the central control equipment cubicle (MTR).

•Supervisor's desk, central control room (CCR).

•Power station telephone operator and receptionist desk.

The supervisor's desk and power station telephone operator's manual controllers have the following facilities, which are not available to the caller initiating a paging call via the PAX or PABX:

•Individual call of all power station pagers with distinctive bleeping or vibrating of the pager to indicate a call from the CCR or the telephone operator.

•Group call of members of operations or emergency teams, i.e., Fire, First Aid or Security. The pagers of the team members will also receive individual paging calls when their discrete number is called.

If a pager is faulty, the user will be provided with a replacement pager. The replacement pager, however, operates on a different calling code to the user's normal pager. To enable the user to be called when his published number is dialled on the PAX, or called from a manual controller, it is necessary to programme the

central control equipment with this information so that calls can be automatically sent to to the replace-

ment pager. This may be accomplished from any of the manual controllers, if not barred, or from the test and programming controller.

7.3.4 Transmitters and antennas

A single transmitter with one or more antennas may be sufficient to provide paging signals in all areas of

a small power station, but large power stations, and particularly nuclear power stations, require a number of synchronised transmitters having multiple antennas with associated radiating cables. The transmitters are synchronised to prevent interaction of signals from two transmitters having radio cover which overlap in some power station areas. The synchronising is accomplished by frequency-di\ iding, the main transmitter radio signal and transmitting the lov, frequency derivative via telephone cable pairs to each of the associated slave radio paging transmitters where the signal is multiplied back to the original frequency, thus producing a synchronised transmitter output.

The siting of the transmitters and associated antennas requires a detailed study of the power station main building and outbuildings including the cooling water (CW) pumphouse (much of which may be underground), coal plant (if a coal-fired power station) and the reactor area (if a nuclear power station). Particular attention is needed for the power station basement area.

A close examination of the power station working areas or, if a new power station, a detailed examination of its layout, is necessary to determine the location of antennas to provide maximum radio paging cover. In underground locations and corridors, it may be necessary to supplement the antennas with 'radiating cable' (sometimes called 'leaky feeder') which provides a low power leakage of transmitted signal along its length to enhance the signal in areas of poor reception. Judicious routing through areas of poor reception of the radiating cables connecting remote antennas to the associated transmitters, will enhance the cover along their routes. Alternatively, radiating cable provided for radiating purposes only but having an electrical connection to the transmitter output signal, will also enhance the paging signal in difficult areas of the power station.

Modern pagers are small compact devices which fit in a breast pocket. The calling signal operates any or all of the following alerting modes; audible tone, lamp or vibrator and also the numeric or alphanumeric displays. The power supply is provided by dry or rechargeable batteries.

Calling the pager from one of the manual controllers or from a PAX telephone will operate the alerting devices of the pager, giving a distinctive alert to indicate the type of call, i.e., group call or individual call. The visual display will instruct the paged person to, for example, ring the manual controller, his emergency control (if the paged person is a member of an emergency team), or to call back a PAX telephone number. [f it is not convenient to ring back the originator of the page, it may be delayed at the discretion of the paged person. The pager will also store a number of paged messages which may be displayed retrospectively.

To call a pager from a PAX telephone, the caller dials the paging system access code, followed by the unique code of the paged person, and then the PAX extension number for the paged person to call back.

7.3.5 Direct speech

A 'paged person receive speech' channel is available as an optional extra on contemporary radio paging systems to enable the caller to transmit a verbal message to a paged person or group of persons. There is also a further optional enhancement to provide return speech from the paged person to the caller. These options are rarely specified for CEGB power stations, the use of alphanumeric displayed information on the paged receiver being preferred. The display gives the paged person unambiguous instructions of the action required and also the option of calling the initiator of the page or delaying this until a more convenient time.

The provision of the received speech facility to a paged person would require a higher standard of reception throughout the power station areas, necessitating a larger number of fixed station transmitters. The provision of transmitted speech from the paged person would also require a large number of fixed station receivers to be positioned in strategic locations throughout the site.

It is CEGB policy to provide roving station personnel who need direct speech communication with a handportable radiotelephone operating on the power station radio system.

7.3.6 Use of paging systems

It is the policy of some power station managers to issue most of the staff with a pocket pager which, for a large station, could be 400 or more. This enables all staff to be contacted wherever they are on site. Other power station managers restrict the issue of pagers to roving personnel; operations, maintenance, security and emergency team staff.

8 Radio systems

8.1 Introduction

Radio systems form part of the telecommunication infrastructure at a power station. They are provided both to supplement and as an alternative to the telephone systems, i.e., radiotelephone systems and also for non-speech communication purposes, viz, crane radio control and anticollision systems.

The increasing use of automatic control for power station plant has resulted in greater centralisation of the operational control and monitoring of the plant, and in a significant reduction in permanently manned, distributed control centres. This has reduced operations staff numbers and permitted greater flexibility and mobility in work patterns. The use of radio enables speech communication to be established with the mobile staff.

As has already been mentioned, modern power stations have a central control room (CCR) from which the operation of the power station is monitored and

a einer,lency services.

ot" the most useful means of communication

:,e[0,een the CCR and roving operations staff is by Ro%.ir.q staff are provided with a handportable

r3di0teieph0ne (handportable) and each control desk in

cdh other using the radio system. Additionally, radio

, ,[eins are used for other duties which involve com-

y

munication between other control centres and mobile e.g., between the maintenance works-control

oflice and maintenance staff. Radio in nuclear power ,:alions is extensively used for similar applications as as for the special requirements of health physics

a:on it oring and use during nuclear emergency activities.

8.2 Radiotelephony systems

I he radiotelephone systems used in power stations are 1, a.sed on commercially available private mobile radio (1 , \IR) equipment. PMR equipment is primarily de- i ned for use with wide area mobile radio schemes. I hese comprise a central fixed station housing a high- ntmered transmitter and a sensitive receiver, a fixed

oion antenna mounted on a mast located at a high point central to the required reception area and vehiclemounted mobile radios (vehicle mobiles). The fixed qation is usually connected by land lines to a remote control unit (controller).

This arrangement is suitable for off-site communications, e.g., emergency communications between the rtmer station and the associated grid control centre ■%hcn other telecommunication links have failed.

However, the standard type of PMR system must ! , c adapted to provide on-site radio cover inside power ':it ion buildings, which contain thick, reinforced con-

.. rete walls, floors and ceilings, large fabricated steel plant items, underground plant rooms and labyrinths ol cable ways and pipework galleries. None of these dre ideal places for radio signal propagation.

. Die design constraints on the use of PMR systems in power stations can be summarised as follows:

•(1 restriction of the field strength transmitted by handportables to reduce the effect of radio frequency

interference (RFI) on sensitive electronic control and instrumentation (C and I) equipment.

•The hot, humid and dirty environment.

•The harsh electromagnetic environment.

•The need for simple control units (controllers) in control rooms for operation by staff not employed as professional radio operators, but using radio as

In order to achieve a suitable radio system, accepting these constraints, it is therefore necessary to adapt the standard PMR equipment for use in power stations.

The radio systems used for power stations have developed from the use of a single remote control unit (controller) located in the CCR and cabled to a fixed station (transceiver) located in the plant area close to the antenna. The antenna has usually been mounted on the roof of the highest building, e.g., turbine hall/ boiler house or the mechanical annexe situated between the turbine hall and boiler house. This arrangement

be supplemented by relatively high-powered handportables in order to provide adequate radio cover in all but the smaller power stations.

As power station buildings have become larger and more complex, and the constraint on the power output of handportables has increased (in order to avoid RH to control and instrumentation equipment), so the radio system has had to become more sophisticated.

The handportable RF power output has been reduced to 0.5 W into the antenna instead of art effective radiated power (ERP) of 0.5 W. The definition of an ERP of 0.5 W is the power radiated from a half-wave dipole antenna when 0.5 W is connected to it. As handportable antenna gains vary from —3 dB to — 10 dB when compared with a dipole antenna, the radiated power has been reduced to between 0.25 W and 0.05 W ERP.

In order to compensate for these reductions, the antenna arrangements of the fixed stations have become more complex. The antenna systems comprise internal and external antennas supplemented by radiating cable (leaky feeder) in confined areas, e.g., basements, tunnels, corridors and CW pumphouses.

In addition, distributed fixed station arrangements are used in medium and large power stations. The arrangements comprise fixed transmitters and receivers located at different plant areas adjacent to the antenna systems required to provide radio cover of the area.

Prior to the availability of synchronising systems for transmitters and voting systems for receivers (which allowed automatic selection of the best received signal), the use of distributed fixed stations required each to operate on a different frequency. This was necessary to prevent interference at the receiving mobile. The interference was most acute when the two received signals were of the same frequency and field strength, and was due to the rapid cancellations taking place in the combined received signal as the two varied in phase with each other.

As more radio frequency channels were required to carry the communications traffic, it became necessary

to adopt new techniques to improve the efficient use of the channels available.

One technique was to use quasi-synchronous operation of the fixed station transmitters to allow the same channel to be used simultaneously at all fixed stations. This provided full cover of the station by each channel and enabled two handportables operating via two different fixed stations to use one channel instead of two, as was previously the case. This technique also dispensed with the need for handportable users to remember which channel had to be used in different areas of the power station. Channels were allocated to the main functional groups, e.g., operations and maintenance, for use throughout the power station.

Details of quasi-synchronous operation and the associated receiver voting system are given later in this section.

The latest development being evaluated by the CEGB is the use of radio channel trunking techniques. A trunked radio system uses the frequency agility of the modern synthesised transmitters and receivers available for handportable and mobile radios to enable them to be remotely switched to a channel.

One of a group of radio channels is used as a control channel in conjunction with a microprocesser-based control equipment (CEQ). The CEQ sets up calls between mobiles, controllers or telephones connected to the system and allocates each call to a traffic channel (the name given to the remainder of the channels in the group). Each call is allocated a traffic channel sequentially on a first come first served basis. Once a call has been allocated a traffic channel the CEQ and control channel are free to deal with the next call. This type of system makes more efficient use of the radio channels and enables sufficient channels to be provided to allow a large number of !ow traffic users to be given radio facilities without the need for them to be skilled at radio procedures to make efficient use of a common channel.

The trunked radio system also provides a number of additional improvements and facilities which are described in detail in Section 8.2.5 of this chapter on UHF systems.

8.2.1 Radio frequency bands used by RMR systems

PMR equipment is manufactured to operate in the VHF and UHF bands of the radio frequency spectrum. The two bands cover the following frequencies:

Band 8 contains the following three PMR frequency bands:

Band 9 contains the following PMR frequency bands:

Section 8.7.1 of this chapter gives an explanation of single-frequency and two-frequency simplex operation.

8.2.2 Comparison of VHF and UHF systems

The antennas used for PMR systems operating in the VHF and UHF frequency bands are similar. These are derivatives of the standard half-wave dipole. The actual antennas used will be dealt with in more detail later. However, it is worth noting at this point that the receiving area of an antenna determines the power received from a radiating signal, which is measured in watts per square metre (see Section 8.5.1 of this chapter).

The area of a receiving antenna is proportional to the square of the wavelength of the radiating signal. Therefore a half-wave dipole is more effective at VHF than at UHF. This also applies to those antennas that are derived from the half-wave dipole. This is one reason for using VHF off-site and UHF for on-site telecommunications in a power station, where there is a requirement to confine the radio cover to the site and immediate area around the site, thus allowing the re-use of the UHF frequency channel elsewhere in the country.

VHF PMR equipment is therefore used for telecommunications between the power station and mobile or fixed stations up to approximately 30 km away, depending on the height of the transmitting antenna and the type of surrounding terrain, while UHF PMR equipment is used for on-site radio systems, i.e., up to approximately 8 km from the power station.

With careful design, it has proved possible to repeat UHF frequency channels at minimum distances of 1216 km and thus make economic use of the UHF radio spectrum. Detailed design requirements to achieve this

8.2.5 UHF systems used in power stations

The frequencies used for power station UHF systems are in the process of change (1988). The existing twelve 25 kHz spaced channels are being replaced by twentyfour 12.5 kHz spaced channels.

The existing group of twelve 25 kHz spaced channels

is:

The proposed new group of twenty-four 12.5 kHz spaced channels is:

UHF systems used for on-site radiotelephone communications are based on conventional PMR systems which use manual selection of the radio channel by a mobile or handportable for speech communications. Automatic allocation of one of a group of radio channels, as used in trunked radio systems, is at present being evaluated by the CEGB.

The conventional UHF systems used for on-site radio communications comprise desk-mounted or table-top controllers, fixed stations containing the base station radio transmitters and receivers (for each of the radio channels used by the power station) and mobile radios. The mobile radios include handportable radiotelephones and radiotelephones mounted in vehicles.

The complexity of the controllers will depend on the overall complexity of the radio system and the ability of the radio contractor to supply customised simplified controllers instead of his standard model.

The controllers are connected to the fixed stations by multipair cables. In modern power stations the cables have short-time fireproof insulation material to improve the security of the radio systems.

Single and multichannel controllers are provided as necessary. A single-channel controller has access to one radio channel only, whereas a multichannel controller has access to a number or all of the channels. The latest controllers have a programming facility, which enables the number of channels accessible to the controller to be pre-programmed, as required by the customer.

The fixed stations are located within the plant areas of the power station, close to the associated antenna systems.

The number of fixed station locations is dependent on the number of antenna systems required to provide good radio cover of the power station. The number of fixed station transmitter/receivers is dependent on the number of radio frequency channels allocated to the power station. This is based on the predicted speech traffic requirements for operation, maintenance and emergency purposes.

To avoid interference between the signals transmitted simultaneously from a number of distributed fixed stations, it is necessary to use one or a combination of the following:

(a)The allocation of different radio frequency channels to each fixed station.

(b)To repeat the use of the same radio frequency channel(s) only if there is no possibility of an overlap of the radio cover between any two of the fixed stations.

(e)The use of dynamic sharing of the radio channels between fixed station locations so that each has access to all channels, but only those channels not in use at other locations being used at any one time.

Alternatives (c) to (e) require a common control equipment (CEQ) to which each fixed station and controller is connected. The CEQ carries out the receiver voting and dynamic sharing operations for the whole system.

The present practice is to provide a single fixed station location for small power stations and distributed fixed stations for medium and large power stations.

Each fixed station is connected to a dedicated antenna system.

The antenna systems comprise a combination of internal and external antennas, supplemented on larger power stations by radiating coaxial cables (leaky feeders). The complexity of the antenna system depends on the level of difficulty experienced in providing good radio cover throughout the station. The design objective is to obtain a level of radio cover better than a minimum sensitivity signal to noise ratio of 15 dB SINAD. SINAD is the acronym for Signal Noise And Distortion.

Signal to noise ratio in dB SINAD =

S + N + D

10 log

N +D

where S = signal power N = noise power and D = distortion power.

For a modern receiver, a sensitivity of 15 dB SINAD would be equivalent to a signal of approximately 1 AV pd at the receiver input.

The mobile radios include up to four radiotelephone units mounted in vehicles, e.g., ambulance, fire tender and four-wheel drive vehicles, and up to sixty handportable radiotelephone units carried by staff working in the power station.

The mobiles can be single or multichannel radios. The multi-channel mobiles, which can be switched to the channel on which the user wishes to operate, are usually provided for those power stations which are allocated multiple radio channels, e.g., up to five.

The number of channels allocated to a power station is dependent on the size of the station and the predicted radio communications speech traffic. The radio requirements for the operations function of a station having two turbine-generator units could be

one channel but a six-unit station could need at least three channels for operational purposes.

•The increased size and complexity of the civil design of a modern power station.

•The necessity to limit the RE output power of handportables to 0.5 W into the antenna to reduce the RFT to electronic control and instrumentation equipment.

Each fixed station is located close to the associated antenna system which comprises a combination of inter- ;i al and external antennas and radiating cable (leaky feeder). The external antennas are mounted as low as possible commensurate with good radio cover of the site, which includes outlying buildings having no antenna system of their own. The internal antennas are mounted in the large open areas within the power station, e.g., turbine hall/boiler house and CW pumphouse. Radiating cable is used in confined areas, e.g., basements, tunnels and enclosed corridors.

In order to prevent mutual interference between fixed stations, it is necessary to allocate different frequencies to each. This is not necessary for those fixed stations which serve a totally confined area. However, in modern power stations the radio cover from each fixed station is so complex that it is difficult to confine the area of cover.

The disadvantages of using different frequencies at each fixed station are:

•Mobile users need to remember which channel has to be selected for use in different areas of the power station.

•Calls to mobiles require a polling facility to enable calls to be made from each fixed station transmitter in turn. This increases the call set-up time.

•Calls between mobiles operating via different fixed stations require the use of two or more channels for a single conversation or conference between three or more mobiles.

•A single emergency channel, to which all mobiles can be switched in order to monitor the emergency situation as it progresses, cannot be provided.

In order to overcome these short-comings in the system, quasi-synchronous operation of the fixed station transmitters and voting of the associated receivers has been adopted.

Quasi-synchronous operation of the transmitters allows the same channel frequency to be used at each fixed station. This provides full radio cover of the power station on all channels.

The system uses high stability oscillators, permitting the channel frequency at each fixed station to be offtuned by one or two Hertz from each of the adjacent fixed stations. The effect of this arrangement is that a mobile which receives signals of similar field strength from two fixed stations will detect a beat frequency of one or two Hertz, but will not experience rapid

fluctuations in signal due to phase cancellations. Theoretically the one or two Hertz beat frequency, bein g sub-audio, will not be heard by the users and therefore speech quality will not be impaired. In practice, in those areas having pronounced quasi-synchron ous effects, a background noise is detected which rises and falls in amplitude at a low frequency of one or two Hertz. This noise is produced by multipath signals arriving at the receiving antenna which vary in pha se , However, the effect does not normally detract from the intelligibility of the speech. On the rare occasions when intelligibility is affected, a small spatial movement by a handportabie user of one or two paces will result in a marked improvement in reception. This is because the points of poor and good reception tend to be located on a small-meshed matrix covering the affected area.

Signals received at two or more fixed stations from the same mobile are compared in the CEQ, the best quality signal is then selected by a receiver voting system and connected to the receive bus of the CEQ. The CEQ bus is then connected to the cable from a controller or to a transmitter if the call is between mobiles. Whe re an area is adequately covered by the fixed station transmitter but not by the associated receiver due to the lower power output of the handportables, one or more distributed fixed station receivers can be provided to i mprove cover.

An alternative to the use of quasi-synchronous transmitters is to use the frequency agility of synthesised transmitters and receivers in the mobile. The CEGB is evaluating trunked radio systems which make use of the synthesised mobiles and fixed stations now available.

Trunked radio systems

Trunked radio systems use automatic allocation of one of a common group of radio channels to a mobile radiotelephone, directly wired controller or telephone each time a radio call is made.

The system dispenses with the need to allocate each radio channel to a specific function, e.g., operations, maintenance or common services, and makes more efficient use of the radio channels available for a power station. Channels can be shared by adjacent power stations, or power station construction sites, which results in more efficient use of channels and makes more channels available to each management unit in times of emergency or abnormal operational activity. In order to share channels between adjacent power stations, or between a power station and an adjacent construction site, a data signalling link has to be provided between the CEQs of each of the radio systems. The link is used to indicate to each CEQ the channels in use at any one time.

Automatic channel selection requires a change to the method of operating the mobile. In a conventional radio system the mobile user switches to the channel and uses a verbal call-sign or the name of the called

calling message to the mobile and a ringing tone to the calling party.

On receipt of the calling message the mobile will emit an audible calling tone. The call is answered by pressing the PTT button on the mobile. On receiving the PTT signal, the CEQ removes the calling message and ringing tone and connects both parties to a common traffic channel. Speech communications can then proceed over the traffic channel while the control channel is free to handle subsequent calls.

If the called ID belongs to a directly wired controller or telephone or to a PAX/PABX tie-circuit, the CEQ sets up a hard-wired path to the called party and rings the controller or telephone, or transmits the necessary dial pulses/DTIvIF signals over a PAX/PABX tiecircuit. On receipt of an answering signal from a directly wired controller or telephone, the CEQ would connect the parties to a common traffic channel. Alternatively, on completion of the dial pulses or DTMF signals over the PAX/PABX tie-circuit, the CEQ would connect the calling party and the tie-circuit to a common traffic channel.

The MPT1327 system has the advantage of off-air call set-up and faster signalling speeds which make more efficient use of the radio channels.

Both the CCIR/CCITT and the MPTI327 systems use timers to control the occupancy time of the traffic channels. Fixed timers are used to limit the occupancy to a customer-determined period, e.g., 2 min. When dynamic timing is provided, the fixed timers are overridden only when there is not a free traffic channel left on the system. This is another method of improving the efficient use of the radio channels.

The timers that are normally incorporated in a system are as follows:

•The dynamic timer which provides a warning signal approximately 10 s before clearing-down a communications channel. The dynamic timer is only activated when there is only one channel free on the system. This ensures a channel is available, or is made available, at worst after a period equivalent to the setting of the minimum fixed timer for emergency or priority calls made from a mobile or handportable.

e.g., 5 minutes, before a forced clear-down sequence is initiated. This timer would be over-ridden by the dynamic timer during periods of heavy traffic.

•A channel activity timer is also provided which is initiated each time a PTT is operated on the channel. If a PTT operation is not detected in a preset time, e.g., 30 seconds, a warning signal is transmitted.

If a PTT is operated then the timer is reset. If a PTT is not operated within approximately 10 seconds of hearing th ,.; warning signal a clear-down sequence is initiated.

The use of microprocessor control in the CEQ of both systems enables sophisticated facilities to be provided. Examples of these are as follows:

•Abbreviated keying — this enables the mobile to u se more easily-remembered one or two digit codes in

place of frequently used or lengthy codes, e.g., calls to the CCR telephone or to telephones on the CEGB CT N.

•Ring-back when free — this enables a calling party to request to be called when a busy party becomes available after a call is completed.

•Group calls — this enables the CCR to call a group of mobiles, e.g., a fire team, by keying a common group code.

•Emergency override — this enables mobiles or directly wired controllers/telephones with the necessary class-of-service to 'knockdown' an existing call of a lower class-of-service if all channels are engaged.

Some of the facilities, e.g., abbreviated keying, can be used to simplify the operation of the mobile for those users not requiring the more sophisticated facilities.

Both trunked systems have the following important features:

•Efficient use of the radio frequency channels which enables more traffic to be handled.

•Confidentiality of radio conversations as controllers and mobiles are unable to select busy channels and monitor them.

•Remote control units (controllers) can be replaced by directly wired telephones which simplify the procedures for accessing and using the system. This is more acceptable to control engineers who, in general, do not wish to be trained radio operators.

•The users have the equivalent of a dedicated channel for each call on the system. This enables a large number of low-level traffic user groups to make use of radio whereas previously, because they were unable to have a dedicated channel, they opted out of using the system. The resulting integrated traffic requirements can be used to justify the allocation of more channels to the power station.

•The use of datalinks between radio systems serving two or more adjacent power stations or between a partly commissioned power station and its adjacent construction site, enables a group of common

channel frequencies to be shared. This provides a larger pool of channels which are available to any