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The equipments that carry out this multiplexing at each end of a line are known as 'Pulse Code Modulation' (P.C.M.) terminal equipments. This approach is the one adopted by telecommunications authorities and some Utilities favour its adoption on their private systems, for economic considerations.

Optical fibre communications are well established in the electrical supply industry. They are the preferred means for the communications link between a substation and a telephone exchange when rented circuits are used, as trials have shown that this link is particularly susceptible to interference from power system faults if copper conductors are used. Whilst such fibres can be laid in cable trenches, there is a strong trend to associate them with the conductors themselves by producing composite cables comprising optical fibres embedded within the conductors, either earth or phase. For overhead lines use of OPGW (Optical Ground Wire) earth conductors is very common, while an alternative is to wrap the optical cable helically around a phase or earth conductor. This latter technique can be used without restringing of the line.

8.7 SIGNALLING METHODS

Various methods are used in protection signalling; not all need be suited to every transmission medium. The methods to be considered briefly are:

a.D.C. voltage step or d.c. voltage reversals b.plain tone keyed signals at high and voice frequencies c.frequency shift keyed signals involving two or more tones at high and voice frequencies

General purpose telecommunications equipment operating over power line carrier, radio or optical fibre media incorporate frequency translating or multiplexing techniques to provide the user with standardised communication channels. They have a nominal bandwidth/channel of 4kHz and are often referred to as voice frequency (vf) channels. Protection signalling equipments operating at voice frequencies exploit the standardisation of the communication interface. Where voice frequency channels are not available or suitable, protection signalling may make use of a medium or specialised equipment dedicated entirely to the signalling requirements.

Figure 8.5 illustrates the communication arrangements commonly encountered in protection signalling.

8.7.1 D.C. Voltage Signalling

A d.c. voltage step or d.c. voltage reversals may be used to convey a signalling instruction between protection relaying points in a power system, but these are suited only to private pilot wires, where low speed signalling is acceptable, with its inherent security.

8.7.2 Plain Tone Signals

Plain high frequency signals can be used successfully for the signalling of blocking information over a power line. A normally quiescent power line carrier equipment can be dedicated entirely to the transfer to teleprotection blocking commands. Phase comparison power line carrier unit protection schemes often use such equipment and take advantage of the very high speed and dependability of the signalling system. The special characteristics of dedicated 'on/off' keyed carrier systems are discussed later. A relatively insensitive receiver is used to discriminate against noise on an amplitude basis, and for some applications the security may be satisfactory for permissive tripping, particularly if the normal high-speed operation of about 6ms is sacrificed by the addition of delays. The need for regular reflex testing of a normally quiescent channel usually precludes any use for intertripping.

Plain tone power line carrier signalling systems are particularly suited to providing the blocking commands often associated with the protection of multi-ended feeders, as described in Chapter 13. A blocking command sent from one end can be received simultaneously at all the other ends using a single power line carrier channel. Other signalling systems usually require discrete communication channels between each of the ends or involve repeaters, leading to decreased dependability of the blocking command.

Plain voice frequency signals can be used for blocking, permissive intertrip and direct intertrip applications for all transmission media but operation is at such a low signal level that security from maloperation is not very good. Operation in the 'tone on' to 'tone off' mode gives the best channel monitoring, but offers little security; to obtain a satisfactory performance the output must be delayed. This results in relatively slow operation: 70 milliseconds for permissive intertripping, and 180 milliseconds for direct intertripping.

P r ot ect ion:

Sig nalling and Int er t r ipping

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8.7.3 Frequency Shift Keyed Signals

Frequency shift keyed high frequency signals can be used over a power line carrier link to give short operating times (15 milliseconds for blocking and permissive intertripping, 20 milliseconds for direct intertripping) for all applications of protection signalling. The required amount of security can be achieved by using a broadband noise detector to monitor the actual operational signalling equipment.

Frequency shift keyed voice frequency signals can be used for all protection signalling applications over all transmission media. Frequency modulation techniques make possible an improvement in performance, because amplitude limiting rejects the amplitude modulation component of noise, leaving only the phase modulation components to be detected.

The operational protection signal may consist of tone sequence codes with, say, three tones, or a multi-bit code using two discrete tones for successive bits, or of a single frequency shift.

Modern high-speed systems use multi-bit code or single frequency shift techniques. Complex codes are used to give the required degree of security in direct intertrip schemes: the short operating times needed may result in uneconomical use of the available voice frequency spectrum, particularly if the channel is not exclusively employed for protection signalling. As noise power is directly proportional to bandwidth, a large bandwidth causes an increase in the noise level admitted to the detector, making operation in the presence of noise more difficult. So, again, it is difficult to obtain both high dependability and high security.

The signal frequency shift technique has advantages where fast signalling is needed for blocked distance and permissive intertrip applications. It has little inherent security, but additional circuits responsive to every type of interference can give acceptable security. This system does not require a channel capable of high transmission rates, as the frequency changes once only; the bandwidth can therefore be narrower than in coded systems, giving better noise rejection as well as being advantageous if the channel is shared with telemetry and control signalling, which will inevitably be the case if a power line carrier bearer is employed.

P r ot ect ion:

Sig nalling and Int er t r ipping

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Voice frequency

Carrier frequency shift

On/off keyed carrier

Digital Optical

Protection relay scheme

Protection signalling equipment

PCM primary multiplex

Frequency division multiplex

Radio transmitter

Optical transmitter

Communication equipment

Power line carrier communication channelPower line carrier Power line carrier

Pilot wires Pilot channel

Transmission media

Optical fibre general purpose

Optical fibre dedicated

Radio

Figure 8.5: Communication arrangements commonly encountered in protection signalling

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