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Unit protection schemes8.2
Teleprotection commands8.3
Intertripping8.4
Performance requirements8.5
Transmission media, interference and noise8.6
Methods of signalling8.7

Introduction 8.1

• 8 • P r ot ect ion: Sig nalling and Intertripping

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8.1 INTRODUCTION

Unit protection schemes, formed by a number of relays located remotely from each other, and some distance protection schemes, require some form of communication between each location in order to achieve a unit protection function. This form of communication is known as protection signalling. Additionally communications facilities are also required when remote operation of a circuit breaker is required as a result of a local event. This form of communications is known as intertripping.

The communication messages involved may be quite simple, involving instructions for the receiving device to take some defined action (trip, block, etc.), or it may be the passing of measured data in some form from one device to another (as in a unit protection scheme).

Various types of communication links are available for protection signalling, for example:

i. private pilot wires installed by the power authority i.pilot wires or channels rented from a communications company i.carrier channels at high frequencies over the power lines iv.radio channels at very high or ultra high frequencies v.optical fibres

Whether or not a particular link is used depends on factors such as the availability of an appropriate communication network, the distance between protection relaying points, the terrain over which the power network is constructed, as well as cost.

Protection signalling is used to implement Unit Protection schemes, provide teleprotection commands, or implement intertripping between circuit breakers.

8.2 UNIT PROTECTION SCHEMES

Phase comparison and current differential schemes use signalling to convey information concerning the relaying quantity - phase angle of current and phase and

• 8 • Prot ect ion: Sig nalling and Intertripping

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P r ot ect ion:

Sig nalling and Int er t r ipping magnitude of current respectively - between local and remote relaying points. Comparison of local and remote signals provides the basis for both fault detection and discrimination of the schemes.

8.3 TELEPROTECTION COMMANDS

Some Distance Protection schemes described in Chapter 12 use signalling to convey a command between local and remote relaying points. Receipt of the information is used to aid or speed up clearance of faults within a protected zone or to prevent tripping from faults outside a protected zone.

Teleprotection systems are often referred to by their mode of operation, or the role of the teleprotection command in the system.

8.4 INTERTRIPPING

Intertripping is the controlled tripping of a circuit breaker so as to complete the isolation of a circuit or piece of apparatus in sympathy with the tripping of other circuit breakers. The main use of such schemes is to ensure that protection at both ends of a faulted circuit will operate to isolate the equipment concerned. Possible circumstances when it may be used are:

a.a feeder with a weak infeed at one end, insufficient to operate the protection for all faults b.feeder protection applied to transformer –feeder circuits. Faults on the transformer windings may operate the transformer protection but not the feeder protection. Similarly, some earth faults may not be detected due to transformer connections c.faults between the CB and feeder protection CT’s, when these are located on the feeder side of the CB. Bus-zone protection does not result in fault clearance – the fault is still fed from the remote end of the feeder, while feeder unit protection may not operate as the fault is outside the protected zone d.some distance protection schemes use intertripping to improve fault clearance times for some kinds of fault – see Chapters 12/13

Intertripping schemes use signalling to convey a trip

Power transmission line

Blocking Communication link

Data

Communication systems

Telecontrol Telephone

Permissive trip

Telemetry

Data

Communication systems

Telecontrol Telephone

Telemetry

Teleprotection command (receive)

Teleprotection command (send)

Protection relay scheme

Protection relay scheme

VI TripTrip

Intertrip

Blocking

Permissive trip

Intertrip

Figure 8.1: Application of protection signalling and its relationship to other systems using communication (shown as a unidirectional system for simplicity)

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P r ot ect ion:

Sig nalling and Int er t r ipping command to remote circuit breakers to isolate circuits. For high reliability EHV protection schemes, intertripping may be used to give back-up to main protections, or back-tripping in the case of breaker failure. Three types of intertripping are commonly encountered, and are described below.

(Parte 1 de 4)

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