8 Protection: Signalling and Intertripping - …

1 Introduction protection schemes commands requirements media, interference and noise of Signalling 8 protection : Signallingand INTRODUCTIONUnit protection schemes, formed by a number of relayslocated remotely from each other, and some distanceprotection schemes, require some form of communicationbetween each location in order to achieve a unit protectionfunction. This form of communication is known asprotection Signalling . Additionally communicationsfacilities are also required when remote operation of acircuit breaker is required as a result of a local event. Thisform of communications is known as communication messages involved may be quitesimple, involving instructions for the receiving device totake some defined action (trip, block, etc.), or it may bethe passing of measured data in some form from onedevice to another (as in a unit protection scheme).

2 Various types of communication links are available forprotection Signalling , for example:i. private pilot wires installed by the wires or channels rented from acommunications channels at high frequencies over thepower channels at very high or ultra fibresWhether or not a particular link is used depends onfactors such as the availability of an appropriatecommunication network, the distance betweenprotection relaying points, the terrain over which thepower network is constructed, as well as Signalling is used to implement UnitProtection schemes, provide teleprotection commands,or implement Intertripping between circuit UNIT protection SCHEMESP hase comparison and current differential schemes usesignalling to convey information concerning the relayingquantity - phase angle of current and phase and 8 protection .

3 Signallingand IntertrippingNetwork protection & Automation Guide 113 Network protection & Automation Guide 114 8 protection : Signalling and Intertrippingmagnitude of current respectively - between local andremote relaying points. Comparison of local and remotesignals provides the basis for both fault detection anddiscrimination of the of Unit protection schemes are given in Chapter methods are covered later in this TELEPROTECTION COMMANDSSome Distance protection schemes described in Chapter12 use Signalling to convey a command between localand remote relaying points. Receipt of the informationis used to aid or speed up clearance of faults within aprotected zone or to prevent tripping from faults outsidea protected systems are often referred to by theirmode of operation, or the role of the teleprotectioncommand in the INTERTRIPPINGI ntertripping is the controlled tripping of a circuitbreaker so as to complete the isolation of a circuit orpiece of apparatus in sympathy with the tripping of othercircuit breakers.

4 The main use of such schemes is toensure that protection at both ends of a faulted circuitwill operate to isolate the equipment concerned. Possiblecircumstances when it may be used feeder with a weak infeed at one end, insufficientto operate the protection for all protection applied to transformer feedercircuits. Faults on the transformer windings mayoperate the transformer protection but not thefeeder protection . Similarly, some earth faults maynot be detected due to transformer between the CB and feeder protection CT s,when these are located on the feeder side of the protection does not result in faultclearance the fault is still fed from the remote endof the feeder, while feeder unit protection may notoperate as the fault is outside the protected distance protection schemes useintertripping to improve fault clearance times forsome kinds of fault see Chapters 12/13 Intertripping schemes use Signalling to convey a tripPower transmission lineFigure.

5 Application of protection Signalling and its relationship to other systems using communication(Shown as a unidirectional system for simplicity)BlockingCommunicationlinkVIDa taCommunicationsystemsTelecontrolTelepho nePermissivetripTelemetryDataCommunicati onsystemsTelecontrolTelephoneTelemetryTe leprotectioncommand(receive)Teleprotecti oncommand(send)ProtectionrelayschemeProt ectionrelayschemeVITripTripIntertripBloc kingPermissivetripIntertripFigure : Application of protection Signalling and its relationship to other systems using communication(shown as a unidirectional system for simplicity)Network protection & Automation Guide 115 8 protection : Signalling and Intertrippingcommand to remote circuit breakers to isolate high reliability EHV protection schemes, intertrippingmay be used to give back-up to main protections, orback-tripping in the case of breaker failure. Three typesof Intertripping are commonly encountered, and aredescribed Direct TrippingIn direct tripping applications, intertrip signals are sentdirectly to the master trip relay.

6 Receipt of the commandcauses circuit breaker operation. The method ofcommunication must be reliable and secure because anysignal detected at the receiving end will cause a trip ofthe circuit at that end. The communications systemdesign must be such that interference on thecommunication circuit does not cause spurious a spurious trip occur, considerable unnecessaryisolation of the primary system might result, which is atbest undesirable and at worst quite Permissive TrippingPermissive trip commands are always monitored by aprotection relay. The circuit breaker is tripped whenreceipt of the command coincides with operation of theprotection relay at the receiving end responding to asystem fault. Requirements for the communicationschannel are less onerous than for direct trippingschemes, since receipt of an incorrect signal mustcoincide with operation of the receiving end protectionfor a trip operation to take place.

7 The intention of theseschemes is to speed up tripping for faults occurringwithin the protected Blocking SchemeBlocking commands are initiated by a protection elementthat detects faults external to the protected of an external fault at the local end of aprotected circuit results in a blocking signal beingtransmitted to the remote end. At the remote end,receipt of the blocking signal prevents the remote endprotection operating if it had detected the external of the communications channel is less serious forthis scheme than in others as loss of the channel doesnot result in a failure to trip when required. However,the risk of a spurious trip is shows the typical applications of protectionsignalling and their relationship to other signallingsystems commonly required for control and managementof a power system. Of course, not all of the protectionsignals shown will be required in any particular PERFORMANCE REQUIREMENTSO verall fault clearance time is the sum relay operating relay operating breaker operating timeThe overall time must be less than the maximum time forwhich a fault can remain on the system for minimumplant damage, loss of stability, etc.

8 Fast operation istherefore a pre-requisite of most Signalling the time allowed for the transfer of a commandis of the same order as the operating time of theassociated protection relays. Nominal operating timesrange from 5 to 40ms dependent on the mode ofoperation of the teleprotection signals are subjected to the noise andinterference associated with each communicationmedium. If noise reproduces the signal used to conveythe command, unwanted commands may be produced,whilst if noise occurs when a command signal is beingtransmitted, the command may be retarded or missedcompletely. Performance is expressed in terms ofsecurity and dependability. Security is assessed by theprobability of an unwanted command occurring, anddependability is assessed by the probability of missing acommand. The required degree of security anddependability is related to the mode of operation, thecharacteristics of the communication medium and theoperating standards of the particular power design objectives for teleprotection systems arenot more than one incorrect trip per 500 equipmentyears and less than one failure to trip in every 1000attempts, or a delay of more than 50msec should notoccur more than once per 10 equipment years.

9 Toachieve these objectives, special emphasis may beattached to the security and dependability of theteleprotection command for each mode of operation inthe system, as Performance Requirements IntertrippingSince any unwanted command causes incorrect tripping,very high security is required at all noise levels up to themaximum that might ever be Performance Requirements Permissive TrippingSecurity somewhat lower than that required forintertripping is usually satisfactory, since incorrecttripping can occur only if an unwanted commandhappens to coincide with operation of the protectionrelay for an out-of-zone permissive over-reach schemes, resetting after acommand should be highly dependable to avoid anychance of maloperations during current Performance Requirements Blocking SchemesLow security is usually adequate since an unwantedcommand can never cause an incorrect trip.

10 Highdependability is required since absence of the commandcould cause incorrect tripping if the protection relayoperates for an out-of-zone performance requirements are shown in Figure TRANSMISSION MEDIAINTERFERENCE AND NOISEThe transmission media that provide the communicationlinks involved in protection Signalling pilots or line fibresHistorically, pilot wires and channels (discontinuous pilotwires with isolation transformers or repeaters along theroute between Signalling points) have been the mostwidely used due to their availability, followed by PowerLine Carrier Communications (PLCC) techniques andradio. In recent years, fibre-optic systems have becomethe usual choice for new installations, primarily due totheir complete immunity from electrical use of fibre-optic cables also greatly increases thenumber of communication channels available for eachphysical fibre connection and thus enables morecomprehensive monitoring of the power system to beachieved by the provision of a large number ofcommunication Private Pilot Wires and ChannelsPilot wires are continuous copper connections betweensignalling stations, while pilot channels arediscontinuous pilot wires with isolation transformers orrepeaters along the route between Signalling may be laid in a trench with high voltage cables,laid by a separate route or strung as an open wire on aseparate wood pole over which Signalling is required varyconsiderably.