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Princip le s and Characte ris tics o f Distance Pro te ctio n (o n p ho to d istance p ro te ctio n re lay typ e SIPROTEC 4 7SA522 fo rtransmissio n l ine s)

by Edvard

Introduction to Distance Protection

Distance relays are one of the most important protection elements in a transmission line.

These relays may sometimes be set based in percentages of the line impedances, f or example a typicalsetting f or zone 1 is 80% of the impedance of the line in order to not reach the remote end, the zone 2 can beset at 120% of the impedance of the line in order to dependably overreach the line, Zone 3 sometimes aredisabled or set to cover an adjacent line.

Distance relays characteristics may be Mho, Quadrilateral, Offset Mho, etc. In the case of the quadrilateralcharacteristic or long reaching mho characteristics, additional care may be required to remain secure duringheavy load.

In the case of parallel lines, the mutual coupling of these lines can cause distance relays to under reach andover reach. For this reason the relay setting must consider this ef f ect, some relays have algorithms tocompensate, but it is necessary to use the current of the parallel line which adds complexity to the installation.

In some countries there criteria that a distance protection can not reach f ault in other voltage levels, because

Fig ure 1 - Ad vantag e s o f d is tance o ve r o ve rcurre nt p ro te ctio n

f ault clearing times in sub transmission levels may be slower than f ault clearing t imes at the transmission level.

The problem of combining fast fault clearance with selective tripping of plant is a key aim f or the protectionof power systems.

To meet these requirements, high-speed protection systems f or transmission and primary distribution circuitsthat are suitable f or use with the automatic reclosure of circuit breakers are under continuous developmentand are very widely applied.

Distance protection, in its basic f orm, is a non-unit system of protection of f ering considerable economic andtechnical advantages.

Unlike phase and neutral overcurrent protection, the key advantage of distance protection is that its f aultcoverage of the protected circuit is virtually independent of source impedance variations.

Distance protection iscomparatively simpleto apply and it can bef ast in operation f orf aults located alongmost of a protectedcircuit. It can alsoprovide both primaryand remote back-upf unctions in a singlescheme. It can easilybe adapted to createa unit protectionscheme when appliedwith a signallingchannel.

In this f orm it iseminently suitable f orapplication with high-speed auto-reclosing, f or theprotection of crit icaltransmission lines.

Principles ofDistance RelaysSince the impedanceof a transmission lineis proportional to itslength, f or distancemeasurement it isappropriate to use arelay capable of measuring the impedance of a line up to a predetermined point (the reach point).

Such a relay is described as a distance relay and is designed to operate only f or f aults occurring between therelay location and the selected reach point, thus giving discrimination f or f aults that may occur in dif f erent linesections.

The basic principle of distance protection involves the division of the voltage at the relaying point by themeasured current. The apparent impedance so calculated is compared with the reach point impedance. If themeasured impedance is less than the reach point impedance, it is assumed that a f ault exists on the linebetween the relay and the reach point.

The reach point of a relay is the point along the line impedance locus that is intersected by the boundarycharacteristic of the relay.

Since this is dependent on the ratio of voltage and current and the phase angle between them, it may beplotted on an R/X diagram. The loci of power system impedances as seen by the relay during f aults, powerswings and load variations may be plotted on the same diagram and in this manner the perf ormance of therelay in the presence of system f aults and disturbances may be studied.

Relay performanceDistance relay perf ormance is def ined in terms of reach accuracy and operating time. Reach accuracy is acomparison of the actual ohmic reach of the relay under practical conditions with the relay setting value inohms.

Reach accuracy particularly depends on the level of voltage presented to the relay under fault conditions.

The impedance measuring techniques employed in particular relay designs also have an impact. Operating timescan vary with f ault current, with f ault posit ion relative to the relay setting, and with the point on the voltagewave at which the f ault occurs.

Depending on the measuring techniques employed in a particular relay design, measuring signal transienterrors, such as those produced by Capacitor Voltage Transf ormers or saturating CTs, can also adverselydelay relay operation f or f aults close to the reach point. It is usual f or electromechanical and static distancerelays to claim both maximum and minimum operating times.

However, f or modern digital or numerical distance relays, the variation between these is small over a widerange of system operating conditions and f ault posit ions.

Distance Relay Characterist icsSome numerical relays measure the absolute f ault impedance and then determine whether operation is requiredaccording to impedance boundaries def ined on the R/X diagram.

Tradit ional distance relays and numerical relays that emulate the impedance elements of tradit ional relays donot measure absolute impedance. They compare the measured f ault voltage with a replica voltage derived f romthe f ault current and the zone impedance setting to determine whether the f ault is within zone or out-of -zone.Distance relay impedance comparators or algorithms which emulate tradit ional comparators are classif iedaccording to their polar characteristics, the number of signal inputs they have, and the method by which signalcomparisons are made.

SIPROTEC 7SA522 p ro te ctio n re lay - Sing le l ine d iag ram (p ro vid e s full-sche me d istance p ro te ctio n and inco rp o rate s allfunctio ns usually re q uire d fo r the p ro te ctio n o f a p o we r l ine )

The common types compare either the relative amplitude or phase of two input quantit ies to obtain operatingcharacteristics that are either straight lines or circles when plotted on an R/X diagram. At each stage ofdistance relay design evolution, the development of impedance operating characteristic shapes andsophistication has been governed by the technology available and the acceptable cost.

Since many tradit ional relays are still in service and since some numerical relays emulate the techniques of thetradit ional relays, a brief review of impedance comparators is justif ied.

Example of Modern Distance Protect ion RelayThis

particulary relay has following ANSI protection functions:

ANSI Description ANSI Description21/21N Distance protection 50HS Switch-onto-f ault protectionFL Fault locator 50BF Breaker f ailure protection50N/51N;67N

Directional ground-f ault protection 59/27 Overvoltage/undervoltage protection

50/51/67 Backup overcurrent protection 81O/U Over/underf requency protection50 STUB STUB-bus overcurrent stage 25 Synchro-check68/68T Power swing detection/tripping 79 Auto-reclosure85/21 Teleprotection f or distance protection 74TC Trip circuit supervision

27WI Weak-inf eed protection 86 Lockout (CLOSE command interlocking)

85/67N Teleprotection f or ground-f aultprotection

Distance Relays (VIDEO)Cant see this video? Click here to watch it on Youtube.

Resource: Network protection and automation guide Areva; SIPROTEC47SA522 Distance Protection Relayfor Transmission Lines; An Example Distance Protection Application with Complicating Factors by Yofre Jacomeand Charles F Henville

Introduction to Distance ProtectionPrinciples of Distance RelaysRelay performanceDistance Relay CharacteristicsExample of Modern Distance Protection RelayDistance Relays (VIDEO)