electrical-engineering-portal.com http://electrical-engineering-portal.com/principles-characteristics-distance-protectionGoogle+Principles and Characteristics of Distance ProtectionPrinciples and Characteristics of Distance Protection (on photo distance protection relay type SIPROTEC 4 7SA522 for transmission lines)Introduction to Distance ProtectionDistance relays are one of the most important protection elements in a transmission line.Theserelays may sometimes be set based in percentages of the line impedances, for example atypical setting forzone 1 is 80% of the impedance of the line in order to not reach the remoteend, thezone 2 can be set at120% ofthe impedance of the line in order to dependablyoverreach the line, Zone 3 sometimes are disabled or set to coveran adjacent line.Distance relays characteristics may be Mho, Quadrilateral, Offset Mho, etc. In the case of thequadrilateralcharacteristic or long reaching mho characteristics, additional care may berequired to remain secure during heavyload.In the case of parallel lines, the mutual coupling of these lines can cause distance relays tounder reach and overreach. For this reason the relay setting must consider this effect, somerelays have algorithms to compensate, but itis necessary to use the current of the parallel linewhich adds complexity to the installation.In some countries there criteria that a distance protection can not reach fault in other voltagelevels, because faultclearing times in sub transmission levels may be slower than fault clearingtimes at the transmission level.The problem of combiningfast fault clearance with selective tripping of plant is a key aim for the protection ofpower systems.To meet these requirements, high-speed protection systems for transmission and primary distribution circuits that aresuitable for use with the automatic reclosure of circuit breakers are under continuous development and are verywidely applied.Distance protection, in its basic form, is a non-unit system of protection offering considerable economic and technicaladvantages.Unlike phase and neutral overcurrent protection, the key advantage of distance protection is that its fault coverage ofthe protected circuit is virtually independent of source impedance variations.Figure 1 - Advantages of distance over overcurrent protectionDistance protection is comparatively simple to apply and it can be fast in operation for faults located along most of aprotected circuit. It can also provide both primary and remote back-up functions in a single scheme. It can easily beadapted to create a unit protection scheme when applied with a signalling channel.In this form it is eminently suitable for application withhigh-speed auto- reclosing, for the protection of criticaltransmission lines.Principles of Distance RelaysSince the impedance of a transmission line is proportional to its length, for distance measurement it is appropriate touse a relay capable of measuring the impedance of a line up to a predetermined point (the reach point).Such a relay is described as adistance relay and is designed to operate only for faults occurring between the relaylocation and the selected reach point, thus giving discrimination for faults that may occur in different line sections.The basic principle of distance protection involves the division of the voltage at the relaying point by the measuredcurrent. The apparent impedance so calculated is compared with the reach point impedance. If the measuredimpedance is less than the reach point impedance, it is assumed that a fault exists on the line between the relay andthe 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 be plotted onan R/X diagram. The loci of power system impedances as seen by the relay during faults, power swings and loadvariations may be plotted on the same diagram and in this manner the performance of the relay in the presence ofsystem faults and disturbances may be studied.Relay performanceDistance relay performance is defined in terms ofreach accuracy and operating time. Reach accuracy is acomparison of the actual ohmic reach of the relay under practical conditions with the relay setting value in ohms.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 times canvary with fault current, with fault position relative to the relay setting, and with the point on the voltage wave at whichthe fault occurs.Depending on the measuring techniques employed in a particular relay design, measuring signal transient errors,such as those produced by Capacitor Voltage Transformers or saturating CTs, can also adversely delay relayoperation for faults close to the reach point. It is usual for electromechanical and static distance relays to claim bothmaximum and minimum operating times.However, for modern digital or numerical distance relays, the variation between these is small over a wide range ofsystem operating conditions and fault positions.Distance Relay CharacteristicsSome numerical relays measure the absolute fault impedance and then determine whether operation is requiredaccording to impedance boundaries defined on the R/X diagram.Traditional distance relays and numerical relays that emulate the impedance elements of traditional relays do notmeasure absolute impedance. They compare the measured fault voltage with a replica voltage derived from the faultcurrent and the zone impedance setting to determine whether the fault is within zone or out-of-zone. Distance relayimpedance comparators or algorithms which emulate traditional comparators are classified according to their polarcharacteristics, the number of signal inputs they have, and the method by which signal comparisons are made.The common types compare either the relative amplitude or phase of two input quantities to obtain operatingcharacteristics that are either straight lines or circles when plotted on an R/X diagram. At each stage of distance relaydesign evolution, the development of impedance operating characteristic shapes and sophistication has beengoverned by the technology available and the acceptable cost.Since many traditional relays are still in service and since some numerical relays emulate the techniques of thetraditional relays, a brief review of impedance comparators is justified.Example of Modern Distance Protection RelaySIPROTEC 7SA522 protection relay - Single line diagram (provides full-scheme distance protection and incorporates all functions usuallyrequired for the protection of a power line)This particulary relay has following ANSI protection functions:ANSI Description ANSI Description21/21N Distance protection 50HS Switch-onto-fault protectionFL Fault locator 50BF Breaker failure protection50N/51N; 67N Directional ground-fault protection 59/27 Overvoltage/undervoltage protection50/51/67 Backup overcurrent protection 81O/U Over/underfrequency protection50 STUB STUB-bus overcurrent stage 25 Synchro-check68/68T Power swing detection/tripping 79 Auto-reclosure85/21 Teleprotection for distance protection 74TC Trip circuit supervision27WI Weak-infeed protection 86 Lockout (CLOSE command interlocking)85/67N Teleprotection for ground-fault protectionDistance Relays (VIDEO)Cant see this video? Clickhere to watch it on Youtube.Resource: Network protection and automation guide Areva; SIPROTEC47SA522 Distance Protection Relay forTransmission Lines; An Example Distance Protection Application with Complicating Factors by Yofre Jacome andCharles F Henville