Power System Protective Relaying: Basic Concepts .
Power System Protective Relaying:basic concepts, industrial-gradedevices, and communicationmechanismsInternal ReportReport # Smarts-Lab-2011-003July 2011Principal Investigators:Rujiroj LeelarujiDr. Luigi VanfrettiAffiliation:KTH Royal Institute of TechnologyElectric Power Systems DepartmentKTH Electric Power Systems Division School of Electrical Engineering Teknikringen 33 SE 100 44 Stockholm SwedenDr. Luigi Vanfretti Tel.: 46-8 790 6625 luigiv@kth.se www.vanfretti.com
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Contents1 Introduction22 Main components of protection systems33 Implementation of protective relays in power systems3.1 Generator Protection . . . . . . . . . . . . . . . . . . . . . . . .3.2 Line Protection . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3 Transformer Protection . . . . . . . . . . . . . . . . . . . . . . .3.4 Load Protection . . . . . . . . . . . . . . . . . . . . . . . . . . .3.5 Short description of Programming and Software Features fromvendors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33466. . . . . . . . . . . . . . . . .different. . . . .164 Communications in power system protection4.1 Physical-based protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.2 Layered Based-protocols . . . . . . . . . . . . . . . . . . . . . . . . . . .4.3 Communication Delays in Data Delivery for Synchrophasor Applications182022275 Summary28
Power System Protective Relaying: basic concepts,industrial-grade devices, and communicationmechanismsThis report provides a survey of protective relaying technology and its associated communications technology used in today’s power transmission systems. This report is dividedin two parts. In the first part, the operating principles of relay applications and the maincomponents of protection systems are briefly introduced. This helps the reader to becomefamiliar with the principles used by most common protective relays. A review and comparison between different vendors is also provided to highlight the industrial state-of-theart in this field. The second part is concerned mainly with power system relaying communication. The various protocols and network topologies used for protective relayingpurposes are explained. Associated communication standards are outlined. The aim ofthis part is to provide background on the communication technologies used by protectionsystem.1IntroductionThe IEEE defines protective relays as: “relays whose function is to detect defectivelines or apparatus or other power system conditions of an abnormal or dangerous natureand to initiate appropriate control circuit action ” [1]. Relays detect and locate faultsby measuring electrical quantities in the power system which are different during normaland intolerable conditions. The most important role of protective relays is to first protectindividuals, and second to protect equipment. In the second case, their task is to minimizethe damage and expense caused by insulation breakdowns which (above overloads) arecalled ‘ faults ’ by relay engineers. These faults could occur as a result from insulationdeterioration or unforeseen events, for example, lighting strikes or trips due to contactwith trees and foliage.Relays are not required to operate during normal operation, but must immediatelyactivate to handle intolerable system conditions. This immediate availability criterion isnecessary to avoid serious outages and damages to parts of or the entire power network.Theoretically speaking, a relay system should be capable of responding to an infinitenumber of abnormalities that may happen within the network. However, in practice,some compromises must be made by comparing risks. It is quite difficult to ensurestability and security of the entire power system if only local measurements are employedin monitoring, protection and control schemes. One promising way is to develop systemwide protection and control mechanisms, complementary to the conventional local and2
zonal protection strategies. In order to implement such mechanisms, synchronized phasormeasurement may serve as an effective data source from which critical information aboutthe system’s condition can be extracted. Synchronized phasor measurement capabilitiesare now one of the features available in the most advanced protective relays commerciallyavailable, and the use of this feature is proliferating.2Main components of protection systemsThe main components of protection systems are discussed briefly below. Current & Voltage Transformer: also called instrument transformers. Theirpurpose is to step down the current or voltage of a device to measurable values,within the instrumentation measurement range 5A or 1A in the case of a currenttransformers (CTs), and 110V or 100V in the case of a voltage (or potential) transformers (VTs/ PTs). Hence, protective equipment inputs are standardized withinthe ranges above. Protective relays: are intelligent electronic devices (IEDs) which receive measured signals from the secondary side of CTs and VTs and detect whether theprotected unit is in a stressed condition (based on their type and configuration) ornot. A trip signal is sent by protective relays to the circuit breakers to disconnectthe faulty components from power system if necessary. Circuit Breakers: Circuit Breakers act upon open commands sent by protectiverelays when faults are detected and close commands when faults are cleared. Theycan also be manually opened, for example, to isolate a component for maintenance. Communication Channels: are the paths that deliver information and measurements from an initiating relay at one location to a receiving relay (or substation)at another location. The topic of communication channels is described in detail inthis report.3Implementation of protective relays in power systemsIn this section, protective relays are categorized depending on the component which areprotect: generators, transmission lines, transformers, and loads.3.1Generator ProtectionThere are different protection schemes used for protecting generators depending on typeof fault to which they are subjected. One of the most common faults is the sudden loss oflarge generators, which results in a large power mismatch between load and generation.This power mismatch is caused by the loss of synchronism in a certain generator - it issaid that the unit goes out-of-step. In this case, an out-of-step relay can be employed
to protect the generator in the event of these unusual operating conditions, by isolatingthe unit from the rest of the system. In addition, microprocessor-based relays have abuilt-in feature for measuring phase angles and computing the busbar frequency from themeasured voltage signal from the VT [2]. Thus, phase angles and frequency measurementsare also available for use within the relay. Figure 1 shows the connection of out-of-steprelays for generator protection.GeneratorCTVTRest of the powersystem278Out-of-stepProtectionRelay Trip Signal from the Out of StepProtection Relay (78) to the CircuitBreaker to protect the Generatorin case of loss of synchronism(fault)Figure 1: Implementation of out-of-step relays to protect generators3.2Line ProtectionTransmission lines can be protected by several types of relays, however the most commonpractice to protect transmission lines is to equip them with distance relays. Distancerelays are designed to respond change in current, voltage, and the phase angle betweenthe measured current and voltage. The operation principle relies on the proportionalitybetween the distance to the fault and the impedance seen by the relay. This is doneby comparing a relay’s apparent impedance to its pre-defined threshold value. Distancerelays’ characteristics are commonly plotted on the R-X diagram are shown in Fig. 2awhereas Fig. 2b represents the Mho relay which is inherently directional [3]. As anillustration in conjunction with the figure, suppose a fault arose, the voltage at relaywill be lower or the current will be greater compared to the values for steady state loadcondition. Thus, distance relays activate when relay’s apparent impedance decreases toany value inside the parametric circle. For this reason, the impedance of the line afterthe fault can also be used to find the location of the fault.Like several engineering constructs, a backup is employed for redundancy. A minimumof two zones are necessary for primary protection of distance relays to address the faultsat the far end of the protected line section near the adjacent bus. Such a criterion providesa safety factor to ensure that any operation against faults beyond the end of a line willnot be triggered by measurement errors. Several protection zones can be built by usingseparate distance measuring units, which provided redundancy since both distance units
XXLineZone 3nZRZone 2nZRZone 1RR(a) Impedance(b) MhoFigure 2: Distance relay characteristicswill operate for faults occurring in Zone 1. The key difference between the two redundantunits is in the time delay; the unit covering Zone 1 would operate instantaneously whereasthe unit designated in Zone 2 would have an added time delay between fault signalingand operation. Also, by modifying either the restraint and/or operating quantities, therelay operating circles can be shifted as shown in Fig. 2b.In some applications, a further setting (Zone 3) is included, which is greater thanZone 2 setting. For a fault generated in Zone 1, Zone 3’s operation occurs after a longertime delay than that associated with the Zone 2. Therefore, the delay acts as a temporaltolerance for the protective schemes within the fault zone. The delayed operation willtrigger if the tolerance is exceeded. Hence, this setting provides a form of back upprotection. Figure 3 depicts protection zones of distance relays. Typically, Zone 1 is setin range of 85% to 95% of the positive-sequence of protected line impedance. Zone 2 is setto approximately 50% into the adjacent line, and 25% into the next two lines for Zone 3as described in [4]. The operation time for Zone 1 is instantaneous whereas Zone 2, andZone 3 are labeled T2 and T3 , respectively.T3T2Gen(1)Bus 2Bus 1Bus 3TimeZone 3T3Zone 2T2Zone 1DistanceFigure 3: Protection zones of distance relays
Most of today’s microprocessor- based relays implement multi-functional protectionfeatures. They are considered as a complete protection package in a single unit. Incase of line protection via distance protection schemes, microprocessor-based relays alsoprovide over current protection, directional over current protection (for selectivity in caseof multiple parallel lines), under/over voltage protection, breaker failure protection (incase the breaker fails to trip even after receiving the trip command), etc [5]. Figure 4shows the connection of a distance relay for line protection.Trip Signal from the Distance Relay(21) to the Circuit Breaker toprotect the Transmission Line (L1)in case of fault Z(ImpedanceRelay)Line L1VT CTRest of the powersystemLine L2Figure 4: Implementation of a distance relay to protect transmission line L13.3Transformer ProtectionEach transformer unit can be protected by a differential relay. The protection principleof this relay is to compare the current inputs at both are high and low voltage sides ofthe transformer. Under normal conditions or external faults (also keeping into consideration of the transformer’s turn ratio), the current entering the protected unit wouldbe approximately equal to that leaving it. In other words, there is no current flow inthe relay under ideal conditions unless there is a fault in the protected unit. Moreover, microprocessor-based relays incorporate other protection functions such as thermaloverload (which tracks the thermal condition of the windings) and over/under frequencyrelays. These two relays work with each other because transformer energy losses tend tobe raised with frequency increases, therefore thermal overload relays are also equipped toprevent the winding insulation damages [6]. Figure 5 shows the connection of a differentialrelay for transformer protection.3.4Load ProtectionElectrical loads are commonly sensitive to the voltage variations which can cause seriousload damages when high voltage fluctuations arise. In that case, loads can be protectedby using over/under voltage relays. Figure 6 shows the connection of over/under voltagerelay for load protection.Table 1 summarizes all the protection schemes that are designed for the primary powersystem components discussed above. The table also states the required inputs for the relay to perform each particular protection function and the output parameters from relayin order to generate a trip command.
Table 1: Protection schemes for common system Under/OverVoltageProtection27/59LOADTRANSMISSION LINEGENERATORRelay TypeTRANSFORMERComponent78Operating PrincipleRelay tracks the impedanceby detecting the variations ofthe voltage/current. Thevariations is small duringnormal conditions however itchanges nearly stepwise inthe case of fault conditions.This means that theimpedance is changedabruptly.Protects the transformerfrom internal faults by takingthe current inputs from bothprimary and secondary sideof the transformer. The sumof these currents (taking intoconsideration transformerturns ratio) is zero undernormal conditions or externalfaults but not equal to zeroin case of fault conditionsA fault in a transmission linewill result in the decrease ofline impedance which iscompared with a pre-definedthreshold value. The tripsignal will be sent to thebreaker if the measuredimpedance is smaller thanthe threshold.A fault in a transmission linewill result in the increase ofcurrent passing through theline which is compared with apre-defined threshold value.The trip signal will be sent tothe breaker if the measuredcurrent exceeds thethreshold.A fault at the load bus willvary the terminal voltage.The measured voltage iscompared with pre-definedthreshold value. The tripsignal will be sent to thebreaker if it is lower/ greatercompare to the threshold.Input ParametersOutput ParametersCurrent and Voltage(V, I)Impedance(Z VI )Currents fromprimary andsecondary side(Iprimary , Isecondary )Current(I)Current and Voltage(V, I)Impedance(Z VI )Current(I)Current(I)Voltage(V )Voltage(V )
TransformerCTCTRest of the powersystem87 TTransformerDifferentialRelayTrip Signal from theTransformer DifferentialRelay (87T) to the CircuitBreaker to protect thetransformer (T2) in case offaultFigure 5: Implementation of differential relay to protect transformerLoadVTRest of the powersystem 27/59 U(Over/UnderVoltage Relay)Trip Signal from the Under/OverVoltage Relay to circuit breaker inorder to disconnect the load underfaulty conditions.Figure 6: Implementation of an over/under voltage relay for load protectionTable 2-5 summarize the different types of protection for system components such asgenerator, transformer, transmission line and motor (load). These tables describe thecauses and effects of various faults which occur frequently in power systems. Moreover,the necessary protection schemes to protect against such faults are also mentioned.In addition, the characteristics of relays such as available measurements, operatingtimes and communication protocols, from different vendors are summarized in Table 6.These relays’ characteristics are obtained from several manufacture product manualsGeneral Electric (GE) [5, 7–10], Schweitzer Engineering Laboratories (SEL) [2, 11–14],Areva-Alstom [6, 15–18], and ABB [19–23].
Table 2: Generator protective relaysImportant Protections for Individual UnitsUnitsType of ProtectionProtection against overloadANSICodes49CausesIncreased power on the generator’s loadEffectStator winding overheatingsideProtection SchemeThermal image relay (keeping track oftemperature) / over current relayGenerator’s full capacity cannot beProtection against unbalanced loadsProtection against reverse powerGENERATORconditionsOut-of-Step protectionProtection against frequencyvariationsProtection against under/overvoltagesProtection against internal faults(differential protection)Stator Earth Fault protection4632788127/598764Sudden loss or connection of heavyloads, or poor distribution of loadsutilized, rise of negative sequenceNegative sequence over current relaycomponents (rotation in reverse(unsymmetrical loads would give rise todirection) leading to heavy currents innegative sequence components)Parallel operation of a generator withthe rotorGenerator behaves as motor and drawsother units may force motor behaviorpower from the network, turbineDirectional power relay with reverse power(due to load unbalance or poor loadconnected to generator will be damagedsetting optionsharing between generators)due to winding overheatingLoss of synchronism due to lineWinding stress, high rotor ironswitching, connection/disconnection ofcurrents, pulsating torques, mechanicalheavy loads, electrical faults, etcresonancesImproper speed control, griddisturbance or sudden load cut offOut of step protection relay which tracks theimpedance calculated from measured voltageand current. In case of fault; there is nearly astep change in voltage/currentSevere speed changes will cause overFrequency protection relay which tracksfluxing , serious damage to the turbinefrequency and trips the breaker in case ofgenerator setabnormal frequenciesSystem disturbance or malfunctioningOver fluxing and winding insulationOver/Under voltage relay with pre-set voltageAVRfailurelimits defined in the settingsInternal faults (phase to phase and 3Gives Rise to Large amount of currentsDifferential protection with CTs on each sidephase to ground faults)that can damage the windingof generator (Unit Protection)Winding insulation failure, inter-turnThermal and magnetic imbalance andIsolated neutral and earth where voltage relayfaultdamage to rotor metallic partsdetect earth faultLoss of synchronism between the rotorLoss of Field protection40Loss of exciter source, open or shortand stator fluxes, draws reactive powerImpedance relay is used to implement thiscircuit at the field windingfrom the grid and provokes severetechniquetorque oscillationsRotor Earth Fault protectionSynchro Check61F25Winding insulation failure, inter-turnThermal and magnetic imbalance andVoltage relay energized by neutral VTfaultdamage to rotor metallic parts(depends on type of neutral connectio
KTH Electric Power Systems Division School of Electrical Engineering Teknikringen 33 SE 100 44 Stockholm Sweden Dr. Luigi Vanfretti Tel.: 46-8 790 6625 luigiv@kth.se www.vanfretti.com
diversity protocols for single-relay scenarios and analyzed their outage performance. Specifically, the authors in [3] proposed fixed and adaptive relaying protocols. Examples of fixed relaying are amplify-and-forward and decode-and-forward relaying. Adaptive relaying protocols comprise selection relaying, in which the relay applies threshold tests
Opportunistic Resource Allocation and Relaying Methods for Quality of Service in the Downlink of . "Relaying in Long Term Evolution: Indoor full frequency reuse," Proceedings of IEEE European Wireless Conference 2009, pp. 298-302, Aalborg, 17-20 May 2009. 2. V. Venkatkumar, T. Haustein and M. Faulkner, "Relaying results for indoor .
Protective Relaying 3 Protective Relaying Architecture 6 Quantifying Protective Relay Performance 9 Protective Relay Configuration and Testing 10 II. PRIOR ART 14 III. SOFTWARE FRAMEWORK IMPLEMENTATION 19 Protection Equipment 19 Protection System Model 22 Automated Protection System Testing 27 Phase Instantaneous Over-current Protection 33
The various protocols and network topologies used for protective relaying purposes are explained. Associated communication standards are outlined. The aim of this part is to provide background on the communication technologies used by protection system. 1 Introduction The IEEE defines protective relays as: "relays whose function is to detect .
Abstract – Directional overcurrent relaying (67) refers to relaying that can use the phase relationship of voltage and current to determine direction to a fault. There are a variety of . (9) Phase to ground and phase to phase faults are more complicated, but have a similar derivation to (9): 1, , -
1 Multipoint-to-Multipoint Protective Relaying Signaling Using Digital Communications and Built-In Logic D. G. Williams1, Member, IEEE, B. M. Dob2, Member, IEEE, and A. G. Morinec3, Senior Member, IEEE Abstract-- A state-of-the-art analog microwave system was installed in the Cleveland area in the 1950s and 60s for pilot protection of 138 kV transmission lines as the transmission system was .
Case Study Protective Relaying over IP/MPLS Myth to Facts Michael A. Nunez, P.E. and James Denman, Lower Colorado River Authority Genardo T. Corpuz, P.E. and Joey B. Melton Jr., Lower Colorado River
OPNET based tools. A set of lab test facilities provide interfaces to commercial IEDs for students to study the characteristics and evaluate the performance. The paper starts with description of newly developed modeling and simulation tools for teaching protective relaying and substation communication concepts.
