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600MW发变组保护的配置与整定
路亮亮
600
MW
发变组
保护
配置
亮亮
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600MW发变组保护的配置与整定 路亮亮,600MW发变组保护的配置与整定,路亮亮,600,MW,发变组,保护,配置,亮亮
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华北电力大学科技学院毕 业 设 计（论 文）开 题 报 告 学生姓名： 路亮亮 班级： 电气07K7 所在系别：电力工程系 所在专业：电气工程及其自动化 设计（论文）题目： 600MW发变组保护的配置与整定 指导教师： 杨明玉 2011年 3 月 6 日毕 业 设 计（论 文）开 题 报 告一、结合毕业设计（论文）课题情况，根据所查阅的文献资料，每人撰写不低于2000字的文献综述。（另附）二、本课题要研究或解决的问题和拟采用的研究手段（途径）：一、本课题要研究或解决的问题：1、 完成所给发变组线路图中的相关的短路计算。2、 根据所给的发变组的参数结合相关原则和原理合理的制定出能够反应各种故障的继电保护装置的配置方案。3、 根据大型发变组保护整定计算的相关原则，进行整定计算和校验等工作。二、拟采用的研究手段：结合发变组保护的基本原理和继电保护及安全自动装置技术规程选取采用RCS-985微机发电机变压器组成套保护装置，学习大型发电机变压器继电保护整定计算导则进行相关短路计算及装置的整定计算和校验。三、指导教师意见：1 对“文献综述”的评语： 2对学生前期工作情况的评价（包括确定的研究方法、手段是否合理等方面）：指导教师： 年 月 日S Q.J. SEPTEMBER 1968The Electrical Protection ofGenerators and TransformersJ. H. SYKES, B.SC. (ASSOCIATE MEMBER)THE object of this paper is to discuss the electricalprotection of generator and transformer units withoutputs greater than 10 MW. At these output levelsIt can be assumed that the generator will bedirectly coupled to the transformer without anintervening circuit-breaker.An essential requirement of the protection is thatit must be able to discriminate between faults onthe equipment it is protecting and faults onadjacent equipment which other relays are protect-ing. In the former case the protection must operateto isolate the equipment, in the latter case it mustremain stable unless it is to act as back-up protec-tion for the relays on the adjacent equipment inwhich case it must allow a discriminating periodbefore operating.Fig. 1 shows a comprehensive generator-transformer protection scheme comprisingdifferential, earth fault, negative phase sequenceand overcurrent protection.The protection can be divided into two broadcategories:(a) Equipment for the detection of insulationfailures.Equipment for the detection of abnormalrunning conditions.Insulation failures usually lead to earth faults,phase-to-phase faults and inter-turn faults,abnormal running conditions include loss of field,loss of load, overload and unbalanced loading.Insulation FailurePhase-to-Phase FaultsPhase-to-phase faults within the generator andtransformer produce substantial fault currentlimited mainly by arc resistance. However, thistype of fault is very rare since the construction ofthese units does not generally allow close proximityof phase conductors. Where this does occur theinsulation is twice the thickness of phase to earthinsulation.Phase faults within the generator can beadequately detected by circulating current differ-ential protection. Two basic. schemes are used.The first employs a high impedance relay connectedas shown in Fig. 2. For stability under maximumthrough fault conditions the relay is given a settinggreater than that which could be produced underthe worst through fault condition. To determinethe worst condition it is assumed that one currenttransformer produces full output whilst the other isfully saturated and can therefore be replacedfor calculation purposes by the current trans-former internal resistance. The voltage producedat the relay can then be calculated fromh (Rti + 2-RH;). The overall setting of the protec-tion is determined by summating the currenttransformer magnetizing currents at the relayvoltage setting, and the relay current and multi-plying these by the current transformer ratio togive the setting in terms of primary current.The relay setting current determines the percent-age of generator winding that can be protectedFig.1. Typical overall protection of generatortransformer unitsince a fault at the neutral point produces zerocurrent, increasing to a maximum for a fault onthe generator terminals.When the lead resistance is excessive the relay,setting may be so high as to cause some embarrass-ment in the provision of suitable current trans-Mr. Sykcs, 26 and married, followed a 1-3-1 course atA.E.I. Manchester and Leeds University, on the heavy currentside. He spent a short time as a development engineer and is nowan Applications Engineer with G.E.C.-A.E.I. Switchgear Ltd. intheir Power Protection and Meter Section. His hobbies includemotor sport and photography.* Abstract of a paper read before the North MidlandSection on 8th February, 1966.S.Q.J. SEPTEMBER 1968formers so a biased differential relay is used. Theprinciple involved is that additional relay coils areconnected in the circulating current path andarc arranged to produce a force in the oppositedirection to that produced in the relay operatingcoils. Fig. 3 shows the principle of operation.a) Basic Differential Protectscoupled with equipment for current trans-former tap changing is a practical impossi-bility.(2) as the primary and secondary current trans-formers are of different ratio the character-istics are likely to differ fairly widely.The effect of both the above tends to result indiffering outputs from the two sets of currenttransformers and hence tends to drive currentthrough the relay operating coil under normal loadand through fault conditions. To ensure stabilitythe relay is desensitized by increasing the effectivebias, e.g. the bias for transformer protection is20/40%, whereas for generator protection it is5%.Overall differential protection will detect bothb ) Circuit showing current flow duringthrough fault condition!For ttcbility through fault conditionsrelay setting voltoge If(RcT4 + 2RW)Primary operating current = (2 lu + I,)x C.T Ratiolp= magnetising current of CXat relay setting vollcgeIn relay opercling currentat setting voltageFig. 2. Differential protectionIt can be seen that under through fault conditionsOperate force ii/, is smallBias force ii+i is largeresulting in non-operation.Under internal fault conditionsOperate force it+it is largeBias force iiit is smallresulting in operation.This scheme approaches the ideal in that it hashigh stability to through faults and a highsensitivity to internal faults.The overall protection of the generator andtransformer is essentially a biased scheme, for tworeasons:(l)on large units on-load tap changing isusually incorporated in the transformer.The provision of taps on the current trans-former to match the transformer turns ratio,Through FaultInternal FaultFig. 3. Biased differential relayphase and earth faults and although for phasefaults practically the whole of the winding isprotected under earth fault conditions the protec-tion is not so effective as the fault current islimited to 300 amps by the generator neutralresistor. In view of this it is usual to providefurther relays for earth fault protection.Generator Earth Fault ProtectionTo detect earth faults over practically all thegenerator winding a sensitive relay with a 5%setting is used in conjunction with a 300/1Aneutral current transformer. This will detect faultcurrents as low as 15A which is considered to beless than the current which could cause damage to5.Q.J. SEPTEMBER 1968the stator core. The sensitivity of this relay is sojow that it could be affected by surges or faultson the external system resulting in slight neutraldisplacement. For this reason the relay has anI.D.M.T. (inverse with definite minimum time)characteristic. As back-up to this relay and thedifferential protection an instantaneous earth faultrelay with a 10% setting is also included and thisprotection is restricted to the generator andtransformer primary winding by virtue of thetransformer delta winding.Transformer Restricted Earth-Fault ProtectionIn generator earth fault protection it wasexplained that better earth fault protection wasobtained by using a sensitive relay in addition tothe differential protection. This is also true forsecondary winding protection of the transformerbut for a different reason. In this case there is noFieldCircuit BreakerFig. 4. Rotor earth fault detectorneutral resistor to limit the earth fault current butthe differential protection is less sensitive.The circulating current scheme with highimpedance relay is used. Current transformers inthe H.V. line are paralleled and circulate theircombined secondary current with one currenttransformer in the neutral. Operation of the relayis restricted to faults within the star winding of thetransformer and the main connection up to thecurrent transformer location.Rotor Earth Fault ProtectionModern practice is to operate a generator withits field winding isolated from earth and thereforea single fault between field winding and rotorbody due to insulation breakdown can be tolerated.It is important, however, to know of the existanceof such a fault so that the generator may be takenout of service at leisure, since the incidence of asecond fault will short-circuit some part of the fieldwinding, resulting in asymmetry of the air-gapfluxes. This can cause severe vibration of therotor with possible damage to the bearings.Fig. 4 shows a modern method of rotor earth-fault detection. The field is biased by a d.c. voltagewhich causes current to flow through the relayfor an earth fault anywhere on the field system.Transformer Inter-turn ProtectionThe use of surge divertors on overhead linesminimizes the effect of surges on the transformerwindings, but even so, there is a risk of breakdownbetween adjacent turns at the end of the winding.This gives rise to a high local current, but a smallterminal current because of the high ratio oftransformation between the whole winding and theshorted turns.The only method of detecting such a fault at anearly stage is to use a Buchholz relay. Fig. 5 showsthe arrangement of the relay which is located in theoil circuit between the transformer tank and theconservator.The heat produced by the high local currentcauses the transformer oil to decompose andproduce a gas. This is trapped in the Buchholzrelay chamber and as the pressure builds up the oillevel is depressed and the float contacts close.A heavy fault gives rise to an explosive genera-tion of gas, a heavy oil surge passes up the oilcircuit towards the conservator tank and displacesVoncAlarm circuit -raTrip Circuit-Fig. 5. Sectional view of Buchholz relaythe relay vane. The contacts on the vane areusually arranged to energize the tripping circuit,whereas the float contacts arc arranged to give analarm.Abnormal RunningUnbalanced Stator CurrentsUnbalanced currents in a three-phase powersystem occur due to single-phase faults, certaintypes of loading, open circuits due to broken linesor pulled jumper joints, or due to the failure of onepole of a circuit breaker to close. Although shortcircuits will normally be cleared by circuit protec-tion, uncleared faults and cases of unbalancedloading arising from one of the foregoing,occasionally cause the unbalanced current condi-tion to be maintained on generating plant.A system of unbalanced phase currents can beresolved mathematically into three componentsystems of balanced currents having respectivelypositive phase sequence, negative phase sequenceS.Q.J. SEPTEMBER 1968and zero phase sequence. Most unbalancedsystems of currents that are likely to occur willcontain positive and negative phase sequencecomponents, zero phase sequence may or may notbe present.The positive sequence component is similarto normal balanced loading and hence producesno abnormal reactions within the machine.The negative sequence component system willproduce an armature reaction field which willrotate in a direction opposite to that of the rotor, and hence will produce a flux which sweepsthrough the rotor with twice the rotational speed.Hence spurious currents of twice the machinefrequency will be induced in the rotor body, in damper windings, and in the field windings. Thesespurious currents give rise to heating which can beexcessive if the degree of unbalanced loading isappreciable.The amount of negative sequence current whichcan be carried by a generator naturally depends+ xFig. 6. Loss of excitation characteristicupon the design, but is usually well below ratedcurrent. In the case of high-speed turbo-generators,the continuous current which can be carried isusually between 10-15% of the positive sequencecontinuous rating.The negative sequence heating follows a normalresistance law and hence is proportional to thesquare of the current. The heating time constantof the machine is largely a function of the coolingsystem employed. This is expressed by designers bya rating equation.1 t = KWhere 72 is the negative sequence currentexpressed on a per unit basis of continuousmaximum rating (C.M.R.), / is the current durationin seconds, and A T is a constant which for turbo-type machines will usually have a value between3 and 20.The problem of protecting against this conditionlies in obtaining a relay characteristic which willaccurately follow this heating characteristic. Theusual arrangement is an inverse with definiteminimum time delay relay connected to a networkwhich segregates the negative phase sequencecurrent from the positive and 2ero sequencecurrents. The relay has a long operating time andhas a range of settings to allow its characteristic tobe accurately matched to those of the machine.Loss of ExcitationWhen a generator loses its field excitation itspeeds up slightly and acts as an induction genera-tor deriving excitation from the system andsupplying power at a leading power factor.If the system to which the generator is coupledis unable to supply the excitation power of theGenerator OverallDifferentialGeierctor TransformerOverall DifferentialStator Earth FaultInstantaneousSloior Earth FaultInverse TimeTransformer H.VRestricted Earth FaultTransformerBuchhoU SurgeHV OvercurrentTurbineStopUnit TransformerOverall DifferentialUnit Transformer LVRestricted Earth FaultUnit Transformer HVOvercurrentUnit TransformerBuchholz SurgeNegative PhosiSequenceLo.s of Exitotionwndtnq TemperatureGeneratorFieldSwitchEciterFieldSwitchUnitTransformLV C!CircutDreaherFig. 7. Protection tripping arrangementmachine then a fall in system voltage will occurand system stability may be upset. There is also thepossibility of overheating of the rotor due toinduced slip frequency currents in the rotor anddamper windings.Loss of excitation can be detected by measuringthe reactive component of stator current; anexcessive value of VAR import indicates eitheractual or prospective loss of synchronism. Toallow for system transients which may cause amomentary reversal of VAR component it isusual to incorporate a fixed time delay of betweenone and five seconds in the tripping sequence of therelay.An alternative solution is to apply an offsetimpedance or mho measuring relay at the generatorS.Q.J. SEPTEMBER 1968terminals. Its operating characteristic is arrangedas shown in Fig. 6 so that during conditions ofextremely low excitation or complete loss ofexcitation the equivalent generator impedance fallswithin the tripping zone.Over cur rent ProtectionAlthough this is self-explanatory and a I.D.M.T.relay is used the application of overcurrentprotection to a generator is not straight forward.As this relay discriminates by time it must bearranged to discriminate with the slowest relayon the system which the generator is feeding. Theslowest relay is usually an I.D.M.T. relay on theoutgoing feeder from the station busbars. It is inthat position because time graded protection relaysmust have higher time settings as they approachthe source and although it not may be the primaryprotection it must be considered.The significance of this slowness of operationwill be apparent when the performance of agenerator under fault conditions is considered.Upon the inception of a fault the generator currentincreases instantaneously to above 10 x full loadcurrent and then decreases rapidly to about 80%full load so that after two or three seconds thegenerator current is less than the full load value.As we are dealing with the slowest relay on thesystem it is unlikely that the relay will operate whenthe fault current is above its setting level. Clearlyan overcurrent relay cannot be set to less than thefull load value and we are therefore faced with thesituation where a normal I.D.M.T. relay would notoperate.To overcome this problem a voltage-restrainedovercurrent relay is used. This is an I.D.M.T.relay with a voltage-energized restraining winding.At normal voltage it has a setting of 125% F.L.which will enable discrimination to be achievedwith other overcurrent relays. For faults close tothe relaying point when the system voltage, andhence the restraint, is reduced the relay setting isreduced. With zero voltage the relay has a settingof 25%.Relay Tripping FunctionsThe tripping function of each relay is shown indiagrammatic form in Fig. 7. Most relays arearranged to energize the main circuit breaker andfield breaker trip coils directly to provide rapidfault clearance and minimize fault damage.
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