Substation Configuration Reliability1Reliability of Substation ConfigurationsDaniel Nack, Iowa State University, 2005IntroductionWhile one of the strongest points in a power system is the electric substation, it still containswhat could be described as weak points or points of failure that would lead to loss of load. Byknowing how to calculate the reliability of different substation configurations, an engineer canuse this information to help design a system with the best overall reliability. But determining thereliability of a substation can also be important for existing installations as it can help locateweak points that may be contributing to overall system unreliability. This paper will present anoverview in determining substation reliability indices and then through the use of an exampleshow how various configurations can be compared.Before embarking on determining reliability, the purpose of the assessment should be clearlyevident as this may affect the choice of which method is used to determine reliability. A methodmay look at how substation reliability affects the overall system reliability, how the systemreliability affects substation reliability , or substation reliability decoupled from the rest of thepower system. Methods may also be better suited to specific types of substations such astransmission and switching, distribution or industrial. Switching and reconfiguration eventstypically will use a more complex method of reliability assessment than those used to look at asingle substation design, This paper will concentrate on determining the reliability of a substationnot including system wide effects.Substation Evaluation Basics adapted from Billington describes what he considers the five essential steps to be carried out when performingsubstation evaluation. While much work on this topic has been carried out since this earlypublication, these steps remain valid and provide a starting point for this discussion on reliabilityevaluation. The steps are listed below followed by a short description of each. The method usedto carry out each of the five steps can vary depending on the chosen reliability assessment. Physical System DescriptionPerformance CriteriaReliability IndicesFailure Mode and Effects EvaluationAccumulation of Failure Effects and SummaryPhysical System DescriptionAn important step when beginning the reliability assessment is to determine the boundaries ofthe system that will be studied. A system study would include not only the substation, but alsothe incoming and outgoing feeders as well as determining the impact the substation has on thesystem and ultimately customer satisfaction. While many of the early reliability studies focusedon transmission and switching substations in isolation from the electrical system, there now are anumber of methods that include the impact of the substation on the system [1,3,4]. More recent
2Substation Configuration Reliabilityworks have extended both the system and decoupled analysis to the distribution system, whichhas its own unique load point reliability indices [8,9].After the boundary has been determined, the next step is to determine in what detail willcomponents be represented. In the simplest case, a two state, up/down model can be used torepresent all components, or if more detail is required higher order models can be utilized. Thedetail needed will be dependant on what type of failure modes being considered. Figure 1 showsMarkov component models of increasing complexity .Figure 1: Component ModelsFinally, the component reliability data must be specified.Performance CriteriaIf any system constraints are needed for the study, they would be added in this step. This wouldinclude items such as transmission line carrying constraints, bus voltages and overloads. Thecriteria specified in this step will vary greatly upon what type of reliability study is being carriedout. A system study may include a large number of operating constraints while an industrialsubstation study may include only a few.Reliability IndicesDuring this step, a level of satisfactory performance must be developed. Billington lists anumber of possibilities ranging from a positive/negative status describing whether or not asystem reaches the operational goal, to a numerical number that describes the “availability of thesystem,” a per-unit time the system meets the reliability goal.Some commonly used substation reliability indices are listed below. Failure rate λ (/yr)Duration (min/yr)Repair time r (hrs)Availability (%)Other indices may be of importance when dealing with a more system wide view or whenconcerned with cost of the loss of load. SAIFI, SAIDI, CAIDI, ASAI (distribution system indices)
Substation Configuration Reliability 3LOLP, LOLE, MELL (load point indices)Failure Modes and Effects EvaluationFor each failure mode, the effects of the failure and what action must be taken to correct thefailure need to be determined. The effect of each failure can then be listed according to thelikelihood of the event. The following steps can provide a framework for gathering the neededinformation from each failure mode.1. Protection system status and resulting breaker action.2. Have breaker actions caused load interruption3. Have any performance criteria violations occurreda. If yes, then determine actions to mitigate violationsi. Transfer possible?ii. Repair required?4. Record all effects by terminal affected, along with the probability of the event and itsduration.Later papers have described failure modes in a somewhat different manner categorizing theminto 4 basic groups [3,4,5] or combinations thereof . Passive failure eventsActive failure eventsStuck-condition of breakersOverlapping failure eventsPassive failure events are component failures that do not activate the protection system such asunknown open circuit conditions or unintentional operation of a circuit breaker.As expected, if a passive failure does not activate the protection system, an active failure is anevent that causes the protection system to operate and isolate a failed component. A simpleexample of an active failure event would be a fault on a bus and the subsequent operation ofbreakers to “seal off” the area from the rest of the station.If during the above fault one of the primary breakers failed to operate and a backup or secondarybreaker had to operate to isolate the faulted area, this would be termed a ‘stuck-condition ofbreakers’ failure mode. The station may still remain in operation, but a larger portion hasbecome inoperable than in the active failure mode.An overlapping failure is when a failure has occurred and before the failure has been fixed,another failure occurs. When carrying out a reliability study, it is common to only look at eventsthat involve two components. According to Allan  the probability of higher order failures isnegligible.A number of methods have been used in determining the final substation indices. The majorityof these methods represent each component as a Markov model, which allows various analyticalmethods to be used to solve for the substation reliability. Another possible method, which will be
4Substation Configuration Reliabilityillustrated by an example presented later in this paper, is the minimal cut-set method based onthe criterion of continuity of service.A downside to using Markov models is that all transition rates must be constant, implying thatthe time spent in a state is exponentially distributed. While this may be true of failure times, therepair times may be considerably different. Billington and Lian have developed a Monte Carloapproach to solving a system with nonMarkovian models. More information on this approachcan be found in .Accumulation of Failure Effects and SummaryThe final step is to list all system failures by the probability of occurrence. This will provide aclear picture of scenarios that will cause the most problems. To find the system reliability (or inthis case, substation reliability), combine the system failure probabilities and frequencies. Eachfailure state is an exclusive state, so the probability of occurrence of system failure is the sum ofall the failure event probabilities. The product of occurrence of failure event and the durationcan be used to determine the probability of the failure state.Substation Configurations PrimerBefore embarking on determining substation reliability indices, it is helpful to be familiar withsome of the common substation layouts and their corresponding names. Certain configurationsmay be more suited to a specific task, so the equipment in each type of substation may vary, butwith the exception of switching stations, they generally will include a transformer, circuitbreakers and isolation switches. This section will give a brief introduction to 6 of the morecommon substation bus configurations followed by a number of advantages/disadvantages . Itwill conclude with a cost comparison of each configuration. Plan and elevation views of eachtype of configuration can be found in the appendix.Typical Bus ConfigurationsSingle BusFigure 2 shows the one-line diagram of a single bus substation configuration. This is thesimplest of the configurations, but is also the least reliable. It can be constructed in either of lowprofile or high-profile arrangement depending on the amount of space available. In thearrangement shown, the circuit must be de-energized to perform breaker maintenance, which canbe overcome by the addition of breaker bypass switches, but this may then disable protectionsystems.Figure 2: Single Bus
5Substation Configuration ReliabilitySingle Bus Advantages: Lowest costSmall land areaEasily expandableSimple in concept and operationRelatively simple for the application of protective relayingSingle Bus Disadvantages: Single bus arrangement has the lowest reliabilityFailure of a circuit breaker or a bus fault causes loss of entire substationMaintenance switching can complicate and disable some of the protection schemes andoverall relay coordinationSectionalized BusFigure 3 shows the layout of a sectionalized bus, which is merely an extension of the single buslayout. The single bus arrangements are now connected together with a center circuit breakerthat may be normally open or closed. Now, in the event of a breaker failure or bus bar fault, theentire station is not shut down. Breaker bypass operation can also be included in thesectionalized bus configuration.Figure 3: Sectionalized BusSectionalized Bus Advantages: Flexible operationIsolation of bus sections for maintenanceLoss of only part of the substation for a breaker failure or bus faultSectionalized Bus Disadvantages: Additional circuit breakers needed for sectionalizing, thus higher costSectionalizing may cause interruption of non-faulted circuitsMain and Transfer BusA main and transfer bus configuration is shown in Figure 4. There are two separate andindependent buses; a main and a transfer. Normally, all circuits, incoming and outgoing, areconnection the main bus. If maintenance or repair is required on a circuit breaker, the associatedcircuit can be then fed and protected from the transfer bus, while the original breaker is isolatedfrom the system.
6Substation Configuration ReliabilityFigure 4: Main and Transfer BusMain and Transfer Bus Advantages: Maintain service and protection during circuit breaker maintenanceReasonable in costFairly small land areaEasily expandableMain and Transfer Bus Disadvantages: Additional circuit breaker needed for bus tieProtection and relaying may become complicatedBus fault causes loss of the entire substationRing BusFigure 5 depicts the layout of a ring bus configuration, which is an extension of the sectionalizedbus. In the ring bus a sectionalizing breaker has been added between the two open bus ends.Now there is a closed loop on the bus with each section separated by a circuit breaker. Thisprovides greater reliability and allows for flexible operation. The ring bus can easily adapted toa breaker-and-a-half scheme, which will be looked at next.Figure 5: Ring BusRing Bus Advantages: Flexible operationHigh reliabilityDouble feed to each circuitNo main busesExpandable to breaker-and-a-half configurationIsolation of bus sections and circuit breakers for maintenance without circuit disruption
7Substation Configuration ReliabilityRing Bus Disadvantages: During fault, splitting of the ring may leave undesirable circuit combinationsEach circuit has to have its own potential source for relayingUsually limited to 4 circuit positions, although larger sizes up to 10 are in service. 6 isusually the maximum terminals for a ring busBreaker-and-a-HalfA breaker-and-a-half configuration has two buses but unlike the main and transfer scheme, bothbusses are energized during normal operation. This configuration is shown in Figure 6. Forevery 2 circuits there are 3 circuit breakers with each circuit sharing a common center breaker.Any breaker can be removed for maintenance without affecting the service on the correspondingexiting feeder, and a fault on either bus can be isolated without interrupting service to theoutgoing lines. If a center breaker should fail, this will cause the loss of 2 circuits, while the lossof an outside breaker would disrupt only one. The breaker-and-a-half scheme is a popular choicewhen upgrading a ring bus to provide more terminals.Figure 6: Breaker-and-a-HalfBreaker-and-a-Half Advantages: Flexible operation and high reliabilityIsolation of either bus without service disruptionIsolation of any breaker for maintenance without service disruptionDouble feed to each circuitBus fault does not interrupt service to any circuitsAll switching is done with circuit breakersBreaker-and-a-Half Disadvantages: One-and-a-half breakers needed for each circuitMore complicated relaying as the center breaker has to act on faults for either of the 2circuits it is associated withEach circuit should have its own potential source for relaying
8Substation Configuration ReliabilityDouble Breaker-Double BusThe final configuration shown is the double breaker – double bus scheme in figure 7. Like thebreaker-and-a-half, the double breaker-double bus configuration has two main buses that areboth normally energized. Here though, each circuit requires two breakers, not one-and-a-half.With the addition of the extra breaker per circuit, any of the breakers can fail and only affect onecircuit. This added reliability comes at the cost of additional breakers, and thus is usually onlyused at large generating stations.Figure 7: Double Breaker-Double BusDouble Breaker-Double Bus Advantages: Flexible operation and very high reliabilityIsolation of either bus, or any breaker without disrupting serviceDouble feed to each circuitNo interruption of service to any circuit from a bus faultLoss of one circuit per breaker failureAll switching with circuit breakersDouble Breaker-Double Bus Disadvantages: Very high cost – 2 breakers per circuitComparison of Bus Configuration CostsTable 1 gives a relative cost comparison of the different substation configurations discussedabove . The comparisons are done with four circuits for each configuration, but do notinclude costs associated with a power transformer. Note that the cost relationships between theconfigurations may change, depending on the number of circuits used and protective relayingdevices that are used.
9Substation Configuration ReliabilityTable 1: Cost Comparison of Substation ConfigurationsConfigurationSingle BusSectionalized BusMain and Transfer BusRing BusBreaker-and-a-HalfDouble Breaker-Double BusRelative Cost Comparison100%122%143%114%158%214%Substation Reliability Comparison Example adapted from The following example will compare the reliability of five different substation configurations asshown in Figure 8. The indices developed for each will be the average failure rate, averageoutage duration, and annual outage time. The components modeled in the example will betransformers, bus bars and breakers. Although the original example included a system studywith distribution indices, only substation indices will developed in this discussion.a.b.c.d.e.Single busSectionalized single busBreaker-and-a-halfDouble breaker-double busRing busTwo lines, either of which can supply the total need ofthe station, feed each configuration. The stationsreliability will be computed ignoring line failure andalso with line failures included.The reliability of each configuration will be evaluatedusing the minimal cut-set method based on thecriterion of continuity of service.A minimal cut-set is a set of components that when allfail, the continuity of service is lost, but if any one ofthe components doesn’t fail, the continuity remains.Figure 8: Studied Substation ConfigurationsThe cut-sets will be categorized according to theirfailure mode and then further divided into active and passive failures. The failure modesconsidered in this example are listed below. First order total failure (both active and passive failures)First order active failureFirst order active failure with stuck condition of circuit breakerSecond order overlapping failure event involving two components
10Substation Configuration ReliabilityWhen the minimal cut-sets have been formed, the reliability indices for each cut-set mode can becalculated. Each minimal cut-set can be represented as a parallel configuration of componentsand the various cut-sets together can be represented as a series configuration [12, 13].To show how the cut-set method would work, the failure rate for the single bus configurationwill be calculated. For this calculation, the incoming lines T1 and T2 will be assumed to have100% reliability. To better recognize the cut-sets, the single bus configuration has been redrawnbelow in Figure 9.Figure 9: Single Bus DiagramThe component reliability data is shown in Table 2. λT is the total failure rate of a componentand λA is the active failure rate of a component.Table 2: Substation Component Reliability DataComponent λT (/yr) λA (/yr) λM (/yr) MTTR MTTM(hours) (hours)Line0.0460.0460.588Transformer 0.0150.0151.015120Breaker0.0060.0041.0496Bus bar0.0010.0010.528The first order total station failure modes are the failures of the high voltage bus and low voltagebus. The failure of either bus interrupts the station continuity.!t 0.001 0.001 0.002(1)The first order active failure modes are B1, B2, B3 and B4. To illustrate this, consider a faultoccurs on L1 and breaker B1 fails to open, breaker B2 will operate, thus breaking stationcontinuity. Similar scenarios can show that B2-B4 are also active failure modes.!a 0.004 0.004 0.004 0.004 0.016(2)The first order active failure plus stuck breakers (p 1) are T1 B3 stuck and T2 B4 stuck.!s 0.015 0.015 0.030(3)
Substation Configuration Reliability11The total failures overlapping total failures are B1 B2, B3 B4, B3 T2, B4 T1 and T1 T2. Theparallel failure rate of each paralleled group of two components can be calculated as follows.!1e" !1 !2 e" !2 " (!1 !2 )e"( !1 !2 )e" !1 e" !2 " e"( !1 !2 )! B1 B2 7.1357 #10 "5!p ! B 3 B 4 7.1357 #10 "5! B 3 T 2 1.772 #10(4)"4! B 4 T 1 1.772 #10 "4!T 1 T 2 4.401#10 "4The total failure rate is then the sum of the paralleled rates.!o 2 " ( 7.1357 "10 #5 ) 2 " (1.772 "10 #4 ) 4.401"10 #4 9.372 "10 #4(5)The overall substation failure rate is then the sum of failure rates for each failure mode.! !t !a !s !o 0.0489(6)Annual outage time for the substation configuration can be found in a similar manner along withthe average outage duration.U U t U a U s U o 3.53Ur 72.15!(7-8)Complete reliability indices for the five substation configurations with 100% reliable source linesare listed in Table 3.Table 3: Substation Reliability Indices (Ignoring Line Failure)Configuration λ (/yr) r (min) U (min/yr)a0.048972.153.53b0.045371.953.26c0.00301 184.560.56d0.00567 124.2160.70e0.017481.881.42For comparison, the indices with the impact of source line failures is shown in Table 4. Noticethat trends seen in Table 3 are also seen in Table 4. Failure rates increase between 0.9% and35% and U increases from between 2.8% and 53%. This show what effect source line failurescan have on the substation indices, but it does not change the relationships between theconfigurations. Configuration ‘c’ is still the most reliable scheme and ‘a’ remains the worst.
Substation Configuration Reliability12Table 4: Substation Reliability Indices (Including Line Failures)Configuration λ (/yr) r (min) U (min/yr)a0.054980.504.42b0.045976.353.50c0.00356 175.760.63d0.00572 125.140.72e0.023592.202.17These results could then be fed into a composite system, which would then ultimately lead toload point 11]W. Li, “Risk Assessment of Power Systems,” IEEE Press, 2005.R. Billinton, “Power-System Reliability Calculations,” Massachusetts Institute of Technology, 1973.R.N. Allan and J.R. Ochoa, “Modeling and Assessment of Station Originated Outages for Composite SystemsReliability Evaluation,” IEEE Transactions on Power Systems, Vol. 3, No. 1, February 1988.R.N. Allan, “Effects of Protection Systems Operation and Failures in Composite System ReliabilityEvaluation,” International Journal of Electrical Power & Energy Systems, Vol. 10, No. 3, July 1988.J.J. Meeuwsen and W.L. Kling, “Substation Reliability Evaluation including Switching Actions withRedundant Components,” IEEE Transactions on Power Delivery, Vol. 12, No. 4, October 1997.D. Koval, “Substation Reliability Simulation Model,”“Design Guide for Rural Substations Design Guide for Rural Substations,” Rural Utilities Service, UnitedStates Department of Agriculture, June 2001.R.E. Brown and T.M. Taylor, “Modeling the Impact of Substations on Distribution Reliability,” IEEETransactions on Power Systems, Vol. 14, No. 1, February 1999.B. Retterath, A.A. Chowdury and S.S. Venkata, “Decoupled Substation Reliability Assessment,” 8thInternational Conference on Probabilistic Methods Applied to Power Systems, Iowa State University,September 2004.R. Billington and G. Lian, “Monte Carlo Approach to Substation Reliability Evaluation,” IEE Proceedings-C,Vol. 140, No. 2, March 1993.T. Tsao and H. Chang, “Composite Reliability Evaluation Model for Different Types of Distribution Systems,”IEEE Transaction on Power Systems, Vol. 18, No. 2, May 2003.J. McCalley, “Analysis of Series/Parallel Systems Comprised of Non-Repairable Components,” Power LearnElectric Power Engineering Education, Module PE.PAS.U14.5, 2005.J. McCalley, “Analysis of Non Series/Parallel Systems of Non-Repairable Components,” Power Learn ElectricPower Engineering Education, Module PE.PAS.U15.5, 2005.
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Bus fault causes loss of the entire substation Ring Bus Figure 5 depicts the layout of a ring bus configuration, which is an extension of the sectionalized bus. In the ring bus a sectionalizing breaker has been added between the two open bus ends. Now there is a closed loop on the bus with each section separated by a circuit breaker. This