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IEEE TRANSACTIONS ON POWER SYSTEMS 1

Probabilistic Evaluation of Substation CriticalityBased on Static and Dynamic System Performances

Armando M. Leite da Silva, Fellow, IEEE, Airton Violin, Cludio Ferreira, and Zulmar S. Machado, Jr.

AbstractIn most practical cases, traditional methods forassessing the reliability of substations, separately from the electricpower network, are not sufficient to a good understanding ofthe system performance and possible reinforcements. This paperpresents a new methodology to evaluate the criticality of electricalsubstations taking into account their possible operating states, as-sociate probabilities, and consequences on the static and dynamicperformance of the electric power system. Two complementaryreliability indices are proposed to express the dynamic securityand static adequacy levels that a substation provides to the powergrid. An application in a test system is presented and the substa-tion criticality ranking is provided and discussed.

Index TermsAdequacy and security analyses, loss of stabilityprobability, power system reliability, probabilistic transient sta-bility, substation ranking, substation reliability assessment.

I. INTRODUCTION

I N general, there are several substations in a bulk electricpower system and they represent a relevant sector of the in-frastructure of any country. Critical substations are those that arevital to keep the integrity of the power grid. They are strategicnodes such that if affected by natural events or even terrorist ac-tions will cause a large disturbance in the system, possibly cre-ating cascading effects all over the network. Natural events in-clude storms, tornados, etc. and their consequences are electricfaults, line disruptions, etc. Following these events, loads aredisconnected, intentionally or not, endangering the security ofthe system operation. Large amounts of load interruption, char-acterizing blackouts, certainly impose huge negative impacts onthe countrys economy and, consequently, on its internationalvisibility.Unexpected changes of the network topology, due to simul-

taneous outages in substations, are usually not properly evalu-ated in traditional system studies, such as power flow and tran-sient stability analyses. The node representing the substationis considered robust and totally reliable. Although the subjectsubstation reliability and its basic concepts have already beendiscussed in relevant academic books [1][6], in practice, thistheme has received little attention. Only when a blackout oc-curs, discussions on this subject emerge, but ending soon, to be

Manuscript received August 01, 2013; revised October 28, 2013; acceptedNovember 26, 2013. This work was supported by the Brazilian agencies CNPqand CAPES, from the Ministries of Science and Technology (MCT) and Edu-cation (MEC), respectively, Brazil. Paper no. TPWRS-00994-2013.The authors are with the Institute of Electric Systems and Energy, Federal

University of ItajubUNIFEI, MG, Brazil.Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TPWRS.2013.2293626

re-discussed after the next blackout. Moreover, the definition ofminimum requirements by some independent system operators(ISO) almost standardizes the arrangements of new substationsto be installed in the grid, creating a sense of security, whichmay be apparent and, sometimes, ineffective from the technicaland economical point of view.An analysis of the technical literature of the last forty years

shows initiatives to improve the methodology for reliabilityassessment of large substations, inserted into meshed powersystems [7][19]. These initiatives can be classified into twogroups. In the first group, the reliability of substation andswitching station arrangements are separately evaluated fromthe network and its possible operating conditions: see, for in-stance, [2], [4], [6], [7], [12], and [19]. In the second group, theimpact of station originated outages on the network reliability isassessed considering different objectives [6], [8][18]. In theseworks, the most frequent goals are the generation and transmis-sion reliability itself, the impact of the protection system on thesystem reliability, and the selection of substation configurationbased on estimate costs. Also, a variety of models and toolswere employed: cut-sets, event and fault trees, Markov, stateenumeration, and Monte Carlo simulation.Two relevant aspects can be identified from the past initia-

tives: 1) all analyses involved static network assessment toolslike load flow and optimization; and 2) the criticality of thenode/substation within the network was not the main issue. Toeffectively assess the criticality of substations, both dynamic(i.e., transient stability) and static (i.e., load flow) analyses haveto be performed to capture the proper weakness of the systemnode. There are also initiatives [20], [21] to integrate adequacyand security in composite reliability assessment, but the sub-station reliability has not been considered. Proposals in proba-bilistic transient stability [5], [22] and transmission equipmentsecurity ranking [23] are also restricted evaluations. In [17],some metrics based on concepts from the Spectral Graph theoryare proposed. Although a very innovative work, some electricalaspects (e.g., substation arrangement, protection system, dis-patching function, etc.) and reliability concepts were ignored,with more emphasis being given to the network topology.This paper proposes a new methodology to evaluate the crit-

icality of substations in a power system based on the static anddynamic consequences of their equipment outages in the net-work. Markov models are used to represent all possible statesconsidering the substation configuration, including all externalbranches (i.e., transmission lines, etc.) connected to it. The mainsubstation equipment (i.e., breakers, switches, buses, and ter-minals) are properly modeled. Terminals include connectionsfor lines, transformers, shunt elements, and loads. All transition

0885-8950 2013 IEEE


2 IEEE TRANSACTIONS ON POWER SYSTEMS

rates among these equipment operating states are properly con-sidered. Finally, the dynamic and static consequences of theseevents on the entire network are analyzed through transient sta-bility, load flow, and optimal power flow (OPF) algorithms. Twocomplementary risk indices are proposed to express the securityand adequacy levels that a substation provides to the power grid.They are the loss of stability probability (LOSP) and the tradi-tional expected energy not supplied (EENS), re-interpreted forthe specific problem being treated. An example of application inthe Brazilian Birds test system (BBTS) [24] is presented and theranking with the most critical substation considering adequacyand security aspects is provided and discussed.

II. PROPOSED METHODOLOGY

The traditional definition of reliability is present throughoutthe electric utility industry, including NERC (North AmericanElectric Reliability Corporation: http://www.nerc.com), andconsists of two fundamental concepts: security and adequacy,i.e., Security (also named operating reliability by NERC) isthe ability of the electric system to withstand sudden distur-bances such as electric short circuits or unanticipated lossof system components;

Adequacy is the ability of the electric system to supplythe aggregate electric power and energy requirements ofthe electricity consumers at all times, taking into accountscheduled and reasonably expected unscheduled outagesof system.

These two concepts can be extended to cope the with networknodes, where all substation fault states, their probabilities, andquantification of the consequences of these failures on the dy-namic and static behavior of system can be properly measured.Thus, two indicators are obtained, one for the security offeredby the substation to the network (i.e., node robustness in viewof system stability) and the other related to the adequacy (i.e.,node robustness in view of keeping the connected loads).To achieve the previous goals, dynamic (transient stability

analysis) and static (power flow and OPF analyses) simulationsare separately performed with the use of traditional programs.It is also important to note that the proposed methodology is ap-plicable to the planning stages, or studies of reinforcements inexisting substations, when assessing the reliability of the substa-tion regarding the system network. The stochastic behavior ofsubstation equipment and connected components are assumedto follow Markovian models, and typical simplifications areconsidered when performing dynamic and static simulations inthe planning phase. The basic models and steps of the method-ology are briefly described as follows.

A. Simulation of Failures in Substations

By using Markovian-based techniques and state space di-agrams, the probabilities of all failure states of a substationcan be properly identified and evaluated [1][6]. These statesand corresponding probabilities are obtained for two stages: 1)post-fault condition, due to the occurrence of active failures thatcan lead to simultaneous circuit outages, turned on by the pro-tection system; and 2) in post-switching condition, when the

Fig. 1. Three-state component model.

Fig. 2. Four-state component model.

Fig. 3. Five-state component model.

component is isolated for repair. Usually, the following failuremodes are considered: Single contingencies (failures and scheduled mainte-nance);

Stuck-breaker occurrences for transmission componentfailures;

Double contingencies (the most critical).The following assumptions were adopted to obtain the failure

states of a substation: Double contingencies involving active faults are ne-glected;

Normally open components are not subject to failures; No component is put into maintenance if there is anotherone being repaired at the substation.

Three component models, illustrated by the diagrams ofFigs. 13, are used to represent switches and lines (Fig. 1),buses and transformers (Fig. 2), and circuit breakers (Fig. 3).These models represent a realistic conciliation betweenmodeling sophistication and data availability. Clearly, somediagrams can become more or less complex [2], [3], dependingon the type of study and data availability [25][28].In the state space diagram shown in Fig. 1, State 1 (with

steady-state probability ) represents the normal operation be-fore the fault. State 2 (with probability ), describes the com-ponent condition after the fault, but before the isolation. Finally,State 3 (with probability ) represents the state condition afterisolation, but before repair is completed. The transition rates inthis diagram are: is the active fault rate in failures per year


DA SILVA et al.: PROBABILISTIC EVALUATION OF SUBSTATION CRITICALITY BASED ON STATIC AND DYNAMIC SYSTEM PERFORMANCES 3

(f/yr); is switching rate, whose inverse, , is the averageswitching time, usually, in hours; is the repair rate whose in-verse, , is the average repair time in hours.Fig. 2 shows a four-state component model used to cope with

the scheduled maintenance. Thus, and are the transitionrates to go in and out from the maintenance state, respectively.Rate is in maintenances per year (m/yr) and the inverse ofrate represents the average maintenance time, , usuallygiven in hours.In order to model the circuit breakers, a fifth state is included

to represent stuck-breaker conditions, as illustrated in Fig. 3. Inthis work, three failure transitions are considered from normaloperating State 1: 1) transition to State 2, with failure rate ,representing the occurrence of an active failure with the pro-tection system response; 2) direct transition to State 3 (repair),without acting the protection system, with passive rate ; and3) transition to State 5, with rate , to represent the stuck-breaker condition.Transition rate is not easily obtained, since it depends on

detailed database that includes protection system failures. Usu-ally, the technical literature provides the stuck-breaker proba-bility, i.e., value in the diagram of Fig. 3. In this model, tran-sition rate is usually known, most frequently equal to , or

. Based on Markovian steady-state conditions, the tran-sition rate can be evaluated as

(1)

Once the rate is evaluated, all stationary probabilities asso-ciated with the circuit breaker model can be calculated.Equations (2) and (3) are then used to calculate the proba-

bilities associated with all states of interest for a given networksubstation, i.e.,

(2)

(3)

In (2), the single contingency (SC) probability, , asso-ciated to substation state is obtained as the product of prob-ability , which corresponds to a not normal (NN) stateassociated to component and the product over all probabili-ties associated to components , in normal (N) operatingstates. The NN state is characterized by the occurrence of anactive fault (post-fault) or when the component is already iso-lated for repair (post-switching) or on maintenance. This evalu-ation process is analytical (i.e., enumeration), covering all com-ponents ( to ) of a substation.In (3), a similar evaluation process is used to assess the

double contingency (DC) probability, , associated tosubstation state . It is obtained as the product of probabil-ities , and the product over all probabilitiesassociated to components , in normal states.It is important to mention that all substation states generated

by combining equipment states (N or NN types) are assumed to

be independent. However, the re-configurations of the substa-tion due to switching actions are properly considered during thiscombination process. Moreover, the failure modes and adoptedassumptions reduce the state space of interest, without loss ofaccuracy of major consequences.

B. Dynamic SimulationsFor failure states in the post-fault condition (where there is a

sudden change of the network topology due to multiple circuitoutages), a standard transient stability analysis program is usedto assess the dynamic performance of the system. The aim is toverify the behavior of the system during the transient process, byevaluating the rotor angle stability. A failed substation state isconsidered secure for the system if survives the transient phase,i.e., if the synchronism among systemmachines is kept, with thepurpose of reaching a new operating point after the disturbance.Usually, a single-phase short circuit in the vicinity of the substa-tion and also inside is considered, followed by the removal ofthe faulted equipment within the time required by the protectionsystem.The criteria used to define if the system is secure encompass:

1) the machine angles, in relation to the center of inertia of thesystem, should not exceed a specified value (e.g., 360 ) that in-dicates loss of synchronization; 2) themachine frequenciesmustnot be changed more than 5% compared to the system nominalvalue; and 3) the system response must be damped. Exceedingone of these limits classifies a fault event as potentially inse-cure for the system. In general, the simulations are carried outwithout considering special protection schemes, security controlschemes or even regional schemes for load shedding. Clearly,these schemes can be included in the studies according to theprocedures established by the ISOs and regulatory agencies.However, studies that define the substation configuration, or theevolving of its bus arrangements, should be conducted with acertain degree of conservativeness, so that the substation mayserve the system with robustness.

C. Static SimulationsFor failure states in the post-fault and post-switching condi-

tions (where the component is isolated for repairs), simulationsare also carried out based on AC power flow and OPF programsto assess the system static performance. These simulations areperformed to establish the new steady-state operating point, and,if necessary, to evaluate the amount of load shedding to ensureequipment capacity limits. The OPF program uses an AC non-linear formulation based on interior point and has several con-trols available. The details related to the mathematical formula-tion and solution of the OPF program can be found in [29][31].In the present work, however, only reactive power and voltagelimits in PV buses were activated. Thus, re-dispatching of ac-tive power to alleviate equipment overloads is not considered, inorder to be consistent with the assumptions usually accepted bysystem engineers at this phase of the planning process. Althoughthis remedial action could be easily turned on in this process,it does not concur with the degree of conservativeness gener-ally used by planners in these studies. Therefore, if overloadedequipment is detected, only minimum load shedding criterion isused to solve the problem.



In order to evaluate the amount of load shedding, the OPFprogram accounts for both constraints: branch capacities andbus voltage limits. In certain failure events, load curtailmentsmay occur due to the disconnection of substation elements (i.e.,islands), and these situations are also accounted for in total loadshedding. It is noteworthy that there are no capacity limits forflows in internal branches of the substation. This assumption isvalid since, in most cases, the switchyard equipment has highrated current values. Moreover, the connections of elements aredistributed along the substation as the system expands, thusmin-imizing the risk of local overloads.

D. Reliability Indices

Reliability indices for ranking substation criticality are ob-tained based on (4) and (5). Equation (4) defines the index lossof stability probability (LOSP), i.e.,

(4)

where is the probability of occurring an insecure state , as-sessed by the dynamic simulation, and the subset of all stateslike this. Index LOSP reflects the degree of insecurity that thesubstation offers to system from the dynamic behavior point ofview; i.e., loss of synchronization during the transient process,due to failures at the substation. This new measure is the sum ofthe probabilities of all failure states substation classified as po-tentially unsafe for the electrical system. The higher this valueis, the greater the risk to the safety of the electrical system willbe. Values above a given benchmark can justify investments insubstations to minimize these risks.Equation (5) is the expected energy not supplied (EENS)

index, i.e.,

(5)

where is the probability of occurring an inadequate state ,assessed by static evaluations, is the corresponding loadcurtailment, and the subset of all states like this. This indexreflects the degree of inadequacy that the substation offers tothe system from the static point of view. This indicator is thetraditional weighted average of the amounts of load sheddingoccurring in the electric system for the various failure statesof the substation under analysis. The higher this value is, theworse the risk of not meeting the system loads. Similarly to theprevious case, values above a given level can justify investmentsin substations to minimize these risks.Both indices are assessed to measure the performance of a

specific node; i.e., substation configuration and its protectionequipment. Notice that, when evaluating a particular substation,including its connections, all other nodes and lines are assumedto be completely reliable. Thus, these indices represent condi-tioned expectations for ranking substation criticality.

E. Algorithm

The previous concepts are combined into an algorithm to de-termine the proposed reliability indices LOSP and EENS. Themain steps are described as follows:1) Based on the component models and failure modes de-scribed in Section II-A, the corresponding probabilities arecalculated for all selected states of a given substation;

2) Considering the post-fault and post-switching conditions,the obtained substation states are grouped according totheir terminal status (connected or disconnected)eachgroup represents the same network configuration;

3) Based on a truncation criterion, a list of grouped statesis defined for analyzing their dynamic and static perfor-mances;

4) For the states grouped in the list as post-fault condition,transient stability simulations are carried out, reproducingthe role of protection and classifying the system conditionas: stable or unstable;

5) For all grouped states in the list, static simulations are alsocarried through AC power flow and OPF programs to eval-uate possible load curtailments;

6) If all grouped states for the given substation are verified,both reliability indices LOSP and EENS are calculated;

7) Proceeding in this way for all substations of the network,the ranking substation criticality is established.

The previous algorithm characterizes a state enumerationprocess for assessing the substation criticality through staticand dynamic evaluations. The truncation criterion is based onthe assumptions described in Section II-A, where the mostcritical double contingencies are included and third and higherorder contingencies are neglected. The criticality of doublecontingencies is established by the engineer knowledge orsimply by a truncation probability.

III. APPLICATION

To demonstrate the application of the methodology, a testsystem is presented in which a substation has been chosen toexemplify the procedure for obtaining the proposed indices.

A. Brazilian Birds Test System

Fig. 4 shows a single line diagram of the Brazilian Birdstest systemBBTS, with thirty buses, whose static and dy-namic data are described in [24]. The system is composed oftwo 230-kV rings interconnected by 440-kV transmission lines,where the power flows go from right to left helping the plantsat Canrio and Sabi generating buses to meet the loads in the230-kV area. Generators and transformers are shown throughequivalents in the diagram. Table I shows the base case dis-patching (at heavy load level) previously set for the standardload flow program. Basically, the system meets the 1200 MWof distributed loads and 59 MW of losses. Hydroelectric powerplant Tucano is the slack bus and concentrates the system re-serve, which represents % of the total generation. In other



Fig. 4. Brazilian Birds Test System.

TABLE IBASE CASE GENERATING POWER DISPATCH

plants, the generation is fixed at the value indicated in the table.

B. Pelicano Substation

Fig. 5 shows the Pelicano substation, at 230-kV voltage level,which is used to illustrate the assessment of the two previouslydefined reliability indices. Its configuration is a typical doublebus with single breaker and four switches per bay, connectingfive terminals. In normal operating configuration, terminals T1,T2, and T5 are connected to bus B1, and terminals T3 and T4are connected to bus B2. All bypass switches and at least onebus selection switch of each bay operate normally open. It isalso important to notice that the system had its base case ad-justed to meet the N-1 criterion; i.e., the loss of an element ofthe system, connected to the substation does not cause viola-tions in its normal operating limits.The dynamic simulations were performed with a standard

transient stability analysis program, using typical models forsynchronous machines, voltage regulators, speed governors,etc.; see detailed data in [24]. The simulation involves the

Fig. 5. Configuration of the 230-kV Pelicano substation.

application of a single-phase short circuit at the substation, withthe subsequent removal of the fault along with the involvedequipment (circuit disconnection) due to the action of theprotection system. The total simulation time was 20 s, the shortcircuit exposure time was 150 ms (primary protection activa-tion), and backup protection (to simulate failure in openingbreakers) of 500 ms.The static simulations were performed with standard AC

power flow and OPF programs. Static controls were enabled toensure reactive power and voltage limits in PV buses. Activepower redispatch to solve overload problems was not consid-ered, and the equipment capacity limits of the system circuitswere set at the emergency level.Typical international data [25][28], shown in Table II, were

used to define the failure rates ( for active faults and



TABLE IISUBSTATION RELIABILITY DATA

TABLE IIISUBSTATION FAILURE STATES AND SYSTEM MULTIPLE OUTAGES

for failures passive), the average switching times , and themean time to repairs , in hours. Information about sched-uled maintenance ( for frequency rate and for durations)were also included. For the probability of stuck breaker, i.e.,, named in (1), a typical value is also considered.Table III shows the simulated cases for the 230-kV Pelicano

substation. Each case represents a set of events that leads to thesame substation terminal disconnected status, from its normaloperation configuration. The failure state probabilities of thesubstation under analysis were obtained with a computationaltool developed for this purpose, where: active failures;

active failures stuck breaker; stevent is a repair or scheduled maintenance and 2nd event is anactive failure; st event is a repair or scheduledmaintenance and 2nd event is a repair.It is important to observe that the events that characterize fail-

ures in the substation are different, although the effects of someof them are the same. For example, Case 3 represents a singlecontingency occurring at the substation which disconnects all itsterminals, causing the complete loss of system node. In cases6 and 8, the same terminals are disconnected, but the eventsor conditions are different with different probabilities of occur-rence.Case 1, for example, represents all single contingencies that

cause simultaneous loss of terminals T1, T2, and T5, that is, theoutput of B1 substation bus, with a probability of

. The dynamic simulation carried out for this condition hasrated the system as potentially unsafe, i.e., unstable, becausethere is loss of synchronism. The static simulation performed

Fig. 6. Angle between Canrio machines and the system COI: Case 1.

Fig. 7. Angle between Canrio machines and the system COI: Case 5.

found an amount of load shedding in the system of 38.41 MW,resulting from this event in the substation.Case 5, for instance, represents the contingencies that occur

in the transmission elements associated with failures in openingbreaker (primary protection in stuck breaker pole condition) andcause the outage of bus B2, due to the action of backup pro-tection. The performed dynamic simulation rated the system assafe, and the static simulation found an amount of load sheddingin the system of 82 MW.The double contingencies were limited to the most critical

ones, whose associated probabilities have magnitudes up to onethousand times smaller than those due to single contingencies.It is possible to include these fault states with lower probabili-ties, but the impact on the proposed indices would be extremelysmall, and in most cases, would not justify additional static anddynamic simulations. Notice that, in bus configuration with dis-tributed connectivity, as in the case of double bus/breaker and ahalf scheme, the number of states in double contingency is al-ready high with these limitations.To illustrate the dynamic simulations, the two previous cases

1 and 5 are discussed as follows. In Case 1, Fig. 6 shows thatthe angle between the machines at hydro power plant Canrioand the center-of-inertia (COI) of the BBTS goes to an irre-versible excursion beyond 360 degrees, indicating the loss ofsynchronism of these machines, compared to the rest of thesystem. Moreover, if machine frequencies exceed the limits of% from the nominal value of 60 Hz or if the system exhibits

poor damping, loss of synchronism is also detected. Therefore,in these cases the system is classified as potentially unsafe,i.e., insecure. In Case 5, Fig. 7 shows that the angle between themachines at Canrio generating station and the COI of the BBTSexhibits a small excursion, and it is quickly damped, defining anew operating point, which indicates a stable system conditionfor this particular substation fault.



TABLE IVSUBSTATION BASIC CHARACTERISTICS AND NUMBER OF SIMULATIONS

Based on the proposed ranking methodology, the followingreliability indices are evaluated through (2) and (3):

and MWh/yr. These indices allowquantifying the degree of security and adequacy that Pelicanosubstation offers to the power system. The LOSP index is aprobability that measures whether the system runs some riskof losing synchronization, which may initiate a cascade processand culminate in a regional failure or a widespread blackout. Onthe other hand, the EENS indicates the expected load curtail-ment under steady state conditions that affects the quality of theutility in meeting the power demand. It rationalizes investmentsin the substation, balancing the cost-benefit ratio over time. Inconclusion, the reliability of the substation is measured by anindex related to large but less frequent shutdowns, with charac-teristics clearly systemic, and another indicator related to morefrequently shutdowns of smaller magnitudes, but causing cus-tomer inconveniences.

C. BBTS Results

Using the procedure previously described for the 230-kV Pel-icano substation, the same reliability indices were obtained forthe other nine substations of the BBTS. As can be found in [24],different sizes and types of bus configuration settings are offeredin the BBTS, which provide a variety of conditions for testingthe proposed methodology. Table IV shows the settings in termsof substation configurations and the number of dynamic (tran-sient stability) and static (power flow and OPF) analyses car-ried out to ensure a good quality enumeration process. The aimof these evaluations is to rank the existing BBTS substations interms of criticality, as shown in Table V.From the dynamic point of view (system security), Table V

shows that Pelicano substation with isthe most critical, followed by Tucano and Arara with

and , respectively. All other LOSPindices are nil for the remaining BBTS substations. From the

TABLE VINDICES AND STATIC AND DYNAMIC RANKING FOR THE BBTS

static point of view (system adequacy), the most critical substa-tion is Tiziu, with MWh/yr. It is followed bysubstations: Bicudo (70.78 MWh/yr), Tucano (42.85 MWh/yr),etc. As could be expected, the ranking list based on LOSP secu-rity indices has nothing to do with a similar list based on EENSadequacy indices. Therefore, static-based ranking procedurescannot be used for selecting substations or nodes to be fullyevaluated using transient stability analyses. This is a relevantconclusion to the security studies in power system analyses.Table V also illustrates a dynamic and static ranking list,

where the priority is given to the LOSP security index, and thenthe EENS is used to classify the remaining substations. Thisranking provides the overall criticality of the system substa-tions, although the individual security and adequacy ranking,with their respective index values, are more essential.It is possible to establish an acceptable value for the proposed

EENS index, in order to define those substations that may bereinforced to ensure this threshold. The amount of investmentsshould be compared to the economic benefits, measured by thereduction in terms of interruption costs, as proposed in the liter-ature; see, for instance, [6], [14], and [16].As previously stated, the disconnections that occur in substa-

tions due to equipment failures can be classified into two maincategories. The first one that encompasses more frequent dis-connections with minor impacts on the system, and the secondone that includes disconnections less frequent but more severeto the network, which can seriously affect the balance of theirpower electromechanical sources. Not often, an indicator is as-sessed to capture the problems related to the latter category.However, security-based indices are absolutely necessary fora realistic approach involving the performance of substationsin the electrical system. Bearing in mind the BBTS, Pelicano,Tucano, and Arara are those substations that should be givenpriority in terms of investments, risk management, and oper-ating practices. Both ranks, dynamic and static, depend on thedispatching function and other operating practices. Therefore,these three substations have definitely to be put at the top of thecriticality ranking lists.



D. Final Remarks

The proposed methodology not only allows applications torank the criticality of substations in bulk power system, butalso studies involving a single substation performance. Thus,using the same procedures described for the 230-kV Pelicanosubstation of the BBTS, a real substation in the Braziliannetwork was analyzed. It is a 230-kV substation, named SoLuis II, belonging to Eletronorte Power Company. This studyarose from the need to evaluate the expansion of this substationto meet local loads, taking advantage of the physical spacestill available and, thus, optimizing investments. Experiencesfrom previous blackout occurrences have justified the use ofthe proposed method in order to evaluate, in a predictive way,the limits of this particular substation with respect to risksposed to the system. Based on adequacy and security indices,different types of reinforcements were compared. Reference[28] describes this work, where the entire emphasis is givento the technical-economical aspects involving different typesof reinforcements. This specific study proves, somehow, thefeasibility of the proposed approach in real power systems.Finally, in a recent report published by NERC [32], the anal-

ysis of their bulk power system (BPS) provided a relevant in-dustry reference for historical BPS reliability, offered analyticalinsights toward industry action, and enabled the identificationand prioritization of specific steps that can be implemented inorder to reduce and manage risks to reliability. Among the rec-ommendations, one can find [32]: 1) A thorough investigationinto the root causes of circuit breaker failures that contributeto disturbance events is a high priority for 2013; 2) A smallsubject matter expert technical group should be formed to fur-ther probe the AC substation equipment failures, particularlycircuit breaker failures, and provide risk control solutions to im-prove performance. The proposed probabilistic approach forranking substation criticality, based on static and dynamic per-formance indices, tries to accomplish these recommendations ina predictive manner, which complements the metrics proposedby NERC.

IV. CONCLUSION

When assessing the reliability of substations and measuringthe consequences of the faults originated in them on the electricpower network, the same type of bus arrangement can result indifferent system performances, depending on their position inthe network topology. The failure effects become systemic, i.e.,the results of the analysis depend on the substation bus configu-ration, its size, the system topology and its operating conditions.Thus, it is compulsory to assess appropriate reliability indices tomeasure the criticality of substations considering both dynamic(security) and static (adequacy) system performances, to ensurea reliable decision making process.The proposed methodology consists of applying Markovian

models to the substations to cope with their associated failures,and, within a standard power system analysis framework, tomeasure through two probabilistic indices both the security andadequacy of the bulk power system.This work can be seen as a new effort to solve the substation

criticality ranking problem. The proposed methodology can be

used to provide several benefits to both system and substation,such as: 1) postpone investments in the planning phase; 2) de-fine bus configurations different from a pre-specified standardto best suit the system; 3) meet and manage risks sometimesunclear to the system planners; 4) prioritize investments in re-inforcements in existing substations; and 5) guide effective op-erating practices and maintenance procedures.Finally, the full integration of the proposed probabilistic

models with the standard power system analysis frameworkwill be possible in the near future, based on new transientstability metrics and programming environment.

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Armando M. Leite da Silva (S77M78SM91F00) was born in Rio deJaneiro, Brazil, in 1954. He received the B.Sc. degree from the Catholic Uni-versity of Rio de Janeiro (PUC-Rio) in 1975, the M.Sc. degree from the FederalUniversity of Rio de Janeiro (COPPE-UFRJ) in 1977, and the Ph.D. degreefrom the University of Manchester (UMIST), U.K., in 1980, all in electrical en-gineering (EE).He worked at the EE Department, PUC-Rio, as a Professor until 1994. From

1990 to 1991, he was a Visiting Researcher at the Research Division of On-tario Hydro, Canada. From 2003 to 2004, he was a Visiting Researcher at thePower SystemUnit, INESC Porto, Portugal. Since 1994, he has been a Professorat the Institute of Electric Systems and Energy, Federal University of Itajub(UNIFEI), Brazil.

Prof. Leite da Silva received the Sebastian Z. de Ferranti Premium Awardfrom the Power Division of the IEE, U.K., in 1992. In 2010, he was recognizedwith the PMAPS Merit Award for his contributions to probabilistic methods.In 2011, he received the IEEE PES Technical Committee (PSACE) Prize PaperAward. In 2012, he was the recipient of the IEEE-PES Roy Billinton PowerSystem Reliability Award.

Airton Violin was born in Dracena, So Paulo, Brazil, in 1956. He receivedthe B.Sc. and M.Sc. degrees in electrical engineering from UNIFEI in 1982 and2003, respectively. He is now pursuing theD.Sc. degree in electrical engineeringat the Institute of Electric Systems and Energy, UNIFEI, Brazil.From 1984 to 1993, he worked as a Senior Engineer in the Electric System

Planning Department of Eletronorte, Brazil. Since 1993, he has been a privateconsultant engineer, mainly involved with the project and design of electricpower substations.

Cludio Ferreira was born in So Loureno, Minas Gerais, Brazil, in 1955. Hereceived the B.Sc. degree from UNIFEI in 1977, the M.Sc. degree from FederalUniversity of Braslia (UnB) in 1983, and the D.Sc. degree from UNIFEI in1998, all in electrical engineering.From 1977 to 1991, he worked as a consultant engineer of Themag Engen-

haria Ltda. He has been involved in many studies for companies like: ONS(Operador Nacional do Sistema), Furnas, Cemig, CPFL, Eletrobrs, Aneel(Brazilian Regulatory Agency), etc. From 1999 to 2000, he was an advisorto the board of directors of Aneel. Currently, he is an Associate Professor atthe Institute of Electric Systems and Energy, UNIFEI, Brazil. His main areasof interest are static and dynamic power system analyses, electromagnetictransients, control, operation and power system planning.

Zulmar S. Machado, Jr. was born in Governador Valadares, MG, Brazil, in1974. He received the B.Sc. degree from the Federal University of Juiz de Forain 1997, the M.Sc. degree in system engineering and computation in 2000, andthe D.Sc. degree in electrical engineering in 2005, both from COPPE-UFRJ.From 2006 to 2007, he worked as a research engineer in the Brazilian

Power System Research Center (CEPELCentro de Pesquisas de EnergiaEltrica). From 2007 to 2010, he worked as senior engineer in the Brazilian ISO(ONSOperador Nacional do Sistema). Since 2010, he has been an AssistantProfessor at the Institute of Electric Systems and Energy, UNIFEI, Brazil. Hismain areas of interest are power system optimization, time domain simulation,security region analysis, and system control.

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