Improving the Reliability of Breaker-And-A-half Substations Using Sectionalized Busbars

Improving the Reliability of Breaker-and-a-Half Substations Using Sectionalized Busbars M. B. Stevens S. Santoso Department of Electrical and Computer Engineering The University of Texas at Austin Austin, TX, USA [email protected] Abstract—In utility power system substation design, the breaker-and-a-half topology is frequently used, primarily because it is considered very reliable. This paper aims to demonstrate that by adding sectionalizing circuit breakers into the busbars of the breaker-and-a-half configuration, the reliability of this substation design can be improved. Using a combinatorics-based approach to combining reliability indices of substation component failures, combined with using minimal cut sets of reliability graphs to identify failure scenarios, overall system failure rates were calculated for various substation topologies. The results demonstrate that the sectionalized breaker-and-a-half topology reduces the substation’s failure rate by 70.8% and its total annual downtime by 28.9% compared to a typical breaker-and-a-half scheme, although at the cost of increasing the mean time to repair of a substation failure by 2.49 hours. Index Terms-- Power Systems, Power System Reliability, Substations.

I.

INTRODUCTION

Reliability studies of utility power system substations have often concerned themselves with comparisons of different architectures used within the substation to connect incoming (source-side) lines to outgoing (load-side) lines. As substations can potentially supply power to a very large number of downstream customers, it is very important that they be very reliable. In most reliability studies, the breakerand-a-half topology is identified as among the most reliable configurations [1]. Many approaches have been developed for the modeling of substation reliability and the derivation of accompanying reliability indices. Of these, most concern themselves primarily with the failure rate of individual components within the substation, identifying which components’ or combinations of multiple components’ failures would cause the substation as a whole to fail. In substation reliability, system failure is defined as no paths existing between the incoming and outgoing lines. Assigning each component a set of reliability indices, aggregate reliability indices can be The work presented herein is supported in part by the U.S Office of Naval Research Contract Number: N0014-08-1-0080.

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created by considering every situation in which the station fails. The components usually considered include transmission lines, transformers, reactors, buses, circuit breakers, reclosers, and switches. Each component has several associated failure states, as applicable, including fault, preventative maintenance, false trip, and stuck. Failures are distinguished as either passive, when they do not trigger the operation of adjacent protective devices, and active, when they do. Two or more component failures occurring in sequence, with the subsequent failures occurring before the initial failure is repaired, are referred to as second- and higher-order failures. Many papers on substation reliability only concern themselves with limited types of third- or higher-order failure events, or neglect them entirely [2], as the likelihood of failures drops off steeply as the order increases. Reliability indices evaluated usually include failure rate, average outage duration, and total annual outage time. Each component has an associated value for each index, with average outage duration being a function of the average repair time and total outage time being the product of the other two indices. Then each first- and second-order failure event can be given a set of reliability indices by probabilistically combining the indices of the components involved in each event. Finally, a set of overall substation reliability indices can be derived by combining the indices of each failure event that results in substation failure [1]. A wide variety of approaches to modeling these indices have been developed, including the use of state enumeration through minimal cut sets [3] or fault tree analysis [4], and calculation of reliability indices through combinatorics-based probability models [1], [5], Markov chains [2], [6], sequential Monte Carlo methods [7], [8], and treating reliability indices as random variables [9]. This paper will utilize a minimal cut set approach to enumerating failure states and a combinatorics-based probability model to calculate reliability indices.

II.

SUBSTATION CONFIGURATIONS

To ensure a more direct comparison between topologies, all configurations considered will have two incoming and two outgoing lines. A. Single Bus, Single Breaker The most basic substation arrangement is the single bus, single breaker. Sources are attached to a single busbar that feeds the outgoing lines, each line protected by a single circuit breaker (see Fig. 1). Cursory inspection shows that this topology is very vulnerable to failures, as any single active failure of any of the circuit breakers or the busbar will result in system failure. This arrangement will therefore be used as our baseline of reliability against which we will compare other, more robust, arrangements.

C. Ring Bus The ring bus configuration consists of four busbars arranged into a ring, with each bus sectionalized by a circuit breaker (Fig. 3). This configuration contains sufficient redundancy that no first-order failure will cause the system to fail.

Figure 3. Ring Bus arrangement.

Figure 1. Single Bus, Single Breaker arrangement.

B. Sectionalized Bus The sectionalized bus arrangement is simply the single bus, single breaker configuration with the addition of a sectionalizing circuit breaker on the busbar separating the two pairs of incoming and outgoing lines (Fig. 2). This adds a layer of protection, ensuring that if a bus fault occurs, or if there is a short circuit in one of the line breakers, only one side of the substation will be taken down, while the other side remains operational. A short circuit in the bus breaker will still result in system failure, as will a number of second- and higher-order failures, but this topology nonetheless represents an improvement in reliability over the single bus, single breaker arrangement.

D. Breaker-and-a-Half The Breaker-and-a-Half arrangement consists of two parallel busbars, connected by two lines. Each connecting line contains three circuit breakers, an incoming line and an outgoing line (Fig. 4). This configuration contains a significant amount of redundancy, even more than the ring bus. However, this increased redundancy also means more components are involved, increasing the number of possible failure points. Like the ring bus, the breaker-and-a-half arrangement cannot fail due to a first-order failure.

Figure 4. Breaker-and-a-Half arrangement.

E. Breaker-and-a-Half with Sectionalized Buses This configuration takes the breaker-and-a-half design and adds a sectionalizing circuit breaker onto each busbar, separating the two connecting lines (Fig. 5).

Figure 2. Single Bus, Single Breaker arrangement with a sectionalized bus.

where 8766 is the number of hours in a 365.25 day year. Similar logic applies to calculating third-order failure rates. If we wish to calculate the failure rate for a scenario in which failures A, B, and C occur before failure A is repaired, we calculate the second-order failure rate for failures A and B, then multiply by the failure rate of failure C (in #/MTTRA). For example, the failure rate of a third-order failure in which the second-order failure described above occurs, followed by a second circuit breaker experiencing an active fault before the busbar can be repaired can be calculated as follows: λbus+CB-P+CB-A = λbus+CB-P ∗ (λCB-A ∗ MTTRbus/8766). (2) Figure 5. Breaker-and-a-Half arrangement with sectionalized buses.

III.

METHODOLOGY

Each potential component failure is assigned a set of reliability indices, failure rate (λ), mean time to repair (MTTR), and total annual downtime, as shown in Table I [1], [4], [10]. Only circuit breakers and busbars are considered as substation components. As their number and position does not vary between different topologies, incoming/outgoing lines, transformers, fuses and other components are considered to be outside of the substation and thus irrelevant to our comparison. Busbars can experience only active failures, such as a fault. Circuit Breakers can experience three kinds of failures: active failures, such as insulation breakdowns resulting in a fault, passive failures, such as false tripping, and stuck conditions, when a breaker fails to open during an adjacent active failure. TABLE I. Component Failure

COMPONENT RELIABILITY INDICES Reliability Index MTTR (hours)

Total Downtime (hours/year)

Failure Rate (#/year)

Busbar – Active

12

0.288

0.024

Circuit Breaker – Active

12

0.12

0.01

Circuit Breaker – Passive

12

0.12

0.01

Circuit Breaker – Stuck

1

0.005a

a. Stuck condition is modeled as a probability. That is, there is a 0.5% chance that a given breaker will not open when needed.

Second- and third-order failures are modeled by multiplying probabilities. All failures are assumed to be independent and uniformly distributed throughout the year. From this assumption, the failure rate (in #/year) for a second-order failure is the failure rate of the first component (in #/year) times the failure rate of the second component (in #/MTTR). This represents the chance that the second component fails before the first can be repaired. For example, the failure rate of a second-order failure in which a busbar experiences an active failure, followed by a circuit breaker experiencing a passive failure before the busbar can be repaired can be calculated as follows: λbus+CB-P = λbus ∗ (λCB-P ∗ MTTRbus/8766),

(1)

As mentioned in the footnote of Table I, stuck breaker failures are modeled as a simple probability. The failure rate of a second- or third-order failure involving a stuck breaker condition is equal to the failure rate of the corresponding first- or second-order failure without the stuck breaker multiplied by the stuck breaker probability shown in Table I. For example, the failure rate of a second-order failure in which a busbar experiences an active failure and an adjacent circuit breaker experiences a stuck condition is given by: λbus+CB-S = λbus ∗ λCB-S.

(3)

Fourth- and higher-order failures are not considered. Failure scenarios are enumerated using minimal cut sets. A reliability graph is constructed, each node representing a component and each edge identifying which components are adjacent to each other. Two nodes on opposite ends of the graph represent the sources and the outgoing lines, respectively. Failure scenarios are identified by removing exactly as many nodes, as well as the edges incident upon them, as is needed to disconnect the source node from the load node. The sets of nodes removed are called minimal cut sets. Different failure types follow different rules for node and edge removal, with passive failures removing only that component’s node and incident edges, while active failures and any accompanying stuck breakers remove all adjacent nodes and their incident edges (as well as further nodes and edges if an adjacent node represents a busbar, which behaves in the same manner as a stuck breaker). Fig. 6 shows an example of a reliability graph and the representation of two fault scenarios. Fig. 6a is a Single Bus, Single Breaker arrangement with a sectionalized bus represented by a reliability graph. Fig. 6b shows a first-order, active failure in circuit breaker 2. The arrows show the failure propagating through bus 2 to circuit breakers 4 and 5, which open, containing the fault. As can be seen, there still exists a path between the source and load nodes, thus this failure does not constitute a cut set. Fig. 6c shows a secondorder failure: circuit breaker 2 experiences an active failure, as in Fig. 6b, and circuit breaker 5 becomes stuck, allowing the failure to propagate through to bus 1 and circuit breakers 1 and 3. The source and load nodes are disconnected in this scenario, so this second-order failure is a minimal cut set of the graph, and thus is a substation failure. Because each failure scenario is a minimal cut set of the reliability graph, the set of component failures included in a

second- or third-order failure does not contain any proper subsets of component failures that themselves represent a substation failure. From this fact and our assumptions that failures are uniformly distributed and independent, the MTTR for a second- and third-order failure is 6 and 4 hours, respectively. These times represent, on average, the time between the final component’s failure and the repair of the first component that failed, which will return the substation to operation. Any failure involving a stuck breaker has a MTTR of 1 hour.

MTTR of the substation is the average of the MTTRs of the failure scenarios, each scenario’s value weighted by its corresponding failure rate. IV.

RESULTS

The reliability indices of the five substation topologies considered are summarized in Table III, as well as Figs. 7-9. TABLE III.

SUBSTATION RELIABILITY INDICES Reliability Index

Substation Arrangement

MTTR (hours)

Total Downtime (hours/year)

Failure Rate (#/year)

11.999974

0.768001643

0.06400027

11.657036

0.120340567

0.01032343

Ring Bus

1.0675151

0.000440691

0.00041282

Breaker-and-a-Half

2.4432985

1.04285E-05

4.2682E-06

Breaker-and-a-Half with Sectionalized Bus

5.9297684

7.41002E-06

1.2496E-06

Single Bus, Single Breaker Single Bus, Single Breaker with Sectionalized Bus

(a)

(b)

(c) Figure 6. (a) A reliability graph of the Single Bus, Single Breaker with sectionalized bus topology. (b) A first-order failure, which the substation survives. (c) A second-order failure, which the substation does not survive.

Substation reliability indices of failure rate and total downtime are calculated by summing the respective indices of each failure scenario. Mean time to repair is calculated through an average of the MTTR of each failure scenario, weighted by each scenario’s failure rate. Table II shows the calculation of the overall substation reliability indices for the Single Bus, Single Breaker topology. Second-order failure rates are calculated using (1). Note that there are no third-order minimal cut sets for this arrangement. The total failure rate and total downtime of the substation are the sums of the failure rates and total downtimes of the individual failure scenarios listed. The total TABLE II.

Figure 7. Substation failure rates.

The two Single Bus, Single Breaker configurations are inferior to the other three configurations in terms of all three reliability indices. However, these configurations contain fewer busbars than the other three topologies, and fewer circuit breakers than the two Breaker-and-a-Half topologies. In situations where cost is of greater importance than reliability, then, these configurations may be preferable. The Ring Bus topology has the lowest mean time to repair

CALCULATION OF SINGLE BUS, SINGLE BREAKER SUBSTATION RELIABILITY INDICES

Bus (A)

12

Total Downtime (hours/year) 0.288

CB1 (A)

12

0.12

0.01

CB2 (A)

12

0.12

0.01

CB3 (A)

12

0.12

0.01

Failure 1

Failure 2

CB4 (A)

Failure 3

MTTR (hours)

Failure Rate (#/year) 0.024

12

0.12

0.01

CB1 (P)

CB2 (P)

6

8.21355E-07

1.36893E-07

CB3 (P)

CB4 (P)

6

8.21355E-07

1.36893E-07

11.99997433

0.768001643

0.064000274

TOTAL

of the five substations, though it fails more often than either of the Breaker-and-a-Half configurations. This is due to the fact that the majority of the failure scenarios in a Ring Bus substation involve stuck breakers. Stuck breaker conditions occur more frequently than overlapping active or passive failures, but can be repaired much more quickly, as can be seen in Table I. Therefore the ring bus may be preferable in situations in which the loads being served can sustain a greater number of short failures better than a lesser number of failures of a greater duration. The low MTTR of a ring bus topology as compared to a breaker-and-a-half arrangement has been observed in previous studies [1], [6].

Figure 8. Substation Mean Times to Repair.

The sectionalized breaker-and-a-half topology, then, would be most useful in a situation where one is trying to minimize the number of failures that occur, regardless of their duration. The traditional breaker-and-a-half topology represents a middle ground between the ring bus and sectionalized breaker-and-a-half. V.

CONCLUSIONS

A minimal cut set approach to failure state enumeration and a combinatorics-based probability model of reliability indices leads to the conclusion that the Breaker-and-a-Half substation topology can be made more reliable by including sectionalizing circuit breakers in the busbars. This configuration has a lower failure rate and total annual downtime than the four other topologies modeled, though it has a higher mean time to repair than the ring bus and the breaker-and-a-half scheme without sectionalized busbars. Of these three topologies, the ring bus has the lowest MTTR and the highest failure rate and total annual downtime, with the traditional breaker-and-a-half falling between the other two topologies in all three indices. In the future, the comparison made here between the sectionalized breaker-and-a-half topology and other substation arrangements can be corroborated and refined though the use of other established approaches to reliability modeling. The comparison can be further refined though the inclusion of economic concerns or comparisons against other, more complex substation arrangements. REFERENCES [1]

Figure 9. Substation Total Annual Downtimes.

The results demonstrate a higher level of reliability in both failure rate and total annual downtime in the sectionalized breaker-and-a-half configuration as compared to the arrangement without sectionalizing breakers. The downside of this change can be seen in Fig. 8, as the failures of the sectionalized arrangement take, on average, longer to repair than those in the standard version. This is because most of the failure scenarios for the standard breaker-and-a-half configuration that are prevented by the sectionalized busbars involve stuck breakers. As mentioned above, these failures, while being relatively frequent compared to active or passive circuit breaker failures, also can be repaired much quicker. However, this increase in MTTR is more than offset by the reduced failure rate, as can be seen in Fig. 9.

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