Potential Reliability Benefits Could be Achieved Through the Implementation of single-phase reclosing and lockout in distribution feeders. However, several issues and misconceptions regarding potential problems must be addressed first.

Despite the potential problems, the improvement to system average interruption duration index (SAIDI) is real and demands attention.

FAULT PROTECTION

Historically, the fault protection of distribution feeders has been accomplished with protective devices such as fuses and reclosers, which are applied using one of two overcurrent-protection philosophies:

• Fuse blowing

  Reclosers and fuses are coordinated so that both temporary and permanent faults are isolated by blowing the fuse immediately upstream of the fault. This is also known as fuse clearing.

• Fuse saving

  Reclosers and fuses are coordinated to allow temporary faults to self-extinguish during the reclosing intervals. Only permanent faults are isolated by blowing the upstream fuse.

RECLOSING SCHEMES

Typically, three-phase reclosers are installed in main trunks and sometimes in long or highly loaded three-phase laterals. Single-phase reclosers are installed in single-phase and two-phase laterals and sometimes in three-phase laterals. Modern three-phase reclosers, such as the one in Fig. 1, are designed to allow the reclosing and lockout of individual phases. Similarly, modern single-phase reclosers can be controlled to reclose and lockout all phases simultaneously.

Reclosers are fundamental to improving the reliability of distribution systems. However, independently of the fault's nature (temporary or sustained and single phase, two phase or three phase), the reclosing attempts and lockout operation of a conventional three-phase recloser affect all three phases of the distribution system, causing momentary or sustained interruptions to all downstream customers.

This sort of three-phase reclosing and lockout is not optimal from the point of view of customer reliability indices. Why do customers served by phase A of a feeder have to be affected by a fault that occurs in phase C of the feeder? This criticism and the availability of modern reclosers have driven the implementation of other reclosing schemes for improving the reliability of the distribution system.

Modern reclosers are capable of providing the classic and most-common protection scheme used by utilities. In this scheme, all phases trip and reclose during the reclosing cycle, and all phases lock out if the fault is permanent.

A second functionality is available providing single-phase reclosing and three-phase lockout. In this mode, only the faulted phase trips and recloses. Lockout, if required, affects all phases. This scheme is similar to that used for transmission lines, and it is starting to become more popular for distribution applications.

ISSUES AND CONCERNS

A third scheme involves both single-phase reclosing and single-phase lockout. This is the most controversial and least-common application of modern reclosers. Nevertheless, some utilities are starting to implement this scheme, which can lead to significant reliability benefits.

The implementation of single-phase reclosing is controversial for several reasons. One is the possible impact of this sort of system operation on the equipment, especially the impact of losing one phase for those customers using three-phase motors. While this can be a valid concern, it is worth noting that the protection against the loss of one phase is part of a typical protection scheme for three-phase motors. And, of course, customers are also responsible to develop protection schemes to protect their equipment.

It is also worth noting that, for decades, utilities all over the world have been installing fuses (one per phase) in three-phase laterals. In this situation, a single-phase fault causes the operation of only one fuse, producing exactly the same effect as the single-phase lockout of a recloser would produce. Similarly, it also has been a relatively common practice to install three single-phase reclosers in long three-phase lines, usually in rural areas. Many of these single-phase reclosers do not have the capability of locking out the three phases simultaneously, which leads to single-phase lockout when a permanent single-phase fault occurs downstream.

Another concern is related to the unbalanced protection of the feeder, specifically with the operation of the ground overcurrent relay. When only one phase of a recloser locks out, it creates an unbalance that can cause the operation of the ground overcurrent relay. This can be overcome in many situations by modifying the relay settings to increase the tolerance for higher unbalance or by limiting the application of single-phase reclosing to sections of the feeder where the unbalance is not significant enough to cause the operation of the ground overcurrent relay.

Additionally, a potential issue to consider, according to the regulations of some states in the United States, is that the loss of service in at least one phase of a three-phase customer is considered a sustained interruption for reliability purposes. While this is also a valid argument, it is worth noting that if the loss of service was due to a fault, the three-phase customer would be affected by an interruption independent of the reclosing scheme being used. Furthermore, if three-phase reclosing were used, not only the three-phase customers would be affected, but so would all the other customers located downstream of the recloser. However, if single-phase reclosing were used, only those customers served by the faulted phase (including the three-phase customers) would be interrupted and counted for reliability purposes.

RELIABILITY BENEFITS

Other concerns are related to backfeed, ferroresonance and the potential interaction of reclosers and distributed generation. These issues are analyzed on a case-by-case basis.

In order to show the advantages and reliability benefits of single-phase reclosing, a comprehensive study was performed for a typical distribution system. This study analyzed the impact of the installation of reclosers for improving the reliability (SAIDI) of the distribution system. The system consists of 36 feeders (12.5 kV), with a total of 36,476 customers, and has the typical four-wire multi-grounded layout used in the United States, with single-phase and three-phase distribution transformers. The study estimated and compared the reliability benefits due to the installation of 24 reclosers in 21 feeders for both protection philosophies — fuse saving and fuse blowing — and two reclosing schemes — single phase and three phase.

The reliability benefits were estimated using a predictive reliability model (PRM) developed using the SynerGEE Electric solution from GL (Hamburg, Germany). The PRM was built from a typical power flow model. The power flow model included all the data about the connectivity of the system, distribution system components and distribution transformer loads. The PRM was built by allocating customers to all distribution transformers and assigning reliability parameters, such as failure rates and mean times to repair (MTTR), to all distribution system components (overhead and underground lines, protective and switching devices, and distribution equipment).

The reliability parameters of the distribution equipment and protective and switching devices were assigned using data available from several sources such as manufacturers, specialized literature and industry data. The reliability parameters of overhead and underground lines were calibrated using historical reliability and outage data for the last five years. These parameters were calibrated in such a way that the reliability indices, SAIDI and system average interruption frequency index (SAIFI), calculated by the PRM match the average value of the historical reliability indices from the last five years.

The historical reliability indices were calculated per the IEEE 2.5 Beta Method for major event exclusion. Within the model limitations, the calibrated PRM represents the current reliability of the distribution system. The calibrated PRM is the “base model” for reliability analysis, and the calibrated reliability indices are the “base reliability” of the system.

SINGLE-PHASE FAULT COMPONENT

The PRM simulates faults in all components according to their respective failure rates and MTTR, and then it calculates the corresponding reliability indices (SAIDI and SAIFI). Therefore, the benefits of installing a set of reclosers can be estimated by comparing the reliability indices calculated by the PRM before and after installing the reclosers. During the simulations and analyses performed in this study, all the reclosers were operated using three-phase reclosing or single-phase reclosing. The reclosers were installed in strategic locations on the system in order to try to increase their reliability benefits and effectiveness. Reliability indices were calculated for all combinations of fuse saving and fuse blowing protection philosophies and for both single-phase and three-phase reclosing and lockout.

One variable that influences the effectiveness of single-phase reclosing is the percentage of single-phase faults that occur in the distribution system. For each condition, results were calculated for three different percentages of single-phase faults: 25%, 50% and 75%. These percentages indicate the ratio of faults that are single phase with respect to the total number of faults in the system.

SIMULATION RESULTS

This variable will have a significant influence. For instance, if all faults that occur in the distribution system were three phase, then both single-phase reclosing and three-phase reclosing would have the same impact on reliability. Nevertheless, since most faults in the distribution system are typically single phase, it is expected that the reliability benefits of single-phase reclosing are significant.

The PRM simulation results show that single-phase reclosing achieves a higher reduction in SAIDI than three-phase reclosing, and that this reduction increases as the percentage of single-phase faults increases from 25% to 75%. This is true for both fuse-saving and fuse-blowing protection philosophies.

These results are explicitly shown in Fig. 2, which presents a plot of the differences between the SAIDI reduction attained by single-phase reclosing and three-phase reclosing. The plots show that for both overcurrent protection philosophies, single-phase reclosing achieves larger improvements in SAIDI than three-phase reclosing.

This shows that the difference in SAIDI reduction attained by both reclosing schemes can be notable for some feeders. For instance, if 75% of faults in feeder F1 were single phase and fuse saving were used, the SAIDI reduction attained by single-phase reclosing would be 33% with respect to the base case. On the other side, the SAIDI reduction achieved by three-phase reclosing would only be 6%, which would lead to a significant difference of about 27% between both reclosing schemes. This is the value shown in Fig. 3.

These results demonstrate the notable potential reliability benefits that can be attained by implementing a single-phase reclosing scheme. These noteworthy benefits justify considering and further analyzing the possibility of implementing single-phase reclosing as part of a standard approach for improving the reliability of distribution feeders. For instance, this application is particularly attractive for feeders with mostly residential customers, where problems related with three-phase motor loads are not a concern.

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Julio Romero Aguero (jromeroaguero@quanta-technology.com)is a senior advisor for Quanta Technology (Raleigh, North Carolina, U.S.). He has more than 14 years of experience working with electric utilities and regulatory boards. His areas of expertise are reliability, planning, maintenance, operations, regulation and analysis of power distribution systems. He holds a Ph.D. in electrical engineering from National University of San Juan, Argentina, and a BSEE degree from Autonomous National University of Honduras.

Laura Whittington (laura.whittington@ComEd.com) is a staff planner in the distribution capacity planning department at ComEd (Oakbrook Terrace, Illinois, U.S.) and has been with the company since 1988. Her experience ranges from metering applications to long-range planning. She holds a BSEE degree from Marquette University and is a member of the IEEE Power & Energy Society.