Electric Distribution Feeder Backbone Protection

Analyses of electric distribution systems repeatedly verify that the most immediate and cost-effective strategy for improving an electric utility’s distribution circuit reliability is protection of  the feeder backbone. The feeder backbone, also referred to as the mainline, main gut, or feeder (though the latter term sometimes refers to the whole circuit), is usually the three-phase part of the circuit that runs unfused from the substation to the normally open ties to other circuits or to the physical end of the circuit (i.e. at a geographical or territory boundary, etc.). Another way to describe it is that the backbone is every part of the circuit that is not behind (i.e. electrically downstream of) a fuse.

Protecting the backbone is typically done in one of two ways:

• Hardening: Focuses on methods of making the infrastructure less susceptible to outages, and
• Sectionalizing: Involves the installation of additional reclosers in targeted protection zones as well as fusing unfused taps in order to decrease the number of service interruptions associated with any backbone outage.

Hardening is aimed at eliminating outages (which in turn cause customer interruptions) and sectionalizing serves to mitigate the impact of outages by minimizing the number of customers interrupted by an outage.

Hardening the Feeder Backbone

Hardening the backbone is aimed at eliminating outages (which in turn cause customer interruptions). There is a number of leading industry practices (summarized below) that support this goal:

• Enhanced Vegetation Management where utilities initially get on cycle (typically a four-year cycle for the entire system), then transition to optimizing the cycle (varying the frequency of tree trimming based on circuit impact on reliability performance), and ultimately target broken limb/fallen tree outages to remove overhang (outside the clearance required by the tree trimming specification) and “danger” trees in their entirety (those trees that are most susceptible to falling on a line).
• Lightning Protection the relative importance of which varies based on a specific location’s tendency to experience lightning. In areas where lightning caused outages are prevalent, the use of lightning arresters is potentially effective, provided a proper lightning protection scheme is implemented.
• Repair Pole and Pole-Top Fault Causing Equipment Problems where utilities integrate their line inspection programs with a prioritization process that categorizes discrepancies based on their relative impact to safety and reliability. Further, many utilities are extending their infra-red inspection programs to distribution lines, providing a preliminary indication of potential failures to pole-top and conductor-related equipment.
• Animal Guarding dealing with the continued challenge of squirrels, birds, snakes, and other animals causing outages on overhead distribution transformers (most common), as well as lines and substation equipment. Animal guards in various forms are effective, particularly when combined with an effective tree trimming program.

Sectionalizing the Feeder Backbone

Sectionalizing the backbone serves to mitigate the impact of outages by minimizing the number of customers interrupted by an outage. This is the single most cost-effective program to mitigate electric service interruptions and improve reliability as measured by SAIFI. The following discussion summarizes these investments and measures:

• Installation of Reclosers: A standard recloser does not have communications capability but uses its own relays to sense current upstream and downstream in order to determine how to operate. It does not know the state of other switches or the event that causes an outage, only the state of the current on the line to which it is attached. It is nonetheless quite effective, and sometimes more so than a fully automated system. The net effect is to minimize the number of customers experiencing an outage as part of a specific targeting strategy.
• Installation of Fuses: Fuses, like reclosers merely sense current and react according to their design. Typically deployed at the taps, they ensure that a fault experienced on a tap does not take the entire feeder out-of-service, thereby minimizing the number of customers experiencing an outage to those on the tap (generally a relatively small number).
• Relaying/Over-Current Protection: Utilities use a variety of relays to protect equipment from damage due to a fault or other operating condition. Those on power transformers can sense high temperature oil, a sudden change in pressure in the oil tank, and/or voltage differentials. For distribution circuits, relays primarily provide protection from an electrical fault on one or more of the phases (the main sources of circuit protection are fuses and over-current relays that open fault-interrupting devices such as circuit breakers and reclosers). Fuses blow and circuit breakers open when a circuit has experienced excessive current due to a ‘short circuit’ (fault). Once the fault is cleared, blown fuses are replaced with another of the same size and type, where as circuit breakers or reclosers are simply reset. Though simple in concept, there are significant variances in how utilities design these over-current protection schemes. The numbers of times a circuit breaker or recloser will automatically re-close and how long will be the delay between re-closings are the variables that account for these differences; the criteria for which is oftentimes focused on achieving the correct balance between saving fuses (eliminating “unnecessary” mobilization) and minimizing momentary interruptions.
• Distribution Automation: Distribution Automation uses a communication infrastructure to effect a high degree of automated switching either between switches (decentralized scheme), or between each switch and centralized control centers (centralized scheme). Though differing views exist regarding the specific communication technology, and the means and extent to which switches should be able to automatically interrupt a fault, there is no question that a fully modernized system with the latest technologies in distribution, substation, and customer premise automation will significantly improve reliability.