by Chris McCarthy, S&C Electric Company
Inclement weather has always worried electric utilities, which together spend billions annually on power outages caused by downed power line poles or branches. Businesses and consumers also incur costs. Storm-related power outages cost the U.S. up to $55 billion annually, according to the Department of Energy (DOE) Office of Electricity Delivery and Energy Reliability.
With traditional grids, all it takes is a single fault to knock the power out along an entire feeder, which can affect hundreds of homes and businesses until the utility can locate and repair the problem. For utility customers, that time represents money, so avoiding power outages and speeding restoration is becoming increasingly important.
But electric grids are getting smarter. Many use sophisticated fault interrupters and smart switches to build resiliency into the system. Smart grids limit the effects of faults by redirecting power to unaffected areas or providing stored energy as backup. The extent to which homes and businesses are affected is dropping considerably.
Smart switches and fault interrupters self-heal by isolating disturbances so nearby devices can perform switching activities automatically to restore service and then communicate fault details to a control center. This process isolates faulted lines, restores services to unaffected lines and reduces the time for repair crews.
The DOE in a November 2014 report highlighted the benefits of smart grids and their ability to reduce the impact that weather-related outages have on homeowners and businesses. The report gives several examples where utilities that use traits inherent to smart grids-accelerated service restoration through automated feeder switching (AFS); fault location, isolation and service restoration (FLISR); and integrated advanced metering infrastructure (AMI)-have helped limit power outages, as well as the number of homes and businesses affected. The report also illustrates how smart grids save utilities and customers millions.
“Business and residential customers experienced fewer financial losses, as shorter outage time limited lost productivity, public health and safety hazards, food spoilage, and inconvenience from schedule disruptions,” the report states with a few caveats. “Some regions suffer severe weather events more than others, and thus experience different costs and benefits from improved storm recovery capabilities.”
Each utility has its own technology starting points, local circumstances and weather conditions, “making it impossible to establish a singular course of action or best practice for applying AFS, FLISR, and AMI for outage management,” the report states. “Instead, each utility must individually evaluate the costs and potential benefits to build a business case and investment strategy suited to its needs.”
Smart grids have improved storm response and recovery. EPB of Chattanooga, Tennessee, estimated the customer impact of power outages at $100 million a year. The utility, which has 172,000 customers, 117 substations, 3,582 circuit miles of distribution lines and a summer peak demand of 1,300 MW, turned to a self-healing system-the IntelliTeam SG Automatic Restoration System. Backed by more than 1,000 IntelliRupter PulseCloser fault-interrupting devices, the system automatically reconfigures power distribution after a fault to restore service quickly to unaffected feeder segments.
During a July 2012 storm, the new system enabled EPB to reduce total restoration time by up to 17 hours, and use of automated feeder switching prevented power loss, or instantly restored power in the case of temporary faults, for up to 40,000 customers. After a February 2014 snowstorm, the utility reduced restoration time up to 36 hours and saved some $1.4 million in overtime costs because it required fewer truck rolls and repair crews, according to the DOE report. In February 2015, EPB restored power to 25,000 homes and businesses 30 minutes after a major winter storm and freezing rain.
EPB’s automated restoration system helped improve its System Average Interruption Index (SAIDI) 40 percent and its System Average Interruption Frequency Index (SAIFI) 45 percent between 2011 and 2014, according to the DOE report.
When tropical storm Isaac hit in August 2012, Florida Power & Light (FPL) used nine automated smart switches serving about 16,000 customers to reduce customer interruptions and upstream momentary disturbances. FPL services 4.7 million customer accounts and operates more than 70,000 miles of power lines over a 27,000-square-mile service area.
In 2014, FPL expected to save more than $30 million through enhanced operational efficiencies. Through the third quarter of 2014, it had deployed 1,000 automated feeder switches and advanced fault interrupters, avoiding more than 300,000 customer interruptions, the DOE report states. Since Isaac, the utility has deployed more than 3,800 remote fault indicators that cover 620 feeders. The utility recently invested in TripSaver II dropout reclosers to continue to reduce momentary outages from storms at the edge of its grid. It is expected to save customers on each main feeder as much as $500,000 annually.
FPL’s automated restoration system reduced its number of customer minutes interrupted from 700,000 in 2012 to 200,000 in 2014 for substation transformers, according to the DOE.
Energy Storage’s Role
Improved outage management is not limited to more sophisticated switching equipment. Energy storage also can play a role.
An energy storage system that went into service in July 2013 is helping BC Hydro reduce the length and frequency of power outages. The utility, which provides one 25-kV distribution feeder to supply power to residents of Field, British Columbia, uses advanced fault interrupters and other advanced switching equipment to alleviate outage problems by automatically transferring Field’s electric load to battery power when the grid is disrupted.
During the first six months of system operations, six major power disruptions occurred from trees’ falling on the lines, broken power poles and other factors. The system avoided outages by supplying Field with battery power for a combined 40 hours. To date, the system has supplied a total of 80 hours of battery backup. The storage system notifies local residents by tweet that they are using battery power and should conserve energy. A follow-up tweet notifies them when the power source returns to normal.
Distributed Intelligence Continues Success
Utilities are beginning to understand the benefits of accelerated service restoration through smart feeder switching and energy storage. But confusion often arises when deciding how best to manage the process.
Some utilities perceive centralized systems as their best method for managing outages. Others see advantages in systems that use distributed intelligence, where fault interrupters team with nearby switches to isolate faults and redirect power to unaffected areas, all within seconds and without the need for centralized control.
Centralized systems offer broad benefits in managing data and multiple applications, but they are less effective than distributed intelligence systems in quickly addressing faults and outages. If a centralized system or communication is down, that represents a single point of failure. With distributed intelligence, teams of switches don’t have to report to a central communication system. Instead, they can talk to one another in small pockets within the grid. If one device is not communicating or is taken out of commission, it affects only the automatic restoration capability of a small local area. Such systems also are closer to the action so they can react faster when faults occur.
Distributed intelligence systems also are more resilient. Centralized control is important when dealing with a storm that affects a broad geographic area. But if that storm knocks out a communication tower, that’s a big, wide area that no longer is visible or accessible by the centralized system. With distributed intelligence, there is no single point of failure so you can keep the power going by automatically limiting the outage only to the affected fault area.
Utilities are best served by supporting a layered approach using the right level of intelligence at the right locations. Utilities often have different levels of communication needs for different centralized applications, handling varying amounts of data and requiring different response times, latency needs and other factors. When it gets down to fault interruption, location and isolation, that’s really best done by the field devices.
Chris McCarthy is director of grid automation and control at S&C Electric Company. He is responsible for product management and global strategy for all automation offerings.More PowerGrid International Issue Articles
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