by Tim Qualheim, S&C Electric Company
Yesterday’s electricity reliability is unacceptable to today’s consumers. Consumers’ growing appetite for digital products, combined with more industrial and commercial use of computerized machinery, is putting performance pressure on electric utilities. Momentary outages, which have been on the rise, are frustrating and increasing costs for users of sensitive digital technology.
Regardless of how brief, a power outage has the potential to generate broad complaints.
Residential customers are growing irritated, for example, at having to reset clocks and security systems more often. Retail businesses are upset at the hassle, costs and lost sales that occur when customers leave rather than wait for cash registers to reboot. Manufacturing plants also incur significant costs because of lost production and idle workers while product assembly line controls are reset. They might even have to scrap material and clean up messes caused when factory processes stop suddenly.
Known as momentary outages, these electric blinks are an inevitable consequence of utility system designs that for 100 years have focused on minimizing sustained outages. Some utilities are addressing customer concerns over these outages. Florida Power & Light Co. (FPL) has committed to focus on reducing momentary outages across its system and to significantly improve its customer service.
Two main utility practices cause blinks: substation breaker-protection schemes and lateral protection practices.
Substation Breaker-protection Schemes
Utilities know that 70 to 80 percent of faults are temporary, meaning they are caused by things, such as squirrels or branches, that fault a line but fall away as soon as the fault is interrupted. An American Public Power Association study found that squirrels cause on average 62 faults per year per feeder with 1,000 customers.
Typical utility practice is to interrupt those temporary faults with the substation breaker or recloser and then reclose to restore power to the system. The reason utilities use this practice is so no one on the feeder endures a sustained outage for a temporary fault. The downside is that almost everyone on the feeder is subjected to a momentary outage for every temporary fault. Some utilities also add reclosers out on the feeder to reduce the number of customers who experience a blink for faults farther out on the feeder.
Here’s a well-kept secret: Voltage sags (voltage that drops below 70 percent of nominal) can be troublesome to customers and affect their electronic equipment as much as a blink. Voltage sags are talked about less, but they are a well-known problem that results from typical substation breaker-protection schemes. Most substation transformers have two to four feeders connected to them. When the breaker for one feeder operates for a sustained fault, it opens and then recloses three to four times. Each time it recloses, it puts the fault back on the system, causing voltage sags on the transformer bus and all the other feeders connected to it.
One large U.S. utility recorded both momentary outages and voltage sags and concluded that there is one voltage sag for every two momentary interruptions. Because a voltage sag can have the same effect on customers as a momentary, this is a big deal.
To eliminate this problem, progressive utilities are replacing breakers and reclosers with innovative PulseClosingà¢â€ž- Technology. These advanced IntelliRupter® Fault Interrupters leapfrog legacy breakers and reclosers, providing fault interruption without the problems caused by conventional reclosing. Instead, they test the line through advanced technology that eliminates voltage sags.
Lateral Protection Practices
A typical 15-kV utility feeder has some 1,000 customers, but only a handful are on most of its 20 to 30 laterals. Utilities usually protect a lateral with a simple fused cutout.
For sustained faults, the fuse operates and isolates the lateral.
For temporary faults on the lateral, utilities set their systems one of two ways: to allow the fuse to blow or so the feeder breaker will operate, saving the fuse.
If a utility sets for the fuse to blow, the lateral experiences a sustained outage for all faults, even for the 70 to 80 percent that are temporary. If it sets the system so the breaker operates to save the fuse on all temporary faults, it will do just that, clearing the temporary lateral fault and saving those customers from a sustained outage. All other customers on the feeder, however, will experience a momentary outage.
The solution to this problem is to use reclosers instead of fused cutouts on laterals. The lateral recloser provides the best of both worlds by isolating sustained faults and interrupting temporary faults, and it does so without affecting other customers on the feeder. Utilities have known this for years, but they have only sparingly applied reclosers to laterals because of the complexities, maintenance demands and costs of some reclosers. FPL chose the S&C TripSaver® II Cut-Out Mounted Recloser as the most cost-effective solution for its systemwide deployment of a lateral recloser protection strategy.
This reclosing technology saves utilities from having to spend considerable sums on fuse-replacement and truck rolls while saving many utility customers from costs associated with momentary faults. Another benefit is significantly improved utility reliability measures: System Average Interruption Duration Index (SAIDI) and Momentary Average Interruption Frequency Index (MAIFI).
Momentary Outage Costs
Historically, utilities have been hesitant to take a broad approach to reduce their momentary outages. The primary reason is the budgetary impact; utilities have not been allowed to recover the costs associated with improving those rates. The reason regulators hadn’t historically approved them is because the industry had not been able to reasonably quantify the cost impact from momentary outages on electricity customers. But this is changing. Unbiased data is emerging and showing the real costs of outages to consumers.
In 2004 under contract for the Department of Energy (DOE), the Lawrence Berkeley National Laboratory published a study, “Understanding the Cost of Power Interruptions to U.S. Electricity Consumers.” This study has been scrutinized broadly and is widely accepted as a reasonable electricity user cost estimator for power outages. More recently, the DOE developed the Interruption Cost Estimate (ICE) Calculator, which takes the study data and allows a user to customize the calculation for geography and system parameters.
With the ICE Calculator, the calculated outage cost for one minute generally is accepted as a reasonable estimate for the cost of a momentary outage. These electricity customer outage costs are beginning to appear in many industry reports and presentations.
More important, they are part of many rate case submissions.
Regulators are accepting the consumer outage cost data as reasonable and are beginning to allow utilities to recover their reliability improvement expenses in their rates.
Utilities are noticing when those such as FPL commit to reduce momentary outages for electricity customers.
Regardless of the utility, most outage-related costs electricity users face stem from momentary outages.
Tim Qualheim is vice president of strategic solutions at S&C Electric Company. He is responsible for the strategy and direction of S&C’s solutions around the world, including switching technology that carries and directs electricity, the software and communications that enable advanced grid control and energy storage. Reach him at firstname.lastname@example.org.