Outages are expensive, soak up resources, weaken the system while they are in effect and aren’t always restored on time as promised to the customer. But, in one form or the other, we have to live with them.
There are two general outage classifications: forced and scheduled (non-forced) outage; different utilities may have different outage classifications. Forced outage: as the name implies is not planned; thus the utility have no control over it and cannot predict (although some system indicators may predict possible failures and thus steps are taken to remove the troubled component from service before it actually fails). The goal is to minimize the number of forced outages by performing several preventive measures and maintenance, installing smart devices and metering to monitor the system behavior. Outages impact the utility’s credibility, public image, reliability, may result in penalties, impede proposed rate increase and have detrimental l impact on customers.
Faults are the main reasons for forced outages: single phase to ground fault is the most common fault. Faults could be the result of a car hitting a utility pole, someone digging into a utility cable, animal contact, bad workmanship, etc.
Fortunately, scheduled outages are more common than forced outages, there are two main categories for scheduled outages: scheduled and emergency.
Common reasons for scheduled outages: (O&M) performing maintenance, some programs are mandated by the governing commission/state, usually driven by prior accidents/events, although utilities do a terrific job minimizing failures, accidents do occur and as a result, several programs are mandated to prevent the initial reason the failure occurred from reoccurring, i.e. a network was shut down due to the failure of several cables of the same type and manufactured year. As a result a program is mandated to replace all the cables in the system that share the same characteristics. New business is another main reason (un-preventable) for taking outages, i.e. to connect the new customer to the utility lines in order to supply them with electric service.
Summer-prep is preparing the system for the next summer where the system loads are high; as a result, outages are taken in the fall and spring to remove the weak parts of the system to ensure that the system can handle the forecasted loads. Outages are taken for environmental reasons as well, i.e. a transformer or cable leaking oil.
Emergency outage is another very critical category; as the name implies, there is an urgent situation where time is of the essence. If the emergency situation is so severe, i.e. transformer on fire, immediate action is warranted; regardless of the consequences; immediate action is taken to prevent more damage. Some system emergency conditions may not be that severe; action is still needed promptly; however, there are usually time for some quick engineering analysis and operating moves.
Issues with taking outages:
- Putting the system in jeopardy (since one element, i.e. cable or transformer is currently out of service during the outage).
- Cost: contrary to common belief, outages cost money and not simply a press of a button, because there are pre-moves required to ensure or safety of the crew that will be working and to ensure continuity of service when the outage starts.
- Diesel generators may need to be deployed and further field switching will be required in case more circuits are lost.
- Accessibility issues, i.e. car parked on top of a manhole, can’t access customer structure/electric vault, etc.
- Unplanned weather changes; think about the polar vortex where there the temperature was really low and the electricity usage increased significantly during the time when several system components were out of service for maintenance purposes; natural gas fired plants suffered.
Recall time is an important factor during the outage, where the party responsible for requesting the outage or the party actually performing the actual work inform the control center about the expected time needed to restore the equipment or feeder back to service, in case there was need. Of course, some outages are only granted a small window to perform the needed work and there is a need to work 24/7.
The higher the voltage, the more critical the outage is; some outages require notification to the regional transmission operator few months in advance. Higher voltage outages may have a major impact on the utility’s energy market and its neighbors. System operators are constantly running next worst contingency analysis. Timing is everything; worst case is the loss of critical components during peak hour; which the utility planning group plans for.
Another issue with outages is the post work tests, i.e. distribution feeders and transmission feeders undergo high voltage tests after the work is complete to ensure the integrity of the cable, splices, insulation, etc. Although those tests are necessary, they subject the cable to extreme conditions, which many believe weaken the cable even though the cables pass the tests.
As a result of the aforementioned, it is to the best interest to all to optimize the outage time and prevent the need to re-request the same component out of service again to perform different work in the near future. It could be as simple as a distribution e-mail list that get sent to all those who may benefit from feeders being out of service, long before the outage is planned. Also, even though the outage may be done during light load period, it is a good philosophy to treat every outage with a sense of urgency, just in case the system conditions rapidly change.
If several forced outages do occur simultaneously or the utility got hit by a major storm, the planning and operations group will prioritize which component will be returned to service first.
In summary, outages are the necessary evil; they are expensive, risky, involve a lot of man power, weaken the system while they are in effect and are not guaranteed to be restored on time as promised.