Electric System Outages

We’ve experienced four significant outages this year.  In three cases, key electric equipment or components failed.  As noted in the attached recap of outages, old equipment at the Downtown substation failed, a high voltage underground terminator failed outside of the Pine Street substation failed, and a large breaker in the Pine Street substation failed when it was damaged by a leaky roof.

 

In the case of the other significant outage, the typical “3-shot” system was switched to a “1-shot” setting designed to shut the power off quickly for safety reasons. The series of breakers were set to 1-shot only for a one week period during the construction on the Main Street culvert. The breaker settings were changed because workers were using cranes very close to power lines. Breakers are now set to normal operation. The outage was cause when a lightning arrestor on Owen Street failed.  Because the breakers were set on one-shot, once the breaker saw the fault and opened, it did not reclose as it would have under normal conditions.  Under normal conditions the outage may not have occurred at all or would have been limited to smaller area.

 

The breaker is an overcurrent device that can have an autorecloser which is used in a distribution system circuit and when tripped, will cause one or several brief outages followed by either normal operation (as the breaker succeeds in automatically restoring power after a transient fault has cleared) or a complete outage of service (as the breaker’s autorecloser exhausts its retries). If the fault is on an adjacent circuit, the customer may see several brief "dips" (sags) in voltage as the heavy fault current flows into the adjacent circuit and is interrupted one or more times. A typical manifestation would be the dip, or intermittent black-out, of domestic lighting during an electrical storm. Autorecloser action may result in electronic devices losing time settings, losing data in volatile memory, halting, restarting, or suffering damage due to power interruption. Owners of such equipment may need to protect electronic devices against the consequences of power interruptions.

 

Breakers may cooperate with down-stream protective devices called sectionalizers, usually a disconnector or cutouts equipped with a tripping mechanism triggered by a counter or a timer. A sectionalizer is generally not rated to interrupt fault current and is therefore cheaper than a recloser. Each sectionalizer detects and counts fault current interruptions by the circuit breaker. After a pre-determined number of interruptions, the sectionalizer will open, thereby isolating the faulty section of the circuit, allowing the recloser to restore supply to the other non-fault sections.