The Big Event

By now, the "big event" has come and gone. And in my humble opinion, writing in early December, Y2K probably will pass uneventfully, without either global disturbances or the end of the world as we know it. Electrons don't know the time, date, or year. For the most part, they just keep flowing happily along. When they do not, we usually call it "an event," "a disturbance," or, in geek-speak, "a power quality phenomena." However, one person's event may be another's nonevent. In addition, what makes up an event to one person may be a series of events to another. The key issues here are system operation and system susceptibility. The electric utility system is designed to respond in a certain manner to "events" on the electrical distribution and transmission systems. Knowing how these systems operate, especially their fault protection, can be very valuable in determining the source of a disruption to a process in an industrial facility, or the computer systems in a data processing center. The electrical generators produce power at a comparatively low voltage, which transformers step up to hundreds of thousands of volts. This higher- voltage power flows more efficiently, with less power loss in the wiring along the way. Transmission systems at 500 KV are not uncommon anymore. These transmission lines feed many distribution feeders, so a transmission level fault can have vast effects, covering hundreds of miles. Fortunately, such events are generally rare, since drunk drivers, tree branches, and critters are less likely to affect these tall towers than they affect distribution system poles. The most common sources of problems on the electric utility system occur on the distribution system, typically less than 69 KV. A typical distribution substation may have three to seven feeders at 13 KV, being supplied off a common voltage buss fed from a transmission line through a step-down transformer. The most common sources of sags found in a 1976 study in Northern Virginia [1] are shown below, where there were an average of 40 thunderstorms per year. Cause of Sags on Distribution System CAUSE No. of OCCURRENCES PERCENTAGE Wind and lightning 37 46% Utility equipment failure 8 10% Construction or traffic accident 8 10% Animals 5 6% Tree limbs 1 1% Unknown 21 26% TOTAL 80 When phase-to-ground or phase-to-phase faults occur, such as when a drunk driver hits a pole, the system protection schemes begin to operate. The typical distribution breaker will wait six to ten cycles before opening, to give the fault a chance to clear itself. It will then stay open for a period, before trying to reclose. If the breaker can reclose, then the cause of the fault is no longer present. This recloser sequence may repeat three to six times, after which the breaker will be "locked out." If the breaker is locked out, human intervention will be necessary for the circuit to be re-energized. Breaker lockout prevents the wire lying on the drunk driver's car after he/she just hit the utility pole from arcing on and off continually. However, if the fault is caused by a lightning-induced flashover, then

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.