Monday, January 30, 2017

Tricks A Dumb Grid Can Play

Inauguration Day in Brookville, Pennsylvania arrived with a bang. Within minutes after Donald Trump swore to preserve, protect, and defend the constitution of the United States, the 911 calls began.  In one house, light bulbs were exploding.  Another resident reported that a power strip was smoking.  At another house, the siding was on fire, and at yet another the electric meter was engulfed in flames.  The main radio transmitter at the police station tripped out, so the 911 call center was unable to contact them until the dispatcher used his battery-powered radio to make contact.  In all, about 400 residents of the small western Pennsylvania town of 3800 suffered some type of damage, ranging from singed carpets to fried computers and exploding fluorescent light fixtures.

The power surge had nothing directly to do with Donald Trump.  A quick investigation by Penelec, the local electric utility, revealed that an insulator on a power line had failed.  What follows here is my extrapolation from the limited details in the Associated Press wire story, but represents what I think is a good guess.

Electric power networks are divided into transmission lines and distribution lines.  The transmission lines are the tall steel-framed towers that span many miles across the countryside, and are the interstate highways of the electric grid, transmitting megawatts of power from generating plants to substations many miles away.  To transmit this much power efficiently, the voltage of these lines is generally above 100,000 volts (100 kV).  For example, there is a 138-kV transmission line that connects Brookville with the rest of the power grid in western Pennsylvania, and there may be others at even higher voltages.
Once power arrives at a substation, it is stepped down in voltage with large, expensive units called transformers to a lower voltage suitable for distribution locally.  Distribution lines, the neighborhood streets of the network, carry voltages in the range of 12 kV to 35 kV, or occasionally higher in rural areas.  My guess is that Brookville has at least two or three separate distribution circuits with voltages in the 25-kV range.  The familiar wooden power poles that carry telephone and cable TV lines also support power-distribution cables, always suspended on the highest point of the poles.  Every few hundred feet, a "pole pig" (distribution transformer), usually a metal can a couple of feet tall, lowers the voltage still more to 240 V or less for delivery to commercial and residential customers. 

What probably happened was this.  One of the high-voltage insulators on a transmission line carrying in excess of 100,000 volts failed mechanically, dropping its conductor on or near enough to one of the town's distribution lines to allow a flashover (an arc) to jump from the 100+ kV transmission line to a 25-kV distribution line.  All power-line insulators are built with a safety margin.  That is, an insulator for a 25-kV line may be able to withstand 50 or even 100 kV, which can happen during situations such as lightning surges and so on.  This is good in normal circumstances, but in this case it backfired.

The insulation of the 25-kV distribution line held just long enough for the high voltage, four or five times normal, to get into the distribution transformers and ultimately the houses of about ten percent of the town.  So for a few seconds or maybe even longer, equipment designed for 120 V was receiving, say, 500 or 600 V. 

There are kooks on YouTube who delight in taking innocent electric appliances such as razors, clocks, toasters, light bulbs, and so on, and connecting them to high-voltage power sources just to see what happens.  They are never pretty.  Every electric appliance has a maximum rated voltage, and when you exceed it by 500% you either blow a fuse or, in the case of equipment that doesn't have fuses such as light bulbs, the excess heating makes something melt or vaporize or explode. 

Many power strips have surge-arresting devices in them meant to absorb fast transient surges caused by lightning.  But those surges usually last only milliseconds, and a surge of several seconds overheats such a device, making it smoke, which explains the reports of smoking power strips. 

Why didn't all the protective devices that a utility normally uses, such as fuses and circuit breakers, operate right away?  Because they are designed primarily for lightning strikes, not an overvoltage that lasts many seconds.  And the fuses probably didn't blow right away because a fuse only slightly over its rated current takes a considerable time to melt. 

Penelec has announced that they will compensate those who have suffered losses as a result of the surge.  Fortunately, no one was injured, but a considerable amount of property was damaged and the mess will take weeks or months to clean up. 

The "dumb grid" in the title refers to the fact that most electric utilities still use protective technology that was developed prior to World War I, for the most part:  fuses and electromechanical relays.  Innovative "smart grid" technology connecting the power grid to the Internet and replacing many electromechanical controls with faster-acting solid-state devices promises a number of good things, mainly pertaining to increased efficiency and reliability.  But it's also possible that if Brookville's grid was smarter—that is, if it could have figured out within milliseconds what happened and cut off the surge then—none of the bizarre damage might have occurred.

Admittedly, situations in which a transmission line arcs over to a distribution line are rare.  But as Brookville shows, they can happen.  If the new Trump administration wants to improve America's infrastructure, encouraging utilities to take the smart-grid path is one way to do it.  Whatever Washington does about it, though, it's too late for a few hundred residents of Brookville, who are still replacing siding, light bulbs, and computers as a result of a freak accident that shows we still have a ways to go in improving electric utility safety and reliability.

Sources:  The Associated Press report on the Brookville incident of Jan. 20, 2017 was carried by a number of news outlets, including at on Jan. 28, 2017.

1 comment:

  1. Quote: " Innovative "smart grid" technology connecting the power grid to the Internet ... promises a number of good things, mainly pertaining to increased efficiency and reliability."

    It also threatens our safety by opening up our power distribution system to any hacker on the planet. Networked control, yes. Doing so via the Internet, no.