Thursday, 29 September 2011

Power failures and why they happen

I'm getting rather fed up with the explanation given by the MD of the JEC that the power failures were due to "complex issues". The public really deserves a better explanation than that.

It's a "black box" explanation - there was a power cut, but it could have been the result of a dark conspiracy in a Dan Brown scenario or a Dennis Wheatley black magic cursing the French operator and causing a flock of crows to fly into power lines.

But really, understanding what happens is quite simple, even if locating what might be termed "the fault in the circuit" is more difficult, and we deserve better explanations than "these are complex issues".

First of all, this is how a power grid works:

At a high level, the power grid is a very simple thing. It consists of a set of large power plants (hydropower plants, nuclear power plants, etc.) all connected together by wires. One grid can be as big as half of the United States. A grid works very well as a power-distribution system because it allows a lot of sharing. If a power company needs to take a power plant or a transmission tower off line for maintenance, the other parts of the grid can pick up the slack.

The thing that is so amazing about the power grid is that it cannot store any power anywhere in the system. At any moment, you have millions of customers consuming megawatts of power. At that same moment, you have dozens of power plants producing exactly the right amount of power to satisfy all of that demand. And you have all the transmission and distribution lines sending the power from the power plants to the consumers.

This system works great, and it can be highly reliable for years at a time. However, there can be times, particularly when there is high demand, that the interconnected nature of the grid makes the entire system vulnerable to collapse.

And that's what happened on Monday and yesterday - something caused the grid to collapse. What happens in such a scenario is that the failure "cascades", and can often only be prevented from bringing other collapses by a process of triage - that is, decisions are made to close down sections of the grid to safeguard the rest, and then, as the failing area is checked over, parts can be brought back on line. Here are some scenarios:

Let's say that the grid is running pretty close to its maximum capacity. Something causes a power plant to suddenly trip off line. The "something" might be anything from a serious lightning strike to a geomagnetic storm to a bearing failure and subsequent fire in a generator. When that plant disconnects from the grid, the other plants connected to it have to spin up to meet the demand. If they are all near their maximum capacity, then they cannot handle the extra load. To prevent themselves from overloading and failing, they will disconnect from the grid as well. That only makes the problem worse, and dozens of plants eventually disconnect. That leaves millions of people without power.

The same thing can happen if a big transmission line fails. In 1996, there was a huge blackout in the western United States and Canada because the wires of a major transmission line sagged into some trees and shorted out. When that transmission line failed, its entire load shifted to neighboring transmission lines. They then overloaded and failed, and the overload cascaded through the grid.

In nearly every major blackout, the situation is the same. One piece of the system fails, and then the pieces near it cannot handle the increased load caused by the failure, so they fail. The multiple failures make the problem worse and worse, and a large area ends up in the dark.

The kinds of problems that cause the failure are not complex, and some of them are listed here:

1) Natural Causes - Weather Related

Numerous power failures are caused by natural weather phenomena such as lightening, rain, snow, ice, wind, and even dust.

2) Other Causes of Outages

Animals coming into contact with power lines, such as large birds, accounted for 11% of outages in the U.S.. Additional causes of failures were primarily man made outages that show up in the form of vehicle and construction accidents with power poles and power lines, maintenance from utilities, and the occasional human error.

3) Short Circuits - A short circuit occurs when an electric current travels along a path that is different from the intended one in an electrical circuit. When this happens, there is an excessive electric current which can lead to circuit damage, fire, and explosion. Short circuits can occur when the insulation of the wiring used breaks down. It can also occur due to the presence of an external conducting material (such as water) that is introduced accidently into the circuit.

4) Electrical treeing is a phenomenon that affects high power installations such as high voltage power cables, transformers, etc. Any impurities or mechanical defects in the equipment used in high voltage installations can lead to partial electric discharges in the equipment. The damaging process manifests itself in a tree-like pattern, hence the name electrical treeing. Over a period of time, if it goes undetected, this phenomenon can lead to a continuous degradation of the equipment and eventually result in a total breakdown.

5) Sudden variations: If certain parts of the grid are carrying electricity at near capacity, a small shift of power flows can trip circuit breakers, which sends larger flows onto neighboring lines to start a chain-reaction failure

But the complexity lies in finding the fault. A circuit breaker may trip in your house, but it is difficult sometimes to know what electrical equipment caused the safety cut out to come into play. Sometimes it is obvious - a light has blown, and all the lights in the house go off as the switch trips. But it could be a bulb blowing, or it could be a fault in the light circuit - I've had a problem with spotlights, and it was the holders that had problems, not the lights themselves.

With a large national grid, it is much more difficult. In August 14th 2003 there was a massive power failure in the USA, but determining what caused it was more difficult. That is where the "complexity" comes in, because if the right cause is not found, the same problem can occur again. This was the story:

Power was restored yesterday for most of the 50 million people caught in the largest blackout in the nation's history, which shut down airports, nuclear-power plants, subways, elevators and traffic signals from New York City to Toronto and Detroit. At least one person died in the United States, and possibly two in Canada. The massive outage, which shut down 100 power plants in eight states and Ontario, was blamed on a power-grid failure that cut electricity service across the northern United States and Canada. There remained no explanation as to what caused the grid failure. It took just nine seconds for the power outage to spread across the region, incapacitating plants in Connecticut, Massachusetts, Michigan, New Jersey, New York, Ohio, Pennsylvania and Vermont, officials said.

It still is not clear what caused the blackout, Mr. Gent said. "This is really the big question. I can't answer that," he said in a telephone conference call. "I personally am embarrassed [that the outage occurred]. I am upset. My job is to see that this doesn't happen, and I failed in my job. That's why I'm upset." Mr. Gent said the North American Electric Reliability Council is focusing on a distribution route of transmission lines known as the Lake Erie Loop, which goes from New York to Detroit to Canada and back to New York. He said about 300 megawatts of electricity were flowing east into New York when something happened on the loop in the Midwest. When that happened, the electricity reversed course, sending about 600 megawatts of electricity west.

The first sign of a problem showed up in the Midwest, he said, where power lines first shut down, but that doesn't mean the outage started there. The event that led the electricity to reverse course lasted no more than 10 seconds, he said. The Lake Erie Loop is "the center of the focus," he said. "This has been a problem for years, and there have been all sorts of plans to make it more reliable," Mr. Gent said, adding that an investigation may not yield clues about the cause of the outage for months, but that there could be preliminary answers within weeks. "The final verdict may be months away," he said. "We will absolutely be able to tell where this happened and why it happened."

This is in fact the sequence of events, as constructed later:

1:58 p.m. The Eastlake, Ohio, generating plant shuts down. The plant is owned by First Energy, a company that had experienced extensive recent maintenance problems, including a major nuclear-plant incident.

3:06 p.m. A First Energy 345-kV transmission line fails south of Cleveland, Ohio.

3:17 p.m. Voltage dips temporarily on the Ohio portion of the grid. Controllers take no action, but power shifted by the first failure onto another power line causes it to sag into a tree at 3:32 p.m., bringing it offline as well. While Mid West ISO and First Energy controllers try to understand the failures, they fail to inform system controllers in nearby states.

3:41 and 3:46 p.m. Two breakers connecting First Energy's grid with American Electric Power are tripped.

4:05 p.m. A sustained power surge on some Ohio lines signals more trouble building.

4:09:02 p.m. Voltage sags deeply as Ohio draws 2 GW of power from Michigan.

4:10:34 p.m. Many transmission lines trip out, first in Michigan and then in Ohio, blocking the eastward flow of power. Generators go down, creating a huge power deficit. In seconds, power surges out of the East, tripping East coast generators to protect them, and the blackout is on.

Our power comes from RFI France, and they produce a report on failures. The most common failures are those of "busbars" -a busbar is a strip of copper or aluminium that conducts electricity within a switchboard, distribution board, substation or other electrical apparatus, and of course, if these fail, an electrical substation can fall out of the system. Their 2010 report noted that:

The number of short-circuits affecting transmission facilities dropped by 19% compared with 2009, continuing a trend observed for several years now. 97% of these short-circuits were transient and therefore did not impact the availability of the transmission facilities.

In terms of reliability, we observed 12 simultaneous faults on double 400 kV lines (11 in 2009), all transient (for 3 permanent in 2009), and three 400 kV busbar faults (versus 8 in 2009), of which two were due to damaged equipment and one to human error.

The level B ESS involved: two, well managed, double busbar faults; one N-k reliability guarantee loss following a busbar fault with a duration limited to one and a half hour; a (very low) risk of the loss of a production volume higher than 3000 MW following a busbar fault during a differential busbar protection scheduling of outage; failure to execute a safeguard order by a transmission control centre, with limited impact.

The evolution of ESS 0 reveals an increase in the number of problems involving busbar switch disconnector in 400 kV substations, knowing that incomplete operation can lead to major risks for the reliability of the electrical system. It is necessary to ensure that the current actions are continued and that they are relevant..

So while we've not had problems, it is interesting to notice that France has not been without some problems.

One of the curiosities about last night's incident is that it was noted in the JEP that "Jersey, Guernsey and significant parts of the Normandy peninsula were affected when the French electricity network developed a serious fault."

However, I've not been able to find any reports in the French newspapers about the power failure effecting Normandy or elsewhere in France. It is very curious indeed.

"The Connexion", Frances English-Language paper has lots of breaking news over the past week, but nothing about power cuts anywhere. French News online doesn't mention it. Le Monde doesn't. Figaro doesn't. Paris Normandy doesn't. France Daily doesn't. Reuters Frances doesn't.

The Washington Times, 2003


Anonymous said...

It would be nice if our local news covered stories of interest from just over the water, but it's as if Normandy and Brittany just don't exist, or nothing ever happens there that could possibly interest us.

OK so with digital TV standards being the same in both countries now (used to be PAL in the UK and SECAM in France to make things difficult) it's easier to point a second aerial at the nearest coast and pick up their telly but it's difficult to follow what's going on with only O-Level French from years ago!

Would it take much to have an agreement with French TV to use their footage, and bring in a bilingual to make sense of it for us?

Mark Forskitt said...

A useful list of online French newspapers, including some in English.

TonyTheProf said...

I tried there Mark - that's where I got the newspapers I looked at. Couldn't see any power cut story though.
If you find a link, please post as a comment.