redundancy in power distribution grid

[siegeld]

I live in a town that is hard hit every time there is a storm. Trees knock out power lines - often the main feeders to an entire area. I'm wondering why the utility doesn't build a little redundancy into the local grid. For example, an area can have two different connection to the main feeders in different areas, so that a tree that takes out one line doesn't cause a total failure. I'm wondering if this is standard practice or if it is never done. Technically I think this should work, am I wrong? The other suggestion people keep making is to try to get the lines buried, but that's never going to happen. It just costs far too much.

 

[bacon4life]

Yes, many areas already have some redundancy in the local distribution grid. However, most of the switching to alternate feeders is done manually. When a crew is called out, the first task is often to reconfigure the feeders to get the majority of customers back in service, then they start working on repairing the radial portion. Some utilities are pursuing automated switches, however they are much more expensive and often require significant investments in control and communication infrastructure.

 

[HornTootinEE]

Like any engineering project, it comes down to cost vs. benefit. Good utility design does attempt to build "looped" distribution systems-meaning they can be switched in the case of an outage on their main feeder. As bacon said though, that is often manual and crews first reconfigure the system to get the most people back on as quickly as possible, then go on to restore critical feeders based on hospitals, emergency services, commerce, etc.

 

[cranky108]

You are correct that underground is expencive, and has about a 30 to 35 year life before it starts failing. The underground failures are difficult to find and result in longer outages due the time it takes to find and replace.

The problem is the common assumption is underground cable lasts for ever. It dosen't. But over head wire lasts as long as a tree dosen't fall on it.

There are some recent NERC reports that point out that many states don't have tree trimming requirements for distribution systems. So i suggest you call up your state rep. and complain (the squeaky wheel gets the grease). Just be aware the utility will be asking for a rate increase to pay for it.

 

[siegeld]

Why do you need switches for the redundancy? If you have extra connections the power will just flow through them all, when some fail the load should automatically redistribute, no? I'd think that the cost of the redundancy is just the extra wire, is that not correct?

 

[cranky108]

No. If you have a loop system with two feeds, and a fault happens, then most likely both breakers on both feeds will operate.

However if you have only one feed, the fault current is lower and one breaker operates outaging half the customers. Someone then has to go and find the fault and open switches to isolate it. Then other switches are closed to restore customers.

If you want a better quality of power, some utilities like the one I work at, will ask you to pay more for that. Few customers do ask for a redundit power feed, and are willing to pay for it. Most ask, but want it for free (you get what you pay for).

Because it costs more to provide redundit service, it is only fair that customers who are not happy with 99.999% power, and want 99.9999% should pay for it.

 

[siegeld]

Well, our town is not 99.999% reliable - I'd be happy with that! I don't have power probably averaging something like 3 days a year. I'm part of ConEd, but not in NYC. I'm paying just about the highest rates in the country for this poor level of service. My guess is that the local grid is not intelligently designed.

 

[wayne440]

It is all relative, "siegeld" - I would be thrilled with 3 days out of the year, I have had 3 outages in the last 30 days.

 

[HornTootinEE]

Perceptions are all in the eye of the beholder. Which is why outage indices (CAIDI, SAIFI, etc.) are prevelent for utilities to measure their "relative" power quality.

The nature of distribution doesn't allow it to operate loop-fed on a continous basis such as transmission systems do. Cranky explained it above.

If you live in rural areas of ND where I come from, a typical ND spring snow/ice storm can tear down miles of rural distribution, leaving farms and residences out for weeks (literally) at a stretch. Those customers all know it comes with the territory.

Many urban customers don't realize that electricity is really one of the most reliable sources of energy they have access too-even if they are out a few times a year, which is often a weather related issue which can't be controlled by anyone. When was the last time you were out (more than once) for seperate system failure events? And "blown transformers" don't count-thats what everyone blames their outages on. I can count on one hand the number of times I was called out on an after-hours call that was NOT nature related (or 25 yr old cable-as cranky also mentions)

 

[cranky108]

Good point weather is not the only factors causing outages. Animals on overhead equipment, or snakes in switchgear, car/bad driver related outages are also on the list. And some EPRI studies have displayed lightnings ability to strike and fail underground cable. And there is also human error.

Of these other causes the bad driver related outages seem to distress me the most. Some how cars seem to aim for those green boxes, and a few have caved in some of our underground junction boxes (they weren't ment to be driven on).

 

[HornTootinEE]

Drivers aiming for green boxes must be true. I was called out once-a van had run over a single phase primary pedestal. When I got there, he had not only run over it, he was half way down his van on top of the pedesal, front wheels pointing 45 degrees in the sky. He had also run over a 1000-pair phone ped at the same time... THAT was interesting.

 

[cranky108]

I have also noticed street lights, fences, trees, mail boxes and fire hydrents as targets. So it is not just green boxes.
The cable TV box near my house has been open for a better part of a year, probally also from a bad driver. The cable company dosen't seem to be concerned.