Just a saftey reminder.

[itsmoked]

A house burnt down on my street the day before yesterday. Pretty rare event nowadays. Maybe 2 houses a year burn in my city of 60k.

Fire started inside the garage and spread via the attic to the house. The cars gas tank ruptured causing a real ramp-up of the disaster.

Faulty wiring was the cause. I watched as the fire inspector dragged the faulty piece of wiring out of the garage thru the 8 fire trucks from our town an two others. He photographed the chunk of faulty wiring that was laying on the ground several times. Then drug it back to the house-remains and hung it on the fence.

Last nite I walked past the remnants and noticed the "wire". I went over and looked at it.

Orange 14AWG extension cord.


Another point in the extension cord column!

We should probably all review our extension cords and toss any with even the smallest defects.

 

[rmw]

Skogsgurra,

Thanks for clarifying that, because that is what I thought I had been taught long ago.

Someone else cleared me up on the reactive power in another thread.

All I have to do now is to figure out what it was that I thought I was taught about that (reactive power).

rmw

 

[felixc]

If the cord already gets warm when the current flows through, coiling it will increase its temperature further. But just for a matter of less conduction of the heat to the air. I lived it.

Right from the basement of my house, I can feel the warmth of the cable that feeds the central vacuum when it is being used. Standard white 14-2, standard Ametek motor, 10 amps, brand new.

felixc

 

[itsmoked]

All this is why you are never supposed to leave cords coiled or buried under insulated stuff like cardboard, carpet, wood, etc. Not to mention the increased likeyhood of injuring the cord walking on it.

Of course the classic is running zip cord under the livingroom carpet. <shutter>

 

[ruggedscot]

Wow reactive power heating power lines ?

I read about something that they were trying after they had the massive outages in Toronto - Get this.....

By co-ordination of breakers they could run the grid up so that a path existed that took in most of the affected lines. Then at one point they would create a bolted short. Go back to the main breaker feeding the transmittion line 750kV and close the breaker. Yep a long line feeding a short at that sort of voltage, it was thought that the resultant stresses on the cables would shake the ice off of the lines. It was meant to be a last ditch effort...

 

[VEBill]

I thought that PF was set by the load (customer end), not the source (Utility Co.).

In other words, I can't see the Utility Co. having a big knob labled "PF" for use in winter to heat transmission lines.

 

[logbook]

Power factor is set by the load.

Reactive VA load does increase transmission loss. If you put on a reactive load you will get more current and then heat the lines more. That's why factories get charged for poor power factor loads.

I love the idea of the shorted line! This would probably make the lines "jump" wildly when you get up into the kiloamp region. I suppose they rely on the main breakers saving the day. Of course the power would go down for the consumers when you did this "ice clearing" until the short was removed.

 

[Skogsgurra]

Sorry again folks. There are so many nice myths walking around out there. And so many teachers that think that a good story could lighten up a dull afternoon. They may even believe in them themselves...

Gunnar Englund

http://www.gke.org

 

[zeusfaber]

Is it true that some utilities keep big rotating machines dotted round the network just spinning light (I think I heard them called rotary compensators)? The idea being that they can over or under excite them to inject or absorb VARs somewhere local to the load to improve the power factor over the main transmission segment.

If these things are around then, in theory, adjusting them (something within the utility's gift) ought to have some effect on the temperature of the upstream transmission line.

A.

 

[rmw]

zeusfaber,

I went to

http://www.power-eng.com

and searched on synchronous condensers and got several hits. Here is one of the more interesting ones on the topic.

http://pepei.pennnet.com/articles/a...ID=192255&KEYWORDS=synchronous condensers&p=6

I had read another article that I thought was in Power-Eng magazine about the problem of IPP's only wanting to sell power and the utilities that transmitted their power having to carry the burden of supplying sufficient reactive power for the grid to be stable.

I wish I completely understood what I just said.

rmw

 

[rmw]

skogsgurra,

When I was in college one of the ways I worked my way through was as a Jr. Engineer at a heavy metal parts manufacturing plant where I learned a lot of practical engineering on the job.

I often had to sit in classrooms with my teeth clamped firmly on my tongue when my college professors-often men who had left the classroom as a student to begin teaching students-made statements that I knew from the real world to be false.

Since I was smart enough to know that the classroom was an autocracy not a democracy, I recognized that what the teacher stated was what was to be regurgitated back to them on the exams. Arguing with them only got you in trouble. I learned that the hard way too.

I didn't have the requisite experience to be able to ascertain what was true and what was BS in my non ME courses, however. So I was just stuck having to take whatever they said at face value.

rmw

 

[Skogsgurra]

rmw,

You have absolutely the right attitude. And the fact that you now question some of the more fantastic "truths" that you learned in the class-room is very promising.

The compensators that you mention are all very real. They are large generators that have no input shaft. They just rotate idly and by magnetizing them more or less, they produce reactive Mvars to support local demand and also stabilize voltage.

They are the equivalents of capacitor banks. In theory, they could be used to load the lines. All that is needed is to underexcite them which will make them draw more current. But, since their capacity is usually a lot less than the lines, the loading effect is probably not enough to heat - or shake - the lines.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

Here is a most interesting short paper on Stanford Linear Accelerator's synchronous condenser.

http://epaper.kek.jp/p95/ARTICLES/TAA/TAA16.PDF

Here's a common picture of one.

 

[zeusfaber]

Thanks for the links. Reassuring to see that something I think I was told about in a college lecture is a genuine product.

A.

 

[Skogsgurra]

There is one little twist to the power loss theme. I grew up not far from a main power distribution station where they had synchronuous compensators. I think that they were two or four-pole machines. So the windage losses would have been heavy if the machines hadn't been totally enclosed and filled with hydrogen (H2 gas).

The hydrogen did not only have a lot less friction than air, it also conducted heat a lot better. It wasn't easy to keep air from getting into the machines and sure enough - one day the gas mixture had reached the critical level where it just exploded. I do not know if H2 is used any more. Helium perhaps? Anyone?

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

Hi Gunnar,

Hydrogen is still very much used for large synchronous machines in the power generation business. Hydrogen cooled machines are less common below about 250MVA because of all the additional equipment required to keep hydrogen safely in the machine, but above this rating air cooled machines can't get rid of the heat.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[Skogsgurra]

Isn't this just wonderful?! I didn't know that. No Helium? Doesn't work?

Gunnar Englund

http://www.gke.org

 

[rmw]

Helium is too light. It makes the generator float up off of its foundations.

On a serious note, Scotty UK makes a good point. The windage/heat transfer characteristics of air vs. H2 are the same whether the machine is a generator or a condenser.

I note that in some of the reading on this topic that I did, and possibly mentioned in the link I gave above that in several cases locations were mentioned where generators were clutched to GT drivers so that they could be used on an either/or basis as required; clutched to the driver for power generation purposes, or de-clutched for the generator to act as a condenser.

rmw