Is this so?

[peter]

I was perusing Wired.com and I came upon this piece about if New York was ready for electric vehicles. Joseph Something, a head honcho of ConEd said:

?The challenge for us is that we have all this copper cable underground that are sending power out to transformers,? he said. ?Those large cables rely on cycling. If they are at a high load for a period of time they can carry a certain amount of electricity but if they don?t cool down during the night, they will start to lose capacity. They need to be cooled to maintain the capacity. If we didn?t cool them down, the next day they couldn?t carry as mich and we?d have to put more cables in the ground. They will fail sooner or they will fail catastrophically when you need them most.?

Now I didn't know that electric wires have to cool down at night. Is this true.
~Peter

 

[iwire]

I am going with yes to some extent.

 

[cadpoint]

I think You read a secondardy article about the newer design for power distrubition cables, this involves cooling as part of the application and is incorporated into the design of the service.

Ok He was thinking he needed to cool his wire to be the latest!

 

[sameguy]

They get tired, that's why the power co. shuts off all the power plants at night. Sounds like bs but? Its not like we are the manufacturing hub we were so wheres the big loads? He could be telling us the feeders are too small and old.

 

[dbuckley]

Heavily based on truth.

Those ampacity tables in the NEC are all based on the ability of a cable to dissapte heat to prevent over-temperature conditions. However, PoCos push much harder than the NEC allows. Especially smaller transformers; they get overloaded to hell and back, secure in the knowledge that their loads are not continuous, and they'll get a chance to recover.

Cabling, switchgear and transformers have an "emergency" rating, which is what they can actually do at the limit do for a few hours if really necessary. The skill of a utility is to actually understand what these ratings are, and how they can be used. These ratings are thermal in nature. When they get it wrong, you get blackouts. Here in NZ the mob in Auckland vastly overestimated the ratings of several of their underground feeders, and subsequently bust them by not appropriately load-shedding during a cascading failure, and thus put the CBD out of power for weeks. Morons.

 

[mivey]

Never pushed a cable such that it had to be cooled down every day. Transformers, yes. The cooling down period just re-coups some of the life of the transformer you stole by running it hot (the de-grading of the transformer insulation). There are limits, of course. For extended transformer abuse, we would add additional cooling.

Not sure how extended abuse of a cable would work out. I would think it would have been more of a issue of loosing insulation life than capacity. I mean once it is hot, it is hot.

They must be riding right on the edge of acceptability. If they were pushing into the upper limits, I would think they would have a cable failure. The earth is a big heat sink, but I would not think it would sink the heat fast enough to allow you to abuse the cable all day long, every day. I would think auxiliary cooling would be needed like forcing cool air through the duct bank or installing chilled pipes in with the conduit.

 

[K8MHZ]

They must be riding right on the edge of acceptability.

Or worse.

 

[LawnGuyLandSparky]

I was perusing Wired.com and I came upon this piece about if New York was ready for electric vehicles. Joseph Something, a head honcho of ConEd said:

?The challenge for us is that we have all this copper cable underground that are sending power out to transformers,? he said. ?Those large cables rely on cycling. If they are at a high load for a period of time they can carry a certain amount of electricity but if they don?t cool down during the night, they will start to lose capacity. They need to be cooled to maintain the capacity. If we didn?t cool them down, the next day they couldn?t carry as mich and we?d have to put more cables in the ground. They will fail sooner or they will fail catastrophically when you need them most.?

Now I didn't know that electric wires have to cool down at night. Is this true.
~Peter

During heat waves in NYC, ConEd sends crews out, sometimes one on every other streetcorner, with the manholes open and the manhole blowers (HUGE ONES, like the fire departments use to get smoke out of a building after a fire) all night long. In some areas of Midtown, during the day too.

In other areas especially uptown and downtown everything below street level is underwater anyway...

 

[esobocinski]

This is no different than the NEC concept of continuous load vs. non-continuous load that everyone here already knows. NEC allows a higher capacity for 3 hours or less because it takes a while for the load to heat up the cables/equipment, so you aren't really reaching temperature limits before the load goes away and everything cools off. If you do reach temperature limits, a short infrequent period at those conditions still has a much smaller impact on overall service life than if those conditions are commonplace.

Utilities play the same game, except that they theoretically know the heating characteristics of their cables/equipment and thus can make more informed decisions instead of relying on a fixed 3 hour rule of thumb. They also can make informed cost/benefit decisions like "reducing the remaining service life of this cable to 5 years is worth not causing outages", whereas NEC expects indefinite serviceability.

That, of course assumes that the utility operators are competent. If not, it's a recipe to go boom as dbuckley relates above.

 

[e57]

One must also take into account much of NYC is underground - no free air cooling. And once a cable is hot it will conduct less... And the next day it will be starting at a higher temp when usage is at peak, get hotter, and conduct even less - creating more heat losses. If it's summer time it would be even hotter - with less cycling of demand, and higher heat losses.

So yeah - I think they are worried that they won't be running large swinging cycles anymore - but this is exagerated since I doubt many people in NYC proper have a car or a place to plug it in. Nor would the taxi fleet allow itself to be dependant on plugging in every few hours.

 

[winnie]

If they are operating the cables at overload during the day (meaning that they are operating at a current level which would cause the conductors to overheat if maintained), then they are depending upon the overnight cooling to maintain their overload capacity.

However I believe that if they were to operate the cables continuously at their maximum continuous rating, then the total energy delivery capacity would be greater.

This is easily shown if you assume a single maximum temperature. Consider a system that is on for half the time and off for half the time, and compare it to a system that is on at half the current level continuously. You get twice the heating in the on-off system as the continuous system. Additionally, with two systems that have the same _average_ heat production, you will get the same _average_ temperature...but the system with steady heat production will have a lower peak temperature.

However I don't think my assumption is valid; while we are used to single 'maximum' temperature ratings, the reality is that conductors can tolerate a certain amount of overtemperature. I don't know how to solve for maximum total energy delivery once you factor in conductor overtemperature allowance...but my gut tells me that the continuos case will still deliver more total energy than the overload/cooldown case.

-Jon

 

[SAC]

The post about the starting temp at the beginning of each day is probably spot on. It's not so much that they are running over capacity, but that the load is such that over the peak usage period there is a net gain in conductor temperature, and they depend on the down period to allow them to cool back to a lower point for the start of the next peak period.

Picture a sine wave - it goes up and down, but the average, peak, and minimum are constant over a large number of cycles. Now picture a sine wave where every "up" part is just a little more "up" than the "down" part is "down" - the average goes up, the peak goes up, and the minimum goes up. At some point the temp gets to the point where it is over the limit.

 

[cadpoint]

...Picture a sine wave - it goes up and down, but the average, peak, and minimum are constant over a large number of cycles. Now picture a sine wave where every "up" part is just a little more "up" than the "down" part is "down" - the average goes up, the peak goes up, and the minimum goes up. At some point the temp gets to the point where it is over the limit.

Feel free to clarify exactly what your saying... I don't know what your exactly saying...

In respects to voltage wouldn't one want a constant peak, or average or medium; whatever wave your talking of will always be constant to whatever the voltage applied to! Frankly it'd be less Voltage not more as more current is applied.

Isn't it the current that plays havic on the sine wave and the wire! This is what will drag down a peak or average or medium measure of the voltage?

In the simplist sense it's still a ratio equation of A=B/C. While there are many meters, there are many many ways to measure what your exactly need.

Sorry I'm missing what your saying!

 

[SAC]

Feel free to clarify exactly what your saying... I don't know what your exactly saying...

Sorry - my comment was much more simplistic and not related to the "sine" in AC at all. My comment was about the average heating of the conductors during the peak electrical usage of a day vs. the cooling that occurs during the down period of electrical usage of the day. If the assumption is that the cooling available is not enough to prevent a rise in the conductor temperature during the peak period, but that it provides a cooling period during the down period, then everything can average out during a day.

For example, consider that for 12 hours a day the peak usage causes a 1 degree rise/hour, and that during the down period it allows a 1 degree fall/hour. On average over a number of days the conductor temperature remains the same with no net increase in conductor temperature. But now increase the "down" current such that it only allows a .5 degree/hour fall over the 12 hour "down" period, and there is now a net 6 degree/day gain in conductor temperature (12 degrees up during the peak, and 6 degrees down during the down). Over time, the conductors may now overheat (or require additional cooling).

 

[weressl]

If they are operating the cables at overload during the day (meaning that they are operating at a current level which would cause the conductors to overheat if maintained), then they are depending upon the overnight cooling to maintain their overload capacity.

However I believe that if they were to operate the cables continuously at their maximum continuous rating, then the total energy delivery capacity would be greater.

This is easily shown if you assume a single maximum temperature. Consider a system that is on for half the time and off for half the time, and compare it to a system that is on at half the current level continuously. You get twice the heating in the on-off system as the continuous system. Additionally, with two systems that have the same _average_ heat production, you will get the same _average_ temperature...but the system with steady heat production will have a lower peak temperature.

However I don't think my assumption is valid; while we are used to single 'maximum' temperature ratings, the reality is that conductors can tolerate a certain amount of overtemperature. I don't know how to solve for maximum total energy delivery once you factor in conductor overtemperature allowance...but my gut tells me that the continuos case will still deliver more total energy than the overload/cooldown case.

-Jon

To be a little more descriptive of what is actually happening.

Let's say that a cable is rated 100A at 70C rise. When operated at 70A the temperature will rise to 55C. If operated at 130A the temperature will rise to 85C, therefore shortening the life of the cable.

You have a fairly repeatable cyclic loading varying between those two numbers of 70A and 130A. The load increase and decrease from low to high is predictable and occurs through several hours then stay at the level for several hours before start changing again. Since your cable is underground, the temperature is constant, unlike if it overhead where you would need to consider the ambient temperature cycling. As we said the high current and temperature is 85C@130A. However the temperature rise is not immediate. It has been shown that if it is started @ 55C the current rise will reach temperature equilibrium of 85C after 7 hours of continuous loading @130A. When the loading cycle is plotted it is shown that the current/time profile does not produce temperatures greater than the rated conductor temperature, but even more importantly the unloading provides sufficient cool down time that the conductor reaches the low equilibrium of 55C.

In addition to the above there is what is called the emergency rating of the conductors that allows a certain higher temperature to exist for so many hours continuously and for so many cumulative hours of the cable's life.

The above is greatly simplified; it is just to give a sense of how these things are determined.

 

[zog]

I was perusing Wired.com and I came upon this piece about if New York was ready for electric vehicles. Joseph Something, a head honcho of ConEd said:

?The challenge for us is that we have all this copper cable underground that are sending power out to transformers,? he said. ?Those large cables rely on cycling. If they are at a high load for a period of time they can carry a certain amount of electricity but if they don?t cool down during the night, they will start to lose capacity. They need to be cooled to maintain the capacity. If we didn?t cool them down, the next day they couldn?t carry as mich and we?d have to put more cables in the ground. They will fail sooner or they will fail catastrophically when you need them most.?

Now I didn't know that electric wires have to cool down at night. Is this true.
~Peter

Many of the NYC underground cales are now superconductors, which have a cooling system, keeping superconductors cool is vital to thier operation. Perhaps that is what he is refering to.

 

[zog]

They get tired, that's why the power co. shuts off all the power plants at night. Sounds like bs but? Its not like we are the manufacturing hub we were so wheres the big loads? He could be telling us the feeders are too small and old.

Shut down power plants at night? It takes hours, sometimes days to do a plant shutdown and start up. Most of the power is supplied by "base load" plants, which are nukes, coal fired, etc... they hardly ever shut down, usually for a short period in the spring and/or fall for maintenence.

"Peaking plants", which are typically gas turbines (Think airplane jet engine with a generator attached) are designed for fast start ups and shut downs and only run during peak load conditions.

 

[winnie]

Laszlo,

Thanks for the further details. I'd love to hear more since on the power distribution side of things I only have theory, not experience. I know how motors operate under overload (since I regularly push motors in overload on the dyno), and I know what motor insulation life is supposed to do with temperature (theory again, since I've not pushed motors to insulation failure (yet)), and I know that the theory is similar to what is happening to cables underground...but there is a wide gap between theory and practise.

Question: when you are talking about cables underground, does the heat produced by the cable change the temperature of the soil enough to matter? I am guessing that the soil adds both to the thermal insulation (less heat for a given temperature rise at equilibrium) but also adds to the thermal mass (takes longer to reach equilibrium)?

Zog,

I don't think that superconductors are the issue here. Superconductors don't have much of a grey area temperature wise. Either they are cold enough to superconduct, in which case they don't self heat, or they are above the superconducting transition temperature, in which case the self heating is so great as to prevent them from superconducting if they are carrying current. Basically, if something happens to stop the line from superconducting just for a moment (either it gets too warm or it carries too much current or it is subjected to an excessive magnetic field), then the line will quench and fall completely out of superconduction.

-Jon