Message Board Thread - "345kv Disconnect - 64C Rise"

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345kv Disconnect - 64C Rise electricpete 6/2/2005
Attached is an image of a 345KV manual disconnect.

This is one of two parallel paths through which flows a total of 1250MW. (You can calculate the total current from that if you want).

(This is the generator bay in a breaker and a half scheme.)

First reading waS 64c rise in a 7.5 mph wind.

Second reading two days later approx 10C rise in a 2.5 mph wind.

The total loading has not changed. Division of load between the two parallel paths is unknown (I assume it splits based on impedance of the two paths).

How serious would you view this? What do you make of the fact that it has gone down? What would you consider the risks would be if switching was done that opened the parallel path to put all the load through this hotspot?

Re:345kv Disconnect - 64C Rise Bob Berry 6/3/2005
Are you absolutely sure this is not a reflection as a change in viewing angle could account for the variation. The wind speed dropping should cause an increase in the delta and this would cause me to be suspicious of my readings.If you are sure then even a 10 degree C delta is serious enough to warrant some further investigation. in the meantime monitor, monitor, monitor.
Re:345kv Disconnect - 64C Rise IRJay 6/3/2005
I would disocunt reflection as the image shows no other signs of this. I would think the connection "fixed" itself by welding. Continue to monitor, it could prove very bad to switch full load to this switch until the integrity can be verified.
Re:345kv Disconnect - 64C Rise Doctir bob 6/10/2005
I wrote a paper some years back analyzing a problem on a 230KV switchyard ring bus. The hinge had, in fact, welded itself and on post mortem exhibited a resistance of close to 100,000 micro-ohm. Doing the analysis, I discovered that hot spots will cool off as their resistance increases when they are in parallel circuits. This is due to the current being shunted to the path of least resistance. As the power dissipated is I-squared times R, the current dominates. It can get so bad that the true problem is actually not the hot spot, as the current in that leg is too low to cause much heating. But there could be a hot spot in the other leg due to so much current being shunted.

My advice to anyone finding a hot spot on a parallel circuit is to next find the current in each leg. The current will tell you where the problem really is. We have had experiences where the hot spot wasn't the big problem.

You open the wrong side, you get an explosion

Re:345kv Disconnect - 64C Rise Carl M 6/10/2005
A very interesting case study, fellows. I do have a novice questin (novice to HV transmission)
What is "Hinge Resistance" as applied to ring circuits?

Thank you,
Carl Marcinkowski
Re:345kv Disconnect - 64C Rise electricpete 6/30/2005
Carl - as far as I know a hinge is a part of the switch.

Bob - One thing I would say is missing from your model is the reactance. Not a small error since high-voltage buses are overwhelmingly inductive. Qualitatively the results are similar but quantitatively you need a tremendous increase in resistance before the load sharing is affected.

Current is stricltly not available unless we clamp on to circuits which can trip 1250MW which wer are not inclined to do.

We examined with infrared the two paths at several points. What we saw were similar temperatures among all three phases within each path. For example if I looked at the path with the hot connection at a different series point, I see 2C rise on all three phases (buswork above ambient). If I look at the other series path I see somewhere around 5C rise. So the other path is carrying more current but it is balanced among phases which leads me to believe the resistance is not affecting the current sharing (the split between the two paths is established by the reactance). Clear as mud?

The temperature remains stable in the range 10-15C on all readings after that first one. Interestingly, we had the highest wind of all during the first survey. Any more thoughts?

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