Message Board Thread - "Metering CT and PT Connection"

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Metering CT and PT Connection neatster77 7/11/2005
This image shows current and potential tranformer connections to a 115 kV distribution line. The load was 271 Amps. The bottom connection is the CT primary. The top connection is the PT's primary.

I'm having trouble understanding why the PT connection could get so hot. The PT primary winding "should" be a very high impedance and draw very little current. If the connection is bad, shouldn't the current choose the least resistive path i.e the power line conductor?
Re:Metering CT and PT Connection IRJay 7/11/2005
The PT does draw some current so if the connection becomes impaired in such a manner as to raise it's resistance you create the circumstance you see. Typically you would talk of the power here as I squared x R. But I like to use E squared / R which reflects the large voltage present. Use the calculator we hand out in our ITC classes and you could calculate the power here on this connector. Remembering that a soldering iron can be 850 degrees with only 18 watts of power.
Re:Metering CT and PT Connection Manuel 7/14/2005
image parameters says Distance 6, i suppose that units are on Feets.

if distance are on feets, for 115kv seems like very dangerous distance.

just a little observation.

roberto cruz
Re:Metering CT and PT Connection neatster77 7/14/2005
Yeah, you got point. Since I took the level I course and found out that setting doesn't effect the temperature reading too much, I don't bother setting that distance value. I was probally about 20 feet (6 m) away when I took this.
Re:Metering CT and PT Connection electricpete 7/24/2005
I agree with your conclusion that normal current of a PT is just excitation current which is [b]designed[/b] to be very low on a PT for accuracy reasons (any current draw creates a voltage drop through the series leakage reactance which decrease effective voltage ratio below the turns ratio).

CT current would be much higher (270A in your case.) But I notice the PT cable isn't that much smaller.

Is it possible that the "PT" is actually a CCVT - coupling capacitor voltage transformer. In that case you will draw capacitive current.

Or maybe possible you have recorded the components wrong.

Or remotely possible that the top connection is a 3-lead connection connecting top, bottom, and PT leads? (I don't think so but just trying to understand).
Re:Metering CT and PT Connection electricpete 7/24/2005
To IRJay if you intended to use 115kv as E and connection resistance as R, E^2/R is a ficticious (incorrect) number. The full voltage E is not dropped accross the connection resistance. In fact exactly the opposite - The connection resistance is a very small resistance in series with a very high impedance transformer, so the voltage drop occurs primarily accross the transformer and very little accross the connection.
Re:Metering CT and PT Connection neatster77 8/2/2005
The connection has been verfied. It is a PT connection. Perhaps the attached visual will help.
Re:Metering CT and PT Connection Falcon 8/4/2005
potential transformers and current transformers differ in the amount of windings they have. voltage measuring devices will always differ in temperature to current measuring devices. You are correct when you say current takes the path of least resistance. That's why a poor connection has a high current reading. Electrons are not readily flowing through the conductor. For an example, 2 conductors have equal loads. 1 is twice as big as the other. The amps will be less in the big conductor because the electrons are flowing easily.
Re:Metering CT and PT Connection Pete 8/17/2005
To answer your intial question: I would say that the connection needs to have maintenance performed on it. If the PT was drawing excessive current I would expect to see the line to the PT to have elevated temperatures, not just the connection. If an electrical connection is "good" you should not see a hot spot. An example of this would be looking at a terminal strip with load on it. The connection should be at appx. the same temp. as the conductor.

Prior to disassembly I would request that the electricians perform a DLRO on the connection.

Are there any like configurations, at appx. the same load, in the yard that you can I/R for comparisions?
Re:Metering CT and PT Connection neatster77 8/17/2005
The outage on this line is set for next Tuesday. I'm not sure we'll be able to DLRO because protective grounds will be on either side of the stinger while they inspect/repair the connection.

I scanned another phase of the same line to use as a reference temperature. The conductor and connections were about 93 degrees F.

Thanks for all the input.
Re:Metering CT and PT Connection electricpete 8/18/2005
That is some useful data you have been gathering - a great case study. I am really surprised to see such a hot connection on a low-current PT circuit. It will be interesting to see what they find. Please keep us posted.
Re:Metering CT and PT Connection electricpete 9/5/2005
neatster77 - did they find out anything about this connection during the outage work?
Re:Metering CT and PT Connection neatster77 9/7/2005
The outage was postponed. I'll post as soon we find something.
Re:Metering CT and PT Connection neatster77 9/23/2005
This connection was repaired yesterday. Plant personnel discovered busted strands of MCM between the PT and CT connection. The short time duration of the outage prevented them from replacing the stinger with new MCM.

They also found a couple loose bolts and corrosion when they removed the PT connection. The area was cleaned and the bolts were tightened. This image was taken about 24 hours after the line went back into service.

Unfortunately, I wasn't able to perform any other tests on the connection or the CCVT.

I was surprised to see that merely cleaning the connection and tightening bolts fixed the problem. I expected the hot spot to remain because the damaged strands of current carrying conductor were not repaired.
Re:Metering CT and PT Connection neatster77 9/28/2005
I meant metering PT not CCVT

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