Is there anything special about wiring a current transformer? 4

[bdn2004]

We've discovered that the field wiring on some large power transformers CTs are wired with aluminium cabling. This is the wiring that goes from the transformer junction box to the 50/51 power relay. We are in the process of replacing it, as well as all of the wiring to the devices on the transformer. My question is...is there anything special about CT wiring? Can it be run with the signaling in the same cable? The digital signals are 120VDC.

Also, do shorting terminal blocks have to be used? There are no shorting blocks on the existing installation, although they are shorted to ground whenever they are worked on. There have been complaints about shorting blocks in the past and the electricians don't trust them. I just wondered if they must be used.

A sketch is attached.

 

[waross]

.is there anything special about CT wiring?
Sudden death if the secondary circuit is opened under load. Less dramatic results may be blindness. Temporary if you are lucky.
Opening a CT secondary circuit under load has been described as "A BIG ORANGE BASKETBALL" be survivors.
If you have an electrician who DOES trust the shorting links, try not to stand too close to him when he is working.
When a CT is to be taken out of service I use the shorting device and then replace the cover. Using the shorting device to "safe" a CT in order to change connections with primary current flowing? NEVER.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rasevskii]

I agree with waross.

bdn24: Obviously the work in this case MUST be done with the transformer out of service and made safe. The CTs should be provided with shorting links at the CT in principle, or the nearest terminal box at the transformer.

The CT secondary wiring should be 4mm2 copper if you are in the IEC world. In principle a separate cable or conduit run.

Hopefully you are not attempting this work with the transformer in service.

rasevskii

 

[bdn2004]

Thanks for the info.

The transformer will be shut down temporarily, the CT jumper wires will be installed and shorted to ground, then the transformer will be re-energized.

4mm2 by conversion appears to be minimum #10 AWG. It would be helpful to have a standard that calls this out and the separate cable requirement.

 

[rasevskii]

The practice in IEC, I believe from site experience, is to run a 4 conductor cable (4 x 4mm2) from the CTs in the field to the control/relay panel, and not to combine the CT circuits into cables carrying other circuits. Just common sense.

If you are in the world of conduit and not cable in trays, then the CT conductors may or may not be combined in the same conduits with other circuits. If you are in North America, then refer to the NEC, to see if there are any references to CT circuits in the standards.

Appalling that aluminum was used in the original installation for control circuits. See to it that the terminal blocks are suitable for copper and not aluminum only.

rasevskii

 

[bdn2004]

Rasevskii,

I couldn't find anything in the NEC or IEEE standards for the size of the wiring. There must be a reason it is sized larger because it would only be carrying 5 amps max.

That is a good point about the terminal blocks. The transformer manufacturer installed copper wiring on his side to the junction box that is mounted on the transformer. However this wiring goes through a couple of other j-boxes that were installed at the original installation where that could indeed be an issue.

If you knew what this company makes you might understand the aluminum being used.

 

[trat1208]

The reason a larger gauge wire (#10) than is needed for 5amps is used is to reduce the burden seen by the CT secondaries. Larger diameter wire has less resistance. Combine the resistance of the wiring with the burden of the connected relay or metering coils (available in manuals) to get total CT burden.

The burden is important because CTs will attempt to produce whatever secondary voltage necessary to provide an accurate scaled down sample of the primary current. For a given primary current, if burden is increased on the secondary circuit, increasing secondary voltage will be required to drive the same amount of secondary current. Under fault conditions when CT primary current is high, the secondary voltage required to drive the large secondary current may go high enough that the CT will saturate and not produce an accurate secondary current. CT's are available in different ratings that give the secondary voltage at which they saturate.

I previously worked at a large electric utility where practice was to have all relay & control panel wiring be #14AWG with the exception of CT wiring which was #10AWG. The only exception I have ever seen to this is where the length of the CT leads to the relay/control house were abnormally long (i.e. like coming from a transformer located two blocks away from the control room in a very large substation). In this case, I have seen larger diameter wiring used. I assume the sizing was based by engineering calculation of the burden of the circuit and it was found that #10 would not be sufficient.

I have always seen CT secondary leads cabled in 4/c #10 (for 3ph sets of CT's) or 2/c #10 (for single CT's). I wouldn't mix CT and control signals in the same cable. High voltages will develop on the secondary leads of the CTs when faults occur and you don't want to introduce the possibility of these high voltages affecting your controls.

 

[LionelHutz]

There must be a reason it is sized larger because it would only be carrying 5 amps max.

But it won't be carrying 5 amps max. The current will be MUCH higher than 5A during faults.

 

[DTR2011]

"There must be a reason it is sized larger because it would only be carrying 5 amps max."

The IEEE Std is 20x Inom

 

[bdn2004]

Smallgreek,

What does "Inom" stand for? I nominal? and if so how does that jibe with a #10? Also can you refer me to the correct IEEE Std? Thanks.

 

[oldfieldguy]

bdn2004-

"Inom" is the nameplate secondary current of your current transformer. In the ANSI world, that's five amps. Your CT is XXX:5. In the IEC it's one amp.

The IEE standard says you need to size your circuit for 20 times that. It doesn't mean your new wiring needs to be rated for 20x5, or 100 amps continuous, but at THAT level of current, you need to insure that the voltage impressed at the CT terminals does not put your current transformer into saturation, where the secondary current stops being a good representation of the primary current.

On short runs, this isn't much of a deal. Most CT circuits on indoor gear are #12 or #10 AWG. But where the problem comes in is in longer runs, such as to an outdoor device, and your lead lengths become dozens and hundreds of feet. In some large transmission yards, it is not unusual to see the current circuits from far-off devices as #4 AWG between the terminal blocks of the device and the terminal blocks inside the control house at the relay panel. Of course, the run from terminal block to CT and terminal block to relay are back to #12 or #10.

In short, though, if you're not familiar with these considerations, you might want to drag an EE with relay experience in on your project.

old field guy

 

[bdn2004]

Oldfieldguy,

"In short, though, if you're not familiar with these considerations, you might want to drag an EE with relay experience in on your project. "

This is good advice, obviously I don't have experience with this. Thanks.

But all we are doing is replacing the existing CT wiring which is #12 or #10 AL with Cu. My question was, when replacing this is there any Codes requiring minimum sizes or something special? They wanted to replace this the next day as a maintenance job when they discovered it. We've found that old AL wiring when shaken or moved slightly can just break off very easily. And it is is well known to everyone involved that CT wiring can be deadly if it is opened energized - thus the panic when this was discovered.

I didn't really expect to have to get the system re-engineered. But we've slowed down and are making this a project now. I do have a couple more questions. Again thanks for the answer.

 

[oldfieldguy]

bdn2004-

"But all we are doing is replacing the existing CT wiring which is #12 or #10 AL with Cu."

No problem, then. If the circuit worked properly with Al, then Cu will work. A few further considerations: Use compression type ring lugs. Proper crimping is a must. Many specifications call for 'complete cycle' or 'ratcheting' crimp tools. Replicate the path of the old wiring, although a few inches difference in conductor length is not critical.

When you're finished with your wiring it is prudent to check your work out. Ideally, this would involve passing primary current through the affected CT's and checking for proper indication on the relays and meters in the circuit. If this isn't available, then connect a secondary current source to the CT terminals and send percentages of that 'Inom' through the circuit to verify your wiring.

old field guy