Grounding of CT's on a new transformer. Where should the ground connection be located?

[bdn2004]

We got our new oil-filled transformer that came with CT's that are each wired out to terminals on the top of the transformer. From there they are wired to a control panel at ground level.
Looking inside the terminal well of a CT on the top of the transformer, and per the transformer drawing, one side of each CT is grounded up there to the case of the transformer.

If these are left in place it seems it would put these CT's in a 3-phase wye configuration.....Is the intent of this, or is this wrong? We would typically ground the CT's at the control panel. Do you have to ground both sides of a CT when de-energized and performing maintenance?

 

[constantlylearning]

When performing maintenance, it's important that the CT secondary be shorted and grounded. I personally don't see the need for multiple grounds and actually think this should be avoided.

Also, my personal preference is to ground the CT at the equipment from which it originates, though I think there are various practices. I prefer this because it's a logical way to avoid multiple grounds. If your policy is to ground at the originating equipment, you don't have to decide which control panel to ground in.

Another practice would be to ground the CT at the first panel the CT enters once it leaves the equipment from which it originates.

 

[dpc]

They need to be grounded somewhere for safety reasons. They should only be grounded in one place to avoid CT current going where you don't want it to go. In the US, the most common grounding location is at the first terminal block inside the relay panel or control building where the relays/metering are located. But this location is by no means a universal standard.

This has been discussed previously - you might try searching the forum.

 

[marks1080]

I would anticipate seeing a new unit to have all of the CT windings shorted and grounding, as that is common practice during the layup or delivery of CTs.

The circuit should only be grounded at a single point. Otherwise you may void any neutral current readings your protections might want to use.

If the CT circuit ground is located in the control / relay building, the circuit will be less susceptible to interference from the switchyard.

Where in the relay building you choose to ground the circuit is up to you. It’s a good idea to allow for multiple grounding points via sliding link terminal blocks on the back of the panels the CT circuit is connected to. If you do this there is also a possibility of the CT ground connection being compromised.

Mark

 

[constantlylearning]

I failed to mention (as "dpc" said) that there should always be a ground (usually in at a single point) on CT circuit for safety reasons. Also, all CT's need a closed path on their secondary terminals, either by shorting or having some type of instrument connected. An open CT is a very dangerous thing.

Sorry if I'm being redundant.

 

[pwrtran]

Manufacturers short all CT's and ground them to the transformer tank when leaving the factory during shipping, that is the common practice for them to ground the CT's. Once transformer be installed, you can ground the CTs per your practice. We ground them inside relay panels.

 

[pwrtran]

Forgot to say not all CTs will be wired up and transformer manufactures do not know, so if a CT will not be even wired up, it is shorted and grounded already.

 

[jghrist]

As others have said, it is common practice for manufacturers to short and ground all CT windings for shipment. I would expect that this would be done at the ground level control panel to make it easy to remove the shorting bar. The OP's description doesn't sound like the windings are shorted for shipment, but that they are connected in wye with the neutral grounded. This is unusual. What if you wanted to connect the CTs in delta for an old fashioned electromechanical differential relay on a wye winding?

Instrument transformer secondaries should be connected to ground at only one point (see IEEE Std C57.13.3). Making the ground connection at the relay or control building has the following advantages:
Voltage rise is minimized near the relay equipment.
The shock hazard to personnel in the building is reduced.
All grounds are at one location, facilitating checking.

 

[bdn2004]

Attached is a sketch of the existing circuit. Also there is a picture of the terminals used at the transformer panel and at the control panel. I had another post about what type of terminal to use. Neither of these terminals are of a ring type or of the preferred types people mentioned. However the wire does fit snug. I'm not sure why these need to be of the testing type which both are. That complicates things too....we'd like to just wire these onto the Ground Bus at the xfmr control panel but it would place (2) wires on the line side on the terminals.

The Vendor did put temporary jumpers to short the terminals at the control panel which were removed so to get the wires to the relays.

I'll post another sketch of the proposed solution.

 

[collies99]

WOW, poor drsign overall.

Some redesign suggestion.
1. Get rid if the isolating type terminal block, the knife switch. That is an accident waiting for a chance to happen.

3. Exchange the sliding link with Weidmuller presdure type TB snd change the vonnectors with one that has a small bend tab; do not know make and model, but had beeb discussed in this forum in the past.

3. The slide link is OK in a CT junction box for acess by a qualified personnel, but you could install one more for every CT so that you can slide it close to short out the CT winding if required without disturbing existing wiring. CT single point ground then goes in the same box.

note: secondary cct schematic is incomplete and is now open circuited, relay polarity not shown. all info is pertinent for proper review.

 

[collies99]

Added note:

Existing CT grounds to be removed.
CT grounding done by jumpering the CT neutral (X2, looks like single ratio on dwg) to each other then 1 connection to the ground bus.

Polarity may be reversed at the relay, typically X1 (incoming as per dwg) goes to polarity input terminal on relay and sum of ABC also to polarity on ground input terminal at relay.

 

[bdn2004]

Oops made a mistake on the first sketch. There is not an additional ground on the top CT.

 

[bdn2004]

Darn...still the wrong one....

 

[waross]

It is bad practice to use the ground connection to carry metering current. Jumper the CTs instead of grounding them and then ground at one location only.
Get rid of any terminal that may open the circuit. Open circuited CTs are dangerous and may be lethal.

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

 

[bdn2004]

Waross,

Are you saying all of these terminals should be replaced with a feed-through style? Also, you are saying all the grounded wires from all three CT's should be jumper-ed together and one wire taken to the ground bus?

Knowing the hazards of an open circuited CT I don't really see how a xfmr mfg has an excuse to supply us with this style of terminal. If there is something else I just learned in this -> specify exactly what you want. You won't get it otherwise!

 

[waross]

OK, I just Googled those terminals.

http://www.clrwtr.com/PDF/Weidmuller/Weidmuller-Screw-Connection-Measuring-Terminals.pdf

https://www.phoenixcontact.com/onli...detail:pid=3035700&library=usen&tab=1#General

They are intended for CT use but possibly not as they are connected in your drawing.
While it is electrically possible to connect a meter to the test points and then open the shorting link, it may not be safe. A safer use may be to use the shorting feature to short the CT leads. Meter connections may then be changed and the shorting link opened for testing.

Good practice is to complete the wiring of the CTs with the same type of wire for both the CT polarities. Then add one jumper to ground.
Never assume that a ground path has a low impedance and never assume that a ground path will never be inadvertently opened.

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

 

[bdn2004]

Waross, Collies99

The attached sketch shows what I think you mean. Top is Demo, Bottom is new. The Jumper numbers are J1, J2, etc. for reference.
I don't like the fact these have 3 wires under a terminal. The ground connection for each one can't go straight to the ground bar?

 

[bdn2004]

Attached is a picture of the control cabinet on the transformer where the CT wiring terminates. The ground bus is right behind it. It would be easier to wire directly to this bus as opposed to jumpering between the terminals.

 

[collies99]

for your consideration; you need some regular feed through terminal block, pressure type with proper connectors, and only 3 slide types.

 

[bdn2004]

Thanks Collies99...I like your design.
But looking at these slide type terminals, they don't seem that substantial to me. I wish there was a better way to do this. I certainly hope they don't get used a lot.

Our guys have commented before they don't like the shorting block type terminals either on the CT circuits. They would rather make up their own jumpers and short them to ground.
I haven't seen them do it, just heard them say this.

What exactly happens when a CT gets shorted? Is it an instantaneous discharge around the coil? And right after it's shorted - it's lost it's potential energy and then harmless?