Grounding Multi-Ratio CTs

[Sunny_92]

The image below depicts the factory wiring for 1200:5 MR CTs connected to an overcurrent relay. If instead you wanted to connect the relay to taps X2-X4 (to obtain a 600:5 ratio), what would be the correct way to do it?

Would you still leave X5 as the grounded tap? Or would you move your grounding to the new center point (X4)?

 

[rian0201]

move the grounding to used tap..


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[meternerd]

Lower number tap is always polarity. Higher number is return (non-polarity), which is usually grounded. Works for any tap combination.

 

[gray.one]

Keep in mind that the physical location of the ground connection can matter. If everything is in the same switchgear, then no big deal. If the CTs are a hundred of meters from the relay, it does. For long runs you ground at the relay. Because that is where people will be doing work. Ground is for personnel safety.


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[Sunny_92]

Thank you for all of the responses!

Keep in mind that the physical location of the ground connection can matter. If everything is in the same switchgear, then no big deal. If the CTs are a hundred of meters from the relay, it does. For long runs you ground at the relay. Because that is where people will be doing work. Ground is for personnel safety.


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This was my next question. So, say there's a breaker out in the yard that has bushing CTs and shorting blocks inside the breaker cabinet, and there's a control building (no more than 100 feet away) that houses the protective relay and another set of shorting blocks. Should the CTs only be grounded in one spot? And would that spot be the shorting block in the control building?

If the return is grounded at both the breaker and the control building, does it create any code violations or potential harm?

 

[rian0201]

i think the answer was provided by gray.one, and you have to ground it at one point.

IEEE C57.13.3 tells about this..

grounding at two points would cause circulating currents due to parallel paths and cause errors on current readings.

for me, as long as the ground is located to where the person usually works during maintenance is already enough.. sometimes we ground at the breaker cabinet since we do the test for relays there..


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[Phil Corso]

Sunny,

X1-X2 has, say=40 turns; X2-X3=30; X3-X4=20; X4-X5=10! Always maintain the ground-connection at X5! Then, connect burden-devices between X1, and X5!

Thus, Max turns-ratio, available, between X1-X5 is 100:1!

Example 2: you want 50:1 ratio, simply short terminals X2-X3-X4!

Example 3: you want 10:1 ratio, simply short terminals X1-X2-X3-X4!

And, you must never leave turns open-circuited!

Regards, Phil Corso

 

[GoldDigger]

Wouldn't shorted turns create a sufficient opposing magnetic field in the core to interfere with the magnetic flux driving the turns that are active?
I can see not leaving the entire CT secondary open, but shorting all unused turns does not seem right to me.

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[Phil Corso]

Wouldn't shorted turns create a sufficient opposing magnetic field in the core to interfere with the magnetic flux driving the turns that are active?

No! Remember, these are Multi-Ratio CTs! Not multiple, but separated-winding, CTs!

The taps have to be carefully positioned to provide an integral-number of turns!

Phil

 

[GoldDigger]

Sorry, but I still don't get it. I figure that only the fact that there are no other current paths to create a magnetic field in the core that allows current to be forced through the active turns against the burden resistance
If there were separate core sections, I would agree with your advice since current in one coil would not affect the flux in the other cores.

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[Phil Corso]

Sorry, but I still don't get it./QUOTE]

Hmm. I'm sure the mfg's of such CTs would be interested in your thesis that it doesn't work!:slaphead:

Sit back and think about it a while. First of all, the turns aren't shorted. (Sounds like a good thread to consider, tho!) What you envision of a shorted-turn is wrong!

I won't keep you in suspense... as turns are added, the excitation curve, i.e., sec'y-volts vs magnetization-current increases!

Phil

 

[GoldDigger]

Sorry, but I still don't get it./QUOTE]

Hmm. I'm sure the mfg's of such CTs would be interested in your thesis that it doesn't work!:slaphead:

Sit back and think about it a while. First of all, the turns aren't shorted. (Sounds like a good thread to consider, tho!) What you envision of a shorted-turn is wrong!

I won't keep you in suspense... as turns are added, the excitation curve, i.e., sec'y-volts vs magnetization-current increases!

Phil

Maybe I am just missing something fundamental, but to me "simply short terminals X2-X3-X4!" is equivalent to shorting the windings that run between those three terminals. (OK, not shorted turns, but shorted groups of turns.)
I guess I will have to find some reference works to clarify it.

 

[rian0201]

Sunny,

X1-X2 has, say=40 turns; X2-X3=30; X3-X4=20; X4-X5=10! Always maintain the ground-connection at X5! Then, connect burden-devices between X1, and X5!

Thus, Max turns-ratio, available, between X1-X5 is 100:1!

Example 2: you want 50:1 ratio, simply short terminals X2-X3-X4!

Example 3: you want 10:1 ratio, simply short terminals X1-X2-X3-X4!

And, you must never leave turns open-circuited!

Regards, Phil Corso

this is different on what i have seen on site. on multiratio CTs, if we use X1-X2 for example, we only wire X1-X2, and that's it... we dont short unused taps..

and for grounding, we used to where that polarity is..not X5..

so far, no problems have arised.. was this wrong? for me, its not.. as seen on site for so many years..


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[gray.one]

Sunny,

X1-X2 has, say=40 turns; X2-X3=30; X3-X4=20; X4-X5=10! Always maintain the ground-connection at X5! Then, connect burden-devices between X1, and X5!

Thus, Max turns-ratio, available, between X1-X5 is 100:1!

Example 2: you want 50:1 ratio, simply short terminals X2-X3-X4!

Example 3: you want 10:1 ratio, simply short terminals X1-X2-X3-X4!

And, you must never leave turns open-circuited!

Regards, Phil Corso

I've never seen that done and don't see any advantage. Just more chance of errors.

Maybe this is an old timers' way of avoiding shorting terminal blocks, but we always have those.


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[mivey]

Sunny,

X1-X2 has, say=40 turns; X2-X3=30; X3-X4=20; X4-X5=10! Always maintain the ground-connection at X5! Then, connect burden-devices between X1, and X5!

Thus, Max turns-ratio, available, between X1-X5 is 100:1!

Example 2: you want 50:1 ratio, simply short terminals X2-X3-X4!

Example 3: you want 10:1 ratio, simply short terminals X1-X2-X3-X4!

And, you must never leave turns open-circuited!

Regards, Phil Corso

No. You will be shorting the CT. This is a multi-tapped winding. You must be thinking of something else.

 

[mivey]

Wouldn't shorted turns create a sufficient opposing magnetic field in the core to interfere with the magnetic flux driving the turns that are active?
I can see not leaving the entire CT secondary open, but shorting all unused turns does not seem right to me.

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Correct. Shorting any unused turn is wrong unless they are all unused. If all are unused, we usually short across the whole winding (X1-X5).

 

[mivey]

No! Remember, these are Multi-Ratio CTs! Not multiple, but separated-winding, CTs!

The taps have to be carefully positioned to provide an integral-number of turns!

Phil

Seems like you have something backwards. Maybe you are thinking about multiple grounds on multi-winding PTs? Or maybe multi-winding CTs like in substations where you have multiple metering and/or protection windings?

At any rate, the CT in question is one winding and when in operation, unused taps should be left open.

 

[mivey]

Maybe I am just missing something fundamental, but to me "simply short terminals X2-X3-X4!" is equivalent to shorting the windings that run between those three terminals. (OK, not shorted turns, but shorted groups of turns.)
I guess I will have to find some reference works to clarify it.

You won't find one to support shorting the turns as Phil proposed.

 

[mivey]

this is different on what i have seen on site. on multiratio CTs, if we use X1-X2 for example, we only wire X1-X2, and that's it... we dont short unused taps..

and for grounding, we used to where that polarity is..not X5..

so far, no problems have arised.. was this wrong? for me, its not.. as seen on site for so many years..


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You don't ground polarity. Polarity is the smaller number and goes to the instrument (X1 for your example). The higher of the used numbers gets grounded (X2 for your example).

 

[mivey]

I've never seen that done and don't see any advantage. Just more chance of errors.

Maybe this is an old timers' way of avoiding shorting terminal blocks, but we always have those.


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I'm an old-timer and we don't do that.