polarity of ground fualt detection ct on the neutral

[mshahwan]

is there a season why the polarity of a GF detection CT on the neutral is setup as if the flow of current is from ground , through the NGR to the neutral point?

 

[dpc]

For a typical GF relay, the polarity is not going to matter. If you are doing some type of differential relaying, you have to match polarity on both sides.

But this is ac current, so actual direction of current flow is not the issue.

David Castor

http://www.cvoes.com

 

[cranky108]

Are you using it for detection, or direction?

As far as CT polarity, it should be away from the device.

 

[mshahwan]

hmmm let me try again. i was testing the CT ratio of the ground CT on the neutral of a 2MVA 4160V/600v delta-wye transformer. this CT in integral to the XFMRs assembly so I have no access to it except to the secondary terminals through the XFMRs control cabinet. so my setup was to inject current through the secondary windings of the power xfmr's windings (+ve lead) to the XO point (-ve lead) and take a measurement. i kept getting a polarity error. i reversed the positive and negative leads so the injection was done from the X0 (+ve) to the secondary windings (-ve) and it worked. I also noticed on the drawing that the CT polarity is setup for current flow from the ground to the XO. this is where my confusion is. that means the potential @ ground is higher than the XO point (through the NGR). which means under ground fault condition, ground potential rises and current flows from ground to the transformer's windings. is this correct?

 

[davidbeach]

How would the orientation of a CT have anything to do ground potentials?

CTs should be connected so that the wye point of the CTs points toward the protected zone and other side gets connected to the polarity terminal of the relay. Same goes for a neutral CT, it should be grounded on the equipment side and connected to the polarity of the relay on the side away from the equipment. It's nice when it happens that the polarity of the CT connects to polarity of the relay, but means nothing really. For the relay connections, forget that the CTs even have polarity marks, just go with the correct wiring relationships. For your testing, current from ground to the X0 terminal should be in phase with current from the CT into the relay.

 

[wilkin1]

It is standard practise to fit cts with P1 to the busbars or machine.
The single line or wiring scematic can then be followed
where REF or DIFF cts require the correct polarity in relation to each other.
For SBEF polarity is not important.

 

[mshahwan]

Davidbeach,
the polarity of the CT is marked so as for the first half cycle the current flows from the ground to the neutral point. this CT is a zero sequence current transformer with only the neutral conductor going through it.
My question is why would charge flow from ground potential to the neutral potential? in what case is the neutral at lower potential than ground?

 

[davidbeach]

Forget polarity, it means nothing, really. Any type of ground fault will cause current to flow between neutral and ground. Maybe neutral is at a higher potential than ground, it alternates every half cycle, and it really doesn't matter which is at a higher potential. What really matters is the relationship of the currents into the relay when comparing the current in the neutral and the residual current in the three phase conductors.

 

[lz5pl]

mshahwan, how exactly you performed polarity test? If you use simples setup - connect a battery to primary side and read analogue miliammeter on secondary - you could get wrong results due to high inductance of the transformer winding. Normally deflection of analogue miliammeter is higher during the breaking of current, so it can get you in confusion.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[jghrist]

As others have said, polarity doesn't matter if you are not comparing the current measured to another current, like in a REF relay.

As to how the current could flow from the ground to the neutral, consider a single phase ground fault. If current flows out of the phase bushing, it flows through the ground and from the ground into the neutral bushing. The ground potential is a consequence of the current flow, not the cause. The driving force of the current is the transformer winding voltage.

 

[stevenal]

Why? By standard. Integral CTs are mounted with polarities away from the device, whether on phase or neutral.