NGR and Current Unbalance

[MajalaLearning]

Hi, I understand how NGRs work to limit fault current however I had a question involving NGRs in the case of current imbalance. Since usually a current imbalance would result in current in the neutral, do NGRs also work to limit the amount of possible current imbalance in a system? Or would a phase imbalance greater than the limited neutral current still be possible?

 

[winnie]

NGRs do nothing at all to neutral current, at least not directly.

If you consider any current flow path (from the source transformer, through circuit wiring and loads back to the source transformer), the NGR can only change the current flow paths that go through the NGR. Normal circuit paths are unchanged.

The indirect way that NGRs can limit current on the neutral is that NEC code prohibits NGR systems to supply L-N loads. So you should only have 3 phase and L-L single phase loads on a system grounded via a NGR. But if you have some sort of utility system that doesn't follow the NEC, or you have an incorrect system that has L-N loads and an NGR, then you will see the exact same sort of phase imbalance of the same system with a solidly grounded neutral.

-Jonathan

 

[MajalaLearning]

NGRs do nothing at all to neutral current, at least not directly.

If you consider any current flow path (from the source transformer, through circuit wiring and loads back to the source transformer), the NGR can only change the current flow paths that go through the NGR. Normal circuit paths are unchanged.

The indirect way that NGRs can limit current on the neutral is that NEC code prohibits NGR systems to supply L-N loads. So you should only have 3 phase and L-L single phase loads on a system grounded via a NGR. But if you have some sort of utility system that doesn't follow the NEC, or you have an incorrect system that has L-N loads and an NGR, then you will see the exact same sort of phase imbalance of the same system with a solidly grounded neutral.

-Jonathan

Thank you! This response made me realize that the way I was asking the question/thinking about it was not correct. Really what I am wondering is this:
Usually I understand that it's typical to use residual ground calculations from the three phase currents to detect a ground fault. But in the case of an NGR, the actual ground current would be limited to a very small value that the residual-ground method may not be sensitive enough for. In this case, is there any benefit to including residual-ground based protection? Would it still be useful for detecting other sources of current imbalance?

 

[winnie]

Thank you! This response made me realize that the way I was asking the question/thinking about it was not correct. Really what I am wondering is this:
Usually I understand that it's typical to use residual ground calculations from the three phase currents to detect a ground fault. But in the case of an NGR, the actual ground current would be limited to a very small value that the residual-ground method may not be sensitive enough for. In this case, is there any benefit to including residual-ground based protection? Would it still be useful for detecting other sources of current imbalance?

What exactly do you mean by using the residual ground method to detect imbalance?

The vector sum of all three line currents should be zero, even if the system is highly unbalanced (eg a large single phase load connected A-B and nothing connected to C).

As I see it, there are many options for detecting fault current: separate CTs on each of the three phases combined with math to calculate the residual current. 1 large CT that covers all 3 conductors in its aperture, directly measuring the residual current. 1 small CT on the NGR conductors. Measuring the voltage across the NGR. Assuming everything is perfect, all these approaches should give the same result.

Could your question be: is there any benefit to using residual current detection on the phase current measurements rather than directly measuring the current through the NGR?

I don't know enough about installed systems to give a good answer to that question. Perhaps some of the industrial installation guys will chime in.

-Jonathan

 

[ron]

Residual ground fault sensing meters the 3 ungrounded conductors and calculates the unbalance (since there are no L-N loads) as the neutral. Impedance grounded systems require a ground detector per NEC 250.4(A)(5). The CT ratio for sensing is typically chosen based on the expected resistor current which makes it sensitive to the extent that is practical.

 

[jim dungar]

Are you looking to measure Ground current on multiple individual feeders or only for the entire system at the NGR?

 

[D. Castor]

It would be quite unusual to have line-to-neutral loads in a system that is resistance-grounded. This is not generally allowed. The neutral point is connected to ground through the grounding resistor. So normally any neutral current flows through the resistor because there is no other (intended) path. So a 4-wire 480/277 V system with 277 loads cannot be high-resistance grounded.