Residual Grounding

[rockman7892]

I am fairly new to the idea of residual grounding and was hoping to gain some better insight. I have a 3-wire three phase 480V system. I recently have come across a breaker on this system that is using a residual grouning configuration with 3 CT's.

I am used to seeing zero sequence CT's where the current in on each pahse is balanced by the others and the total sum of current at any time in the zero sequence CT window is 0. Even if the phase currents are unbalanced in a zero sequence CT the summation of these currents is still zero because of the fact that what goes in one phase must be balance out by the other two.

With residual grounding, does the same case apply that the current of one pase is balanced by another even if the phase currents are not equal? For instance if I had line current os A=100 B=90 C=110, would I still see an overall sum of 0A when added together in a resuidual gounding scheme?

What if instead of a 3-wire system, I had a 4-wire system with a nuetral? If I had unbalanced currents would the three phase currents stil cancel when summed at a point or would some of this current return on the nuetral?

 

[dpc]

First, keep in mind that you cannot simply add the currents together. The relative phase angle of the current has to be factored in, so you are adding vectors (phasor) together, not simple scalar values.

Having said that, the residual ground fault detection and the zero sequence (flux summation) work on much the same principle. With the zero sequence CT around all three phases, the fluxes are being summed within the inside of the toroid. For balanced currents, the resultant flux is zero, so no current is induced in the CT. For the residual connection, the three CT secondary currents are connected in wye connection, effectively summing the three currents, with the resulting "residual" current being monitored by a separate overcurrent device or input. If all three currents are balanced, Kirchoff's current law says the residual current is zero. If there is an imbalance, the ground relay detects the resulting current as ground current.

If you have a four wire system, the three currents may not sum to zero, since imbalance current can flow in the neutral. But the same is true for the zero sequence method UNLESS you run the neutral through the zero sequence CT along with the phase conductors.

A CT can be placed in the neutral and this current can be added to the three phase currents in the residual method. It gets a little tricky when there are multiple sources, but it can be done.

 

[rockman7892]

dpc

Thanks for the explanation. I am still a little unclear though about the unbalanced concept. If I have three phase currents that are unbalanced due to single phase loading will the residual CT's still sum to something other than zero? If this is the case the wont the residual grounding trip a relay with unbalanced single phase currents even though there is really no ground fault?

When I speak of balance in this case I mean unbalanced phase currents like the ones shown above, and not unbalanced in the sense that what is one one line current is balanced by the others. Hope what I'm asking makes sense regarding the two uses of the word balance.

 

[DiscoP]

On a three wire system, at any instant the 3 currents in all phases will add up to zero, unless there is an earth fault (or some other odd condition)

As dpc said, on a 4 wire system you need to also add the neutral current, either by including the neutral inside the torroid, or a neutral CT.

It doesn't matter if the currents are balanced, they will still sum to zero. If they don't sum to zero - zero sequence current is present.

However if it is a 4 wire system, with only 3 CTs used (or the torroid doesn't include the neutral), then protection may see zero sequence current for unbalanced loading.

If the load is unbalanced on a 3 wire system (or 4 wire system with neutral CT), then negative sequence current is present.

 

[dpc]

If the neutral current is not accounted for, the residual ground detection AND the zero sequence CT will both register neutral current as imbalance current. It won't cause a trip unless the neutral current exceeds the pickup setting. This situation is quite common at utility substation distribution feeders.

As I mentioned, it can be dealt with by summing the neutral current along with the three phase currents.

 

[controlsdude]

grounding articles look here

http://ecmweb.com/grounding/

 

[rockman7892]

So if I have a 4-wire system and all the phases are balanced then they will all add to zero and there will be no nuetral current, on the nuetral ( I have heard that you dont even need a nuetral in this case) However if the three phases are unbalanced then there will be current on the nuetral resulting from this unbalance.

As I mentioned above, in a 4-wire system if there is phase unbalance then there will be current on the nuetral wire. In a 4-wire system, where does this current go, resulting from an unbalance? From my understanding it returns on the other two phases. Does any current return on the other two phase conductors in a 4-wire system, and is the nuetral stricly used for just unbalanced currents?

 

[jghrist]

It all depends on the load. If you have load connected between two phases on a 4-wire system, then there will be unbalance, but no current in the neutral. The phase currents will add to zero.