Current on transformer tank ground CT's during an external fault

[rockman7892]

I had mentioned in a previous thread that we have a utility transformer (4.16kV wye secondary) equipped with a tank ground CT that is picking up current during an external fault in our faciltiy. The transformer has a CT around a connection to ground on each side of the transformer (same direction) and the utility claims that each of these CT's is series connected at the relay so the currents cancel. (We proved wiring to relay is correct with currents canceling during normal operation)

Both myself and the utility are confused as to why we are seeing a tank ground current of aproximately 350A in the relay looking at these CT's during an external fault in our plant. Except for an internal winding faiure the only way I see these CT's current can add in the relay are if current is passing through each one of these CT's from ground and then exiting trhough the transformer pad as opposed to the other side of the tank where the opposite CT is located. Has anyone had similar experiences with tank ground CT's picking up on external ground faults or have any ideas what can be happening here? Should current even be flowing through these CT's during an external fault? (Possible low impedance path?)

Another issue that may or may not be related to this tank ground issue is the fact that the NGR inside the substation is somehow being shunted and allowing more than the design limited 400A of current to flow during a ground fault. We are trying to locate where this NGR may be shunted and see if this problem is related to the current being seen on the tank ground CT's.

Also during normal operation we notice about 10A of current on each of the tank ground CT's canceling in the relay to 0A. We also notice about 10A of current at the CT around the netural bushing of the transformer. Is this 10A of current considered normal during operation? We have a 3-wire system so the only place I can see it coming from is system charging current due to capacitive coupling.

 

[PHovnanian]

Except for an internal winding faiure the only way I see these CT's current can add in the relay are if current is passing through each one of these CT's from ground and then exiting through the transformer pad as opposed to the other side of the tank where the opposite CT is located.

I think this is might be the case. Multiple connections to ground will result in circulating currents should there be any voltage gradient in the grounding system. The voltage gradient could result from normal operation neutral earth return through the grounding system or due to ground fault currents.

A grounding study might reveal the expected magnitudes of these voltage gradients. And with some guesses as to the tank to pad contact resistance, perhaps an estimate of these currents could be made. Its possible that either fault or even the normal operation CT currents observed could be a result of the degradation of the station grounding system. As the system deteriorates, these circulating currents seek alternate, higher resistance paths through transformer tanks, pads (fences and other objects) instead of the ground grid. So that might be one place to look.

The 10A at the transformer neutral suggests an alternate possibility. How is the neutral connected to the station ground grid? Solid or through some resistance? Its possible that a ground fault inside the transformer at a point on a winding very near the neutral point could cause low (~10A) currents to flow under normal circumstances. But during an external fault, the return current would be through the tank and internal short in addition to the neutral bushing. Eliminate this possibility before digging up the station grounds.

 

[rockman7892]

PHovnanian

"As the system deteriorates, these circulating currents seek alternate, higher resistance paths through transformer tanks, pads (fences and other objects) instead of the ground grid"

I assume with this statement you meant to say "lower resistance" paths instead of "higher resistance"

Interesting point about possible voltage gradiants causing these ground loops. You mention these voltage gradiants possibly being a result of normal operation neutral earth return through the grounding system. Where would this earth current come from during normal operation? Charging current?

I guess it is possible that there could be small winding fault inside of transformer. To confirm this is there a way to tell what direction current is traveling between transformer tank and ground? If current direction is found to be flowing from ground up to transformer tank would this confirm that current is not coming from fault inside transformer?

 

[PHovnanian]

Normal operation neutral (or ground) currents will come from unbalanced loads, in the case of 4 wire wye systems, or charging currents for balanced load and/or three wire systems.

And yes, my 'higher resistance paths' refer to what were originally higher resistance, but are now lower in comparison to a failing ground system.

The direction of the normal operation circulating currents may provide no meaningful information. They could be leading due to capacitive charging currents, or lagging due to leakage reactances of the windings in question. And then there's the question of what is leading which phase when they are only 120 degrees apart. One could do some analysis, but I think the different types of faults would result in overlap between the possible measured values.