Measuring ground faults (This article will work for WYE systems only; you may reference RESL190365 for DELTA systems)
Many causes including shorts and power quality instances
There are two ways to measure ground faults.
1. Measure the neutral point of the WYE side of the transformer
2. Measure the sum of all the currents.
Below is a diagram that might help explain an application of V4. The V4 PT ratio can be set independently. It may be that no PT is necessary (i.e. 1:1) because the voltage will likely be fairly small. If the transformer star-point is resistance grounded a much larger voltage would develop across the grounding resister during a ground fault. The customer should do the calculation to determine what the maximum expected voltage might be.
-V4 measures the star point (neutral) voltage with respect to ground.
-During a phase to ground fault, depending on the transformer MVA and star point to ground resistance, V4 may be a few tens of voltage.
-More effective for resistance-grounded transformer secondary applications
In an ideal situation the sum of the currents will equal the phase currents. But in the case of a ground fault they are different. In order to see if the summation is different, the diagram below can be used to measure the sum of the currents and that sum will be displayed in I4.