Grounding Bank Installation

[student444]

My boss at my internship challenged me the other day and I'm stumped. He asked me how exactly a grounding bank worked. What he wants is a detailed description of how the Wye-Delta connections of the transformers creates the nuetral line used far away from a substation. Anybody out there have a clue as to what I can tell him or where I can look for a detailed answer. If you need more info just ask. Thanks a lot.

 

[tryinghard]

Re: Grounding Bank Installation

The line side of a 3 phase transformer will have H1, H2, & H3, the load side (of a 4 wire) will have X1, X2, X3 & X0. The neutral terminates on the load X0

On a Wye load the X0 neutral is the tap in the center of all three phases causing an even voltage reading between every phase to neutral in other words an even return path.

On a Delta load the X0 neutral is the tap between X1 & X3 (L1 & L3) causing an even voltage only between phase 1 to X0 and phase 3 to X0 but the phase 2 (X2) to X0 will be a higher voltage (208V) because it has a farther return path.

A transformer is considered a separately derived system because there is no direct electrical connection between the primary circuit conductors and the secondary circuit conductors. Because it is a separately derived system you need to treat it like a service main, in that you also have to bond the X0 (neutral grounded conductor) to the case connecting it to the grounding electrode system, in other words you must bond the neutral X0 to the case frame.

The destination end of the load feed from the transformer (usually a sub panel) will NOT bond the neutral to the case, these MUST remain separate or you will create a parallel path for neutral current.

 

[jtester]

Re: Grounding Bank Installation

student444
The connection you are asking about is a grounded wye/delta transformer bank. There isn't any load connected to the delta(low) side. Because the wye(high) side is grounded, it will supply ground fault current based on the impedance of the bank in the grounding portion of the calculation.

Many years ago, I worked on a power line that ran for miles through the forest to a mine. We started on a grounded wye system, but only ran 3 phase wires, no neutral. When we got to the mine, we installed a bank of 250 kva transformers in a grounded wye/delta configuration with nothing on the delta side, just the delta connection. We pulled a 4 wire system from that point just as if the neutral had been there all along. We thought the engineer was nuts. Many years and a college education later, I understand that it works.

Jim T