Residential Pole-Mounted Utility Transformer Grounding

[jtester]

Jim,
Let me start by saying the transmission portion of the "SYSTEM" is delivered DELTA, in the voltage range of 110kv, 115kv, 230kv, and 500kv three phase with a 120 degree shift. When that transmission voltage is tapped at a substation, we step down to a distribution voltage of 34.5kv, 13.2kv, or 4.1kv. At this point it is DELTA three phase that leaves the station after regulation and protection schemes are in place.

Let's look at three phase primary running down the road on x-arms. Lets stop at the first customer on line. At the pole you see a tx (obviously OH construction). The primary of the three phases are DELTA. The tx requires 19.9kv, (34.5 / sq rt of 3) = 19942 or 19.9kv. So in order to achieve this, we ground the primary coil at the tx (H1 bushing connected to one phase of 34.5kv, H2 bushing to ground). This connection gives us a primary connection which is WYE connected. The secondary is DELTA connected, thus the WYE-DELTA connection. Now if we were to utilize the 34.5kv, we would connect the primary side of the tx DELTA (H1 connected to one phase, and H2 connected to one of the other two phases). The secondary is DELTA connected, thus we have a DELTA-DELTA connection. The WYE-DELTA or DELTA-DELTA is referring to the tx connections only, not the main line delivery primary.

You seem to be mixing up terms. As Don said, you don't have a ground to tie the H2 bushing to in a delta. You have described a primary wye system 19.9/34.5 kv.

You don't have a secondary delta until you have at least 2 transformers in a bank. You have described a single phase secondary system, not either a delta or a wye.

Each of the primary voltages you identified is a wye system. Interestingly the 4160 volt system originated as a 2400 volt delta.

I'd also encourage you to look at the actual transformer connections in your transmission system. While there often isn't a neutral run on a transmission line, they are almost always wye systems also.

Jim T