Buried Underground Single Phase Transformer Primaries

[Kartracer087]

Hi,

I am wondering, how do they Y connect the primaries of single phase pad mounted transformers for homes and small businesses. What I am curious to know is if they use the concentric neutral contained in the medium voltage cable or do they use a separate neutral brought down from the utility riser? If they use the concentric neutral, is it connected to the utility pole's multigrounded neutral through a cable splice at the riser pole? I am mainly thinking about radial systems namely systems where one riser feeds one transformer. I know they have loop systems in the single phase pad mount transformers where they can series connect each primary of the transformer until it reaches the end of the line where it would either connect to another phase on the pole or the neutral would connect to the final terminal much like connecting receptacles/light fixtures in a house.

Thanks

 

[charlie]

See Kartracer087's question.

Wye splices are rarely done. 1? transformers have 2 primary bushings for elbow terminators and all the transformers are loop fed. That statement works for all installations except for some radially fed transformers. I do not believe any electric utilities are using single bushing 1? transformers anymore. In the case of 3? transformers, there are some 3 bushing, radial fed transformers but most are also loop fed.

The concentric neutral is used and a separate neutral is rarely used. We use 15 kV, JCN cable with a 1/3 size concentric neutral for a shield. If three cables are used together, we get a full sized neutral. The concentric neutral is connected to the MGN (multi-grounded system neutral) at the pole and to the grounding system at the transformer.

At the transformer, we install a ring of #2 solid copper around the inside of the HV/LV compartment and connect the LV neutral bushing, HV concentric neutrals, the transformer ground pad, the lightning arrester tails if it is radial or it is an open point, and the ground rod to the #2. These are basically the same whether it is a radially fed transformer or loop fed. :smile:

 

[Kartracer087]

I see now.

Now about the transformer bushings, is each bushing connected to each end of the primary coil or is there some other connection to the other side of the transformer coiling?

I guess a picture would help explain what I mean:




If this is correct, then the red dots would be the bushings then the black wire would be a phase wire and the white wire would be either another phase wire going to another transformer (loop) or would be a neutral for a radially fed transformer. Do I have the right idea here?

 

[charlie]

. . . Now about the transformer bushings, is each bushing connected to each end of the primary coil . . . Do I have the right idea here?

No and No.

Each primary bushing is connected to an internal bus so the bushings feed through the same as a straight splice. If it is radially fed, a lightning arrester is plugged into one of the bushings and the tail is connected to the grounding bus. The transformer fuse holder is connected between the internal bus and one end of the transformer primary coil. The other end of the coil is connected to the X0 bushing or to the internal tank ground. 3? pad-mount transformers are done in the same manner except there are six primary bushings and three primary fuses.