Medium-Voltage Transformer

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sam34

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I'm reading some specifications regarding MV Tranformers and it was indicated two types for primary high voltage switch; 1) arranged for radial feed with 3-phase and 2-position. 2) arranged for loop feed with 3-phase and 4-position. What are the main different between the two system? Thank you,
Sam
 
The two position is just "On" and "Off." It takes power from upstream, and either does or does not provide power downstream. Thus the notion of "radial."

The four positions are essentially, "connected to power from the left," and "connected to power from the right," and "connected to power from both left and right," and "off." It enables you to have a group of such switches arranged so that power is connected in a complete circle. Thus, you can still have power to everything in the loop, even if you open one switch. Also, the "off" position turns power off to the load of that switch, but still connects left side to the right side. That enables you to work on one load, without breaking the loop configuration. Finally, if you need to work on one of the switches in the loop, you select "left only" on the switch to the left of the one you wish to turn off, and you select "right only" on the switch to the right of the one you wish to turn on. This takes away all power from the switch in the middle, but keeps power to everything else in the loop.
 
Charles, thanks for your reply, that was very informative. I have one more question for you; How many surge arresters do I need for the loop feed circuits? I know I'll need one for each phase, is that means three or six arresters would be needed?
 
I don't think I will be able to answer that. Perhaps someone else can chime in.

But I think more information will be needed from you. Examples: How many switches? How many transformers? How does the utility power connect to the loop?
 
Sam:

You can order your transformers with station class arrestors in them from the factory. It's not an extremely expensive adder and is a good idea. Also, you will probably need arrestors at the utility metering cubicle, assuming you have one.
 
A typical application is to just use arresters at an open point or end of the line. At that point, a voltage wave will double when it reflects back. Transformers along the way usually have no need for individual arrester installations.

You can install external arresters, elbow arresters for example, instead of internal arresters. I would wonder about station class inside a transformer. Station class arresters handle large energy discharges, and the energy in the underground, if you have good riser pole arresters, is not very high, just impulse voltage.

Jim T
 
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