Note: This question is about power systems in the kV, MVA range.

Many distribution and transmission systems are impedance grounded, which means that (some of) the transformer neutrals are connected to ground through an impedance.

It's increasingly common that there are two ground wires along an overhead transmission line; one regular ground wire (typically Fe, FeAl or Al), and one optical ground wire (OPGW). I know that they are often bundled together at the last pole, and I guess connected to the transformer as if they're the same wire.

I wonder how exactly are the ground wires connected to ground?

  • Is the impedance between the ground wire and the transformer?
  • Is the ground wire connected directly to the transformer neutral, and the neutral is connected to ground through an impedance?

How is the optical wire connected to the rest of the communication system? Does it continue as a ground wire until it reaches the control room (or something similar), or is it just a fiber between the transformer and control room? If so, how is it connected to the transformer, how do they get the fiber out, and how is it protected?

Below are some figures with some options. I believe the first and last image are the correct ones, but I'm not entirely sure.

A few notes:

  • A typical single line diagram shows the transformer neutral as in figure 1, but the neutral wire is not included in the SLD, so it might just be a simplification.
  • I appreciate it very must if someone can explain how it's done in HV substations, and not how it could be done. Please only answer if you know the answer.

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1 Answer 1


The ground wires are not connected to transformer neutrals. They are typically connected to the steel towers along the transmission line. At the substation ends of the line they are connected to the sub ground grid. There are variations in practice from utility to utility but none that i know of try to directly connect to the transformer neutral.

The transformer neutral impedance connects directly between the star point (neutral) and the substation ground grid. The overhead ground wires (OPGW or anything else) are tied to the ground grid, not the transformer star point.

The idea of the overhead ground wire (static wire or shield wire) is lightning protection on transmission lines. Distribution is a different animal and practices vary widely.

Here is an old good post by @Transistor on this topic, and here is a paper that discusses the various approaches to use of these "shield wires". The end of this video shows that the OPGW is brought down the tower (in substation) and the fiber is connected to an interface box (for isolation).


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