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Why inductance to 1m?
2

Why inductance to 1m?

Why inductance to 1m?

(OP)
I have often seen manufacturer's data for transmission line conductors specify inductive reactance to 1m. Anybody know the reason why this info is provided? After all, the actual line X is a factor of tower configuration, etc.

Thanks.

RE: Why inductance to 1m?

2
Inductance is a function of conductor configuration, spacing, arrangement, and type of conductor (ACSR, AAC, bundle conductor, size, etc.).

Line impedance Z= ra +j(Xa + Xd)

The ra & Xa terms are from the tables you mentioned. Xa is the reactance within the conductor and external out to a radius of 1 meter (1 foot in ANSI tables). That impedance is not affected by spacing to other conductors, distance above the ground or other external factors. Given a wire size and type, we can look up Xa in a table wihtout adjusting for the installation specifics.

Xd is the reactance from 1 meter out and is only a function of the separation from the return conductor(s). Another table gives the reactance Xd based only on conductor separation. That table can be used for any type or size of conductor.

Basically our predecessors figured out a way to simplify a complex problem by looking at the conductor and the configuration separately.

Note that sometimes Xd is negative if spacing is less than 1 meter.

RE: Why inductance to 1m?

(OP)
rcwilson,

Am I correct in stating that your expression Z = ra + j (Xa + Xd) is the phase impedance of the line and not the positive sequence impedance?

Does Xd factor in overhead earthwires?

Thanks.

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