Looking for HSS Impedance Values?

[hhhii]

I am looking for Ra and Xa values for 3/8" High Strength Steel (HSS) wire. Normally I have found Ra values for conductors @ 25, 50, and 75 degrees. I have looked for these values for 3/8" HSS with little success. I have a couple of venders looking into this for me, but I have not heard back yet. So I will try here next.

The ONLY source of Ra and Xa values I have in the "Southwire Overhead Conductor Manual". Table 1-39 has impedance data for various grades and diameters of steel conductor. However, instead of Ra and Xa @ different degrees, the table list Ra and Xa @ 10, 40, 70, and 100 amps. So I am not sure which value, if any, to use.

I need these values for impedance calculations for various transmission (69-138 kV) structures. If anyone has any info one this, I would be very thankful. I need to know Ra @ 25, 50, 75 degrees and Xa for 3/8" HSS. Or at least maybe which values in Table 1-39 to use.

In the Table 1-39 there is a 20 degree DC value for Ra but I do not know where to find the conductivity constant (228.1=aluminum, 241.5=HD Copper)of HSS Grade A wire is.

Thanks
Howard

 

[jstickley]

I'd be somewhat surprised to find the values at different temperatures as you've requested. I've only seen impedance values for static wires at various amperages. I'm not 100% certain why they are stated as such, but my assumption would be that because the static wire typically carries little to no current, it makes more sense to state impedances at various amperages. We typically use values at 10 A, as follows for your particular conductor.

Ra: 6.59 ohms/mile
Xa: 1.10 ohms/mile (60 Hz, 1 ft spacing)
Xa': 0.1244 Megaohms/mile (60 Hz, 1 ft spacing)

Hope this helps.

 

[jghrist]

The following values are from the SES FDIST program used to calculate the fault current splits in grounding design, at 100A, 60 Hz, 25°C:

Rdc = 6.51 ohm/mi
Rac = 7.71 ohm/mi
X = 2.15 ohm/mi at 1 ft spacing

Hope this helps more than confuses things, considering jstickley's response. There are also (small and difficult to use) curves of resistance and GMR for steel wire in the Westinghouse T&D Handbook, Chapter 3, showing variation with current, but without stating the temperature. The current magnitude determines flux and therefore the iron or magnetic losses inside magnetic conductors. This is why the resistance varies with current.