Use of cables with semiconductive shields in VSD/motor applications

[LiteYear]

From this site and other sources, I've come to understand that semiconductive screens in cables are generally used in medium and high voltage transmission and distribution applications. There they reduce uneven capacitive voltage distribution by providing a relatively low impedance discharge path, rather than relying on an insulator to be perfectly distributed. The semiconductive layer is less susceptible to cracks and voids and therefore maintains a more consistent admittance. The trade-off for the reduction in voltage stress in insulation is that you get some additional losses through the semiconductive layer. Please correct me if I'm wrong.

What I want to know is what happens when you use these cables between a VSD and a motor? The Olex Type 241 cable, for example, is reasonably popular in mining environments, and it's possible that it will find its way into VSD installations.

Would I be right in supposing that the semiconductive screens will present a higher capacitance path between phases and earth, and therefore conduct a greater amount of high frequency current? What is the effect on the inductance of the common mode return path?

 

[Skogsgurra]

Without knowing much about the 241 cable, I would still say that it isnt a very good idea.

While MV installations have to deal with 60 or 50 Hz voltage distribution, where resistive voltage distribution still is important, the VFD cables mainly have to handle capacitive voltage distribution and there, the resistive paths need to be of quite a low resistance to have any influence. So low, that you probably would experience insulation problems, heating and such things.

I think that semiconducting outer layers on the individual cores in the cable could help fighting inter-core ionization and ozone production. But that is a somewhat different problem.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[PHovnanian]

The capacitance of a semicon shielded cable will behave the same as the capacitance of any other cable. For a given frequency, it is a shunt impedance from phase to ground. At higher frequencies generated by VSD drives, that shunt impedance will be lower. As long as you stay within the specs of the drive, the drive will be OK. But the shunt currents will depend on the voltage magnitudes at each harmonic.

However, the semiconductor shield within the cable represents a loss element in series with the capacitive shunt impedance. And at higher frequencies, this loss may cause heating and possibly reduced life of the cable.

You might want to check with the cable manufacturer to see if they have any data or analysis regarding losses in the semiconductor shields at higher harmonics.

 

[LiteYear]

Data and analysis is rare and light on. That's why I come to you learned folk!

I'm sure the drive will be fine. I'm more interested in non-ideal effects like touch potentials and circulating currents.

Thanks for your comments so far. I'm interested in hearing more!