Zero sequence impedance of the cable laid in the steel pipe 3

[Krustabas]

If it’s possible I’d like to ask advice. We have three XLPE cables in trefoil formation laid in the steel pipe (the pipe was previously a part of high-pressure fluid-filled pipe-type cable system). The pipe and cable screens are directly earthed at both ends of the cable (see attached image).

I have several uncertainties:

1. How to calculate zero sequence impedance Z0 of a cable in this case?

2. Can I use Carson's equation? Assumption about equivalent earth conductor at a variable complex depth according to frequency and earth resistivity might not be valid here, as earth current most probably will flow in the steel pipe.

3. Is there a mutual coupling between cable conductor and steel pipe (it would influence the direction of the current induced in the pipe)?

4. Does anyone have similar cable layouts in their power systems?


Any help, links or suggestions appreciated.

 

[JJHorak]

Take a look at the SEL paper "Protection of High-Voltage AC Cables." It reviews issues associated with Zo of cable.

 

[jghrist]

From Westinghouse T&D Reference Book, 4th Edition

When cables are installed in iron conduits or steel pipes, the zero-sequence resistance and reactance are affected by the magnetic material because it closely surrounds the phase conductors and forms a likely return path for zero-sequence current. No method of calculating this zero-sequence impedance is available, ...

There are some tables included for low voltage cables based on tests. You may have to get data from manufacturers based on tests. Maybe steel armoured cable test data could be used.

 

[7anoter4]

I agree with jghrist. The cable manufacturer may indicate how to calculate the reactance for cables in magnetic [steel] conduit. See:

http://www.okonite.com/engineering/voltage-regulation.html#Reactance of conductors at 60Hz

But, as you remarked, the problem is more complicated. So Timesabroom proposal to see SEL paper "Protection of High-Voltage AC Cables." is still valuable.See:

http://selinc.biz/techpprs/6221_ProtectionACCables_DT_20060131.pdf

Also you may consult "Handbook of Electrical Power Calculation" by H. Wayne Beaty see:

http://books.google.com/books?id=eP...X&oi=book_result&resnum=5&ct=result#PPT236,M1

 

[jghrist]

The Schweitzer paper if very informative, and gives more detail about how difficult it is to determine the zero-sequence impedance of cable in pipe. It doesn't give a method of calculation, however. Note that the zero-sequence impedance of pipe-type cable is dependent on the zero-sequence current.

 

[Krustabas]

It seems we will have to estimate the parameters of this cable by the help of on-site experiment.

Thank you all for your answers and links. I was hoping there is some kind of established calculation methodology for this case, but it appears the things are quite difficult.

 

[ePOWEReng]

Also keep in mind that the zero sequence impedance changes depending on the return path of the current. If the return path is through the neutral conductor of the cable, then the zero sequence impedance will be significantly lower compared to if the return path is through remote ground. Care must be taken in choosing what the "worst case" zero sequence impedance is for particular protection elements. If this is a radial distribution application, then one calculation based on the larger zero sequence impedance will suffice. If this is a looped application also consider what the lowest zero sequence impedance could be so that you don't over-reach too far.