×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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

Zero sequence impedance of the cable laid in the steel pipe

Zero sequence impedance of the cable laid in the steel pipe

(OP)
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.
 

RE: Zero sequence impedance of the cable laid in the steel pipe

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

RE: Zero sequence impedance of the cable laid in the steel pipe

From Westinghouse T&D Reference Book, 4th Edition

Quote:

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.
  

RE: Zero sequence impedance of the cable laid in the steel pipe

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%20of%20conductors%20at%2060Hz
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=ePwF-PT_1QIC&pg=PT250&lpg=PT250&dq=J.+H.+Neher,+%E2%80%9CThe+Phase+Sequence+Impedance+of+Pipe-Type+Cables,%E2%80%9D&;source=bl&ots=0KZW-681X0&sig=7kKfeAQJWstKBfY43m5EozW_chY&hl=en&sa=X&oi=book_result&resnum=5&;ct=result#PPT236,M1
 

RE: Zero sequence impedance of the cable laid in the steel pipe

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.

RE: Zero sequence impedance of the cable laid in the steel pipe

(OP)
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.

RE: Zero sequence impedance of the cable laid in the steel pipe

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources