Copper conductor screens for 33kV cables

[Ajay1971]

On a recent project I have had to use 33kV cables to distribute power to many island plants from the Main Receiving Susbstation (MRS). It should be noted that the 33kV power system has a solid ground connection.

My problem is related to the rating of the conductor screen...
a) Do I rate the screen for the calculated L-G fault current of 26kA?

b) What should be the withstand time for the screen to carry 26kA - 0.25seconds or 0.5 seconds or 0.75 seconds or even 1.0 second?

c) Can I use the cable manufacturer's standard screen and bond it with the cable steel armour?

Option (c) has been used prediminantly due to resistance earthed networks where the earth fault currents are restricted to 800A.

Thank you.

 

[alehman]

a. Yes, if you are concerned about protecting the unfaulted portions of cable from damage. Also, the shields of the non-faulted phases may carry a portion of the return fault current.
b. It depends on how fast your overcurrent protection operates. You can obtain a time-current damage curve from the cable supplier and plot it with the overcurrent protection device time-current curve to verify the screen is adequately protected.
c. Theoretically yes. Is you screen otherwise grounded at mulitple points? Parallel paths cause circulating current to be induced which requires derating of the cable ampacity. The steel armor will probably not improve the situation much. Consider a separate parallel copper ground conductor.

http://okonite.com/engineering/shielding-currents.html

http://okonite.com/engineering/shielding.html