Reclosing (re-energizing) underground transmission cable after a fault

[Eam1]

I am trying to research the requirement to wait or not a predetermined amount of time (“x” minutes) to re-energize an underground transmission line that was taken out of service (tripped) by a relay action.

The case I am after is the one where the cause of the outage or trip has been determined and confirmed to be external to the underground cable; in other words the UG cable is in good operating condition. We are assuming that the fault was momentary or the faulted piece has been removed, bypassed and/or repaired.

I understand that there is trapped energy in the cable even after it is de-energized; would this trapped energy have any detrimental effect on the good cable if the cable is re-energized without dissipating it?

Thank you!

 

[jghrist]

The trapped energy would be the same as that in a capacitor when it is de-energized. By the time that you determine that the fault was external or the faulted piece removed, it will be negligible and of no concern.

 

[Eam1]

Hi,
Thanks for the reply. That is my understanding. Do you know of any books, web pages or documents that may explain what would be the maximum voltage spike a cable would see if re-energized within a short amount of time?
Thank you!

 

[submonkey]

Hello,

Check to see whether you have a magnetic voltage transformer
or other transformer connected to the cable. The winding
impedance at DC will be low, and will discharge the cable
capacitance.

Thanks,
Submonkey.

 

[Eam1]

Hi - In some cases we do have them, but in some other cases we don't. For the cases we don't, should we wait? IEEE C37.104-2012 "Guide for Automatic Reclosing of Circuit Breakers for AC Distribution and Transmission Lines" allows reclosing for underground transmission lines as long as the fault has been removed and the UG cable has not been impacted, BUT I have not been able to find anything that discusses the trapped energy. I am assuming it is fine because IEEE states, BUT I have not been able to find documentations that goes into explaining the energy stored in the cabole.

Thanks!

 

[cvance]

To answer your question simply, the capacitive charge that occurs on an energized line would dissipate naturally as a standard decay of your RCL circuit. This RCL circuit would be determined as a model of your transmission line parameters. If your transmission line is less than 50 miles, the shunt capacitance is typically ignored completely in calculations.

Look into pi equivalent model (or lumped equivalent) of a transmission line for anything longer.

I personally would expect the decay of voltage to occur within cycles or seconds, not minutes. Although, when you go to re-energize, something to consider is the voltage rise at your receiving end of a lightly loaded line.

Note that capacitance is determined as a function of length of line.

Allan Greenwood: Electrical Transients in Power Systems
Lou van der Sluis: Transients in Power Systems
J.D. Glover, M.S. Sarma: Power System Analysis and Design
L.M. Faulkenberry, W Coffer: Electrical Power Distribution and Transmission

 

[Eam1]

Thank you very much for the information and references!