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Performance of Earth Continuity Conductor in Fast Wavefront Transients
4

Performance of Earth Continuity Conductor in Fast Wavefront Transients

Performance of Earth Continuity Conductor in Fast Wavefront Transients

(OP)
Hi,

I'm doing a Surge Assesment study for a Substation fed via an U/G 132kV Cable (~200m). Cable is single-point bonded with 3kV Sheath Voltage Limiter. The Transmission Line to U/G Cable transition point has a 132kV Surge Arrester and clips off the Over Voltage surge due to a lightning strike on a nearby tower (3rd out). A 300sq.mm ECC runs along the cable bonding the substation earth grid & 1st the tower earth.

Ive modelled the main cable in EMTP-RV. Not sure what the role of ECC is for FWTs? Should the same be modelled as well?

Regards,

Engg_Downer

RE: Performance of Earth Continuity Conductor in Fast Wavefront Transients

I think that the purpose of the ECC would be for power frequency earth fault return current, not for any surge protection. The ECC may affect capacitance to earth from the cable, however. How are you modeling the earth in EMTP-RV?

RE: Performance of Earth Continuity Conductor in Fast Wavefront Transients

Rules of thumb for lightning protection conductors.
No sharp bends.
Use braided cable, not twisted strand cable.
Always run in the general direction of the ground grid.
The thinking is that at lightning discharge frequencies, the spiral twist in conventional multi strand cable, a sharp bend or a return bend in a conductor will exhibit self induction.
This may cause enough impedance that the current leaves the conductor and flashes over to an adjacent surface.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Performance of Earth Continuity Conductor in Fast Wavefront Transients

Check out lecture 24 at this website.

http://www.ece.uidaho.edu/ee/power/ECE524/

The connection to ground should have as little impedance as possible so the wave front goes to ground. The ground connection has to be low enough to try to prevent voltage doubling from happening when the lightning wave front reflects off the ground and potentially doubles the voltage and causes a flashover.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

RE: Performance of Earth Continuity Conductor in Fast Wavefront Transients

I agree with jghrist.See:
IEEE 575/2014
6.3.3 Parallel ground continuity conductor
During a ground fault on the power system, the zero-sequence current carried by the cable conductors
returns by whatever external paths are available. Since a single-point, bonded cable shield/sheath is
grounded at one position only, it cannot, except in the case of a cable fault, carry any of the returning
current. This being so, unless some parallel external conductor is available or is provided to serve as an
alternative path, the return current can flow only by way of the ground itself. Because the resistivity of the
ground is very high compared with that of good conductors, the return current is widely diffused through
the ground and the mean effective depth of the current is hundreds of meters deep. Because the returning
current path is significantly remote from the cable, the voltage induced along parallel conductors, including
the cable shields/sheaths, tend to be very high.
Accordingly, it is recommended that single-point bonded and multiple single-point bonded cable
installations be provided with a parallel GCC that is grounded at both ends of the route as shown in Figure
2. The spacing of this conductor from the cable circuit should be sufficiently close to limit the voltage rise
of the shield/sheath to an acceptable level during a single-phase fault. The size of this conductor must be
adequate to carry the full, expected fault current for the cable system.
For limiting transient voltage see chapt.6.9 Transient voltage analysis .See also Annex E:
Transient voltages and voltage withstand requirements of protective
jackets

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