Earthing connections between transformer substations

[hhhansen]

Hello
One of our clients have multiple 10/0,4 kV substations at the industrial area with Dyn- transformers.
Now the client wishes to establish an earthing connection between the substations by mesns of 240 mm2 cable connection.
In my opinion this is an advantage regarding overall safety.

However Do you see any potential problems/Challenges in regards to this?

B.R. Hans-Henrik

 

[7anoter4]

The grounding scope is to reduce the current through the grounding grid in order to improve touch and step potentials.
Only 10 kV short-circuit current could participate in potential rising.
So single phase to ground short-circuit could be 10-30 A
The phase-to-ground-to-phase could be high-usually 0.75*I"k3.In your case a maximum 20 kA it could be.
If the substation circuit breaker trip delay is not more than 3 sec. 240 mm^2 copper temperature will be less than 250oC.
I cannot say if this cable will significantly reduce the current or not.
An example of calculation is as follows:

Where:
Zsys-supply system impedance.
ZR=cable phase R including phase-to grounding reactance.
Zgw=grounding wire impedance
ZpR =grounding resistance at R phase-to-ground fault
ZpS=grounding resistance at S phase-to-ground fault
If three phase short circuit current will be 30 kA the system impedance could be 10/sqrt(3)/30=0.2 ohm.
The grounding resistance could be 1 ohm.
Let's say the distance to substation will be 1 km.
Let's say the soil resistance 100 ohm.m then the grounding depth Dp=2.085/sqrt(f/ro/10^9)/10^3=93 m
XL=0.145*LOG10(Dp/rc)=0.145*log(93000/0.779/10)=0.59 ohm
The cable 240 mm^2 copper resistance will be 0.1 ohm.
A simple calculation the current through ground according to the following sketch:
If no grounding wire exists Ip=I"k2
ZR and Zgw include wire-to-ground reactance.ZS=0 –the S fault is located at the source.
Let's say the supply cable 15 kV 3 copper conductors 240 mm^2 XLPE insulated
ZR=0.1+j0.085 and parallel 2 phases-to-ground reactance 0.5 ohm
The I”k2 in the first case when no grounding wire is present =9200 A and this is the current through grounding
grid.
In second case in presence of grounding wire I”k2=9200 A but Ip=4420 A.