Earthing System Impedance
Earthing System Impedance
(OP)
We recently conducted an earthing audit of a local substation. Part of the testing was fall of potential testing to determine overall ground Impedance. The measured value for this test was 10.2Ohms. This value is much higher than any other values previously measured in other locations. Due to the high earth Z and the high fault currents our EGVR is in excess of 40kV!! Has anyone else come across such high EGVR's/Earth impedance values?
Thankyou
Thankyou






RE: Earthing System Impedance
What is the line-to-ground voltage? The GPR cannot exceed this value.
RE: Earthing System Impedance
You should check some references to be sure that the method is reasonable for your application.
Jim
RE: Earthing System Impedance
RE: Earthing System Impedance
RE: Earthing System Impedance
Same test method as used every other time where results have always been acceptable...
Line to ground voltage of 110kV / sqrt3 (~63.51kV.
Its a relatively small switching station with no TX. Only a couple of isolators (disconnectors) and a circuit breaker.
RE: Earthing System Impedance
Your GPR must have considered all of the current going through the grid. Most of it will return through the shield wire. This is what saves the day and makes the design practicable.
RE: Earthing System Impedance
Thats long enough, have you considered soil treatment?
RE: Earthing System Impedance
Hence all earth fault current must return via earth.
We performed a soil resistivity test, not exactly sure what is meant by soil treatment though.
The surrounding land is undisturbed bushland with just a transmission line track heading out in either direction from the station. The station has been there for many years so the ground has had plenty of time to 'settle'.
Current was injected down one transmission line track with the voltage readings taken 180deg in the opposite direction.
RE: Earthing System Impedance
I have tested lots of substations, from 345 kV on down. Some come close to or exceed this 10 Ohm number. Some utilities spend a lot of money, time and effort to get below 2 Ohms - some don't.
Soil resistivity, hence substation resistance varies with soil moisture and consequently the season and recent rainfall. Yes, fault current, and step and touch potential are all affected by soil resistivity (and fault paramaters). It all depends on where the emphasis is placed - and how much money you have to throw at the problem.
RE: Earthing System Impedance
Its seems very high at ~42kV with a worst case fault clearance time of 1 second.
However, as this station is not the source all fault current would be returning to the remote stations which are much larger with more substansual earthing systems.
This station is in a very remote location with little or no known underground assets in the area, ie. Gas, Water, Telecommunications, etc.