Ground Grid Resistance - Hydro Electric Plant
Ground Grid Resistance - Hydro Electric Plant
(OP)
On an upcoming project we are required to measure the ground grid impedance of a ground grid installation for an outdoor 230kV substation connected with a hydro electric plant. The ground grids longest dimension is a little over 100 meters. A penstock approximately 600 meters long exists which is connected to the plant. We are required to measure the ground impedance with and without the penstock connected.
With the penstock not connected we will place the remote current electrode 1000 meters away from the substation and run a fall-of-potential test using frequency selective equipment capable of measuring under and over the power system frequency.
To my understanding the ground grid was modelled using ETAP, therefore no complex analysis exists such as would be obtained using CDEGS.
With the penstock connected would I have to consider the ground grid being 100 meters plus the 600 meter penstock?
If this is the case I would likely have to enquire about using the tranmission line as the current return path and place the remote current electrode at least 6km away. Your thoughs and comments are appreciated.
With the penstock not connected we will place the remote current electrode 1000 meters away from the substation and run a fall-of-potential test using frequency selective equipment capable of measuring under and over the power system frequency.
To my understanding the ground grid was modelled using ETAP, therefore no complex analysis exists such as would be obtained using CDEGS.
With the penstock connected would I have to consider the ground grid being 100 meters plus the 600 meter penstock?
If this is the case I would likely have to enquire about using the tranmission line as the current return path and place the remote current electrode at least 6km away. Your thoughs and comments are appreciated.






RE: Ground Grid Resistance - Hydro Electric Plant
RE: Ground Grid Resistance - Hydro Electric Plant
At this point I would have to have discussion with the engineer and utility to determine feasibility of isolating the shield wire for testing if interference is a concern. My past readings on tranmission station grounding have been performing the test with the shield wire connected since this influences the ground grid impedance.
The terrain is difficult and only practically allows for following the transmission line since the station is in a mountainous region.
RE: Ground Grid Resistance - Hydro Electric Plant
My concern with mutual coupling is between the phase wire used for the current lead and the one used for the potential lead. See Influence of Inductive Coupling Betwen Leads on Ground Impedance Measurements Using the Fall-of-Potential Method, L. Ma and F.P. Dawalibi, IEEE Transactions on Power Delivery, Vol. 16, No. 4, October 2001, pp 739-743. Large errors are possible with large grids or low soil resistivity.
RE: Ground Grid Resistance - Hydro Electric Plant
RE: Ground Grid Resistance - Hydro Electric Plant
RE: Ground Grid Resistance - Hydro Electric Plant
We could measure the mutual coupling of the overhead line with the Omicron equipment and take this into account to evaluate the ground grid impedance.
RE: Ground Grid Resistance - Hydro Electric Plant
RE: Ground Grid Resistance - Hydro Electric Plant
RE: Ground Grid Resistance - Hydro Electric Plant