I am looking at an surface mining installation which has a 66/22kV substation suppling a large mobile electrical load (5-10MVA) via a 22kV above ground cable (max 2.5 miles long). The cable earth is connected to the 22kV neutral at the substation. The mobile load has operators and there is no earthing grid associated with it.
The concern is that a 66kV fault will cause a Ground Potential Rise (GPR) at the substation which will be transfered via the 22kV cable earth to the mobile load resulting in step and touch voltage issues at the load. Worst case being a 66kV fault at the substation itself.
Limiting the 66kV earth fault is expensive (requires the installation of a >30MVA 1:1 Delta Wye isolating transformer with a NER). The effectiveness of reducing the earthing resistance at the substation (using copper in the ground and parallel paths connected via the 66kV line overhead earth is also limited).
Does anyone have a suggestion for another suitable means of reducing the possible step and touch voltages at the mobile plant arising from earth faults on the 66kV side?
The concern is that a 66kV fault will cause a Ground Potential Rise (GPR) at the substation which will be transfered via the 22kV cable earth to the mobile load resulting in step and touch voltage issues at the load. Worst case being a 66kV fault at the substation itself.
Limiting the 66kV earth fault is expensive (requires the installation of a >30MVA 1:1 Delta Wye isolating transformer with a NER). The effectiveness of reducing the earthing resistance at the substation (using copper in the ground and parallel paths connected via the 66kV line overhead earth is also limited).
Does anyone have a suggestion for another suitable means of reducing the possible step and touch voltages at the mobile plant arising from earth faults on the 66kV side?
