Why earth resistance varies for different fault locations in solar farms

Background

Solar farm earthing systems are large and when performing earthing system modelling, we simulate fault at different locations using ELEK SafeGrid Earthing Software. For example, we might be simulating medium voltage faults at different transformer locations.

Outcome

For large solar farms (or wind farms), the grid impedance and the grid potential rise (GPR) will often be significantly different for different fault locations. Note that for small earthing systems that are meshed, moving the fault location will often have negligible effects on grid impedance and GPR.

The reasons why there is a significant difference for large grids is as follows:

• The apparent (Thevinen) impedance as seen from the fault location may be significantly different.
• The voltage drops are significant for long conductors

Example

The images provided below is displaying the surface voltages simulation for a typical PV solar farm. The same fault current is applied in both cases but the location of fault has been changed each time. We can observe that when the location of the fault is changed, the grid impedance as seen from Ifault1 (Zgrid 1) and Ifault2 (Zgrid 2) locations is 0.505 Ohms and 0.472 Ohms respectively.

References:

[1] ELEK SafeGrid Earthing Software, V8.0.