One of our biggest challenges in designing high voltage protection is determining the grid resistance at a substation after it has been connected to the Multi-grounded neutral (MGN) network. This is due to the MGN and other infrastructure providing parallel paths for return current, typically skewing the readings down, often significantly. In many instances, the original measured grid resistance is simply not available, or is so old that the validity of the reading is suspect. In such a case, the power company must calculate the grid impedance using resistivity data coupled with software modeling of the grid. There are a couple of software products that will do this, but that method has always been held as somewhat suspect by most Inductive Coordination and Electrical Protection (ICEP) Engineers as being inferior to actual measurement.
With the advent of the Smart Ground Multimeter (SGM), that challenge has been made a little more clear. This unit developed by EPRI injects a current in a range of frequencies, then analyzes the signature of the current returning to the meter from a remote probe. By way of the analysis and the meters algorithms, a resistance measurement is determined for the portion of that current that flows through the earth. Its ability to do so lies in the fact that the grid resistance to remote Earth is nearly purely resistive with very little or no reactive component.
This presentation will show some of the capabilities of the SGM and how they can be applied in practical applications, and will include examples of actual readouts from the meter with an overview of how those readings were obtained and applied.