Q120 : Using MALZ/HIFREQ to Model Fall-of-Potential Grounding System Impedance Measurements
Question How can MALZ/HIFREQ be used to Model Fall-of-Potential Grounding System Impedance Measurements?
Answer To simulate with grounding system impedance measurements conducted according to the Fall-of-Potential Method with MALZ or HIFREQ is quite straightforward. Proceed as follows:
1. Model your conductor system, being sure to include the buried structure (typically a ground rod) which you are using as the return current electrode for your fall-of-potential test. Although you may choose to ignore them initially, nearby grounds, such as transmission line tower and pole grounds, distribution line grounds, including water pipes associated with distribution services, and the like, which are electrically continuous with ground paths leaving the substation, may have a significant effect on the fall-of-potential profile. After an initial run, you may find that you need to significantly extend your model in order to match the measured apparent impedance curve.
2. Inject 1 A at 180° into your return electrode and 1 A at 0° into your grounding system (at the same location this was done in the field). Specify the return electrode energization first (Energization 1) and your main grounding system energization second (Energization 2). This way, the main grounding system energization becomes the default reference for the generation of the fall-of-potential curve after the program has been run.
3. Define a potential profile which coincides with your measurement traverse.
4. Run MALZ or HIFREQ.
5. In the Output Toolbox, request a touch voltage plot, using all the default settings: in particular, leave the reference GPR as "user-defined" and do not specify a value. Because you have energized the main grounding system with Energization 2, the GPR of this system at the injection source will automatically be used. The resulting curve is your apparent impedance curve in ohms, as a function of distance from the potential profile starting point. Although the y-axis is labelled "touch voltage in volts" or something similar, it represents apparent impedances in ohms, because you are using a unit injection current in your MALZ/HIFREQ run.
6. If this computed curve is similar to the measured curve, then this confirms that your computer model is close to reality. Now, simply compute the ground impedance of your substation in the usual way (i.e., without a return electrode), locate the point on your apparent impedance curve corresponding to this ground impedance and see what is the measured impedance at this point along the measurement traverse. This is your measured impedance.
Note that the following factors can make the measured ground impedance different from the computed one:
1. Measurement frequency versus modeled injection current frequency (if not the same): for the best match, both should be the same. A subsequent run at power frequency can be made to adjust the measured value for frequency.
2. Non-modeled buried metallic structures: buried structures near the measurement traverse, or anywhere around the substation for that matter, can influence the measurements. It is important to obtain as much information about these before the measurements begin: this way they can be avoided as much as possible during the measurements, and then considered adequately during the modeling.
3. Interlead coupling (between current injection and voltage-measuring circuits): magnetic field induction can exaggerate voltage readings if not accounted for. Some instruments do a good job of eliminating most such noise automatically.
4. Electrical noise: voltages measured by the instrumentation when the injection current is off may incorrectly inflate readings if they are of a comparable magnitude (or higher) to the voltages measured with the injection current on.
5. Incorrect modeled soil structure: were soil resistivity measurements made to large enough pin spacings? Were measurements made along more than 1 measurement traverse for a consistency check?