Evaluating & Mitigating Transferred Voltage Hazards
In the event of a fault, voltages may be transferred from a grounding system through the soil, resulting in hazardous conditions for personnel, the public, and adjacent facilities. This webinar, given by David Lewis, P.E, at EasyPower, uses the XGSLab simulation software to highlight the evaluation of transferred voltages, providing several case studies and mitigation efforts. Identification and evaluation of several transferred voltage scenarios provides opportunities to view cost effective strategies to reduce these risks.
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Answers to Q&A Following Live Webinar
|Question||Answer (David Lewis, PE at EasyPower)|
|Is XGS able to model the nonductive and conductive portions of the fence?||Yes, with XGS you can approximate the conductive fence mesh and fence posts in the software. The non-conductive portions were simply removed from the model, but another approach possible with XGSLab is showing the nonconductive fence as a 'background'. A 'background' would allow the fence to be visually represented, but it does not effect the electrical characteristics of the modeled system.|
|Please elaborate on 'equipotential' software capable of doing this transferred potential evaluation.||There are methods that allow engineers to determine a grounding system impedance using simple hand calculations. These methods use an equipotential assumption meaning the whole ground grid will have the same voltage potential, and does not account for the self nor mutual impedance of the conductors. There are software options that use this assumption as well which may be an acceptable assumption in some sites with insignificant voltage drop from one edge of a ground grid to the other, but is likely to cause erroneous conclusions when doing a transferred voltage study. transferred voltages often examine long linear objects where the system will have to account for voltage drop along the recipient object. You often need to have a software like GSA_FD or XGSA_FD that will calculate self and mutual impedance of your modeled systems.|
|Typically the DC side of the inverter is isolated from ground. The AC side of the inverter is also connected with a cable to the transformer. In that case there is no ground fault current through the soil which creates STEP & TOUCH potentials and GPR. Therefore do you really need such a large ground grid for the PV station?||The faults that are typically evaluated in the utility scale PV site, like that of the presentation, are located on the medium voltage (the 12-34 kV) at the inverter-GSU. A local fault on the LV side of the GSU would not produce a GPR as they have a metallic connection back to the source, as you noted. One of the challenges of PV grounding design is understanding how the system under analysis is actually connected, as there are different configurations. In many utility scale PV systems the grounding system is common from the DC grounding conductors and the AC grounding conductors.|
|The Canadian code & several provincial codes allow GPR up to 3000V. If this GPR is transferred to another remote sub through the shield/screen of a cable, and if this shield is touched by a person what will be the situation then? How to mitigate this transferred potential to safe touch voltage of 50V?||That situation is precisely a transferred touch voltage hazard, and the mitigation will often be scenario dependent. One approach could be use of a gradient control mat to remove any potential between the shield/screen and the person's feet.|
|Is there any recommended limit for the GPR value?||With regard to personnel safety, there is not a GPR limit, but you can consider the GPR in relation to the touch voltage limit for determining the likelihood of transferred voltage hazards. You may see regions with guides or requirements on the maximum GPR, but that really relates to the likelihood of transferred voltage hazards and/or a concern of equipment damage. Communication equipment and cable insulation are examples of systems that may be damaged from the GPR at a site.|