Q121 : Why are Computed Step Voltages Asymmetrical in a System with Four-Sided Symmetry (i.e., a Transmission Line Tower)
Question Why are Computed Step Voltages Asymmetrical in a System with Four-Sided Symmetry.
Answer Here are some possible reasons why computed step voltages are asymmetrical in a system (a transmission line tower) with four-sided symmetry:
1. If the "Gradient" type of step voltage is selected, then step voltages are computed only in the direction of the potential profile and not in any other. As a result, step voltages along two parallel sides of the tower can be the same, but quite different from those on the other two insides. The regular step voltage option should be selected to obtain true step voltages.
2. If the spacings between points along each profile (as specified in the PROFILE definition) are different from the spacings between the profiles (as specified in the SURFACE definition), then step voltages in the two different directions may be based on different point spacings and therefore different. Remember that at each profile point, the program searches within a default radius of 1 m (or a user-specified radius, if any) for all other points: the voltage between the central point and each other point is computed and the maximum is retained. If the point furthest from the central point in one direction is much further than the furthest point in the perpendicular direction (or maybe there are no points within the search radius in the perpendicular direction!), then this will introduce an asymmetry between the step voltages along different sides of the tower.
3. Your profile points may not occur at the same distances from the tower foundations/grounding on all four sides: i.e., there may be an asymmetry in the positions of the profile points with respect to the four sides of the towers.
Note that earth potentials, and therefore touch and step voltages, are computed only at the profile points you define. Depending on the points you choose, the results may vary somewhat.