MATLAB: Is there an inconsistency in the usage of term “alpha” in the Surge Arrester Model documentation

Question

There seems to be an inconsistancy between the equation, the voltage versus current logarithmic plot, and the values given for alpha 1-3 in the SURGE ARRESTER block help documentation in SimPowerSystems 4.5.

The logarithmic plot of voltage versus current shows the slope as alpha. However, looking at the equation provided relating V, Vref, I and Iref, the slope of the logarithmic plot seems that it should be 1/alpha.

Also, the V-I curve presented in the documentation for the Surge Arrester model indicates that the model switches between an alpha 1, alpha 2, and alpha 3 region. However, there is no specific information defining where the boundary between these regions would occur.

Answer 1

The documentation in Release 2009a has been updated for more clarity. Please refer to the most recent documentation here;

http://www.mathworks.com/access/helpdesk/help/toolbox/physmod/powersys/ref/surgearrester.html

For previous releases, read below for any additional information:

Documentation on the SURGE ARRESTER block in SimpowerSystems 4.5 is lacking few details. Here is additional information on this block:

The "alpha1", "alpha2", "alpha3" terms of the voltage versus current logarithmic plot should be replaced by "segment1", "segment2", and "segment3" respectively to avoid any confusion. The alpha terms were incorrectly used to identify the 3 segments, and not the slope of it . With this modification in place, the slope of the 3 segments matches the three alpha terms in the equation provided relating V, Vref, I and Iref.

Also, it should be noted that the boundaries between alpha1, alpha2 and, alpha3 regions occur where the exponential curves cross (so the V-I curve is continuous). If you take a look under the mask for the SURGE ARRESTER block, then, inside the SOURCE block you will see that the three segments are modeled by three FUNCTION blocks and three switches are used to pass form one segment to the other, depending on the value of the current.