I've had a look at your file but there are a few problems;
I got a message that it couldn't load an object because the object was not available on my machine. Closer examination showed a missing reference to Microsoft Office Web Componenets 11.0 which is an Excel 2003 reference and I only have Excel 2010 here. So when I try to run code it balks.
There is commented-out code left right and centre and I'm not sure which code I should be testing (it all refuses to run anyway).

So I'll confine myself to general comments.

When it next errors out, choose Debug and then hover the mouse over the following variable names in the errant (highlighted in yellow) line (by the way, the one with NO quotes around WorksheetName):
WorksheetName
Column
VoltageDropColumn

Their values should pop up in a little box next to the cursor, and check they contain the values you expect, especially the WorksheetName, as this should contain a string with the EXACT (no extra spaces etc.) name of an existing sheet in the workbook.

I see in some place you have:
WorksheetNumber = "3"
where you've assigned a string to the variable, but later you have:
WorksheetNumber = WorksheetNumber + 1
where you're treating it as a number. I expect that Excel will coerce the value to the right type, both in the above line and in the later:
WorksheetName = WorksheetName & WorksheetNumber
but if you have Dimmed the variables to one type and you have an On Error Resume Next line somewhere, the WorksheetName will not contain quite what you expect.