in Powerworld there is a function called ''line flow pie'' which indicate the percentage of load of the line. But what is it? I think that it is the ratio between the current on the line and its nominal value times 100. Is this correct?
From the documentation:
The line flow pie charts are used to indicate the percentage MVA, MW, or Mvar loading of a transmission line or a transformer. The degree to which the pie chart is filled shows how close the device is to its limit (provided the device has a nonzero limit). A line flow pie chart becomes completely filled when the device's flow meets or exceeds 100% of its rating.
So rather than current, it is indicating power (Real, reactive, or true).