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In PSSE the machine speed is given in pu speed deviation, i.e. Hz/fbase - 1. The frequency base is either 50 or 60 Hz.

At nominal speed the pu speed deviation is 0. The speed deviation is positive at overfrequency and negative at underfrequency.

The formula to calculate the frequency in Hz is fbase*(spd+1).

You have to apply a formula to plot the rotor speed in Hz. You can do that in PSSLT with activity FUNC, as jconto described. Another alternative is to use a function in Plotbook, see example below:

Here, A is a speed channel with machine pu speed deviation.

Personally, I prefer to read the outfile with dyntools.CHNF() and plot the channels with matplotlib. Then I can calculate the frequency in Hz for all spd-channels before plotting.