It seems to be a mistake in sign in the help section of the Performance Ratio https://www.pvsyst.com/help/performance_ratio.htm. In particular the help section shows a weather-corrected PR equation as:
PR (corr) = E_Grid / ( PNomPV * Σ hours ( GlobInc / GRef * (1 - muPmpp * (Tarray - TArrayAver) )
While the NREL paper (cited in the help section) https://www.nrel.gov/docs/fy13osti/57991.pdf shows the equation with a diferent sign mainly exchanging (TArrayAver - Tarray ):
PR (corr) = E_Grid / ( PNomPV * Σ hours ( GlobInc / GRef * (1 - muPmpp * (TArrayAver - Tarray ) )
Assuming muPmpp is a negative number.
I will be happy to receive any feedback on this subject.