Hi Siddarth,
The latest version of the technical reference for SAM's detailed photovoltaic model was published in 2018, so you may want to use that as your reference instead of the older 2015 version. It is available on the SAM website at:
sam.nrel.gov/photovoltaic/pv-sub-page-2.html
The equation you mention in the 2018 version of the manual is Equation 10.33. According to the code in the lib_pvmodel.cpp source code, when the power output value resulting from that equation is negative, the model sets the value to zero:
You can find the source code on the GitHub.com repository for SSC:
github.com/NREL/ssc/blob/develop/shared/lib_pvmodel.cpp
Best regards,
Paul.