why we connect triangular wave across the voltage sensor in solar module example

This test circuit is to generate the P-V curves for the solar cell for two values of light intensity input to the solar cell.

This voltage source is to define the solar cell voltage. 

For the total simulation time, this triangular wave is actually a ramp. It increases from 0 to a certain value. At this value, the output power curve drops back to zero with the light intensity=1000 (W/m^2).