Another way to represent the voltage of a PV array in change of irradiance and temperature?
The equation   

          Vmt = VocCv(Tm-Tr)


is just plain wrong. 

You need to intelligently compare the curves and the formula instead of blindly plugging things in and being puzzled.   

You cane see from the AV at various temperature curves (right hand graph) that Voc ~= 48V at 5 degrees C and about 38V at 65 degrees C. ie it decreases by 10V as temperature increases by 60 C.
So a 1st approximation is
Voc = 48 - 10 x (T-5)/60
So at 5C Voc = 48 - 10 x (5-5)/60 = 48v  and
at 65C   Voc = 48 - 10 x (65-5)/60 = 48-10 = 38V.

Look at the formulae you have and see how they can be made to reflect the graph based result.