Reactive-Voltage (QV) Droop Curve and droop constant of Statcoms
What are the differences between these values?


\$V_{max}\$ and \$V_{min}\$ are primarily depend on the specification and capability of the STATCOM elements (i.e. converter, capacitor, insulation etc.) On the other hand, the droop is specified by the STATCOM operator where the main purpose of installing STATCOM may play the main role. A lot of research papers have been published to calculate the optimum droop (and other control parameters) for different purposes, for example 1 2 3 4 5.


  How can I find a right value of it?


The is no "right value" as such. You can take a value from literature and apply it if the aim of your work is just to implement STATCOM. However, if your aim is to optimally tune it, then you have to take a look at the research done in that area and see how you can develop it further.


  Edit


In the thesis linked in the comment, they are not dealing with STATCOM, though the converter act almost the same. In the thesis, the main purpose of the converter controller is to deliver the active power from the source (solar, wind, battery .. etc.), while the rest of the margin that left from the converter capability is utilized to compensate reactive power (this margin is specified by power factor of converter (i.e. 0.99)).

Now to your question Is that correct way to do the calculation?. I would say it is a logical way, but again not the optimum for other purposes. They are using a very simple principle where the converter should supply

1) All possible reactive power (\$PF_{max}=0.99\$) when the PCC voltage is at the minimum accepted value.
2) 0 reactive power when the PCC voltage is nominal.

So each point of (1 and 2) gives two coordinates (voltage and reactive power). and the droop is the line linking between them.