Thank you, Gussan, what about selection of reference values for implementing a control scheme for VSI inverters??

in dq reference frame you have a vector that includes 3-phase voltage. by selecting speed of rotation for the vector, you can create the three phase frequency. in other word vector amplitude in the DQ reference frame is three phase voltage amplitude and the vector rotation speed is your three phase frequency. then your frequency reference is phase frequency and your amplitude reference is your amplitude of three phase voltage.

For d-q control of grid connected inverters you should use the stationary frame. The vector rotation can be implemented with equations, or using a PLL (phase-locked-loop).

In addition to the above, VSI are utilized at distribution level in most of the cases. At distribution level, there is a coupling between active power and voltage (P/V coupling). Similarly, there is a coupling between reactive power and frequency (Q/f coupling). these couplings are due to small X/R ratio at distribution levels. In order to decouple these couplings, VSI  are modeled in DQ-synchronous rotating reference frame. Based on this DQ model, we can design controllers. Coming to selection of reference parameters of d & q axis components, its mainly depends on which ratings we are going to design. We can set reference value of the q component to zero for ensuring control of only active components.

@ E S M Raju P- I would like to confirm, the coupling of powers in power system, Active Power is coupled with frequency NOT voltage to the best of my knowledge. Reactive with voltage. Did you mean some thing else or any special condition??

Dear Amit Kumar Roy,

Generally, Active Power and Reactive Power are (Depends on ) coupled to Voltage as well as frequency. However, In case of Power system transmission, R/X ratio is small and due to this, we are assuming decoupling between Active Power  and voltage, Reactive power and frequency (i.e Active Power is coupled with only frequency and Reactive with voltage). In case distribution, this assumption is not realistic. As mentioned in above comment, to decouple these, we are modelling in DQ frame and designing controller.