Everywhere I am finding trial and error method for these gains value selection. Matlab has an example for grid connected PV system modeling simulation
power_PVarray_grid_avg or power_PVarray_grid_det . But I have no idea of how values of PI controller were selected for input dc voltage control.
you need to check symmetrical optimum method, ITAE criteria and root locus design.
there is also a research paper on it as well.
here it is i guess it can help.
and according to my knowledge the pid gains in example of Matlab it has used Sisotool to automatically tune the pid controllers.
I really appreciate your help. I will have a look at it. In Simulink automatic tuning is there but since It's an honors project so, I need to have deep understanding of all these things.
For the entire control scheme of the inverter do you have any resource, in which everything is explained in detail?
Be careful if you want to use the classic Ziegler Nichols tuning rules. The use a global gain and then some integrating time constant for gain "1" but they omitted a clear transfer function. Matlab uses a different transfer function definition which results in different values. If the measurement gain changes in Matlab all terms have to be adapted. Why not simply use Nyquist with at least 40° phase margin often gives satisfactory results, Pay attention to model the time delay between measurement and control also as a phase margin. Other methods use Z-transform, but also the (parasitic) time between measurement and action is important.
you can use either matlab tool, nyquist or zeigler nichols methods to find approximate/closer values of pi and then tune manually using these values i.e. use trial and error to find best results by varying these values.
I see the following steps in finding the gains of the PI Controller:
1) One need to make an accurate plant model of the grid and the converter.
2) Obtain the transfer function of the loop only including current PI controller. By making necessary arrangements in the equations, Gain factor can be obtained.
3) Then approximate the current loops and find an overall DC link loop transfer function and repeat Step 2.
Try to understand the following research paper which gave step by step approach over calculation of PI gains.
Vector control of three-phase AC/DC front-end converter by
J. S. Siva Prasad
Tushar Bhavsar
Rajesh Ghosh
G. Narayanan
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