the conventional design of PI controller is depends on so many parameters like plant, control strategy and the limitations of system, we need to solve the equations of order the plant we have and the control loops also. their is much literature in google related to this PI design, you can follow that things. but generally most of the cases trail and error method is used because only you can decide the what parameter is important to control.

Hi,

I would advise you to take a look at Parks transformations for three phase systems. They are generally used to describe induction and synchronous machines in machine  theory, but useful thing is that they can be applied to any three phase waveforms as mathematical transformations.

These transformations will give you conversion from three phase sinusoidal waveform transform into two DC components. You can apply control theory on these DC signals now and than convert them back using inverse Park transformation to three phase form. If you use same transformations for all your signals (reference, input, output) you should be able to convert your AC problem into DC problem and thus your regulator design would be in DC.

hi,

You can also design sinusoidal reference controller which can able to track sine wave.

For saturation problem, its depend on actuator design. Depend on that you can set limit for the controller. 

You can refer this paper for sinusoidal reference and harmonic reference controller design.

Conference Paper 2-DOF Controller Synthesis for Tracking of Harmonic Referenc...

If you were able to design and understand a PI controller then do help me ........ I am on the same project ........