I have simulated a microgrid in Simulink MATLAB; DC link voltage is converted to AC voltage by PWM inverter; The control system is implemented using proportional-resonance or PR controllers and includes two voltage and current control loops. Current and voltage references are applied to the controllers by fixed reference frame (alpha-beta-zero).

The simulation steps are as follows:

In the first step, the inverter must be synchronized with the local network with the help of filters and loops designed in the system (goal: soft start of the inverter).

In the second step, after synchronizing the inverter with the grid, the inverter must feed the balanced three-phase ac load.

In the third step, current control loop references are applied to the inverter, and the inverter should be able to inject current into the local grid without problems. (ID and iq references are converted to alpha-beta-zero format and then applied to the flow controller.)

Until the third step, when the flow control loop references were not applied, the system performed the first and second steps well; But by applying the references of the flow control loop, the performance of the system becomes unbalanced and even the first and second steps do not give the same results as before.

What is the cause of this problem? And why does the performance of the system suffer by applying references related to the flow control loop? It is necessary to explain that we did not have such a problem in single-phase studies.

Thank you for your guidance