Optimal selection of controllers for three-phase four-leg inverter-based distributed generation units

This paper presents an optimal selection of linear controllers for the three-phase four-leg inverter operating in island-mode. The inverter uses dual-loop linear control strategy to control the output AC voltage. For each loop following linear controller combinations are tested: proportional (P), proportional-integral (PI), proportional-resonant (PR), and proportional-integral-resonant (PIR). Elimination of steady-state error under DC and AC waveforms, output impedance value, system stability and execution time are compared. Results show that only an outer loop PIR controller is able to eliminate steady-state error in the case of both symmetrical and non-symmetrical load regardless of the inner loop controller type. For the inner controller, both PI and PIR controllers show superior characteristic for tested criteria. Finally, the control system is implemented in TI DSP ControlCARD and tested using hardware-in-the-loop setup under symmetrical and non-symmetrical loads.