A Comparative Study of PI and PR Controllers for Current Control in Grid-Connected Multilevel Inverters

This work mainly investigates the performance of two current control strategies for grid-connected inverters: the first operates in a synchronous reference frame and uses proportional–integral (PI) controllers, while the second control solution is implemented in a stationary reference frame and makes use of a proportional–resonant (PR) controller. The PI controllers are widely adopted due to their limited dependence on an accurate plant model, whereas the PR controllers are particularly suited for sinusoidal reference tracking. The comparison of the two control strategies has been realized considering different performance metrics, such as the ì command tracking capability, load disturbance rejection, and noise sensitivity. The PI controller operates on constant quantities and, therefore, requires a synchronous reference frame transformation and decoupling terms to compensate for the effects of the plant. Variations in system parameters may compromise its stability. In contrast, the PR controller does not require reference frame transformations or decoupling terms, but it achieves optimal performance only at the resonant frequency, corresponding, in our case, to the grid frequency. Stability analysis for both control strategies is carried out by including the output LCL filter in the current control loop, resulting in resonance peaks in the transfer functions. Simulation results show that the PI controller offers a favorable trade-off between robustness and dynamic performance when the grid angle is accurately provided by the phase-locked loop. Otherwise, the PR controller demonstrates superior performance due to its independence from grid angle estimation.