A simplified DQ Controller for Single-Phase Grid-Connected PV Inverters

Synchronous dq-frame controllers are generally accepted due to their high performance compared to stationary alpha beta-frame ones, as they operate on dc quantities, achieving zero steady-state error. In single-phase systems, however, PI-based dq controllers cannot be directly applied due to the reduced number of input signals available compared to three-phase systems. The common approach in single-phase systems is to create a synthesized phase signal orthogonal to the fundamental signal of the single-phase system so as to obtain dc quantities by means of (alpha beta to dq) transformation. The orthogonal components in conventional approaches are usually obtained by phase shifting the real signals by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. In this paper, an alternative dq controller scheme which referred as to simplified dq controller is proposed. The proposed scheme does not require orthogonal quantities to be generated, making it easier to be implemented. The simplified dq control method is experimentally compared to the conventional delay-based dq control method and shown to improve the poor dynamics of the conventional approach while not adding excessive complexity to the controller structure. A single-phase five-level diode-clamped grid-connected PV inverter is considered as an example in this paper.