Reduced-order Aggregate Model for Parallel-connected Grid-tied Three-phase Photovoltaic Inverters

Ratings of utility-scale grid-tied photovoltaic (PV) inverters are typically no more than 1 MVA. This implies that a large number of inverters are usually installed in utility-scale PV energy-conversion systems. Given the complexity of inverter dynamic models (typical models are high dimension and nonlinear), reduced-order models for inverters are critical for performance assessment and accurate representation of PV-system dynamics in the bulk power grid with limited computational burden. In this paper, we formulate a reduced-order model for parallel- connected grid-tied three-phase PV inverters that has the same structure and model order as a single inverter. We adopt a singlediode model for the PV modules, and each inverter is assumed to be a single-stage dc-ac voltage-source converter with an input dc- link capacitor and an LCL output filter. The control architecture includes a maximum power point tracking (MPPT) algorithm, a dc-link voltage controller, a reactive power controller, a current controller, and a phase-locked loop for grid synchronization. Numerical simulations demonstrate the computational benefits and accuracy of the reduced-order model.