A Hybrid Passivity-Based Control Strategy for Three-Level T-Type Inverter in LVRT Operation

The fast dynamic performance of a photovoltaic (PV) inverter is one of the most important issues in low-voltage ride-through (LVRT) operation, as the voltage dip period is short. Nevertheless, it is difficult to control the output currents with conventional methods, especially during transients or under low-power factor operation. In order to satisfy the LVRT requirements and improve the performances of a three-level T-type inverter (3LT
²
I) in a PV system, the hybrid passivity-based control (HPBC) strategy with the fast transient response and strong robustness to system parameter changes and external disturbances is proposed in this paper. First, the Euler- Lagrange model of the 3LT
²
I under the LVRT condition is derived and analyzed. Then, the 3LT
²
I system is theoretically proved to be strictly passive by using some ingenious arithmetic and differential calculation techniques. Furthermore, based on the error energy function and the Lyapunov stability criterion, a passivity-based controller including a negative sequence of grid voltage feedforward control terms is designed to suppress the negative-sequence currents effectively. The balanced grid currents are obtained, which improve the stability of the system in LVRT operation. Compared with the conventional methods, the proposed HPBC scheme has the superior characteristics of fast and non-overshoot response in a transient state and accurate current tracking performance in a steady state. The theoretical analysis and effectiveness of the proposed strategy have been validated by means of both simulations and experiments.