Comparison of NPC and T-Type Single-Phase String Inverters Using WBG Devices for Grid-Tied PV Systems

Single-phase string inverter has been widely applied to grid-tied photovoltaic (PV) rooftop applications for its renewable energy. However, the inherent attribute of intermittency in solar energy may induce instability and unqualified power. To meet the grid-interconnection standards, high demands of efficiency and harmonic distortion need to be imposed on DC/AC inverters. This paper compares two most popular multilevel inverter topologies, neutral point clamp (NPC) and T-type, in terms of loss distribution, power loss, and total harmonic distortion (THD). Both inverters adopt the same highly efficient level-shifted (LS) PWM modulation scheme, and could achieve partial soft switching for their synchronous switches with the utilization of optimal dead time algorithm. However, due to the distinct commutation and switching process, two inverters possess totally different device losses and dead time effect. Thus, a trade-off among the loss distribution, power loss and THD performance will be presented. The losses on per-switch are calculated by curve-fitting loss model obtained from double pulse test (DPT), and then are incorporated into respective system-level simulation to evaluate the total losses. A 1-kW all gallium nitride (GaN) HEMT-based NPC and a 1-kW hybrid silicon carbide (SiC) MOSFET/GaN HEMT-based prototypes are presented with the input of 800 V and the rated output of 220 V / 4.5A.