Application of grid-connected 3-phase 4-leg advanced t-type i̇nverter for renewable energy generation systems with seamless transition

In order to increase efficiency of the inverter, 3-level T-type Neutral Point Clamped (T-NPC) inverter structure is frequently used in low voltage applications. Recently, Advanced T-NPC (AT-NPC) inverter structure has been developed by using Reverse Blocking IGBT (RB-IGBT) with low conduction losses in the middle leg bidirectional switch. When the inverter disconnects from the grid in the event of a grid failure, the phase voltages of conventional 3-phase 3-leg inverters are affected by unbalanced loads. The object of this thesis is to design, control and implement a 3-level high efficiency inverter in 3-phase 4-leg AT-NPC topology capable of being operated in islanding and grid connected modes and of seamless transferring between these modes, for low voltage renewable disturbed generation systems. With the proposed 3-phase 4-leg inverter system, 1-phase and unbalanced 3-phase load groups in islanding mode can be fed by distributed generation units. The inverter is designed as current controlled based on Proportional Resonant (PR) controller in grid connected mode operation and designed as voltage controlled based on Proportional Multi Resonant (PMR) controller in islanding mode operation. In order to provide seamless transfer between these operating modes, cascade seamless transfer method is proposed by combined these control techniques in this work. For the 3-level inverter, 20 kHz switching signals are generated with Carrier Based PWM (CBPWM) technique. A laboratory prototype 2,5 kVA rated power of 3-pahse 4-leg AT-NPC type 3-level inverter system has been installed. Simulation studies of the proposed system have been carried out via PSIM software and model-based embedded codes generation for Digital Signal Controller (DSC) have been carried out with PSIM/SimCoder software. The real time controlling of the inverter system is achieved with TMS320F28335 DSC card. As a result, with the proposed control method, the inverter is operated in both island mode and grid connected mode and seamless transfer between these operating modes has been realized.