Performance comparison of three-level and multi-level for Grid-connected photovoltaic systems

The most Grid-connected inverters for photovoltaic (PV) system use the three-level grid-connected inverter, which generates the three voltage level at the output of inverter. It can inject the sinusoidal current to utility grid through a set of filter as shown in Fig. 1. Even though, the three-level grid-connected inverter can be operated in high efficiency, but it still has some drawback, such as, the high total harmonic distortion (THD) of output voltage, the high switching power losses cause by high switching frequency in order to reduce the filter size, and power losses cause by mismatching of PV module power in the same PV string. The cascaded H-bridge multi-level inverter topology is an important choice to reduce some drawback of the three-level grid-connected inverter for PV system as shown in Fig.2. This paper presents the comparison studies the performance of using three-level and cascaded H-bridge multi-level inverter topologies for grid-connected PV system. The both inverter topologies have to operate as a single-phase grid-connected inverter in the same condition of filter size and under the IEEE standard of lower 5% of total harmonic distortion of grid current. The compared performance indicators consist of the grid injected power while the irradiation is changed in two cases (The same size and different size of PV power changing between two sets of PV string). The both systems have to be tested in the same testing condition. The MATLAB/SIMULINK simulation results are verified the accuracy of the both topologies performance.