Single phase multilevel string inverter for solar applications

The proposed study deals with modeling and implementation of a multilevel string inverter with non-isolated dc-dc converter design. The modeled system is intended to be used for solar applications that the photovoltaic panels are connected in string structure. However, the dc-dc converter and multilevel inverter are capable to be used in distributed energy generation applications owing to the proposed control infrastructure. The entire design includes dual buck converter at the dc coupling input where both of the converters simultaneously operate to regulate photovoltaic string voltages. Each dc converter is capable to manage a string comprised of two photovoltaic arrays at the input. The maximum power of each solar array is allowed to be 1.2 kWp while the maximum value of total input power is limited to 4.8 kWp regarding to the related international standards such as AS4777-2013, IEC62109, and IEC61727. The output voltages of dc converters are set to 120V by the proportional-integral controller and are coupled over dc busbar to supply 9-level string inverter. The multilevel string inverter is comprised of two H-bridges in symmetrical topology where it is arranged according to regular 5-level symmetrical inverter. Furthermore, the regular sinusoidal pulse width modulator is enhanced to increase voltage level of inverter to nine instead of regular 5-level output. The inverter provides unity power factor with 0.63% total harmonic distortion ratio.