Dual DC-DC Converter Design for String Inverters Used in Solar Plants

4th International Conference on Renewable Energy Research and Applications (ICRERA) -- NOV 22-25, 2015 -- Palermo, ITALYWOS: 000379126300004This study deals with the design of a dual buck converter where both of them are simultaneously operated to regulate photovoltaic (PV) string inputs. The proposed control and converter structure is intended to improve the dc-bus interface for solar arrays and to control the dc bus voltage of a commercial grid-tie inverter that is generated by four PV arrays. The control algorithm is improved by including PI control to the regular perturb-and-observe algorithm. Regarding to the international standards on single-phase commercial string inverters such as AS4777-2013, IEC62109, and IEC61727, the maximum power of each solar array is allowed to be 1.2 kWp while the maximum total input power is limited to 4.8 kWp. Individual MPPT controllers that are improved in this study control each one of the dc converters handling dual solar arrays at the input connection. The dc interface voltages regulated by converters are coupled on a unique dc bus to generate the supply voltage of inverter section. The controllers perform a robust operation against the freely fluctuating irradiation values of each solar arrays

Single Phase Multilevel String Inverter for Solar Applications

4th International Conference on Renewable Energy Research and Applications (ICRERA) -- NOV 22-25, 2015 -- Palermo, ITALYWOS: 000379126300003The 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

Dual DC-DC Converter and Monitoring Interface for Asymmetrical String Inverters

4th International Conference on Renewable Energy Research and Applications (ICRERA) -- NOV 22-25, 2015 -- Palermo, ITALYWOS: 000379126300273This paper proposes monitoring of the dual dc-dc converters that independently operate of each other in the same topology (Buck-Boost). The dual converter structure is proposed to implement asymmetrical string inverter that is intended to be used variously arranged PV strings. The converters are operated by independently running MPPT algorithms. The converters are supplied by programmed dc sources emulating freely arranged solar arrays, and different loads are connected to each converter output. The converters allow using independent power sources like hybrid power techniques owing to these features. Current and voltage sensors measure the output powers of each converter, where the obtained values are evaluated by P&O algorithm on MCU and the PWM signals are generated. At the same time, obtained values are transmitted to computer by a microcontroller on optically isolated UART to USB unit. The received values are saved in a database to enable historical and instant analyse capability of the converters