In recent years, the energy generation with regenerative sources such as wind energy, photovoltaic, etc. has increased rapidly. The connection of such energy sources to the grid requires power electronics conversion systems. In the present work a new approach for the hybrid connection of solar panels and wind turbines is proposed. The system examined in the research work utilizes a 5-Level Cascaded H-bridge Voltage Source Inverter (5L-CHB-VSI) for this purpose and features advantages regarding the cost, the maintenance, and, especially, the availability of energy generation under changing weather conditions. The flexibility of the hybrid system allows different modes of operation depending on the available PV or wind power, which are considered in all details in the frame of the work. It is expected to be a good solution for the grid integration of small generation systems in rural or remote areas in developing countries.
Each phase of the 5L-CHB-VSI consists of two single-phase H-bridges, fed by separate DC-links. Therefore, a hybrid operation is possible by feeding the DC-links out of different energy sources. In the examined case the bridges are connected either to a photovoltaic (PV) string or to a rectifier, fed by an electrical generator that is driven by a wind turbine (WT). The grid connection was performed by using a voltage oriented control scheme with additional feedforward modulation index compensation (FFMIC). The proposed scheme controls the active and reactive power in the point of connection.
Two types of the feedforward compensation methods are presented, aiming at two targets. The first one, which is based on the injection of a zero-sequence signal, leads to balanced transfer of the power to the grid in case of unsymmetrical energy production among phases, due to fault degradation, model mismatch, partial shading at the PV string or weather conditions. The second one deals with the maximum power point tracking for each connected energy source without using the additional DC-DC converters that are necessary in conventional approaches.
The proposed control scheme - including feedforward modulation index compensation - was realized by using a single DSP. Furthermore, phase-shifted Pulse Width Modulation (PS-PWM) as well as Space Phasor Modulation (SPM) techniques were implemented to control the inverter. Experimental results on a laboratory setup confirm the theoretical considerations. The PV panels were emulated in the laboratory by using programmable DC supplies, while the wind turbines were emulated using permanent magnet synchronous generators. The results show the flexible operation of the system due to the individual MPPT operation for each H-bridge cell and the balanced injection of the power, even under unsymmetrical generation conditions. The possibility to switch between star- and delta-configuration of the inverter topology was proposed and analyzed in order to enhance the performance of the grid integration and the efficiency of conversion.