Othman Abdulhadi Taha

In recent years, the energy generation with regenerative sources such as wind energy, photo-voltaic, etc. has rapidly increased. 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 re-search work utilizes for this purpose a 5-Level Cascaded H-bridge Voltage Source Inverter (5L-CHB-VSI) and features advantages regarding the cost, the maintenance and especially the availability of energy generation under changing weather conditions. The flexibility of the hy-brid system allows different modes of operation depending on the available PV or wind power that 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 consist 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 an additional feedforward modulation index compensation (FFMIC). The proposed scheme controls the ac-tive 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 (e.g. model mismatch, partial shading at the PV string, etc.) or weather conditions. The second deals with the maximum power point tracking for each con-nected 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 in-verter. 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 inte-gration and the efficiency of conversion.