Dynamic String Interconnection with Buck Converter Topology
Wurster, T., R. Merz, Markus B. Schubert · EU PVSEC · 2010
Inhomogeneous illumination, e.g. due to partial shading of one photovoltaic module in a photovoltaic string, limits the common current of all modules of the string. Depending on the string configuration, partial shading reduces the output voltage of the string dramatically by bypassing the affected module. The voltage drop in one string of a parallel connection forces the inverter to decide whether to operate all strings at the reduced voltage or to force the affected string to the optimal voltage of the remaining strings. Both operating points lead to high mismatch losses that additionally decrease the output power of the photovoltaic system. The Dynamic String Interconnection (DSI) provides independent maximum power point tracking of multiple photovoltaic strings. We present a prototype of the dynamic string interconnection with an electrical conversion efficiency up to DSI = 97.8 %. Simulations of the circuit and a test system at the rooftop of the ipe prove the individual maximum power point tracking of the strings and demonstrate a gain in efficiency of about 1.2 % compared to the parallel connection of the strings. This initial efficiency gain results from the compensation of the string tolerances. If one of the modules of the test system is bypassed, the gain in system efficiency rises to 9.5 %.