Module-level DC/DC conversion for photovoltaic systems

Photovoltaic (PV) systems are increasingly used to generate electrical energy from solar irradiance incident on PV modules. Each PV module is formed by placing a large amount of PV cells, typically 60, in series. The PV system is then formed by placing a number, typically 10-12, of PV modules in series in a string and sometimes by placing multiple strings of series-connected PV modules in parallel, depending on the desired output voltage and power range. In practical cases, differences will exist between output powers of the cells in the various PV modules, e.g. due to (part of) the modules being temporarily shaded, pollution on one or more PV cells, or even spread in cell behaviour. Due to the current-source-type behaviour of PV cells and their series connection these differences will lead to a relatively large drop in PV-system output power. This paper addresses this problem by adding DC/DC converters on PV-module level. The concept, named 'delta conversion', aims at averaging out differences in output power between groups of PV cells within modules and between modules inside the PV system. As a result, all these groups of PV cells can output their maximum available power, such that a drop in output power of the total system is prevented. This paper compares the delta-conversion concept with other state-of-the-art module-level power-conversion concepts and presents first measurement results obtained with a demonstrator system.