Multicell PWM Converters for Renewable Energy DC Micro-Grids

Within the applied research project multi-cell PWM converter topologies shall be analyzed for the usage in renewable energy systems and DC micro grids. DC-based energy distribution concepts (used, e.g., for emergency lighting since many years) will gain attractiveness also for common power distribution as a consequence of the fact that most renewable energy sources (e.g., solar-cell or fuel-cell systems etc.) as well as most modern loads (appliances, electronic equipment etc.) actually are DC-based. Hence, DC grids with power electronic grid- coupling of all load and generation units will offer higher flexibility at reduced overall system losses. The proposed multi-cell converters are ideally suited for DC micro-grid applications because modern low-voltage power MOSFETs can be used as switching devices. The extremely low on-resistance of such semiconductors results in a very high converter efficiency, especially in the important region of partial load. A further advantage of the proposed multi-cell concept is that it directly reflects the fact that most renewable energy sources and storage devices are formed by "stacked" DC systems (e.g., series connected batteries/capacitors or solar modules/arrays). By using multi-cell converters, it is possible to operate the individual string elements at different (i.e., locally optimized) power levels leading to an optimized performance/efficiency of the total system. The project is focused to two renewable energy key applications: 1. A multi-cell battery storage unit (BSU) which will be used in DC micro grids for power leveling and voltage guidance. 2. A "distributed" solar converter (DSC) where each solar generator module/array (similar to each battery module of the BSU) is equipped by a dedicated non-isolated DC/DC converter (switching cell). The outputs of all converters are connected in series to form an "intelligent" solar string feeding a DC power bus of an AC inverter system or, alternatively, favorably immediately a DC micro grid. The advantage of the proposed DSC concept is that each solar module/array can be operated at its individual maximum-power-point (MPP) resulting in a substantially increased energy yield, especially in case of partial shading. By proper control (based on low-power micro-controllers) the MPP operation of all switching cells can be achieved inherently without any specific communication measures. This, and the fact that in principle no (temperature and ageing sensitive) electrolytic capacitors are required, will ease the integration of the converter cell directly into the solar module. The BSU in turn takes benefit from the multi-cell concept by the fact that a charge equalization of the individual battery modules can be guaranteed. In addition, a specific control of the DC/DC converter allows a loss-free emulation of a desired output v/i-characteristic of the BSU which will be used to achieve a characteristic-based primary power flow management within the DC micro grid.

Within the research project multi-cell PWM converter topologies for application in renewable energy systems and DC micro grids are analyzed. DC-based energy distribution concepts (used, e.g., for emergency lighting since many years) will gain attractiveness also for common power distribution as a consequence of the fact that most renewable energy sources (e.g., solar-cell or fuel-cell systems etc.) as well as most modern loads (appliances, electronic equipment etc.) actually are DC-based. Hence, DC grids with power electronic grid-coupling of all load and generation units will offer higher flexibility at reduced overall system losses. The developed multi-cell converters are ideally suited for DC micro-grid applications because modern low-voltage power MOSFETs can be used as switching devices. The extremely low on-resistance of such semiconductors results in a very high converter efficiency (typically 99%), especially also in the important region of partial load. A further advantage of the proposed multi-cell concept is that it directly reflects the fact that most renewable energy sources and storage devices are formed by "stacked" DC systems (e.g., series connected batteries/capacitors or solar modules/arrays). By using multi-cell converters, it is possible to operate the individual string elements at different (i.e., locally optimized) power levels leading to an optimized performance/efficiency of the total system. A battery storage unit (BSU), e.g., takes benefit from the multi-cell concept by the fact that a charge equalization of the individual battery modules can be guaranteed. In addition, a specific control of the DC/DC converter allows a loss-free emulation of a desired output v/i-characteristic of the BSU which will be used to achieve a characteristic-based primary power flow management within the DC micro grid using a varying DC voltage level as a quantity communicating the load status of the grid.