Online Monitoring of the Power Transfer in a DC Test Grid

One solution for increasing the use of renewables is to find new strategies to promote the connection of distributed energy resources (DERs) within the existing power system. A solution that allows a flexible integration of dispersed generation (DG) into energy networks is the use of dc at the user layer. This seems a reasonable choice since several renewable sources and the presently available storage systems deliver electricity in dc form (e.g., fuel cells and solar cells). Furthermore, other generators (wind turbines and microhydro or microgas turbines) deliver electricity in ac form but mostly at variable or nonstandard frequencies. Consequently, the output of these generators must be rectified, converted again into ac, and conditioned to meet the nominal grid parameters. Anticipating the extensive use of low-power DC-based intelligent devices, one can avoid losses in energy transfer by using a dc layer within the distribution networks, at least where the energy is produced in dc form. DC grids in buildings are expected to be one of the main applications and challenges in the future. The development of a dc test network, including measurement and communication, is presented in this paper. The development of promising applications showing the effective integration of DG into intelligent networks with low-power loads is illustrated and discussed. Specific indicators are formulated to characterize power quality issues of dc systems.