Power Theory as Basis for the Control of Grid-Connected Converter Systems

Sustainable energy harvesting, above all wind energy and solar energy, uses self-commutated converters for grid connection. Modern drive systems also use self-commutated converters instead of diode bridges for grid connection to attain constant working conditions and also to improve power quality. So-called active filters base on the same technology. In this way, the total number of controlled power-electronic assets (CPE) connected to the grid increases continuously. Consequently, the relation between conventional short-circuit power and the total power of CPE decreases. The resulting interaction between the CPE at comparatively weak grids becomes more and more relevant and may lead to resonances. This paper analyzes reasons for such interaction based on a qualitative assessment of reduced short-circuit power in the presence of CPE. It then utilizes power-theory concepts to define an optimal type of load as well from a purely theoretical as from a praxis-oriented point of view. Basic rules for load characteristics at weak grids are derived.