Active infeed converter with integrated power-quality improvement applying pole-restraining control

Abstract

Solar-, biomass- and combined heat-and-power generation (CHP) connect in increasing numbers to local power grids. With growing power generation in the local grid and decreasing power (and short circuit power) from the transmission grid, the converters used to feed energy into the grid (usually called Active Infeed Converters, AIC, or Active Front Ends, AFE) have to guarantee power quality within these grids. Concerning reactive power, voltage levels and frequency control conventional power plant control mechanisms can be adapted which is not discussed here. Concerning harmonics, the characteristics of converters allow effective harmonic mitigation. The set-point value generation for the converter current harmonics is here based on the FBD power theory. Its basic idea is to load all harmonics contained in the power grid voltages with clearly defined resistive currents. In this way a damping effect is ensured. In contrast to usual active filtering technologies which compensate one load or a group of loads based on measured currents, the suggested set-point value generation bases only on the voltages at the connection point of the converter. The paper demonstrates that this methodology is superior to fundamental current generation with regard to grid voltage harmonic mitigation. The methodology is not restricted to converters feeding energy into the grid. However, from the point of view of stability these converters are much more problematic than converters taking energy from the grid. Consequently, the more demanding case is treated here. A further aspect covered in this paper is the quality of control of the converters: Increasing infeed by converters increases the coupling between the converters. The local power grid no longer decouples the converters. Advanced converter control, here applying pole-restraining, solves this potential problem. (6 pages)