Improving grid voltage quality by decentral injection of current harmonics

This research project deals with the development of an adjustable converter for injection of fuel cell power. A DC/DC converter stage increasing the voltage level is connected to a DC/AC inverter for grid current injection via a high capacitance DC link. This decouples the momentary power drawn from the energy source from the momentary power being injected into the grid, allowing arbitrary output current waveforms without an increase of input current ripple. Conventionally, pure fundamental frequency currents are considered the optimum waveform regarding the injected currents impact on the grid. This paper demonstrates how grid voltage harmonics caused by nonlinear loads can be minimized by adapting the converters injected current shape. In contrast to harmonics compensation in industrial applications where the compensation current is determined by measuring the local loads current harmonics, the intended decentral application in private houses suggests a different approach to improve overall grid voltage quality. The digital signal processor used for the converters control and monitoring functions calculates the frequency spectrum of the measured grid voltage run by digital Fourier transformation (DFT) and determines the current harmonics necessary to counteract the present voltage harmonics. The resulting current frequency spectrum is modulated into a time dependant current shape that is added to the fundamental frequency current run determined by the requested amounts of active and reactive power. This way, harmonic currents of the local loads are compensated without changes in the house installation for measuring the loads currents. Also current harmonics of neighbouring houses are compensated.