Hierarchical fuzzy-logic control for a single-phase voltage-source UPS inverter

Fuzzy logic controllers (FLCs), in general, have the potential of operating successfully under a wide range of load variation since their working principles do not require precise knowledge of the load parameters. However, they are computationally intensive, thus requiring a very powerful processor for real-time implementation. Nonetheless, hierarchical FLCs are much less computationally demanding than standard FLCs without compromising the controller performance. This paper proposes the application of hierarchical fuzzy logic controllers to UPS applications. The proposed control scheme comprises two fuzzy controllers implemented in a nested fashion to form two control loops. The first fuzzy logic controller employs the actual filter capacitor voltage, its reference waveform, and the output signal of a soft start circuit. The inputs of the second fuzzy logic controller are the output of the first fuzzy logic controller, the reference waveform of the capacitor current, and its actual value. Simulation results of the proposed control scheme show that the system has fast dynamic response and is capable of producing sinusoidal load voltage with low harmonic content.