Start-Up Operation of Active Three-Phase Third Harmonic Injection Rectifiers

Passive three-phase diode (B6) bridge rectifiers are widely used in industry for three-phase AC-to-DC power conversion due to simplicity of circuit and design, high efficiency as well as robustness and cost-efficiency. Such B6 rectifiers with DC-side smoothing inductor optionally can be upgraded/ equipped by an additional active power electronics network - based on the third harmonic injection (THI) principle - in order to guarantee unity power factor (lambda > 0.99) and high quality input currents (THDi < 5%). As the THI circuit offers an optional current shaping capability, the active extension has to be able to boost and control its DC-link irrespectively of instantaneous output DC-link voltage level (vo) and load situations (Po). This is necessary to assure proper current generation during nominal load operation of the B6 rectifier. As the initial voltage of the active circuit (after B6 start-up) is much below the predefined minimum required DC voltage, a specific short-term voltage/current control algorithm is required during start-up of the active concept in order to prevent the system from irreversible damage of power components and to guarantee undistorted input currents. The fundamental principle, design considerations and feasible parasitic effects of such a controller concept are therefore analyzed and discussed in this paper.