Modeling and Improvement of DC-link Capacitor Lifetime in a Regenerative Cascaded H-bridge Motor Drive

Abstract

Motor drives represent electric equipment used for speed control of electric motors. Varieties of industrial applications, such as assembly, pumps, fans etc., require motors and they consume huge amount of electric energy. Compared with traditional motor drives, which can only send energy from grid to motor, a regenerative motor drive can achieve bi-directional power flow control between motors and utility grid. Regenerative motor drives are excellent candidates for reducing power loss in motor-related applications. One of the most essential parts of a regenerative motor drive power cell is dc-link capacitors. They create suitable dc-link voltages and smooth the voltage waveforms. Reliability, or lifetime of dc-link capacitors highly affect power cell lifetime, and power loss in dc-link capacitor is also another issue that worth noticing. This thesis focuses on the lifetime modeling and lifetime improvement of dc-link capacitors in a regenerative cascaded H-bridge medium-voltage motor drive. The lifetime modeling bases itself on the mechanisms of dominant lifetime stresses in practical operations. A proposed method is used to reduce a dominant current harmonic component in dc-link capacitors. With the proposed lifetime model and harmonic-reduction method, dc-link capacitor lifetime improvement can be anticipated in this motor drive model. Less power losses in those dc-link capacitor banks can also be achieved.