A New Low Switching Frequency Control of Regenerative CHB Drive With Low-Voltage Ride-Through Capability

In conventional medium-voltage high-power cascaded H-bridge (CHB) drive, a three-phase rectifier is adopted in each power cell to provide isolated dc-bus voltage for the output H-bridge. This limits the application of the conventional CHB drive, where regeneration capability is desired. The regenerative CHB drive can be made possible by replacing the diode rectifier with a three-phase IGBT-based active-front-end (AFE) rectifier in each power cell. However, due to thermal constraints in the high-power medium-voltage drives, the first challenge of the regenerative CHB drive is to deal with the extra switching losses introduced by the IGBT devices. Another challenge of the grid-tied regenerative CHB drive is to handle the low-voltage sags in the power grid without triggering unnecessary downtime in transients. In this article, a novel low-switching frequency control strategy is proposed for the AFEs in the regenerative CHB drive with low-voltage ride-through (LVRT) capability. The main harmonic contents generated by the AFEs with the proposed control strategy can be eliminated by the existing phase-shifting transformer in the CHB drives. This allows meeting with IEEE STD 519-2014 requirement with a 60 Hz switching frequency to minimize the extra introduced switching losses in steady state. Moreover, LVRT capability is integrated into the proposed control scheme with reduced current sensor count. Accompanied with videos, the experiments on a seven-level regenerative CHB drive are implemented to validate the feasibility of the proposed control scheme, which can be extended to regenerative CHB drives with any voltage levels.

I. Introduction

Electric motor systems are by far the most important type of load in industry, using about 70% of the consumed electricity in the European Union [1], [2]. Medium-voltage (MV) high-power adjustable speed drives (ASD) have found widespread applications in various heavy industries, such as in the oil and gas sectors, production plants, and process industries [3], [4]. The conventional cascaded H-bridge (CHB) multilevel inverters prevailed in the MV industrial drive domain due to their modularity, scalability, and reliability [5], [6]. CHB inverter, shown in Fig. 1(a), is composed of a number of H-bridge power cells, which is cascaded on the motor side to achieve the full MV range with low harmonic distortion. In the conventional CHB drives, the isolated dc-bus voltages are obtained through a phase-shifting transformer and three-phase diode-front-end (DFE) rectifiers, which are shown in Fig. 1(b). The phase-shifting transformer can produce a set of three-phase secondary voltages shifted by a certain angle with respect to the primary voltages. The phase-shifting angles can eliminate low-order harmonics of the currents at the grid point of common coupling (PCC) with an improved harmonic profile [7]. Fig. 1.

Seven-level regenerative CHB drive. (a) CHB configuration. (b) DFE (non-Regen) power cell. (c) SCR Regen power cell. (d) IGBT-based AFE Regen power cell.