Cascaded H-Bridge Based Solid State Transformer (SST) Technology for Grid and Motor Drive Applications

#huntsville #engineering #power #grid

This talk will focuse on three-stage (MV/HF DC-DC/LV) CHB-based Solid State Transformer (SST) technology-based architecture for both grid integration and motor drive applications. The research focuses on developing advanced control strategies to enhance the performance of SST applications. They are mainly wind and solar farms, data centers, microgrids, EV charging stations, and propulsion drives. SST technology is becoming the most attractive solution for direct integration of MVAC grid with LVAC/LVDC grid. It offers high power density with advanced grid support, which is not possible with conventional Low-Frequency Transformer (LFT) based solution. This work addresses the associated challenges, including control complexity, power quality issues, and higher semiconductor losses in MV stage of CHB-based SST for aforementioned applications.

The MV grid-connected SST introduces power quality issues (poor current THD) due to the dead time effect and harmonics present in grid voltage. The power quality issues, especially at light load conditions are more significant. A novel hybrid control technique is proposed to ensure perfectly sinusoidal MV grid current even under light load conditions, resulting in improved grid current THD. Experimental validation using a 1.2kV(MV)/400V(DC)/200V(AC), 20kW CHB-based SST prototype demonstrates the effectiveness of this approach. The proposed hybrid control technique is further extended to address the trade-off between cost and performance of the SST that explores a cost-effective alternative utilizing both Si-IGBTs and SiC MOSFETs in the MV stage. The extended  hybrid control approach simultaneously improves semiconductor losses (nearly 50 %) as well as power quality with higher effective switching frequency.

SST technology holds immense potential for next-generation MV motor drive applications, particularly in marine propulsion systems, because the lighter and compact electric drive is the preferred choice in this application. This work also proposes a modified CHB-based SST-fed high-frequency propulsion drive with a new modulation technique. The proposed drive offers high power density and achieves a higher effective switching frequency with reduced semiconductor losses. It is validated in a 7-level modified CHB-based SST-fed field-oriented controlled induction motor drive prototype. To further enhance the reliability of variable speed drives, this research also addresses their sensitivity to power supply disturbances. A simple and cost-effective regenerative ride-through technique is proposed for CHB inverter-based vector-controlled induction motor drives, ensuring smooth operation during power interruptions or voltage sag. This improves overall system resilience.