| MS Seminar

Name of the Speaker: Mr. SOURABH CHAHAR (EE22S050)
Guide: Dr. Kamalesh Hatua
Online meeting link: http://meet.google.com/njx-xhjr-wng
Date/Time: 18th June 2025, at 11:00 AM to 12.00 PM
Title: Advanced Control Strategies for Cascaded H-Bridge based Solid State Transformer under Unbalanced Grid Conditions.

Abstract :

Recent advancements in power electronics have made Solid State Transformers (SST) a highly promising solution for modern smart grid systems in generation and distribution networks. SST enables efficient interfacing between MVAC grids and LVDC/LVAC grids, offering enhanced controllability, modularity, and power density, along with advanced grid support features. SST offers a multitude of advantages over conventional low-frequency transformers, such as galvanic isolation, higher power density, improved power quality, regulated bidirectional power flow, active and reactive power compensation, enhanced system dynamics, and grid stability. This work focuses on the development of control strategies to enhance the overall power quality of a three-stage CHB-based SST operating under unbalanced grid conditions.

Unbalanced voltage conditions in both MV and LV grids result in oscillations in active and reactive power, which lead to oscillations in LVDC link voltage at twice the grid frequency. Additionally, these conditions cause distorted input currents and increased odd-order harmonics, thereby degrading the overall power quality of SST. This work addresses the challenges posed by unbalanced grid conditions in the MV and LV grid of a three-stage CHB-based SST, focusing on improving overall power quality and ensuring smooth operation. A control strategy is proposed to operate and improve the overall power quality of the CHB-based SST under unbalanced grid conditions. The effectiveness of the proposed control architecture is validated through experimental testing on a 600V (MVAC)/200V (LVDC)/100V (LVAC), 20 kW CHB-based SST prototype.

Unbalanced grid operation in MV grid connected SST introduces odd-order harmonics in the input currents. In addition, dead time effects and harmonics in the MV grid voltage worsen these distortions, resulting in degraded power quality and increased current THD. An improved modulation scheme is proposed to enhance the overall power quality of MV grid-connected SST while operating under unbalanced grids. The proposed control ensures that the grid current remains sinusoidal across all loading conditions, thereby enhancing power quality. The proposed improved modulation scheme is validated experimentally using a 600V (MVAC)/200V (LVDC)/100V (LVAC), 20 kW CHB-based SST prototype. This work also proposes a control strategy in the stationary frame to operate a three-stage CHB-based SST under unbalanced grid conditions. The proposed control aims to enhance the overall power quality while reducing control complexity and computational burden on controllers. The proposed improved modulation scheme is validated through MATLAB Simulink on a 1.65kV (MVAC)/500V (LVDC)/300V (LVAC), CHB-based SST.