| PhD Viva

Name of the Speaker: Mr. Shekhar Bhawal (EE16D014)
Guide: Dr. Kamalesh Hatua
Online meeting link: https://meet.google.com/zio-qegj-nku
Date/Time: 20th June 2024 (Thusday), 5:00 PM
Title: Solid State Transformer Based on Naturally Cell Balanced Series Resonant Converter for Medium Voltage Application

Abstract :

The Solid-State Transformer (SST) provides higher power density solutions (reduced weight, size, volume, and so on) to interface Medium-Voltage AC (MVAC) to Low Voltage DC (LVDC) with high-frequency galvanic isolation in the DC-DC stage, in comparison to traditional 50Hz transformer-based power electronic systems. High power density solutions for connecting MVAC to LVDC systems are critical, particularly for supplying EV charging stations, data centers integrated with renewable energy sources, and other rising DC loads/sources. SST also brings numerous other benefits on the power supply side, including bidirectional power flow control, VAR compensation, harmonic compensation, grid-side disturbance isolation, fault-tolerant operation, and many other features. The Input Series Output Parallel (ISOP) architecture with a Cascaded H-Bridge arrangement on the Medium Voltage (MV) side is one of the most widespread SST architecture. However, as the system's power rating increases, so does its hardware and control complexity, making it more difficult to implement practically such a cutting-edge solution due to higher control complexity, cost, and loss number. Thus, this work proposes a simple yet effective closed-loop control architecture for the SST system to control its LVDC, eliminating closed loop requirement from the entire isolated DC-DC stage making the practical realization of the SST system simple and convenient. The proposed SST has two essential stages: Medium-Voltage (MV), and DC-DC stage. The SST system uses a CHB configuration to realize the MV stage. Each CHB cell is connected to the corresponding Series Resonant Converter (SRC) cell in DC-DC stage. SRC converters are operated at resonance and open loop conditions. This condition offers natural voltage balancing (among the DC links of CHB units) without any closed-loop control in DC-DC stage. This intrinsic advantage decreases control complexity in the MV stage as well. The proposed control architecture is modular so it can be extended for any power and voltage level. The feasibility of the proposed control architecture is validated experimentally in a 1.65 kV (MVAC)/600 V (LVDC), 8 kW prototype SST system.

Although the resonance function ensures soft switching in DC-DC stage under all loading conditions, the MV stage still faces hard switching, reducing overall system efficiency. When SiC MOSFET devices are used in the MV stage over Si IGBT devices (to reduce semiconductor loss), the semiconductor cost of the system increases drastically. As a result, Si IGBT and SiC MOSFET are introduced appropriately in the MV stage and are switched based on their merits using a novel line frequency modulation technique that reduces semiconductor losses without increasing semiconductor cost while ensuring a high effective switching frequency (high power density and fast system response is also ensured). The line frequency modulation results in unbalanced voltage and power-sharing among CHB cells (in AC side of it), resulting in asymmetrical component ratings. The limitation of unsymmetrical voltage, power, and component ratings is overcome using a circulation logic. The line modulation method for SST system was initially implemented for a three-stage SST system (application limited to Grid-to-grid) which has an additional LV stage. Later, it is extended for a two-stage SST system (without LV stage) making it suitable for Grid-to-grid as well as Grid-to-LVDC interface, which further improves the practical feasibility of the SST system. Later, the line frequency modulation method has been further extended to reduce further hardware complexity (eliminates a single SRC unit). The proposed control methodology is proven experimentally in a 600 V (MVAC)/200 V (LVDC), 5 kW prototype SST system.