The demand for high-power converters are surging due to applications like renewable energy, motor drives and grid-interface applications. Typically, these converters’ power ranges from tens of kilowatts (kW) to several megawatts (MW). To reach such high power levels the converter voltage ratings must increase, as the current ratings cannot be reached by the available devices or because the system losses become excessive. To address this, two strategies can be utilized: multilevel topologies (e.g. Multilevel Modular Converter or Flying Capacitor Multilevel Converter) and high voltage switches. For medium voltage applications, the most commonly employed switches are the IGBT and the IGCT. Both are silicon-based technology and are limited to a rated voltage of 6.5 kV and 4.5 kV, respectively. Often, these devices switching frequency are limited to less than 1 kHz.
To expand the frontiers of medium voltage converters and to demonstrate the capabilities of wide band gap devices in medium voltage, a 7-level 13.8 kV AC 22 kV DC 1.1 MW flying capacitor multilevel converter based on 10 kV SiC MOSFET with 2.5 kHz switching frequency was designed and constructed. Given the complexity of a multilevel topology, the high voltage levels, and the critical nature of the loads, a failure in a high-power converter can incur significant costs, long service downtime, and safety risks to personnel. Hence, understanding the failure modes of these converters is essential for designing protections and mitigation strategies to prevent or reduce the risks of failures. Furthermore, the adoption of 10 kV SiC MOSFET introduces additional challenges in terms of protection. Despite their well-known benefits, these devices exhibit shorter energy withstanding time compared with their silicon counterpart, and increased insulation stress resulting from the high dv/dt imposed by the fast-switching transient at higher voltages.
In this context, a failure mode analysis was conducted for the converter aforementioned. The analysis examined the fault dynamics and evaluated the protections schemes at the converter level. The study identified a failure mechanism between cells, so called Cell Short- Circuit Fault (CSCF), capable of damaging the entire phase-leg. In response, a protection scheme based on TVS (Transient Voltage Suppression) diodes was designed to prevent extremely imbalanced cell voltages and failure propagation. Because of the high electric field intensity environment of the converter, an FEA (Finite Element Analyses) simulation is performed to verify and control the electric field (E-field) intensity within the protection module itself and in the converter assembly. Next, the protection module insulation design was successfully verified in a Partial Discharge (PD) experiment. In sequence, an experimental verification utilizing an equivalent circuit based on the fault model demonstrated the efficacy of the protection module. Waveforms extracted while the converter was operating showing the protection module acting during a fault are presented and analyzed. Finally, the influence of the protection module in the switching of the 10 kV SiC MOSFET was evaluated via a double pulse test (DPT), revealing negligible effects on the converter performance. / Center of Power Electronics Systems (CPES)
Department of Energy (DoE) / Master of Science / Due to governmental policies and market opportunities renewable energy (e.g. solar and wind energy) is increase its share in the electricity generation in the US and around the world. This scenario poses challenges regarding the stability of the grid and variation in the generation along the day. One of the alternatives to alleviate the problem is to use highpower converters that provides a interface between grid and manufacturing plants. This type of converter have bidirectional capabilities and can store the energy generated by solar farms during the day and return it to the grid at night for example. Moreover, it can provide grid support capabilities in terms of variation of frequency and voltage.
To expand on the grid interface converters application concept, a medium voltage power converter in 22 kV DC and 13.8 kV AC is designed utilizing novel techniques and the latest technologies in semiconductors, 10 kV SiC MOSFETs. The benefits of this design are a small form factor, high efficiency, immunity to electromagnetic interference and power quality. This work presents a failure mode analysis of the power converter aforementioned, the analysis examined the fault dynamics and an evaluation of the protections schemes at the converter level.
The failure analysis revealed the need of a protection scheme extremely imbalanced cell voltages and failure propagation. Hence, a protection module based on TVS (Transient Voltage Suppression) diodes was successfully designed and tested. Due to the high voltages present in this equipment, an FEA (Finite Element Analyses) simulation is performed to verify and control the electric field (E-field) intensity within the protection module itself and in the converter assembly. Experimental results are provided for insulation design integrity (partial discharge test), for the efficacy of the protection module against the fault, and for the impact of the protection module on the operation performance.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/120889 |
| Date | 01 May 2024 |
| Creators | Mendes, Arthur Coimbra |
| Contributors | Electrical and Computer Engineering, Burgos, Rolando, Dong, Dong, Zhang, Richard |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | ETD, application/pdf, application/pdf |
| Rights | Creative Commons Attribution 4.0 International, http://creativecommons.org/licenses/by/4.0/ |
Page generated in 0.0028 seconds