Design and Assessment of a Grid Connected Industrial Full-SiC Converter for 690 V Grids

Die Bedeutung von Leistungshalbleitern mit großem Bandabstand (Wide Band Gap, WBG) nahm in den letzten drei Jahrzehnten kontinuierlich zu. Diese Bauelemente haben das Potenzial, Silizium (Si) - Bauelemente in bestimmten Anwendungen sowie Leistungs- und Frequenzbereichen zu ersetzen. Siliziumkarbid (SiC)-Leistungshalbleiter sind die gegenwärtig am Weitesten entwickelten WBG-Leistungshalbleiter. Dank besonderer Materialeigenschaften zeichnen sich SiC-Leistungshalbleiter im Vergleich zu Si-Bauelementen durch einen geringeren spezifischen Widerstand, eine höhere Schaltgeschwindigkeit, geringere schaltverluste sowie eine höhere maximale Sperrschichttemperatur aus. Die deutlich erhöhten Herstellungskosten limitieren den Einsatz von SiC-Leistungshalbleitern auf Anwendungen, in denen die Vorteile dieser Bauelemente die höheren Kosten überkompensieren und Systemvorteile ermöglichen. Heute werden SiC-Leistungshalbleiter z.B. in Solarwechselrichtern oder in Elektrofahrzeugen verwendet. Für Stromrichter industrieller elektrischer Antriebe ist die Kosten-Nutzen-Bilanz des Einsatzes von SiC-Leistungshalbleitern gegenwärtig nicht bekannt. Diese Fragestellung motiviert diese Arbeit. Die Auslegung sowie die daraus resultierenden Vor- und Nachteile eines Stromrichters mit SiC-Leistungshalbleitern für elektrische Industrieantriebe ist der Untersuchungsgegenstand dieser Arbeit. Zu diesem Zweck wurde unter Einhaltung industrieller Auslegungskriterien ein 240 kVA SiC-basierter Stromrichterdemonstrator als aktiver Gleichrichter am dreiphasigen 690 V Niederspannungsnetz untersucht. Auf der Basis einer Stromrichterauslegung für SiC- und Si-Leistungshalbleiter wurde ein theoretischer Vergleich von Kosten, Effizienz, Größe und Gewicht durchgeführt. Die Arbeit stellt zunächst den Stand der Technik für SiC-Leistungshalbleiter dar. Anschließend wird ein geeignetes SiC-MOSFET Module für den industriellen Stromrichter ausgewählt und bezüglich des Schaltverhaltens sowie der Parallelschaltung charakterisiert. Der Auslegung des Stromrichterleistungsteils liegen industrielle Anforderungen zu Grunde. Ein realisierter Demonstrator für einen netzseitigen Stromrichter (Active Front End) ist durch eine symmetrische Parallelschaltung von zwei SiC-Modulen, geeignete Ansteuerschaltungen (Gate Drive Units), eine niedrige Streuinduktivität im Kommutierungskreis sowie ein LCL-Filter mit Standard-Kernmaterialien gekennzeichnet. Der Stromrichtervergleich zeigt, dass der betrachtete Stromrichter mit SiC-Leistungshalbleitern im gesamten Betriebsbereich geringere Verluste verursacht als ein vergleichbarer Stromrichter mit Si-Leistungshalbleitern. Der SiC - basierte Stromichter ermöglicht auch eine deutliche Gewichtsreduktion bei ca. 89% der Systemkosten. Somit stellen SiC-Leistungshalbleiter eine attraktive technische Lösung für die untersuchte Anwendung eines aktiven Gleichrichters für industrielle elektrische Antriebe dar.Wide bandgap (WBG) power semiconductors have drawn steadily increasing interest in power electronics in the last three decades. These devices have shown the potential of replacing silicon as the default semiconductor solution for several applications in determined power and frequency ranges. Among them the most mature WBG semiconductor material is silicon carbide (SiC), which presents several characteristics at the crystal level that translate in the potential of presenting lower resistivity, be able to switch faster with lower switching loss, and present both higher characteristics to tolerate and dissipate heat when com pared with silicon. However, the same characteristics that make it great also present a different set of drawbacks to be considered, which aligned with its increased cost make it challenging to assess if its advantages are justified for a particular application. Applications that highly value efficiency and/or power density are the most benefited, and converter solutions featuring the technology have already breached into these application markets. However in other applica tions, the line from which silicon carbide starts making sense in the cost/benefits/drawbacks balance is not clear. This is typically the case of industrial applications, which were the main focus and motivation of this work. Hence, in this work the main goal has been to determine the basic characteristics, advantages and limitations that SiC technology designs for industrial low voltage high power grid connected converters present. To that end, a 690 V, 240 kVA SiC-based grid-tied converter demonstrator following industrial design criteria has been developed. Then, based on this design procedure a theoretical comparison between a 690 V, 190 kVA SiC-based converter against a silicon-based converter designed for the same power output has been performed to compare them regarding cost, efficiency, size and weight. This work also comprises a thorough revision of the state of art of SiC devices, which led to the selection of the switching device. Additionally, a characterization of both single and parallel-connected operation of the semiconductor modules was performed, to determine the module characteristics and its suitability to build the SiC converter demonstrator. Results show that the converter demonstrator operates as designed, proving that is possible with the corresponding precautions to achieve: a low inductive power loop, balanced parallel connection of SiC modules, adequate driving circuits for the parallel-connected modules and an adequate filtering solution in compliance with grid-codes based on standard core materials for the selected switching frequency. Finally, the theoretical comparison between the two designed power converters shows that, attained to the conditions of the comparison, the SiC converter solution presents efficiency gains over the whole operating range, while presenting substantial weight savings at 89% of the costs of the Si-IGBT design, presenting itself as the cost-effective solution for the presented application requirements under the given design constraints

Power Semiconductors for An Energy-Wise Society

This IEC White Paper establishes the critical role that power semiconductors play in transitioning to an energy wise society. It takes an in-depth look at expected trends and opportunities, as well as the challenges surrounding the power semiconductors industry. Among the significant challenges mentioned is the need for change in industry practices when transitioning from linear to circular economies and the shortage of skilled personnel required for power semiconductor development. The white paper also stresses the need for strategic actions at the policy-making level to address these concerns and calls for stronger government commitment, policies and funding to advance power semiconductor technologies and integration. It further highlights the pivotal role of standards in removing technical risks, increasing product quality and enabling faster market acceptance. Besides noting benefits of existing standards in accelerating market growth, the paper also identifies the current standardization gaps. The white paper emphasizes the importance of ensuring a robust supply chain for power semiconductors to prevent supply-chain disruptions like those seen during the COVID-19 pandemic, which can have widespread economic impacts.The white paper highlights the importance of inspiring young professionals to take an interest in power semiconductors and power electronics, highlighting the potential to make a positive impact on the world through these technologies.The white paper concludes with recommendations for policymakers, regulators, industry and other IEC stakeholders for collaborative structures and accelerating the development and adoption of standards

Modeling, Measurement and Mitigation of Fast Switching Issues in Voltage Source Inverters

Wide-bandgap devices are enjoying wider adoption across the power electronics industry for their superior properties and the resulting opportunities for higher efficiency and power density. However, various issues arise due to the faster switching speed, including switching transient voltage overshoot, unstable oscillation, gate driving and evaluation difficulty, measurement and monitoring challenge, and potential load insulation degradation. This dissertation first sets out to model and understand the switching transient voltage overshoots. Unique oscillation patterns and features of the turn-on and turn-off overvoltage are discovered and analyzed, which provides new insights into the switching transient. During the experimental characterization, a new unstable oscillation pattern is found during the trench MOSFET\u27s turn-off transient. The MOSFET channel may be falsely turned back on, resulting in severe oscillation and possible loss of control. Time-domain and large-signal analytical models are established, which reveals the negative impact of common-source inductances and unconventional capacitance curve of trench MOSFET. Besides the devices themselves, another determining part in their switching transient behavior is the gate driver. A programmable gate driver platform is proposed to readily adapt to different power semiconductors and driving schemes, which can greatly facilitate the evaluation and comparison of different devices and driving schemes. The faster switching speed of wide-bandgap devices also requires more demanding measurement and monitoring solutions. A novel combinational Rogowski coil concept is proposed, which leverages the self-integrating feature to further increase the bandwidth. Prototypes achieved more than 300 MHz bandwidth, while keeping the cross-sectional area less than 2.5 mm$^2$. Finally, the very high voltage slew rate of wide-bandgap devices may negatively impact the motor load insulation. Attempting to fully utilize the higher switching frequency capability, sinewave and $dv/dt$ filters are compared. It is shown that sinewave filters can achieve higher efficiency and power density than $dv/dt$ filters, especially for high frequency applications

AC Voltage Control of a Future Large Offshore Wind Farm Network Connected by HVDC

The offshore wind resource around the seas of the UK is a very large renewable energy resource. Future offshore wind farm sites leased by the Crown Estate for Round 3 development will need high power capacity grid connection, but their remote location presents a challenge for the electrical connection to the grid. Long distance AC cable transmission is not practical due to the large cable capacitance which leads to reactive power loss. This thesis considers the voltage source converter and high voltage direct current (VSC-HVDC) technology as the future grid connection for the offshore wind farm network, which is more controllable and has better transmission efficiencies for long distance and high power cable transmission applications. The offshore AC network is weak with very little inertia and has limited rating at the HVDC converter substation. The dynamics in key variables in the offshore wind farm AC network and how they affect certain components in the system were studied. Without proper control, the offshore voltage and the frequency will be sensitive to change. The capacitor of the AC filter at the offshore VSC-HVDC station was found to be vulnerable to over-voltage, therefore a closed loop AC voltage controller was proposed here to maintain a constant capacitor voltage and to prevent tripping or over-voltage damage. The tuning of the control gains were optimised with a pole placement design method and small signal analysis for observing the system eigenvalue damping. The control parameters were then tuned for a fast and well damped controller. The AC voltage controller was evaluated and compared to an open loop system. The controller was able to limit the resonance in the LC filter that can be triggered by large and fast disturbances in the current, voltage and frequency. Current saturation could be implemented within the control structure for device protection from over-currents. Insight on how the wind turbine fully rated frequency converters and controllers may interact with the VSC-HVDC converter station is also discussed