Reliability Modeling and Improved Redundant Operation for MMC-Based MTDC Systems

• Main Author: Isik, Semih
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2023
• Subjects: Alternative Energy; Alternative energy sources; Control algorithms; Controllers; Design; Electricity distribution; Electronics; Energy; Failure; Marketing; Sustainability; Systems design; Systems science; Systems stability
• ISBN: 9798379871390

Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) transmission is one of the critical alternatives to carry bulk power, integrate a large number of renewable resources, and provide flexible power sharing. Power electronics-based converter stations are adopted to change the transmitted power level. VSC topologies are mainly adopted for this purpose in HVDC transmission. Modular Multilevel Converter (MMC) topology is distinguished from the other topologies with its modular and scalable structure, especially for HVDC transmission. However, power module failures are the most common failures in MMC applications because the chances of failure increase when many modules are connected in an MMC phase to meet the desired voltage level [1]-[3]. Although different structure power modules are available for MMC applications, the essential components are the same for each structure. For instance, a half-bridge power module consists of two switching elements, e.g., IGBTs, and anti-parallel diodes and an energy storage element, e.g., a capacitor.In contrast, a full-bridge power module doubles the number of IGBTs while keeping single energy storage. The essential components inside a power module are prone to malfunction under increased gate leakage current, the gate threshold voltage degradation, excess temperature, aging, environmental reasons, and many other mechanical causes. On the other hand, it is highly preferable not to shut down a converter system even if it is subjected to internal or external faults. Shutting down a converter system under any fault may cause high downtime, low reliability, and availability indexes. An alternative way to keep indexes smaller and the financial losses less during a fault condition is to operate the system in a lower power mode until the fault is cleared or sustain the operation with a fault-tolerant controller [4],[5].This thesis investigates several control aspects of an MMC system for HVDC applications. This thesis aims to improve the reliability and availability of an MMC application using the control structure. Therefore, this thesis investigates the reliability modeling of an MMC and identifies the leading causes for components’ life-time reduction. Accordingly, a circulating current control method is proposed to minimize the magnitude of the circulating current and reduce the ripple in the capacitor voltage to extend the life-time of the components so that higher reliability can be achieved. Even though higher reliability can be achieved, this does not mean that the converter is fully resilient to failure. Using the scale-up methodology, fault-tolerant distributed controller approaches are presented for hot and cold redundant based MMC structures to sustain the operation. Finally, an MMC-based multi-vendor Multi-Terminal DC transmission grid is modeled to investigate converter control interoperability issues. The Nyquist and the impedance-based stability criterion approaches are adopted to identify possible resonant points to a black-box converter control tuning approach. After determining the MTDC system stability based on different operating points with different system control methods, a DC fault clearance and grid restoration method is suggested without needing DC circuit breakers [6].