Modeling, Control, and Benchmarking of Multiple Modular VSC Topologies in Multiterminal DC-Grids

Abstract

Future power-transmission systems should be efficient, flexible, robust, and resilient to overcome the ever-growing challenges such as accessing remote locations, handling intermittency ranging from long to short term, and withstanding to the fast rate of changes. The ongoing rapid transformation of power systems is driven by increasing global energy consumption, high penetration of renewable energy and distributed energy resources. Modular voltage source converters (VSCs)-based high-voltage direct-current (HVDC) systems and multiterminal HVDC (MTDC) systems can fulfill these requirements improving the reliability of power systems. This transition may eventually lead to global super-grids consisting of MTDC networks and dominated by diverse types of modular VSC power electronics topologies from multiple vendors.

Challenges of modular VSC topologies for the formation of HVDC and MTDC systems include the control of the power flow, regulation of voltages and currents, frequency support capabilities, energy balancing and regulation, losses distribution amongst submodules (SMs), internal SM faults, operation under parameter variations and unbalanced conditions, and DC-fault tolerance. The main research efforts of the thesis focus on performance enhancement of both established and emerging modular VSC topologies. This is necessary to ensure fully-compatible operation of multiple VSC types, integrating their unique characteristics that improve the overall performance, flexibility, reliability, robustness, and resiliency of future HVDC and MTDC systems. The state-of-the-art modular multilevel converter (MMC) and the emerging DC-fault tolerant alternate arm converter (AAC) are considered for detailed investigation.

Measuring electrical parameters in modular VSC-based HVDC applications is challenging due to circuit complexity and hardware requirements with their associated cost. Estimation of various electrical parameters enables similar functionality with reduced sensor requirements and an added level of reliability. The performance of different DC-voltage estimation methods for indirect DC-voltage control of MMC-based HVDC systems is analyzed. The dependency of the accuracy of DC-voltage estimations on the circulating current control techniques, and interactions between grid- and converter-level control of the MMC are challenging for the reliable operation of MMCs in MTDC systems. Grid- and converter-level control interactions are shown to be avoided and robust control of MMC-based HVDC is ensured, developing an instantaneous DC-voltage estimation method that is independent of circulating current control techniques.

Another critical issue is that the typical MMC with half-bridge SMs lacks the DC-fault tolerance, requiring costly DC-breakers to provide the DC-fault tolerant capability to MMC-based HVDC and MTDC systems, also limiting the use of overhead transmission lines. Therefore, the topology and operation of the AAC are analyzed in detail. The SM requirements of the AAC is thoroughly investigated comparing bipolar SMs based on SM-complexity, voltage balancing, and detailed loss analysis. Limited energy balancing flexibility at lower modulation indexes is a prime challenge of the AAC for HVDC applications. Also, providing zero-current switching (ZCS) operation for the director switches (DSs) of the AAC is another major challenge associated with AAC internal current control. Novel energy balancing methods and overlap period-based circulating current control techniques are developed, extending the operating region of the AAC to lower modulation indexes and providing ZCS for the DSs. Alternative approaches for ZCS of the DSs based on the coordinated operation of the converter transformer and AAC are also investigated.

The scale and required investment of HVDC and MTDC systems necessitate detailed validation and testing of concept designs before the actual implementation, requiring the development of benchmark HVDC systems. Unlike the MMC, AAC lacks such benchmark HVDC models. A fully AAC-based HVDC station and an HVDC transmission system model are developed, ensuring the compatibility with widely used MMC-based benchmark HVDC test systems. Finally, the thesis develops an MTDC system based on both MMCs and AACs, including the developed control methods. The performance of MMC- and AAC-based HVDC and MTDC systems are investigated under steady-state, during transient conditions, and contingencies.

Detailed simulation models aligned with widely accepted DC benchmark test systems are implemented in a real-time digital simulator (RTDS) to validate all of the proposed control methods and to evaluate the performance of proposed MMC- and AAC-based HVDC and MTDC systems. Extensive RTDS results demonstrate the reliable operation of the proposed control methods, and fully compatible operation of both established and emerging modular VSC topologies, enabling the formation of future super-grids consisting HVDC systems and MTDC networks based on multiple VSC topologies.