The MMC-based MTDC systems are considered a promising solution for long-distance power transmission, integration of renewable energy sources, and interconnection of power grids. Nowadays, MMC-based MTDC systems have been successfully developed in various projects worldwide and are expected to play a significant role in future electrical power transmission systems.

Despite the benefits provided by the MMC-based MTDC system, various technical problems emerge. For example, in case of a DC fault on HVDC transmission lines, the DC voltage suffers a deep sag, and the fault current increases to the peak value after several milliseconds, the system stability is seriously affected. The fault currents will easily damage the power electronics and may lead to a collapse of the entire system if the faults are not cleared promptly. Thus, it is crucial to implement a fast, selective, and reliableDC fault protection technology in the system for fault detection. Once the fault is cleared, it is important to know the exact fault location to repair the faulty sections and to restore the system. Hence, an accurate DC fault location technique is of utmost importance for the MTDC system, which would significantly minimize electricity loss and expedite the system restoration process in the event of power outages. In addition, there is a lack of standardization in MMC control, and the majority of HVDC projects are constructed in a vendor-specific manner. As of today, it is unclear how MMC converters from different manufacturers will interoperate with each other. These pose new challenges to the performance of HVDC protection and MMC control and need to be addressed to manage, safeguard, and accelerate the practical feasibility of this system.

The research in this thesis aims to address the shortcomings that have not been addressed in the state of the art, mainly related to the challenges arising when DC faults occur in the MMC MTDC systems and, as such, could provide promising solutions for future practicalMTDCapplications. The main topics areMMC control&interoperability, Protection, and Fault location for the MMC-based MTDC system. The thesis deals with designing a robust protection scheme, a fault location method, and an investigation of the interoperableMMC controllers...
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For this study, we investigated efficient strategies for the recovery of individual links in power grids governed by the direct current (DC) power flow model, under random link failures. Our primary objective was to explore the efficacy of recovering failed links based solely on topological network metrics. In total, we considered 13 recovery strategies, which encompassed 2 strategies based on link centrality values (link betweenness and link flow betweenness), 8 strategies based on the products of node centrality values at link endpoints (degree, eigenvector, weighted eigenvector, closeness, electrical closeness, weighted electrical closeness, zeta vector, and weighted zeta vector), and 2 heuristic strategies (greedy recovery and two-step greedy recovery), in addition to the random recovery strategy. To evaluate the performance of these proposed strategies, we conducted simulations on three distinct power systems: the IEEE 30, IEEE 39, and IEEE 118 systems. Our findings revealed several key insights: Firstly, there were notable variations in the performance of the recovery strategies based on topological network metrics across different power systems. Secondly, all such strategies exhibited inferior performance when compared to the heuristic recovery strategies. Thirdly, the two-step greedy recovery strategy consistently outperformed the others, with the greedy recovery strategy ranking second. Based on our results, we conclude that relying solely on a single metric for the development of a recovery strategy is insufficient when restoring power grids following link failures. By comparison, recovery strategies employing greedy algorithms prove to be more effective choices.@en

This article presents an accurate DC fault location method that applies parameter fitting. This technique first discusses the traveling wave (TW) propagation process in the decoupled line-mode network. We obtain the exact fault distance equation based on the analytical expressions for the wavefront of backward line-mode voltage TW. The adaptive multi-step Levenberg- Marquardt's (AMLM) algorithm is used for parameter fitting due to its fast processing speed and accuracy. The exact fault location can then be estimated using the parameter fitting results. The proposed fault location method is validated using a three-terminal HVDC system modeled on a real-time digital simulator (RTDS) platform. Based on the experimental results, the proposed method accurately detects the fault location, with all estimated errors smaller than 1%, and can withstand 40 dB noise interference. Moreover, the proposed method does not need a high sampling frequency and communication device. Its accuracy is independent of fault resistance and type compared to existing methods.@en

Meshed offshore grids (MOGs) present a viable option for a reliable bulk power transmission topology. The station-level control of MOGs requires faster dynamics along with multiple objective functions, which is realized by the model predictive control (MPC). This paper provides control, and protection design for the Modular Multilevel Converter (MMC) based multi-terminal DC (MTDC) power system using MPC. MPC is defined using a quadratic cost function, and a dqz rotating frame voltage inputs are represented using Laguerre orthonormal functions. MPC has been applied for the control of both grid forming and grid following converters in a four-terminal MTDC setup, implemented for real-time Electromagnetic Transient (EMT) simulation. By applying numerous time-domain simulations, the advantages of the MPC when dealing with AC and DC side disturbances are investigated. The investigation highlights the MPC's inherent feature of fast response and high damping during- and post-disturbance, which is compared to the traditional PI controller performance. The analysis provides a comprehensive insight into the transient behavior of the MTDC during disturbances.

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The DC transmission line protection technology is crucial for the development of multi-terminal Voltage Source Converter (VSC)-based HVDC systems. This article proposes a robust non-unit traveling wave protection (TWP), which deals with the DC fault area identification and fault type discrimination for high impedance fault conditions. The authors applied the traveling wave (TW) reflection and refraction method for the line-mode network. The distinctive features of high-frequency components contained in the line-mode and pole-mode voltage TWs at different relay units are used for the algorithm modeling. Discrete Wavelet Transform (DWT) is selected as the time-frequency analysis tool. The performed simulations are conducted for a four-terminal VSC-HVDC system, and validate the protection feasibility and robustness. More precisely, the proposed protection scheme identifies the internal and external DC faults within 2 ms, and provides correct operation during high-impedance faults (HIF) with a 25 dB level noise interference. This protection scheme makes use of a VSC-assisted resonant current (VARC) direct current circuit breaker (DCCB), that successfully interrupts the fault currents in less than 10 ms after the fault inception. The authors also comprehensively compared the proposed scheme with the existing methods. The obtained results show that the proposed protection scheme is superior in terms of sensitivity and selectivity performance.

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This paper presents the modular-multilevel-converter (MMC) control interoperability (IOP) and interaction within the High Voltage Direct Current (HVDC)-based power system. IOP is a crucial issue in the large-scale HVDC grid with different suppliers. To accommodate future multi-vendor HVDC grids developments, this article comprehensively investigates the MMC control IOP issue. Firstly, the most commonly adopted proportional-integral (PI) control and other non-linear controllers, e.g., model predictive control (MPC), back-stepping control (BSC), and sliding mode control (SMC), are constructed for MMC. Then, the IOP simulations are carried out in a multi-terminal direct current (MTDC) system in real-time digital simulator (RTDS) environment. The most frequent transients of the practical projects, e.g., power flow changing, wind speed changing, and DC/AC grid faults, are simulated with eight different scenarios. Each scenario presents different control capabilities in maintaining system stability, more precisely, the scenarios with non-linear controllers show faster settling time and fewer DC voltage and power variations. Controller switchings are also achieved without bringing large system oscillations. This paper provides the optimal allocation strategy of controllers to cope with system transients.@en

Existing line protection methods for multi-terminal direct current (MTDC) systems are constrained by the placement and values of boundary elements. To overcome this limitation, this paper proposes a non-unit DC line protection method based on the normalized backward traveling waves (BTWs) of the 1-mode voltage. Firstly, this article studies the traveling wave characteristics and derives the expressions for the normalized BTWs. Then, the Levenberg-Marquardt algorithm is used for amplitude fitting and normalization calculation. Based on the normalized BTWs, a non-unit protection method is proposed. Finally, the proposed method is evaluated with a simulation model on the PSCAD/EMTDC platform. The results demonstrate that the proposed method can accurately identify faults of different resistances and distances without requiring boundary devices, and is robust against noise disturbances (35 dB).

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Due to its excellent performance, VSC-based high voltage direct current (HVDC) power systems draw significant attention. They are being heavily used in modern industrial applications, such as onshore and offshore wind farms, and for interconnection between asynchronous networks. However, the traditional proportional-integral (PI) control method is not robust enough to track the reference signal quickly and accurately during significant system disturbances. This paper proposes a robust adaptive back-stepping control (BSC) method that secures vulnerable power-electronic equipment. The adaptive BSC controller regulates the sum of capacitor energy, and the AC grid current through decoupled and closed control-loop design. The major advantage of the proposed control approach is the smooth transient response and accurate tracking ability, which is superior to classical control methods. In addition, the proposed methods have the merits of systematic and recursive design methodology and demand a low processing burden for Lyapunov functions and control laws. Moreover, the implementation particularities of the proposed approach are illustrated and verified for a power system digital twin using real-time digital simulator (RTDS).

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