A secure, equitable, and environmentally sustainable energy system is a critical global challenge in pursuing a sustainable future. As dependence on fossil fuels wanes and the integration of renewable energy sources escalates, traditional power grids are confronted with unprecedented challenges, particularly in system inertia. This situation necessitates a substantial upgrade in energy transmission infrastructure. Such infrastructure must evolve to bridge the expanding geographical gap between renewable generation sites and consumption centres, all while maintaining power system stability. The solution to this complex puzzle may well reside in developing the 'Express Energy Highway'—a concept that envisions a trans-national High Voltage Direct Current (HVDC) grid as the backbone of a modern, digital-era energy network. HVDC grids are at the forefront of this transformation, providing an efficient solution for long-distance power transmission and capable of integrating a diverse range of energy sources with lower losses compared to traditional HVAC power systems. The expansion of HVDC technology, especially within Europe's ambitious energy targets and the phased decommissioning of nuclear power plants, underscores the pressing need for advanced control systems and real-time simulation models to navigate the complexities of these next-generation energy highways.

This thesis delves into modern HVDC technologies, focusing on Voltage Source Converter (VSC) technology. VSC-HVDC grids are renowned for their flexibility, functionality, and efficiency—essential for reducing carbon footprints and integrating massive renewable energy sources. Additionally, the thesis explores the operational dynamics of Multi-Terminal HVDC (MT-HVDC) systems. It develops a comprehensive framework for large offshore multi-terminal HVDC grids, concentrating on three focus areas: Advanced Control Algorithms, Fault Management, and Real-Time Simulation. At its core, the thesis emphasizes Advanced Control Algorithms, especially Model Predictive Control (MPC). This technique optimizes control actions by minimizing an objective function under dynamic system constraints, which includes enhancements like Grid Forming Control for integrating fluctuating energy sources into systems with low inertia and Control Interoperability to ensure stable operations under varying conditions. The second focal area, Fault Management, is critical for ensuring the reliability of HVDC grids. It includes strategies like Fault Current Suppression to assist DC circuit breakers and minimize damage during DC faults, alongside DC Fault Ride Through Control, which maintains system operation during DC cable faults and facilitates rapid recovery, eliminating any resonance in the MT-HVDC grid. The third area, Real-Time Simulation, offers a testing platform for these control strategies through standalone and integrated simulations, including Hardware-in-Loop, Control-in-Loop, and Software-in-Loop setups. These simulations validate the effectiveness of the controls in real-time scenarios, ensuring robust performance under different operating conditions. The convergence of these areas underpins the thesis's goal to Design and implement robust, stable, and rapid control systems for large offshore MT-HVDC grids.

The findings from this thesis confirm significant advancements in HVDC technology, mainly through implementing MPC strategies within the MT-HVDC grid. The research demonstrates that MPC enhances operational stability, efficiency, and responsiveness to system disturbances and fluctuations in renewable energy inputs—crucial in settings with low system inertia where traditional methods are less effective. Moreover, the thesis highlights the critical role of real-time simulation platforms in validating advanced control strategies, ensuring their robust performance under real-world operational scenarios. In fault management, strategies such as Fault Current Suppression and DC Fault Ride Through Control prove essential in mitigating the impacts of faults, thereby enhancing system resilience and reliability. Overall, these findings underscore the critical role of sophisticated control technologies and real-time simulations in mastering the complexities of modern HVDC grids, supporting the transition toward a sustainable and dependable global energy infrastructure.@en

Real-time simulations have become a crucial tool for life cycle studies of VSC-based HVDC systems. This paper introduces real-time Multi-Terminal HVDC (MTDC) power [1] system network models with real-time wind pro le feedback. It addresses the shortcomings of existing benchmark network models and lls the modeling gaps. ® RSCAD/RTDS environment represents the real-time modeling techniques for studying the life cycle of Bipolar Metallic Return con guration of HVDC systems. This paper evaluates the performance of the proposed network model using unscheduled events, startup, and black start events. Future studies can be conducted using the proposed network models by mimicking the actual performance of cable-based DC grids while considering the computational insights from this paper. The ndings of this paper shall enable the identi cation of various stress points that can be utilized to specify technical requirements for component design and AC/DC protection studies concerning startup and black start sequence.

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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 radial topology of the Multi-terminal High Voltage Direct Current (MTDC) power system is a preferred connection for the gigawatt- renewable power due to its scalability and reliability. However, a radial topology with a metallic return bipolar converter configuration MTDC network possesses technical challenges regarding DC fault current interruption and grid expansion. Furthermore, such HVDC networks are energized in a specific manner, usually involving a separate energizing controller. This paper proposes a design of DC Hubs with direct current circuit breakers (DCCBs) along with a network energization sequence without requiring a separate controller. Additionally, a PI-based controller for post-DC fault circulating current in MTDC's metallic return is proposed. This control operates after DCCB recloses, removing any offset in the metallic cable by regulating the power setpoint in the converters. The proposed control is investigated under a pole-to-ground fault occurrence in the DC Hub. The proposed solution is validated by RSCAD/RTDS^@ simulation by applying detailed and average equivalent models of turbines, DCCBs and converters. The results of this simulation show a successful suppression of the DC circulating current, which results in a balanced operation of the MMCs in the post fault steady state conditions.

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This paper explores the possibilities of providing fast frequency support as an emergency support service to the disturbed AC system through the MTDC grid. A two-layer hierarchical control structure of the MTDC grid is proposed to assure the minimum cost of the frequency control actions, the minimum voltage deviations, or the minimal impact on the frequencies of not-affected AC systems while ensuring the stable operation of MTDC grid. An optimization algorithm is executed at the secondary control level to find the optimal reference values for the voltage-droop characteristics of the voltage-regulating converters, and consequently their DC voltages and active power references. Then, at the primary control level, the reference values are tuned with the optimization results. Implemented control structure confirms that MTDC can provide set values at its terminals without endangering its stability. The secondary control layer is implemented in MATLAB, while the performance of the controller is successfully evaluated through simulation in RSCAD.

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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

In a multi-terminal direct current (MTdc) system based on a modular multilevel converter (MMC), high-speed and large interruption capability direct current circuit breakers (dc CBs) are required for dc fault interruption. However, commercializing these breakers is challenging, especially offshore, due to the large footprint of the surge arrester. Hence, a supplementary control is required to limit the rate of current rise along with the fault current limiter. Furthermore, the operation of the dc CB is not frequent. Thus, it can lead to delays in fault interruption. This study proposes the indirect model predictive control (MPC)-based zero-sequence current control. This control provides dc fault current suppression by continuously controlling the zero-sequence current component using circulating current suppression control (CCSC) and by providing feedback to the outer voltage loop and inner current loop of MMCs. The proposed control is simulated for pole-to-pole and pole-to-ground faults at the critical fault location of an MTdc system. The simulation is performed in Real Time Digital Simulator (RTDS) environment, which shows that the predictive control reduces the rate of rise of the fault current, providing an additional 3 ms after the dc fault occurrence to the dc CB to clear the fault. Besides, the energy absorbed by the dc CB's surge arrester during the pole-to-pole and pole-to-ground fault remains the same with the proposed control.

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The modular multilevel converter has become a popular topology for many applications in medium and high-power conversion systems such as multi-terminal direct current power systems. In this paper, Modular multilevel converter structure and related equations are presented. Then, control methods for circulating current, output current, and energy balance among legs and upper and lower arms of each phase for conventional proportional-integral control and sliding mode control are described. Notably, this study concentrates on the multi-terminal direct current configuration link with masterslave control by presenting a π model for the high voltage direct current transmission line. Moreover, dq-frame is used in the control strategy with a modified first-order sliding mode control and a second-order sliding mode control for preventing chattering. The results show that applying the proposed method in a hybrid power system can provide fast transient responses, zero overshoot, and better stability. Finally, the results are verified by simulations in MATLAB/Simulink.

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Direct current circuit breaker (DC CB) is the key component to provide reliable operation of Multi-terminal Direct Current (MTDC) system. Fast, effective and accurate DC CB models are needed for system-level studies. Due to large number of components in the DC CB, its detailed modeling is needed in order to simulate current interruption process correctly. However, the simulation time may be longer depending on the network complexity. This paper proposes an average model which is compared to a detailed model of a Voltage-source-converter resonant current (VARC) DC CB in an MTDC system in terms of its performance and computation time for two typical simulation cases. The average and the detailed model are modelled and simulated in PSCAD/EMTDC environment. An accurate response of the average model during fast transient event is presented, showing additional computational advantage.@en

Due to their weak nature, such as low inertia, offshore energy hubs are prone to unprecedented fast dynamic phenomena. This can lead to undesired instability problems. Recent literature, with main focus on onshore systems, suggests that electrolysers could be an attractive option to support wind generators in the mitigation of balancing problems. This paper presents an Electromagnetic Transient (EMT) model for real-time simulation based study of the dynamics of active power and voltage responses of offshore hubs due to wind speed fluctuations. The purpose of this study was to ascertain the ability of an electrolyser to support an offshore energy hub under different scenarios and with different locations of the electrolyser. Two locations of Proton Exchange Membrane (PEM) electrolysers were considered: centralised (at the AC common bus of the hub) or distributed (at the DC link of the wind turbines). Numerical simulations conducted in RSCAD® on a 2 GW offshore hub with 4 × 500 MW wind power plants and 330 or 600 MW PEM electrolysers show that electrolysers can effectively support the mitigation of sudden wind speed variations, irrespective of the location. The distributed location of electrolysers can be beneficial to prevent large spillage of wind power generation during the isolation of faults within the hub.@en

The multi-modular converter (MMC) technology is becoming the preferred option for the increased deployment of variable renewable energy sources (RES) into electrical power systems. MMC is known for its reliability and modularity. The fast adjustment of the MMC’s active/reactive powers, within a few milliseconds, constitutes a major research challenge. The solution to this challenge will allow accelerated integration of RES, without creating undesirable stability issues in the future power system. This paper presents a variant of model predictive control (MPC) for the grid-connected MMC. MPC is defined using a Laguerre function to reduce the computational burden. This is achieved by reducing the number of parameters of the MMC cost function. The feasibility and effectiveness of the proposed MPC is verified in the real-time digital simulations. Additionally, in this paper, a comparison between an accurate mathematical and real-time simulation (RSCAD) model of an MMC is given. The comparison is done on the level of small-signal disturbance and a Mean Absolute Error (MAE). In the MMC, active and reactive power controls, AC voltage control, output current control, and circulating current controls are implemented, both using PI and MPC controllers. The MPC’s performance is tested by the small and large disturbance in active and reactive powers, both in an offline and online simulation. In addition, a sensitivity study is performed for different variables of MPC in the offline simulation. Results obtained in the simulations show good correspondence between mathematical and real-time analytical models during the transient and steady-state conditions with low MAE. The results also indicate the superiority of the proposed MPC with the stable and fast active/reactive power support in real-time simulation.

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This chapter presents a general overview of the experience learned with the use of DIgSILENT PowerFactory in the design of theoretical lectures and practical sessions of a power system dynamics course at postgraduate level. This chapter focuses on the experiences acquired in the course that is part of the MSc program in Electrical Engineering of TU Delft, Department of Electrical Sustainable Energy. The discussion provided in this chapter focuses on power systems application with special focus on (i) Steady-state, Dynamic, (ii) Voltage Stability and (iii) rotor angle stability. The main objective of using PowerFactory at MSc level is to expose the postgraduate students to real-life application, however, without lack of generalisation this chapter is dedicated to the is to expose to the application above by using a very well-known two area-four machine test power system (2A4G), it gives students insights and experience with cases closer to actual power systems. Results of this pedagogical experience demonstrate the importance of incorporating appropriate power system simulations tools in the postgraduate level.

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