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In this paper, a reliable methodology is proposed in order to implement and validate a Model Predictive Control (MPC) scheme on an actual Voltage Source Converter (VSC) integrated in a scale-down multi-terminal DC grid. The objective of the investigated MPC controller is to enable AC frequency support among two asynchronous AC areas through a High Voltage Direct Current (HVDC) grid, while considering physical constraints, such as maximum and minimum DC voltage. A systematic and accurate implementation strategy is proposed, based mainly on the Hardware In the Loop (HIL) and Power Hardware In the Loop (PHIL), leading to the real-life testing on VSC, controlled by a classical microcontroller. The technical problems during the implementation process, as well as the proposed solutions, are described in detail through this paper. This procedure is deemed valuable to bridge the gap between offline simulation and the actual implementation of such advanced control scheme on experimental test rig. [ABSTRACT FROM AUTHOR]

This paper presents an evolution of control systems of Modular Multilevel Converters (MMCs) focusing on the internal voltages and currents dynamics. MMCs have passive components inside the converter that create extra dynamics compared to conventional VSCs. Some control schemes that do not consider these internal dynamics may still stabilize the system asymptotically thanks to the linearisation in the modulation step. However these control schemes are less robust because they are prone to poor damped oscillations on the dc side of the converter. The MMC circuit and energy relationships are presented in this paper. Along with a gradual development of the energy based control, the important roles of each internal dynamics are clearly demonstrated. Experimental results are presented to show the impacts of the linearisation in the modulation step on the system behaviour. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) represents the important technological innovation that emerged among the diverse available topologies of voltage source converter and is avowedly the most suitable solution for converters in high voltage direct current (HVDC) transmission and multiterminal direct current grids. Special focus is given through this paper to the dynamic performance of an MMC-based, point-to-point HVDC system. Using an optimal guaranteed cost control theory, a robust control approach is designed in order to reject the impact of the unmodeled uncertainty, mainly in the ac side of the MMC. For this aim, a small-signal state-space linear model is derived for the control design of an advanced local controller of each MMC station. Furthermore, a new optimal guaranteed cost controller is proposed based on the convex optimization problem using linear matrix inequality optimization approach. The proposed strategy leads to regulate simultaneously the ac grid and differential currents as well as total stored energy per phase in abc frame. To ensure the energy balancing between upper and lower arm per phase, an outer control loop is used in order to control the energy difference per phase between upper and lower arms of MMC. For the MMC linked to HVDC system, the active power reference is generated through an outer classical dc voltage controller. This combined control strategy between classic and advanced robust regulation methods allows exploiting the advantages of both control methods. Effectiveness of the proposed optimal robust control strategy for point-to-point MMC-HVDC system is evaluated across accurate and skillful simulation study under MATLAB/SimPowerSystem environment. The simulation results convince satisfactory dynamics responses of a two-terminal MMC-HVDC system based on the robust control approach under various operating conditions, even under unbalanced ac grid conditions (e.g., asymmetrical fault). [ABSTRACT FROM AUTHOR]

This paper presents the finite time stabilisation strategy of two problems: the first one is the control of the high voltage direct current based on voltage source converter, while the second is the control of the multi-terminal direct current transmission systems. Subject to finite-time control design strategy, a linear and nonlinear dynamic model are derived based on the state-space description. Furthermore, continuous or discontinuous finite-time feedbacks are proposed to ensure the tracking of the output variables and to enhance the stability of the studied high voltage direct current system. In addition, the proposed control strategy is extended for the multi-terminal direct current system. A comparative study between various approaches (Proportional-Integral control, continuous or discontinuous stabilising finite-time controllers and control by backstepping) is presented and shows that the finite-time continuous feedback gives an excellent transient response. [ABSTRACT FROM AUTHOR]

This article presents a backstepping control design strategy for the voltage source converter (VSC)-based high-voltage direct current (HVDC). First, a dynamic model is derived based on the state space description. Subject to the backstepping control design procedure strategy, a non-linear control scheme is developed in the sense of Lyapunov stability theory in order to satisfy various objectives of a stable HVDC system and guarantee a grid connection with a unity power factor. Then, the proposed control method is extended for multi-terminal (MT) HVDC transmission systems based on VSCs. In order to improve the dynamic behavior of the controlled DC bus voltage and the stability of MTDC systems, a backstepping control strategy accorded to each VSC is proposed and integrated into the voltage droop control strategy. The designed advanced controller allows to improve the overall DC grid stability and to reach the droop values, designed on static considerations, with satisfying dynamic behavior. Compared to the conventional control, the use of a backstepping control allows to exhibit excellent transient response over a wide range of operating conditions. [ABSTRACT FROM PUBLISHER]

Summary: Modular Multilevel Converter (MMC) is becoming a promising converter technology for high‐voltage direct current transmission systems due to its high modularity, availability, and power quality. It is a multi‐input‐multi‐output nonlinear system. The control system for MMC is required to simultaneously achieve multiple control objectives. Existing control strategies for MMC are complex and the controller parameter design is not straightforward for the nonlinear systems with highly coupled states. In view of this, a steady‐state model for the MMC is developed on bilinear deviation state‐space model around a working point. Based on linear quadratic regulator and least squares methods, a nonlinear polynomial feedback law is designed to simultaneously control the grid and differential currents, and the global stored energy and energy balancing between total upper and lower arms. To generate the optimal current references, a multivariable linear quadratic controller is used to regulate the total energy per leg, energy difference between each upper and lower arms, and the DC bus voltage. The proposed high‐level controller depicts an original advanced control structure of MMC converter. The performance of the proposed strategy for a detailed model of 400‐level MMC is evaluated using simulations in MATLAB/SIMULINK/SPS software environment. [ABSTRACT FROM AUTHOR]

This article deals with dc voltage dynamics of multi-terminal HVdc grids (MTDC) with energy-based controlled modular multilevel converters (MMCs) adopting the commonly used power-voltage droop control technique for power flow dispatch. Special focus is given on the energy management strategies of the MMCs and their ability to influence on the dc voltage dynamics. First, it is shown that decoupling the MMC energy from the dc side causes large and undesired dc voltage transient after a sudden power flow change. This occurs when this energy is controlled to a fixed value regardless of the dc voltage level. Second, the virtual capacitor control technique is implemented in order to improve the results. However, its limitations on droop-based MTDC grids are highlighted. Finally, a novel energy management approach is proposed to improve the performance of the later method. These studies are performed with detailed MMC models suitable for the use of linear analysis techniques. The derived MTDC models are validated against time-domain simulations using detailed electro-magnetic transient MMC models with 400 sub-modules per arm. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is becoming a promising converter technology for HVDC transmission systems. Contrary to the conventional two- or three-level VSC-HVDC links, no capacitors are connected directly on the dc bus in an MMC-HVDC link. Therefore, in such an HVDC link, the dc bus voltage may be much more volatile than in a conventional VSC-HVDC link. In this paper, a connection between the dc bus voltage level and the stored energy inside the MMC is proposed in order to greatly improve the dynamic behavior in case of transients. EMT simulation results illustrate this interesting property on an HVDC link study case. [ABSTRACT FROM PUBLISHER]