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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.

The increasing importance of cascaded multilevel converters (CMCs), and the sub-category of modular multilevel converters (MMCs), is illustrated by their wide use in high voltage DC connections and in static compensators. Research is being undertaken into the use of these complex pieces of hardware and software for a variety of grid support services, on top of fundamental frequency power injection, requiring improved control for non-traditional duties. To validate these results, small-scale laboratory hardware prototypes are often required. Such systems have been built by many research teams around the globe and are also increasingly commercially available. Few publications go into detail on the construction options for prototype CMCs, and there is a lack of information on both design considerations and lessons learned from the build process, which will hinder research and the best application of these important units. This paper reviews options, gives key examples from leading research teams, and summarizes knowledge gained in the development of test rigs to clarify design considerations when constructing laboratory-scale CMCs.

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 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.

Control of Voltage Source Converters (VSC) based HVDC transmission systems is developed in this paper. The proposed approach aims the stabilization of VSC, which is characterized by non-linearities due to requirements of power flow and DC bus voltage. The steady state average model for the VSC-HVDC system is developed on linear and bilinear deviation state space model around the working point. Based on poles placement and Least Squares (LS) methods, linear and nonlinear polynomial feedback are considered. The proposed approach leads to regulate simultaneously the dq grid currents and the DC bus voltage. By upon the time domain simulations in MATLAB environment, effectiveness of the proposed control strategies are tested on a VSC and a point-to-point VSC-HVDC transmission system. The simulation results show the robustness of the studied systems under various conditions.

The Modular Multilevel Converter (MMC) represents the important technological innovation that emerged among the diverse available topologies of VSC and is avowedly the most suitable solution for converters in HVDC (High Voltage Direct Current) transmission and MTDC (Multi-Terminal Direct Current) grids. Special focus is given through this paper to the dynamic performance of an MMC-based, back-to-back 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 in the AC side of the MMC converter. 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 convex optimization problem using LMI optimization theory. The proposed approach 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 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 input 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 back-to-back MMC-HVDC system is evaluated across accurate and skillful simulation study under Matlab/SimPowerSystem environment.

This article deals with DC voltage dynamics of Multi-Terminal HVDC grids with energy-based controlled Mo\-dular Multilevel Converters (MMC) 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 by controlling the energy to a fixed value, regardless of the DC voltage level, causes large and undesired DC voltage transient after a sudden power flow change. 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 EMT MMC models with 400 sub-modules per arm.

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 stabilizing finite-time controllers and control by backstepping) is presented and shows that the finite-time continuous feedback gives an excellent transient response.

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.

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 proce- dure 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 volt- age and the stability of MTDC systems, a backstepping control strat- egy 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 con- trol allows to exhibit excellent transient response over a wide range of operating conditions.

The modular multilevel converter (MMC) is be- coming 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.

This work focuses on the steady-state analysis of three types of MMC based STATCOM. For a given STATCOM rating, Double-Star Full Bridge, Single-Star Full Bridge and Single-Delta Full Bridge have been compared in terms of design and losses. In this approach, the number of submodules is chosen according to the voltage and current ratings of semiconductor devices while the submodule capacitor value is obtained by following an energy storage criterion to maintain the submodule voltages within an acceptable voltage range.

This paper presents a comparative analysis of the dynamic behavior of a Modular Multi-level Converter (MMC) with grid-forming control either with or without controlling the MMC internal energy. It has been demonstrated that the internal energy of the MMC in low-level control interacts with the high-level control, which is performed by a grid-forming scheme. In case of controlling the internal energy of the MMC, this interaction is mitigated. Moreover, it has been shown that the dynamic behavior of a grid-forming controlled MMC with internal energy control is identical to an equivalent 2-level Voltage Source Converter (VSC).

The MMC is the solution today to connect HVDC grids to the current HVAC grid. This paper proposes to evaluate the impact of Zero Sequence Voltage Injection variants, which until now, have not been extensively studied. Such techniques can involve, for example, a reduction of the SM capacitor value, the number of SM requirement and converter losses. The paper presents MMC current model, control and highlights the implication of the zero-sequence voltage. Grid current control structure with the introduction of the zero-sequence voltage is presented in different techniques. These modulation schemes are compared through two main quantities in MMC, the energy requirement defining the SM capacitance value and the power losses.

The Modular Multilevel Converter (MMC) represents the recent development among the diverse available topologies of VSC and is allegedly the most suitable solution for converters in HVDC transmission systems. This paper investigates the stability of modular multi-level converters based HVDC system connected to a weak ac grid. Small signal stability based on eigenvalues analysis is used to study the interaction between the weak ac grid and the converter. The impact of control parameters, mainly the synchronization system (i.e., Phase Locked Loop) on the stability of the MMC is also considered in frequency domain. Finally, time-domain simulations and frequency domain analysis are carried out using MATLAB/Simulink and symbolic toolbox to validate the effectiveness of the proposed study.

The modular multilevel converter (MMC) is recognized for many advantages over conventional 2-level or 3-level voltage source converters (VSC). One of the features is that it is possible to control the stored energy in the distributed submodule (SM) capacitors. The aim of this paper is to propose a methodology to develop the control system implemented on a 21-level MMC mock-up. This control must be first validated in a real-time (RT) simulation environment with an accurate MMC model representing the behaviour of the physical MMC mock-up. Then it is performed in the hardware-in-the-loop (HIL) simulation before the implementation. The power system is emulated in a real time simulator from Opal-RT Technologies, which is connected to an external control system programmed in a dual-core microcontroller. The developed HIL platform facilitates the tests of the control and protection system in all possible scenarios.

This paper deals with the conception and the development of a detailed EMT Model for MMC based on experimental results obtained from a mock-up. The main purpose is to illustrate how to exploit the performances of EMT simulation tools to develop a detailed model that represents accurately the behaviour of a physical MMC. According to step-by-step identification of the MMC element parameters, the idea is to perform a systematic method, which allows expanding an accurate EMT model considering the behaviour of the prototype and its environment. The first part depicts the MMC topology and the modelling approach of the Half-bridge Sub-Module (SM) using a detailed IGBT-based model. The second part of the simulation model conception concerns both control levels such as high-level and low-level controllers. The last part of the EMT model conception involves the modelling of measurement process, ADC (Analogue Digital Converter), sensors dynamics, the communication delays and especially the quantization effect. Finally, the obtained results from the final detailed EMT model is compared to the experimental behaviour for different active and reactive powers operating points in order to prove the effectiveness and the capability of the EMT modelling to reach a detailed and accurate model.

The subject of this paper is the experimental validation of a recently proposed advanced control scheme for Voltage Source Converters (VSC) based on Model Predictive Control (MPC). The main purpose of the investigated advanced controller is the frequency support from an AC grid to another after significant disturbance through HVDC Grid. The paper reports on the implementation methodology on a small-scale 3-terminal DC mock-up grid consisting of several physical low-scale VSCs, actual DC cables. These components are coupled with realtime simulation tools simulating the adjacent AC grids. The different steps for the validation process of the MPC strategy are illustrated, starting from offline simulation based on a model of the DC grid, up to the actual implementation of the controller in the mock-up of the DC grid.

Over the last 15 years, VSC-based HVDC (High Voltage Direct Current) has become a mature technology for HVDC transmission schemes. The Modular Multilevel Converter (MMC) represents the recent development among the diverse available topologies of VSC and is allegedly the most promising solution today. In fact, the MMC topology offers significant benefits compared to the traditional two-level VSC (Voltage Source Converter), such as lower losses, distributed storage of capacitive energy, improved scalability to higher voltage ratings, a modular design, low total harmonic distortion and, hence, the potential lack of passive fillers on the AC-side of the converter. For the control design, some simplifying assumptions are made to derive an energy based Average Arm Model (AAM) that takes into account the internal dynamics (i.e. the total energy stored in the converter) which do not exist in the 2-level VSC, as well as the AC and DC side dynamics. This additional internal dynamics implies that the control system of the converter must possess additional control loops that govern the DC current and the total amount of stored energy in the SM capacitors of the MMC. The total stored energy in the MMC is then decoupled from the DC bus, but can also be potentially shared depending on the reference signal of the energy control loop. So, the Energy-based controller strategy is introduced, where extra control loops in cascade are added to regulate the dynamics of interest. In this work, an energy based control method is developed for the OPAL-RT’s 10 levels MMC prototype model (5 kW – 400 V). Only the high-level control is proposed and implemented thanks to real time simulation model on the CPU and ARTEMiS solver based on State-Space Nodal Solver. As mentioned above, the energy-based control is based on the possibility to control the AC and DC power separately. Thus, if the power that flowing through the converter is controlled by the AC (resp. DC) power reference, it will be possible to drive the energy level thanks to the other power reference DC (resp. AC). Therefore, to control globally the MMC inner dynamics and state space variables, additional controllers should be added. Thereby, energy based controls have been developed where all the state variables are controlled. In this configuration, the high-level control is composed of outer and inner closed loop controls, which allow controlling the power and the internal MMC energy. Then, based on the designed energy-based controller, the OPAL-RT’s MMC prototype model has been simulated under different operation conditions arising a good performance in steady state as well as during transients. Moreover, the advantages of the Per-unit approach such as the control design as well as the power converter sizing have been carried out across this work. Thus, the per-unit approach is performed on one hand for high voltage characteristic of MMC where the per-unit parameters are derived from EMTP library (based on INELFE project) and on the other hand for the low scale OPAL-RT mockup. As conclusion, the per-unit approach can be used for designing the control as previously said but it can be also very useful for the sizing process of the converter. While the per-unit approach has been used for many years for the classical power system elements (transformer, synchronous machines), it can be extended also to the sizing of power converter. So, in this work, the per-unit quantities of the main elements (capacitor, inductances) will be highlighted. Then, the high voltage characteristic will be considered as a reference and the low voltage characteristics will be compared and some conclusions will be underlined.

The frequency support provided by a DC grid to an interconnected AC grid and integrated in VSC-HVDC controller is studied in this paper. The advantages of a combined control of voltage droop and frequency droop are investigated on a hybrid AC/DC system for the case of a sudden loss of large generation unit in the AC grid. The coupling between both droops control and the correction factor of the frequency droop coefficient are discussed. An experimental validation on VSC converter prototypes and under realistic operation conditions is conducted using Power-Hardware-In-the-Loop simulation. The practical results showed a good match with the theoretical calculation and confirmed the offline simulation results presented in previous work.

This paper proposes an advanced control strategy for Modular Multilevel Converters (MMC) integrated in Multiterminal DC grid. In this present work, a three terminal MMC-MTDC system connecting onshore AC systems with an offshore wind farm is setup. Firstly, the voltage droop control associated to the conventional cascaded controllers for MMC stations is studied, the dynamic behavior of the DC voltage is analyzed and some drawbacks are outlined. In order to improve the dynamic behavior of the controlled DC bus voltage and the stability of MTDC system, an optimal multivariable control strategy of each MMC converter is proposed and integrated in a voltage droop controller strategy. The designed advanced controller allows to improve the overall DC grid stability and to reach the droop values designed on static considerations with acceptable dynamic behavior. By means of numerical simulations in EMTP-RV software, it is shown that the proposed control strategy performs well the stability of MTDC grid with 400- level model for MMC compared with the classic existing control methods.

Modular Multilevel Converters (MMC) are becoming increasingly popular with the development of HVDC connection and, in the future, Multi Terminal DC grid. A lot of publications have been published about this topology these last years since it was first proposed. Many of them deal with converter control methods, other address the method of estimating losses. Usually, the proposed losses estimation techniques are associated to simple control methods For VSC (Voltage Sources Converters) topology, the losses minimization is based on the limitation of the RMS currents values. This hypothesis is usually extended to the control of MMC, by limiting the differential currents to their DC component, without really being checked.