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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.

Energy management in modular type converters constitutes a key aspect of their operational stability. This paper introduces a full energy management structure for the Extended Overlap-Alternate Arm Converter (EO-AAC) ensuring both equal energy distribution across all six stacks and the maintaining of ripple-free DC current during steady state. The performance of the control structure against active power step events is validated by detailed simulations using EMTP-RV software. Moreover, the full energy management allows the EO-AAC to have an equivalent controllability to Modular Multilevel Converter (MMC) through only two conducting stacks in overlap mode ensuring the power balance. From this observation, it is demonstrated that the use of control strategy like the virtual capacitor to support HVDC system, originally designed for MMC, is possible. Gathering all these controllers leads to a general conclusion which is the dynamic equivalence between EO-AAC and MMC.

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.

Modular Multilevel Converter (MMC) is an established technology for HVDC or Multi-Terminal DC (MTDC) systems, due to its advantages over classical Voltage Source Converters (VSCs) such as two or three level VSCs. To achieve a full control of all state variables, it is essential to implement energy-based method in which a cascade control loop is employed to regulate all state variables including ac and differential currents, and stored energy within MMC arms. In addition to normal operation control, dc Fault Ride Through (DC-FRT) capability of the MMC is a crucial and challenging control issue especially for overhead line HVDC system where non-permanent dc fault occurrence is statistically more probable. Furthermore, the main problematic technical obstacle to develop HVDC/MTDC grids is the lack of mature dc fault protection. Since conventional control in normal operation cannot be employed in case of dc fault, an efficient control strategy is indispensable. The principal objectives of the novel control methodology are to (i) obtain DC-FRT capability, (ii) decay short circuit current to zero, (iii) secure the MMC through leg and arm energy balancing, (iv) support ac grid as a Static Synchronous Compensator (STATCOM) and (v) resume normal operation after dc fault clearance. The simulation results verify the validity of proposed control strategy to fulfill the abovementioned objectives in dc fault operation of the hybrid MMC.

This paper presents the modelling of the DC-DC Multilevel Modular Converter (DC-DC MMC) with half-bridge Sub-Modules (SM) and the control based on the inversion of its model. The DC-DC MMC structure presents many advantages such as its modularity, the absence of capacitors on the high DC bus voltage and a very low switching frequency due to the large number of SMs. This topology also preserves the intrinsic disadvantages of the MMC as the complexity of modelling and controlling due to the large number of semiconductors and state variables to control. The control of this converter cannot be symmetrical due to the interconnection of the two parts by an internal AC grid. The control strategy of one part of the DC-DC MMC uses the conventional control scheme with currents controls and stored energy control. The second one uses the energy control and produces the waveform of the three-phase internal AC bus voltage linking the two parts of the converter. The explicit control for the generation of internal AC voltages guarantees the correct operation of the converter even in a critical DC voltage dip on one or the other DC buses. Thus, it avoids the need of a DC circuit breaker or the use of full bridge MMC sub-modules. The validity of the proposed control is verified by simulation using Matlab-Simulink.

Volatile productions and consumptions generate a stochastic behavior of distribution grids and make its supervision difficult to achieve. Usually, the Distributed Generators reactive powers are adjusted to perform decentralized voltage control. Industrial controllers are generally equipped with a local affine feedback law, which settings are tuned at early stage using local data. A centralized and more efficient tuning method should aim to maximize the probability that all the node voltages of distribution grids remain within prescribed bounds. When the characteristics of the stochastic power forecasts are known, the centralized algorithm allows to update the settings on a regular time basis. However, the method requires to solve stochastic optimization problem. Assuming that stochastic variables have Gaussian distributions, a procedure is given which guarantees the convergence of the stochastic optimization. Convex problems drastically reduce the difficulty and the computational time required to reach the global minimum, compared to nonconvex optimal power flow problems. The linear controllers with optimized parameters are compared to traditional control laws using simulations of a real distribution grid model. The results show that the algorithm is reliable and moreover fast enough. Hence, the proposed method can be used to update periodically the control parameters.

The M2DC exploits the interleaving between the three legs of an MMC to realize a promising uninsulated DC/DC converter to interconnect HVDC grids. This paper details a current and energies decoupled model of the M2DC. The major idea proposed in this paper is focused on the full energy control generating optimal current references to minimize the internal currents magnitude. The energy sum and difference models are fully detailled. Both current and energy control loops are based on the model inversion principle in order to control all the state variables. The proposed control is based a dynamic control developed with the model inversion principle associated on an optimization of the current magnitude deduced from a quasi static analysis. All dynamics of the system are then explicitly controlled, which guarantee a good dynamic behavior during the transient. Therefore, current and energy controls are presented in details. Simulation results show the dynamic behavior of the converter for various operating points.

The present paper deals with the post-fault synchronization of a voltage source converter based on the droop control. In case of large disturbances on the grid, the current is limited via current limitation algorithms such as the virtual impedance. During the fault, the power converter internal frequency deviates resulting in a converter angle divergence. Thereby, the system may lose the synchronism after fault clearing and which may lead to instability. Hence, this paper proposes a theoretical approach to explain the dynamic behavior of the grid forming converter subject to a three phase bolted fault. A literal expression of the critical clearing time is defined. Due to the precise analysis of the phenomenon, a simple algorithm can be derived to enhance the transient stability. It is based on adaptive gain included in the droop control. These objectives have been achieved with no external information and without switching from one control to the other. To prove the effectiveness of the developed control, experimental test cases have been performed in different faulted conditions.

Grid-forming inverters usually use inner cascaded controllers in order to regulate the output AC voltage and the converter output current. Yet, at the power transmission system level where the power inverter bandwidth is limited (i.e.; low switching frequency), it is difficult to tune its controller parameters to achieve the desired performances because of the control loops interactions. In this paper, a direct AC voltage control based state-feedback control is applied. Its control gains are tuned using the linear quadratic regulator. In this paper, a sensitivity analysis is proposed in order to choose the right cost factors that allow the system to achieve the imposed specifications. Conventionally, the system based on direct AC voltage control has no restriction on the inverter current. Hence, in this paper, threshold virtual impedance has been added to the state-feedback control in order to protect the inverter against overcurrent. The robustness of the proposed control is assessed for different short circuit ratio using small signal stability analysis, then, checked in different grid topologies using time domain simulations. An experimental test bench is developed in order to validate the proposed control.

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.

This paper addresses the volt-var control of distribution grids embedding many distributed generators (DGs). Specifically, it focuses on the compliance of powers to specified PQ diagrams at the high voltage/medium voltage (HV/MV) interface while the voltages remain well controlled. This is achieved using a two-stage optimization corresponding to two different classes of actuators. The tap position of capacitor banks is selected on a daily basis, given a stochastic model of the input powers prediction, which allows infrequent actuation and increases the device lifespan. In a second stage, a confidence level optimization problem allows to tune on an hourly basis the parameters of the DGs reactive power affine control laws. Results on a real-size grid show that the combined tuning of these actuators allows the ability to comply with European grid codes while the control effort remains reasonable.

Although the probability of occurrence of ac grounding faults at the valve-side of the interface transformer of a high-voltage direct-current (HVDC) link is low, they may cause high risks to the converter when compared to grid-side ac faults. This paper analyzes the characteristics of valve-side ac single-phase-to-ground faults in full-bridge modular multilevel converters (FB-MMCs) based bipolar HVDC systems. Overcurrents in the converter arms are analyzed and it is shown that overvoltages in FB submodules (SMs) occur without an appropriate protection in place. Two strategies are investigated to protect the FB-MMC during the fault and corresponding controllers are designed. The effectiveness of the presented strategies for the prevention of overcurrents and overvoltages, upon non-permanent and permanent faults, and system post-fault restoration are investigated. For completeness, the strategies are also verified by conducting simulations in PSCAD/EMTDC.

The dq impedance stability analysis for a grid-connected current-control inverter is based on the impedance ratio matrix. However, the coupled matrix brings the difficulties to derive its eigenvalues for the analysis based on the General Nyquist Criterion. If the couplings are ignored for simplification, the unacceptable errors will present in the analysis. In this paper, the influence of the couplings on the dq impedance stability analysis is studied. For taking the couplings into account simply, the determinant-based impedance stability analysis is used. The mechanism between the determinant of the impedance-ratio matrix and the inverter stability is unveiled. Compared to the eigenvalues-based analysis, only one determinant rather than two eigenvalue s-function is required for the stability analysis. One Nyquist plot or polemap can be applied to the determinant for checking the right-half-plane poles. The accuracy of the determinant-based stability analysis is also checked by comparing with the state-space stability analysis method. For the stability analysis, the coupling influence on the current control, phase-locked loop and the grid impedance are studied. The errors can be 10% in the stability analysis if the couplings are ignored.

Distributed generators (DG) reactive powers are controlled to mitigate voltage overshoots in distribution grids with stochastic power production and consumption. Classical DGs controllers may embed piecewise affine laws with dead-band terms. Their settings are usually tuned using a decentralized method which uses local data and optimizes only the DG node behavior. It is shown that when short-term forecasts of stochastic powers are Gaussian and the grid model is assumed to be linear, nodes voltages can either be approximated by Gaussian or sums of truncated Gaussian variables. In the latter case, the voltages probability density functions (pdf) that are needed to compute the overvoltage risks or DG control effort are less straightforward than for normal distributions. These pdf are used into a centralized optimization problem which tunes all DGs control parameters. The objectives consist in maximizing the confidence levels for which voltages and powers remain in prescribed domains and minimizing voltage variances and DG efforts. Simulations on a real distribution grid model show that the truncated Gaussian representation is relevant and that control parameters can easily be updated even when extra DGs are added to the grid. The DG reactive power can be reduced down to 50 % or node voltages variances can be reduced down to 30 %.

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.

This paper addresses the relevance of using reactive power from Medium Voltage (MV) networks to support the voltages of a High Voltage (HV) rural network in real-time. The selection and analysis of different optimal coordination strategies between the HV and several MV grids is investigated. The algorithms will control the reactive powers that can flow between HV/MV networks after a request from the Transmission Network Operator in case of an emergency situation such as a line outage. From a case study, the relevance of the coordination is enlightened and recommendations are given on how to tune and to combine the optimal algorithms with the advanced Volt Var Controllers of the distribution grids.

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.

The Modular Multilevel Converter (MMC) is a power electronic structure used for high voltage adjustable speed drives applications as well as power transmission applications and high-voltage direct current (HVDC). MMC structure presents many advantages such as modularity, the absence of a high voltage DC bus and very low switching frequency. It presents also some disadvantages such as modeling complexity and control due to the large number of semiconductors to control. The objectives of this paper are to present the methodology to design a laboratory MMC converter and its control system. This methodology is based on an intensive used of real-time simulation, to develop and test the control algorithm is proposed. This MMC prototype must be as realistic as possible to a full scale MMC, with a large number of SM (i.e. 640kV on the DC side, a rated power of 1GW and 400 sub-modules). A control hardware integrating distributed processors (one for each arm) and a master control is presented. The protocols to validate sub-modules, arms and the converter are explained.

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.

Purpose Self-commuted voltage source converter (VSC) can significantly extend the flexibility and operability of an HVDC system and be used to implement the concept of multi-terminal HVDC (MTDC) grid. To take full advantage of MTDC systems, its overall behaviour must be characterized in quasi static and dynamic states. Based on the numerous literatures, a dedicated two-level VSC model and its local controllers and DC grid voltage regulators are developed for this purpose. Furthermore, the requirement of the system to guarantee all the physical constrains must be well assessed and concrete demonstrations must be provided by numerical simulations. Design/methodology/approach First, a two-level VSC model and its local controllers and DC grid voltage regulators are developed. Then, DC cable models are investigated and their characteristics are assessed in the frequency domain. Those developed models are combined to form a three-terminal HVDC grid system on Matlab/Simulink platform. To analyze the stability of this electrical system, the dynamics of the system against variations of power dispatch are observed. Findings To analyze the stability of this electrical system, the dynamics of the system against variations of power dispatch are observed. The differences in the DC grid voltage dynamics and the power flow of the converter stations coming from the embedded primary controls are analysed, and the technical requirements for both cases are assessed. Originality/value In this paper, the dynamic stability of an MTDC system has been analysed and assessed through an adequate simulation model, including its control scheme and the cable models. The interest of the improved PI model for cables is highlighted.

This paper presents the design, development, control and supervision of a hardware-based laboratory Multi- Terminal-Direct-Current (MTDC) test-bed. This work is a part of the TWENTIES (Transmission system operation with large penetration of Wind and other renewable Electricity sources in Networks by means of innovative Tools and Integrated Energy Solutions) DEMO 3 European project which aims to demonstrate the feasibility of a DC grid through experimental tests. This is a hardware-in-the-loop DC system test-bed with simulated AC systems in real time simulation; the DC cables and some converters are actual, at laboratory scale. The laboratory scale test-bed is homothetic to a full scale high voltage direct current (HVDC) system: electrical elements are the same in per unit. The test-bed is supervised by a Supervisory Control And Data Acquisition (SCADA) system based on PcVue. Primary control based droop control method to provide DC grid power balance and coordinated control methods to dispatch power as scheduled by transmission system operator (TSO) are implemented. Since primary control acts as converter level by using local measurements, a coordinated control is proposed to manage the DC grid power flow. The implemented system is innovative and achievable for real-time, real-world MTDC-HVDC grid applications.

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.

To ensure power system security with very high wind generation (WG) penetration, the participation of wind generators in primary frequency control is essential. Previous studies have shown the technical capability of wind turbines to participate in primary frequency regulation at a wind farm level. In order to analyze, the contribution of wind power to primary frequency regulation at system level one needs to quantify the amount of primary reserve from conventional sources that can be displaced. This amount of reserve depends on the aggregated variability of WG during each reserve provision time-interval. This paper presents a statistical approach to assess the impact of intrahour wind power variability on the volume of primary reserve that can be provided from WG. Furthermore, the effectiveness of different reserve allocation strategies is compared. The proposed approach is applied to a case study based on real-wind data measurements from the French island of Guadeloupe. Results show that for a small isolated system neglecting WG intrahourly variability leads to an overestimation of its contribution to primary reserve.

Modular multilevel Converters(MMCs) may contain numerous insulated-gate bipolar transistors. The modeling of such converters for electromagnetic transient-type (EMT-type) simulations is complex. Detailed models used in MMC-HVDC simulations may require very large computing times. Simplified and averaged models have been proposed in the past to overcome this problem. In this paper, existing averaged and simplified models are improved in order to increase their range of applications. The models are compared and analyzed for different transient events on an MMC-HVDC system.

A modular multilevel converter control system, based on converter energy storage, is proposed in this paper for two different control modes: active power and dc voltage. The proposed control system decouples the submodule (SM) capacitor voltages from the dc bus voltage. One of the practical applications is the management of active redundant SMs. A practical HVDC system with 401-level MMCs, including 10% redundancy in MMC SMs, is used for validating and demonstrating the advantages of the proposed control system. This paper also presents a novel capacitor voltage balancing control based on max – min functions. It is used to drastically reduce the number of switchings for each SM and enhances computational efficiency.

The aim of this paper is to present a linear quadratic Gaussian (LQG) controller designed with two main objectives: to allow the contribution of wind turbines (WTs) to the primary frequency regulation of an island power system, and to reduce the WTs drive-train mechanical stresses. The designed LQG1_CPC_Track controller is compared, in terms of the mentioned objectives, to a more classical controller containing two uncoupled control loops. The comparison is carried out in a simulation model of the Guadeloupian island power system taken as a case study. The model is implemented in Eurostag software. Simulation results show that the contribution of both controllers to the primary frequency regulation is satisfactory, and that the LQG1_CPC_Track allows reducing drive-train mechanical stresses significantly. Thus, thanks to the LQG1_Track, on top of allowing the integration of more wind energy in the grid with the contribution to primary frequency regulation, WTs would have less maintenance costs and could be manufactured with cheaper material.

Grid operational challenges are significant to increase securely the wind penetration level. New embedded control functions are therefore required in order to make participate wind generators in power system management. In this paper the implementation of inertial response and primary frequency control in a wind turbine controller are investigated. Main factors affecting the performances of the frequency regulation are identified and characterized. The influence of control parameters and the turbine operating point on the inertial response are analyzed through obtained performances in an islanded power system. The combined control scheme using both controllers is also developed and the potential of the obtained grid service at partial load is discussed.

Real-time testing of new and more sophisticated distributed resource interfaces during transients, representing the different physical parts (mechanical, thermal, hydraulic, chemical, electrical, electronics) of a nonconventional generator in a single platform, or analyzing the interactions of distribution systems with distributed generators, energy markets, and customer behaviors are scenarios that cannot be studied with current software packages. This paper analyzes the present status and discusses the future development of tools that could cope with these simulation challenges. This paper includes test cases that will illustrate the scope of some of these simulation tools.

The control of a wind energy conversion system can be decomposed in two parts: a local control depending on the power structure and a global control (strategy) deduced from global considerations. The local part ensures an efficient energy management of each component of the system. The local control structure can be deduced from the Energetic Macroscopic Representation, which is a graphical description of the system according to action and reaction principle. Using inversion rules, the deduced control structure is composed of a maximum of control operations and measurements. The global control part is independent of the power structure. This strategy part leads to achieve power objectives (active and reactive power targets) and system constraints (machine efficiency and DC bus limitation). Several strategies can be defined for the same system. These control decomposition is applied to a wind generation system composed of a permanent magnet synchronous generator and two three-phase converters. Simulation results are provided for a 600 kW wind energy conversion system.

The Modular Multi-Level DC-DC Converter (M2DC) is an attractive non-isolated DC-DC converter topology for HVDC grid. In order to carry out MTDC grid stability studies, the development of reduce order models of converters is necessary. This article first presents the M2DC converter. Then, the reduce order model will be developed in the second part. The development of the control of this model will be carried out in the third part. Atlast, the comparison of the reduce order model and its control with the average arm model will be performed in the later section of the paper.

The Modular Multilevel DC Converter is an attractive non-isolated topology to interconnect High Voltage DC Links. This paper presents the interaction among control, component design and efficiency of this converter. The impact of the two degrees of freedom on the design and the efficiency is analyzed.

As distributed generation increases, it is essential to study its impact on the grid dynamics. This paper focuses on understanding the influence of the emergent technology of Grid-Forming converters on the electromechanical oscillations of the power system. Interactions among synchronous generators and gridforming converters are analyzed thanks to simplified models. These highlight the similarities of both sources, and thus, explain the participation of the converter in the oscillation. They also revealed the differences that justify the damping effect of Grid-Forming converter. This conclusion, obtained with simplified models, is validated with a small-signal stability analysis of a detailed model in the dq0-frame

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.

In recent years, several works on grid-forming converters in transmission networks have been performed. Since these high voltage grids are mainly inductive, a natural decoupling exists between active and reactive power controls. This decoupling facilitates the design and the performance of the grid-forming controllers. However, in distribution applications, it is well known that the R/X ratio of the line impedance is substantially higher, preventing the system from a natural decoupling effect. This paper proposes an original solution to decouple the control and gives an application to a low voltage connection of a grid-forming converter.

Nowadays, the production of electric energy is evolving towards decentralized systems by an increasingly advanced integration of new active loads such as electric vehicles and renewable energy sources. This energy transition involves the use of power electronics converters to regulate energy exchanges. In this context, this paper brings the L2EP knowledge on grid-forming control developed in a high-voltage context to the ValeoSiemens reversible charger. This comprises two significant differences in the sizing of the system: a lower grid connection impedance and a resistive aspect of the distribution network. To overcome these challenges, this paper proposes two techniques: the virtual impedance to compensate the low value of connection impedance and dynamic decoupling of active and reactive power to consider the resistive effect of the distribution network.

This paper introduces a Type-IV wind turbine interfaced to a grid-forming converter. In order to retain the stable operation of a wind turbine in the presence of a grid-forming control, the classical control of a back-to-back converter has to be modified. The modification of this control creates a strong link between a wind turbine and grid dynamics. From the grid side perspective, this link allows provision of the inertial response from a wind turbine during transient events. On the wind turbine side, this coupling causes the appearance of the torsional vibrations within the drivetrain structure. These vibrations are then propagated to the grid as power oscillations. As a result, there is a negative impact on the mechanical components of a wind turbine as well as on the power system operation. In this work, a solution is introduced in order to suppress the undesired vibrations by applying a damping technique to the control of a back-to-back converter combined with a grid-forming control. Based on the conducted analysis, the addition of a damping filter results in the mitigation of torsional vibrations.

This paper proposes the analysis of the frequency support from a Type-IV wind turbine interfaced to the grid via a grid-forming converter. The frequency support including the inertial response and the fast primary frequency support is implemented by integrating a grid-forming control to the control a back-to-back converter. Knowing that the inertial response from a grid-forming converter can be tuned by varying the inertia constant of a grid-forming control, it is possible to extract different amounts of kinetic energy from the rotating masses of a wind turbine and positively impact the grid frequency dynamics during a grid frequency event. However, the increased inertial response may lead to an increased loading imposed on the mechanical components of a wind turbine. To elaborate and verify these statements, the analysis is aimed at two objectives. Firstly, it is estimated through time domain simulations whether the inertial response from a grid-forming control-based wind turbine has a significant active power contribution or can be neglected compared to the share of the primary frequency support. Secondly, based on the analyzed impact of the inertial response on the grid frequency and the mechanical loading, a value of the inertia required from a wind turbine with a grid-forming converter is recommended.

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 question of grid forming control is very different depending on the connection to a low voltage or high voltage grid. In case of higher power application, the low switching frequency may induce some stability issues. This question has been studied and some solutions have been proposed through new inner current and voltage control tuning methods. However, the possible interactions between the inner and the outer controls have not been discussed yet. Actually, in large power system, the phasor modeling approximation is used in order to ease the analysis and reduce the time computations. It assumes a good decoupling between the control loops, which allows neglecting the inner loop dynamics. This paper investigates the effectiveness of this assumption by taking some examples of tuning methods proposed in the literature and showing the ability of each method to guarantee the decoupling between controllers. In this paper, small-signal analysis tool, participation factors and parametric sensitivities are used.

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.

This paper investigates the effect of using phaselocked loop (PLL) on the performance of a grid-forming controlled converter. Usually, a grid-forming controlled converter operates without dedicated PLL. It is shown that in this case, the active power dominant dynamics are highly dependent to the grid short circuit ratio (SCR). In case of using PLL, the obtained results illustrate that the SCR has a negligible effect on the dynamic behavior of the system. Moreover, the power converter will not participate to the frequency regulation anymore; therefore, the converter response time can be adjusted independently to the choice of the droop control gain, which is not possible without PLL. A simple equivalent model is presented which gives a physical explanation of these features.

The insertion of stochastic renewable energies in distribution grids generates important voltage fluctuations and reactive power exchange between distribution grids and transmission grids. These variations need to comply with European grid codes. The control strategy involves the actuation of the On Load Tap Changer and Capacitor Banks tap position, and the reactive powers of the DGs. This paper focuses on the stochastic estimation of the powers at the interface and the influence of the Capacitor Bank tap position. The main results are that, using a simplified stochastic model of the grid, one can obtain the distribution of the powers, and select, on a daily basis, the tap position which minimizes the level of the prescribed PQ diagram violation risk. This study paves the way for a volt-var stochastic optimization of the distribution grid.

Because of the throng of control strategies based Voltage Source Converters (VSC) recently proposed in the literature; their classification and characterization are becoming a trending topic. The high similarities of the proposed control strategies may lead to confusions and a misunderstanding of vocabulary. Therefore, this paper seeks first to highlight the possible features fulfilled by power converters in a large power system. The combination of these features is used to classify power converters. Furthermore, power converters can be seen by a power transmission system operators as black boxes, and they may have the same inputs and outputs, which makes their characterizations more difficult. This paper looks to show that only the fundamental nature of the source has an influence on the system dynamic behavior, thus, power converter can be characterized from their transient behavior in response to grid disturbances.

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.

In the near future, power converters will be massively introduced in transmission grids due to renewable energy sources and high voltage direct current (HVDC) increase. Voltage Source Converter (VSC) control laws assume that Synchronous Generators (SGs) build a stiff AC voltage which allows the synchronization of converters. This is one of the major reasons that limit the high integration of currentsource converters in transmission grid. This constraint is no longer relevant when power converters operate as a voltage source based on the grid-forming concept. This concept uses an inner cascaded PI controllers in order to regulate the output AC voltage. However, it is difficult to tune its controller parameters for stable operation in grid-connected mode. This paper proposes an alternative state-feedback control with integral compensator based linear quadratic regulation (LQR) in order to ensure a stable operation and to get a better AC voltage transient and good decoupling between reactive and active power. The proposed control will be fully analyzed and compared to conventional methods.

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.

The virtual synchronous machine concept (VSM) has been developed initially to reproduce the synchronous machine stabilizing effect by providing inertia with the emulation of swing equation, whereas droop control is developed initially to ensure load sharing and has no inertia. An introduction of a low pass filter to droop control has been motivated to filter the active power measurement and ensures a time decoupling with the inner control loops, whereas, this low-pass filter can also provide inertia to the system. This functionality is limited due to its negative impact on the active power dynamic. This paper proposes an analysis of the conventional droop control by showing its limitations and proposes an improved inertial droop control that allows providing the inertia to the system and ensures a good dynamic behavior of the active power at once in simple manner, and without modifying the load sharing capability. The results obtained are compared to the conventional method (Droop control and VSM) in various topologies in order to show the relevance of the proposed method.

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 insertion of stochastic renewable energies in distribution grids generates important voltage fluctuations. However, the influence of the On Load Tap Changer and Distributed Generators (DGs) controllers, and specifically the existence of dead-bands in the control laws, has been seldom evaluated. Under the assumptions of Gaussian inputs and a linear model of the grid, it is shown that node voltages can be approximated either by Gaussian variables or sums of truncated Gaussian variables. A procedure is necessary to select the Probability Density Function (PDF) which fits best each node voltage. A signal based method and another algorithm relying on the grid topology are presented and compared when the modeling is applied to a real distribution grid. The model is accurate and can be used for confidence level or chance-constrained optimization of control parameters.

The role of Modular Multilevel Converters (MMCs) in HVDC grid greatly differs depending on whether it is an offshore or an onshore station. From the common point in their control schemes, an unexploited ability of the MMC—the controllability of the internally stored energy—is identified in both offshore and onshore applications. The virtual capacitor control, previously proposed by the authors, makes use of this degree of freedom to provide energy contribution to the DC grid. The impact of this control is demonstrated by time-domain simulations of a five-terminal HVDC grid.

The use of DC transmission is particularly advantageous for long-distance transmission and interconnection of asynchronous AC networks. Several converter topologies can be used for HVDC. Multilevel Modular Converters (MMCs) are the most favored given their technological advantages over other converters topologies. Due to their industrial maturity, they have become essential for all AC / DC conversion. So far, they have always been studied with a voltage source on DC side. However, when the converter is equipped with DC breaker, a series inductor is associated to limit current variations. This has consequences in terms of modeling and control determination. This article aims to propose a modification of the control law in order to take into account this inductor. To facilitate the control organization, the Energetic Macroscopic Representation (EMR) is used.

From the origin of the grid, energy has been delivered to electrical loads mainly by synchronous machines. All the main rules to manage the grid have been based on the electromechanical behavior of these machines which have been extensively studied for many years. Due to the increase of HVDC link and renewable energy sources as wind turbine and PV, power converters are massively introduced in the grid with a fundamentally different dynamic behavior. Some years ago, they were connected as simple power injector. Then, they were asked to provide some ancillary services to the grid, in the future, grid forming capability will be required. Even if gridforming converters had been extensively studied for microgrids and offshore grids, it has to be adapted to transmission grid where the topology may be largely modified. This paper presents an algorithm for calculating the controller parameters of a gridforming converter which guarantee a stable behavior for many different configurations of the grid.

The interconnections of offshore wind farms have raised the interest of Multi-Terminal DC (MTDC) grids with voltage source converters, specifically with the Modular Multilevel Converter (MMC) topology. For controlling the DC bus voltage, the droop control strategy proves to be one of the most effective since it allows a shared effort between the different converters connected to the DC grid. Furthermore, these MTDC grids may likely result in multivendor schemes, where each converter could have different control strategies among them. Specially on the way that the internal energy of the MMC is managed, which may have an important impact on the DC voltage dynamics. This paper proposes a simplified model to represent the dynamic behavior of the DC grid including converters which highlights the droop gain and the energy management of the converter as main influential parameters. The performance of this model was assessed by comparisons with EMT simulations on a representative case study with different parameters.

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 deals with a centralized tuning of the local controllers parameters in a distribution grid with many Distributed Generators. The optimal controllers settings are obtained by minimizing the confidence level of voltage specification violations. The confidence level optimization problem uses Gaussian uncertainties of short-term forecasting and OLTC errors along with an accurate linear power flow approximation. Considering these assumptions, the optimization problem is shown to be convex and the characteristics of uncertainties are reduced to their means and standard deviations. The proposed method optimizes the full network while keeping the droop-like industrial structure of the controllers and allows to consider feeders which can have low and high voltages. The merits of the method are illustrated via a modified real distribution network showing a decrease of voltage variances and violations.

The modular multilevel converter (MMC) is the most accepted solution to connect a HVDC grid to an AC transmission grid. The Alternate Arm Converter (AAC) is another promising structure since it allows a DC short-circuits blocking capability similarly to the Full Bridge MMC while having a small impact on the power losses. Its footprint is smaller to the MMC since the needed number of modules is closer to 50% and the SM Capacitors are about three times smaller. The AAC is a hybrid structure between a 2 level VSC converter and an MMC one. Elements hampering the development of the AAC are its complexity to model and control, in particular the opening procedure of the director switches (DS) since these DS are directly connected in series to the arm inductance. This paper proposes a fast method to control the opening of the DS at zero current. The first part is focused on the instantaneous model and current control of the converter and AAC. The second part is focused on the opening method of the DS without generating overvoltage in the converter and taking into account the technical parameters of the various elements of the AAC. Finally, simulation results validate the DS opening control of the AAC converter.

The average value model of the Modular Multilevel Converter (MMC) is in general non-linear with time periodic variables. Recent developments demonstrated how the MMC model can be transformed into a a Steady-State Time Invariant (SSTI) representation allowing for linearization of the model. While previous modeling efforts for small-signal eigenvalue analysis considered mainly the classical Circulating Current Suppressing Controller (CCSC), this paper presents an approach for representing a complete energy-based control system in a set of Synchronously Rotating Frames (SRFs). This is obtained by separating the state variables according the their frequency components and applying corresponding Park transformations. The resulting model is based on existing controllers implemented in the stationary abc frame, and enables small-signal stability studies of MMCs with such control systems. Simulations results comparing an EMT type MMC model with the complete SSTI system validate the proposed approach.

European Grid codes define new network management rules. In order to answer these decrees, it is important to estimate accurately the voltages and powers inside distribution networks. The intermittent nature of renewable sources leads to consider stochastic variables in power flow algorithms. A review of power flow methods and their ability to comply with these requirements is done, which shows that computationally demanding nonlinear methods have to be discarded when dealing with stochastic data and considering limited calculation time. A combination of linear methods is proposed, for which average errors in power and voltage are quite low, when applied to a real-life distribution network. The validity domain of the method is also presented.

The modular multilevel converter (MMC) is one of the available solution to connect a HVDC grid to an AC transmission one. The Alternate Arm Converter (AAC) is a promising structure since it allows a DC short-circuit blocking capability as the Full Bridge MMC while having equivalent losses to the half-bridge MMC. Its footprint is smaller than the MMC since the needed number of modules is closer to 50% and the SM Capacitors are about three times smaller. The AAC is a hybrid structure between a 2 level VSC converter and an MMC one. The main drawback of the AAC is the complexity of its control. This paper presents first, the instantaneous model and current control of the AAC. The second part is focused on the energetic model of the AAC and its control. Finally, simulation results validate the quality of the proposed control.

Modular multilevel converters (MMCs) offer several advantages, such as high scalability and power quality, which are convenient for high-voltage DC transmission systems. The MMC structure contains high number of sub-modules (SMs) per phase that imposes high requirements on the control system. Therefore, it can be implemented in two control levels: the control of the switches that includes the balance of hundreds of voltage on the elementary SMs, the higher level control whose aim is to control the currents, power and energy in the system. This paper focusses on the latter control, which reveals interesting challenges in the management of the MMC stored energy. To illustrate this, the present paper proposes an overview of the different ways of controling the MMC energy. The impact of these energy based control strategies are verifierd by EMTPRV time domain simulation and their comparison highlights that the chosen control may have large influence on the MMC dynamics, MMC losses and exchange of the energy between MMC and DC link.

This paper presents a small signal eigenvalue analysis applied to a droop-controlled HVDC terminal based on the Modular Multilevel Converter (MMC) topology. The applied linearised model is derived from previous modelling efforts recently proposed in the literature, which rely on the application of three Park transformations at different frequencies (w, −2w and 3w) applied to associated variables defined within the MMC model. The investigated configuration is controlled under the well-known Circulating Current Suppression Controller (CCSC). The developed small-signal model is utilized to evaluate two different approaches for calculating the insertion index for modulation of the MMC, and to reveal potential stability problems in the system. It is demonstrated by participation factor analysis that the potentially unstable modes of the system under the investigated control strategy are linked to the uncontrolled zero-sequence component of the common-mode current resulting from the CCSC

Voltage Source Converter-based transmission grids might be the future of power system with the increasing share of interfaced electricity generation. These systems should be able to support the utility grid autonomously. The present paper focuses on the VSC output filter design and internal control of filter states. Nested AC current and voltage loops are the state of the art for power converters, controlled as voltage sources, in Microgrid or UPS applications. These internal controls are candidates for future industrial implementation at high power level. However, in a VSC based transmission system, those controls will face various topologies while requiring robustness against transient events (load changes, line tripping, etc.). The following development aims at providing keys for designing a versatile internal control suitable for VSC-based transmission system.

The multiplication of interfaced generation units connected to the utility grid might form VSC-based transmission grids in a near future. To support the utility grid autonomously, the converters must provide grid-forming capabilities and be connected in parallel. As the structure of a transmission grid is changing and unknown, these requirements presume a robust local voltage control for each highpower VSC, to face various network topologies. Indeed, the stability is challenged when voltagecontrolled VSCs are close to each other, namely, when the tie-impedance is low. The present contribution describes an impedance-based criterion method as a convenient and effective tool for assessing the stability of the voltage control of converters in a VSC-based transmission grid. The method is successfully applied to a typical two-VSC system to anticipate unstable situations

The Modular Multilevel Converter (MMC) is a most promising converter technology for the High Voltage DC application. The complex topology of the MMC requires several additional controllers to balance the energy in the capacitors which are distributed all over the converter. Typically, there is a requirement of two controls; one is the regulation of the total energy in each leg, and the other is the distribution of the energy between the upper and the lower arms. This paper presents control strategies for the latter one being capable of distributing the energy only by internal power flow, so that undesired interference with the associated grids can be completely avoided. The proposed controls are achieved by forcing the common mode currents to be balanced while keeping the classic cascaded control structure as much as possible. The effectiveness and advantage of the proposed solutions are demonstrated by simulations.

The Modular Multilevel Converter (MMC) has enhanced the feasibility of Multi-Terminal DC grids (MTDC). For controlling the DC bus voltage in the MTDC grids, the droop control is the most promised technique. This paper evaluates the dynamic impact of the way of controlling the MMC on the MTDC grids. Results are compared with a simplified model that highlights the key elements for the dynamic behavior of the DC bus voltage, the droop parameter and the equivalent DC bus capacitor.

The complex topology of the Modular Multilevel Converter (MMC) requires some additional controllers to keep its functionalities. One of the important requirements on the MMC control is to balance the energy stored in the distributed capacitors in the arms on the three legs. However, due to the superimposed internal DC and AC power flows in the converter, the energy stored in the arms contains intrinsic oscillations. This paper provides a thorough analysis on those intrinsic oscillations on the internal energy of the MMC. Based on the analysis, a novel analytic filter is proposed, which enables to extract average value of the energy while keeping other internal dynamics stable. The proposed filter is implemented on an EMTP-RV platform. The simulation demonstrates its improved dynamic response and reduction of the internal losses compared to the existing solutions.

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 transmissions. This paper presents an Average Value Model (AVM) of the MMC that still includes the characteristic internal dynamics that are non-existent in traditional 2-level VSCs. This AVM and its control are described and linearized in order to obtain a state-space model of the MMC that can easily be used as a subsystem for multi-terminal HVDC (MTDC) grids. A case study showing a 401-level MMC-based HVDC link simulated in the EMTP-RV software validates the proposed state-space representation of the MMC.

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.

This article deals with power flow coordination of a HVDC grid used to connect offshore wind farms to several mainland grids. The coordination of the DC grid is achieved thanks to a centralized control which monitors and sends proper setpoints to manage power flow. This controller is equipped with a dedicated algorithm which enables to guarantee as much as possible the desired power transfer and cope with wind power forecast errors. Moreover, this HVDC grid controller is used as interface for the AC transmission system operator to redistribute power flow among grid side converter stations in order to de-risk AC contingencies and avoid wind power spillage. Simulations results obtained from an EMT model of a five-terminal MTDC grid with Modular Multilevel Converters prove the effectiveness of the proposed methodology in normal operation as well as the system restoration after a wind farm disconnection.

This article deals with power flow coordination of a HVDC grid used to connect offshore wind farms to several mainland grids. The coordination of the DC grid is achieved thanks to a centralized control which monitors and sends proper setpoints to manage power flow. This controller is equipped with a dedicated algorithm which enables to guarantee as much as possible the desired power transfer and cope with wind power forecast errors. Moreover, this HVDC grid controller is used as interface for the AC transmission system operator to redistribute power flow among grid side converter stations in order to de-risk AC contingencies and avoid wind power spillage. Simulations results obtained from an EMT model of a five-terminal MTDC grid with Modular Multilevel Converters prove the effectiveness of the proposed methodology in normal operation as well as the system restoration after a wind farm disconnection.

This article deals with power flow coordination of a HVDC grid used to connect offshore wind farms to several mainland grids. The coordination of the DC grid is achieved thanks to a centralized control which monitors and sends proper setpoints to manage power flow. This controller is equipped with a dedicated algorithm which enables to guarantee as much as possible the desired power transfer and cope with wind power forecast errors. Moreover, this HVDC grid controller is used as interface for the AC transmission system operator to redistribute power flow among grid side converter stations in order to de-risk AC contingencies and avoid wind power spillage. Simulations results obtained from an EMT model of a five-terminal MTDC grid with Modular Multilevel Converters prove the effectiveness of the proposed methodology in normal operation as well as the system restoration after a wind farm disconnection.

This paper focuses on simplified models of the Modular Multilevel Converter suitable for large-scale dynamic studies, in particular simulations under the phasor approximation. Compared to the existing literature, this paper does not a priori adopt the modeling approach followed for the original twolevel or three-level Voltage Source Converter. On the contrary, a model is derived following a physical analysis that preserves its average internal dynamic behavior. An equivalent control structure is proposed and various alternatives are highlighted. The proposed model with its controllers has been implemented in a phasor simulation software and its response has been validated against a detailed Electromagnetic Transient model. Finally, an illustrative example is presented with the application of the proposed model on a large grid consisting of AC areas interconnected with a multi-terminal DC grid.

The modular multilevel converter (MMC) is the most promising solution to connect HVDC grids to an HVAC one. The installation of new equipment in the HVDC transmission systems requires an economic study where the power losses play an important role. Since the MMC is composed of a high number of semiconductors elements, the losses estimation becomes complex. This paper proposes a simulation-based method for the losses estimation that combines the MMC averaged and instantaneous model in a modular way. The method brings the possibility to compare performances for different modules technologies as well as different high and low level control techniques. The losses characteristics within the MMC are also discussed. The passive losses are taken into account for the first time.

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.

In the near future, transmission grids will include an increasing share of directly connected voltage source converters. These systems should be able to support the utility grid autonomously. The present paper reviews the most popular approaches to address the challenge of proper load sharing amongst multiple parallel inverters applied to the utility grid case. Originally developed for Microgrid or UPS applications, these decentralized controls mimic a conventional synchronous generator based system steady-state behavior and showed suitable effectiveness to reach the desired operating point. However, the operations at the transmission grid level bring additional stress on the converters and require transient capabilities. Physical limitations of semi-conductor based converters fundamentally differ from a conventional synchronous generator. Therefore the paper illustrates how the usual real power controls may lead to undesirable behavior during transient events. This raises new issues for reliable operations of interconnected VSCs at transmission level.

This paper analyses the evolution of reactive power flows at the interface of French transmission and distribution systems. The influence of the insertion of distributed generation (DG), the replacement of medium voltage (MV) overhead lines by underground cables as well as the evolution of the reactive part of the load are investigated. Numerical simulations are performed with a real French HV/MV substation for illustrative purpose. The technical consequences of such evolutions are investigated, e.g. the saturation of On Load Tap Changers (OLTC). The use of DGs connected to the MV network to regulate reactive power exchange between Transmission System Operator (TSO) and Distribution System Operator (DSO) is also discussed.

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. Few of them are addressing explicitly the two different roles that are held by this converter in a HVDC link: controlling the power or controlling the DC voltage level. Most of the time, the DC-bus voltage is supposed to be constant. In an HVDC link, this corresponds to the substation which controls the power. This paper addresses the cases when the voltage is regulated by the converter and presents the different ways of voltage control.

This article deals with the evolution of voltage and reactive power control strategies in distribution networks with distributed generation. First a comprehensive review of voltage control and reactive power management techniques is given. The gap between actual or short-term industrial possibilities of distribution grids and techniques proposed in literature is highlighted. A realistic control based on local rules using the reactive power capabilities of distributed generators, capacitor banks and on load tap changer (OLTC) voltage setpoint is then presented. Its objective is to control both distribution network voltages and reactive power flows at the Transport System Operator (TSO) - Distribution System Operator (DSO) interface. The proposed control allows limiting OLTC bounds and conflicts between local rules. At last, this paper presents simulations results based on French MV distribution network in order to depict the efficiency of such a control.

Modular Multilevel Converters 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. Few of them are addressing explicitly the 2 different roles that are held by this converter in a HVDC link: controlling the power or controlling the DC voltage level. Moreover, for a given function, different ways of controlling this converter may be considered. This paper proposes an overview of the different solutions for controlling the MMC and proposes a methodology to synthesize the control architecture.

This papers deals with the Modular Multilevel Converter (MMC). This structure is a real breakthrough which allows transmitting huge amount of power in DC link. In the last ten years, lots of papers have been written but most of them study some intuitive control algorithms. This paper proposes a formal analysis of MMC model which leads to the design of a control algorithm thanks to the inversion of the model. The Energetic Macroscopic Representation is used for achieving this goal. All the states variables are controlled to manage the energy of the system, avoid some instable operational points and determine clearly all the dynamics of the different loops of the system.

This paper deals with the technical feasibility of a multi-terminal VSC-HVDC (MTDC) grid in the North Sea, connecting several offshore wind farms to several substations of the onshore AC grid. A topology of meshed DC grid with five terminals is chosen. Normal operations as well as fault events are investigated in order to estimate the flexibility and the reliability of the MTDC grid. Therefore, this study will comprise two distinct parts. First, the paper goes through the control performance of the system and then, the protection scheme is discussed. The simulation of the considered model is developed and will be computed using EMTP-RV.

This paper describes a design process of advanced grid equipment including both dynamic simulations and power-hardware-in-the-loop. A case study concerning the insertion of an energy storage system in a large isolated grid is used as an illustrative example. First, the methodology developed to achieve a real-time simulation of the studied grid from its original dynamic model is presented. The real-time platform used within the framework of this project is based on the RT-LAB environment and is interfaced with real ultracapacitors through a power amplifier. Some experimental results are analyzed in the final part. The advantages of this methodology including power-hardware-in-the-loop are emphasized by illustrating its contribution to dynamic model validation and the obtained improvement in control system performances.

To facilitate the increasing penetration of DG, voltage regulation may be required in a near future. This paper therefore presents a local DG voltage regulation. According to the performed simulations, a significant increase in admissible installed capacity of DG is possible. Nevertheless, the impact on existing grid components must be carefully taken into account.

Optimisation du plan de tension d’un système électrique insulaire en présence de production éolienne

Abstract – A Hardware-In-the-Loop (HIL) simulation of a wind energy conversion system has been developed using Energetic Macroscopic Representation. Wind, turbine and mechanical power train are emulated by controlled DC drive to impose the dynamical behavior of the actual process on the generator shaft. In this paper, this HIL simulation is extended to two different wind turbines. The first one leads to extract a rated power of 900 kW using fixed blades. The second turbine leads to extract a rated power of 2 MW using adjustable blades. Experimental results are provided on reduced power experimental set-up for both wind turbines.

Abstract -This paper describes the development of a real-time digital simulation platform for studying the integration of distributed generation in electrical network. A hardware in the loop application with a gas micro turbine is presented. We highlight the methodology used to achieve this new type of simulation and present the real-time performance of the power system simulator.

Abstract-- This paper describes the development of a real-time digital simulation platform for testing the integration of distributed generation in electrical network. A “Hardware in the loop” VSC modelling is presented with an application to a gas micro turbine. We highlight the methodology used to achieve this new type of simulation and present the real-time performance to illustrate the method.

Abstract-- In this paper, we propose to study the possibility of using a photovoltaic system combined with a high speed micro-turbine. This hybrid system can work as stand-alone system or grid connected system as it will be a part of a micro-grid. Initially, we propose simple dynamic models of photovoltaic and micro turbine systems. Then, we carry out a comparison between simulations and measurements of the two systems. At last, simulation results show the effectiveness of the suggested hybrid system.

Abstract. In this paper, we propose to study the possibility of using a photovoltaic system combined with a high speed micro-turbine. This hybrid system can work as stand-alone system or grid connected system as it will be a part of a microgrid. Initially, we propose simple dynamic models of photovoltaic and micro turbine systems. Then, we carry out a comparison between simulations and measurements of the two systems. At last, simulation results show the effectiveness of the suggested hybrid system.

Abstract – For reducing the torque ripple, the nonsinusoidal currents should be injected into the PMSM to compensate for the non-sinusoidal back electromotive force (EMF). The multiple reference frame theory can be applied to tracking the desired currents, while it requires too many computations. In this paper, another approach is proposed by using the multi-frequency resonant controllers in the stationary reference frame. The reference currents deduced from the reference torque are suggested to compensate for the non-sinusoidal back EMF. Thanks to the resonant controllers with adaptive coefficients according to the input current frequency, the regulated currents can perfectly tracking their references with variable frequency. Thereby, the torque ripple in the PMSM can be significantly reduced. The simulated and experimental results demonstrate the validity and efficiency of the proposed approach.

Abstract – The resonant controller has been proved effective for AC input current control in the stationary reference frame. In this paper, we introduce a new type of resonant element. Associating this element in series or in parallel, we can creat the resonant controller for multiple frequencies. By using the pole assignment theory, the design of multi-frequency resonant controller becomes easier than that by frequency domain analysis. A criterion polynomial for this approach is given by this paper, with which the designed controller is efficient and robust to the variation of system parameters. The digital realization of the resonant controller is also discussed, and by recalculating coefficients according to the input frequencies, it can perfectly regulate the AC input with variable frequencies. The validity and effectiveness of the proposed approach are verified by simulation and experimental studies.

As the share of renewable energies increases, it is essential to study its impact on the power systems. A major concern is the frequency dynamics after an event due to the loss of inertia in the system. To support the frequency with converter based generation,the grid-forming control has been proposed. This paper focuses on the application of such control on wind turbines. Thanks to simplified models, the inertial response of grid-forming wind turbine is analyzed. It is concluded that the MPPT control of the wind turbine reduces the inertial effect of the converter. Delaying the MPPT action with a filter can improve the inertial effect. Finally, if the wind turbine operates in the speed limitation region, interactions with synchronous machines may emerge.

Afin de réaliser des études de stabilité des réseaux MTDC, le développement de modèles d’ordre réduit des convertisseurs s’avère nécessaire. Cet article présente un modèle réduit du convertisseur DC/DC modulaire multiniveaux (M2DC) : une topologie de convertisseur DC-DC non isolée attrayante pour le réseau HVDC. Cet article présente d’abord le convertisseur M2DC. Dans un second temps, le modèle réduit sera développé. Le développement de la commande de ce modèle sera effectué dans la troisième partie. Enfin, la comparaison du modèle réduit et de son contrôle avec le modèle moyen des bras sera effectuée dans la dernière section de l’article.

Le convertisseur DC-DC Modulaire Multi-niveaux (M2DC) est une topologie attrayante de convertisseur DC-DC non isolée pour les réseaux haute tension à courant continu (HVDC). Cet article présente dans un premier temps, le modèle et la commande du M2DC. Une étude de l’utilisation des degrés de liberté phares est menée dans un second temps pour réaliser une analyse de leurs impacts sur le dimensionnement des éléments du convertisseur tels que les condensateurs de sous modules ainsi que sur les pertes liées aux semi-conducteurs

Cet article présente la modélisation et la commande basée sur l’inversion de ce modèle, du Convertisseur Modulaire Multiniveaux DC-DC (MMC DC-DC) en demi-ponts. La structure DC-DC MMC présente beaucoup d’avantages tels que sa modularité, l’absence de condensateur sur le bus DC haute tension et une fréquence de commutation très faible étant donnée le grand nombre de SM. Elle conserve aussi les inconvénients intrinsèques du MMC comme la complexité de modélisation [2] et de contrôle [3] dû au grand nombre de semi-conducteurs et de variables d’état à contrôler. La stratégie de contrôle utilise le schéma de contrôle classique avec contrôle d'énergie et contrôle de puissance pour une partie du MMC DC-DC. La seconde partie utilise le contrôle d'énergie et génère la forme d’onde de la tension du bus triphasé AC liant les deux parties du convertisseur. Le contrôle explicite de la génération de l’onde de tension AC permet de garantir le bon fonctionnement du convertisseur même en cas de creux de tension DC critique sur l’un ou l’autre des bus DC et ainsi évite la nécessité de disjoncteur DC ou d’utiliser un MMC Full bridge. La validité du contrôle proposé est vérifiée par simulation à l’aide de Matlab-Simulink.

L'utilisation de la transmission DC est particulièrement avantageuse pour la transmission à longue distance et l'interconnexion des réseaux AC asynchrones. Plusieurs topologies de convertisseur peuvent être utilisées pour le HVDC. Les convertisseurs modulaires multiniveaux (MMC) sont les plus favorisés étant donné leurs avantages technologiques par rapport aux autres topologies de convertisseurs. Du fait de leur maturité industrielle, ils se sont imposés maintenant pour tous les convertisseurs AC/DC à transistors de forte puissance. Jusqu'ici, ils ont toujours été étudiés avec une source de tension côté DC. Or, lorsqu'ils sont équipés de DC breaker, on associe une inductance en série pour limiter les variations de courant. Ceci a des conséquences en terme de modélisation puis de détermination de la commande. Cet article a pour objectif de proposer une modification de commande afin de prendre en compte cette inductance.

Le Convertisseur Modulaire Multiniveaux (MMC) est une structure d’électronique de puissance utilisée dans des applications de variation de vitesse des machines électrique haute tension mais aussi des applications de transport de l’électricité à très haute tension et courant continu. La structure MMC présente beaucoup d’avantages tels que sa modularité, l’absence de bus DC haute tension et une fréquence de commutation très faible étant donnée le grand nombre de SM. Elle présente aussi des inconvénients comme la complexité de modélisation [2] et de contrôle [3] dû au grand nombre de semi-conducteurs à contrôler. Cet article a pour objectifs de présenter le dimensionnement d’un convertisseur MMC de laboratoire ainsi que son système de contrôle, le plus réaliste possible d’une structure échelle une, avec un grand nombre de SM. Le dimensionnement de ce dernier prendra en compte les contraintes et les caractéristiques son fonctionnement dans un réseau HVDC 640kV 1GW. Une architecture de contrôle, contraintes par le nombre de de sous module, sera présentée. Les protocoles de validation des sous modules, d’un demi-bras puis du convertisseur seront présentés.

Le système étudié dans cet article est un convertisseur Modulaires Multi-Niveaux. Dans une première partie, l’utilisation de la Représentation Energétique Macroscopique (REM) permet de mettre en évidence les couplages importants qui existent au sein de ce système. L’inversion du modèle conduit à une architecture générale de la commande amenant à avoir autant de correcteurs que les variables d’état. Avec la solution proposée, il est possible de contrôler chaque tension de condensateur équivalent. Dans une seconde partie, la méthodologie permettant de déterminer le diagramme PQ du convertisseur MMC est présentée. Quelques points de fonctionnement aux limites de ce diagramme sont validés par simulation. La maitrise des dynamiques ainsi que la connaissance du diagramme PQ sont des étapes nécessaires pour l’intégration de ce type de convertisseur dans un réseau alternatif.

This paper deals with small signal stability analysis of Voltage Source Converter based Multitermial HVDC grids (VSC-MTDC). First, a simplified model is used to predict the voltage dynamic behavior. Next, a modular method to build the state space model of the overall MTDC system is described. Model validation is achieved in time domain by a comparison with an electromagnetic simulation in EMTP-rv. As concrete example of effective application, a parametric study on the droop value is carried on a 3-terminal meshed HVDC grid. This example attests to the effect of the droop parameter on the stability of the overall system.

L’apprentissage de l’Electronique de Puissance (EP), domaine essentiel à la réalisation de systèmes électriques efficaces, est une tâche délicate, surtout pour un ingénieur généraliste. Dans cet article, un projet étudiant, dont le but est d’exploiter l’énergie solaire afin de réduire les émissions en CO2 de véhicules automobiles, est décrit. Il expose l’approche scientifique, les résultats et comment une telle activité permet un apprentissage actif et efficace de l’EP.

In the context of increasing demand for green energy, offshore wind plants, particularly in the North Sea, are a good means to meet the challenge and reach 20 % of renewable energies by 2020 in Europe. Since these wind turbines will be located quite far from the shore; DC links may be needed to transfer the power to the main grid. Instead of building several independent DC links in the same region, for economic reasons, it seems more advantageous to con- nect these DC links together and create a meshed DC grid among different countries. This idea is attractive but a lot of technical challenges have to be overcome. Some solutions may be found in the latest technological development, especially with the evolution of power electronics. However, all the solutions are not found yet. In this paper, we are presenting the state of the art of Voltage Source Converter (VSC) based HVDC grid technology. The generic structure of Modular Multilevel Converter (MMC) is presented. The basis of the control methods for a point- to-point link is first analyzed and then extended to a meshed DC grid. We are also presenting the major breakthroughs still to be done in terms of management and protection of the DC grid.