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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.

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

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

The modular multilevel converter (MMC) 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.

This study presents control strategies of offshore wind farms with dc series collector systems and modular multilevel converter based high voltage direct current (MMC-HVDC) transmissions. Series connection of dc wind turbines enables the collector system to establish the HVDC transmission voltage level with neither transformer nor centralized converter platform, indicating huge savings on offshore wind farm investment. However, severe wind variation imposes overvoltage on the wind turbines and may induce a cascade of failures of other series connected units. This paper first explains the wind farm design constrains bound by overvoltage limitation of wind turbine units. Then two kinds of control strategies, classical control and proposed HVDC global control, are presented to ensure every unit operating within safety limit. Wind farms with both control strategies are simulated. Finally, the two kinds of strategies are compared in terms of technical feasibilities, economical benefits and annual energy production rates. The results demonstrate that DC series offshore wind farms with proposed HVDC global control strategy imply competitive economic advantages of future integration of remote energy generation.

to ensure the operation of each wind turbine under a safe voltage level while no power production is lost in the wind farm. This paper first introduces the whole wind generation system. Then the steady state analysis of the dc series offshore wind farm and the overvoltage phenomenon are described, which lead to the proposal of the global control strategy. The detailed realization of the global control strategy by use of the MMC is given after a short recall of the MMC arm average model.

The exploitation of the abundant offshore wind energy far from the coast relies on the High Voltage Direct Current (HVDC) transmission which requires a dedicated offshore substation for converting power from AC into DC. The investment on the bulky offshore platform is high and its installation is complicated. This thesis deals with a pure DC offshore wind farm topology which exports its energy to onshore without using an offshore centralized power conversion substation. This wind farm is called Series Parallel Wind Farm (SPWF), which is comprised of several clusters of wind turbines interconnected in series, so that the output converters of the wind turbines step up the voltage to be high enough for direct power transmission. However, the series connection indicates that the independent operation of wind turbines becomes impossible. Unbalanced power production of wind turbines due to uneven wind speed distributed in the wind farm leads to wind turbines output voltage variation. The overvoltage limitation control strategy is then developed to ensure safe operation of the series connected wind turbines. It is first validated by being applied to a Point-to-Point (P2P) system using Multilevel Modular Converter (MMC) based HVDC transmission. After that, the SPWF is integrated into different Multi-Terminal DC (MTDC) systems. The results validate the feasibility of proposed strategy, and show its advantage that no power curtailment is required to limit the wind turbines output voltage.