is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. This is an author-deposited version published in: https://sam.ensam.eu Handle IDAbstract-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.

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Abstract-The DC-side dynamics of Modular Multilevel Converters (MMCs) can be prone to poorly damped oscillations or stability problems when the second harmonic components of the arm currents are mitigated by a Circulating Current Suppression Controller (CCSC). This paper demonstrates that the source of these oscillations is the uncontrolled interaction of the DC-side current and the internally stored energy of the MMC, as resulting from the CCSC. Stable operation and improved performance of the MMC control system can be ensured by introducing closed loop control of the energy and the DC-side current. The presented analysis relies on a detailed state-space model of the MMC which is formulated to obtain constant variables in steady state. The resulting state-space equations can be linearized to achieve a Linear Time Invariant (LTI) model, allowing for eigenvalue analysis of the small-signal dynamics of the MMC. Participation factor analysis is utilized to identify the source of the poorly damped DC-side oscillations, and indicates the suitability of introducing control of the internal capacitor voltage or the corresponding stored energy. An MMC connected to a DC power source with an equivalent capacitance, and operated with DC voltage droop in the active power flow control, is used as an example for the presented analysis. The developed small-signal models and the improvement in small-signal dynamics achieved by introducing control of the internally stored energy are verified by time-domain simulations in comparison to an EMT simulation model of an MMC with 400 sub-modules per arm.

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.

This article deals with DC voltage dynamics of Multi-Terminal HVDC grids (MTDC) with energy-based controlled Modular 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, causes large and undesired DC voltage transient after a sudden power flow change. This occurs when this energy is controlled to a fixed value regardless of the DC voltage level. Second, the Virtual Capacitor Control technique is implemented in order to improve the results. However, its limitations on droop-based MTDC grids are highlighted. Finally, a novel energy management approach is proposed to improve the performance of the later method. These studies are performed with detailed MMC models suitable for the use of linear analysis techniques. The derived MTDC models are validated against time-domain simulations using detailed EMT MMC models with 400 sub-modules per arm.

This paper describes the characteristics of a series parallel wind farm (SPWF) topology and investigates the control strategy to ensure its safe operation. The SPWF was found to have advantages over other pure dc wind farm architectures in terms of lower construction cost and lower power losses in the collection system. However, unbalance power productions among the wind turbines cause the variations of their output voltages, which may endanger the safe operation of the entire wind farm. This paper proposes a global control strategy that prevents wind turbines from operating above their overvoltage capabilities. With an active participation of the onshore converter, the proposed strategy allows maximum power point tracking (MPPT) of the wind turbines. The practical limitations of this strategy are discussed and improvements are given. The feasibility of the proposed method is validated in a simulation of 300 MW wind farm developed in EMTP-RV.

is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. AbstractThis 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.

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.