Transmission system operators (TSOs) are increasingly requesting frequency support, reactive power compensation, and grid-forming capability to power plants interfaced by power converters. In these plants, a large number of converters is operated in parallel and all of them should be adequately coordinated to provide these services. For that purpose, the virtual synchronous machine (VSM) control technique is commonly adopted. However, the dynamic characteristics of the whole plant depend on the number of converters connected, their size, control parameters, and electrical interconnections. This makes it difficult to know the aggregated dynamic characteristic of the plant and how each VSM contributes to the plant response. In this article, the operation of VSMs in parallel configuration for island and grid-connected applications is studied. The developments make it possible to adapt the VSMs parameters so that the grid sees the power plant as an equivalent generator whose low-frequency characteristics do not vary with the number of converters in operation. The developments were validated in a laboratory consisting of four 15 kVA converters operating as VSMs, and a grid emulator.

LCL filters are commonly used to connect Voltage Sourced Converters (VSCs) to the grid. This type of filter is cheaper than a single inductor for the same current THD, but it generates resonance problems if no active or passive damping method is applied. Active damping methods are becoming popular in the literature because they improve efficiency, but they are sometimes difficult to implement and additional measurements are required. This paper proposes an active damping method for VSCs connected to weak grids that is based on making zero the open-loop phase at the resonance frequency. It will be shown that this strategy provides adequate damping of oscillations and that it can be achieved in two different ways: at the design stage (if the design constraints make it possible) or with an all-pass filter in series with the current controller. Two methods to design the all-pass filter are proposed. Also, the proposed active damping technique is compared with three alternatives already proposed in the literature. All the control algorithms are verified by simulation and in a 15 kW prototype of a three-phase VSC connected to a configurable weak grid via a LCL filter.

This work presents a comparison of three different modulation techniques applied to modular multilevel converters (MMCs). The three modulation strategies studied in this paper are the phase-shifted sinusoidal pulse width modulation (PS-SPWM), the space-vector modulation (SVM) and the nearest level modulation (NLM). This paper focuses on analysing the particularities and implementation of each modulation technique. The modulation technique largely defines the generated harmonic content, making this is a key point that must be studied in depth. The paper briefly describes the three modulation techniques and analyses the harmonics generated by each one of the methods. In addition, the paper presents and compares the digital implementation of the three modulation methods in a Field Programmable Gate Array (FPGA). The proposed approaches are validated using a real processing platform and experimentally evaluated in a real high-power six-level MMC.

In recent years, there has been a growing interest in using model-free deep reinforcement learning (DRL)-based controllers as an alternative approach to improve the dynamic behavior, efficiency, and other aspects of DC–DC power electronic converters, which are traditionally controlled based on small signal models. These conventional controllers often fail to self-adapt to various uncertainties and disturbances. This paper presents a design methodology using proximal policy optimization (PPO), a widely recognized and efficient DRL algorithm, to make near-optimal decisions for real buck converters operating in both continuous conduction mode (CCM) and discontinuous conduction mode (DCM) while handling resistive and inductive loads. Challenges associated with delays in real-time systems are identified. Key innovations include a chattering-reduction reward function, engineering of input features, and optimization of neural network architecture, which improve voltage regulation, ensure smoother operation, and optimize the computational cost of the neural network. The experimental and simulation results demonstrate the robustness and efficiency of the controller in real scenarios. The findings are believed to make significant contributions to the application of DRL controllers in real-time scenarios, providing guidelines and a starting point for designing controllers using the same method in this or other power electronic converter topologies.

This article presents a systematic robust current control design approach for three-phase voltage-source converters. Robustness is guaranteed by combining intrinsic passive properties of the impedance uncertainty at the point of common coupling together with stability results from a passivity-based control theory. This approach ensures stability against typical uncertainty sources at mid and high frequencies, such as cable resonances or other converters interaction, with significant less conservative performances than the obtained with the traditional robust control theory. The approach uses multiobjective controller synthesis formulation that allows us to logically combine robustness requirements with performance objectives avoiding heuristic iteration over the control structure and parameters. The controller synthesis is performed by means of a nonsmooth $\mathcal {H}_{\infty }$ optimization technique that tunes all free parameters of a vector-based controller function, which constraints its structure. This results in a synthesized controller with lower order than those obtained with convex optimization definitions of the $\mathcal {H}_{\infty }$ control problem. The design methodology is validated in time and frequency domain by means of theoretical analysis and experimental results with three usual grid filters: L , LCL, and LLCL .

LCL-filter-based grid-tie inverters require damping for the current-loop stability. There are only software modifications in active damping, whereas resistors are added in passive damping. Although passive damping incurs in additional losses, it is widely used because of its simplicity. This paper considers the active damping in medium-voltage parallel inverters for wind turbines. Due to cost reasons, only minimal software changes are allowed and no extra sensors can be used. The procedure must be robust against line-inductance variations in weak grids. Double-update mode is needed so the resonance frequency is under the Nyquist limit. The bandwidth reduction when using active damping is also required to be known beforehand. Moreover, the design procedure should be simple without requiring numerous trial-and-error iterations. In spite of the abundant literature, the options are limited under these circumstances. Filter-based solutions are appropriate and a new procedure for tuning the notch filter is proposed. However, this procedure requires that the resistance of the inductors is known and a novel filter-based solution is proposed that uses lag filters. The lag filters displace the phase angle at the resonance frequency so that the Nyquist stability criterion is fulfilled. Simulations and experiments with a 100-kVA prototype validate the analysis.