For modular multilevel converters (MMCs) applied to medium-voltage dc distribution grids, using the traditional nearest level modulation (NLM) as in HVdc systems can lead to severe current distortion due to significantly reduced module numbers. This paper proposes a hybrid modulation method combining NLM and pulse width modulation (PWM) where only one module per arm operates under PWM mode. The proposed nearest level PWM (NL-PWM) method not only significantly reduces the current distortion, but also avoids the complicated voltage balancing control in each module. The harmonic characteristics of NL-PWM are derived using double Fourier transform, which provides a theoretical basis for selecting the module number and switching frequency for medium-voltage applications in accordance with grid harmonic requirements. Finally, the harmonic characteristics and feasibility of the proposed modulation method are validated by simulation and experimental studies on an MMC with six modules per arm. The simulated and experimental results reveal that NL-PWM has better voltage and current harmonic characteristics over NLM and CPS-PWM, thereby suiting the application of MMC with few models.

An integrated controller of wind turbines with both inertial response and primary frequency regulation (PFR) to provide complete dynamic frequency support for the grid with high wind power penetration is investigated. The wind turbine control governor contains two cross‐coupled controllers: pitch controller and maximum power point tracking (MPPT) controller. First, as a precondition for the PFR, a de‐loading pitch control scheme is proposed to reserve capacity required for frequency regulation. Then, by optimizing the MPPT scheme, the rapid virtual inertia response is achieved even under de‐loading operation condition. Based on the analysis of the steady‐state characteristics of wind turbines with frequency droop control, the primary frequency control strategy, which enables the adjustment of frequency droop coefficient, is further proposed through pitch angle changes. Thus, the PFR and inertial response can be both achieved by the proposed de‐loading pitch controller and optimized MPPT controller. A three‐machine prototype system containing two synchronous generators and a Doubly Fed Induction Generator (DFIG)‐based wind turbine with 30% of wind penetration is implemented to validate the proposed integrated control strategies on providing inertial response and subsequent load sharing in the event of frequency change.

In a DC grid, the inherent inertial support from the DC capacitors is too small to resist step changes or random fluctuations from the intermittent power resources, which results in lower DC voltage quality. In this paper, an adaptive droop control (ADC) strategy is proposed to achieve an increased inertia from the droop controlled converter. The adaptable droop coefficient according to the DC voltage variation enables fast swing of the droop curve, so that the converter can provide inertial power for the DC grid like synchronous generators in AC grids. The design of the ADC including the calculation and limitation of the adaptable droop coefficient is analyzed in detail. The small-signal analysis of the DC grid with ADC is provided to identify its stability issue. Experimental tests on a controller hardware-in-the-loop (HIL) platform of a low-voltage (LV) DC grid are carried out to validate the proposed method. In this LV DC grid, the proposed ADC is implemented on the energy storage system (ESS) which provides inertial support to improve the DC voltage quality under different power fluctuations, and smooths the power transmitted to AC grid.

Virtual synchronous generator (VSG) control strategies have been widely researched and applied in grid-forming converters for utility grid inertia support under high level penetrations of renewable energies. Traditional VSG control encounters coupling issues between active and reactive power loops, especially in low voltage resistive feeder line conditions. This power coupling effect degrades not only the effectiveness of inertia support during transients, but also the system stability. In this article, the coupling mechanism between active and reactive power loops is analyzed and a full-state feedback control method is proposed to release the coupling issue in traditional VSG control. In the proposed control, the state feedback matrix is designed to eliminate the power coupling between active and reactive power control loops, and fully retain the capability for active and reactive inertia response. Disturbance injection is applied to avoid dependence on line parameters. Root locus analysis shows that the decoupled power control endows system with better stability and damping performance. Hardware experimental results indicate that the power decoupling VSG significantly releases the coupling between active and reactive power, contributing to better system stability and providing improved transient power inertia response to power grid.

Permanent magnet synchronous generators (PMSGs) equipped with back‐to‐back grid‐forming converters have the ability to form a 100% renewable energy power grid. Black‐start is the key capability of grid‐forming converters when restoring the system from a blackout. It is necessary for a grid‐forming converter of the PMSG to operate in black‐start and grid‐connected active support modes. This paper investigates an improved damping adaptive grid‐forming (DA‐GFM) control method for PMSG wind turbines to suppress system frequency fluctuations and achieve a smooth black start. The proposed DA‐GFM control introduces the frequency variations and natural constant function to adaptively adjust its damping coefficient in the dynamic operating progress. The implementations and effects of the proposed DA‐GFM control method on the grid‐forming converter are theoretically evaluated. Additionally, the detailed black‐start operation process is completely studied through a microgrid system with two PMSG wind turbines and energy storages. Compared with the conventional grid‐forming control, the proposed control scheme can not only suppress frequency fluctuations better, but also achieve a smooth forming power grid process. Finally, experiment tests on a controller‐level hardware‐in‐the‐loop simulation platform are carried out to validate the proposed control strategy for achieving a smooth black start.

The compact design of the modular multilevel converter (MMC) has drawn great attention due to the miniaturization requirement of the distant offshore wind power integration system. Only 50% submodules (SMs) in the phase leg are inserted at any instant, which results in lots of redundant SMs in the conventional MMC. Combining modular structure with device series switches can effectively reduce the number of modules by improving their utilization. However, a fixed modulation ratio is necessary for some compact topologies to balance arm energy, and additional complicated control algorithms are required to obtain a limited voltage regulation range. This article develops an arm multiplexing MMC (AM-MMC) with multiple arms, in which the SMs in the middle arms are assigned to operate in time-division multiplexing (TDM) modes. A multiplexed nearest level modulation method applied to the AM-MMC is presented to achieve full-range voltage regulation without additional energy balance control. To ensure reliable operation of TDM modes, the structure design and zero-voltage switching scheme of arm selection switches are given in detail. The feasibility and performance of the developed AM-MMC has been demonstrated on a small-scale 150 V/2.75 kW experimental prototype and an 85 MVA/±35 kV AM-MMC high-voltage direct current simulation system based on MATLAB/Simulink.