This paper presents a decoupled power control strategy for a modular multilevel converter (MMC)-based hybrid ac-dc grid integrated with a hybrid energy storage system (HESS). This system can mitigate the active power fluctuation caused by intermittent renewable generation and also realize reactive power compensation as required by voltage regulation. The proposed hybrid ac-dc system is novel in that batteries and supercapacitors are distributed into the upper arm and lower arm of MMC, respectively. It is therefore possible to design fully decoupled power control and simultaneously optimize the performances of batteries and supercapacitors. Firstly, the system modeling and operational principles of MMC are briefly introduced in this paper. Based on these analyses, the power decoupling between upper arm and lower arm is presented to realize different control objectives. Moreover, battery state-of-charge (SOC) balancing control and supercapacitor voltage control are presented also, which are crucial for ensuring the normal operation of these energy storage components and the proposed MMC-based hybrid ac-dc grid. Finally, the proposed system and its control strategies are verified by laboratory experimental results.

Repetitive control strategies have been commonly applied in pulse-width-modulated (PWM) voltage source inverters (VSIs) for many industrial applications. This paper presents a repetitive controller for voltage harmonic mitigation of VSI-based islanded microgrids. The phase delay in the overall control system, e.g., the delay caused by the digital duty cycle calculation, PWM generation, and repetitive controller, has to be compensated by elaborately designed phase lead filters in order to prevent control performance deterioration and system instability. Nevertheless, quantificational analysis and practical design of the time advance unit in such filters are hardly found in existing literature works. In view of this, this paper proposes an explicit analysis of the phase lead filters and a novel design method of the time advance unit in repetitive controllers to ensure system stability. Moreover, with the help of the proposed method, the overall system stability margin is predictable and improved controller performance is achieved as well. The proposed method is implemented experimentally to show the accurate stability margin calculation as well as the excellent steady state and dynamic performances of the repetitive control scheme.

This paper proposed a novel battery energy storage system based on modular multilevel converter (MMC), which has several merits compared with two-level and cascaded multilevel battery energy storage systems. The proposed system can manage the state-of-charges (SOCs) of all batteries to be equal to avoid the overcharge or over discharge of single battery stack as traditional. Besides, the inherent power exchange characteristics using circulating current could increase the control flexibility for SOC balancing, and improve output waveform quality, maximize storage capacity and reduce internal losses. The key issues regarding injected dc current control, SOC balancing control and circulating current control are discussed in this paper. Finally, the proposed system were verified through Matlab/Simulink simulation and a scaled down experiment prototype.

The modular multilevel converter (MMC) has become the most attractive converter technology for medium/high-power applications, specifically for high-voltage dc (HVdc) transmission systems. One of the technical challenges associated with the operation and control of the MMC-based system is to precharge the submodule (SM) capacitors to their nominal voltage during the startup process. In this paper, considering various SM circuits, a generalized precharging strategy is proposed for the MMC-based systems, which can implement soft startup from the dc or ac side. The proposed precharging strategy can be applicable for various SM circuits and MMC configurations. The proposed startup strategy does not require extra measurements and/or auxiliary power supplies. The charging current is controlled by adjusting the changing rate of the number of blocked and bypassed SM capacitors. Based on the proposed startup strategy, the startup processes of MMC/MMC-HVdc systems with various SM circuits are analyzed and a generalized startup procedure for various MMC-HVdc systems is proposed. In addition, the uncontrollable steady-state SM capacitor voltages of various MMC-based systems are analyzed and the associated precharging time is also investigated. Performance of the proposed strategy for various MMC-HVdc systems is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment and experimental results are based on a scaled-down prototype.

This paper investigates and compares various modulation methods and capacitor voltage-balancing algorithms of modular multilevel converter (MMC) for solid-state transformer (SST) applications. Characteristics of capacitor charging and discharging are analyzed for the existing single-step alternating voltage-balancing and the conventional sorting algorithms with phase-shift (PS) modulation and nearest-level control (NLC) methods. Based on the analysis, the single-step alternating algorithm leads to a low-frequency voltage ripple while the conventional sorting algorithm introduces additional switching actions. To address these issues, two new voltage-balancing algorithms are proposed, i.e., a multi-step alternating voltage-balancing algorithm and a current-less sorting algorithm. The performance of the proposed algorithms with the PS and NLC modulation methods are comprehensively investigated and compared based on theoretically analysis and experimental results. The study results demonstrate the capability of the proposed methods to balance capacitor voltages while reducing the low-frequency voltage ripple and avoiding additional switching actions.