Renewable Energy Sources (RESs) are growing rapidly and highly penetrated in Microgrids (MGs). As a result of replacement of the synchronous generators with a large amount of RESs, the overall system inertia might be dramatically reduced which negatively affected the MG dynamics and performance in face of uncertainties, leading to weakening of the MG stability, which considers being a serious challenge in such grids. Therefore, in order to cope with this challenge and benefit from a maximum capacity of the RESs, robust control strategy must be applied. Hence, in this paper, a new application of robust virtual inertia control-based Coefficient Diagram Method (CDM) controller is proposed in an islanded MG considering high-level RESs penetration for enhancement the system’s validity and robustness in face of disturbances and parametric uncertainties. The proposed controller’s proficiency has been checked and compared with H-infinite controller using MATLAB/Simulink which approved that the CDM controller achieved superior dynamic responses in terms of accurate reference frequency tracking and disturbance reduction over H-infinite in all test scenarios. Thus, the proposed controller alleviates the difficulties of H-infinite controller such as the experience and necessary abilities to design the form of the weighting functions for the system. Consequently, the frequency stability is improved and approved that the proposed CDM based virtual inertia controller can significantly support a low-inertia islanded microgrid against RESs and load fluctuations.

Harmonic state space (HSS) is seen as an effective impedance modeling method to precisely characterize the internal harmonic features of the modular multilevel converter (MMC). However, the existing MMC impedance models assume the ideal grid, ignoring the grid impedance, and they also do not incorporate the widely used dual loop control and phase locked loop (PLL). In this paper, a complete MMC impedance model based on HSS is proposed to reveal the grid impedance coupling effect of MMC. The model analysis results demonstrate that the MMC impedance is coupled with the grid impedance due to the internal harmonics. This coupling causes MMC to be affected by the grid impedance and may cause instability. On the other hand, the proposed model not only consists of the ac current and the circulating current control, but also incorporates dc voltage outer loop and PLL with a clear physical meaning. Based on this, the paper illustrates the factors that will enhance the coupling, which shows that the proposed model has to be used to improve the accuracy of the analysis. Finally, effectiveness of the proposed model is verified by simulation and experimental results.

Bipolar LVdc distribution is a promising solution for better integration of renewables and enhancement of system efficiency and reliability, but it also faces the challenge of pole voltage imbalance. By simple modifications of the bidirectional CLLC dc/dc converter, this article presents a dc/dc converter with integrated pole voltage balance capability. The converter presents less number of semiconductors, smaller overall footprint, and lower power loss compared to the employment of additional pole voltage balancers. Operating principle of this dc/dc converter and its inherent pole voltage balancing capability are explained, while further control scheme is also developed to compensate the small pole voltage difference caused by the dead time effect. Simulation results with 150 kW/±375 V bipolar output demonstrate the effectiveness of the converter, and the experimental results from a 500 W/±50 V downscaled prototype further prove the proposed concept.

The modular multilevel converter (MMC) has attracted extensive research in recent years. An appropriate model is necessary to analyze stability or to design MMC controllers. Several published MMC models have been derived in single-phase form to simplify the modeling mathematics. However, little attention is given to the zero-sequence voltage, which introduces coupling in the single-phase model and leads to significant error. In this paper, after revealing the mechanism behind the zero-sequence voltage, a more accurate generalized single-phase MMC model is proposed, which eliminates the zero-sequence voltage coupling effect and is linearized based on harmonic state space (HSS) theory to precisely characterize the internal harmonic features of MMC. A systematic HSS modeling process is presented for both open-loop and closed-loop conditions. And the proposed model is generalized as it can incorporate different control strategy by controller transfer function substitution. Hence it is valuable to analyze MMC stability and dynamics. Model effectiveness is verified by simulation results.

As the state-of-the-art voltage-source topology, the modular multilevel converter (MMC) is attractive for various medium- or high-power applications. In MMC, reliability and uninterruptable operation are always seen as primary concerns, which requires the MMC can continue operating even though some of the SMs are failed. In this paper, a novel fault-tolerant modulation and control strategy is proposed for MMC in medium voltage applications. Compared with previous works, the proposed strategy provides optimum performances under healthy conditions and minimum performance degradation after SM failures. During healthy conditions, the redundant SMs are fully utilized to decrease SM capacitor voltage for higher reliability and to reduce switching frequency for higher efficiency. While during fault-tolerant operation, by appropriate modification of the modulation and controllers, no extra healthy SMs need to be bypassed and the output voltage harmonics of MMC remain unchanged without causing voltage mismatches. Feasibility and effectiveness of the proposed strategy have been verified by simulations in MATLAB/Simulink software and experiments on a downscaled three-phase MMC prototype.

Circulating current is an undesirable phenomenon in modular multilevel converter which should be attenuated to reduce current stress and power losses. This letter presents a comparative study of the active and passive circulating current suppression methods. Voltage penalty of the active circulating current control is derived, and design of the passive circulating current filter parameters is provided. Simulations and experiments are performed to validate the analysis.