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Multiple insulated gate bipolar transistor (IGBT) open-circuit faults severely affect the reliable operation of modular multilevel converters (MMCs). The existing literatures have not provided an effective diagnosis algorithm for MMCs when multiple IGBT open-circuit faults simultaneously appear in one arm. This article presents a diagnosis strategy to handle that condition. Fault detection and estimation of the faulty submodule (SM) number is implemented by checking the value of the arm voltage error. The faulty SMs are located by observing when the transition of their switching functions leads to an obvious change of the arm voltage error. Meanwhile, the errors between the measured and predicted capacitor voltages are utilized to accelerate the localization. All of the faulty SMs can be quickly located one by one within several control cycles by using the proposed localization method. Simulation and experimental results demonstrate the validity of the proposed diagnosis strategy. [ABSTRACT FROM AUTHOR]

This article first analyzes the effect of the level-increased nearest level modulation (NLM) to the circulating current of the modular multilevel converter (MMC). The total inserted submodule (SM) number in one phase-leg of the MMC is variable using the level-increased NLM, which can significantly increase the peak-to-peak value of the circulating harmonic currents. Then, a circulating harmonic currents suppression method is proposed by applying deadbeat control for the MMC with level-increased NLM. Compared with the existing direct circulating current control approaches for the MMC, the proposed method presents specific extension and adjustment principle of the total inserted SM number. Therefore, the proposed method can improve the dynamic control performance and handle large circulating harmonic currents at steady state. And it can well coordinate the modulation and circulating harmonic currents suppression stages to avoid disturbing the ac-side output voltage while regulating the circulating current. The effectiveness of the proposed method is evaluated by a single-phase industrial-level simulated MMC system and a laboratory experimental platform. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is a popular research topic in power electronics and is a promising topology for high-power applications. Many steady-state analysis models have been investigated for MMCs. The circular interaction among the electrical quantities in an MMC has been established and improved models have been proposed, with consideration of the second-order circulating current controller and line harmonic currents in the analysis. So far, no steady-state analysis models have addressed the effect of averaging voltage regulation control, which influences switching function of MMC upper and lower arms, and finally, the accuracy of steady-state analysis. In this article, we discuss the validity of existing models with and without averaging control. After extensive simulations, it is found that the accuracy is affected, showing the weakness of existing models and verifying the effect of averaging voltage regulation control on steady-state analysis of MMCs. Then, to solve this problem, an improved analysis model is proposed. In this model, averaging control is considered at the beginning of circular-interaction-based analysis. The new model presents a higher accuracy compared with the existing ones. [ABSTRACT FROM AUTHOR]

The power electronic transformer (PET) or solid-state transformer (SST) is a key technique in the future distribution grid. One of PET's future possible application is the soft normally open points (SNOP) to handle the power and voltage challenge brought by the renewable energy sources. In this paper, a PET-based three-terminal medium-voltage SNOP topology is proposed. The topology consists of three cascade H-bridge (CHB) stages and a multiactive bridge (MAB) stage. The dc voltage control strategy for the proposed PET-SNOP is studied. Two strategies – the CHB+CHB+CHB strategy and CHB+MAB strategies are developed from the existing strategies but they both have some drawbacks. In order to overcome the drawbacks, a novel dc voltage control strategy—CHB×3+MAB strategy is proposed, which combines the advantages and avoids the drawbacks of these two strategies, moreover, it also brings more flexibility and robustness to the dc voltage control. The effectiveness of the proposed PET-SNOP topology as well as the proposed CHB×3+MAB dc voltage control strategy is verified by both simulation and experiment results. [ABSTRACT FROM AUTHOR]

The solid-state transformer (SST) has the ability to operate with only two input phases, which can be called single-phase open-circuit (SPO) operation. With this ability, the SST can maintain operation under permanent single-phase-to-ground faults, where the fault phase must be isolated, providing a better power supply sustainability than that of the conventional low-frequency transformer. In the absence of research on the SPO operation of an SST, this paper presents a study of it. The influence of SPO operation on the grid and the SST itself is analyzed. Under the SPO state, the SST will inject the negative-sequence current to the grid; meanwhile, the maximum load power will decline; thus, the optimized power factor under the SPO state is unity. The maximum power of △-SST is higher than that of the Y-SST under the SPO state owing to the △ connection. A control strategy for the SST medium-voltage cascaded H-bridge stage is proposed. SPO current control can be achieved by controlling the positive-sequence current. A proper negative-sequence voltage can be injected to synchronize the cluster voltages to obtain a lower current total harmonic distortion and smaller dc voltage ripple. In the △-SST, a proper zero-sequence current can be injected to fully use the power rating of the double-cluster branch. [ABSTRACT FROM AUTHOR]

It is considerable to reduce the computation load while keeping a good total harmonic distortion (THD) performance for the modular multilevel converter (MMC) with model predictive control (MPC). In this paper, the discrete-time mathematical model of the MMC is derived and the output voltage ripples of the cascaded submodules (SMs) are analyzed in detail. Accordingly, a modified increased-level MPC method A (IL-MPC-A) is proposed with 2N + 1 ac-side output voltage levels. The computation load is significantly reduced without losing the harmonic circulating currents suppression and capacitor voltage ripples compensation capabilities. In this method, the cost function minimization step and capacitor voltage sorting algorithm are combined to determine the optimal control option. In addition, to avoid the selection of weighting factors and further reduce the computation load as the increasing of the SM number, another modified IL-MPC-B is proposed by directly calculating the optimal ac-side output voltage level and arm summation voltage according to the discrete-time mathematical model. As a result, the ac-side output current and circulating current controls are decoupled. Furthermore, the capacitor voltages are balanced with a proposed control scheme by adjusting the circulating current reference. The effectiveness of the proposed MPC methods is verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

The grounding of low-frequency distribution transformers is a critical concern that ensures the proper operation and protection of the distribution system. The power electronic transformer (PET), as a future alternative low-frequency distribution transformer, also needs grounding. This paper studies PET operation with arc-suppression-coil (ASC) grounding in the Y-connected medium-voltage (MV) stage, focusing on the following two aspects. [ABSTRACT FROM AUTHOR]