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This paper proposes a startup sequence for a modular multilevel converter (MMC) HVDC-connected offshore wind farm (OWF) and demonstrates how it can be simulated using an electromagnetic transient-type tool. The objective is to analyze inrush currents of transformers and cables (HVAC and HVDC). Modeling accuracies and computational performances are examined using three existing modeling methods for the MMCs. [ABSTRACT FROM PUBLISHER]

A recently reported modeling methods for a switched capacitor converter either assume output as “firm” voltage or a resistor to facilitate calculation. When the size of output filter capacitor is moderate, accompanied by noticeable output ripple, previous modeling results may not provide accurate prediction and design guidelines. In this paper, a new modeling technique taking into account the output capacitor effect is proposed for simple dual-phase switched capacitor converters without complex coupling loops. The proposed model employs transient calculation, takes charge redistribution phase into account, and includes the duty cycle, switching frequency as well as output capacitor simultaneously in the final output impedance formula, which enhances accuracy of the model. It is suitable for interleaved and noninterleaved switched capacitor converters. The simulation and experimental results for the unity gain SC converter are provided to verify the proposed theory. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) is a newly introduced switch-mode converter topology with the potential for high-voltage and high-power applications. This paper focuses on the design and control methods for fault-tolerant operation using redundant submodules (SMs), which is one of the most important features in the MMC topology. By comparing three design schemes of redundant SMs, the most economic and reliable scheme is identified. In addition, a mathematical model of the MMC with arms containing different numbers of SMs is developed. Based on the identified scheme and mathematical model, an energy-balancing control strategy is proposed to keep the MMC operating normally under SM fault conditions. Finally, time-domain simulations of a 61-level MMC system are performed, using the PSCAD/EMTDC software, while experiments are carried out on a 41-level MMC prototype. The simulation and experimental results validate the design scheme, mathematical model, and proposed energy-balancing control strategy. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMC) are considered a top converter alternative for voltage-source converter (VSC) high-voltage, direct current (HVDC) applications. Main circuit design and converter performance evaluation are always important issues to consider before installing a VSC-HVDC system. Investigation into a steady-state analysis method for an MMC-based VSC-HVDC system is necessary. This paper finds a circular interaction among the electrical quantities in an MMC. Through this circular interaction, a key equation can be established to solve the unknown circulating current. A new steady-state model is developed to simply and accurately describe the explicit analytical expressions for various voltage and current quantities in an MMC. The accuracy of the expressions is improved by the consideration of the circulating current when deriving all the analytical expressions. The model's simplicity is demonstrated by having only one key equation to solve. Based on the analytical expressions for the arm voltages, the equivalent circuits for MMC are proposed to improve the current understanding of the operation of MMC. The feasibility and accuracy of the proposed method are verified by comparing its results with the simulation and experimental results. [ABSTRACT FROM AUTHOR]

In this paper, three dc/dc ladder multilevel converters are compared. The first one is the classical ladder topology and the two other topologies presented are based on the classical one. A mathematical calculation of the output resistance and the gain of the converter as a function of the number of levels is carried out for the three topologies in order to estimate the voltage drop due to the output current. These calculations are validated with simulations and experimental results. Finally, the behavior of the three topologies is compared through experimental tests. Results show higher performance for the presented converters compared to the classical ladder. [ABSTRACT FROM AUTHOR]

The matrix converter (MC) is arousing considerable attention as an alternative for conventional ac/ac converters due to the advantages it offers. However, the control and modulation of this converter is complex. This, together with the fact that the MC usually operates at high modulation frequencies, makes the computational load of the platform to be simulated excessively high. All this makes the simulation time of models including the MC excessively long, even more so when both the transient and steady state of the system must be analyzed. This paper presents a new MC simulation technique called Switching State Matrix Averaging (SSMA). Although this is a fixed-step technique, a long simulation step can be used without forfeiting the accuracy of an ideal variable-step simulation. Likewise, the SSMA drastically speeds up the simulation, reducing the amount of required resources and the tuning time of the complex platforms in which the MC is used. A series of simulations has been performed in order to verify the proposed method. Moreover, a comparison between experimental and simulation results has been made, demonstrating the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]