Abstract
Modular multilevel converters (MMCs) play an important role in the power electronics industry due to their many advantages, such as modularity and reliability. In the current research, the simulation method is used to study the system. However, with the increasing number of sub-modules (SMs), it is difficult to model and simulate the system. In order to overcome these difficulties, this paper presents a universal mathematical model (UMM) of MMC using half-bridge cells as SMs. The UMM is a full-scale model with switching state, capacitance, inductance, and resistance characteristics. This method can calculate any number of SMs, and it does not need to build a simulation model (SIM) of physical MMC—in particular, parametric design can be realized. Compared with the SIM, the accuracy of the proposed UMM is verified, and the computational efficiency of the UMM is 8.7 times higher than the simulation method. Finally, by utilizing the proposed UMM method, the influence of the parameters of MMCs is studied, including the arm induction, SM capacitance, SM number, and output current/voltage total harmonic distortion (THD) based on the UMM in the paper. The results offer an engineering insight to optimize the design of MMCs.
Highlights
With the rapid development of offshore wind farms, the demand for a high power, high-quality transmission system becomes more urgent
Before 0.06 s, the system was at a transient state, and the universal mathematical model (UMM) and simulation model (SIM) results had shown similar behavior with a slight amplitude difference
The open circuit fault of SM was assumed to study the effect of the internal fault of sub module on the Modular multilevel converters (MMCs) system
Summary
With the rapid development of offshore wind farms, the demand for a high power, high-quality transmission system becomes more urgent. The methods above do not reflect the switching state and the transient process of the SM capacitor voltage To address this problem, an efficient model was proposed by Udana and Gole in [21], which is referred to as the detailed equivalent model (DEM) in this paper; yet, a drawback of the DEM is that the individual converter components are invisible to the user. A universal mathematical model for MMC is proposed which can reflect the steady-state and dynamic process of MMC. Using the proposed MMC, the output voltage and the current THDs of MMC have been analyzed under different parameters (such as module number, capacitor voltage, arm inductance).
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