Abstract
In power electronics, the modular multilevel converter (MMC) is an easily scalable topology with an high output voltage quality. It is suitable for the transmission of large amounts of electrical power over long distances, which supports the realization of the ongoing energy transition. State-of-the-art methods require a comparatively large total cell capacitance in the system for energy pulsations during operation. In the present paper, in order to minimize this total capacitance, first a new method is developed to model the system, and second, by help of this model, optimal current trajectories are calculated. These currents are used for control to reduce the energy pulsation over the complete operating range, and thus, to better utilize the hardware. The new method independent on the Clarke transformations is implemented on a laboratory scale setup, and measurement results are presented which validate the new method. Furthermore, the new method is compared to the state-of-the-art method of the compensation of the 2nd harmonic and outperforms the latter significantly. This applies to the entire operating range for different power factors. A total reduction of up to 44% of the energy pulsations is achieved.
Highlights
Grid expansion is the backbone of the energy transition.Especially in Germany, renewable electricity must be transported from the sea and windy coasts to the consumption areas in the centre of the country
The Modular Multilevel Converter (MMC) as shown in Fig. 1 was presented in 2002 [1], [2] and has become established in the last years as the system of choice, when it comes to the transmission of large amounts of power over long distances [3]
The MMC is a promising topology for the application in high and extra-high voltage networks
Summary
Grid expansion is the backbone of the energy transition. Especially in Germany, renewable electricity must be transported from the sea and windy coasts to the consumption areas in the centre of the country. Degree of model understanding of the controlled system and the electrical network This enables control strategies to be developed that support the grid and ensure the safe operation of MMCs. Different methods provide approaches for energy control or voltage control in systems based on the MMC [13], [16]–[23]. Different methods provide approaches for energy control or voltage control in systems based on the MMC [13], [16]–[23] In this present paper a new systematic, control engineering based derivation of the model equations is presented. Using the degrees of freedom, optimal current trajectories are calculated, that allow a significant reduction of the energy pulsations compared to state of the art methods [13], [52], [58] This enables a far better utilization of the installed capacitance and reduces overall system costs.
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