Modeling, analysis, and control of circulating currents in half-bridge modular multilevel converters
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Date
2020-06-23
Authors
Marzoa Montalvo, Cesar Gabiel
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Abstract
In operating Modular Multilevel Converters (MMCs) the common practice is to eliminate circulating currents using a Circulating Current Suppression Controller (CCSC). This practice reduces converter losses and submodule capacitor voltage ripple; it also allows usage of semiconductors with lower ratings. Although preliminary research has pointed to the possibility of minimizing capacitor voltage ripple or converter losses by injecting circulating currents, little formal analysis has been conducted on how the improvement of one will affect the other, or on how to control the ripple or the losses. This thesis presents a detailed analysis on how to inject a precise amount of circulating currents in order to obtain certain submodule capacitor ripple. A mathematical model that describes the capacitor voltage ripple based on the second-order circulating current contents is used to determine the ripple profile of the converter at a given operating point. Then it is possible to determine what amount of circulating current needs to be injected to obtain a certain ripple. An analysis of how such circulating current injection affects losses is also made. The proposed method is validated using PSCAD/EMTDC simulations, where a high level of accuracy is observed. Validation using a Real-Time Digital Simulator (RTDS) using a Control Hardware-in-Loop (CHIL) scheme is also presented. An implementation in a down-scaled MMC prototype further validates the proposed method. This thesis shows that the optimal operating point (regarding losses and capacitor voltage ripple) for MMCs is not achieved by using a conventional CCSC, as is the common practice today; instead injecting a certain amount of second-order circulating current can reduce both ripple and converter losses simultaneously.
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Keywords
Modular multilevel converter (MMC), HVDC VSC converter, Circulating currents