Modelling of Modular Multilevel Converter Using Input Admittance Approach

Abstract

With increasing Distributed Energy Resources (DER) penetration and increased High Voltage Direct Current (HVDC) applications, Modular Multilevel Converter (MMC) is becoming an integral part of the power system due to its advantages in terms of scalability, harmonic performance and efficiency. One of the most challenging topics as with regard to MMC lies in its modelling as the detailed model offers computational challenge during transient simulations. Thus, the main purpose of this study is to derive an input admittance model that can be used for stability studies and controller designs. In this thesis, the topology and operating principles of MMC are studied and an input admittance model is derived analytically. The derived input admittance is then compared with the input admittance of two level converter. Moreover, an average model of an MMC with four Sub Modules (SM) per arm is simulated in PLECS and used to validate the analytical model using frequency domain approach. Furthermore, the impact of different controller parameters, switching schemes and switching frequency variations on the derived input admittance is investigated. It is found that the analytical and the simulated models agree very well for the entire frequency range with the exception of slight discrepancy at low frequency ranges (between 1 and 10 Hz). It is demonstrated that the input admittance model remains the same regardless of the changes in modulation schemes. Moreover, the derived input admittance is able to predict the changes in the input admittance of MMC due to control parameter variations.