Modelling of MMC-HVDC system for EMT study

Student thesis: Doctoral Thesis

Abstract

The modular multilevel converter (MMC) technology has been a subject of increasing importance for high voltage direct current (HVDC) systems due to its technical advantages in terms of scalability, performance and efficiency. There are already several MMC-HVDC projects in construction and operation worldwide. Offline and real-time simulations of MMC-HVDC systems prior to project construction are important for both power system operators and electrical equipment suppliers. In electromagnetic-transients (EMT) simulations, significant simulation challenges are brought by MMC-HVDC systems since a large number of sub-modules (SMs) in each MMC have to be simulated individually. Therefore, a detailed investigation on MMC-HVDC modelling methods is of great theoretical and engineering values.In this thesis, the half-bridge (HB) MMC offline (PSCAD) and real-time (RSCAD) models with adjustable number of SMs per arm with generic and customized control functions are developed using different MMC modelling methods, namely, detailed switching, switching function, Thevenin equivalent and averaged models. The developed models are validated against benchmark models from PSCAD software libraries for accuracy during steady-steady operation, and AC and DC short circuit faults. Simulation efficiencies of different HB-MMC models are compared for offline PSCAD simulation. The comprehensive comparison of the offline simulation waveforms proves that the averaged and switching function HB-MMC models produce largely identical results as those from the Thevenin equivalent MMC model in the PSCAD MMC library, but with much greater simulation efficiency.In this thesis, interoperability of different MMC topologies, namely, the HB, full-bridge (FB) and hybrid MMCs are assessed quantitatively. Based on the aforementioned modelling methods, FB and hybrid MMC simulation models are developed and validated. Offline and real-time three-terminal DC grid models closely resembling the Caithness-Morey-Shetland HVDC system are developed to investigate DC grid power flow control during steady-state and AC faults. Interoperability of different MMC topologies in the DC grid is assessed quantitatively using offline PSCAD and real-time RSCAD simulations.Furthermore, a frequency-domain small-signal impedance measurement tool in RSCAD is presented to extract the impedances of MMC and connected AC networks for stability assessment. The scenario of a simplified GB AC network connected to an MMC is provided in RSCAD for real-time simulation use. Focusing on grid-connected converter system, the effect of AC cables and MMC time-domain modelling methods on their impedance are investigated. Through impedance measurement and comparison, it has been found that the different time-domain modelling methods of MMC have no significant effect on the impedance of the MMC. Since most frequency-domain analysis for MMC is based on the averaged model, this conclusion provides a theoretical basis for the frequency-domain MMC modelling research.
Date of Award27 May 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorLie Xu (Supervisor) & Derrick Holliday (Supervisor)

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