Generalized Voltage-based State-Space Modelling of Modular Multilevel Converters with Constant Equilibrium in Steady-State

TL;DR

This paper introduces a generalized voltage-based state-space model for modular multilevel converters (MMCs) that achieves constant equilibrium in steady-state, enabling accurate internal dynamic analysis and control design.

Contribution

It presents a novel modeling approach using voltage sum and difference variables with multi-frequency Park transformations for steady-state analysis of MMCs.

Findings

Model accurately captures internal voltage and current dynamics.

Model can be linearized for eigenvalue-based stability analysis.

Validation through time-domain simulations confirms high accuracy.

Abstract

This paper demonstrates that the sum and difference of the upper and lower arm voltages are suitable variables for deriving a generalized state-space model of an MMC which settles at a constant equilibrium in steady-state operation, while including the internal voltage and current dynamics. The presented modelling approach allows for separating the multiple frequency components appearing within the MMC as a first step of the model derivation, to avoid variables containing multiple frequency components in steady-state. On this basis, it is shown that Park transformations at three different frequencies ($+\omega$, $-2\omega$ and $+3\omega$) can be applied for deriving a model formulation where all state-variables will settle at constant values in steady-state, corresponding to an equilibrium point of the model. The resulting model is accurately capturing the internal current and voltage dynamics of a three-phase MMC, independently from how the control system is implemented. The main advantage of this model formulation is that it can be linearised, allowing for eigenvalue-based analysis of the MMC dynamics. Furthermore, the model can be utilized for control system design by multi-variable methods requiring any stable equilibrium to be defined by a fixed operating point. Time-domain simulations in comparison to an established average model of the MMC, as well as results from a detailed simulation model of an MMC with 400 sub-modules per arm, are presented as verification of the validity and accuracy of the developed model.