Impedance-based AC/DC Terminal Modeling and Analysis of MMC-BTB system

TL;DR

This paper develops improved impedance models for MMC-based systems, enabling comprehensive stability analysis at both AC and DC terminals, considering transformer effects and high-frequency resonance, validated through simulations.

Contribution

It introduces a numerical method for steady-state analysis and derives new impedance models that incorporate transformer effects and control loop delays in MMC systems.

Findings

Impedance models accurately predict system stability.

Transformer effects significantly influence low-frequency impedance.

Simplified models may be inadequate for high-frequency resonance analysis.

Abstract

Impedance-based small-signal stability analysis is widely applied in practical engineering with modular multilevel converters (MMCs). However, the deficiencies of existing impedance models (IMs) and the idealized extension for the single MMC influence the analyses in multiterminal systems. Such gaps are filled by focusing on an MMC-based back-to-back system in this paper. To obtain the steady-state trajectory of the system, a numerical method is first proposed based on Newton-Raphson iteration in the frequency domain. Then, by substituting the shared terminal dynamics with active or passive devices, theoretical AC/DC IMs that consider typical control loops with the pure time delay, are directly established based on the multiharmonic linearization. Further aided by the derived IMs, two neglected aspects in the current literature, i.e., the influence of power transformers on low-frequency impedance characteristics and the rationality of using simplified IMs for high-frequency resonance studies, are investigated. It is confirmed that the stability of interlinking systems should be comprehensively analyzed at both AC and DC terminals. This helps position the instability source, obtain the stability margin, and guide the supplementary control strategy. All IMs and analyses are verified by frequency scans and simulations in PSCAD.