Low-Frequency Transient Model of Single-Phase Four-Limb Converter Transformer Considering the Nonlinear Excitation Characteristics of the Iron Core

Transformer modeling is a crucial method for analyzing transient phenomena such as inrush currents. The primary characteristic of a transformer transient model is its ability to reflect how the transformer’s structure and material properties influence the magnetic and electric fields. In high-voltage direct current (HVDC), the single-phase converter adopts a double-core-limb and double-side-limb configuration, whose core structure, magnetic flux distribution, and ferromagnetic materials differ from conventional power transformers. This paper conducts research on low-frequency transient modeling of single-phase four-limb converter transformers. This study first determines the magnetic field distribution of the single-phase converter transformer with the inclusion of leakage flux. Subsequently, a corresponding model is derived from the principle of duality. Due to the laminated structure, the iron core exhibits different excitation characteristics from those of a single silicon steel sheet. For the excitation branch, AC-DC hybrid excitation is used to measure incremental excitation inductance and the nonlinear excitation curve is calculated based on this inductance. Furthermore, the allocation method of this curve in the core limb, side limb, and yoke is proposed to establish the converter transformer model. The results of no-load and inrush current tests based on the scaled model validate the effectiveness of this model, which can accurately calculate the inrush current under different remanence and closing conditions.