15. Engineering-Oriented Modeling and Experimental Research on DC-Biased Transformers

Due to routine preventative maintenance of equipment and operation commissioning of converter station, the direct current (DC) transmission system may operate in a monopolar ground return mode or an unbalanced dipole mode, which may cause a large direct current to flow into the neutral grounded power transformers. The asymmetrical saturation of the core, the loud noise, the serious vibrations, the local loss density concentration, and the local overheating may occur in a DC-biased transformer when the DC is flowing through the transformer winding. In this chapter, the electromagnetic properties of transformer core, the exciting current under DC bias condition, and the harmonic distribution are experimentally investigated based on two sets of verification transformer models. Furthermore, a large-scale numerical analysis of the 3-D electromagnetic fields and the power loss are carried out. The different DC injection tests are also performed on a 500 kV autotransformer to further demonstrate the harmonic characteristic, no-load loss, and noise level under DC bias. Finally, an estimation method of the abilities to withstand the DC bias is proposed.