Magnetizing model and simulation of transformer behaviour under DC bias
Date of Issue2014
School of Electrical and Electronic Engineering
Under the influence of unbalance high voltage direct current (HVDC) system and geomagnetically induced current (GIC), transformers with neutral grounding point may saturate in half cycle due to the injection of DC current. The vibration of transformer, reactive power demand and harmonic components of power system increase significantly, which are harmful to the safety operation of power system. The nonlinear characteristics of iron core are described by the magnetizing model, the accuracy of magnetizing model is of vital importance in the analysis of DC bias. Jiles-Atherton model is derived from physical process of magnetization. With only five parameters, this model has an accurate description of magnetizing process, and easy to implement. Therefore, Jiles-Atherton model is widely accepted and used in the modeling of magnetization. However, with the development of Jiles-Atherton model, there are several editions of mathematical description, some of them are even contradict. The different expressions can easily lead to confusion and inaccuracy of modeling. This work compared several editions of Jiles-Atherton model, clarified the mathematical expression of the model based on physical properties. A modified energy conservation equation was proposed and modified Jiles-Atherton model was thus established. Numerical simulation of modified Jiles-Atherton model was carried out, the variation of B-H characteristics under different levels of DC bias were analyzed. The transformer behaviour under DC bias was analyzed with UMEC model, the result show that the magnetizing current is seriously distorted due to saturation, the harmonic components increase dramatically. The reactive power demands also increase with DC bias. Influencing factors of DC bias are also discussed. The withstanding ability of transformer to DC bias mainly depends on temperature rise and output voltage harmonics under DC bias, the influence of DC bias on temperature rise and power quality were analyzed based on filed data and limit of DC bias was given according to relevant standard. Finally, possible restraining methods to DC bias were analyzed. DC bias can be restrained by injecting reverse direct current, inserting in series with the neutral point a resistance and install capacitors and bypass device to the neutral point of main transformer.
DRNTU::Engineering::Electrical and electronic engineering