Study On Transformer DC Bias Due To Uhvdc Transmission

UHVDC transmission has the virtue of "High-capacity, Long-range" which can effectively alleviate the contradictions that the uneven distribution of energy and load. Along with the general policy "West to east power transmission, North-south power exchange and Nationwide interconnection", the UHVDC is the main selection of long distance transmission. However, when the UHVDC transmission system works in mono-polar operation, DC flows through the neutral point and causes DC bias of transformer around UHVDC grounding polar, which will influence the operation security of transformers. The direct current component of flux and exciting current arisen in a power transformer is called DC bias magnetization. It causes the increasing of magnetizing current, harmonics leakage magnetic flux, and thus the losses, which in turn results in the structures overheating and insulation ageing. Therefore, it is necessary and important to research DC bias of transformer.The reason and mechanism of DC bias and the calculation of excitation current are particularly analyzed in this paper. If the equivalent resistance circuit is used to calculate the DC current component of transformer, the system operation point and the nonlinear characteristics of ferromagnetic materials will not be considered which will bring error, so the system simulation model of transformers under DC bias is established in this paper by utilizing Matlab/simulink. Changing of the excitation current under the over excitation and DC bias is both analyzed in chapter two, and also studied changing of harmonics of the two instance.The magnetic-circuit coupling of transformer model is established under exists of potential difference between neutral ground of AC power network by utilizing ANSOFT MAXWELL. Due to the value of DC is not fixed and irregular change with time the Two-dimensional transient field is used in this paper to analyze characteristics of different internal state of several typical transformer cores under the DC bias, including magnetic induction, magnetic field strength and magnetic line distribution. Through analysis the effects of DC bias of three-phase main transformer, three-phase and three-leg transformer and three-phase and five-leg transformer to assess their ability to withstand the DC value. The loss of the three-phase main transformer is theory calculated and simulated in this paper. The criteria of allowed DC value through transformer and the estimation method is analyzed, and put forward some suggestions to increase anti-bias magnetic of transformer from structure design. The DC bias endurance among the different structural transformers is compared. Finally the reference value of allowable level of DC bias in the conventionally designed transformer is given and the methods of enhancing the ability of enduring DC bias are also suggested.