Analysis Of The Influence Of DC Bias On Power Transformer And The Application Of Suppression Technology

High voltage direct current(HVDC)transmission has the advantages of long-distance or ultra long-distance transmission and good economy.In China,the energy distribution is not balanced and the distribution of energy use is not balanced.The energy supply is the main energy supply area in the west,while the East is the main energy use area.The East-West span is large.Based on the advantages of DC power transmission,in order to alleviate the imbalance between the power supply and the use of power in China,the domestic West to East power transmission adopts multiple UHVDC power transmission,which has become the main way of UHVDC power transmission in China.It is possible for DC transmission electrode to be overhauled or broken down.DC transmission will operate in the way of single pole grounding.During the operation of single pole grounding,DC current will stay in the ground,and different potential will be generated at the transformer neutral point of the nearby AC substation.The potential difference will be generated,and DC current will flow through the transformer neutral point to the main transformer(in the case of transformer neutral point grounding),so as to It leads to DC bias of transformer.If the DC bias of transformer is serious,it will lead to high magnetic saturation of transformer core,increase of magnetic leakage of transformer,abnormal sound,abnormal vibration and local overheating of transformer,which will affect the normal operation of main transformer and reduce the operation life of transformer.Therefore,the problem of DC bias of main transformer in AC substation caused by DC transmission cannot be ignored.Based on the UHVDC transmission from Northwest Yunnan to Guangdong,firstly,a simulation model is built according to the main transformer of Qujiang substation.PSCAD software is used to simulate the phenomenon of DC bias of power transformer,to control different DC bias current input of transformer,and to analyze the excitation current waveform and harmonic current variation characteristics of transformer.The relationship between hysteresis loop and magnetization curve of transformer core under different DC input is analyzed.This paper analyzes the generation of DC bias current,analyzes and compares various suppression measures,and puts forward the practical application of small resistance suppression in the connection of transformer neutral point.Combined with the actual situation of Shaoguan Power Grid,the target value is that the DC quantity of the main transformer of the target power grid in the whole Shaoguan area exceeds the standard.At the same time,make the small resistance as small as possible,establish the double objective optimization,establish the network model of the small resistance,and use particle swarm optimization algorithm to optimize the small resistance.The algorithm is used to optimize the small resistance connected to the neutral point of the main transformer,so as to avoid the large DC current and the large small resistance of some stations,so as to achieve the global optimization.In this paper,combined with the operation of the isolation device in Qujiang station and the actual analysis of the operation of the isolation device,the effectiveness of the isolation measures to suppress DC bias is verified.