The Impact Research Of UHVDC Transmission Under Different Operation Modes On Converter Transformers

The feature that resource distribution and energy demand unbalanced in our country determines UHVDC transmission is the key development direction of the power grid in the future20years.Since the DC transmission system have different system organization and operation modes with AC transmission system, DC transmission system has its unique characteristics by introducing the equivalent continuous DC current in the converter transformer winding. Converter transformer is the core equipment of mutual transformation of the alternating current and direct current, its reliable operation is very important for the high-voltage direct current transmission system. The invasion of DC current in converter transformer can lead to the happen of direct current bias magnetization,which will cause a series of adverse effects to converter transformer itself and the DC transmission system,and the phenomenon of DC bias magnetization is the direct reason that lead to the increasing vibration and noise of converter transformer. This paper analysis the impact of converter transformer DC bias current which is caused by UHVDC transmission system under different operation modes from the perspective of DC bias magnetization.Firstly,proceeding from theUHVDC transmission system, this paper analyzes the reason and mechanism of converter transformer DC bias magnetization,then the calculation method of excitation current that exist in Single phase double winding transformer has been particularly studied,and the harmonic components are theoretically analysed under DC bias magnetization through the formula deduction.Secondly, In this thesis, The±800kV UHVDC transmission system simulation model has been founded in the environment of EMTDC/PSCAD,and the simulation of system different operation modes were carried out to verify the accuracy of this model. On the basis of this model, this paper use the method of equivalent shunt resistance to establish a simulation model that can evaluate the realationship between converter transformer neutral point DC bias magnetization and operation mode of UHVDC transmission system,the realationship between transmission power and converter transformer neutral point current has alse been built.Then based on UMEC transformer model,this paper did a series of quantitative researches about the influence on converter transformer excitation characteristic, the network side and the valve side winding harmonic characteristics, reactive power consumption characteristics with the function of different level DC bias magnetization.Thirdly, using the2-D nonlinear field-circuit coupling method of finite element analysis a2-D finite element analysis model about converter transformer dc bias magnetization is built by the finite element software-Ansoft/Maxwell to simulation analysis the magnetic field distribution characteristics inside converter transformer with different DC bias magnetization, the simulation results proved the principle analysis of DC bias.The flux density distribution at different part of the converter transformer has been particularly analyzed by chosing four representative path.Finally, several restrictive measures towards DC bias have been analyzed and compared at this chapter, including transformer neutral point series resistance method, series capacitor method, reverse injection current method and voltage compensation method, and do a more detailed explanation for their advantages and disadvantages.Take solving the actual problem as the research target, some proposals have been put forward about the related research of dc bias suppression measures.