Research On Insulation Structure Of ±600kV Converter Transformer

Converter transformer,as one of the indispensable devices in the UHVDC transmission system,its reliability can affect working safety of whole system directly.During operation of converter transformer,it will be withstand the operation over-voltage,AC voltage,lightning impulse effect,DC,AC and DC superposition and polarity reversal voltage.Because the DC electric field is distributed in the converter transformer oil and insulation paper by resistive distribution law,the conductivity of insulation medium is the main factor which affects the DC electric field.At the same time,the dynamic DC electric field distribution depends on dielectric coefficient and electric conductivity.The conductivity is affected by environment and the uncertainty of DC insulation design is large.Therefore,the insulation structure is one difficult point of converter transformer design.Firstly,this paper expounds the development status of converter transformer,introduces the background,purpose and significance of the research.The research on main and minor insulation structure,lead and outlet device for domestic and overseas is introduced.The research on main and minor insulation structure,lead and outlet device of ±600kV converter transformer are studied,and then the reasonable and reliable insulation structure is confirmed.This insulation structure is used to the first unit UHVDC converter transformer in China,and has passed the entire factory test and the new production technical evaluation by CEC.This paper will be introduced the research in following aspects for ± 600kV converter transformer.For the main insulation structure design,basing on the theory of thin film and small oil gap,the main insulation structure model is established,the main insulation structure of the whole domain of AC,DC and polarity reverse transitions are calculated by using the Russian VEI special electric field and Canada Infolytica/ElecNet general electric field calculation software,analysis for electric potential and field intensity contours are obtained,making intuitive understanding of the main insulation electric field distribution,specific potential line is selected to be analyzed,the electric field intensity at the interface is controlled and improved,the reasonable and reliable main insulation structure is confirmed.For minor insulation structure design,the principle of wave process and its equivalent circuit are introduced.The winding wave process analysis is finished by establishing model and the axial distribution of oil duct,coil turn-to-turn capacitive compensation structure and end electostatic ring structure is determined.For the structure of barrier,by using the oiled paper composite insulation which is adopt the method of electrode covering and oil gap separating,the model of barrier is established,the permitted electric field intensity of oil-gas structure is analyzed by simulation analysis to judge whether its insulation structure is reliable.Reliable structure of barrier is determined by improving the electric field distribution,and it ensures that the DC barrier has not appeared breakdown and harmful partial discharge during the insulation test to provide basis for the design of converter transformer leads.