Synthesis And High Frequency Insulation Property Analysis Of Corona Resistant Nano-organosilicon/PI Films

With the introduction of the global energy Internet,how to properly connect distributed generation to the power grid has become one of the most important issues to be solved urgently in the development of power systems.Power electronic transformer has advantages in terms of the grid connection of distributed generation due to its high level of controllability.However,the power electronic excitation of high-frequency and pulse waveform with short rise time,large amplitude and high frequency can pose a greater threat to the insulation while achieving the high controllability of power electronic transformer.The polyimide(PI)film,which is the internal insulating material of the power electronic transformer,has insulation defects under the power electronic excitation of high-frequency and pulse waveform.And these defects become the key factor that restricts the development of power electronic transformer into high reliability,high voltage and large capacity.Therefore,improving the high-frequency insulation property of polyimide film through nano-modification and molecular structure modification is of great significance for promoting the further development of the power electronic transformer.In order to prepare a PI film with strong corona resistance and high quality,the Nano-organosilicon(1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane(GAPD)with the shortest molecular chain and modified by NH2 at the chain end was selected as a dopant to improve the corona resistance of the PI film while avoiding the "trap" effect caused by phase separation.The introduction of phenyl sulfide and GAPD reduces the dielectric loss while increasing the resistance of the PI to corona.At the same time,based oin the original preparation method,the coating preparation was carried out in the nitrogen atmosphererdust-free environment and in a milder manner of vacuum defoaming.Thereby,it can prevent the relative molecular mass of the polyamic acid from the influence of moisture in the air so that it increases the viscosity of polyamic acid.What’s more,it also advoid the defects introduced into the PI film caused by the bubbles in the solution and the dust in the air so that it improves the quality of the PI film prepared in the laboratory.In order to study whether the physical and chemical properties of nano-organosilicon/PI film can meet the operational requirements and then come to the best modification method,nano-organosilicon/PI films’ several physical and chemical properties,such as its infrared spectrum,scanning electron microscope,ultraviolet-visible absorption spectrum,frequency domain dielectric spectrum,thermogravimetric curves and so on,were tested in this research.There are three main results of the text which can be listed as following:firstly,compared with inorganic nanoparticles,the addition of GAPD can form a network structure connected by Si-O-Si in PI film,which has better dispersibility and hardly agglomeration;secondly,Although the addition of GAPD can improve the dielectric constant and dielectric loss of the films to a certain extent,reduce the thermal stability of the films and change the stacking structure of PI intemiolecular mixing layer,the modified nano-silica/PI films still have excellent properties and can meet the requirements of practical operation.;thirdly,in the process of preparation,the PI film made by replacing 40%of 4,4’-diaminodiphenyl oxide(ODA)with 4,4′-diaminodiphenyl sulfide(SDA)has better dispersibility,lower dielectric loss and tighter intermolecular stacking structure,which has a greater advantage in improving the high-frequency insulation properties of the PI film.Based on this,a modification scheme of PI with low dielectric loss,corona resistance and high frequency insulation performance is proposed.In the synthesis process of PI,40%ODA is replaced by SDA,and 4%moore score of GAPD is added to modify it.In order to study the high-frequency insulation properties of nano-organosilicon/PI film,a needle-plate electrode was used to conduct a surface-discharge experiment at the power supply voltage frequency of 15 kHZ.The initial discharge voltage and flashover voltage along the surface and insulation life of the PI films were tested in the experiment.The process of creeping discharge was photographed by a camera,and the discharge data were collected and analyzed by a self－compiled collection system.Based on the results of physical and chemical properties test,the mechanism of the influence of nano-silica on the high frequency insulation properties of PI thin films was proposed.The study results show the nano-organosilicon/PI films have higher surface discharge inception voltage and flashover voltage,and their corona resistance is improved(the film’s life is increased by 53.7%),this is because the addition of GAPD will lead to changes in PI molecular structure,molecular bond energy and the number of space traps;in addition,according to the creeping discharge process recorded by the camera,the creeping discharge can be divided into four stages:the initial stagerthe stable development stage the rapid development stage and the flashover stage.The addition of GAPD will increase the duration of the first two stages,thus increase the lifetime of PI film.But at the same time,the pre-flashover voltage will be intensified.The discharge phases of PI films at high frequency sinusoidal voltage occur in the region where the voltage polarity reverses,which is related to the electric field formed by space charge in the injection trap.Compared with the traditional PI film,the discharge phases of PI film with the addition of GAPD are more concentrated.