Noise Reduction Mechanism Of Converter Transformer Based On Absorption And Shunt Capacitance Commutated Converter

With its advantages in transmission distance,transmission capacity and operation cost,HVDC has gradually become an important carrier of power transmission and global energy interconnection.There are many defects in the traditional mode of power grid commutation,such as a large number of reactive power compensation equipment,easy commutation failure,no power supply to passive network and high noise level of converter station.The noise pollution problem of the converter station is becoming more and more serious,especially the electromagnetic noise emitted by the converter transformer,which has a great influence on the surrounding ecological environment and the life of the residents.However,the existing way of noise reduction in the converter station mainly restrains the noise by blocking the transmission way,and can not eliminate the noise from the source.In order to solve the above problems,a high voltage direct current transmission mode based on Absorption and Shunt Capacitance Commutated Converter(ASCCC)is proposed.The topology and working principle of ASCCC are studied,and the principle of transformer magnetostrictive theory is used to study the noise reduction mechanism of the converter transformer.First of all,this paper analyzes the mechanism of transformer noise.The main source of transformer noise is its own body vibration.The main reason for the body vibration is the electromagnetic noise produced by magnetostriction effect of silicon steel sheet.Based on the structure and working principle of converter transformer,combined with magnetic field equation and elastic mechanics equation,the stress of silicon steel sheet in alternating magnetic field is analyzed.The expression of the magnetostriction force and acceleration of the silicon steel sheet is derived.It is shown that the high harmonic current through the transformer will enhance the magnetostrietive effect of the silicon steel sheet,thus aggravate the vibration of the iron core,resulting in the higher noise level of the converter transformer compared to the ordinary power transformer.Secondly,on the basis of the traditional grid switching mode,the absorption and shunt capacitor commutation mode is proposed,that is,a shunt capacitor is parallel between the converter transformer and the commutation valve.Due to the shunt of the capacitor banks,the commutation current is superimposed by the side current of the transformer valve and the current of the capacitor.Compared with the commutation voltage,the overlay angle of the commutation is reduced,and the commutation is more rapid and reliable.With the equivalent shunt model,the load side is equivalent to the harmonic source,and the shunt capacitance shunt ratio is analyzed.It shows that the shunt capacitor can absorb the high order harmonic and have good filtering effect.Therefore,the harmonic components are greatly reduced by the converter transformer injected into the current of the AC network.Combined with the influence of harmonic on magnetostrictive effect,we get the conclusion that the way of ASCCC commutation is to filter high-frequency harmonics and achieve the purpose of transformer noise reduction.Taking the Three Gorges to Guangdong±500kV HVDC transmission project as an example,the simulation model is built by Matlab-Simulink software to verify the theoretical analysis.Finally,based on the absorption and shunt capacitor commutation mode,a set of 100kW rectifier load prototype test system platform is designed,and the noise and vibration test of the Danish B&K noise source identification system is used to test the noise and vibration.According to the principle of acoustics,the noise of converter transformer is measured by the sound pressure method.By changing the DC current of the system,two cases are collected in different working conditions,unconnected and connected to the capacitor bank,the noise data of the converter,the vibration acceleration of the iron core,and the quality of the electric energy.Through the data comparison,it is proved that the ASCCC method can effectively reduce the noise level of the converter transformer and improve the power quality of the system.