Research On The Distribution Characteristics Of Short Circuit Radiated Electromagnetic Field Of MMC Converter Valve Sub-module

In this paper,the electromagnetic simulation software is used to build the radiation model of the MMC converter valve antenna,put into the typical frequency fed power supply,calculate the field strength distribution and propagation law of the valve tower in the normal and short-circuit state,and monitor the electric field strength change of the system in the solution domain.The simulation results in this paper provide the basic simulation basis for realizing the short-circuit fault diagnosis of non-contact converter valve.Firstly,the basic topology and three sub-modules of MMC converter valve are analyzed theoretically.Taking the half-bridge sub-module as the follow-up key research object,the mathematical model of MMC is deduced,and the mathematical relationship between module output current and voltage and bridge arm current and voltage is obtained.The FEKO antenna model of the valve tower is built with the moment method.When the sub-module is short-circuited,the frequency spectrum of the fault capacitor voltage is analyzed,and the typical frequency feed power supply is extracted as the field source of the radiated electromagnetic field of the valve tower.The law of field strength distribution in near field,middle field and far field is deduced by using dipole theory.Finally,the electromagnetic simulation software is used to simulate and calculate the spatial electric field intensity change law and field strength distribution characteristics during the converter valve short-circuit fault.The simulation results show that in the transverse field strength solution domain,the same numbering sub-module in different phases is short-circuited,and the distribution trend of the electric field strength difference curve in the space under multi-frequency feed power supply is almost the same.the field strength reaches the peak in the nearest monitoring area to the short-circuit sub-module,and the electric field energy mainly oscillates and propagates symmetrically up and down along the positive and negative directions of the X axis.The magnetic field intensity difference curve of the three-phase system is different.In phase A,the magnetic field difference curve is positive,from the peak along the positive and negative direction of the X axis,until the magnetic field energy attenuates to zero.In phase B,the magnetic field difference curve monitored in the X-axis solution domain is both positive and negative,and the magnetic field intensity attenuates smoothly along the positive X-axis at the peak until it attenuates to a negative maximum and then rises to zero.In C phase,the magnetic field difference curves are all negative,and the magnetic field intensity decreases steadily from the peak to zero along the positive and negative directions of the X axis.In the longitudinal field strength solution domain,the opening of the curve at 0.1MHz frequency is the smallest and the change of the curve is the most obvious.With the increase of frequency,the opening angle of the curve increases gradually until it is approximately 180 °.Compared with the field strength propagation distance at each frequency,the field strength propagation distance at 0.1MHz frequency is the farthest.There are two peaks in the magnetic field intensity difference curve,and the peak appears in the Y-axis monitoring domain closest to the short-circuit sub-module.The electric field distribution in the near field can be divided into three evolution stages:longitudinal distribution,transition period and transverse distribution,and the peak values of electric field intensity are also different in the three stages.When the frequency is 5,10 and30 MHz,the peak value of electric field intensity in the short circuit state exceeds the peak value in the normal state.The characteristics of the field strength distribution in the far field are mainly shown in the radiation lobe of the electric field intensity and the propagation direction in the space,the action of the feed power source of different frequency,the radiation lobe of the electric field intensity is different,which is arch bridge type,spherical,oblate,respectively.The frequency of the feed power supply increases,and the number of lobes increases.The simulation results of FEKO are approximately consistent with the theoretical research,which shows the effectiveness of analyzing the short-circuit fault of converter valve from the point of view of electromagnetic field.The research results of this paper provide a new solution for the location and detection of short-circuit fault of MMC converter valve,and have important theoretical significance.