Design Of Transient Electromagnetic Transmitting System Based On Modular Multilevel Converter

Transient electromagnetic method(TEM)is an effective geophysical detection method,which has been widely used in geological survey and resource exploration.To meet the increasing demand of energy,it is urgent to detect deep mineral resources and some resource-rich extreme environments,so higher requirements are placed on the performance indicators of TEM instruments.Based on the working principle of TEM,the transmitting current has a direct impact on the detection effect.Increasing the transmitting voltage can ensure the excitation current under the condition of poor grounding,which helps to improve the adaptability to extreme environments.The traditional transmitting system uses an H-bridge inverter as the transmitting circuit.The voltage stress of the device will be high when the transmitting voltage level is high.However,the existing high-voltage IGBT is not only expensive,but also limited by the technological level,causing serious heat generation and high efficiency,which cannot meet the existing detection requirements.Modular multilevel converter(MMC)is a novel multilevel inverter,which has advantages such as high modularity,low voltage stress of switching devices,and easy cascading expansion.In this paper,MMC is used to replace the traditional H-bridge inverter to overcome the shortcomings of the traditional transient electromagnetic transmitting system.The main research contents are as follows:1.The structure of the MMC transmitting system is introduced.Through the simplified analysis of this circuit,the working principle of the circuit is discussed in detail,and it is further equivalent to a graded adjustable voltage source.Based on the analysis,the circuit can control the slop of the output current to adjust the quality of the transmitting current waveform,which is the basis of the study of the control strategy.The current ripple characteristics of this novel circuit and its suppression methods are discussed.And to ensure the voltage balance of the floating capacitor,the voltage self-balancing method is discussed.2.Based on the equivalent model analysis,the segmented transmitting current control strategy of the MMC-based transmitter is studied,in which the rising and falling edge speeds of the transmitting current can be adjusted,the ripple is suppressed in the steady current stage and the slow turn-off at the falling edge can be realized.The simulation model of the MMC transmitter is built in Simulink,and the feasibility of the segmented current control strategy is verified preliminarily.The key factors affecting the output current waveform quality of the MMC transmitter are analyzed by comparing the transmitting current waveforms under different MMC transmitting circuit stages,different transmitting frequencies and different current reference values.3.On the basis of theoretical analysis and control strategy research,the key hardware circuit of MMC transmitting system and the program design based on STM32 are completed.The design of key hardware circuit includes SM unit board,isolated drive circuit,auxiliary power supply and so on.The program design part introduces the overall program design process and the segmented current control strategy design process in detail.4.The prototype of the MMC-based transmitting system is developed,and the output waveform of the key hardware circuit,the the overall system and the SM capacitor voltage waveform were tested respectively.The experimental results are consistent with the theoretical analysis and simulation results,which further verifies the feasibility of the MMC-based transmitter and its control strategy.