Design Of Power Cable Insulation Level Online Monitoring Device

With the development of city,the demand for electricity increases sharply and the safe operation of state grid has been paid more attention.In the safe operation of the state grid,the biggest threat is system electric shock.,it means system voltage sag in a moment.However,the phenomenon of electric shock is caused by short circuit between phases after the breakdown of insulation of power equipment.Therefore,good insulation of electrical equipment is very important for the safe operation of state grid.Meanwhile,in large power cable accident,mostly caused by damage to the insulation layer of the cables.In this case,designing a new online insulation monitoring device suitable for 6 ～ 35 kV power system has great significance.The research results of this paper mainly include the following three points:Firstly,this paper analyzes the detection methods of power cable insulation level on-line monitoring at home and abroad,and compares the measuring principles,application scenarios,advantages and disadvantages of these methods.In this paper,dc superposition method is selected as the research method of the subject.According to the working principle of dc superposition method,the power cable insulation level online monitoring device is designed.Secondly,the hardware circuit design of online insulation monitoring device is based on the principle of dc superposition method.The device is mainly composed of two parts: ATT7022 EU as the busbar insulation online monitoring part of A/D sampling chip and MSC1211 as the branch insulation on-line monitoring part of A/D sampling chip.At present,the existing power cable online insulation monitoring devices on the market cannot monitor the insulation of the branch in real time,it is necessary to disconnect the branch and measure the insulation of the branch.Therefore,real-time on-line monitoring of branch insulation level is the innovation point of this device.The hardware includes A/D conversion circuit,stm32,measuring gear switch circuit,signal hardware filter circuit,device power supply circuit,peripheral display and alarm circuit and so on.It is necessary to analyze the selection principle and working effect of these circuit devices.Thirdly,according to the hardware circuit design of the whole system software,The part of measuring busbar insulation level is based on the harmonic analysis which provided by chip manual.The insulation level of the branch is determined by measuring the current of the branch.A/D converter chip is used to extract the collected signals,and then the direct flow is obtained by Fourier decomposition algorithm.The decomposed results are processed and converted to obtain branch current.The sampling resistance of each branch is the same,so the insulation level of the branch is judged by comparing the size of each dc current.After the design of the device is completed,the installation and commissioning of the device are simulated in the laboratory environment to verify the working performance of the device,and the software and hardware of the device are corrected by analyzing and finding the deficiencies in the design of the device,so as to constantly improve the function of the device.The online insulation monitoring is carried out on line in the actual application field until it has satisfied in the simulated field.