Research On Insulation Monitoring And Fault Tracking Device Of DC System

In recent years,the power plants and substations have been shut down by mistake due to the decrease of insulation of DC system,and the malfunction of protective equipment has occurred from time to time.The insulation degree of the dc system can be monitored in real time at the running site,and the fault branch can be quickly warned and accurately positioned when the insulation drops,so as to avoid large-scale accidents,which is of great significance for improving the reliability of the power grid.This paper studies the "dynamic difference method",which measures the bus voltage and bus-to-ground resistance by putting the detection resistance into the positive and negative buses separately,and realizes the function of fault line-finding and branch-to-ground resistance detection by installing leakage current sensors in each branch,which effectively reduces the influence of zero drift and temperature drift of leakage current sensors.In view of the high accuracy of AC sensor measurement,in this paper a frequency conversion secondary injection method for detecting branch grounding resistance is proposed.Short-circuit capacitors are put into DC buses to eliminate the influence of buses on ground resistance and distributed capacitance when detecting branch insulation.The DC system is injected with AC signals to meet the requirements of the national standard,so that the branch current can be measured by AC sensors,and the measurement accuracy can be further improved than that by DC sensors.The maximum detection error of grounding resistance of 100k? inner branch is 8%,which meets the design requirements.It provides theoretical support for the next research of the project.In this paper,the insulation monitoring device of DC system is designed to complete the theoretical verification and hardware implementation of bus parameter detection.According to the demand of branch fault line-finding,this paper designs a highly integrated open-ended digital sensor based on the principle of pulse width change.The leakage current of the fault branch can be accurately identified by converting the current change to the square wave duty cycle change of the output of the oscillating circuit,and then by filtering,amplifying and other circuit processing.The test results show that the output of the sensor is basically linear and meets the requirements of operation.The device can accurately complete fault line-finding.In view of the communication needs between the main machine and the sensor and the difficulties in substation field dc system wiring,this paper designs the fault line finding dc carrier communication technology,so that the information transmission between each leakage current sensor and the main machine is combined with the power supply cable to complete the communication and power supply functions with two power supply cables.In addition,the communication technology transmits digital signals,which are less affected by external interference and have better anti-interference ability.By setting up an analog DC system in the laboratory to debug the designed system online,the device runs smoothly and all the measured data meet the requirements.According to the actual operation of the device in Chengnan substation,Jiaohe substation and Baiyun substation,the measurement error of bus parameters is less than 1%,and the measurement error of branch resistance is less than 10%,which satisfies the design requirements well.