Research On Key Technology Of Insulation Monitoring For DC System

With the rapid development of the new energy industry,DC power systems are widely used in different walks of life.Since the DC floating ground system cannot use AC leakage protection measures,insulation monitoring technology has become an important and necessary means of safety protection for the DC system,especially in the new energy industry.Especially in the new energy industry widely used in the new energy industry.With the frequent safety accidents of electric vehicles in recent years and the rapid development of DC power grid industry,newer and higher requirements should be put forward for insulation monitoring technology.Existing insulation monitoring methods can be summarized as electric bridge method and signal injection method.Based on these two methods,this paper summarizes the existing problems and puts forward the corresponding optimization strategy,so as to improve the real-time,accuracy,robustness,stability and universality of the insulation monitoring technology.The characteristics of different topological structures and applicable occasions of electric bridge method and signal injection method are described,the formation mechanism of the monitoring results error of the two methods are analyzed.On this basis,the unified circuit model and different characteristics of electric bridge method and signal injection method are summarized.Considering the influence of the non-idealized characteristics of the circuit switch in the unified model on the DC insulation monitoring results,the difference between optocoupler MOS and IGBT is studied,and the influence of the turn-on voltage drop and the turn-off leakage current on monitoring error is analyzed.A compensation strategy based on linearized estimation is proposed to reduce the monitoring error.This strategy is to establish the linear simulation equation according to the corresponding switch,estimate the turn-on equivalent resistance and turn-off equivalent resistance,and then simulate the bridge resistance value under the influence of the nonideal switch.The experiment shows that the higher the DC voltage is,the more obvious the effect of this strategy is to improve the monitoring accuracy.In view of the problem that the monitoring accuracy of traditional insulation monitoring method can be greatly reduced when the overall DC voltage range is wide,a dynamic compensation strategy based on sampling voltage is proposed.This method cancels out the common-mode component in the sampling voltage through D/A converter,so that more effective components can be included in the A/D converter withlimited resolution,so that the sampling error has less impact on the monitoring results.In addition,the traditional insulation monitoring method cannot be accurately measured under the condition of DC voltage change or insulation resistance position change.Therefore,a state observer based on optimal tracking is proposed to make the estimated value of each parameter approximate to the actual value.The state observer can reduce the influence of the sudden change of uncontrollable components such as DC voltage on the monitoring results.Combining the state observer with the sampling voltage dynamic compensation strategy can make the insulation monitoring system more accurate and robust.In view of the problem that the monitoring period can be greatly prolonged if there are line reactance and grounding capacitance in the monitoring loop,a quasi-constant frequency insulation monitoring method based on iterative sampling is proposed.This method is to take a finite number of samples at the same time interval of the grounding current to estimate the variation curve of the grounding current.In order to eliminate the calculation deviation caused by the sampling error,it is necessary to filter and track the variable parameters with half a monitoring period as the interval,and use the least square method for curve fitting,and calculate the final result by Newton iteration method.When the sampling area is in the stage of climbing and stabilizing,this method can realize the fixed monitoring period.Finally,the experimental verification proves that the method can detect the insulation resistance more quickly without affecting the accuracy compared with the traditional method under different system parameters.For the DC insulation monitoring device installed on the DC side of three-phase AC/DC converter,the common-mode interference model of grid-connected converter and off-grid converter and its interference form to DC insulation monitoring are deeply analyzed.In view of the problem that the decrease of AC side insulation will affect the results of DC insulation monitoring data,the influence of AC side insulation resistance on ground current of grid-connected and off-grid converters under uncontrolled and controllable conditions is analyzed,and the relationship between ground current and zero vector switching pattern is obtained.An insulation monitoring method based on zero vector dynamic modulation is proposed,which makes the two zero vector switching patterns switch with each other,and it is interlaced with two modes of insulation monitoring,so as to obtain the insulation resistance of AC side and DC side respectively.Finally,the feasibility of the method is verified by simulation and experiment,and the influence of different insulation resistances on the monitoringresults of this method is tested.The research content of this thesis can promote the further development of DC insulation monitoring technology and play a positive role in promoting the safe and healthy development of new energy and related industries.93 figures,7 tables,138 references...