Research On Grounding Dispersed Current Model And Risk Assessment Method Of Transmission And Distribution Lines

In recent years,with the continuous advancement of urbanization,the length of transmission and distribution lines in China has maintained a rapid growth.However,the AC/ DC hybrid transmission and distribution network will inevitably pass through densely populated areas.Under the action of strong thunderstorm,insulation deterioration,external force and other factors,it is very easy to cause the grounding of transmission and distribution overhead lines.The grounding dispersed current will not only form the dispersed current potential difference on the earth surface,but also generate high-density current in the ground through the line grounding device,resulting in frequent accidents that the grounding dispersed current of the transmission and distribution line endangers human body and surrounding biological safety.Therefore,targeted and effective safety assessment and protection methods of grounding dispersed current of the transmission and distribution line are studied.It is important to ensure the stable operation of power transmission and distribution system and the surrounding biological safety.However,in the research of site engineering,there are mainly the following problems: 1)Transmission and distribution line corridor and the surrounding earth environment are complex.In practice,most of them are heterogeneous soil,and the soil will show differentiated physical characteristics in different frequencies.The soil electrical parameter structure under different characteristics will have a serious influence on the calculation of grounding dispersed current of transmission and distribution lines.2)The existing electrical grounding standards are mainly for substations or power plants,pay less attention to the risk of dispersed current under extreme grounding of transmission and distribution lines,and lack the theoretical model of grounding dispersed current under corresponding conditions and the matching quantitative evaluation method.In order to deal with the above problems,through the construction of physical and mathematical models and the combination of theory,simulation and experiment,this paper carries out the research on the grounding dispersed current model and its risk assessment method of transmission and distribution lines.The main work includes:1)Aiming at physical properties of grounded soil impulse condition and the estimation method of horizontal multi-layer electrical parameters,based on the experimental analysis of its nonlinear characteristics and critical breakdown field strength,an improved heuristic algorithm is used to estimate the electrical parameters of multi-layer horizontal layered soil,and the estimation of three-dimensional soil layered structure model is realized through a new line layout method.Based on the discrete complex image method and heuristic optimization,a simplified estimation method of multi-layer soil frequency domain parameters considering that the optimal thickness of soil model is insensitive to frequency variation is obtained.2)Based on the theory of electrical network and the analysis of spatial electromagnetic field,the coupling model of power frequency grounding current of distribution network disconnection considering the above ground circuit of horizontal layered earth and underground electric field is established.The availability of coupling model is proved by field test,the influence laws of different factors on the potential difference are found out,and the power frequency grounding dispersed current characteristics of distribution network disconnection under actual extreme working conditions are clarified.3)Considering the frequency-domain parameters of grounding conductor and soil,and soil breakdown characteristics,the transient analysis model of lightning grounding conductor is established and verified by the measured data.Based on the dispersed current measurement sensor,the dispersed current test platform is built and verified by the dispersed current experiment.The effects of soil environment,grounding device structure and other factors on the distribution of impact dispersed current density in the ground are studied.4)Based on the power frequency fault grounding dispersed current model,the calculation method of human ventricular fibrillation probability based on finite element collaborative structured sampling is studied.By considering the relationship between human impedance and applied voltage,the current flowing through human body is calculated,and the personal safety risk of fusion multi factors under power frequency fault grounding dispersed current is quantified.Based on the lightning impulse grounding dispersed current model and by building a current receiving test platform for aquatic organisms,the response threshold of aquatic organisms under impulse condition are studied,and a method considering the distribution characteristics of lightning current amplitude and the Stress-Strength Interference model is proposed to evaluate the lightning impulse grounding dispersed current safety of aquatic organisms.Based on the actual accident case,the quantification and optimization of the risk of aquatic biological under grounding dispersed current are realized,which provides decision support for active protection.