Study On The Calculation Method And Distribution Law Of Ground Potential Near Grounding Devices Under High Frequency Current

With the operation of Gas Insulated Substation,the frequency of fault current,operation current and other transient current is getting higher,up to 100 MHz.The influence of fault,operation and lightning transient current on grounding system is gradually increasing.Thus,the grounding system faces new transient security problems.Since the steel is mostly used as grounding conductor in China,the conductivity is smaller and the permeability is larger than that of copper.When the high frequency fault current is dispersed through the grounding device,the longitudinal impedance of the steel grounding conductor increases and the grounding device presents high impedance characteristics.The transient ground potential increases,causing potential dangers to the safety of personnel and equipment.At the same time,due to the influence of skin effect,the electric field is concentrated on the surface of the grounding conductor,and the effective length of the grounding conductor decreases.Leading to unequal potential distribution in the grounding grid of ultra high voltage and large capacity substation.In addition,as a dispersion medium,soil electrical parameters such as soil resistivity and permittivity change with frequency.Therefore,the traditional empirical formula has been unable to meet the safety requirements of the grounding system.It is necessary to propose a method to analyze the high frequency characteristics of grounding devices.In traditional analysis of grounding system,soil parameters are usually set as constant.The actual fault transient current frequency is very high,and the range is very wide.The influence of high frequency fault current should be considered in the numerical calculation of the grounding system.In this paper,the experimental study of soil electrical parameters at different frequencies is carried out.The complex impedance of soil samples is measured by Novocontrol broadband dielectric impedance spectroscopy.The curves of soil resistivity and relative permittivity with frequency are obtained from the water content range of 5% to 30%,and the experimental data is fitted by the expression of Cole-Cole model.Thus,the expression of power function of soil resistivity and relative permittivity on frequency is obtained.Then based on the full wave Maxwell equation,the finite element analysis model of the high frequency characteristic of the grounding devices is established.The model takes into account the influence of the induction current and the displacement current.The space geometric coordinate transformation is used to solve the contradiction between the finite computing area and the infinite dispersion space,and the impedance boundary condition is applied to deal with the skin effect under the high-frequency fault current.In this paper,a high frequency finite element analysis model of horizontal grounding electrode,vertical grounding electrode and horizontal grounding grid are established.The rules of ground potential distribution of three typical grounding devices under the high frequency fault current is calculated.Finally,an example of a factory that produces GIS equipment is analyzed,and the transient ground potential rise of GIS fault current is obtained.The influence of vertical grounding electrode and shielding net on ground potential distribution is discussed.Thus,the optimal design scheme of grounding grid is proposed.It is found that with the increase of the fault current frequency,the inductance effect in the grounding conductor is enhanced,the grounding device presents higher ground potential,and the ground potential distribution along the conductor direction is more uneven.In addition,adding vertical grounding conductor or shielding net can increase the disperse area of the grounding grid,significantly improve the high impedance problem of grounding grid under the condition of high frequency fault current,and make the ground potential distribution more uniform.