The Safety Analysis And The Study On Reduction Measures Of Substation Grounding System In Seasonal Frozen Soil Area

In China,seasonal frozen soil mainly exists in high altitude or high latitude areas,it has an important effect on the grounding system of seasonal frozen soil,the main reason is that when the soil is frozen,the soil resistivity will increase significantly,which not only affect the grounding resistance,contact voltage and step voltage value,but also affect the allowable value of the step voltage and contact voltage.In this paper,the changes of soil temperature in permafrost regions and its development process were simulated and seasonal frozen soil structure were measured by experimental study;The effect of seasonal frozen soil on grounding resistance,step voltage,contact voltage,soil insulation layer and shunt coefficient under different soil parameters was discussed.Finally,the reduction measures and effective reduction schemes in seasonally frozen soil area were given.Firstly,by establishing the soil model of seasonal frozen soil area,the development process of frozen soil and the temperature change of each layer were studied by using finite element simulation software.Simulation results show that the development of seasonal frozen soil is a process which the lowest temperature layer goes gradually down,the main reason is that the temperature change in the deeper layer behind the temperature change of upper layer;the maximum depth of frozen soil occurs when the surface soil begin to melt,at this time,the lowest depth of temperature in the soil reaches the lowest level;the maximum depth of permafrost is influenced by the difference between the temperature amplitude and the average temperature.When the difference is equal,the freezing depth is almost equal.The duration time of permafrost is influenced by the average temperature and the amplitude of soil temperature in the surface.Secondly,the changing trend of grounding resistance,contact voltage and step voltage of standard grounding network under frozen soil model was simulated.The results show that if the depth of freezing is greater than the buried depth of the grounding network,it will directly affect the grounding safety of the area.The soil internal resistance is mainly determined by the soil structure between the surface and the grounding network,the contact and the step voltage allowed by the soil resistance value are more reasonable.The value of step voltage and contact voltage determined by soil resistance is about 1.6 times the safety value determined by IEEE Standard 80 – 2000,under appropriate conditions,the requirement of the permissible value of the seasonal frozen soil area can be appropriately relaxed.Thirdly,the influence of frozen soil on the short-circuit current of grounding network in the seasonal frozen soil area was studied.The study find: because the tower grounding body depth is smaller than the burial depth of the grounding grid,frozen soil have greater impact on tower grounding device,resulting in the substation shunt coefficient increasing sharply,making the original design of the grid can't meet the safety standards;by the way of adding vertical grounding poles or increasing buried depth of the grounding devices of the towers close to the substation,the shunt coefficient of the grounding grid will be reduced and the grounding current will be reduced.Finally,the influence of different resistance reduction measures and resistance reduction schemes on grounding safety parameters in seasonal frozen soil area is studied.It is concluded that vertical grounding electrode is the most effective way to reduce grounding resistance in seasonal frozen soil area,because it can use the lower resistivity soil to leak current.For the grounding network,the most effective reduction scheme is to sink the frame of the ground network to the unfrozen soil layer,and to add the vertical grounding electrode inside the ground network to reduce the resistance.