Study Of Graphite Composite Electrical Grounding Material And Its Application In Power System

Equipment of power system should be grounded well to decrease the number of lightning stroke trip and to ensure a steady operation of power grid and personal safety. Nowadays, most grounding rod or grounding grid is made with metallic conductor for their high electric conductivity, including steel, copper and steel with alloy coating. Yet, the metallic grounding electrode or grounding grid is proved that it is not fit well with the soil and also easy to be corroded in soil, which shortens their service life. The grounding resistance is difficult to be reduced to the satisfied value in areas where there's high soil resistivity such as mountain, It' Ialso a difficult engineering to construct a grounding grid in fieldwork. Besides, the metallic grounding electrode of tower may be stolen by human that may cause a great risk to the operation of power system. Therefore, a new grounding material is proposed to avoid these technological difficulties in this paper. The new grounding material is called as "flexible graphite composite grounding material". It provides a novel way to the advantage of grounding material in power system. The mainly work conducted in this paper is about preparation, test and application of the new material as shown below.(1) The raw material includes flake graphite of high purity, organic and inorganic fibers, and adhesive and so on. The flake graphite of high purity is selected to be the main conductive material. This raw material is processed through acidification, extrusion, molding and so on. Finally, a new grounding material is made with Leeds line hierarchical structure, namely flexible graphite composite grounding material.(2)Finite element method is applied to simulate the skin effect and inductive effect of steel, copper and graphite composite grounding material. The simulation results indicates that the utilization ratio of graphite composite grounding material is higher than metallic material for there's smaller skin effect; the inductive effect of each conductive material decreases with their radius and the inductance of graphite composite grounding material is smaller than steel resulting in a smaller grounding resistance with high-frequency current.Furthermore, simulation tests on active length of leakage current in soil were done to different grounding material, including mandarin electrode of diameter ?10mm, steel electrode of diameter ?10mm and graphite composite ground electrode of different diameter ?10mm??22mm??28mm. The effect of frequency and soil condition was considered in the simulation test. The simulation data indicated that the active length of leakage current in soil of ?28mm graphite composite ground electrode was closed to that of ?lOmm mandarin electrode, which was longer than that of ?10mm steel electrode.Optimized design of graphite composite grounding material is conducted with numerical calculation and graphite composite grounding material is made with expanded radius. Calculation results indicates that the skin effect and inductive effect decreases with the expanded radius ratio and tend to level off; it is appropriate to made graphite composite grounding material with expanded radius ratio of30%-40%; the grounding resistance of graphite composite grounding grid is decreased effectively with expanded radius ratio which increases its utilization ratio.Simulation tests are conducted to verify the feasibility whether graphite composite grounding material can be used in tower grounding grid. The calculation results show that the AC grounding resistance of graphite composite grounding grid is almost the same as that of other metallic material when the soil resistivity is larger than100?·m; yet their difference increases with the size of grounding grid.Simulation tests are conducted to verify the feasibility whether graphite composite grounding material can be used in substation. The calculation results show that there's difference of AC grounding resistance between graphite composite grounding grid and other metallic material; the difference of network potential difference, step voltage and touch voltage between graphite composite grounding material and other metallic material tends to decrease with the increasing soil resistivity and increase with the size of grounding grid. To improve the grounding characteristics of graphite composite grounding material in substation, measures are proposed that(3)A series of tests are conducted to graphite composite grounding material to get the knowledge of its electric characteristics, thermal characteristics, mechanical characteristics and corrosion characteristics. The resistivity and resistance between graphite composite grounding material and its connection is measures and there's good structure stability with impulse and short-circuit current of large amplitude. Circulating temperature rise test is conducted between-40?and200?and the results indicates that there's a negative temperature characteristic of the graphite composite grounding material resistivity that is much different from metallic material resistivity. The mechanical test results show a stability of graphite composite grounding material and its connection with bending force, torsion and tensile force tests. Comparison is conducted to the corrosion rate of steel, copper and graphite composite grounding material in5typical soil simulation solutions. Accelerated corrosion test results show that graphite composite grounding material performs well.(4)To solve the problem that partial parameters can not be scaled according to the scale ratio in scaled model test, a new extrapolation method called prediction coefficient correction method is proposed to provide guidance to AC grounding simulation test and impact grounding simulation test with non-scaled radius and material resistivity. Analysis is conducted to the effect of soil resistivity on the distortion item with numerical calculation. The effect of non-scaled radius on test results decreases with the increasing soil resistivity. Numerical calculation results indicate that forecast error decreases with the decreasing scaled ratio, but decreases the error caused by grounding material resistivity. Simulation tests results indicate that prediction coefficient correction method can effectively correct the error caused by non-scaled parameters. (5) For prediction coefficient correction method can not achieve series of distortion test, another new extrapolation method is proposed to decrease the error caused by non-scaled parameters. It makes other parameters to be a non-scaled one to compensate the error caused by test conditions. Numerical calculation is conducted to get the knowledge of application range of the two correction methods.(6) Simulation test and real test is conducted to grounding grid with graphite composite grounding material. The above two correction methods are applied to these tests. The test results indicate that there's good adaptability of two correction methods.(7) Lightning impulse simulation test was conducted to1:1model tower grounding grid of220kV transmission lines. The model grounding grid was made with graphite composite ground electrode of diameter ?28mm. The maximum amplititude of applied current impulse of8/20?s was Ich_max=80.33kA. The lightning impulse simulation test results indicated that the impact coefficient of graphite composite ground electrode decreased with increasing lightning current and tended to level off to0.92. Besides, the calculation results of local electric field intensity showed that the spark discharge occurred when the local field intensity was larger than170kV/m.(8)The real test results on horizontal grounding rod, tower ground grid and combined ground grid show that there's good adaptability of composite grounding material in practical field.The composite grounding material is applied to110kV and220kV tower grounding grid and the field test results indicate the new grounding material can achieve a satisfying feature in complicated geological condition.