Neutral Grounding via Low Resistance Step Voltage Calculation and Simulation

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1 6 5 th International Conference on Materials Engineering for Advanced Technologies (ICMEAT 6) ISBN: Neutral Grounding via Low esistance Step Voltage Calculation and Simulation Yanan Ci, Shengsuo Niu, Zhirui Liang North China Electric Power University, Baoding, China ABSTACT: In this paper, the neutral point grounding via low resistance short circuit fault current single-phase ground short circuit fault were calculated and analyzed. The ground current will cause large step voltage, thus threatening the safety of pedestrians. In this paper, this step voltage uneven soil model of theoretical calculations and the use of finite element software to simulate the step voltage. Studying area neutral resistance grounding influence into the soil depth, soil resistivity, capacitance and other factors on the step voltage, step voltage underground cables for single-phase ground fault simulation. And it puts forward the measure of reducing step voltage, the step voltage to minimize the harm of human body.. INTODUCTION With the rapid development of economic, Low esistance Grounding the proportion is increasing distribution network construction is increasing, the city increased load, the cable distribution line capacitive current increases, when ground fault occurs it will produce step voltage. The need for rapid removal of fault, low resistance grounding have this advantage. Therefore, low resistance grounding method began to be applied. Neutral grounding via low resistance power cable is mainly used in large-scale network capacity to earth ground and fault current is too difficult to compensate for metropolitan power supply system. Low resistance grounding system of single phase grounding it, since the total impedance of the circuit is small, causing the ground fault current is large, resulting in a high step voltage around the point of failure, the study is significant for step voltage. Step voltage current literature only for uniform soil resistivity case gives the quantitative analysis, the paper step voltage uneven soil resistivity calculation and simulation, gives the theoretical formula, and underground cables perform step voltage of single phase simulation, measures of reducing the step voltage are put forward.. NEUTAL POINT VIA SMALL ESISTANCE GOUNDING FAILUE OCCUS FAULT CUENT CALCULATION I E A E B E C I IC I B I A Figure. Small resistance grounding system of single-phase grounding fault principle diagram. According to the node voltage equation + + jωc + jωc + jωc U + jwc EB jωce A jωce C () By the power of three-phase available: EA + EB + EC () According to the above two type available: U E B + 3 jω C + (3) When the B phase occurs Single phase short circuit fault: 65

I EB 3 jωc + U + E B + 3 jωc + (4) According to the actual line parameters Step voltage: when one-phase ground fault happens or after the wire falls to the ground, current to ground fault point as

into, it would have a step voltage between two feet below for current scatter diagram.

2 I EB 3 jωc + U + E B + 3 jωc + (4) According to the actual line parameters Step voltage: when one-phase ground fault happens or after the wire falls to the ground, current to ground fault point as the center, along the radial direction of the hemisphere flat out on the far side of the earth, and in the soil near the point of failure surface can produce a larger potential, when people walk into, it would have a step voltage between two feet below for current scatter diagram. ñ U( AB) U( AC) + U(CB) Iρ dao + dac Iρ dco + dcb d + d π dao π dco Iρ dac Iρ + π (9) ( d )( d + d ) π ( d )( d + d ) AO AO AC CO CO CB 3. SIMULATION Model of hemisphere do with the soil, the hemisphere with a radius of 4m, divided into three layers of soil modeling, along with soil depth increases, the resistivity decreases, the election center of the sphere to the point of failure. Skin thickness D m, ρ 3Ω m; the second layer thickness D 3m, ρ Ω m; Bottom 5Ω m. d CB Ñ r Figure. When earthing fault current scatter diagram. Assuming uniform soil resistivity for ρ, Electrical conductivity, O as the core(the point of fault), Set to ground fault current is I, the distance is current density of meters ground fault current I J π r The electric field intensity of the earth is: (5) J I I ρ E γ πγ r π r (6) Thus, potential for meters distance point O: I ρ I ρ U ( ) Edr dr π r π (7) Let the distance between the feet of people A, B the distance between the two points, the distance to the center of the grounding O, when A, B two points at the same soil layer, step voltage is: I ρ d AO + d AB U ( A) U ( B) d π d AO I ρ d AB π ( d )( d + d ) AO AO AB (8) When soil resistivity A, B two points are not at the same soil layer. (a) soil model sectional view (b) three-dimensional model of the soil Figure 3. Soil model. In the circuit module according to the node potential method to build a good circuit, plus the terminal, and soil module coupled to the wire. And then the soil model freely split. In the result the module to measure the earth fault near the surface meters of potential. Figure 4. Simulation model of soil plus circuit module. As can be seen from the simulation data,the closer to the point of failure greater the electrical potential, the potential of the faster unit distance attenuation, indicating the greater the closer to the point of failure step voltage. () Effect of the step voltage in the allowable range by increasing to reduce the step voltage. Walk. 66

3 changing the depth of soil resistivity and grounding, we give priority to practical application to change the resistance, resistance to change comple operations. Changing the depth and soil resistivity grounding relatively easy to operate, can also be combined in several ways would be better. (3)Heights on the step voltage grounding Ω, Changing the height of the grounding Figure 5. The relationship between the step voltage and the resistor. esistance grounding is.85ω, In this case, calculated by the theoretical formula - meters step voltage obtained using equation (4) given I 76. 3AI is put into the formula (9) U( A ) U ( B) 43V The Simulation of step voltage is 55V, the gap between theory and simulation results, since the theory does not consider the impact resistance of the wire, and other factors, the simulation closer to the actual situation. () Effect grounding the radius of the step voltage Figure 6. Grounding the radius of the earth surface potential. Increasing the height of the ground, increasing the radius is equivalent to an increase grounding area, reducing ground resistance, the ground area is inversely proportional to the ground resistance, grounding, resistance is inversely proportional to the radius of the grounding of the grounding, grounding resistance and cross step voltage proportional to the radius and height of the grounding are inversely proportional to the pressure and stride. Effect radius and height of the step voltage are same. The above three methods by changing the size of the step voltage drop of the resistor, followed by Figure 7. Ground structure on earth potential influence diagram. Increasing the height of the ground, due to the increase of the area of the grounding of the grounding body resistance decreases, step voltage decreases. The length of the vertical ground generally between -3 meters, when the length of more than three meters, reduced ground resistance effect was not significant. Height should choose 3 meters, the minimum step voltage. In order to more clearly understand the impact of these factors on the step voltage, the introduction of the step voltage (for step voltage between - meters) the concept of the rate of decline, with the representation, U represents change factors before step voltage, U represents the step voltage change factors after. U U λ % U ) Changing the reduction rate of the resistor λ % ) When the grounding by the radius of.4 meters and.8 meters to increase the step voltage reduction rate: ( ) ( ) λ 6. 6% ) When the height of the grounding by two meters to three meters of the decrease rate λ 3 6. % 4) When the depth of the grounding by.7 meters by.7 meters into rate λ 3 6. % 5) Changing the resistivity of the soil when the decrease rate 5Ω.m 4 λ 9. % 67

The above four methods by changing the size of the step voltage drop of the resistor, followed by changing the depth of soil resistivity and grounding, we give priority to practical application to

4 The above four methods by changing the size of the step voltage drop of the resistor, followed by changing the depth of soil resistivity and grounding, we give priority to practical application to change the resistance, resistance to change comple operations. Changing the depth and soil resistivity grounding relatively easy to operate, can also be combined in several ways would be better. 4. UNDEGOUND CABLE STEP VOLTAGE SINGLE-PHASE GOUND FAULT SIMULATION Underground cabling saves floor space, laying convenient, rational use of land resources. However troubleshooting, maintenance inconvenient. Cause cable failure into mechanical damage, insulation aging, over-voltage, etc., will result in single-phase ground fault, phase fault. Three cable laying in the ground simulation, the cable line is divided into a conductive layer, an insulating layer and a protective layer three. Figure 8. Cable Meshing. 5. THE MEASUES OF EDUCTION STEP VOLTAGE () Increasing the grounding area Grounding resistance and capacitance C is inversely proportional to the ground, earth capacitance C is determined mainly by its area, the capacitance is proportional to the area of the ground. () Increase the vertical ground (3) When a ground fault occurs, select the appropriate line selection device, so that the fault can be accurately cut in the short term, as far as possible to shorten the time of ground fault. (4) Improving soil resistivity (5) To the laying of the insulating layer can be effectively suppressed grounding current flows into the earth topsoil. (6) Select good quality cable installations, when a power failure occurs, timely troubleshooting and fault handling quickly restore power. 6. CONCLUSION Through the step voltage finite element analysis, simulation results and theoretical calculations derived by comparing the two results are basically the same, indicating that the step voltage theoretical formula is correct. The simulation targeted measures to lower the step voltage. Proportional to the size relationship between the step voltage and ground resistance. By reducing the grounding resistance to reduce the step voltage, increasing the contact area of the ground and soil, the depth increases, decreases and other aspects of soil resistivity. In a small resistor to the etent permitted, increasing to reduce the step voltage. Step voltage underground cables from the ground fault break into the area is relatively large, the area of the tear increases, the potential to grow. By comparison step voltage cables than overhead lines produce small, need to be further popularized. Figure 9. Cable single-phase ground fault simulation. EH, PQ, MN segment is laying underground cables rectangular soil model, the cable applied load. In the middle section of the cable that is EF FG at a cable break, cable break and soil contact simulation of single-phase ground fault. PQ near the maimum potential. The longer the cable, the greater the step voltage; break increases, the step voltage also increases. Single-phase cable fault generated step voltage is smaller than the step voltage overhead line failures. EFEENCES [] Li Gaolong. kv grid single-phase step voltage and short circuit current calculation with finite element analysis [D]. Hunan University,. [] Winding Open to Bo, Xu plus column, and so the step voltage simulation and eperimental study [J] single-phase ground fault kv low resistance grounding system Electric Power Automation Equipment, 3, 33 (6): -6. [3] Zhao Zhibin, Cui Xiang, Zhang Bo and other multi-layered soil with different resistivity Finite Grounding Network Analysis [J] Chinese Society for Electrical Engineering, 4, 4 (9): 8-3. [4] National Chung Chen, Shang Gong Fu electrical grounding technology [M] Beijing: China Electric Power Press, 3;

5 [6] People's epublic of China Ministry of Power Industry grounding DL/T6-997 AC electrical installations [S]. Beijing: China Electric Power Press, 997. [7] Chen Zhimin, Gao Pei multi-physics finite element modeling and analysis [M] Beijing: China Communications Press, 7: [8] Zhang Bao hui, Yin root power system relay protection [M] Beijing: China Electric Power Press, 5: [9] Marcelo Valdes, PE. IEEE Senior Member GEPlainville, Connecticut Ground fault location in low-voltage high-resistance grounded systems via the single-processor concept for circuit protection. Conference ecord of the 6 IEEE IAS Pulp and Paper Conference. [] He Jinliang, Yin Han, Zhang, Hu Jun, to the study of laying insulation reduce the DC grounding step voltage method [N] High Voltage Engineering, 4, 4 (7):

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