Ruifang Li

A simple formula of grounding grid resistance in vertical two-Layer soil

Large ground grids usually were located in vertical two-layer soil. A study was done in this paper to get a simple and accurate grounding resistance expression of it. At first, some simulations were done about a same grid buried in different vertical two-layer soil to get resistances of it. An analysis was given about the relationship of grounding resistances when the grid buried in homogeneous medium or vertical two-layer medium. Then, the curve of the relationship was fitted and a formula was gotten. By discuss the mathematical and physical significance of the formula; the coefficients of it were determined. At last, an error analysis of the formula was done and an example was given to show how to calculate grounding resistance of a grid locate in vertical two-layer soil using this formula.

New Method for Calculating Ground Resistance of Grounding Grids Buried in Horizon Two-Layer Soil

With the development of ultra high voltage AC and DC power system, fault current of power plants, substations and convertor station are great than before. Larger grounding grids are needed to meet the safety regulation. Grounding grids usually located in horizon multilayer or vertical multilayer soil structure which parameters such as thickness, resistivity of each layer can be estimated. This paper presents a new method for calculating ground resistance of grounding grid buried in horizon two-layer soil. It is consisting of three chapters. In the first part, simulations were done to obtain ground resistance of grounding grid in horizon two-layer soil and uniform soil. Then, results were preprocessed to two variables, one is the ratio of resistance of grid in horizon two-layer soil to uniform soil, the other is the ratio of soil resistivity about the top layer to the borrow layer. The relationship curve between the two ratios was plotted out. In the second part, an engineering estimation formula was simplified for calculating ground resistance in horizon two-layer soil. Results of the formula proposed are compared with results of simulation software and its error is acceptable. In the last part, examples of how to use this formula are illustrated.

Optimization Function for Grounding Grid in Uniform Soil

Unequally spaced method of grounding grid can economize metal material on the premise of ensuring the safety of equipment and personnel. A new optimization formula of grounding grid buried in uniform soil is presented in this paper. An analysis of grid conductor spacing of unequally spaced grounding grid was done. Serial number was given to each conductor as a specific numbering. A conversion formula of the number to variable X was put forward. Variable Y was defined as the percentage of the distance between the conductor to the edge and the side length of grounding grid. A result was obtained that X and Y obey a specific relationship no matter how many grid conductors were used. The relationship was fitted and a new optimization formula was obtained. When the side length and conductor quantity of grounding grid were given, the position of each grid conductor can be calculated with the formula.

New Division Method on Lightning Scope and Lightning Trip-Out Rate Calculation for Catenary

Lightning trip-out rate of catenary is high in China. It seriously affects the reliability of the traction power supply system. Electro-geometric model of catenary is built to evaluate the lightning failure performance of catenary in this paper. The formula on lightning scope of earth, return lines and catenary is deduced respectively. Furthermore, lightning strike probabilities of return lines and catenary wire are obtained, the trip-out numbers of induction lightning and direct lightning of 100 kilometers lines are calculated. At last, with the combination of calculation formula on the lightning current amplitude distribution and arc establishing rate recommended by DL / T620-1997, the total lightning trip-out rate and the proportion of a variety of lightning types are obtained.

Modeling Lightning Performance of Transmission Systems Using PSCAD

Lightning flash is one of the natural causes of faults in overhead transmission lines. The critical stroke current which causes flashover is a basic parameter characterizing the lightning performance of transmission lines. In recent times, there has not been any mature experience to be referred to either in China or other foreign countries of the study on lightning withstand level of UHVDC transmission lines. Using power system computer-aided design/electro-magnetic transient in DC system (PSCAD/EMTDC), the models of lightning current generator, transmission lines, towers, insulator strings and other associated components have been established in this paper. Meanwhile, According to the criterion of insulation flashover generally adopted in engineering in China, the dependence of lightning withstand level of ±500 kV transmission lines on tower impulse footing resistance has been investigated under the influence of DC polar working voltage, when a negative descending lightning strikes to a tower top. The simulation results agree with the theoretical value in an acceptable way, substantiating that the simulation models are reasonable. The study of this paper would be referential for lightning protection evaluation of UHVDC transmission lines in China.

Study on lightning protection of 110kV transmission lines in Qinghai-Tibet railway

Lightning protection and grounding is an important problem in Qinghai-Tibet railway. A Study based on analysis of thunderstorm activity was given to find out a way to improve lightning withstand level of transmission lines in Qinghai-Tibet railway. This paper analyzed thunderstorm activity to know its characteristics and rules. Then cloud-to- ground (CG) lightning and lightning current amplitude were analyzed. For Qinghai-Tibet Railway more in frozen region and very difficult to decrease grounding resistance of grids, a simulation was done about the lightning withstand level of 110 kV transmission lines and a discussion was given about effects of it by different spanning method of lightning arrester. At last, a conclusion was given that the lightning withstand level can be increased by spanning lightning arrester properly and an analysis was given to the reliability of the transmission lines after spanning lightning arrester. The result of this paper provided a feasible method to improve lightning withstand level about transmission lines and a theory supporting about the design and construction of Qinghai-Tibet railway.

Study on Enhancing Lightning Protection Scheme of Catenary in Subway Viaduct Section

Viaduct increases the height of subway catenary, namely magnifies lightning attraction scope that lead to higher possibility of suffering lightning stroke. Therefore, it is necessary to analyze performance of lightning striking to catenary of subway in viaduct section and propose an improving lightning protection scheme. In this paper, using ATP-EMTP simulation software to establish an associated model to evaluate lightning withstand level of catenary with existing lightning protection schemes including arrester and grounding point, an improving lightning protection scheme is proposed — every pillar ground earth and arresters are installed with some installing spacing between 200m to 400m based on lightning damage degree and reliability requirements — according to analyzing results: while lightning withstand level is lowest for lightning striking to the neutral pillar, lightning withstand level is greatest for lightning striking to the both-ends pillar that arrester and grounding point are both installed; grounding point could obviously improve lightning withstand level for lightning striking to ground wire while arrester could obviously improve the lightning withstand level for lightning striking to catenary; every pillar ground earth could enhance the lowest lightning withstanding level up to 2.5 times than of that pillar ground earth across every 200m.

A study of influencing factors about vertical grounding rods performance in two-layer soil

In high resistivity soil and small grounding grid acreage, grounding resistance of power plant and substations may not meet the relative national regulation just by using horizontal grounding grid, so the resistance reducing measures should be adopted. The influences of the reflective coefficient of the double-layer soil, the top-layer soil thickness and the length of vertical grounding rods on the resistance-reducing ratio were analyzed by the CDEGS software package. The results show that if the reflective coefficient is identical, the difference of the top- layer soil resistivity has little effect on the resistance-reducing ratio. The resistance-reducing ratio decreases as the reflective coefficient increases. If the length of vertical rods is less than the top-layer soil thickness, the reflective coefficient influences the resistance-reducing ratio feebly. The resistance-reducing ratio increases with the increase of the length of vertical rods and gradually reaches saturation state. If the reflective coefficient is negative, the vertical grounding rods which are long enough to reach the bottom-layer soil can effectively decrease grounding resistance.