Weimin Sun

Novel method of corrosion diagnosis for grounding grid

The steel grounding grid buried in earth will corrode and become a hidden danger to the safety of the power system. It is difficult to get an accurate status of corroded ground grids with traditional methods. A novel method is proposed, by which the corrosion status of a grounding grid can be diagnosed from measured port resistances among the touchable down-lead conductors connected to the grounding grid. First, the mathematical relationship between the corrosion status of the grounding grids all grounding conductors and the measured port resistances is deduced in detail based on circuit models and theory. Then, an appropriate mathematical model is discussed for practicable computation. The completed corrosion diagnosis system, including hardware device of data measurement, diagnosis and analysis software, is also introduced concisely. Finally, a field test report is provided to validate this new method.

Seasonal influences on safety of substation grounding system

The low or high resistivity soil layer formed in raining or freezing season affects the safety of the grounding system, and leads to changes of grounding resistance of the grounding system, step and touch voltages on the ground surface. This paper systematically discusses the seasonal influence on the safety of grounding system by numerical analysis method. If the thickness of the low-resistivity soil layer formed in raining season is smaller than the burial depth of the grounding system, the touch voltage would increase. On the other hand, the raining season leads the limit of touch voltage is smaller than the actual touch voltage, even if a surface granite layer is used. The influences of raining on the safety of grounding system must be comprehensively considered. When the high resistivity soil layer is formed in freezing season, the grounding resistance, step and touch voltages would increase with the thickness or resistivity of the freezing soil layer. When the thickness of high resistivity soil layer exceeds the burial depth of the grounding system, the grounding resistance, step and touch voltages would increase sharply, and the touch voltage would exceed the limit of touch voltage decided by a surface granite layer. In order to ensure the safety of the grounding system in freezing season, the grounding system must be designed carefully, and special measures should be used, long vertical grounding electrodes added to the grounding system can effectively attenuate the seasonal influence, and improve the safety of the grounding system.

Optimal design of grounding system considering the influence of seasonal frozen soil layer

Frozen soil leads to the change of the soil model. This would affect the safety of the grounding system. The design of the grounding system considering the seasonal frozen soil should be based on the full investigation of the actual maximum depth of the frozen soil and the actual layered soil models. The final design scheme of the grounding system should be determined synthetically from two phases-first, the grounding system is designed in the normal soil model and its safety is checked in the frozen soil model, second, it is designed in the frozen soil model and its safety is checked in the normal soil model. The influences of the frozen soils on the optimal design of grounding systems in homogeneous and double-layer soil models were systematically analyzed, and the respective design methodologies of the grounding systems were proposed. The influence of vertical electrodes on the design of the grounding system considering the seasonal frozen soil was discussed. The analyzed results state that the safety of grounding system can be sufficiently improved by vertical grounding electrodes added to the horizontal grounding grid.

Two-stage algorithm for inverting structure parameters of the horizontal multilayer soil

The multilayer soil structure model is the fundament to design grounding systems. A fast algorithm is presented to invert the structure parameters of the horizontal multilayer soil. The procedure is divided into two independent stages. First, Fredholm equation of the first kind with respect to the apparent resistivity is solved by using the technology of decay spectrum, which can reduce the computation time greatly. Second, the structure parameter of soil is determined by the generalized Newton-Kantorovich method, which is more robust and less noise sensitive because of using the generalized inverse algorithm to solve the nonlinear equation system. The numerical results show the validities and main features of the proposed approach. The numerical results by using the proposed method are in a good agreement with ones from geophysical exploration.

Analysis on influence of long vertical grounding electrodes on grounding system for substation

A three dimensional grounding system is introduced to decrease the grounding resistance, step and touch voltages of grounding systems in areas with high soil resistivity or with limited areas for grounding systems. The relationship between the number of vertical grounding rods and the grounding resistance is discussed. The new means to add grounding rods to the grounding system with an explosion grounding technique, have efficiently decreased the grounding resistance in the actual grounding engineering. Reasons why vertical grounding rods can efficiently decrease the maximum touch and step voltages of the earth surface above the grounding system are analyzed. Calculated results shows the vertical grounding rods can also effectively decrease the influence of seasonal factor on safety of grounding system. This paper provides the rule to choose the vertical grounding rods in multi-layer soil based on the relationship analysis between the number and the length of the vertical grounding rods.

Optimal design analysis of grounding grids for substations built in nonuniform soil

The optimal design of grounding system can ensure the safety of a substation. The influences of reflective coefficient of double-layer soil and the thickness of top-layer soil and the grounding grid area on the optimum arrangement of grounding system were analyzed by a numerical method. It states the optimum compression ratio (OCR) should be determined by the target function of touch voltage reaching the minimum value. The OCR decreases with the increase of reflective coefficient, when the soil resistivity of the bottom-layer soil is smaller than that of upper-layer soil, the arranged horizontal conductors are more uniform than that when the soil resistivity of the bottom-layer soil is higher than that of upper-layer soil. The formulae to calculate the OCR of double-layers soil structure are obtained from analysis results, which can provide reference for engineers to design the grounding systems of substations with high safety.

Safety analysis of grounding grid for substations with different structure

The influences of the grounding conductor number for a ground system in different designs on the touch and step voltages on the earths surface are analyzed in detail in this paper. The result of this study can be used to determine the rational number of grounding conductors, which can equalize the leakage current distribution and markedly decrease touch voltage and step voltage on the earths surface. Therefore, the designers can design the grounding system for substations and power plants safely and economically. The same analysis is also performed on the unequally spaced grounding grid where the grid conductor spacing increases progressively from the periphery to the center instead of an equally spaced grounding grid.

Novel measurement system for grounding impedance of substations and power plants

A new grounding impedance measuring method based on short lead wire of current electrode and a frequency inverter AC source is proposed. The compensating site of the voltage electrode to obtain the real grounding resistance of the grounding system is analyzed according to soil structure, topological structure of the grounding system under test, and the length of current electrodes lead wire. This new method measures the ground impedance at a variety of frequencies other than power frequency, then interpolates them to reach the ground impedance at power frequency. The mutual inductance between the potential and current wires is analyzed and computed. The Intel 80C196KC cored grounding impedance measurement system is developed and the software is programmed. A field test was performed in the substation and the system worked stably.

Fast algorithm for inverting structure parameters of the horizontal multi-layer soil

A fast algorithm is presented to invert the structure parameter of the horizontal multi-layer soil. The procedure is divided into two independent stages. First, Fredholm equation of the first kind with respect to the apparent resistivity is solved by the technology of decay spectrum to reduce computation time greatly. Second, the structure parameter of soil is determined by the generalized Newton-Kantorovich method, which is more robust and less noise sensitive because of using the generalized inverse algorithm to solve the nonlinear equation group. The numerical results show the validities and main features of the proposed approach.

Multi-layer soil structure analysis for substation and power generating station sites

The multi-layer soil structure model is fundamental to design of grounding systems of substations and power generating stations. Potential distribution of ground surface generated by a point current source is derived using electromagnetic field theory, and also the relationship between the apparent soil resistivity measured by Wenner four-electrode configuration and the measurement electrode span is deduced. A nonlinear target function is introduced, and the problem to analyze structure parameters is transferred into how to obtain the extremum of the nonlinear target function. A suitable calculation method is used to acquire soil structure parameters. The studied analysis software is applied to actual substation sites; the analyzed result has a good agreement with the actual engineering result.