High Soil Resistivity Areas, Power Plants, Substation Grounding Design

The basic demands for electric power system safe operation are to ensure the personnel and equipment safety, and the reliable system operation. These demands are much close related with the design of grounding system. According to the previous power system design standards, it is only required that the grounding resistance on the step voltage and touch voltage is less than 0.5 Ohm. However, in the newly published standard "Grounding for AC electrical installations" (DL/T621-1997), higher demands for grounding resistance are put forward. Moreover, short-circuit current increases largely with the growth of power network, which also multiply the difficulties of grounding system design. In the area of high soil resistivity, this problem is even more evident. Therefore, it is necessary to take effective measures to reduce the grounding resistance.Up till now, the principle measures adopted are as follows: expanding the area of grounding net, increasing grounding rod, soil renewal and use of chemical preparation etc. In the practice of engineering, it is very common to take measures on resistance reduction willfully, neglecting investigation of external conditions, geographic location and geology conditions. These measures often do not meet the requirements and even bring dangers to safe operation of electric power systems.In this thesis, the conventional approaches for grounding computation are briefly introduced. In Chapter 3, it lays special stress on analyzing the Vertical Grounding Rod (VGR) method. In the past, Short VGR was widely used. Based on lots of references and the computational software of CDEGS, the grounding characteristics for long VGR and short VGR are analyzed in this chapter. Long VGR presents sound effect on resistance reduction, by inhibiting the spread of fault current through grounding conductors. Meanwhile, short VGR proves to be not effective on resistance reduction. Moreover, with the increase of long VGR, most fault current flow into the earth rather than spread in grounding conductors, thecurrent density decreases in the direction of horizon and the electric-fieldstrength is reduced, as a result the step and touch voltage is limited. Formulas for grounding trunk computation with long VGR are described; the design of long VGR is discussed in details, including the selection, location, quantity and depth, as well as the construction method.In Chapter 4, the chemical preparation used for resistance reduction are discussed, including the characteristic, classification and operating principles, as well as the related computation. In view of the present situation of disordered market, this chapter centers on the selection of chemical preparation. Its characteristics, corrosiveness and stability are put forward. The construction method is also described.In Chapter 5, other methods for resistance reduction are discussed, including outward extension and underwater grounding conductor etc. Related computation formulas are briefed.Finally, the selection of grounding method is much important to the safe operation of electric power systems, willful use of resistance-reduction methods proves to be harmful. Based on the study and analysis of each method, this thesis sums up its respective characteristics and scope of application, and clarifies some common mistakes in practice. Results of this thesis provide the help for further study and takes notable effect in electric power engineering.