Studies Of The Grounding Design In Power Plants And Substations

The grounding problem of power systems seems very simple, but is much complex and important for personnel and equipment safety. Especially, with the development of power systems and the growth of power networks, the short-circuit current increases largely. Due to the common use of micro-computer monitoring devices, the limit for the grounding current becomes more rigorous. There are a series of requirements, not only on the 50Hz grounding resistance, but also on the impact grounding resistance, the device hot stability, the device touch voltage, the step voltage and the ground voltage disturbance. Many faults, such as the main equipment damage and the lose power in a power substation, arise from the grounding device problem. Therefor, enough attention should be put on the grounding problem of power system.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 andgeology conditions. These measures often do not meet the requirementsand 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 the general requirement for the grounding device in power plants and substations. Formulas for grounding trunk computation are described; the design for the grounding net in power plant and substations is discussed in details, including the design step, requirement, as well as the construction method.In Chapter 4, several measures for resistance reduction are discussed, including the external expanding method, the deep well method, the expanding net method, the grounding net under water method, and the chemical preparation method. 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, the main problems in the grounding net of power plants and substations are discussed. The improvement step and approach are presented.In Chapter 6, as examples, the grounding nets of two substations in Zhumadian are designed and reconstructed. After improvement, the grounding resistance, the touch voltage and the step voltage arrive to the requirement. These two practical examples will provide guidance and reference for solving the grounding problem.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.