Cooling Structural Design Of MOA And Research On Heat Dissipation

As the increase of the load density and capacity of the power network, the short-circuit current becomes larger and larger. Owing to its strong ability of the limitation on short-circuit current, good reliable in operation, good technical and economic performance, and the auto-switching realization without additional control, economic type of the fault current limiter based on Metal Oxide Arrester (MOA) is expected to be applied in high-voltage system.When short circuit happens, tremendous short circuit energy enters into MOA and there is strong short-circuit current on Metal Oxide Varistor (MOV), leading to the rapid temperature increase of MOV. In order to insure the reliability of MOA operation and extend its service life, the temperature of MOV need to be reduced as soon as possible. Therefore, both the design of cooling structure with the excellent cooling efficiency and the knowledge for the temperature distributions are very important.In this thesis, the process of heating transformation of MOA is analyzed, the main principle of some thermal analysis methods, such as Tominage Model Method, Equivalent Thermal Circuit Diagram Method, Finite Difference Method, Finite Element Method (FEM), are introduced, and the power loss property of ZnO resistor in the different current regions which including low current section, turning area section, operation impulse current section and lightning surge current section are discussed. Also, the heat transfer theories such as Fourier's law and conduction differential equations are summarized, the calculation principles of temperature field based on finite element method are briefly described, and the general steps for solving temperature field problem by using ANSYS are given.In order to improve the cooling effect of the cooling structure in MOA, a novel cooling structure is suggested. In this structure, special aluminum pad is added between two ZnO valve plates, and cooler channels are made on aluminum pad. The 3-D FEM model is established to solve the temperature field of the traditional and novel MOV, and the transient temperature field is simulated by using ANSYS code. Accordingly, the distribution characteristics of the temperature field and heat dissipation law are obtained. The systematical calculations and analyses on the influences of the diameter, numbers and distributions of the cooling channels on the heat transfer are performed, and the optimized geometric parameters with better cooling effects is presented. It is shown that the efficiency of the cooling structure can be changed by changing its diameter, there is an optimum diameter of 10 mm, where the good cooling efficiency is obtained, and the efficiency of cooling structure with the four channels is better than those with the two channels. In addition, the temperature distributions of MOV are calculated when the environment temperature is higher under stable equilibrium state after a long period running of MOA. The calculated results shows that the risen temperature is within the allowable range and MOA can reliably run in the short circuit fault. Finally, the heating and cooling properties of MOA are analyzed under different residual voltages, and the minimum time interval of two continuous faults and heating of MOA for two continuous faults are calculated.The goal of this thesis is to provide the theoretical evidence and data support for cooling structural design and heat dissipation analysis of MOA.