A new method for internal over-voltage protection of electrical equipment is presentd with MOV, but MOV submerged in insulation material can interact each other on some conditions, consequently the MOV performance of limiting over-voltage is influenced. To guarantee effectiveness of the protection method, the MOV characteristics of lightning in air, transformer oil and liquid nitrogen are experimentally investigated, and the ANSYS software is applied to calculate the thermal performance of MOV in liquid nitrogen when MOV is struck by lightning especially. Experimental results indicate that response voltage and current increase slowly along with quick raise of lightning voltage, so MOV in several media effectively restricts lightning over-voltage, and can protect operation safety of electrical equipment. The optimum limiting effects come from MOV in liquid nitrogen, and mean voltage is less than that in air by about 200V. The impair of low temperature on MOV microstructure is main reason. Simulation results of the ANSYS software show that,at the temperature above boiling point of liquid nitrogen on the contact surface between MOV and liquid nitrogen, the gasification of liquid nitrogen on local contact surface is likely and severely threatens the dielectric breakdown intensity of liquid nitrogen. Several methods are discussed to improve thermal gradient distribution and avoid local gasification of liquid nitrogen on MOV surface.

To calculate the breaking voltage and current of the high voltage current limiting fuse,and to discuss the impact on the short circuit test system,it is proposed in this paper to build a simulation model of a nonlinear circuit including fuse and metal oxide arrester(MOA) based on the electromagnetism transient program EMTP/ATP.The nonlinear circuit is a short circuit test system of the high voltage current limiting fuse.The high voltage current limiting fuse transient is investigated using this model.To simulate the complex change event of the fuse resistor exactly,the fuse resistor is replaced by several time varying nonlinear resistors in series instead of a single one.Therefore,the simulation of the fuse transient becomes more accurate.The important parameters which characterize the fuse transient such as short switching voltage,short switching current and I2t of the fuse are obtained.They are consistent with the real test results.The correctness of the method is verified by using the simulation the high voltage current limiting fuse transient.The voltage of the generator can also be acquired in simulation.The switching voltage and I2t of the fuse,voltage of the generator can be used as the principle to design the protection of the short circuit test system.

To protect the impulse generator,which was the core of high voltage apparatus test system,a metal oxide arrester would be designed. When current-limiting fuses(CLFs) breaking in fuse breaking test system,fuse element melt and gasified,the arc voltage generated by CLFs will transfer arc energy into the inductive elements of the CLFs breaking test system,such as inductor and capacitor.The fuse elements melt so quick that the overvoltage was higher than other switch-operating overvoltage.The melting course of the CLFs was studied,and an electric-thermal model was established with the finite element method to obtain the resistance-time graph for CLFs,and a circuit model of fuse breaking test system was set up.With the calculation results of electric-thermal model,the overvoltage was calculated with EMTP/ATP.For the appointed device to be protected,the design included electric design and thermal design.Electric design could assure the device to run safely without insulation damage and thermal design should maintain the life time of MOA without thermal crash.Considering the electric and thermal demands,the MOA is picked.Guidelines of MOA was developed for selection and application of the non-electric power systems surge arresters.