The significance of research on transient temperature rise of DC voltage divider

Author： GOZ Electric Time：2024-06-27 09:20:37 Read：13

The DC voltage divider has a short working time in traceability and laboratories, and its transient temperature rise will cause measurement errors. The heat transfer process of the laboratory DC voltage divider with fluid as the insulating medium is constructed by multi-field coupling of electricity, heat and fluid mechanics. The transient temperature rise and cooling characteristics of the 500kV laboratory oil-insulated DC voltage divider are calculated and analyzed. The results show that the non-isothermal flow of transformer oil will cause the upper end of the voltage divider to be higher than the lower end. The too small inner diameter of the insulating bushing causes the non-isothermal flow of transformer oil to be concentrated in the interval between adjacent voltage divider resistors, resulting in an increase in the maximum temperature rise of the voltage divider, the maximum temperature difference on the voltage divider resistor, and the thermal time constant. Under the calculation conditions, after the DC voltage divider with a smaller inner diameter of the bushing works for 2 hours under rated conditions, the maximum surface temperature of the voltage divider is as high as 76.5 ℃, and the temperature can only be reduced to 60 ℃ after 11 hours of natural cooling. This study has guiding significance for the structural design and laboratory application of DC voltage dividers.

High-voltage direct current transmission has the advantages of long transmission distance, large transmission capacity and high stability. It is currently widely used in long-distance, large-capacity transmission and power system networking. With the continuous development of ultra-high voltage direct current transmission, the frequency of DC experiments is increasing, and the measurement of DC parameters has also received more and more attention. The DC voltage divider is an indispensable and important equipment in high-voltage direct current experiments. It plays an important role such as voltage measurement, and its performance directly affects the reliability of the experimental results. The DC voltage divider usually uses resistors to divide the voltage. During the experiment, the temperature of the DC voltage divider will gradually increase, and the heat energy generated by the heating element will be brought to the upper end of the voltage divider by the fluid insulating medium, resulting in a large temperature gradient between the upper and lower ends of the DC voltage divider. Depending on the voltage level, this temperature difference can reach 30K or 40K or more. Since the resistance of the voltage divider resistor is related to the temperature, this longitudinal temperature gradient will cause measurement errors. After the experiment, if the experimenter approaches the DC voltage divider with too high a temperature, it will cause burns. Therefore, calculating the transient temperature rise of the DC voltage divider during the DC voltage measurement process is helpful to analyze the measurement uncertainty, improve the structure of the DC voltage divider, and ensure the personal safety of the experimenter.

The steady-state temperature field inside the ±1 100kV RC SF6 insulated DC voltage divider was optimized by using Ansys fluent software, and the optimization method of coaxial double air chamber overall circulation heat equalization method and the top flange cover plate equipped with a high-efficiency eddy current heat exchanger was proposed. The steady-state temperature rise of the traditional DC voltage divider was calculated using the thermal equilibrium condition, and the temperature distribution and heat dissipation of the RC voltage divider as well as the influence of air temperature and voltage divider power on the transformer oil temperature were studied. However, the flow characteristics of the transformer oil were not considered in this paper, and it was assumed that the temperature of the transformer oil in the voltage divider was uniform, which would lead to a large model error. The above studies are all for the calculation of the steady-state temperature field inside the DC voltage divider, and the thermal time constant of the DC voltage divider is large. During the experiment, it takes a long time to reach the steady-state temperature distribution. Therefore, the research on the internal temperature distribution of the DC voltage divider for laboratory use should focus on the transient temperature rise. For the DC voltage divider whose insulating medium is a solid material with high thermal conductivity, the time domain calculation and measurement test of the temperature field were carried out. The results show that it takes a long time for the DC resistor voltage divider to reach thermal equilibrium, about 10 hours, but the transient temperature rise when the insulating medium is a fluid is not calculated and analyzed in this paper. This paper analyzes the heat transfer process of the laboratory DC voltage divider with the insulating medium as a fluid, and constructs a mathematical model of its transient temperature rise by using the multi-field coupling of electricity, heat and fluid mechanics. Taking the 500 kV laboratory oil-insulated DC voltage divider as an example, its transient temperature rise is calculated, and the transient temperature rise characteristics and the influence of the inner diameter of the insulating bushing on its temperature rise characteristics are analyzed.

Heat transfer process of DC voltage divider: The laboratory DC voltage divider is in the shape of a long cylinder, and the internal voltage divider resistors are arranged in a spiral or zigzag shape and are placed vertically in a cylindrical insulating bushing filled with insulating medium. The insulating medium often uses transformer oil, SF6, etc. The upper and lower sides of the insulating bushing are sealed with circular metal flanges. The voltage divider resistor arm is usually composed of multiple resistors connected in series, and the connected resistors are connected by wires with good conductivity. If they are directly modeled and numerically calculated using the finite element method, a large number of small conductors will appear, which are difficult to split, and the convergence and accuracy of the calculation will be greatly reduced. In order to facilitate modeling and calculation, the voltage divider resistor is simplified to a cylinder with the same diameter as the actual one. At the same time, in order to ensure that the heating power before and after simplification remains unchanged, the simplified equivalent volume resistivity is calculated based on the principle that the total resistance value of each voltage divider resistor arm remains unchanged.