Analysis and conclusion of transient temperature rise of DC voltage divider

Author： GOZ Electric Time：2024-06-29 09:29:55 Read：12

The main heat transfer process inside the DC voltage divider. When the voltage divider is used to measure DC voltage, DC current will flow through the upper and lower metal flanges and the voltage divider resistor, converting electrical power into thermal power. The heat energy generated on the voltage divider resistor increases the temperature of the nearby fluid insulating medium through convection heat transfer, and the density decreases due to thermal expansion, forming a non-isothermal flow, and then the heat energy generated by the voltage divider resistor inside the voltage divider is transferred to the upper and lower metal flanges and insulating sleeves through thermal convection and heat conduction. The upper and lower metal flanges and insulating sleeves transfer heat energy through heat conduction, and finally release the heat energy generated by the voltage divider into the environment through heat exchange with the external environment.

This paper analyzes the heat transfer process of the laboratory fluid insulating medium DC voltage divider, and constructs a mathematical model for calculating its transient temperature rise. The model simultaneously considers the electric field, flow field and temperature field involved in the voltage divider heating process, as well as the relationship between them, and gives the calculation steps for solving the transient temperature rise. For a 500kV laboratory oil-insulated DC voltage divider, the transient temperature rise process was calculated and analyzed using the mathematical model constructed in this paper, and the following conclusions were obtained:

1) Due to the non-isothermal flow of transformer oil, the heat energy at the lower end of the DC voltage divider will be brought to the upper end, resulting in a higher temperature at the upper end than at the lower end. For the voltage divider with an inner diameter of 375mm in the bushing studied in this paper, the maximum temperature is 69.5℃ after working for 2h under rated conditions at an ambient temperature of 35℃, located on the voltage divider resistor 220mm away from the upper flange, and the maximum temperature difference on the voltage divider resistor is 32.3℃. The longitudinal temperature difference will lead to measurement errors. It is recommended to reduce the heat energy transferred from the lower end to the upper end of the fluid insulation medium through measures such as segmented insulation.

2) It takes a long time for the voltage divider to reach a steady-state temperature distribution. When analyzing the voltage divider with a shorter working time, the transient temperature rise should be the main focus. For the voltage divider with a bushing inner diameter of 375mm studied in this paper, under rated working conditions, the temperature of the highest temperature point changes with time. It is estimated that the highest temperature that the DC voltage divider can reach is 70.64 ℃, and the time required for the highest temperature point to reach 99% of the total temperature rise is 187.26 min. With the increase of the temperature rise of the DC voltage divider, the longitudinal temperature difference of the voltage divider resistor increases, and the influence of temperature on the measurement error will also increase accordingly. Therefore, when selecting the resistor of the voltage divider arm of the voltage divider, a resistor with a lower resistance temperature coefficient should be selected. The resistance temperature coefficient should generally not exceed 5×10-6 ℃-1.

3) The excessively small inner diameter of the insulating bushing causes the non-isothermal flow of the transformer oil to be concentrated in the interval between adjacent voltage divider resistors, which will lead to 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 in this paper, compared with the DC voltage divider with a bushing inner diameter of 375mm, the voltage divider with a bushing inner diameter of 200mm has a maximum temperature increase of 22.7℃, a maximum temperature difference increase of 17.3℃, and a thermal time constant increase of 29.02min after working for 2h under rated conditions. The determination of the radius of the insulating bushing of the voltage divider should comprehensively consider the requirements of temperature rise and cost. The larger the radius, the higher the cost and the lower the temperature rise;

4) Under rated voltage conditions, the initial temperature rise rate of the voltage divider is relatively fast. In order to reduce the measurement error caused by the longitudinal temperature distribution of the DC voltage divider, the continuous use time of the DC voltage divider should be shortened as much as possible during the experiment. The specific limit should be controlled according to the requirements of measurement accuracy. For example, the continuous use time of the calibrated voltage divider under rated conditions should be controlled within 10min, during which the longitudinal temperature distribution of the DC voltage divider is less different.

5) The natural cooling process of the voltage divider is relatively slow. For the voltage divider with a sleeve inner diameter of 200 mm studied in this paper, after working for 2 hours at an ambient temperature of 35 °C and rated conditions, it is recommended to cool naturally for at least 11 hours before the experimenter can approach. To speed up the cooling process, it is recommended to increase the heat dissipation area of the voltage divider or speed up the air flow to shorten the cooling time.