Digital Twin Model of Converter Transformer Output Device Based on Real-time Inversion Calculation of Transient Temperature and E-field of HVDC Bushing

The technical conditions and main performance indicators of the outlet bushing of ±800kV ultra-high voltage converter transformer are important basis for its insulation structure design. The equalizing ring, conductive tube, and oil equalizing ball are all of high potential, and the rated voltage of output bushing of the ±800kV ultra-high voltage converter transformer is 844kV (DC phase to ground 721kV, AC phase to ground 123kV). The elevated seat, the connecting flange, the earth and far-field boundary are at zero potential. When the ±800kV ultra-high voltage output bushing operates at the rated voltage, the central conductor heats up due to carrying the large DC current, and the oil temperature of the transformer inside the converter is also high. Research has shown that during operation of the converter transformer bushing, the temperature of the central conductor can reach 90°, the oil temperature in the transformer riser exceeds 65°, and the ambient temperature is 25°. From the calculated values, applying a DC voltage of 200kV to the bushing results in a radial field strength of approximately 3.4kV/mm and an axial field strength of approximately 0.242kV/mm. The radial field strength is lower than the control value of 10kV/mm and the axial field strength is lower than the control value of 2.5kV/mm. Temperature value of 3D bushing could remain constant for about 2 hours. If the temperature remained constant, the dielectric loss factor(tanδ) can be inferred. There will be no change and it has indeed remained at 0.353% throughout the test, indicating good thermal stability of the bushing.