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The Qinghai-Henan ±800kV UHV DC power transmission project is an UHV channel specially constructed for clean energy delivery, ±800kV converter transformers of the project are assembled and tested in Xining at an altitude of 2500 m, full-wave lightning impulse test is one of the main means to evaluate converter transformer insulation performance, its valve side lightning impulse voltage waveform parameters often fail to meet IEC60076-3 standards because of the large winding to the ground capacitance. In order to effectively assess the lightning tolerance of converter transformer, a simulation model of impulse test circuit of ±800kV converter transformer was built based on the existing 6000kV/560kJ impulse voltage generator and ATP-EMTP simulation software, three kinds of optimization methods are proposed, which include increasing the wavetail resistance, increasing load capacitor, and connecting the wave head in parallel with different inductors, the results show that the single optimization method can not meet the waveform requirements. Therefore, a combinatorial optimization test method of increasing the wavetail resistance and connecting the wave head in parallel with different inductors is proposed, which is completed and verified in the full-wave lightning impulse test on the valve side of Qinghai ±800kV high-end converter transformer, the results show that the optimized test waveform parameters meet the standard requirements.

During the impulse voltage comparison test, it is important to keep the waveforms smooth for obtaining consistent experiment results, especially for the front time parameter. However, steps or oscillations are often found on the front of the waveforms. The authors propose a theory that these oscillations can be caused by the resistors in the damped capacitive voltage divider, which often serves as the load capacitor in the impulse voltage generation circuit. The theoretical analysis, numerical simulation, and experimental results are performed, and a good agreement has been found in support of the proposed theory. Furthermore, the waveform parameters measured by two resistive reference standard systems were found to be influenced by the shape of the waveform and the oscillation on the front. It is recommended to avoid using a damped capacitive divider as the load capacitor but use an impulse capacitor with low equivalent series resistance instead. This would help to eliminate this common cause of oscillation on the front of the lightning impulse voltage waveform. Waveform uniformity and uncertainty of the result could then be improved during the comparison of reference measuring systems. [ABSTRACT FROM AUTHOR]

• Discharge experiments for 5–8 m long transmission line-tower gaps show the critical wave front time increases with air gap distance. • An approximate U-shaped curve was found between the front time and the average velocity of the continuous leader propagation. • Choosing standard switching impulses (250 μs/2500 μs) to conduct the discharge test in a ±800 kV transmission line-tower gap is both suitable and effective, however when the operation voltage further increases, the front time should be increased appropriately when conducting switching impulse tests. The front time of switching impulses has a significant influence on long air gap discharge performance, but most testing experiments have been conducted using standard impulses. Here, we carried out experiments investigating the discharge characteristics of a ±800 kV transmission line-tower gap (5–8 m) under positive switching impulses with different front times and obtained important physical parameters, such as the time and voltage of the leader inception, the leader propagation velocity, and the 50% breakdown voltage (U 50). The results show that (1) the critical wave front time increases with the air gap distance; (2) for the tested gaps, the time and voltage of the leader inception show minimum dispersion when the front time is 250 μs; (3) an approximately U-shaped curve occurs between the front time and the average velocity of the continuous leader propagation, and the velocity is the slowest with a front time of 500 μs. The results illustrated that choosing standard switching impulses (250 μs/2500 μs) to conduct a discharge test in a ±800 kV transmission line-tower gap is both suitable and effective; however, when the operation voltage further increases, the front time should be increased appropriately when conducting switching impulse tests. [ABSTRACT FROM AUTHOR]