318 / 2022-03-15 13:08:12

Diagnosis and Elimination of an Overheating Fault for Valve-Side Structure in Converter Transformers

converter transformers,overheating fault,valve-side structure,contact fingers,DC resistance

IEEE ICHVE 2022 / 1-PAGE ABSTRACT

Diagnosis and elimination of an overheating fault in the valve-side structure of ±800kV converter transformers

Xinyu Luo¹, Rui Liu¹, Zhenze Long¹, and Longjiao Li<sup>2

1</sup>Electric Power Research Institute of State Grid Sichuan Electric Power Company

²State Grid Sichuan Electric Power Company

Email :luoxinyu@cqu.edu.cn



Purpose and Aim

At present, the longest service time of ultra-high-voltage (UHV) converter transformers has exceeded ten years and the number of faults is increasing, which seriously threatens the reliable operation of the DC system. Recently, it was found that the valve-side DC resistance of ±800kV converter transformers increased abnormally in annual maintenance, which led to an outage of converter stations. This type of fault in UHV converter transformers was discovered firstly in the world and deserved to report.

Experimental methods

In the annual maintenance of converter transformers, DC resistance testers were used to measure the DC resistance of the valve-side and grid-side windings with bushings of transformers. Furthermore, the DC resistance was measured after the transformers were heated based on low-frequency heating. Finally, the valve-side structure was disassembled for visual inspection and even the scanning electron microscope (SEM) was employed to diagnose.

Results and discussion

The main reason for the overheating fault was the damage of the part of contact fingers on the tail plug of valve-side bushings, as shown in Figure 1. One explanation is that the fingers were overloaded and turned black when the electric current mainly flowed through a part of contact fingers in earthquakes and other cases. Another one is that since the contact between the contact fingers and the socket was poor, the contact finger turned black due to the increased contact resistance. The on-site solution is to enlarge the load capacity of the tail plug to suppress overheating by increasing the number of contact fingers.

Conclusions

The problem-solving ideas in the report will provide valuable experience for subsequent similar works in other electric companies. Moreover, the evolution mechanism of the fault is still controversial and deserve in-depth discussion and research in academia and industry.

Appendix

Figure 1 shows valve-side structures of converter transformers and the damaged contact fingers.



Figure 1. Valve-side structure of converter transformers and the damaged contact fingers