Reliability comparison between electrical and mechanical linkage of high voltage switch

In this paper, we analyze the reliability of high-voltage circuit breakers that use both mechanical and electrical linkage systems. By considering the failure probability distribution and the respective advantages and disadvantages of each linkage type, we aim to provide a comprehensive evaluation of their performance in real-world applications. 1. Introduction High-voltage circuit breakers play a critical role in power systems by controlling and protecting electrical networks. Their reliability is essential for ensuring the safe and stable operation of the entire system. To prevent non-full phase closing, some installations require three-phase mechanical linkage. While some users believe that mechanical linkage offers significantly higher reliability than electrical linkage, a detailed analysis is necessary to confirm this assumption. 2. Circuit Breaker Failure Probability Statistics According to CIGRE's reliability survey on 66kV and above single-pressure SF6 circuit breakers from 1978 to 1991, a total of 70,708 years of operation were analyzed. It was found that 64.8% of all failures were caused by operating mechanism issues, with 21% attributed to secondary electrical control and auxiliary circuits, and 43.8% due to mechanical failures. From 1989 to 1997, data from the national power system showed 458 recorded faults, with 304 being related to operating mechanisms. The operating mechanism consists of two main parts: the mechanical transmission system and the control and auxiliary circuits, including components like terminals, contactors, switches, coils, and gas relays. A CIGRE report (WGl3.06) also categorized failure types based on the operating mechanism. Spring-operated mechanisms showed fewer failures compared to hydraulic or pneumatic ones, indicating higher reliability. To avoid bias, this study focuses on spring-operated SF6 circuit breakers. 3. Analysis of Failure Rates in Electrical and Mechanical Linkage 3.1 Electrical vs. Mechanical Linkage Three-phase electrical linkage circuit breakers typically use three independent operating mechanisms connected via an electrical communication box, allowing synchronization through control signals. In contrast, mechanical linkage relies on a single operating mechanism connected to all three poles via rods. According to SDJ5-85, the minimum phase-to-phase distance for outdoor power devices must meet specific standards, such as 1000mm for 110kV, 2000mm for 220kV, and so on. 3.2 Failure Probability Analysis By analyzing Table 3, we can assess the likelihood of failure in both electrical and mechanical linkages. While both types face similar risks for insulation breakdown or operational failure, the key difference lies in internal mechanical failures within the spring mechanism and between the mechanism and the breaker body. 3.3 Failure Analysis 3.3.1 Failure Between Mechanism and Body Mechanical linkage systems are more complex to install and require precise alignment between the three poles. On-site conditions often lead to misalignment, increasing the risk of failure. Data from Table 1 shows that 23% of mechanical failures involve deformation, which becomes even more significant when excluding hydraulic and pneumatic mechanisms. This highlights the challenges in on-site installation and commissioning, which can increase fault rates after operation. Additionally, mechanical stress varies across poles, with those closer to the mechanism experiencing higher loads. Temperature changes also affect metal expansion and contraction, altering the position of the breaker during operation. These factors contribute to increased stress and reduced reliability, especially when phase distances exceed 2.5 meters. For SF6 breakers, where the opening distance is small, even minor mechanical variations have a large impact on performance. 3.3.2 Failure Within the Mechanism Although spring mechanisms are more reliable than hydraulic or pneumatic ones, they still require more maintenance than electrical linkages. The greater mechanical stress and vibration associated with three-phase mechanical linkage result in higher failure rates. While manufacturers vary in quality, the general trend indicates that the failure rate of mechanical linkage (P4) is significantly higher than that of electrical linkage (P3). 4. Conclusion The failure rate of three-phase mechanical linkage is generally higher than that of electrical linkage. Unless there are special requirements, electrical linkage should be preferred. For 110kV and below, where phase distances are typically under 2000mm, mechanical linkage may still be suitable. However, for 220kV and above, where phase distances range from 3000 to 4000mm, electrical linkage is more appropriate due to its better reliability and performance.

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