What are the criteria for changing insulator strings on high voltage towers?

The criteria for replacing insulator strings on high-voltage towers is not solely based on the number of broken pieces nor determined by a single calculation. Instead, it involves a comprehensive consideration of multiple factors. Below are the main considerations that determine whether an insulator should be replaced:

• Physical Damage: If there is evident physical damage to the insulator, such as fractures, cracks, surface spalling, or skirt damage, then even if the predefined number of broken pieces has not been reached, replacement should be considered.

• Degradation of Electrical Performance: The electrical performance of insulators can degrade over time due to environmental influences. Regular inspections (such as measuring leakage current, salt density tests, etc.) can assess their electrical performance. When inspection results indicate that the insulator no longer meets safety operation requirements, it should be replaced.

• Reduction in Mechanical Strength: Due to prolonged exposure to wind, ice loads, and other external stresses, the mechanical strength of insulators may decrease. This can be determined through static and dynamic load testing. If the mechanical strength falls below specified values, replacement is necessary.

• Service Life: Although different types of insulators have varying design lifespans, preventive replacement is typically scheduled after a certain service period to prevent issues caused by aging.

• Number of Broken Pieces: Indeed, in some cases, the number of broken pieces can serve as a reference indicator. For example, for composite insulators, if a single skirt breaks, it is often recommended to replace the entire insulator string; for porcelain or glass insulators, if a certain percentage (such as 5% to 10%) of units in a string fail, this may trigger a decision to replace.

• Industry Standards and Regulations: Different countries and regions have their own maintenance standards and guidelines for power systems, which specify the condition assessment and replacement conditions for insulators. For instance, China's State Grid Corporation has developed standards like the "Technical Guidelines for Live Working on ±800kV DC Transmission Lines" to guide practical operations.

• Economic Analysis: Besides technical factors, the cost-effectiveness of replacement is also considered. Sometimes, even if an insulator can still be used, if the maintenance cost is too high or there are potential risks, a preemptive replacement may be chosen.

In summary, the replacement of insulator strings on high-voltage towers is the result of a multifaceted evaluation, involving aspects of safety, reliability, and economic efficiency. In practice, the operation and maintenance units will make the most appropriate decision based on the actual situation, combining all these factors.