Testing is crucial for ensuring the reliability of transformers in power grids. While their electrical function may seem simple, transformers are actually complex machines that require careful design, manufacturing, and testing to ensure their long-term reliability. Many large power transformers remain in service for over 40 years, making them among the few products designed to last that long in today's fast-paced technological world.
As the core components of power transmission and distribution networks, transformers' reliability under both normal and abnormal service conditions is critical for ensuring a reliable power supply, which is essential for society at large.
In addition to thermal stresses resulting from transmitting the rated power, transformers are also exposed to abnormal stresses, including dielectric stress caused by lightning and switching phenomena, as well as electrodynamic stress from short-circuits occurring in the network due to natural causes or equipment failures. While it's not possible to completely avoid these abnormal stresses, transformers must be designed and built to withstand them without being damaged or impaired.
Often, calculations are provided as evidence of the transformer's capability to withstand electrodynamic stresses. However, these calculations are typically based on theoretical stresses applied to ideal materials in static conditions, unlike the actual dynamic stresses the complete transformer experiences during testing.
Experience gathered over the past two decades at KEMA Labs - CESI Group’s Testing Division - reveals that 25-30% of transformers tested for short-circuit fail during these tests. Failures often occur in unexpected ways not accounted for by the "successful" calculation models. The failures caused by electrodynamic forces are often mistakenly attributed to failures caused by dielectric stresses because they manifest as failures of the insulating materials.
Testing is an integral part of the transformer design process as it verifies that the product meets specifications and requirements. It ensures that the final product functions properly, is reliable, and safe to use. Reducing testing requirements increases the risk of design flaws or defects going unnoticed, which can lead to unforeseen failures in transformers within power networks.
Continuous developments in transformer design, driven by evolving requirements such as, for example, reduced weight, less oil, compact designs for wind turbine applications, varying and unbalanced loading for solar applications, have introduced numerous changes in winding design, insulating liquids, insulation materials, and production processes. Consequently, thorough testing of transformers under both normal and abnormal conditions becomes necessary. Worldwide experience demonstrates that full-scale testing is the most comprehensive method to ensure that transformer designs comply with the requirements of the standards.
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