Issues related to reliability design of power transformers

Power transformers are critical components within the power grid. Once quality issues occur, they may not only result in significant economic and property losses but also endanger lives and cause immeasurable negative social impacts.

Generally, the reliability of a power transformer is primarily influenced by its design, technology, materials, and manufacturing standards. Among these, design—being the foundation of product quality—plays a crucial role in determining the overall reliability of power transformers.

Statistics indicate that "design defects" are the primary cause of major quality incidents historically experienced by the industry, accounting for over 80% of such events. Therefore, the reliability of transformer design is both a prerequisite and a fundamental guarantee for achieving overall product reliability. This article discusses several key aspects of transformer reliability design.

Short-Circuit Withstand Capability Design Principle

Short-circuit withstand capability is a key indicator of power transformer reliability. Damage due to insufficient short-circuit strength is not uncommon in power systems, and failures during random short-circuit tests are also frequently reported.

As a special test, only a very small proportion of power transformers—less than 1% of total production—undergo actual short-circuit testing. Hence, design validation remains the most practical approach to ensure adequate short-circuit withstand capability.

The fundamental principle of short-circuit design should focus on minimizing the actual short-circuit stress as much as possible, rather than blindly increasing the allowable stress limits. The latter approach overly depends on material properties and manufacturing processes and represents an uncontrollable design strategy.

Design Considerations for Hot Spot Temperature Rise

The hot spot temperature rise in various components of a power transformer is closely linked to its service life and directly affects long-term operational reliability. As a type test, temperature rise testing is not performed on every unit. Thus, design analysis and verification remain essential to ensure that hot spot temperature rises across all components remain within safe limits.

Transformer hot spot temperature rise design should focus on three critical areas: winding hot spots, core hot spots, and hot spots in metallic structural parts. Accurate calculation of leakage magnetic field distribution and loss density, based on product structure and parameters, provides a vital foundation for the rational selection of component materials, effective implementation of stray flux control measures, and optimized cooling oil circuit design—ensuring that all component hot spot temperature rises remain within safe values.