Live Webinar Series

Dave Stelmach, Director of Business and Technical Development, intellirent

When a power transformer unexpectedly trips offline, the investigation begins before any electrical test equipment is connected. Modern protective relays capture a wealth of information that can help determine whether the trip resulted from an internal transformer fault, an external system disturbance, or another operating condition. 

This presentation examines the diagnostic information available immediately following a transformer trip and demonstrates how technicians can use it to guide their investigation. Topics include relay event reports, oscillography, sequence-of-events records, and the protective element that initiated the trip. Together, these data sources provide valuable insight into why the transformer was removed from service. 

Rather than treating every transformer trip as a potential equipment failure, technicians should first establish the cause of the trip and use that information to select the most appropriate electrical tests. By understanding the event before testing begins, investigations can be more focused, testing can be prioritized, and the most likely failure mode can be identified more efficiently. 

After reviewing the protective relay records and determining why the transformer was removed from service, the next step is identifying the most likely failure mode. Before performing electrical testing, technicians should gather and evaluate the available evidence to develop a logical understanding of what may have occurred inside the transformer. 

This presentation explores common transformer failure modes, including mechanical, dielectric, electrical, and thermal failures, and discusses how available diagnostic information can be used to identify the most probable cause. Topics include dissolved gas analysis (DGA), oil quality test results, visual inspections, maintenance history, and system operating events, all of which provide valuable insight into transformer condition. The presentation also examines how factors such as through faults, overloading, overvoltage, moisture ingress, insulation aging, and cooling system deficiencies contribute to specific failure mechanisms. 

Rather than performing every available electrical test, technicians should first establish the most likely failure mode and use that understanding to select the most appropriate diagnostic tests. This systematic approach improves the efficiency of the investigation, prioritizes testing, and increases the likelihood of accurately identifying the root cause of the transformer failure.

After reviewing the available diagnostic information and identifying the most likely failure mode, the final step is confirming the transformer's condition through electrical testing. Like putting together a puzzle, each electrical test provides another piece of evidence. By understanding the suspected failure mode, technicians can focus on the electrical tests and specific test results that provide the strongest evidence to confirm the diagnosis. 

This presentation focuses on choosing the right electrical tests based on the suspected failure mode. We'll discuss the purpose, strengths, and limitations of common transformer electrical tests, including turns ratio (TTR), winding resistance, excitation current, leakage reactance, power factor, insulation resistance, sweep frequency response analysis (SFRA), dielectric frequency response (DFR), bushing testing, load tap changer (LTC) testing, and partial discharge testing. Attendees will learn when each test provides the greatest value and how multiple test results can be used together to confirm or eliminate potential failure modes. 

Rather than applying every available electrical test, technicians should develop a testing strategy based on the information gathered throughout the investigation. This approach helps focus and prioritize testing, allowing technicians to concentrate on the tests and results that are most meaningful for confirming the suspected failure mode and completing a comprehensive transformer condition assessment.