A transformer winding consists of a paper insulated conductor, wound around the core, supported by insulated structure, and clamping to ensure rigidity, tensile strength and minimal movement. A large percentage of transformer failures can be attributed to weakening of the core and coil’s mechanical integrity due to through faults generated by external events. Power-protection systems are designed to record and limit both the amplitude and duration of these events. However, due to the reliability of these protective systems, transient faults are often not taken into consideration when determining overall transformer condition. Even when functioning as intended, protective systems can allow several cycles of fault current to pass through the transformer placing excessive thermal and mechanical stresses on the core and winding assemblies; which can shorten the life of the transformer.
How often do through faults occur?
Through faults happen all the time and do not necessarily mean a failure is imminent. Potential generation of through-faults can vary greatly across short spans of any electrical grid, and factors such as circuit length, type (overhead, underground), condition of protective equipment, degrading infrastructure, and terrain are all factors that can affect the frequency of fault occurrences.
How are through faults generated?
Underground systems are designed to limit exposure to potential fault sources, however, when these underground faults do occur, they are usually persistent, causing a lock out of the protection system which requires a closer investigation before power can be restored.
In overhead systems these faults tend to happen more frequently and are often transient in nature, resulting in the fault being cleared in a few milliseconds by the power protection system. Learn more in this article.
The Effect of Through Faults on a Transformer
The amount of energy flowing through the transformer during a through fault places a lot of stress on the core and coil assembly. It does not cause an immediate failure. Yet the frequency of these events can have an aggregated effect on the transformer’s mechanical structure which can result in a decrease of the transformer’s fault withstand capability and an increase in insulation aging rate. In other words, the more these faults occur, the less likely the transformer can handle them.
Because the power-protection system operates effectively, there is not a lot of consideration given to the weakening of the transformer’s clamping system or core. It is hard to quantify the level of through faults a specific transformer can sustain and remain fit for service because there are so many variables to be considered when performing the assessment.
Through Fault Monitoring
Offline Testing is Not Enough
The presence of the effects of through faults is hard to detect using routine maintenance testing such as power factor and dissolved gas or furan analysis. Detection would require more in-depth testing procedures such as sweep frequency response analysis or winding induction testing. Nor are existing vintage electro-mechanical relays much help to understand the amount and duration of through faults. They do not have the capability of modern digital relays that can capture the events.
Online Transformer Through Fault Monitoring
To maximize the useful life of a transformer, it is key to minimize its exposure to through fault currents. Continuous online transformer monitoring helps minimize exposure by using data to measure and compare parametric values of the bushings, OLTC, cooling windings, DGA and many other subsystems. Monitoring allows for proactive testing and maintenance to be implemented before failure occurs.
Circuit Breaker Monitoring Helps Protect the Transformer
If a circuit breaker has slow interrupting times, the duration in which transformers are exposed to excessive fault current increases, effectively aging the transformer prematurely.
Online circuit breaker monitoring has proven to be highly effective at detecting operational abnormalities that may often go unobserved through traditional maintenance testing. Learn more about circuit breaker monitoring.
Introducing the E3 Transformer Monitor with Through Fault Monitoring
The E3 Transformer Monitor is a comprehensive, customizable solution for transformer asset management. The E3 monitors all key transformer components including through faults and harmonics. It consolidates data, manages communication to third party IEDs, and provides a single point of communication for alarms.