Transformer Through Fault Monitoring | Monitoring Through Faults

Through Faults

Through fault monitoring helps minimize exposure to through fault currents. 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 record and limit both the amplitude and duration of these events. However, due to the reliability of protective systems, transient faults are often not taken into consideration when determining overall 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. Several factors affect the frequency of fault occurrences: circuit length, type (overhead, underground), condition of protective equipment, degrading infrastructure, and terrain.

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How are through faults generated?

When a fault occurs, there is an excessive amount of current induced within the transformer’s windings. Introducing a low impedance path to the ground into the circuit results in faults. 

Underground systems limit exposure to potential fault sources. However, when these underground faults do occur they are usually persistent. This causes 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. They 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 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. This 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.

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Online Transformer Through Fault Monitoring

To maximize the life of a transformer, it is key to minimize its exposure to through fault currents. 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 helps prevent failure and allows time for proactive testing and maintenance.

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

E3 Transformer Monitor

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.


Additional Resources


This video shows what is happening inside a transformer when  through-faults occur.


This webinar discusses best practices for determining and limiting the effects of through faults.


Are through-faults and slow breakers damaging your transformer?