Condition of Transformers Using Through Fault Monitoring


While much attention is given to the monitoring of transformer condition based on dissolved gas analysis, partial discharge, and power factor, a large percentage of transformer failure 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, these 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; cumulatively shortening the life of the transformer.

Single cyle 60Hz at seven times nominal RMS

Through Faults

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 must be considered as factors which can affect the frequency of fault occurrences. Faults are generally the result of a low impedance path to ground being introduced into a circuit, such as dielectric failure of protection equipment or tree limbs striking overhead lines, causing the current to exceed the transformer’s rated base. 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. However, 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 system protection. The amount of energy flowing through the transformer (i2t) for the duration of these transient faults places excessive mechanical and thermal stresses on the core and coil assembly of the affected transformer, but due to the power-protection system operating effectively, little consideration is given to the weakening of the transformers clamping system or any core deformation that may have occurred as a result of the increased electromagnetic force. The frequency at which these events occur can have an aggregated effect (cumulative i2t) on the transformer’s mechanical structure, ultimately resulting in a decrease of the transformer’s fault withstand capability. The presence of these cumulative effects is problematic in that they are not easily detected using routine maintenance testing such as power factor and dissolved gas analysis but would require more in-depth testing procedures such as sweep frequency analysis or winding induction testing.


The increased implementation of renewable energy sources connected to the electrical grid and GIC induced core saturation are two factors known to introduce undesirable harmonic frequencies onto the electrical grid. The presence of these harmonics, which can often go undetected, have a detrimental effect on the overall life expectancy of a transformer as they cause an increase in operating temperature. This increase in operating temperature can be attributed to the direct correlation of frequency with levels of eddy currents and iron losses. This increase in the transformer’s operating temperature accelerates degradation of the cellulosic insulation system, causing a loss of both mechanical and dielectric strength. Once this increased thermal condition exceeds 1500C, it would likely be detected via dissolved gas analysis due to an increase in the production of hydrogen and methane, however the increased thermal condition could have gone undetected in ranges below this threshold for a considerable period, allowing irreparable reduction in the degree of polymerization and fault withstand values.

E3 Transformer Monitor

E3 Harmonics and Fault Counter Cards

The harmonics and fault counter card expand the E3’s monitoring capability to include the capture of total fault count, maximum fault current, accumulated fault current [Per Unit=Fault i2t/Rated i2t], THD, and the ratio in percent of even/odd harmonics. Figure 1 is an example of a single cycle fault with an RMS at seven times nominal captured and displayed via the through fault and harmonics card on the E3’s webpage.]

Though this graph only illustrates a single cycle fault, power-protection systems generally require a minimum of five cycles to clear excessive fault current, and in many instances, this time is even greater due to slow operation of protection equipment, exposing the transformer to an increased risk of potential mechanical damage. While it is difficult to quantify the level of cumulative i2t a specific transformer can withstand and still remain fit for service due to the wide range of variables that must be considered when performing assessment, knowledge of a particular transformer experiencing excessive through fault levels could trigger a decision to perform more in depth testing such as sweep frequency analysis, allowing slight deformations of the core and coil assembly to be identified, which would be undetectable by more routine testing methods. Through faults are a common occurrence on most electrical systems, yet the cumulative effects of these events, which can reduce the withstand capability of a transformers mechanical systems, are rarely taken into consideration when determining overall equipment condition. The data provided by the E3 with through fault and harmonics monitoring, raises awareness of through fault and harmonic distortion’s cumulative values, allowing proactive testing and maintenance to be implemented before transformer failure occurs.