Capacitor bank failures

Capacitor banks are a vital tool for industrial environments that require a large amount of power. However, when things go wrong, the failure can result in a catastrophic explosion. Although the transformer blow out that recently bathed New York City in an eerie blue light was the result of an “electrical arc flash” rather than a catastrophic failure of capacitor banks, there are similarities between the two phenomena.

Based on a technical article recently published in Electrical Engineering Portal, here are the most common causes of capacitor bank failure.

Inadequate voltage rating

The most common cause of catastrophic failure in capacitor banks is due to an inadequate voltage rating. This problem occurs when the voltage across the capacitor units exceeds the design values of the inductors connecting the capacitors.


This is not to be confused with a blown fuse, the second most frequent cause of capacitor bank failure. When capacitor units blow their fuses, the short circuit is a result of overvoltage, or harmonics, and most commonly occurs due to “fatigue, incorrect application and improper branch protection,” according to EEP.

Hot in Here

Thermal failure is the third most common reason for a broken capacitor bank. Excessively hot temperatures—whether from high ambient temperature, radiated heat from adjacent equipment, or extra losses—is often behind these failures.

Ferroresonance is another frequent culprit. Ferroresonance is produced when capacitor banks interact with the source or transformer inductance, which results in undamped oscillations in the current or voltage. Connecting a minimal resistive load on the transformer secondaries or using a 3-phase interrupting device, like a ganged (3 pole) circuit breaker, can help avoid ferroresonance.

Faulty harmonics can also fell capacitor banks. According to EEP, “any nonlinear load in the system such as an arc furnace or converter equipment produces harmonics.” While filters are used to control these harmonics, capacitor banks may overheat and fail if they’re not sharp enough. Therefore, it is crucial to install the capacitor banks in the best possible location on the network.   

Our Bad

As with all sophisticated technology, there’s always the possibility that we silly humans will screw it up. This can manifest itself in a number of ways, including the possibility of a manufacturer defect.

Typically, manufacturer defects can be identified when testing the capacitor units at the factory. The failure of most electrical components is represented in a “bathtub curve,” where problems are most likely to occur early as “infant mortality” or during the “end of life wear-out period.”

Human error can come into play during other points in the capacitor’s lifecycle. It’s important to select the proper replacement fuses and to follow the correct procedures when working with circuit switchers and breakers.

For the complete list of EEP’s findings, follow this link.

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