Lurking In Your UPS Systems

An Overlooked Danger: The facts about routine replacement of AC and DC Capacitors in UPS Systems

Did you know UPS systems contain consumables that require maintenance as well as regular replacement? Over the years, we have found that many UPS/Critical Power users are not fully aware of these critical components of the average/conventional UPS System, aside from Batteries.

The AC and DC Filter Capacitors are among the consumables within the UPS System (along with Cooling Fans) that have an expected “useful life” which results in them being the most neglected for routine replacement.  As such, they can be the most damaging and costly to a UPS System (and Critical Loads) when they fail unexpectedly or catastrophically. Few consider the fact that these UPS Systems are On-Line, 24X7X365 days a year, for year after year after year.  Due to this extreme usage, capacitors suffer from wear and tear like any mechanical or electro-mechanical device and, as a result, require routine service and replacement.

The AC and DC Capacitors are relatively simple but vital components in the AC/DC/AC Conversion process that is the primary function of any UPS System. These Capacitors also have expected/designed operational life to maintain operational integrity ranging from 4 to 8 years at the most.

A brief description of what Capacitors do in a UPS System

On the AC Input (Line) side, these AC Input Filter Capacitors receive the brunt of the Input Voltage waveforms distortions, high/low transients, spikes, etc.  They also assist with the “filtering” of the Input Power by removing these harmful effects and improving the power factor and providing a cleaner waveform for the rectification of ac to dc.  This removes much of the unwanted harmonic content of the dc waveform provided to the Batteries and the Inverter Input.

The DC Filter Capacitors are next in line to provide isolation, remove as much of the remaining ac content from the DC Waveform, providing a cleaner DC Waveform.  This float charges the Batteries and provides brief DCV support to the Inverter during very fast Input fails and sags that normally are too quick for the Batteries to begin discharge.

Once the Inverter converts the DC back to AC, it actually is creating a “stepped” or “square” AC Waveform which is unacceptable for AC power. This “stepped” AC Waveform is then passed thru the AC Output Filter {comprised of AC Capacitors and Inductors) and sometimes a transformer to remove unwanted harmonic content and smooths out the final AC Waveform to the UPS Load.

All of these functions are critical to getting clean, consistent power to your Critical Load making the reliability of your capacitors essential to the UPS.

How do failures happen?

Occasionally, AC or DC Capacitors can have isolated internal failures, containment breaches, and can suffer from fatigue. Sometimes, one or two capacitors may “open” quietly and without fanfare or load affecting incident.

When this happens on the Input side, less complete harmonic filtering is encountered, there is less isolation from spikes, sags and surges, the power factor is reduced, and the Rectifier receives “dirtier” power to rectify.

On the DC Side, one or two failed DC Capacitors can and does affect the Batteries directly by losing the ability to adequately remove any leftover AC content from the rectifier, allowing damaging AC content to pass thru to the DC Batteries, effectively reducing the life and performance of the Battery Plant.

When a UPS has one or two or more AC Output filters “open”, when fail, the AC Output Waveform is directly affected in a negative way, the Harmonic filtering ability is reduced, waveform distortion develops, power factor is reduced and this can lead to damaging sensitive critical load devices.

But when they fail in a catastrophic manner, they generally do considerable and extensive damage to the UPS System, resulting in loss of the Critical Loads as well as lengthy downtime and a costly repair.

An example of when good Capacitors go bad

Lurking In UPSDC Filter capacitor after -Lurking In UPS

The pictures below show the failure of only 1 DC Filter capacitor which suffered an internal catastrophic failure while energized and online supporting critical load. These pictures speak for themselves. This UPS was destroyed in the blink of an eye, and the Critical Load went down as a result. This explosion of highly energized plasma not only fatally damaged the UPS, but also damaged the Input Feeder Circuit Breaker and the Output Circuit Breaker, resulting in an extended outage. No Utility or Generator could save them at this point.

These capacitors were 9 years of age and had been recommended twice in two years to be replaced. However, due to budgeting constraints, the site chose not to replace them. The $6,000.00 saved in remedial maintenance and replacement ended up costing this facility over $50,000 in an expedited, replacement UPS System, plus electrical installation and repair costs.  This failure also cost the site hundreds of thousands of dollars in downtime, lost data, data recovery, and productivity. 

Proper maintenance and proactive replacement are key

When clients ask how to justify the expense of a Capacitor replacement, we ask them to consider how much is it worth to maintain a functional data center/server room to the company. What would an unexpected load loss, extended downtime, and lost productivity mean to your lines of business? Your employees? Your clients?

During the regular maintenance, the capacitors are not always visible.  Be sure to perform one Annual offline/bypass maintenance service per year so that the capacitors can be properly examined for any signs of premature failure – including sweating, swelling/bulging, or excess heat. The next time your UPS Service Provider or Technician recommends a Capacitor Replacement, heed the warning and avoid the inevitable capacitor failure and ensuing downtime. It’s not a question of if, but when.

Contact Odyssey Power today for more information on this subject, specific manufacturer’s replacement intervals, and your UPS’s specific age recommendations.