Jason Yates, Technical Services Manager at Riello UPS, explores the key role capacitors play by storing and filtering energy in uninterruptible power supplies and explains why proactive replacement can breathe new life into your power protection system.
Capacitors play a crucial role by helping to smooth, filter, and store energy. A typical UPS contains dozens of capacitors ranging in size from a miniature drinks can through to a tube of Pringles. These aluminium or chromium-plated cylinders contain a pair of conducting surfaces, typically electrodes or metallic plates. A third element, known as the dielectric medium, separates and insulates these other elements.
The volume of charge that a capacitor can store is measured in farads – named after the renowned physicist Michael Faraday – and is determined by the thinness of the dielectric layer and the surface area of the aluminium plates.
You’ll find various types of capacitor in the power section of your UPS:
- AC input capacitors, which form part of the UPS input filter and/or power factor correction stage. They are subjected to the incoming mains supply and have the crucial role of smoothing out input transients, reducing output-reflected switching noise and harmonic distortion.
- AC output capacitors, which form part of the UPS output filters. Their role is to connect to the critical load output, helping to control the waveform of the UPS output voltage and provide reactive power.
- DC capacitors, which form part of the rectification system and energy storage. Their role is to help smooth out any fluctuations in voltage and provide short-term energy storage for mains to battery transitions to guarantee there’s no break in supply to your critical load.
Each UPS capacitor is subjected to potential high frequency switching, as well as all the stresses caused by the physical and electrical operational environment.
Such pressures explain why, along with batteries, capacitors are the UPS components most prone to failure. They age over time as the electrolyte, paper, and aluminium foil inside degrades physically and chemically.
The signs of a failing capacitor
Capacitor service life is shortened by any detrimental environmental extremes, for example, hot or cold operating temperatures, along with workload.
The harder a capacitor has to work (i.e. the more it has to filter unusual levels of voltage noise or transients), the faster it will deteriorate. Similarly, regularly exposing the capacitors to currents that exceed the manufacturer’s recommended rating will quickly cause damage.
Extreme heat, caused by either high ambient temperatures or potentially from blocked filters that limit airflow, will also start to evaporate the solution inside the capacitor. Over time, this builds up an unsafe amount of pressure.
There are several tell-tale signs of impending capacitor failure. Service engineers may notice some oil leakage or deformation caused by excess heat. They might also see a protrusion on the valve cap, which is a clear sign of stress probably caused by a short circuit inside the capacitor.
The use of thermal imaging cameras can also detect an increase in temperatures or any scorched wires connected to the capacitor, which tends to be the result of overcurrents.
Engineers can also use a capacitance meter, which are typically included in any quality digital multimeter, to measure the drop off in tolerance as the capacitor ages.
The cost of capacitor failure
UPS capacitors fail in three distinct ways. Firstly, they can just gradually fall out of tolerance. Secondly, there are ‘open’ failures where they simply stop working but there’s actually little visible evidence that something’s gone wrong. Finally, there’s a ‘short’ failure, where there’s an obvious leak of the dielectric medium. Sometimes this is even accompanied by a loud popping sound like a firework.
Most modern power capacitors are fitted with pressure release valves or complete pressure release caps, which enables the capacitor to alleviate the internal pressure should a failure occur.
This does result in a rise in temperature, though, which in turn causes the internal pressure to increase. In certain instances, a sudden change in pressure leads to the capacitor releasing at an extremely high rate, discharging electrolyte across the surrounding area.
If a single capacitor fails, the rest of the set must pick up the slack. This has the knock-on effect of placing them all under greater stress. It downgrades the ability of the UPS to filter power, increasing the likelihood of problems with harmonics and noise. The energy storage volume reduces, while it can also potentially damage your battery strings.
In the worst-case scenario, a serious capacitor failure will trigger the UPS to switch to bypass mode, which leaves the critical load unprotected and exposed to unstable power. If capacitors are left to completely dry out, they can even become a serious fire risk.
How to reduce the risk of capacitor failure
The areas most under your control are general good housekeeping and robust UPS maintenance. Try to stick to the recommended ambient temperature and humidity levels, as well as keeping air filters clean so air can flow freely and keep the UPS cool.
During regular preventive maintenance visits, field service engineers should visually inspect the capacitors and also scan with thermal imaging devices to highlight any potential problems. Analysis of maintenance field readings will also help to identify the capacitors most likely to fail.
There’s no standard age where a UPS capacitor will fail, and in favourable operating conditions they can reach a service life of up to 10 years. But it’s become accepted industry best practice to treat capacitors as consumables and proactively look to replace them between years four to eight of service life.
It often makes sense to combine swapping out UPS capacitors whilst replacing another key consumable, namely the fans that keep the inverter and rectifier cool enough to operate safely.
Commonly known as a UPS overhaul, proactively replacing ageing capacitors and fans before their end of service life can breathe new life into your UPS.
Naturally, the new parts will perform at a much higher level than the ageing ones. Not only does this reduce the likelihood of a major system failure, but it also makes sure that your unit runs more efficiently, which helps to cut your energy and operational costs.
The potential consequences – not to mention the cost – of a serious capacitor failure are simply far too high for you to ignore. An overhaul offers the most cost-effective way to safeguard the ongoing performance of your UPS at the same time as maximising your equipment’s lifespan and reducing the total cost of ownership.
