UPS capacitor introduction

Capacitors are an integral part of any uninterruptible power system (UPS). Responsible for smoothing, filtering and storing energy, these UPS components are fundamental to the overall health of your UPS. However, much like batteries, capacitors degrade over time and because of this, represent a common point of failure in uninterruptible power supplies. Understanding the factors that influence the lifespan of AC and DC capacitors, as well as the signs of impending capacitor failure, will go a long way toward ensuring that your UPS systems are optimally performing.

What is a UPS capacitor?

UPS capacitors are tasked with helping to adjust voltage fluctuations so they do not adversely impact the equipment connected to the UPS. Designed to store and release electrical energy, capacitors range in size and type, and the number of capacitors inside a UPS varies greatly depending on the kVA rating of the unit. Even the smallest single-phase UPSs have dozens of capacitors, while a 750 kVA three-phase UPS will likely have hundreds.

In UPS systems, AC capacitors form part of the 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, on the other hand, comprise part of the rectification system. Their role is to help smooth out voltage fluctuations and deliver short-term energy storage to ensure there is no break in supply to the critical load.

What is the lifespan of a UPS capacitor?

Like batteries, capacitors also have an expected useful life and will require replacing. As the materials within capacitors age and degrade, they lose the ability to effectively perform their job. The electrolyte, paper and aluminum foil inside the capacitor degrades physically and chemically. Several factors can speed up the rate of deterioration but on average, most capacitors need to be replaced every five to eight years. However, their actual lifespan is influenced significantly by environmental circumstances. While a capacitor might be rated by the manufacturer for seven years of round-the-clock operation, it is likely to actually run for a much shorter period if there are unfavorable operating conditions. Conversely, capacitors that receive regular maintenance and operate in ambient conditions could potentially deliver up to 10 years of useful life.

What factors affect UPS capacitor lifespan?

Similar to UPS batteries, the lifespan of capacitors is reduced by various environmental extremes. All capacitors inside a UPS system are impacted by the potential for high frequency switching, as well as stresses caused by the physical and electrical operating conditions. The three primary factors affecting a capacitor’s useful life are:

1. Excessive current: Capacitors can be damaged when they are regularly exposed to steady currents that exceed the manufacturer’s rating. This often occurs in unstable electrical environments. Short periods of high current are usually harmless, as long as the capacitor isn’t forced to overheat in order to compensate.

2. Overuse: Not surprisingly, the harder a capacitor has to work in order to do its job, the faster it will deteriorate. The rate of failure will be more rapid in environments in which there are high levels of voltage noise or transients, as these conditions force capacitors to overwork.

3. Excess heat: Overheating will eventually cause the solution inside a capacitor to evaporate, causing a buildup of unsafe pressure. It’s important to note that heat not only arises from high temperatures where the UPS is installed, but from inside the capacitor, as well, such as a blocked air filter that results in limited airflow.


Signs of a failed capacitor

Unfortunately, sometimes there are no visible signs before a capacitor fails. You may not realize that one has gone down, but when it does, other capacitors are forced to assume the workload, which in turn will shorten their lifespan. However, by scheduling regular preventive maintenance visits from a UPS maintenance provider, the likelihood is much higher of catching an ailing capacitor before it disrupts operations. Service engineers are professionally trained to identify the signs of impending failure, which include:

1. Oil leakage ── which increases the temperature and impedance in the capacitor

2. Deformation ── which is usually caused by excess heat or internal fluid leaks

3. Scorched wires ── often caused by over-current that can be identified during thermal imaging

4. Burnt valve cap protrusion ── a clear sign of stress that is often caused when a short circuit in the capacitor leads to a build-up of pressure; this will likely result in failure when the protrusion breaks

5. Temperature increase ── When a capacitor’s internal temperature starts to increase, it’s a definitive sign of impending failure

6. Capacitance ── as the capacitor ages, it will slowly fall out of tolerance as the internal structure degrades, similar to the process with a battery.

Most often, capacitors will fail in one of three ways. First, in an “open” condition, where they essentially stop working, often with little to no visible evidence of malfunction. Alternatively, a capacitor may suffer a “short” failure, in which there is clear leak of the dielectric medium and the capacitor may even make a popping sound. Third, the capacitor may gradually fail as it falls out of tolerance.

The risks of not replacing a UPS capacitor

Capacitor failure has a negative impact on the greater UPS system.  The consequences of capacitor failure will vary in large part based on where the capacitor is located, the number that are working in tandem or parallel, and the overall health of the UPS’s other capacitors.  

If capacitors are operating well below their voltage rating, one or two might be able to take over for a failed capacitor without significantly affecting UPS operation. But in many cases, a capacitor failure will trigger the UPS to switch to bypass mode. When this occurs, the power stream bypasses the UPS’s filtering electronics and although the UPS remains operational, it isn’t protecting downstream equipment.

Furthermore, because the capacitor’s filtering ability will suffer, harmonics and electrical noise will be greater. In addition, energy storage volume decreases and battery strings can be damaged. When capacitors are unable to perform effectively, the operating conditions are impacted, resulting in excessive heat and current.

How to prevent UPS battery capacitor failure

To maximize the performance and reliability of your UPS system, it is essential tohave the capacitors inspected on a regular basis by a professional service technician who has the expertise, training and tools to detect issues before they become expensive problems. For example, service engineers are able to measure changes in tolerance using a capacitance meter, while thermal imaging equipment is capable of detecting even small variations in operating temperature. Using these and other tools, technicians can accurately assess the condition of capacitors and, if needed, perform a full or partial replacement of capacitor banks. In addition, you should proactively plan UPS capacitor replacement near the end of their rated service life.

In addition to regular UPS maintenance and proactive capacitor replacement, you can help avoid capacitor failure by adhering to recommended ambient temperature and humidity levels, as well as keeping air filters clean so air can flow freely and keep the UPS cool.