Capacitors are key power components in a UPS unit; they are used to smooth out electrical waveforms by filtering noise and storing electrical energy.

Capacitors are used all over in a UPS, from small units on electronic control boards to large units in the power section of the UPS rectifier (DC Capacitors) and inverter (AC Capacitors). Power capacitors vary hugely in size from the size of a pill bottle up to the size of a 1 litre coke bottle.

  • Power capacitors consist of an aluminium or chrome foil inside with two conducting surfaces, either being electrodes or metal plates. These are separated and insulated from one another by using a dielectric medium such as electrolytic paper saturated in electrolyte oil. The amount of energy stored in a capacitor is known as Farads after their inventor Michael Faraday and the amount of stored energy is determined by the dielectric layer and the aluminium surface area.
  • The input section to the UPS contains AC capacitors which are used for filtering as well as part of the IGBT power factor correction circuit for the rectifier to ensure a sinusoidal voltage and current draw on the mains supply.
  • The output stage of the UPS inverter uses power capacitors to clean and filter out noise in the sinewave output to the critical load as well as provide a reactive power component to the system.
  • The DC circuit uses the power capacitors to filter out noise, i.e., (Super-imposed AC) after the AC mains supply is rectified to form DC power to charge the battery backup as well as provide a short-term energy storage system.

The power capacitors in a UPS are part of a high voltage system and subjected to many system disturbances and a lot of stress during their daily operation inside the UPS.

How long does a capacitor last?

Batteries, fans, and capacitors all have a working life and are all medium-term consumable electrical items used within a UPS. As capacitors age, they dry out due to loss of the electrolyte, and as this happens, they don’t work as well until they eventually pop open or even catch fire.

Capacitor service life depends on use, operational temperature, and quality, with life expectancies of up to 10 years. They are more commonly accepted to be changed as part of preventative maintenance every 4 to 6 years. They generally have long delivery times in SA due to their short shelf life.

What affects the service life of a capacitor in a UPS?

  • High and low temperatures outside of the normal room temperature.
  • Over-working the capacitors beyond their design criteria, causing them to overheat.
  • High heat will cause the electrolyte to evaporate, and they will dry out and can explode.

What signs tell you the capacitors are busy failing and need replacement?

  • Oil seepage from the capacitor causing an increase in impedance and internal temperature.
  • Casing deformation.
  • Hot connections picked up during thermal IR analyses.
  • Increase in ripple voltage on DC capacitors.
  • Change in actual measured capacitance versus rated capacitance.

What happens when they fail?

There are one of two scenarios that can occur:

  • They go open circuit with nobody knowing as they generally don’t show any physical signs and stay undetected. This is why it is very important to perform regular preventative maintenance on your UPS so such faults can be measured and detected.
  • They go short circuit, in which case they start leaking and can blow up. Generally, such capacitors have a weakened point in their housing, causing them to leak slowly rather than splattering electrolyte all over the UPS.
  • Lastly, they can fail slowly over a period of time where their capacitance value will slowly change.

How is a UPS affected when the capacitors fail?

A UPS is negatively affected by capacitor failure, and its operation and inverter load is jeopardised.

Capacitor failure can have the following effects:

  • Increased distortion of the inverter output waveform.
  • System instability in parallel systems.
  • Breakdowns with catastrophic effects such as burning of control wiring.
  • Loss of load up-time and production.
  • High unplanned repair costs.
  • Other component damage within the UPS.

DC capacitor failure can have the following effects:

  • DC bus failure and inverter load loss under power failures.

AC capacitor failure can have the following effects:

  • Loss of load uptime and production.
  • High unplanned repair costs.

How to minimise the risk of catastrophic capacitor failure?

Capacitors can fail at any time as they are consumable items that last according to usage. It is therefore advisable to opt for preventative maintenance. They are to be replaced before they fail, at the slightest sign of deterioration, or at least every 4 to 6 years, for peace of mind and to prevent load downtime and UPS damage.

During preventative maintenance, Standby Systems service engineers will perform visual, mechanical, and electrical checks on the capacitors to ensure they show no signs of damage or aging. Later analysis of the readings taken while on site will also help indicate the condition of the capacitors.

The better the UPS operational environment and the more frequent the preventative maintenance, the longer the capacitors will last.

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