What is Uninterruptible Power Supply (UPS) Input Current Harmonic?

Most UPS systems inherently distort the input current and voltage powering them as they electronically endeavour to harmonise the wave patterns.
This has been well known for years and is the root cause of having to oversize generators backing up UPS Systems.
This non-linear current causes the notches seen in the voltage wave shape form.
This distortion is overcome by either a 6 pulse rectifier or the larger 12 pulse rectifier within most UPSs.
Generators and non-UPS connected loads see this distorted voltage as their power source. Since most loads desire a clean voltage source, the UPS-caused voltage notches create a potential problem

By sizing the generator on the basis of 2:1 with the UPS this can be overcome
For example, a 100kVA UPS will require a 200kVA Generator as both standby and prime power source.
The clean waveform is apparent as well as nearly unity power i.e. 1:1.

Additionally, it is wise NEVER wise to put laser printers or lighting circuits onto any UPS, as these will cause greater harmonic distortion.

Harmonic distortion is less of an issue when using True ON-Line UPS which produce their own power from AC to DC and then back to AC.

When installed any UPS system always have a clear strategy to ensure that only items requiring CLEAN power will ever be connected to the UPS.

This avoids distortion issues in the future as well as preventing the UPS from going into bypass to protect itself against wild or extreme wave form patterns.

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