Why do some UPS systems introduce a certain level of Total Harmonic Distortion (THD) to the input power source, and what are the solutions:

https://www.hatchpower.com/posts/ups-inducing-harmonic 

Why do some UPS systems introduce a certain level of Total Harmonic Distortion (THD) to the input power source, and what are the solutions:

UPS (Uninterruptible Power Supply) systems typically generate harmonic distortion due to the nature of their operation and the type of power electronics they employ. Here are a few reasons why UPS systems can induce harmonics into the input source:

  1. Rectifier Section: Most UPS systems use a rectifier stage to convert incoming AC power into DC power for battery charging and inverter operation. The rectifier stage typically consists of diodes or thyristors, that inducing harmonic distortion into the AC input due to their non-linear characteristics. These harmonics are caused by the abrupt switching of the rectifier components.
  2. Inverter Section: The inverter section of a UPS system converts the DC power from the batteries back into AC power for supplying the load. Inverters commonly use pulse-width modulation (PWM) techniques to synthesize the AC output waveform. The rapid switching of the inverter transistors can result in high-frequency harmonic content in the output voltage, which can then propagate back to the input source.
  3. Non-linear Loads: UPS systems are often used to power sensitive electronic equipment, such as computers and servers. These loads themselves can be non-linear, meaning they draw non-sinusoidal current from the UPS. Non-linear loads introduce harmonic currents into the system, which can interact with the UPS and further contribute to the distortion of the input source.

The harmonic distortion generated by UPS systems can have several undesirable effects on input power source. It can cause overheating of up-stream transformer, breakers failure and reduced lifespan of electrical equipment are fed from same input source to UPS, interference with other sensitive equipment, voltage and current waveform distortion and increased power losses in electrical distribution systems.

To mitigate harmonic distortion, some UPS systems may incorporate input passive harmonic filters to reduce the levels of 3rd and 5th harmonics. However, it is still recommended to install an external active harmonic filter or an advanced passive harmonic filter to further reduce the total harmonic distortion (THDI) to the desired level. Another solution is to select a 12-pulse rectifier UPS for capacities over 500 KVA. Additionally, choosing a UPS that has a rectifier designed based on IGBT (Insulated Gate Bipolar Transistor) instead of diodes or SCRs (Silicon-Controlled Rectifiers) can also help in reducing harmonic distortion.

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