Critical Power System Design Coordination

Critical Power System Design Coordination

When working to reduce downtime through your critical power system design, there are two key questions to ask to ensure various elements work well together:



  1. Will the UPS interact correctly with your generator?
  2. Will the UPS continue to provide clean power to your distribution transformers during transformer startup?


The following is a short list of UPS specifications that should be used to answer these questions when designing a reliable system.



Limit UPS systems to 3% total harmonic distortion (THD) or less at 100% load; 5% or less at 50% load:

Every power supply generates current harmonics as a natural response to the AC to DC conversion process. Over time, input current harmonics will damage your distribution system and decrease the reliability of the system.

UPS systems should mitigate the input leading power factor to less than 5% of the rating of the UPS, specifically at loads less than 10%: To mitigate harmonics, most UPS systems utilize input filters which consist of an inductor and a parallel capacitor bank. As the UPS load decreases, the capacitors generate more kVAR in relation to the true kW of the system and will produce a leading power factor. If the generator realizes too much leading kVAR, it will shut down.

UPS specifications should be limited to +/- 2% during step load conditions: Inverter step load transient response shows the amount of voltage deviation from the inverter when a step load from 0-100% is applied to the UPS inverter. An immediate in-rush of current, or removing the current, tends to cause the UPS inverter voltage to fluctuate.




Dual input sources: Provide increased reliability to the system and prevent a single breaker failure from disrupting both the UPS converter source and the bypass source.


DC power supply: The reliability of the UPS during a power outage directly relates to the reliability of the DC power supply. So, the specification and runtime for the lead acid batteries, lithium ion batteries, or flywheels is critical.

  • The run time for a battery-based solution depends on the battery’s end of life (Run-time vs. battery life is not linear: 80% battery life does not correlate to 80% run-time).
  • If VRLA batteries are used, configure in a parallel string configuration so an open cell does not eliminate the entire system.




Each specification above could have a significant impact on the reliability and operation of the UPS in your overall critical power system.

Find a UPS that meets the spec requirements of your critical power system design