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Understanding UPS Sensitivity and Calibration

So-called standby, line-interactive, and offline UPS systems are popular due to their lower price and do not belong in mission-critical applications.

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randy-collier (002)

Randy Collier is the owner of Comp-Utility Corporation.

There are thousands more Uninterruptible Power Supply (UPS) systems under 10kVA in use than systems over 10kVA. So-called “standby,” “line-interactive” and “offline” systems are popular due to their lower price, and do not belong in mission-critical applications; therefore, they are rare in applications over 10kVA.  However, if you are stuck with an off-line system, this article discusses the problems and remedies associated with random UPS and battery failures of these systems.  

In a perfect world, the power coming from your utility would be at constant voltage and frequency. In that case, these systems would remain in standby all the time and never go into alarm. In the real world, utility power fluctuates. These UPS systems need prompting before switching your critical load between conditioned battery back-up power and utility power. This is accomplished through constant monitoring of distortions in the input utility power, and when the utility power falls out of range, the UPS switches to battery power until the utility returns to acceptable limits. Every manufacturer has its own set of parameters as to when, how and how long this switching will occur, with some advertising zero clamping time to/from battery power. 

Power Problems

Providing back-up power design would be easy if utility power was either on or off. Here are some common power problems:

  • Complete Power Failure (Voltage = 0)

  • Power Sag: Momentary Voltage drops to below an “acceptable limit”

  • Power Surge: Momentary Voltage increases to above an “acceptable limit”

  • Undervoltage: Voltage remains below an “acceptable limit” for an extended period of time

  • Overvoltage: Voltage remains above an “acceptable limit” for an extended period of time

  • Line Noise: Electromagnetic interference or Improper grounding, resulting in random disturbances

  • Frequency variation: Sine wave frequency goes above or below an “acceptable limit”

  • Switching Transient: Momentary changes in voltage and current due to lightning or switching on/off of other nearby devices

  • Harmonic Distortion: Transmitted by unequal loads and can result in resonance, overload, and overheating

Often, these systems will switch to battery when the power appears to be on, or if running on generator. During a brown-out, for instance, the lights may be on, but the UPS may be designed to go to battery power at a low-voltage threshold. Generator power varies more than the utility, so you can expect these systems to go to battery more often while running on generator power.

Trial and Error

If your UPS goes to battery often even when there seems to be available power, you may be able to adjust the sensitivity using its controls. Widening the input power parameters will allow the utility power to run your critical load without going to battery as often. Of course, this subjects your critical load to a wider set of variables, which could be problematic depending on how sensitive your critical load is. The ability to adjust the UPS input parameters varies by manufacturer and model. You can find this in the user manual or contact your manufacturer with your model and serial number for advice on fine tuning.

When running on generator power, the UPS may not be able to track the sine wave produced by the generator, so it thinks it is protecting the load from "dirty" power when it shuts down the output after the batteries have expired. That could be a problem with the UPS or the generator. Sometimes sizing of the UPS and generator can be the issue. Determining that kind of fault is time consuming and costly.

Another fine point with these systems is, that at the very moment you need it is when your batteries and UPS electronics are fully tested. Unless they have a self-test, they will remain silent until you have a power problem, and then they may work as designed. If you cannot obtain the balance with these UPS systems or if your load is truly critical, you should consider “Double Conversion On-Line“ topology. These systems are designed to always run the load from the UPS' inverter, while the electric utility power keeps the battery charged. Therefore, no "transfer" to batteries upon a power fluctuation and, complete elimination of utility power problems. Also, the rectifier, batteries and inverter are always powered up, so even when a rare UPS fault occurs, the utility power is usually available.

Final Thoughts

Using a reactionary UPS design by default tests the metering, transfer switch, batteries and inverter at that critical moment when back up power is needed. Where have a system with adjustable input, you can make these adjustments and perhaps solve issues of multiple alarms and low battery life. Battery life is dependent on the health of the system’s rectifier and also depends on the number of discharges. So, if your battery is wearing out prematurely, or you are experiencing an abnormal number of UPS alarms, it may be a defective UPS, need an adjustment or, in problematic areas where you experience many outages, the distorted utility power is simply wearing out the batteries.

Opinions expressed in the article above do not necessarily reflect the opinions of Data Center Knowledge and Informa.

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