Shedding Light on a Latent Threat to Your Data Center Backup Power

Let’s say you’ve got a 1MW data center and enough generators to supply that much backup power, along with UPSs to tide you over until the generators kick in. You’ve got everything on a maintenance schedule, including regular testing to ensure it all works as intended. You should be all set in the event of an interruption in utility power, right?

Well, probably, but according to a new Schneider Electric white paper, there’s another little wrinkle to consider: instability caused by IT devices that exhibit electrical input current with a condition known as “leading power factor.” In certain instances, typically when the data center is heavily loaded, the condition can cause generators to shut down entirely – leaving you high and dry when you need them most.

The white paper, “Impact of Leading Power Factor on Data Center Generator Systems,” goes into detail on what leading power factor is. It uses words like “sinusoidal” and describes different sorts of currents, including in-phase (good) as well as out-of-phase and harmonic (both not so good). The paper describes how circuit currents come into play in IT power supplies and ultimately cause the leading power factor problem.

Generators can tolerate small amounts of leading power factor, so things may be fine for a while. But over time, as you add more IT equipment to the data center, the risk of problems rises.

In short, if the leading reactive current becomes too great, the generator can lose control out of its output voltage regulation, tripping an overvoltage situation which causes the generator to shut down. As a general rule, the paper says, generators must operate with leading reactive current of less than 20% of max rated current in order to avoid instability.

Now the problem only occurs when the UPS is somehow taken out of the picture, because one of the functions of a UPS is to correct current fluctuation issues such as this, and deliver “clean” power to the IT load. But if the UPS is in a bypass mode, or if the data center uses a tier 3 or other design where one of the power paths to the IT load does not have UPS protection, the leading power factor issue can occur.

The paper walks through how to assess your data center for susceptibility to the problem, both in a current data center under existing and expected future conditions, and for a data center that’s still in the planning stages.

Most importantly, the white paper offers a number of potential mitigation techniques, including simple ones like removing load from the generator or removing offending IT devices, to more involved solutions like installing an “inductive load bank” (a group of large inductor coils) or an electronic power factor correction system.

The electrical engineers among you will definitely want to check this paper out, as you’ll no doubt be interested in the in-depth explanations. But any data center operator should also give it a read, as it sheds light on a problem that, if it comes to roost, may one day leave your data center in the dark.

Click here to download Schneider Electric white paper number 200, “Impact of Leading Power Factor on Data Center Generator Systems.”

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