How to Speak Data Center: IT Power Supplies
December 14, 2012
I mentioned Power Factor Correction (PFC) in a previous posting, but I didn't explain it. If you can understand Power Factor and Power Factor Correction, you can really build some credibility with the electrical engineers. It is not an easy thing to understand, so don't get frustrated.
If you think of a car burning a gallon of gas, efficiency is similar to miles per gallon. In other words, how efficient the car is at converting the gallon of gasoline into miles traveled. Power factor is analogous to a leaky fuel line: It's the fuel that never reaches the engine. This isn't measured in the computer; it is only evident upstream. Using the car analogy, the car will calculate the same MPG, but you will only notice the gas leaking from your tank if you check the gallons supplied at the pump compared to the gallons used by the car.
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We can see the PFC feature in some power supplies in the DoE and LBL graph above. "Efficiency" above shows how watt-hours consumed are wasted by the power supply itself. PFC is about getting more watt-hours out of your existing power entering the power supply.
As discussed, the conversion of VoltAmps to Watts includes a Power Factor. The Power Factor describes how "reactive" the load is by how far the load lags the voltage waveform. In English, this means that an ideal load would match voltage wave perfectly. Any deviation from ideal creates two problems: stranded capacity and harmonics. PFC is important to deal with both of these problems.
The first problem of a leading or lagging power factor is that it reduces the amount of power that can be used on a power circuit. For example, if you assume an IT power supply is on a 20 amp breaker (rated 16 amps continuous load) at 120 volts, which operates at a 0.80 power factor (very poor), the power supply can only consume 1.536 kWh in an hour (16A*120V*0.8PF). At a 1.0 power factor, the power supply can consume 1.920 kWh per hour (12*120*1.0); an increase of 25%. The 384 watt-hours is the stranded power capacity. The electrical infrastructure is present in the data center to support the full 1.920 kW, but your power supply can't extract the energy. A crude explanation of the limitation is that power supply is lagging and can't take advantage of the peak voltage. When the power supplies provide PFC, they take much better advantage of the peak voltage.
Another problem to understand when discussing PFC power supplies with the data center staff is harmonics. When the power supplies lag or lead the voltage, they create "echoes" on the power circuits. These echoes can resonate at specific frequencies and are called harmonics. (like feedback on an audio system). The details aren't important, but the best way to think about harmonics is that it is "noise" on the electrical system.
Power harmonics can cause many types failures, such as overheating conductors, damaging power supplies or causing logic chip failures due to voltage conditions outside specifications. You can track the historical progression of this issue by reviewing the increasing specification by engineers for Z-rated or "Harmonic Mitigating" transformers.
Why should IT care? Not only can you correct a major source of the harmonics by using Power Factor Correction power supplies, you get more out of your electrical infrastructure without any upgrades. By breaking into the data center code, you can show how IT use of PFC power supplies leverages existing investments in data center facilities while reducing noise on the electrical system inside the data center.
Ken Miller is data center architect with the IT Infrastructure and Operation Services division of Midwest ISO, developing mission-critical facilities.