Power Supplies – Friend or Foe?

I thought a high efficiency power supply was going to fix my problems?

Maybe. And you have a choice to make.

As we have written elsewhere, carbon cap-and-trade will focus on total energy consumption, and we’re looking at all corners of the data center for gross energy cutting efforts. As long as you an make a sensible and systemic decisions, you’re going to be fine. Maintain your overall system availability and don’t compromise your critical infrastructure’s ability to operate in all modes (normal, maintenance and failure). The issue with cap-and-trade and reducing energy has always been your total energy consumption and how and where it’s measured.

Today, there are basically two types of power supplies. Your commodity power supplies are not terribly efficient, but they serve a large section of the market. What most folks don’t realize is that, while you are spending millions of dollars and a significant effort in improving the infrastructure that serves the IT systems, the power supplies on your platforms could be trumping that effort. While it’s easy to tar everyone with the same brush, but most common power supplies often operate in the 70% efficiency range. It makes it easy to turn to high-efficiency power supplies as a ready solution as newer systems are employed old systems are sweated and replaced.

Our concern is that all of the plant and equipment changes made to the serving electrical and mechanical infrastructure are not being reinforced by the power supplies on the platforms. It’s a great thing to make the upstream electrical and mechanical systems as efficient as possible – it’s still money and time well spent. The argument we make is that the IT system power supplies are the “last mile” for energy in the data center, and this should not be ignored. So, it’s easy to simply specify high-efficiency power supplies for IT platforms. We’ve been preaching on this for years, so what’s the issue here?

For the past few years, high-efficiency power supplies have been our benign friend in reducing power consumption. Sadly, we’re seeing a sinister turn in the construction of these HE power supplies. Up until a year or so ago, the power supplies did not exhibit unusually high power factor. The “older” HE power supplies operated in the low to mid 90’s for power factor. Today, current power supplies have increased their power factor to nearly unity (1.0 neither leading or lagging). This has been accomplished by increasing the capacitance in the power supply (a routine and crude method for raising efficiency and power factor).

This would not be such a terrible thing, except for two issues. While UPS systems can address this type of load, generators can not. There is growing list of sites that are experiencing leading power factor on their generator sets while in bypass (not on UPS inverter). So, instead of increasing efficiency by reducing impedance as most electrical systems do, the power supplies are adding capacitors to their systems.

Second, very high power factors in the CP system may lead to derating of static UPS systems listed for .9 or .8 lagging power factor operation. Again, choices to be made in a measured and sober manner.

The solution to these high-cap HE power supplies is very easy. Add some reactive load to the UPS bypass buss, such as a synchronous motor or using a mechanically-coupled UPS system, whether that be rotary or flywheel. A rule of thumb would be 15% of the CP kW demand load.

Like everything else in the data center, you need to make choices in your facility that assure AVAILABILITY for all modes of operation. The good news is that the fix for high-cap HE IT power supplies is easy. The bad news is that the HE power supplies need some support from elsewhere in the electrical system to allow generators to operate properly.

Next post – how everyone cheats on PUE and what is the “True PUE” of the facility.