DC Power Plant for Data Center: Choosing the Plant

The hot topic for data centers today is the use of DC power distribution. Using DC power to operate servers and infrastructure can save as much as 20% of power costs and reduce heat load in the data center. In this article, I’ll be discussing choosing a DC power plant for the data center.

The first design choice is the DC voltage that will be used. 48V is the long time standard for telecom central office power plants. The advantage of this voltage is that equipment for it, both supply and load, is commonly available, reducing the cost significantly over other options. The disadvantage is that the lower voltage requires larger conductors, which may add to the expense dramatically if there is a large distance between the DC power plant and the equipment that it is supplying. In all cases, it’s good design practice to keep the DC power plant as close as possible to the equipment it’s intended to power.

120V is also a telco standard but is likely to be harder to locate equipment for (and consequently more expensive), as well as the fact that next to no equipment is going to the able to use that voltage directly, requiring extra equipment to be usable. High end DC power plants for the data center use up to 575V. The higher voltages reduce the conductor size at the expense of not being touch safe and requiring proprietary (and expensive) equipment. The rest of this article will presume the choice of standard 48V DC power.

The next design choice is the type of supply. It’s tempting to use surplus ferroresonant rectifiers, since these are readily available and generally inexpensive. Switching rectifiers, however, are far lighter, more compact, and more efficient. Switching rectifiers are also commonly used in modular designs, making it easy to achieve redundancy.

How much redundancy do you need? The minimum starting point is n+1 (the number of rectifier modules you need to meet load requirements plus one spare). This insures that one module failure will not interrupt service. After this minimum, you need to consider how likely modules are to fail, how long it takes to replace a failed module, and how critical uninterrupted service is. The more likely modules are to fail, the longer it takes to replace a failed module, the more critical it is to avoid service interruptions, the more spare modules should be added above and beyond the number required to actually operate the load (n+2, n+3, etc)

The next consideration is features. Choose a power plant with battery maintenance and protection features such as equalization and low voltage disconnect. During an AC power failure, low voltage disconnect (LVD) disconnects the load if the battery voltage drops below a certain point. This avoids damaging the batteries by over discharging them and allows the power plant to first recharge the batteries when AC power is restored, before attempting to repower the load. If the power plant is expected to recharge the batteries and power the load at the same time, it may overload and trip breakers or blow fuses.

The final consideration is to choose a power plant with the capability to remotely disconnect the batteries. This feature allows the data center fire alarm control panel to automatically disconnect the batteries so that, in case of a fire, the fire department may disconnect AC power to the facility and be sure that nothing is still operating before entering to fight a fire. This is critical for the safety of emergency crews, since the water used for firefighting does not mix with electricity.

While this is not an exhaustive list of considerations for designing a data center DC power plant, they certainly are the most important ones.


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