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	12V/24V/48V/130V DC power plants Data Center Power Systems MAN Wireless Access Point Networks Telecom Central Office Telecom Remote Terminal Rectifier, Battery Plant, CEMF Solid state or rotary DC-AC inverters Engineering, Installation, and Maintenance

Much has been made recently of the case for increasing the power efficiency of data centers by the use of DC power. In a previous paper, I addressed the case for the use of DC power in the data center. In this paper, I will discuss design considerations for such a system.

Typical DC power plants involve the use of a rectifier system, one or more backup battery strings, one or more CEMF cells, battery distribution and fuse bays (BDFB), and local fuse or breaker panels. DC power plants may also include DC-AC inverters to power equipment that has no native DC power option.

The first consideration is to choose the operating voltage for the system. Typical voltages for DC power plants are 24VDC or 48VDC. 48VDC is the most common, being used in telco service for more than 80 years. 24VDC is typically found in use with cellular phone equipment. From the standpoint of having the most options available for it, 48VDC is the best choice.

Some DC power manufacturers are promoting extremely high voltage systems (up to 575VDC) to reduce the voltage drop caused by long cable runs at lower voltages, thereby requiring smaller cables. I don't recommend this approach for several reasons:

• 48VDC can be worked on safely while energized. 575VDC poses a high safety risk should anyone come in contact with it and must be denergized to safely work on it.
• 575VDC requires specialised and expensive proprietary equipment. 48VDC utilises commonly and widely available equipment, reducing costs.
• 575VDC requires an extra layer of DC-DC conversion, to bring the voltage up at the rectifier and down at the load. This means more expense, more potential things to fail, and more space taken away from revenue generation.
• 575VDC pushes standard insulation materials right to the limit, increasing the risk of insulation failure and arc over.

The best way to reduce loss from cable length is to keep the DC power plant as close as possible to the load equipment. Also keep the feed as large as possible by locating the BDFB and/or the local fuse panel as close to the load as possible. In a building LAN environment, I have also used 120VDC to power remote equipment at long distances. 120VDC is a standard telco voltage so the equipment required is inexpensive and easily available, the equipment being powered requires only small amounts of power, and, properly protected, it's safe to work with.

For choice of rectifier, I recomend a modular system that can be configured in a minimum of N+1 (the number of modules required to support the load plus 1). In locations where module failures can be addressed quickly, fewer extra modules may be appropriate. In locations where there may be a delay in replacing failed modules, the number of extra modules in the system should be increased to prevent the possibility of a second failure before the first failed unit can be replaced.

Aside from capacity characteristics, the two most important considerations in battery string design are choosing a non-venting battery (eliminating the potential for flammable hydrogen gas to be generated) and use of multiple smaller strings that can be used in an N+1 configuration. This allows individual strings to be shut down for maintenance while still protecting the system from failure.

All possible equipment should be natively DC powered for the best efficiency, eliminating the need for the DC-AC inverters. All DC powered equipment should be chosen for a wide range of DC input voltage (modern DC power supplies typical work from 36-70VDC). This allows the system to be run without the need for CEMF cells to drop voltage (typically, 48VDC power plants actually run at 52VDC to maintain proper charge on the batteries).

DC power plants should also be integrated into the facilities emergency systems. If there is a fire in the facility, emergency personell may denergize commercial AC power to the facility and enter, expecting to find all power off, when the equipment is still running from the battery string. I typically configure the facility alarm panel to disconnect the battery string on a smoke alarm and disconnect the rectifier plant on any heat detection. This provides the minimum chance of service interruption and maximum protection for any emergency personell as well as the facility.

The final tip I have for engineering a DC power plant for a data center is to upsize both the battery return circuits and the ground circuits. Eliminating the possibility of transients on either from power switching and other sources will increase the quality of the power and decrease possible glitches in the load equipment or false trips of fuses or breakers.

There are certainly more details to fully engineering any DC power plant but I've addressed the most important things that make a DC power plant for data center use different that most other applications.