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An EPRI White Paper

DC Power Production, Delivery and Utilization

is converted to DC at the uninterruptible power sup- ply, to facilitate storage, then is converted again to push out to the servers, and is converted one more time to DC at each individual server. These conversions waste power and generate considerable heat, which must be removed by air conditioning systems, resulting in high electricity costs.

In a 10–15 megawatt (MW) data center, as much as 2–3 MW may be lost because of power conversions. As these centers install ever more dense configurations of server racks, DC power delivery systems may be a means to reduce skyrocketing power needs.

Improved inverters and power electronics allow DC power to be converted easily and efficiently to AC power and to different voltage levels. Component im- provements enable greater efficiency than in the past, and improve the economics of hybrid AC/DC systems. Although improved electronics also enhance AC-only systems, such enabling technology makes the DC pow- er delivery option feasible as well.

The evolution of central power architecture in com- puters and other equipment simplifies DC power de- livery systems. At present, delivering DC to a computer requires input at multiple voltages to satisfy the power

needs of various internal components (RAM, proces- sor, etc.) Development of a central power architecture, now underway, will enable input of one standardized DC voltage at the port, streamlining delivery system design.

DC power delivery may enhance micro-grid system integration, operation, and performance. A number of attributes make DC power delivery appealing for use in micro-grids. With DC distribution, solid-state switching can quickly interrupt faults, making for better reliability and power quality. If tied into the AC transmission sys- tem, a DC power micro-grid makes it easy to avoid back- feeding surplus generation and fault contributions into the bulk utility system (by the use of a rectifier that only allows one-way power flow). In addition, in a low-volt- age DC system, such as would be suitable for a home or group of homes, a line of a given voltage rating can transmit much more DC power than AC power.

Of course, while DC circuits are widely used in energy-con- suming devices and appliances, DC power delivery systems are not commonplace, and therefore face the obstacles any new system design or technology must overcome. For any of the benefits outlined above to be realized, testing, development, and demonstration are needed to determine the true potential and market readiness of DC power delivery, as outlined in the section “Potential Future Work and Research” on p. 26.

High-voltage direct current (HVDC) transmission

Although in Edison’s time, direct current was impractical for trans- mission beyond the distance of a mile, today high-voltage direct current (HVDC) can transmit bulk power over very long distances and also enables interconnection of incompatible power grids.

Valves that can convert high voltage AC to DC and back again were needed for HVDC to work, so such conversion was enabled with the development of static converters and mercury arc valves ca- pable of handling high voltages.This technology was first deployed in 1954, for a transmission system between the island of Gotland and the Swedish mainland.The system was rated at 20 MW and 100 kV and transmitted power over 57 miles. Development in the 1960s

of high-voltage thyristors made with semiconductors helped boost transmission capacity, increasing the cost-effectiveness of HVDC. Technology advances continue apace, with refinements in micro- processor control and other developments enhancing performance. Nearly 100 HVDC systems, including many in North America, are now in service around the world.The largest, in Brazil, is rated at 600 kV.

According to ABB, a key supplier of HVDC systems, newer designs have expanded the power range, extending the economical power range of HVDC systems from 90 megawatts to 1 gigawatt.

June 2006

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