AC Versus DC in PQ

wikicommons public domain
wikicommons public domain

In the 1880s, Tesla (not the car) and Edison (not the city) debated which method of electric current distribution should be used to deliver electricity from generators to homes and businesses. It was a less-than-friendly debate (including the electrocution of an elephant), with much at stake for the inventors and the investors behind them. In the end, the dominant form of distribution became Tesla’s alternating current method rather than Edison’s direct current.

Then, and for the following century, the advantage of the AC method made economical sense. Rotational generating equipment, such as turbines connected to a prime mover such as water or steam, produces electric fields in a sinusoidal manner, which is generally equated to the AC method. The voltage increases in magnitude in one polarity, goes back to zero and then reverses to the same magnitude but in opposite polarity. AC voltage can be stepped up to a much higher voltage level by using of a transformer with different turn ratios of the input and output windings. A higher voltage requires less current for the same power delivery.

Double the voltage gives four times the power for half the current, which requires a smaller size wire. Lower current means fewer losses in the wiring. The economics are apparent when going from 14 kilovolt (kV) stepped up to 345 kV or even 765 kV at transmission levels. It eventually backs down to 13 kV for distribution to 480V for industrial facilities and 120/240V for residential.

Even with the U.S. Department of Energy’s 2016 transformer efficiency requirements of 97–99%, there is still a loss with each transformation, which can become a long multiplication string. Four transformations can result in more than a 10% loss or waste of power. When the dominant electric load was electric motors, there was generally just one more loss in the system, the motor’s efficiency. Now the dominate load is what is called nonlinear, or electronic loads, which translates as equipment that uses DC power instead of AC. IT equipment (servers, printers, laptop computers, cellphones, etc.) operates almost exclusively on DC power. Newer alternative power sources, such as solar or photovoltaic power, produce DC power.

Data centers are among the largest consumers of electrical power as a single industry, and they are a DC haven. The 480V AC incoming power is converted to 120V AC for power supplies used by most of the loads throughout the facility, which must convert it to DC of various voltages to be used in the equipment.

With each conversion comes more losses. Even an Energy Star VI rating is less than 90% efficient. While an adjustable-speed drive for the HVAC system may convert the AC to DC and then back to AC, DC motors could be employed for moving the air. A typical data center also requires backup power capabilities that are most often batteries, which are DC power sources. The batteries must be kept charged, which is another inefficiency, since they are charged with DC current from an AC source.

About 10 years ago, a push came due to the rise of the 380V DC distribution system for data centers. Some traction was made, but many obstacles needed to be overcome. Breakers, fuses, connectors, switches, PDUs, bus systems and others needed to be rated and listed for DC applications. Standards on how to build and inspect needed to be detailed. Like most business decisions, the economics mired down the progress. Industry groups, such as eMerge Alliance, IEC, UL, NEC and NEMA, began working on standards with regards to 380V DC distribution systems, and some have been published. Large manufacturers, such as ABB, Emerson, HP and Schneider Electric, have such products.

DC power is not new. It has been used in the telecom industry for more than a century. With environmental and global warming concerns from our undiminished power-hungry world, maybe this time the overall impact of saving 5–10% of wasted power may give Mr. Edison’s ideas a second chance. Plus, his patents have all expired. Then maybe the Federal Chief Information Office targets for a power usage effectiveness of less than 1.4 for data centers can become standard practice. Less power wasted is fewer greenhouse emissions and lower electrical bills—a win for everyone.

About the Author

Richard P. Bingham

Power Quality Columnist

Richard P. Bingham, a contributing editor for power quality, can be reached at 732.248.4393.

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