Electrify Your Classroom with a Discussion on the War of the Currents, Past and Present
You turn a light on in your house with a flick of the switch. Energy that powered that light-bulb was likely generated many miles away before delivery to your house via high-voltage alternating-current (AC) power lines. Ever wonder why electric current—the flow of electric charge—is delivered as AC instead of direct current (DC), the other type of electric current? Well, the engaging answer to that question is rooted in history.
In the late 19th century a bitter rivalry between George Westinghouse and Nikola Tesla (Team AC) and Thomas Edison (Team DC) was dubbed the “War of the Currents.” Edison publicly campaigned against AC, citing its dangers and even demonstrating the dangers by electrocuting an elephant at Coney Island. Meanwhile, Westinghouse successfully proved the economical use of AC for transmission of electricity and transmitted electricity generated in Niagara Falls to Buffalo, NY, in 1896. Team AC eventually won the war, which led to AC being the primary means for transporting power in the United States today. However, technological advancements in the past few years have reopened the debate. You'll need some background information to understand why.
In alternating current the flow of electrons periodically changes direction. The power grid in the US uses 3-phase AC, each phase offset from the others by 120°. AC voltage can be increased or decreased through use of a transformer. It is possible to transmit higher voltages of electric current more efficiently across long distances. However, high voltage has disadvantages, primarily the increased need for insulation and the difficulty working with the electricity safely.
How are AC and DC power transmitted?
No matter the energy source of the power plant, AC is created by a spinning electric generator. Power is generated at voltages compatible with the type generator used. Each phase is transmitted along a different wire. This arrangement allows for the most stable power transmission with the fewest number of wires. At the power plant, the 3-phase power leaves the generator and enters a transmission substation where it's stepped up to a higher voltage for transmission across long distances. Typical voltages for long-distance transmission range from 155,000 to 765,000 V. Another transformer located near the transmitted power's point of use (referred to as the load), steps the power down to 120 V.
This is in contrast to the first DC distribution system—the Pearl Street Station designed and built by Edison in lower Manhattan in 1881—that utilized a grid composed of copper wires laid inside brick tunnels. Edison's system maintained almost the same voltage throughout, with a 110-V generator used to power 100-V lamps, which caused a slight drop in voltage during transmission. The system used a 3-wire distribution system. Each wire functioned at +110 V, 0 V, or –110 V, with the current carried on the +110-V and –110-V lines and the 0-V wire only carrying the unbalanced current. At the time, this system required generation plants at about every mile because no cost-effective means existed for increasing or decreasing DC voltage as did for the AC system.
Why choose AC for use in the grid?
In the 1890s AC had clear advantages over DC for use in a large distribution grid. First, large electrical generators operated on AC, but a DC distribution grid required an extra step to convert generated electricity from AC to DC. Additionally, converting AC power to DC was relatively easy, but converting DC to AC was difficult and more expensive. Electric demands in the late 19th century did not require DC power. However, Edison and other proponents of a DC electric grid argued that DC was safer than AC, often conducting dramatic demonstrations (as with the elephant) of the dangers of AC. Those arguments weren't enough to overcome the economic benefits of the AC system favored by the Westinghouse-Tesla camp. By 1893 in North America and most of Europe, AC was the clear winner of the War of the Currents. Yet there were areas in the US where people did invest in DC generators. So well into the 20th century some use of DC continued, even in parts of New York City, with a few customers still dependent on DC at the turn of the 21st century.
Today, the power distribution networks (also known as electric grids), operate on AC and are capable of routing power from any power plant to any load center, thus allowing power plants to be located great distances from load centers. Recent technological advances have also made high-voltage DC (HVDC) feasible for long-distance transmission of bulk electric current. Many now see DC as the current of the future because it is more compatible with renewable energy sources. The hope for HVDC is that it will be able to connect wind farms and solar installations spread over a large area and balance out natural fluctuations in these energy sources. In fact, HVDC is more efficient than AC: a 1,000-mile HVDC line loses about 6 to 8% of its power, compared to 12 to 25% for an AC line of similar length. HVDC is also suitable for transmitting power under bodies of water, including 65 miles of line that connect a Pennsylvania grid to Long Island.
In the modern digital age, DC has certainly persevered as a major type of electric current, applied in our daily lives when distances are small or energy storage is necessary. Some common DC-powered items include: electronic devices (laptop computers, cameras, and cell phones), corded home telephones, and some electric vehicles. The current is typically converted from AC to DC, but the items can be operated directly using isolated power generated from wind or solar.
In recent years there has been much discussion over the future of the electrical grid in the United States. Some assert that the United States’ electrical grid needs major overhaul to curb brownouts and blackouts, now common in some of our nation’s largest cities, due to the grid's overuse and susceptibility to natural disasters. Meanwhile, others wish to keep the system as is to avoid the cost associated with converting to new technology. Over 100 years later, will there be a new winner of the War of the Currents?
"The History of Alternating Current: AC Power History and Timeline," The Edison Tech Center Online Resources, 2010, http://edisontechcenter.org/AC-PowerHistory.html.
Patrick J. Kiger, "High-Voltage DC Breakthrough Could Boost Renewable Energy," National Geographic Daily News, December 12, 2012, http://news.nationalgeographic.com/news/energy/2012/12/121206-high-voltage-dc-breakthrough.
"AC/DC: What's the Difference?" Edison's Miracle of Light, American Experience, Public Broadcasting Service Online, 1999–2000, http://www.pbs.org/wgbh/amex/edison/sfeature/acdc.html.
Joel Achenbach, "The 21st Century Grid. Can we fix the infrastructure that powers our lives?", National Geographic Magazine, July, 2012, http://ngm.nationalgeographic.com/2010/07/power-grid/achenbach-text.