Thomas Edison must be smiling. He long held that direct current (DC) was a better and safer way to transmit and distribute electricity. Edison lost the argument to Nikola Tesla, George Westinghouse and other proponents of alternating current (AC). But now, more than 100 years later, we’re in the process of rethinking those long-held assumptions about power.
A convergence of factors—from growing use of renewable energy sources, to an explosion in the number of power-hogging data centers, to recent safety regulations for industrial control panels, even consumer aesthetics—are driving renewed interest by utilities and industries in using both high- and low-voltage DC electrical systems.
Much of this interest comes from efforts to reduce the energy lost between AC and DC conversions—which can be 10 percent or more—especially when multiple conversions are required between an AC electrical distribution system and actual use of the electricity by equipment. While no one expects a wholesale switch to DC power, there are applications and industries where direct current provides an advantage.
Renewables lead the way
Its turns out that high-voltage DC (HVDC) is a more efficient way to transmit electricity over long distances, in part because it reduces the energy lost during such transmissions. Better yet, HVDC transmission systems can be buried out of sight, underground or under water over long distances. That solves one of the major dislikes people have with AC transmission systems: the large towers that march across the landscape like so many Martian landers from “War of the Worlds.”
HVDC transmission is ideal for renewable energy systems, like wind, solar and water, whose sources are often located at great distances from population centers. California was the first state in the U.S. to take advantage of HVDC transmission for renewable energy, in the form of hydroelectric power. The Pacific HVDC Intertie relied on the mercury-arc valve, a technology breakthrough from automation and power-generation vendor ABB. In 1965, along with GE, ABB was awarded a federal contract to build two converter stations for a 1,440 MW, 400 kV transmission line from the Columbia River in Oregon to Sylmar in the northern Los Angeles basin.
ABB is now working on its fourth upgrade to the Pacific Intertie, raising the voltage to 560 kV and the capacity to 3,800 MW at the HVDC converter station in Celilo. The company is supplying valves, cooling, transformers, harmonic filters, and control and protection systems for the project, which will be commissioned in 2016.
New HVDC projects
According to Roger Rosenqvist, vice president of business development for ABB’s Power Systems Division, Grid Systems, some of the recently proposed HVDC transmission projects will serve the densely populated U.S. Northeast, including the Northeast Energy Link, which plans to move wind power from Bangor in northern Maine to eastern Massachusetts along the existing Interstate 95/295/495 corridor. Another project is the Champlain-Hudson Power Express, a 330-mile line to be built within waterways and existing railroad corridors to transmit hydropower from Canada to New York City.
To help construct these new transmission systems, Rosenqvist says ABB has invested more than $100 million in a new plant near Charlotte, N.C. That facility’s 35-story extrusion tower is able to make high-capacity polymer insulated cables for both HVDC and AC underground transmission lines. ABB also has invested heavily in DC converter and control system technologies for both transmission- and facility-level applications.
Besides transmission distances, another reason for applying DC systems to renewable energy power generation is because the amount of power produced varies by weather conditions and time of day. Utilities must store the energy in large, often tractor-trailer-sized batteries, which can produce only direct current. This stored power can then be transmitted as needed to the grid.
DC is also increasingly used for distribution of electric power. “Now that we have an infrastructure for DC power, it’s something companies building new facilities may need to consider,” says Rosenqvist. “Many pieces of equipment in a factory today already contain devices that convert the AC power from the distribution system to direct current, so a common DC distribution system for the entire facility could turn out to be more economical. There are also safety benefits, since the fault currents in a DC power distribution system can be controlled and limited by the converters.”
Pointing to the growth in DC power availability, Rosenqvist expects continued technology developments to improve DC systems. “Once something starts, people get innovative in terms of products and applications,” he says.
Anything that contains a transistor can only use DC power. As more and more of us rely on computers, smartphones, tablets and the Internet in our personal and work lives, the more DC power systems make sense. This is especially true for the large server farms and other electronic devices used for datacom and telecom networks.
Datacom industry equipment is typically powered by AC mains with uninterruptible power supplies (UPSs). Telecom facility equipment is powered by DC power plants with battery reserve power. As more telecom operators house datacom equipment to provide video, voice and data services, multiple power systems can cause problems, including different installation and maintenance requirements, reserve times and grounding systems. This can negatively affect network reliability and service availability.
A mixed power environment also uses more batteries and leads to more potential points of failure and more parts to maintain. DC power plants are simpler, with fewer failure points.
According to a report by Emerson Network Power, a recent study by NTT Facilities found that data centers with DC power systems in Japan achieved a 20 percent improvement in efficiency and a 10 percent reduction in cooling costs compared with AC systems. In addition to reducing the amount of power lost to conversions, there were also fewer transient and harmonics issues with DC systems.
Propulsion systems show how it’s done
The future of industrial power could be found at sea. One of the newest developments in the shipping industry is the introduction of DC propulsion systems to increase energy efficiency. This not only allows engineers to shrink the overall size of the propulsion machinery, equipment and cables that power the propellers, it also leaves more space for cargo.
Read about the first UHVDC Project from Xiangjiaba Shanghai. Click here for more information.
Oliver Simmonds, lead naval engineer at GE Power Conversion, says that DC architecture offers greater flexibility, allowing designers to maximize the potential for using efficient, variable-speed drives. Not only does DC propulsion reduce fuel use, but DC power electronics and architecture also allow the system to transmit as much as 23 percent more power than an equivalent AC system, he says.
Battery storage benefits
Simmonds says AC generators are still used to produce power, since they are less complex than DC generators, but the power is converted to DC for efficient transmission around the ship. This design leads to another benefit: the ability to store DC power in batteries for backup use in an emergency.
“Harbor safety regulations often require ships to run more than one power generator at a low load, which is inefficient, even though they use just a fraction of the electricity they generate,” says Simmonds. “With a DC system, you could use just one generator plus an integrated battery system in case something goes wrong. This represents potentially large fuel savings.”
The short-circuit current rating system approved by UL about seven years ago has led to the widespread use of 24 VDC power for control panels, says Coman Young, global product manager for power supplies and UPS products at Rockwell Automation.
“When there’s a discharge, such as when large drives and motors pull power off the line, capacitors need to be charged and discharged,” Young says. “Energy needs to go somewhere, and power is often pushed back to the device—and the control panel.”
Young says today’s trend toward the use of DC power started in Europe as a way to protect workers who must open or touch control panels. “The standard used to be 120 VAC power, but it’s very difficult to control when equipment is live,” he explains. “DC power never falls lower and never crosses zero.”
Railways are another industry that relies on DC power, using 60, 72 or 120 V, adds Young. “It’s wasteful to go back and forth between AC and DC power, so now all [railway] equipment uses DC busses,” he says.
Read how TenneT Offshore GmbH is using DC for Wind Power. Click here for more information.
Rockwell Automation has recently been moving into the electric utility space, according to Young, to address power quality issues using devices that monitor incoming power and send alarms to plant operators. “Voltage dips can affect plants and they don’t even know it,” he says. “There can be multiple dips in voltage every day. Even though each one may only last for a millisecond, these repeated dips can shut down equipment and even the plant.”
As an outgrowth of its power monitoring business, Rockwell Automation has developed a dynamic sag corrector for its UPS products to mitigate problems caused by voltage sag. The corrector can be used in industrial, data center and IT applications.
Young says there are a number of barriers to wider use of DC power for industrial applications, including the current AC infrastructure; the lack of components that are able to handle DC current; and fusing, because DC power is more difficult to break. “It’s not easy to find 4 or 5 A fuses, which is why they’re costly. It’s now seen as a custom product,” he says.
On the plus side for the growth of DC power, “people are looking at ways to reduce the cost of redundant systems. The price and size of large AC transformers is also an issue. If you lose power completely with AC current, then there’s no power to restart anything. But since DC power can be stored in batteries, it’s always available,” Young explains. “The development of electronic device switches is also increasing the potential for using 120 VDC mains to eliminate AC-DC conversions.”