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Can AC and DC Power Systems Integrate?

  • George Putic

More than 100 years after alternating current (AC) defeated direct current (DC) in the so-called Battle of Currents and became the standard way electricity was delivered, new technologies have opened up the possibility and even the necessity for the two systems to co-exist.

In his Power Systems Lab at the University of Pittsburgh, professor of electrical and computer engineering Gregory Reed is exploring the most efficient ways of integrating the two previously almost incompatible systems.

“For many, many, decades, everything we used was run off of 120 volts AC, 60 hertz,” Reed pointed out. “Well, today, most of our end-use devices, our home electronics, our data servers, our business systems, our lighting systems now with LEDs, so much of our loads only need low voltage DC. Look at your computer. And so what do we do? We plug everything into 120 volt AC which is our legacy system. That has to be converted to low voltage DC for these devices to operate. So we're going back to DC then, at the very, very end for end-use devices today and that creates a lot of inefficiency.”

Do We Need Both AC and DC?

Electric energy can run through wires in two ways: in one direction, sort of like water flowing through a pipe, or in alternating mode, almost like pushing and pulling back and forth.

The first is called direct current, DC for short, and that is the sort of energy we get from batteries, with a plus and minus pole. The second is known as alternating current, or AC, and that is the type we get from our wall sockets and almost everywhere else.

Both types of current can run electrical motors, power lightbulbs, heat our houses, and both have advantages and disadvantages.

In the beginning, the advantages of AC were greater, so electric power companies opted for the one that made more economical sense.

“It really was a fierce battle between Thomas Edison, who advocated his system of DC technology, and Westinghouse and his partner Nikola Tesla who were advocating AC,” Reed explained. “The disadvantage Edison really had with DC back then was he had no way of controlling the current on the DC cables. As he began to deliver DC to longer and longer distances, as he got to beyond a mile (1.6 km) or more, currents got so large that it wasn't safe. The cables had to become so large that it wasn't economical. So in his system he needed a power plant about every mile to feed this DC network.”

This problem has now been solved. We can safely control the currents in DC delivery lines. Suddenly, converting AC to DC for long distance transportation opened up new possibilities.

Utilizing Both Systems’ Advantages

“Beginning in the 1970s, we started to build high voltage DC transmission systems which provided a much more efficient way than AC for long distance transmission,” Reed said, “because we could use this power electronics conversion to control the DC. With AC systems, we use what's called three phases, so we use three cables and large towers to support those cables and all the forces on them, and that's very costly over the long distance. With DC systems we only need two of those. We call them poles, so we only need two cables for DC. So right away we have 33 percent less infrastructure than we have with AC systems.”

Another advantage of a DC power system is that when used as a connection between two AC systems, it prevents the so-called ‘cascading blackouts,’ when a problem in one AC power line, such as a lightning strike, triggers a domino effect that spreads through other AC lines, leaving vast areas without power.

As Reed explains, “With DC systems, we can actually interconnect pockets of AC networks and create subareas of AC systems through DC inter-ties where, if we do have these problems within the network, we can separate them instantly through the DC converters and really eliminate the possibility of having these cascading blackouts over large regions in the future.”

DC In Our Homes and Cars

So DC is increasingly appealing for transporting energy over long distances. But what about in our homes, factories and other businesses? Reed says in the future, they may contain what he calls ‘DC micro-grids.’

The electricity generated by alternative energy sources, such as solar panels and fuel cells, is DC. It can be stored in batteries and safely integrated with 120 volt 60 hertz AC.

“And a part of that battery storage will be in the form of electric vehicles, and we are seeing a huge potential growth in the electric vehicle market. So, continue to put all those batteries in for the network, it's more and more growth of DC entities all around us, for both loads and end-use, and so that growth has become very strong as well,” Reed said.

The System Needs Standards

In a large trailer truck loading facility just outside Pittsburgh, Reed and his students installed an experimental micro-grid. It integrates AC power coming from the local electric utility and from a small wind turbine on the site, with DC power generated by solar panels on the building’s roof. DC is stored in battery banks and either converted to AC or used for appliances running on DC.

Graduate student and researcher Ansel Barchowsky, who participated in building the lab, says it is a collaborative project between the University of Pittsburgh and a local trucking company, Pitt Ohio. The facility has modern, low-power LED lighting fixtures that could run on DC, he says, but "unfortunately, in this facility the lighting existed before we started the DC project so it’s AC lighting. We invert through this 40 kilowatt inverter back to AC from our DC backbone to power that load.”

And that points to another set of problems that must be resolved before the large scale integration of the two systems can commence.

Reed points out, “Since this is so new, we really don't have complete standards around what are the right voltage levels for DC at the end-use level, at the building level. What's a safe voltage? What type of protection should we have around it? Products like DC circuit breakers and DC protection. These are all new developments and we're working very closely with many industrial partners and the utilities are trying to solve some of these problems.”

The Electric Power Systems Lab at the University of Pittsburgh is not the only facility dealing with this, says Reed. Many large power companies and electrical equipment manufacturers, such as Eaton Corporation, Mitsubishi Electric and ABB Siemens, are rapidly developing DC systems, and the standards needed for the full integration of AC and DC power that seems to be inevitable in the 21st century.

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