Power Flow Control Devices: Hardware for the Smart Grid

by David Niebauer

A significant amount of attention (and money) is directed at the communications and IT upgrades necessary to empower a Smart Grid.  The very concept of a more intelligent power transmission system implies a vast increase in data.  The more that can be “known” about conditions of the system, the better and more efficiently the system will operate.  This is the world of software, of information gathering, of machines “talking” to machines.

But there is the other side of the equation, a side that is not as often discussed in the media and research reports on the Smart Grid:  hardware.  One improvement to the electricity transmission infrastructure that unquestionably falls in the category of hardware is the construction of new transmission lines.  The limitations on building new transmission lines, however, become apparent as soon as one starts to think about it.  Not only do zoning, environmental and NIMBY (Not in My Backyard) concerns add delay and uncertainty to the construction of new lines, the cost is truly exorbitant.  Estimates from the National Counsel on Electricity Policy in a 2004 report (Electricity Transmission: A Primer) peg the cost at between $285,000 and $1.71 million per mile, depending on terrain and line type.

Power Flow Control

Another (far less expensive) solution in the category of hardware for the Smart Grid is comprised of devices designed to control the flow of electrons.  Power flow control devices increase the capacity of the overall system without the need to construct new transmission lines.  These devices are an integral component of the Smart Grid and utilize a variety of strategies to modulate the flow of power and therefore increase the efficiency of the power grid.

To understand how power flow control devices work, one must first understand the “meshed” nature of the Grid.  Historically, the power grid was built in a radial structure.  That is, transmission and distribution lines were constructed to directly connect the generating facility with the ultimate load centers.  This is the simplest structure and provides the most control:  essentially one on/off switch is all that is needed. However, it is also highly unreliable – a fault along any part of the line can cause the entire down-stream system to collapse.  System operators have begun to address this problem by constructing transmission and distribution networks in a meshed structure.  A meshed system is more reliable because congestion and faults can be isolated in discrete segments of the mesh without affecting other segments.  See D. Divan, H. Johal, “A Smarter Grid for Improving System Reliability and Asset Utilization,” in Proc. IEEE Power Electronics and Motion Control Conference, 2006.

Power Flow Control Increases Capacity

The increased reliability of meshed networks is obtained at the cost of capacity underutilization and inefficiency.  In a radial structure, transmission and distribution lines are kept at or near capacity. In a meshed system, capacity is limited by the lowest-capacity segment. Electricity always follows a “path of least resistance” (lowest impedance), so the first line to reach its thermal capacity limits the capacity of the entire system, even though a majority of the lines of the system are significantly below their limit.  It is estimated that US grid capacity utilization rates are typically only 45% to 60% of theoretical capacity.

Power flow control devices steer the current in a line in order to balance the loading on all the lines allowing the overall system to operate at its theoretical maximum capacity.  Visualizing electricity flowing like water through a hose helps to understand how this works.   Power Flow Control devices are like valves on the water hose.  One important difference: electricity flows at the speed of light, so controlling the flow requires highly technical solutions.

FACTS devices

A number of solutions have been proposed and are being deployed by utilities and transmission operators.  The most common such solutions are power electronics based Flexible AC Transmission Systems (FACTS) devices.  FACTS devices work by either controlling the voltage or modifying the impedance of transmission lines, thereby controlling the power flow.  For under-utilized lines, increasing voltage allows additional current to be pulled into the line; in congested areas, increasing line impedance pushes excess current into other parallel paths.  The combined effect results in an increase in system capacity and line utilization.

FACTS devices are expensive, but not when compared to the cost of construction of new power lines.  FACTS devices are priced based on increase in capacity of existing lines, and generally range from $150 – $300 per kVA. New distributed FACTS devices are being developed for significantly less and may soon be deployed.


Software is not the only Smart Grid play.  Developments in Power Flow Control – hardware wedded to power electronics – promise to increase the capacity of the existing electric transmission grid, thereby allowing the system to operate more efficiently for lower infrastructure costs.  Controlling the flow of electrons in order to improve the existing system can and is being done.   As the Smart Grid is built out, watch for companies that design and build the hardware that all the software is being designed to control.

David Niebauer is a corporate and transaction attorney, located in San Francisco, whose practice is focused on clean energy and environmental technologies.  www.davidniebauer.com.

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