Edison’s Revenge: Could DC Carve Out a Place in Our AC Grids?
- Written by Steven Collier
George Westinghouse and Nikola Tesla won the “war of currents” a century ago, but Thomas Edison may have the last word. Profoundly new circumstances and revolutionary new technologies favor direct current in addition to or even instead of alternating current power for many applications.
There are many good reasons to think direct current (DC) may have a place in the smarter power systems of the near future. This is not to suggest wholesale conversion of today’s AC grid to DC, but rather the emergence of a hybrid AC–DC power system, both in the grid and within the premises of residential, commercial and industrial customers. After all, an increasing amount of energy is produced by DC sources such as photovoltaics, fuel cells and batteries. At the same time, a rapidly increasing share of energy consumption is DC, in the form of electronics devices, electric and hybrid-electric vehicles and batteries. In fact, almost all uses of electricity—motors, resistance heaters, lighting, electronics—could just as easily have been designed to be supplied by DC in the first place.
Why did AC prevail in the formation of the electrical industry? Is it because a rotating machine innately generates AC? No, with multi-section commutators and properly configured windings they can generate DC with minimal pulsation.
The advent of AC stemmed primarily from the need to transport large amounts of power from ever-larger central generating plants to distant load centers. This in turn required very high-voltage transmission lines to reduce current flow and therefore ohmic losses as well as the physical size and capacity of conductors and associated equipment. The need to convert low-voltage generation to high-voltage transmission and then back to low voltage for distribution and consumption could only be accomplished through alternating current transformers. The motor-generator sets necessary to accomplish the same task for DC were prohibitively expensive, inefficient and unreliable. Power electronics were unavailable.
AC had other benefits. Current and voltage are zero twice every cycle, making it easier to interrupt normal and fault-induced currents. This was important in the early days of the industry when the only way to turn off a generator or transmission line was to open a physical contact switch—there were no air blast or SF6 breakers. AC generators operating in a synchronous network can be readily controlled to ramp up and down in direct response to changes in system frequency or voltage caused by changing loads.
But today, DC is becoming more important for many grid applications, both supply and demand. PV panels, fuel cells, batteries and electrified vehicles produce and use DC power. LED lighting, which is energy-efficient and increasingly prevalent, actually works better and lasts longer on DC. Essentially all electronics devices from consumer electronics to commercial and industrial applications work only on DC. And a rapidly increasing proportion of total U.S. energy consumption is electronic.
For DC sources and loads to be used “on the grid,” the power must first be inverted to AC at any DC source to be transmitted or distributed and then rectified back to DC at any DC end use. This increases the complexity of the customer’s system, hardware expenses, space requirements and direct and tertiary losses, while generally reducing overall system reliability, efficiency and sustainability. The DC switching transients also can adversely affect utility system power quality.
Distributed generation and energy management systems are supplanting the old paradigm of huge central station power plants serving distant load centers through bulk transmission corridors. And the legacy bulk power grid has suffered declining reliability and efficiency. If customers could use distributed solar PV, fuel cells and batteries for DC power directly for their electronics needs, communications, computing, lighting and even transportation (which together make up a steadily growing proportion of their total consumption), they could increase local efficiency and perhaps reduce grid reliance for these critical loads. The same goes for the nation’s immense and growing Internet infrastructure, from data centers to telecom networks.
DC has benefits for transmission and distribution, as well. DC lines aren’t subject to susceptance (inductive and capacitive reactance), which translates into greater power transfer capacity and therefore smaller size for a given power rating. The effective value of DC voltage and current is the same as the peak value whereas for AC it is only 71 percent of the peak value. This means more current flow and power delivered for a given voltage level in a DC system (which, by the way, is why DC is often wrongly described as having higher ohmic losses as a function of distance than AC: it’s transferring more power for a given voltage).
DC power is not subject to frequency variation, nor is it affected by leading/lagging power factor. DC voltage can be changed by power electronics switching without the ohmic and inductive losses and phase shift caused by AC transformers. Unlike an AC system relying on synchronous operations, a DC system is asynchronous, just like the Internet, sharply reducing the possibility of cascading system transients or outages.
DC does not induce current in neighboring conductors and so does not have the adverse EMF/EMI effects that accompany AC such as interference with telecommunications and sensitive electronic devices (and that still arouses some anxiety, however misplaced, about adverse health effects). A DC system is more accommodating to high-frequency telecommunications than an AC one, and not just because of reduced EMF/EMI impacts. Today’s AC grid is optimized for 60 Hz AC operations so that higher frequency telecommunications signals superimposed on the network (in connection, for example, with powerline communications and broadband over powerlines) attenuate due to shunt capacitors and transformers. Line to line and shunt capacitance and higher order AC harmonics interfere with these high-frequency communications signals.
It would be prohibitively difficult and expensive to convert today’s utility grid to DC operation. However, bypassing the AC system entirely for some or all of an individual customer’s DC loads (in the form of a “nanogrid”) or several proximate customers’ DC loads combined (in a “microgrid”) is technically feasible and could be economically desirable. Not only could the output of solar PV, fuel cells and batteries be used directly and more efficiently, but the loads they serve might continue to be served in part or in whole during a grid outage.
The hybrid AC/DC concept can also apply to the transmission and distribution grids. Deploying AC-DC-AC links in the legacy AC grid would make the grid more controllable and therefore more stable, reliable and economical. Think in terms of Texas’s DC ties linking ERCOT to the Southwest Power Pool and Mexico’s CFE . Besides connecting major AC grids asynchronously, these DC interties make it possible to control the power that flows over them. Changing the phase angle on an inverter-rectifier-inverter combination (a so-called “uniform power controller”) creates a valve-like function whereby the voltage and current can be controlled. Using this capability will require real-time monitoring, analysis and control of the combined AC/DC system, but, again, steady, dramatic improvements in high-speed digital electronics monitoring, control and information technologies make this possible.
In conclusion, think about a new grid model, a hybrid AC / DC system, for the 21st century. It’s already happening in some parts of the world, and it is consistent with broad trends we’re already seeing in transmission and distribution. Distributed generation and energy management systems are supplanting the old paradigm of huge central station power plants serving distant load centers through bulk transmission corridors. And the legacy bulk power grid has suffered declining reliability and efficiency. If customers could use distributed solar PV, fuel cells and batteries for DC power directly for their electronics needs including communications, computing, lighting, even transportation (which together make up a steadily growing proportion of their total consumption), they could increase local efficiency and perhaps reduce grid reliance for these critical loads. The same goes for the nation’s immense and growing Internet infrastructure, from data centers to telecom networks.