DC power and renewable resources

The use of DC for technology that could consume renewable energy (RE) or for an RE-related interconnection into the electric utility is of particular interest.


The consulting specifying engineer, by definition, is interested in the here and now and has limited bandwidth for monitoring emerging technologies. One trend the CSE should not miss, however, is the advent of greater use of DC power for many applications. DC’s significant advantages over AC in key instances make this trend “when,” not “if.”

By the way, if you haven’t seen it, I recommend getting your hands on the November/December issue of Power & Energy magazine put by the IEEE Power & Energy Society, which is devoted to "Plugging into DC."

The use of DC for technology that could consume renewable energy (RE) or for an RE-related interconnection into the electric utility is of particular interest. Solar photovoltaics (PV) produce DC energy. Wind power is more efficient and less capital intensive if it is done in DC versus AC. In most environments today, however, we take the DC output of a renewable source, invert it into AC, then connect it to facilities and or to the grid. In many cases, ironically, once that DC power has been inverted to AC, a myriad of devices require that it gets rectified back into DC, whether it's computers, electric drives, lighting. Those DC-AC-DC conversions waste as much as 15 percent of the original renewable energy.

Eliminating 15 percent inefficiency would push renewable energy much closer to cost parity with fossil fuel sources. And, for example, one of the most costly and high-maintenance elements of a wind turbine is the complex transmission that is required to allow the unit to generate AC power. If a turbine only needed to output DC power, the mechanism would be simpler, less expensive and lower maintenance.

For these and other reasons, the use of DC is being explored for facility distribution purposes and even interconnection into the electric utility. So-called solid state transformers are currently under development right now and they were described in a recent CSE webcast in early November available here. (Registering for the webcast enables replay on demand.) A DC transformer can transform power to different voltages of DC or perform a DC-to-AC conversion. The fact that this is done via power electronics, versus traditional copper and steel magnetism, opens all kinds of possibilities.

For example, by controlling the characteristics of the power electronics it's possible for solid state transformers to be used for voltage and VAR support, power factor correction – the sorts of things that utilities frequently need when connecting renewables into the grid at both transmission and distribution levels.

To be sure, these sorts of commercial applications remain a couple of years off. Today we do not have building codes that address how DC should be wired, protected, etc., in either a home or industrial environment. Further, while the DC solid state transformers themselves are currently under commercial development, a number of standards addressing safety and reliability need to be developed for their deployment.

Despite the current immaturity of this technology, when a CSE is planning a major industrial project with an industrial substation, he or she would be well to discuss these emerging technologies with their client, particularly when a solar PV installation is involved. This is the lowest hanging fruit, because eliminating the need for an AC inverter greatly reduces the cost of the PV array. DC can be used directly to recharge battery banks for energy storage. It can be used directly for office lighting, for heating, for electronics products. In the near future we’ll see many consumer electronics, from flat-screen TVs to PCs, sporting both AC and DC inputs. So the first applications may well be rooftop solar PV feeding DC current into an office building.

The knowledge of this new technology coming along suggests that, at the very least, provisions be made or consideration be given not only to the photovoltaic panel design itself but also to how the system is wired and connected. The design should accommodate the eventuality of DC generation feeding DC consumption. It would be an absolute disaster at this point to put up a PV array that had an integral DC to AC conversion in the panel itself, obviously, because then the conversion waste is built into the system.

The use of DC may take a number of directions. Some visionaries in the industry believe that DC codes would require no changes to existing wiring practices, because the voltage level on DC might be different. Others think we’ll need separate wiring, separate facilities. There are two efforts afoot at IEEE to resolve these issues.

Sam Sciacca is an active senior member in the IEEE and the International Electrotechnical Commission (IEC) in the area of utility automation. He has more than 25 years of experience in the domestic and international electrical utility industries. Sciacca serves as the chair of two IEEE working groups that focus on cyber security for electric utilities: the Substations Working Group C1 (P1686) and the Power System Relay Committee Working Group H13 (PC37.240). Sciacca also is president of SCS Consulting.

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