integrate variable renewables by managing reactive power. FACTS devices are currently available and are already deployed in limited applications.

Power electronics also can be used for lower-voltage applications on distribution systems, where the equivalent to FACTS is known as custom power. Custom power devices can provide for significant improvement in power quality on the customer side by controlling voltage and frequency distortions. High power quality is needed for many modern applications, especially in industries with automated production, which could benefit from more economical local solutions to improved power quality. Power electronics also plays an important role in smart metering with two-way power flow (to encourage local power generation) and in real-time pricing (to shift loads away from expensive peak demand periods).

Custom power technologies that offer such solutions exist now, but their application is restricted to situations where their high cost is offset by significant benefits. R&D could help reduce costs and expand their use by 2020.

AC and DC Lines and Cables

A cost-effective way to obtain extra transmission capacity is to upgrade transmission lines and corresponding substations along existing corridors. Transmission capacity can be increased by “reconductoring” existing lines (using materials such as composite conductors that can carry higher current). These materials are presently available but not widely deployed; taking lines out of service for reconductoring is difficult, and new materials are expensive. In addition, all overhead lines can carry current higher than their nominal rating when weather conditions are favorable, and real-time rating that could be continuously adjusted would increase available capacity.19

HVDC becomes cost-effective at long distances, where the reduced capital costs of the lines and reduced energy losses can compensate for the cost of the converters.20 For example, long-distance, high-power HVDC transmission could

19

The nominal current rating of overhead lines is based on assumed worst seasonal conditions. Conductors have some resistance (except for superconductors) and heat is produced as current flows through them. If the line gets too hot, it expands and sags excessively. High air temperature with no wind is usually the design condition. Under less severe conditions, more current can be carried, but existing transmission controls cannot account for this.

20

HVDC lines may be warranted for overhead lines longer than 800–1000 kilometers and underground or underwater lines longer than 60–80 kilometers. A 65-mile long undersea and underground HVDC cable began commercial operation in 2007, carrying 660 MW of power from New Jersey to Long Island.



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