have promise for lower cost and higher energy density. It is likely that these batteries would be available for deployment in the T&D systems after 2020.

In addition to batteries and CAES, there are several other possibilities for energy storage. For example, supercapacitors have been used as energy storage devices in power-quality and similar short-time applications, including HVDC and FACTS. They have very long life as well as very high efficiency compared to batteries. A second example is superconducting energy storage (SES), whereby energy is stored in a magnetic field created by circulating DC current through a coil made of superconducting material. SES, which has high in-out efficiency and cycle life, has been demonstrated for stabilizing power systems and used for power-quality applications, but its application for storage will require more advances in materials science. Energy can also be stored in flywheels, which are particularly suitable for power-quality applications and have a very long cycle life. Given their high costs and low energy-storage density, none of these three technologies is currently suitable for storage in the grid. However, if advances are made, particularly in materials, they may become suitable for use in distribution systems during the 2020–2035 and post–2035 time periods. Because no one type of storage fits all applications, R&D is needed for all of these technologies.

At the distribution and customer levels, the loads being protected or leveled are generally much smaller in size (a few kilowatts to a few megawatts). Thus devices such as ultracapacitors, flywheels, batteries, and uninterruptible power supplies can be used. The choice will normally be determined by the load characteristics. Figure 9.A.4 shows the various types of storage and their applications.

Transformers

Electrical transformers are devices used to raise or lower AC voltage. For example, a transformer near the generating plant increases the voltage (steps it up) at the transmission line, and a transformer at the distribution substation decreases the voltage (steps it down) from transmission levels to those appropriate for the distribution system. This voltage is subsequently reduced as the power travels to the consumer. All told, power from the point of generation to the customer’s meter may flow through four transformers stages, causing total energy losses of about 4 percent in the process. Though utilities procuring transformers generally take estimates of such losses into account, there is always a trade-off between capital costs and operating costs which can push the buyer toward lower first cost. Thus



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