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EPRI Proprietary Licensed Material Figure 8 A large liquid helium refrigerator (JAERI) SMES manufacturers design their systems so that both the coil current and the allowable voltage include safety and performance margins. Thus, the PCS power capacity typically determines the rated capacity of the SMES unit. In particular, as energy is removed from the coil, the current decreases. As a result, the PCS must be designed to deliver rated power at the lowest operational coil current, which is about half of the maximum current. Equivalently, about a quarter of the stored energy remains in the coil at the end of a typical discharge (see the equation for energy stored in a coil in Section 1). The PCS provides an interface between the stored energy (related to the direct current in the coil) and the AC in the power grid. Several different designs have been suggested for the PCS, depending on the application and the design of the SMES coil. The power that can be delivered by the SMES plant depends on the charge status (the current I) and the voltage capability of the PCS, which must be compatible with the grid. Control System The control system establishes a link between power demands from the grid and power flow to and from the SMES coil. It receives dispatch signals from the power grid and status information from the SMES coil. The integration of the dispatch request and charge level determines the response of the SMES unit. The control system also measures the condition of the SMES coil, the refrigerator, and other equipment. It maintains system safety and sends system status information to the operator. Modern SMES systems are tied to the Internet to provide remote observation and control. Technology Attributes Capacity The power capacity for a SMES system is dictated by the application, e.g., power quality, power system stability, or load leveling. The manufacturer uses this to select system design and components. In general, the maximum power capacity is the smaller of two quantities: SMES Page 7
EPRI Proprietary Licensed Material • The PCS power rating. • The product of the peak coil current and the maximum coil withstand voltage. The manufacturer must design the SMES plant so that the current in the superconducting coil and the operational voltage are adequate for the power delivery requirement. The capacities of existing individual micro-SMES installations range from 1 MW to about 3 MW. These are discussed under technology status in the next section. A much larger unit is now being installed by the Center for Advanced Power Systems (CAPS) at the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida. The PCS for this coil will initially have an installed capacity of 5 MW. Future enhancement to 25 MW is planned. The superconducting coil, however, was designed to deliver 100 MW, i.e., the product of the design current and design voltage is 100 MW. Energy Storage Rating The stored energy in the SMES plant depends on the requirements of the application. It is the product of the power capacity and the length of time the installation is to deliver this power. The micro-SMES plants listed above deliver 3 to 6 MJ (0.8 to 1.6 kWh) somewhat more than a leadacid automotive starting battery. Because the power capacity of these units is so high, this entire quantity of energy can be delivered (i.e., the coil can be fully discharged) in a second or so. The larger, 100 MW coil to be installed at NHMFL, mentioned above, was originally designed for a one-second discharge in conjunction with the unified power flow controller (UPFC) operated by American Electric Power (AEP) at its Inez Substation. This coil thus stores about 100 MJ (28 kWh). When the converter at NHMFL is upgraded to 25 MW, the coil will be discharged in about 4 seconds. Physical dimensions of the SMES installation The physical size of a SMES system is the combined sizes of the coil, the refrigerator and the PCS. Each of these depends on a variety of factors. The coil mounted in a cryostat is often one of the smaller elements. A 3 MJ micro-SMES system (coil, PCS, refrigerator and all auxiliary equipment) is completely contained in a 40-ft trailer. Efficiency The overall efficiency of a SMES plant depends on many factors. In principle, it can be as high as 95 % in very large systems. For small power quality systems, on the other hand, the overall system efficiency is less. Fortunately, in these applications, efficiency is usually not a significant economic driver. The SMES coil stores energy with absolutely no loss while the current is constant. There are, however, some losses associated with changing current during charging and discharging, and the resulting change in magnetic field. In general, these losses, which are referred to as eddy current and hysteresis losses, are also small. Unfortunately, other parts of the SMES system may not be as efficient as the coil itself. In particular, there are two potentially significant, continuous energy losses, which are application specific: SMES Page 8
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