Page 252 - Power Electronic Control in Electrical Systems

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240 Power electronic equipment

Recondenser (Two Stage)
Shield Coldhead

Voltage Regulator
and Controls

Room Temp (~293K)

Thermal Link (~60K)

HTS Leads
(High Temperature
Superconductor)
Recondenser
Thermal Shield (~35K) Compressor
Superconducting (~4K)
Coldhead
Compressor
Fig. 6.66 A high efficiency cryostat containing the superconductor magnet. (Courtesy of American
Superconductor Inc.)

magnet is shown in Figure 6.66. The three-phase VSC topology (Figure 6.32) can
be used to transfer power between the superconductor and the power system/load.

6.6.3Other energy storage systems

There are a number of other energy storage systems. A hydroelectric pumped-storage
plant for instance is a quite popular one with power utilities. It works like the
conventional hydroelectric station, except for the fact that the same water is used
over and over again to produce electricity. There are two reservoirs at different
altitudes. When power from the plant is needed, water from the upper reservoir is
released driving the hydro turbines. The water is then stored in the lower reservoir. A
pump is used then to pump the water back from the lower reservoir to the upper one,
so that it can be used again to generate electricity. Pumped-storage plants generate
electricity during peak load demand. The water is generally pumped back to the
upper reservoir at night and/or weekends when the demand is lower and hence the
operating costs of the plant can be reduced to meet the economics of the method. Of
course the advantage of using the water again and again requires the building of a
second reservoir therefore increasing the cost of the overall plant. Since the plant uses
electricity when the water is pumped into the upper reservoir, the concept behind the
development of such plants is based on the conversion of relatively low cost, off-peak
electricity generated by thermal plants into high value on-peak electricity when the
hydroelectric plant generates electricity to assist with demand management for
utilities.
Another energy storage system may be based on batteries. Such a system of
course requires a great deal of maintenance and periodic replacement. Furthermore,