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238 Power electronic equipment

power supply during long non-availability of the AC mains. However, the battery
depends on the times and duration of charge and discharge and for short voltage sags
and other line problems a flywheel can extend its life by dealing with such problems
rather than the batteries.
Finally, another potential application for small flywheel systems is renewable
energy systems and specifically wind power systems. The flywheel may act as a buffer
supplying energy for short intervals of time and mainly between wind gusts since the
flywheel systems can charge and discharge quickly. Batteries cannot compete with
such short demands of energy supply and take more time to charge.
6.6.2 Superconducting magnetic energy storage (SMES)

Superconducting magnetic energy storage (SMES ) is defined as: a superconducting
electromagnetic energy storage system containing electronic converters that rapidly
injects and/or absorbs real and/or reactive power or dynamically controls power flow
in an AC system.
A typical SMES system connected to a utility line is shown in Figure 6.65. But
before explaining the power electronics technology available for such systems, let's
examine first the phenomenon of superconductivity, a natural phenomenon and
probably one of the most unusual ones.
Superconductivity is the lack of resistance in certain materials at extremely low
temperatures allowing the flow of current with almost no losses. Specifically, super-
conductors demonstrate no resistance to DC current and very low to AC current.
They also exhibit quite strong diamagnetism, which simply means they are strongly
repelled by magnetic fields. The levitating MAGLEV trains are based on this principle.
The materials known today as superconductors must be maintained at relatively low
temperature. There are two kinds of materials:
1. low temperature superconductor (LTS)
2. high temperature superconductor (HTS).
The latter has been discovered recently and has opened up new opportunities for
commercial applications of the SMES systems. Applications of such material range
from the microelectronics area such as radio frequency circuits to highly efficient
power lines, transformers, motors, and magnetic levitating trains just to name a few.
The Dutch physicist H.K. Onnes first discovered superconductivity in 1911 at the
University of Leiden. Progress was made later by others, but it was only in 1957 when
the American physicists J. Bardee, L.N. Cooper and J.R. Schrieffer introduced the
BCS theory named after their initials which explained the phenomenon for the first
time in history. This theory provided the first complete physical description of the

Fig. 6.65 A typical SMES system connected to a power network.