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Power electronic control in electrical systems 239

phenomenon known as a quantum phenomenon, in which conduction electrons
happen to move in pairs thus showing no electrical resistance. For their work they
won the Nobel Prize in 1972. However, until 1985, the highest temperature super-
conductor was niobium germanium at 23 K (23 K or 418 F). The temperature of

23 K was very difficult to achieve, as liquid helium is the only gas that can be used to
cool the material to that temperature. Due to its nature, liquid helium is very
expensive and inefficient and that was the main obstacle for commercial applications
of superconductors. In 1986, K.A. Miller and J.G. Bednorz discovered a super-
conducting oxide material at temperatures higher than ones which had been thought
possible. For their discovery they won the Nobel Prize in physics in 1987. They had
effectively raised the temperature for a superconductor to 30 K ( 406 F). In 1987,
P. Chu announced the discovery of a compound (Yttrium Barium Copper Oxide)
that became superconducting at 90 K. Even higher temperatures were achieved in
later times with bismuth compounds at 110 K and thallium compounds at 127 K.
When materials were discovered that their critical temperature to become super-
conductors was raised above 77 K, the low cost and readily available liquid nitrogen
could be used to cool the superconductor. This made some products and applications
commercially viable.
Superconductivity offers two interesting and different ways of energy storage,
namely SMES systems and flywheels based on superconductive magnetic bearings
as presented in the previous section (Hull, 1997).
The characteristics of an SMES system can be summarized as follows:
. Very quick response time (six cycles or simply 100 ms for a 50 Hz system, for large-
scale systems, and fewer cycles for smaller-scale units).
. High power (multi-MW systems are possible, for instance 50±200 MW with
30±3000 MJ capacity).
. High efficiency (since no conversion of energy from one form to another, i.e. from
mechanical or chemical to electrical and vice versa does not occur, the round trip
efficiency can be very high).
. Four quadrant operation.
The above mentioned attributes can provide significant benefits to a utility for a
number of cases as follows:

. load levelling
. improve the stability and reliability of the transmission line
. enhance power quality
. extend line transmission capacity
. provide voltage and reactive power control
. spinning reserve.
A SMES system belongs to FACTS, which can exchange both real and reactive
power with the grid. Therefore, it can be used successfully to manage the perfor-
mance of the grid at a given point. It is based on a well-known concept of DC
current flowing through a coiled wire (Figure 6.65). However, the wire is not a
typical type which has losses through the conduction of current. The wire is made
of a superconducting material. An important part of any SMES system is the
required cryocooling. A highly efficient cryostat containing the superconductor