164
7 Concluding Remarks
appliances, such as televisions, are driven by DC electricity. In the case of refrigerators, washing machines, and air conditioners, commercial AC electricity is converted
to high-frequency AC via DC. Hence, if DC electricity could be supplied to each
home, the conversion from commercial AC to DC would not be necessary.
For superconducting power transmission, we need to cool the superconducting
cable to a sufficiently low temperature such as liquid nitrogen temperature (77.3 K).
The heat that invades the transmission line from the surface is about 1 W per unit
length, and we need electric power of 15 W/m to remove this heat power from
the region at 77.3 K. If this power is smaller than the electric power consumed
in the usual copper transmission line, the superconducting power transmission is
advantageous from the viewpoint of operation cost. If we assume an electric power
transmission of 3 kA through a copper cable, the cross-sectional area of the copper
wire is 3,000 mm
2 , and the electrical resistance per unit length at room temperature
is 5.7 µ, which results in electric power consumption of 51 W/m. Hence, even if
we take the rate of operation into account, the superconducting power transmission
is economical. When the current is increased, the advantage of introducing superconducting power transmission increases, since the cost for cooling does not change.
We now have the problem of the fabrication cost of superconducting cable. This
will be solved, however, by reduction of the cost of superconducting tapes through
improvement of the fabrication technology, with mass production based on the necessity of introducing superconductivity to reduce CO 2 emissions. The fairly high cost
of copper brought about by global economic growth also supports the introduction
of superconductivity. The above comparison has been made for the case where the
power transmission voltage is kept the same as for the present system. If we increase
the current, which increases the advantage of superconductivity, we can reduce the
voltage. This is another advantageous point.
When a network of power transmission lines over a wide area is constructed,
large fault currents caused by short circuits may happen. It is necessary to prevent
such fault currents from propagating. The fault current limiter is the instrument used
for this purpose, and application of superconductors is also expected. There are two
types of superconducting fault current limiter, the resistive type and the magnetic
shield type. The large impedance when the superconductor goes into the normal
state is used to reduce the fault current for both types. The large electrical resistance
in the normal state is directly used in the former type. In the latter type, a transformer
is designed so that the magnetic flux in its iron core produced by the primary and
secondary windings is usually cancelled. When the transport current exceeds the
critical value in one winding, the resistance that appears in this winding reduces the
current, and magnetic flux appears in the iron core, resulting in dramatic increase in
the inductance. Thus, the magnetic shield type is used only for alternating currents,
but the resistive type is used for direct and alternating currents. Rare earth barium
copper oxide (REBCO) thin films are used for the resistive fault current limiter.
Since high resistance is needed when the superconducting thin films go into the
normal state, the use of copper for protection of the thin films is limited, and the
possibility that the limiter will be damaged is fairly high. On the other hand, since
the superconducting cable is very long, it is fairly easy to obtain high resistance. For
7 Concluding Remarks
appliances, such as televisions, are driven by DC electricity. In the case of refrigerators, washing machines, and air conditioners, commercial AC electricity is converted
to high-frequency AC via DC. Hence, if DC electricity could be supplied to each
home, the conversion from commercial AC to DC would not be necessary.
For superconducting power transmission, we need to cool the superconducting
cable to a sufficiently low temperature such as liquid nitrogen temperature (77.3 K).
The heat that invades the transmission line from the surface is about 1 W per unit
length, and we need electric power of 15 W/m to remove this heat power from
the region at 77.3 K. If this power is smaller than the electric power consumed
in the usual copper transmission line, the superconducting power transmission is
advantageous from the viewpoint of operation cost. If we assume an electric power
transmission of 3 kA through a copper cable, the cross-sectional area of the copper
wire is 3,000 mm
2 , and the electrical resistance per unit length at room temperature
is 5.7 µ, which results in electric power consumption of 51 W/m. Hence, even if
we take the rate of operation into account, the superconducting power transmission
is economical. When the current is increased, the advantage of introducing superconducting power transmission increases, since the cost for cooling does not change.
We now have the problem of the fabrication cost of superconducting cable. This
will be solved, however, by reduction of the cost of superconducting tapes through
improvement of the fabrication technology, with mass production based on the necessity of introducing superconductivity to reduce CO 2 emissions. The fairly high cost
of copper brought about by global economic growth also supports the introduction
of superconductivity. The above comparison has been made for the case where the
power transmission voltage is kept the same as for the present system. If we increase
the current, which increases the advantage of superconductivity, we can reduce the
voltage. This is another advantageous point.
When a network of power transmission lines over a wide area is constructed,
large fault currents caused by short circuits may happen. It is necessary to prevent
such fault currents from propagating. The fault current limiter is the instrument used
for this purpose, and application of superconductors is also expected. There are two
types of superconducting fault current limiter, the resistive type and the magnetic
shield type. The large impedance when the superconductor goes into the normal
state is used to reduce the fault current for both types. The large electrical resistance
in the normal state is directly used in the former type. In the latter type, a transformer
is designed so that the magnetic flux in its iron core produced by the primary and
secondary windings is usually cancelled. When the transport current exceeds the
critical value in one winding, the resistance that appears in this winding reduces the
current, and magnetic flux appears in the iron core, resulting in dramatic increase in
the inductance. Thus, the magnetic shield type is used only for alternating currents,
but the resistive type is used for direct and alternating currents. Rare earth barium
copper oxide (REBCO) thin films are used for the resistive fault current limiter.
Since high resistance is needed when the superconducting thin films go into the
normal state, the use of copper for protection of the thin films is limited, and the
possibility that the limiter will be damaged is fairly high. On the other hand, since
the superconducting cable is very long, it is fairly easy to obtain high resistance. For