The present invention relates to a current limiter and in particular
to a resistive superconducting current limiter.
Current limiters prevent unacceptable large current surges in high
power systems during power fluctuations, lightning strikes and short circuits and
thus protect expensive electrical equipment from damage. The need for current
limiters is associated with the continuous growth and interconnection of modern
power systems which results in a progressive increase of short circuits to levels
far beyond the original design capacity of the switchgear.
Current limiters can be grouped as resistive, inductive or hybrid,
which operate by changing their impedance from nearly zero during normal operations
to a current limiting value during fault conditions. The ideal performance characteristics
of a fault current limiter include; zero impedance under normal operating conditions,
high impedance under fault limiting conditions, fast transition from normal to
fault limiting conditions, fast recovery to normal protection after interruption
of a fault, high reliability over long periods with minimal maintenance, low volume,
low weight and low cost.
Superconductors, high temperature (HTC) or low temperature (LTC),
offer attractive potential as fault current limiters due to the great contrast
between the superconducting and non-superconducting states.
The application of low temperature metallic superconductors to power
engineering is limited by their low operational temperature which required liquid
helium refrigeration for large scale devices. The cryogenic engineering of liquid
helium is sophisticated, costly and demands specialised technical support.
In contrast high temperature ceramic superconductors remain superconducting
at transition temperatures above 77K, which is the saturation temperature of liquid
nitrogen at one atmosphere. The low cost, reliability and simplicity of refrigeration
at liquid nitrogen temperatures makes high temperature superconducting materials
very attractive to the power engineering industry.
High temperature superconductors are however inhomogenous, anistropic
and brittle materials. Their use as fault current limiter encounters problems of
local heating and mechanical failure when a fault current is applied.
Resistive superconducting fault current limiters incorporate a superconducting
element connected in series with the system to be protected. When the system is
carrying normal operating current the element is in the superconducting state
and thus has near zero resistance. The element is driven to the resistive state
when a system fault occurs. The increase in current exceeds the critical current
of the superconducting material which quenches to a resistive state. The impedance
of the device (which is predominately resistive) increases rapidly providing the
fault limiting effect.
A problem with resistive superconducting fault limiters occurs if
part of the superconducting element becomes resistive before the rest of the element.
This is known as "local quenching" and is due to a non-uniformity of superconducting
properties along the element length. The quenched part of the superconducting element
overheats and may burn out leading to a catastrophic failure.
To avoid the problem of local quenching it is known to use triggering
techniques to obtain a fast and uniform transition of the superconductor to the
resistive state. Known triggering techniques include laser heating to exceed the
critical temperature of the superconductor, discharging capacitors to exceed the
critical current density of the superconductor and external sources of magnetic
field to exceed the critical flux density of the superconductor. The triggering
technique has to respond within a few milliseconds in order to limit the current
before it reaches its peak value and must provide sufficient energy to quench the
whole of the superconductor.
British patent number 1,236,082 discloses a resistive superconducting
fault current limiter which uses a low temperature metallic superconductor. In
this patent a magnetic field produced by a helmholtz coil is used to quench the
low temperature superconductor. A problem with this fault current limiter is that
in the preferred embodiment, the magnetic field is radial and introduces an external
Lorentz force on the superconducting elements. This arrangement is therefore unsuitable
for use with high temperature superconductors as the Lorentz forces generated
by the radial field would cause mechanical failure of the brittle high temperature
The present invention seeks to provide a triggering technique suitable
for use with high temperature superconductors. A fault current limiter in accordance
with the present invention solves the problem of local heating and mechanical
fracture failure in superconductive elements.
High temperature superconductors offer an attractive cost effective
design due to the reduced cooling costs compared to low temperature metallic superconductors.
The superconductive fault current limiter in accordance with the present invention
has flexible design features to satisfy operating specifications by easily varying
the physical, electrical and magnetic parameters. Uniform quenching of the superconductor
is achieved by the combination of the critical current density and critical magnetic
field intensity. The design has the advantage of being light weight, compact,
high impedance ratio and can be easily upgraded to a higher rating fault current
According to the present invention there is provided a resistive
superconducting current limiter comprising an element maintained in a superconducting
state when carrying an electrical current under normal operating conditions, and
means for generating a magnetic field in the region of the element, the element
comprises a plurality of superconducting elements arranged to be parallel with
one another, and the means for generating the magnetic field are connected in
series with the superconducting elements so that, when a fault occurs, the increase
in the electrical current through the means for generating the magnetic field causes
a substantially uniform magnetic field to be generated parallel to the superconducting
elements which exceeds the critical magnetic field density of the superconducting
elements to assist triggering of the transition of the superconducting elements
to a resistive state characterised in that the superconducting elements are of
a high-temperature ceramic material and the means for generating the magnetic
field comprise a foil-wound coil.
The application of a magnetic field parallel to the superconducting
elements offers the advantage that no external forces are introduced. Mechanical
failure of the superconducting elements does not therefore occur. The uniform
nature of the magnetic field prevents local quenching, which can cause thermal
and mechanical failures due to local heating in high temperature superconductors,
and also increases the rate of change of resistance of the superconductor, which
in effect helps to produce a fast response fault current limiter.
The uniform magnetic field parallel to the superconductor is generated
by a winding through which an electrical current passes and which is preferably
connected in series with the superconducting element.
The winding is preferably a wound conductor foil of for example copper
or aluminium. The winding is preferably connected in series with the superconducting
element so that when a fault occurs the increase in electrical current through
the winding causes a magnetic field to be generated which exceeds the critical
magnetic field of the superconducting element.
Foil-wound coils are simple to produce and self-supporting. The production
of the foil windings can be automated to produce a cost effective fault current
In the preferred embodiment of the present invention the superconducting
elements are located in a cryostat which is filled with a fluid at a temperature
low enough to maintain it in the superconducting state. The cryostat is conveniently
non-metallic and is placed within the winding.
The superconducting elements may be a non-inductively arranged set
of superconducting elements.
Preferably means, such as a reactor or resistor, is connected in
parallel with the fault current limiter to limit the transient overvoltage in the
superconducting elements when in the resistive state.
The present invention will now be described with reference to figure
1 which shows a partially broken away view of a fault current limiter in accordance
with the present invention.
Referring to figure 1 a superconductive current limiter is generally
indicated at 10. The superconducting element 12, consists of a non-inductively
arranged (low self-field) resistor elements which are placed inside a non-metallic
cryostat 14. The cryostat 14 is filled with a cooling medium such as liquid nitrogen
which has a temperature low enough to maintain the superconducting element 12 in
a superconductive state.
The cryostat 14 is placed inside a conventional line winding 16,
a foil winding, connected in series with the superconducting element 12. The superconducting
element 12 is connected to the foil winding 16 by means of current leads 13 and
15. The pair of vertical current leads 13 and 15, each carry current in the opposite
direction and are located inside the winding 16 in the region of predominately
uniform magnetic field.
The line winding 16 is a foil winding, of for example copper or aluminium,
and provides a magnetic field parallel to the superconducting element 12. The magnetic
field assists triggering and the number of turns in the foil winding 16 is selected
to provide low inductance and low magnetic field when the superconducting element
12 is carrying its normal operating current.
In the event of a fault, such as a short circuit, the increase in
electrical current in the foil winding 16 causes a magnetic field to be generated
parallel to the superconducting element 12 which exceeds the critical magnetic
field of the superconducting element 12. The magnetic field produced by the foil
winding 16, in combination with the increased current in the superconducting elements,
triggers the superconducting element 12 to the resistive state.
The magnetic field produced by the foil winding 16 is uniform and
parallel to the superconducting element 12. It is used as a means of additional
triggering and uniform quenching. Uniform quenching solves one of the acute problems
of local quenching, which causes thermal and mechanical failures due to local
heating in most high temperature superconductors used in power systems. The application
of a uniform parallel field is also used to increase the rate of change of resistance
of the superconducting elements 12, which in effect helps to produce a fast responding
fault current limiter.
Since the introduction of a uniform and parallel magnetic field does
not introduce an external force on the brittle superconducting elements 16, the
overall mechanical design can be less costly. This invention uses a foil winding
16, which is simple to produce and self-supporting and the production of the foil
windings 16 can be automated to produce a cost effective fault current limiter
A metal oxide varistor 18 is connected in parallel with the current
limiter 10 in order to provide protection against the transient over-voltages and
to assist in preventing excessive dissipation in the superconducting element 12.
A superconducting current limiter 10 in accordance with the present
invention is light weight, compact, fast, has lower losses, has lower overall cost
and the impedance ratio is superior to other designs. By arranging the superconducting
element 12 in the non-inductive manner, the stored energy in the superconducting
element 12 is greatly reduced. This means that power dissipation in the superconducting
element 12 following a resistive transition is also greatly reduced.
The foil winding 16 generates the magnetic field automatically when
a fault occurs due to the increase in current through the foil winding 16. When
the magnetic flux density exceeds the critical value it assists a uniform and
fast quenching of the superconducting element 12.
It will be appreciated by one skilled in the art that an alternative
arrangement could be used in which the whole system, winding 16 and the superconducting
elements 12, are placed inside the cryostat 14. In this arrangement however allowances
for power losses in the winding 16, current leads 13 & 15 and the cryostat
14, if metallic, has to be made.