APPARATUS FOR AUTOMATIC DISCONNECTION OF CURRENT LIMITER

FIELD OF THE INVENTION

The present invention relates to apparatus for disconnecting current limiters, and relates particularly to apparatus for disconnecting current fault limiters utilising superconducting components maintained at a low temperature by immersion in a cooling medium, typically liquid nitrogen.

BACKGROUND OF THE INVENTION

Fault-current limiters have been implemented using superconducting material to carry the current in an electricity supply network. Such networks normally are expected to carry currents of a few hundred amperes, but if a short-circuit (fault) occurs, the current rises to levels which can be several tens of thousands of amperes. When a fault occurs, the current density in the superconducting material exceeds the critical current density of the material, which ceases to be superconducting and becomes resistive. This process is known as quenching. The presence of resistance in the circuit causes the current to be reduced, or "limited" , reducing the potentially damaging effects of excessively high currents in the network.

Networks for the transmission and distribution of electricity are generally three-phase, so three conductors are required and for each of said three conductors, current -limiting means must be provided. Typically a superconducting fault -current limiter will comprise three limiters as described above, housed either in a single vessel or in a plurality of vessels. In the

following, it should be assumed that the means described to interrupt current in the limiter be duplicated as necessary in a multi-phase limiter.

A typical fault current limiter 10 is shown in Figure 1. The current limiting components 12 are made entirely or in part from superconducting material and are immersed in a liquid coolant (cryocoolant) 14 which is contained in a thermally insulated vessel 16. The thermal insulation of the vessel 16 is typically provided by a vacuum layer 18 surrounding the volume containing the cryocoolant 14. The vacuum- insulated vessel 16 is also sealed from the external atmosphere and is designed to withstand a predetermined change of the internal pressure. The vessel 16 may also be fitted with automatic pressure relief valves (not shown) because, in the event of a fault current, the "quenching" superconducting material 12 generates sufficient heat to boil the cryocoolant 14 in its vicinity causing a pressure rise inside the vessel 16. Thermally insulated bushings 20 may be used to allow the current leads 22 to pass into the vessel causing minimal heat loss. A cooling machine 24 ensures that the cryocoolant 14 is kept at a sufficiently low temperature so that the superconducting parts 12 maintain a superconducting state in the absence of fault current .

At the present time it is difficult to design current limiting parts to be able to carry the limited fault -current for time periods exceeding a few hundred milliseconds without excessive heating which has two consequences (1) the temperature of the current limiting parts rises to an extent that there is a risk of said parts becoming damaged and (2) the liquid coolant reaches its boiling temperature, causing the pressure in the vessel containing the superconducting parts to rise.

Means to prevent the above are known. A typical arrangement is shown in Figure 2, in which a circuit breaker 1 is connected in series with a superconducting fault current limiter (SFCL) 2. A current transformer 3 is provided between the SPCL 2 and the circuit breaker 1, which causes an overcurrent relay 4 to provide a signal 5 tripping the circuit breaker 1. If necessary, a parallel impedance 6 of sufficient dimensions to withstand the current may be arranged as shown in Figure 3, allowing limited fault -current to continue to flow. This may be desirable to assure correct operation of protective devices in the network.

There is however a finite risk that the series- connected circuit-breaker will fail to operate. This could be due to failure of components in any stage of the process by which the circuit -breaker is made to trip. The process includes (i) current measurement, <ii) determination that the current is abnormal (fault- current) , (iii) generation of a signal to trip the circuit-breaker, (iv) operation of the circuit-breaker tripping solenoid, (v) release of the circuit-breaker tripping catch, (vi) healthy operation of the circuit- breaker opening mechanism etc. The result of the circuit -breaker failing to open and interrupt the fault current is that current continues to flow through the current limiting parts of the fault -current limiter, giving rise to boiling of the cryocoolant and continuing heating of said current limiting parts. Thermal damage to said parts could result, rendering the limiter unusable subsequently and potentially leading to loss of the cryocoolant due to excessive pressure -rise within the vessel .

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art .

According to the present invention there is provided an actuator device for a current limiter apparatus having :

(i) at least one first conducting lead;

(ii) at least one second conducting lead;

Ciii) at least one superconducting element connected between at least one respective said first conducting lead and at least one respective said second conducting lead; and

(iv) at least one vessel adapted to enclose at least one said superconducting element immersed in cooling liquid therein, the actuator device comprising actuator means and at least one magnetic bistable latching mechanism, wherein said actuator means and said magnetic bistable latching mechanism are adapted to cause at least one circuit breaker to interrupt flow of current through at least one said first and/or at least one said second conducting lead in response to an increase in pressure in at least one said vessel above a predetermined threshold value.

By providing an actuator device for the current limiter apparatus that is capable of causing at least one circuit breaker to interrupt the current flow in response to an increase in pressure inside a current limiter vessel, this provides the advantage that the circuit breaker is operated as a direct consequence of the pressure increase inside the vessel, therefore improving the response time of the circuit breaker as well as minimizing the risk of structural damage to the current

limiter apparatus . Furthermore, the actuator device is very simple in design, allowing currently available current limiter apparatus to be easily updated. Also, the actuator device is more hard wearing and less costly to manufacture. Furthermore, magnetic bistable latching mechanisms provide a very low maintenance solution utilising permanent magnet technology that is self- sufficient and adapted to directly respond {trip) to movement of its operating shaft, therefore allowing it to be directly coupled to the actuator rod of the present invention.

The actuator means may comprise at least one actuator rod adapted to engage at least one circuit breaker through said at least one magnetic bistable latching mechanism.

The actuator rod may be adapted to be moved axially in response to said increase in pressure.

This provides the advantage of a very simple direct connection between the actuator means and the circuit breaker allowing the circuit breaker to respond without any delay.

The magnetic bistable latching mechanism is adapted to be operated through said axial movement of said actuator rod.

The actuator rod is adapted to engage at least one circuit breaker only in a direction interrupting the flow of the current through the corresponding said at least one first and/or second conducting lead.

This provides the advantage that once the circuit breaker is tripped and the current flow is interrupted, the circuit breaker cannot be automatically set back into its conducting state, therefore minimizing the risk of further damage and improving the safety of the current limiter apparatus.

The actuator means may comprise at least one bellows adapted to expand or contract in response to said increase in pressure.

The actuator means may comprise at least one cylinder and at least one piston slidably received and adapted to move axially within said cylinder in response to said increase in pressure.

This provides the advantage of at least two simple means capable of transforming the pressure change inside the vessel into a longitudinal movement of the actuator rod, which is also hard wearing and easy to manufacture.

According to another aspect of the present invention, there is provided a current limiter apparatus comprising :

(i) at least one first conducting lead;

(ii) at least one second conducting lead;

(iii) at least one superconducting element connected between at least one respective said first conducting lead and at least one respective said second conducting lead;

(iv) at least one vessel adapted to enclose at least one said superconducting element immersed in cooling liquid therein; and

(v) at least one actuator device as defined above.

This provides the advantage that a circuit breaker coupled in series to the current limiter apparatus is tripped in a direct response to the pressure increase inside the vessel, therefore minimizing the response time of the circuit breaker as well as minimizing the risk of structural damage to the current limiter apparatus. Furthermore, the current limiter apparatus is very simple

in design which makes it more hard wearing and cheaper to manufacture .

The current limiter apparatus may further comprise at least one circuit breaker.

The circuit breaker may comprise switching means adapted to be operated through said axial movement of said actuator rod.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:

Figure 1 shows a sectional view of a known current limiter apparatus;

Figure 2 shows a first schematic arrangement of a known circuit breaker and current limiter apparatus;

Figure 3 shows a second schematic arrangement of a known circuit breaker and current limiter apparatus;

Figure 4 shows a partial sectional view of an actuator device of a first embodiment of the present invention in a non-tripped state. The actuator is coupled to a plurality of vacuum interrupters shown as 150a and 150b; this is representative only. Normally there will be three or six vacuum interrupters in a three phase implementation of the invention;

Figure 5 shows a partial sectional view of the actuator device of Figure 4 in a tripped state;

Figure 6 shows a partial sectional view of an actuator device of a second embodiment of the present invention in the non-tripped state;

Figure 7 shows a sectional view of a magnetic bistable latching mechanism of the device of Figure 6 in the non-tripped state;

Figure 8 shows a sectional view of the bistable magnetic latching mechanism of Figure 7 in a state between the non-tripped state and the tripped state,-

Figure 9 shows a close-up sectional view of the bistable magnetic latching mechanism of Figure 7 in the tripped state;

Figure 10 shows a partial sectional view of the device of Figure 6 using a bistable magnetic latching mechanism having two additional solenoids;

Figure 11 shows a sectional view of the device of Figure 4 using a single vacuum switch as circuit breaker; and

Figure 12 shows a sectional view of the device of Figure 4 using two vacuum switches as circuit breaker.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to Figure 4 and 5, an actuator device 100 for a current limiter apparatus 10 embodying the present invention includes actuator means 110, e.g. bellows, adapted to cause at least one circuit breaker 120 to interrupt the flow of current through a first conducting lead 22a and a second conducting lead 22b in response to an increase in pressure δP in the vessel 16 above a predetermined threshold value. In Figure 4, the actuator device 100 of the present invention is shown in a state when no fault current is present. In particular, a bellows 110 is arranged in the wall of vessel 16 or a lid

130 of the vessel 16 such that the bellows 110 is acted upon on the outside of vessel 16 by the atmospheric

pressure Pa and on the inside of the vessel 16 by gaseous cryocoolant having a pressure Pl.

If the pressure Pl rises significantly above the atmospheric pressure Pa, a force will be applied to bellows 110. This force is applied to the mechanism of the circuit breaker 120 through an actuator rod 140. However, the movement of the circuit breaker mechanism is prevented unless the force exceeds a predetermined level . The restraining force which must be overcome to allow movement of the circuit breaker mechanism may be produced in a variety of ways. In this particular example, the restraining force is provided by a permanent magnet or magnets within the bistable magnetic latching mechanism 120. The bellows 110 is arranged to move a sufficient distance to cause the mechanism of circuit breaker 120 to directly open the circuit-breaking contacts 150a and 150b as shown in Figure 5. The mechanism of circuit breaker 120 maintains the contacts 150a, 150b in the open position until it is moved back in its non-tripped position.

As shown in Figure 6, instead of bellows 110, a cylinder and piston arrangement 160 can be used to transform the increase in pressure inside the vessel 16 into sufficient longitudinal movement of the actuator rod 140.

The operation of the bistable magnetic latching mechanism 120 is described with reference to Figures 7, 8 and 9.

In its non-tripped state a ferromagnetic core 200 is attached to the lower end of the ferromagnetic yoke 210, allowing the magnetic field 230 of the permanent magnet

220 to be formed in a circuit between the lower end of the ferromagnetic yoke 210, the lower part of the ferromagnetic core and the permanent magnet 220, providing an attractive force of predetermined magnitude between the core 200 and the yoke 210.

A longitudinal force applied in the upward direction to the magnetic core 200 via the non-magnetic actuator rod 140 will not result in upward movement of the core 200 unless the longitudinal force is larger than the attractive force between the core 200 and the yoke 210. When the pressure increase inside the vessel 16 exceeds a predetermined value, the longitudinal force of the actuator rod will be sufficient to oppose the attractive force between the core 200 and the yoke 210. The core 200 therefore starts to move and the magnetic field 230 diminishes quickly due to the higher reluctance introduced into the magnetic circuit by the air gap between the lower end of the core 200 and the yoke 210, as shown in Figure 8. When the core 200 has moved sufficiently so that the air gap between its lower face and the upper face of the lower limb of the yoke 210 is greater than the air gap between its upper face and the lower face of the upper limb of the yoke 210, the attractive force across the smaller, upper air gap exceeds the attractive force across the longer, lower air gap, propelling the core 200 upwards and attaching it to the upper end of the yoke 210.

Figure 9 shows the core 200 attached to the upper end of the yoke 210 allowing the magnetic field 230 of the permanent magnet 220 to be formed in a circuit between the upper end of the ferromagnetic yoke 210, the upper part of the ferromagnetic core 200 and the permanent magnet 220, therefore providing an attractive force between the core 200 and the yoke 210. Thus, the contacts 150a, 150b of the circuit-breaker 120 are secured in the open position.

In order to prevent automatic movement of the circuit breaker 120 into its non-tripped position when the pressure decreases due to cooling of the cryocoolant 14, the actuator rod is only adapted to engage the core

200 of the circuit breaker 120 in a direction that moves the contacts 150a and 150b into its open position.

As shown in Figure 10, the function of the bistable magnetic latching mechanism can be enhanced by including additional solenoid coils 240a and 240b.

In addition, one or two vacuum switches 300a, 300b may be used to interrupt the current flow through the first conducting lead 22a and/or the second conducting lead 22b as shown in Figures 11 and 12.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.