GB2111774A

GB2111774A - Current sensing device

Info

Publication number: GB2111774A
Application number: GB08231962A
Authority: GB
Inventors: Colin Griffiths; David Richard Aubrey
Original assignee: Brush Switchgear Ltd
Current assignee: Brush Switchgear Ltd
Priority date: 1981-12-15
Filing date: 1982-11-09
Publication date: 1983-07-06
Also published as: GB2111774B

Abstract

Each of three saturable current transformers 10 produces an alternating secondary current whose magnitude is dependent upon the magnitude of the current flowing in a respective phase conductor 11 of a three phase electricity supply. A current-sensitive (e.g. bimetallic) switch 12 and an actuator 14 (common to all three phases) are connected in parallel across the output of each transformer 10. Under normal conditions, the secondary current is insufficient to open the switch 12 or operate the actuator 14. However, when the current in any one of the conductors 11 exceeds a predetermined fault value, the appropriate switch 12 opens in a time dependent upon the magnitude of the secondary current. The respective transformer 10 saturates, and the whole of the secondary current flows through the actuator 14 and causes the latter to operate switchgear (not shown) to interrupt the electricity supply in all three phases. <IMAGE>

Description

SPECIFICATION Current sensing device This invention relates to a sensing device responsive to the alternating current flowing in an electrical conductor, and to apparatus including such a sensing device for tripping electrical switchgear.

In an electrical distribution network, separate low-and high voltage fuses are often provided to protect the low- and high voltage parts of the network, respectively. Typically, the low voltage fuses are placed adjacent to the secondary terminals of a distribution transformer, while the high voltage fuses are connected to high voltage conductors which supply the distribution transformer, in order to protect the transformer. The fuses are chosen so that, for any fault current which flows in the low voltage network, the low voltage fuses will melt before the high voltage fuses. The choice of further relative capabilities of the low- and high voltage fuses is influenced by other technical requirements and by the legal requirements in different countries.

Each time a fuse melts, it must be replaced and the cost of such fuses (particularly high voltage ones) can be very large. The post-fault costs can be greatly reduced by employing a circuit breaker in place of a set of high voltage fuses, which also achieves other advantages for the management of the distribution network. Such a circuit breaker can employ a sensing device comprising a protection current transformer which produces a secondary current in accordance with the current flowing in the aforesaid high voltage conductors, and a fuse and an actuator connected in parallel with one another and across the transformer output. In the event of the current in a high voltage conductor exceeding a predetermined fault level, the fuse melts and the transformer secondary current is or will be sufficiently large to operate the actuator and thereby trip the circuit breaker.The initial cost of the protection current transformer and the actuator can, however, be prohibitively high, so that the circuit breaker is not often a viable alternative to the high voltage fuses from the point of view of overall cost.

It is an object of the present invention to overcome the above-described problems and disadvantages.

According to the present invention, a sensing device responsive to the alternating current flowing in an electrical conductor comprises a saturable current transformer arranged to produce at an output thereof an alternating secondary current whose magnitude is dependent upon the magnitude of said current flowing in the conductor, and an actuator and a current-sensitive switch connected in parallel across the output of the transformer, the current-sensitive switch being arranged to open in a time dependent upon the magnitude of the secondary current when said current flowing in the conductor is above a predetermined value, the arrangement being such that, when the currentsensitive switch opens, the actuator is operated and the transformer saturates to a degree dependent upon said secondary current.

Because it must saturate when the currentsensitive switch opens, the transformer will be small and therefore relatively inexpensive to produce. In addition, because the secondary current which flows through the actuator after the current sensitive switch has opened is limited by said saturation of the transformer, the size and cost of the actuator can be reduced also. Therefore, the initial cost of these components is considerably reduced as compared with conventional constructions, making the circuit breaker to which it is fitted a viable economic alternative to high voltage fuses.Even though the transformer is saturable, it can still supply sufficient energy to operate the actuator when the current in said conductor exceeds the predetermined value, while at the same time enabling the response of the current-sensitive switch to be controlled by a secondary current which is proportional to the current in said conductor when the latter is below said predetermined value. The invention also provides the advantage that the current sensitive switch can be changed to vary the current time response of the circuit breaker, and this can be performed by a user in the field as his protection requirements change.

Desirably, the electrical resistance of the actuator is large compared with that of the current-sensitive switch, preferably not less than five times greater than the latter.

Advantageously, the current-sensitive switch operates on thermal principles, and may be re-settable to avoid the need for replacement. Such re-setting preferably occurs automatically after the switch has opened to avoid the need for manual re-setting.

The actuator may include a solenoid coil, in which case a rectifier is provided between the solenoid coil and the output of the current transformer.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram showing the physical lay-out of a sensing device according to the present invention; and Figure 2 is a circuit diagram showing the electrical lay-out of the sensing device illustrated in Figure 1.

The sensing device shown in the drawing is intended for use in tripping under fault conditions electrical switchgear controlling a three-phase supply. For each phase of the supply, the sensing device comprises a current transformer 10 positioned coaxial with a conductor 11 which carries the respective supply phase. The current transformer produces at an output thereof an alternating secondary current whose magnitude is dependent upon, and also less than, the magnitude of the alternating current flowing in the conductor 11. The transformer 10 is not however of the normal protection current type, but is instead an inexpensive toroidal, grain-oriented silicon steel transformer having few turns.

Connected across the output of each transformer 10 is a thermal switching device 12 which is arranged to open when the current passing therethrough exceeds a certain value, the time taken for the device to open being dependent upon the magnitude of this current. The device 12 may for example include a bimetallic contact member through which the current is passed directly, the contact member when heated to a given temperature operating to switch off the current. Afull-wave diode bridge 13 is connected across the respective transformer output in parallel with the switching device 12, with the outputs of the bridges 13 of all three phases being commonly connected to a solenoid coil 14 such that the latter is connected in parallel with each switching device 12.The solenoid coil 14forms part of an actuatorwhich when energised operates a tripping mechanism (not shown) to trip the electrical switchgear. The electrical resistance of the coil 14 is large compared with that of the switching devices 12, being preferably not less than five times the latter.

During normal operation of the sensing device, i.e.

when the current flowing in the conductors 11 is below a predetermined fault level, each switching device 12 draws off a major portion of the secondary current produced by the respective transformer 10, since its resistance is much smaller than that of the solenoid coil 14. The remainder of the secondary current which flows through the coil 14 is insufficient to cause the actuator to operate.

When the current in any one of the conductors 11 exceeds the predetermined fault level, the secondary current produced by the respective transformer 10 is raised to a level sufficient to open the associated switching device 12. When this happens, the transformer 10 saturates and the full secondary current flows through the solenoid coil 14, thereby energising the actuator sufficiently to cause the latter to operate and thus trip the electrical switchgear. For its forceful operation, the solenoid coil requires a minimum power input. Taking into account the resistance of the solenoid coil, the transformer 10 can be so designed that it produces just sufficient secondary current to give this minimum power input when the current in the conductors reaches the said predetermined fault level.By matching the transformerto the solenoid coil in this manner, the size of the transformer can be kept to a minimum, thereby enabling the sensing device to be made compact in size.

As indicated previously, the time taken for each switching device 12 to open is dependent upon the current flowing therethrough, which is in turn dependent upon the secondary current and hence the current flowing in the conductors 11. If however the current in any of the conductors 11 exceeds the level necessary to commence saturation of the respective transformer 10, the magnitude of the secondary current will be less than would be the case if the transformer were continuously linear in its operation, and accordingly the response time of the associated switching device 12 for a given fault current will be increased. Thus, by suitably matching the switching device 12 and the transformer 10, the response time of the sensing device for a given fault current can be set as desired according to the particular requirements of the switchgear to which it is fitted.Moreover, it is possible to alterthe response time simply by replacing the switching device 12 by another having different characteristics. Thus, the sensing device is advantageously positioned on the exterior of the associated switchgear so that it is readily accessible and the switching devices 12 are provided in modular form so that they can simply be plugged into the sensing device, whereby their replacement by other switching devices having different characteristics can easily be performed. It will be manifest that the switching devices 12 will automatically re-set after the fault current in the conductors 11 has subsided. Accordingly, unlike fuses, there is no need to replace the devices 12 each time the sensing device has tripped the switchgear.

In the event of a high value of fault current occurring in any one of the conductors 11, the power output of the respective current transformer 10 will be increased to such a degree that the solenoid coil 14 will be energised directly without the respective switching device 12 first opening. This is an advantage because the actuator will be operated by the quicker of the two responses. To further this effect, a resistance can be connected in series with the switching device to reduce the ratio between the current which flows through the switching device and the current which flows through the solenoid coil. By suitably choosing the value of this resist ance,the level of primary current flowing through any one of the conductors 11 at which the solenoid coil will be energised directly, can be selected.The current in the conductors 11 which initiates this effect should be chosen having regard to the equipmenu to which the electrical switchgear is connected.

In the case of a transformer, for example, this current should be no less than the magnetising inrush current of the transformer.

From the above description, it will be manifest that the transformers 10, switching devices 12 and rectifier bridges 13 associated with the three phases are connected in parallel across the solenoid coil 14.

Accordingly, the actuator of which the coil 14 forms part will be operated by whichever of the three phases has the highest fault current.

The sensing device of the invention can be used with any suitable type of electrical switchgear, such as that disclosed in our copending UK patent application No. 8102210. In one particularapplica- tion, sensing devices are fitted to a feeder circuit breaker and circuit breakers at T-off connections of 11kV ring main equipment. In such an arrangement, the transformers 10 and the switching devices 12 are so matched that the sensing devices trip their associated switchgear at a minimum fault current of around 50 amps, and saturation of the transformers 10 commences at a current of a few hundred amps in the conductors 11.

Claims

1. A sensing device responsive to the alternating current flowing in an electrical conductor, comprising a saturable current transformer arranged to produce at an output thereof an alternating secondary current whose mangitude is dependent upon the magnitude of said current flowing in the conductor, and an actuator and a current-sensitive switch connected in parallel across the output of the transformer, the current-sensitive switch being arranged to open in a time dependent upon the magnitude of the secondary current when said current flowing in the conductor is above a predetermined value, the arrangement being such that, when the current-sensitive switch opens, the actuator is operated and the transformer saturates to a degree dependent upon said secondary current.

2. Asensing device as claimed in claim 1, wherein the electrical resistance of the actuator is large compared with that of the current-sensitive switch.

3. A sensing device as claimed in claim 2, wherein the electrical resistance of the actuator is at least five times greater than that of the currentsensitive switch.

4. A sensing device as claimed in claim 1,2 or 3, wherein the characteristics of the transformer are matched to the characteristics of the actuator, such that when the magnitude of said current flowing in the conductor reaches said predetermined value, the secondary current which flows through the actuator after the current-sensitive switch has opened is just sufficient to cause the actuator to operate.

5. A sensing device as claimed in any preceding claim, wherein a resistance is connected in series with the current-sensitive switch, the value of the resistance being chosen such that, when the magnitude of said current flowing in the conductor exceeds a desired second predetermined value, the current flowing through the actuator is sufficiently high to operate the latter before the current-sensitive switch has had time to open.

6. A sensing device as claimed in any preceding claim, wherein the current-sensitive switch operates on thermal principles.

7. A sensing device as claimed in any preceding claim, wherein the current-sensitive switch is resettable.

8. A sensing device as claimed in claim 7, wherein re-setting of the current-sensitive switch occurs automatically after it has opened.

9. A sensing device as claimed in any preceding claim, wherein the actuator includes a solenoid coil, and a rectifier is provided between the solenoid coil and the output of the transformer.

10. A sensing device as claimed in any preceding claim, wherein three such saturable current transformers are provided each of which is associated with a conductor carrying a respective phase of a threephase electricity supply, three such current-sensitive switches are provided each of which is connected across the output of a respective one of the transformers, and the actuator is common to all three transformers, being connected in parallel with each current-sensitive switch.

11. A sensing device responsive to the alternating current flowing in an electrical conductor, substantially as hereinbefore described with reference to the accompanying drawing.