CIRCUIT BREAKERS

Abstract: A circuit breaker comprises an interrupter (1) coaxial with a current sensor or transformer (2), the combined assembly being encapsulated (3) within solid dielectric material and supported by an earthed tube (11) mounted on an earthed housing (14). Detection by the sensor (2) of a current overload is communicated over conductors (13) to a circuit within the housing (14) to cause an actuator (12) to pull a dielectric linkage (10) in direction A so as to move armature (1') through bell crank (5) and open interrupter (1), thereby opening the main current path between conductors (6, 7) and flexible coupling (4). The circuit breaker requires no insulating gas or oil.

Description: CIRCUIT BREAKERS Field of the Invention The present invention is concerned with circuit breakers, particularly of the type comprising current sensing devices or current transformers to detect primary current overloads or short-circuits for monitoring and protection. Background Circuit breakers at medium voltages usually need to employ current sensing devices or transformers to detect primary current overloads and short-circuits for moni- toring and protection. In indoor applications, these may be incorporated in the equipment of which the circuit breaker is a component part. On the other hand, in outdoor applications, particularly in rural electrification schemes, where, the circuit breaker is often used in overhead line applications, the current sensor or transformer is preferably incorporated as an integral part of the circuit breaker. This is usually achieved by mounting a current sensor, such as a ring-type current sensor or transformer, coaxial with an insulated conductor or bushing. Typical examples of conventional use are shown in Figures 1 and 2. The interrupting device could be typically a vacuum or gas switch. The design in Figure 1 usually requires some form of additional liquid or gaseous insulation, such as oil or SF6, to keep the size of the circuit breaker to acceptable levels and also to ensure that the internal components are maintained free of moisture and contamination. A more recent design is depicted generally in Figure 2. Here, the need for a tank filled with oil or SF6 is removed. The current transformer or sensor is mounted at the side of the switch and electrically in series with it. This example uses a vacuum switch and current transformer encapsulated in solid insulation. In both cases, however, it is still necessary for the insulation exposed to outside environmental conditions to have additional "creepage" length compared to insulation that is protected from the external environment. Thus, although the typical design illustrated in Figure 2 does not need liquid or gaseous insulation material to minimise the overall dimensions, it is still necessary to protect the internal surface 1 of the insulation below the switch from the effects of condensation. In exposed hostile environments, this can only be done in a practical manner by filling the volume below the switch with a controlled environment such as dry nitrogen or SF6. This requires additional seals and monitoring and regular maintenance to ensure that the internal surface does not become contaminated. It is vitally important to ensure that the internal surface is kept clean and free from condensation and contamination, otherwise there is a risk of internal electrical discharge from the live conductor down the insulation to earth. In Figure 1 , the current flowing through the device is carried by conductors 1 , encapsulated in suitable electrically insulating material 2, such as epoxy resin or polymer concrete. Connection 9, flexible connection 4 and switch 7 provide the internal con- ducting path. Operation to open or close the switch 7 is performed by actuator 8 and lever 5. The integrity of the internal insulation surfaces is maintained by using SF6 gas or oil. In Figure 2, the current flowing through the device is similarly carried by conductors 1 and switch 2 through current transformer 3. In order to maintain the integ- rity of internal surface 4, some form of controlled environment is required, such as SF6 gas or dry nitrogen. Thus, in both cases it is necessary to protect the internal insulation surfaces by using a controlled environment, leading to additional costs and also risks of degradation and failure if the controlled environment is dissipated due to failure of seals and leakage. Summary of the Invention Accordingly, the invention aims to provide a circuit breaker that does not suffer from the above disadvantages. To that end, the invention provides a circuit breaker comprising an assembly consisting of a circuit interrupter (1) mounted coaxially within a current sensor or transformer (2), said assembly being encapsulated within solid dielec- trie material (3) and supported at one end of an earthed electrically conductive tube (1 1) whose other end is mounted on an earthed metal housing. The circuit breaker is preferably a vacuum interrupter. The circuit breaker is conveniently operated by means of a mechanical linkage of insulating material extending between said interrupter and an actuator, said linkage be- ing mounted externally of the metal tube and said solid dielectric material. The operating mechanism for the circuit interrupter may be selected from any of the group consisting of a permanent magnet actuator, a spring-type actuator, a hydraulic actuator, a pneumatic actuator or a solenoid actuator. The mechanical linkage preferably comprises a rod of solid dielectric material. The ends of the rod preferably pass through flexible bellows, at one end into a space within the dielectric material encapsulating the circuit interrupter/current sensor or transformer assembly, and at the other end into the said earthed housing. Brief Description of the Figures The invention will be described with reference to the following drawings, in which: Figure 1 is a typical prior art circuit breaker; Figure 2 is a later development of a prior art circuit breaker; and Figure 3 is an example of a circuit breaker in accordance with the present invention. Detailed Description of the Illustrated Embodiments In general, a solution to the problems discussed above in connection with known circuit breakers is to provide a circuit breaker where the circuit interrupter 1 is mounted coaxially within the current sensor or transformer 2. The combination is encapsulated within the main electrically insulating body 3, as shown in Fig 3. By encapsu- lating the interrupter and current sensor or transformer in this way, the secondary winding can be supported by an earthed metal tube 11. There is then no internal insulation exposed to high voltage stress, either between parts at high voltage and earth or across the terminals of the circuit breaker. This removes completely the need for additional protection and regular maintenance. Referring now in more detail to Figure 3, an interrupter 1 is connected between conductors 6, 7 constituting the main current path. Coaxially located around the interrupter 1 is a current sensor or transformer 2. The interrupter 1 has an armature 1 ' connected via a flexible coupling 4 to one of the main current path conductors 6. Both the interrupter 1 and the current sensor or transformer are encapsulated in a housing 3 of dielectric material. The sensor or transformer 2 is supported at one end of an electrically conductive tubular body 11 , eg of metal, whose other end is elec- trically and mechanically connected to an electrically conductive housing 14. Secondary wires 13 from the current sensor or transformer 2 can be fed through the metal tube 11 to a suitable terminal board (not shown) mounted in the housing 14. The housing 14 and the metal tube 11 are connected to an earth terminal (not shown). An actuator 12 is located within the housing 14 and is coupled to a linkage 0, preferably comprising a dielectric rod. One end of the rod 10 is coupled to the actuator, for example via a spiral spring, and the other end is coupled to a bell crank mechanism 5. The bell crank mechanism 5 is also coupled to the armature V of the interrupter 1. When the current sensor or transformer 2 senses an overload current, circuitry on the terminal board senses the overload condition and activates the actuator 12 in the housing 14 so as to tend to pull the linkage 10 in direction A. The pull on the link 10 is translated into movement of the bell crank 5 so as to tend to pull the armature 1 ' of the interrupter 1 in a rightwards direction (in Figure 3), whereby to open the interrupter 1. In this way, the circuit breaker opens the main current path through conductors 6, 7 in response to detection of the overload current. The interrupter 1 is restored by the action of the actuator 12 pushing the linkage 10 in direction B to close the interrupter 1 via the bell crank lever 5, whereby to close the circuit breaker and restore it to its dormant position. The linkage 10 is preferably terminated mechanically by flexible bellows 8 and 9 to provide weather protection where the linkage or drive rod 10 enters the housings 3 and 14. The push-pull motion can be achieved by using a suitable operating mechanism, such as a permanent magnet actuator as described in UK Patent No 2297429 or any other form of suitable actuator 12, such as spring, hydraulic, pneumatic or solenoid types.

Claims: C AIMS 1. A circuit breaker comprising an assembly consisting of a circuit interrupter (1) mounted coaxially within a current sensor or transformer (2), said assembly being encapsulated within solid dielectric material (3) and supported at one end of an earthed electrically conductive tube (11) whose other end is mounted on an earthed metal housing. 2. A circuit breaker as claimed in Claim 1, wherein said interrupter is a vacuum interrupter. 3. A circuit breaker as claimed in Claim 1, wherein said interrupter is oper- ated by means of a mechanical linkage of insulating material extending between said interrupter and an actuator, said linkage being mounted externally of the metal tube and said solid dielectric material. 4. A circuit breaker as claimed in Claim 3, wherein said actuator is selected from the group consisting of a permanent magnet actuator, a spring-type actuator, a hydraulic actuator, a pneumatic actuator or a solenoid actuator. 5. A circuit breaker as claimed in Claim 3 or 4, wherein said mechanical linkage comprises a rod of solid dielectric material. 6. A circuit breaker as claimed in Claim 5, wherein the ends of the rod pass through flexible bellows, at one end into a space within the dielectric material encapsu- lating the said assembly, and at the other end into the said earthed housing.

Inventors: MCKEAN BRIAN, MCKEAN BRIAN ANDREW RICHARD, MCKEAN NIGEL TIMOTHY, MCKEAN, BRIAN, MCKEAN, BRIAN, ANDREW, RICHARD, MCKEAN, NIGEL, TIMOTHY
Applicants: MCKEAN BRIAN, MCKEAN BRIAN ANDREW RICHARD, MCKEAN BRIAN ASS LTD, MCKEAN NIGEL TIMOTHY, BRIAN MCKEAN ASSOCIATES LTD., MCKEAN, BRIAN, MCKEAN, BRIAN, ANDREW, RICHARD, MCKEAN, NIGEL, TIMOTHY
Application year: 2001
Main classification: H02B13
IPC codes: H01H33