Medium- or high-voltage circuit breaker or isolator, provided with improved fixed contacts, and method of use

A high- or medium-voltage circuit breaker, in which a movable contact (1) is moved in sliding in order to separate stationary contacts (11, 12) from its sliding surface (2). In accordance with the invention, the stationary contacts (11, 12) have switching portions (9) that separate from the movable contact more or less simultaneously as a result of a setback (19) in the movable contact (1), thereby causing two simultaneous switching arcs to appear (14, 15), which arcs are extinguished much more quickly than a single arc, before the current is transferred to the conventional arcing contacts (5). This disposition enables the stationary contacts (11, 12) and their environment to be less exposed to damage.

The present invention provides a medium- or high-voltage switch or circuit breaker, provided with improved stationary contacts, and it also provides a method of using said circuit breaker.

In such switchgear, switching between a closed state and an open state is performed by moving a movable contact along a stroke. The stationary contacts touch a sliding contact surface of the movable contact, and electricity flows from one stationary contact to another through the movable contact, until one of the stationary contacts reaches a break in the sliding surface and loses contact.

It is known that an electric arc is then formed between the movable contact and the isolated stationary contact. This electric arc has damaging effects, in particular by tearing off conductive particles and therefore eroding the contacts between which the arc extends, as well as deteriorating the surrounding atmosphere. That is why other contacts, called arcing contacts, are very frequently associated with the above-described stationary contacts (also called permanent contacts), which arcing contacts touch each other at the beginning of the stroke towards the open state, and separate only after the permanent contacts have already separated. The electric current is then transferred to pass through the arcing contacts while abandoning the permanent contacts; the arc appears between the arcing contacts, at a location in the device that can be selected so as to ensure less damage is suffered; and the arc may then be blasted by a flow of gas that has been compressed in a chamber by the stroke of the movable contact and then released by opening a valve, in known manner.

Although adding arcing contacts is beneficial, it should be stressed that electric arcing between the permanent contacts is not completely eliminated, and there remains a short-lived arc, known as a switching arc, that remains until the current passes only via the arcing contacts. Said switching arc may also damage the switchgear because of its high temperature, which is capable of rising to a few hundred or a few thousand degrees, thereby producing local melting and erosion of the contact material, and also spraying particles into the surroundings. This results in the dielectric strength of the switchgear being weakened via the solid elements or the surrounding gas, and leads to a risk of breakdowns and sparkovers or flashovers. That is why the French patent application published under number 3 001 575 envisages constructing the movable contact in two portions separated by insulation, and constructing one of the stationary contacts as a main portion and an auxiliary portion. The current flows through the main portion of the stationary contact during normal service, but the switching arc extends starting from the auxiliary portion. The advantage of that device is that the location at which the switching arc appears may be sufficiently isolated for the arc to cause less damage, but without reducing the arc itself.

It is also possible to consider applying arc-blasting techniques to switching arcs, but that is not always convenient, nor is it permitted in all kinds of switchgear; and the use of a dielectric arc-quenching gas, involving the switchgear being hermetically sealed, is thus often necessary.

The invention proceeds from another idea: weakening the switching arc, and its effects, by splitting it into two, and causing it to appear simultaneously or almost simultaneously at both of the stationary contacts.

In general form, the invention provides switching or circuit breaking switchgear, including a movable contact provided with a switching stroke switching from a closed state to an open state of the switchgear and with a sliding surface, a first stationary contact and a second stationary contact touching the sliding surface, a first break in the sliding surface being crossed by the first stationary contact during the switching stroke; the switchgear being characterized in that the sliding surface has a second break that is crossed by the second stationary contact during the switching stroke, at essentially the same time as the first break is crossed by the first stationary contact.

In other words, the stationary contacts cross the breaks of the sliding surface together, i.e. at the same time, or with a small time lag chosen to make it possible for the switchgear to interrupt the current where, as a function of a suitable speed of the movable contact during the stroke from the closed state to the open state, simultaneous switching arcs are struck between the switching portions and the breaks in the sliding surface of the movable contact.

This sharing of the switching arc between both stationary contacts promotes fast extinction, and thus reduces the amount of heat that the arc produces and the amount of damage to the permanent contacts. This is particularly visible in a particularly preferred embodiment of the invention, where each stationary contact includes a main portion and a switching portion, the main portion and the switching portion being electrically connected, both touching the sliding surface, but being arranged in such a manner that electricity passes preferentially via the main portion between the stationary contact and the movable contact; and where, for each stationary contact, the main portion reaches the break in the sliding surface before the switching portion. Again in this example, the condition to be satisfied is that the stationary contacts disconnect together, simultaneously or almost simultaneously, from the movable contact by crossing a setback therein.

FIG. 1is a diagram showing a circuit breaker of the prior art, where a movable contact1, in the form of a tube, slides to the left, under the action of a control mechanism, in order to open the circuit breaker. It includes sliding surfaces that touch two stationary contacts3and4(via the same sliding surface2in this example, but that is not necessary). The circuit breaker is also provided with mutually sliding arcing contacts5that are electrically connected respectively to the movable contact1and to the first stationary contact3. The sliding surface2stops at one end6of the movable contact1. When the electric circuit controlled by the circuit breaker opens, the first stationary contact3is released from the movable contact1, and a switching arc briefly appears between them, until current flow is established via the arcing contacts5. Said contacts also separate after an additional stroke of the movable contact1, which truly opens the circuit breaker, while at the same time creating an arc between the arcing contacts5, which arc is extinguished conventionally by blasting.

The stationary contacts of the invention may have the appearance shown inFIG. 2. Each of them includes a main portion7, connected directly to an electricity transmission line8, and a switching portion9, which is an auxiliary portion that is also connected to said line8, but in a way in which it is more difficult for current to pass. The switching portion9may thus be welded to the line8via an endpiece10and it may be made of a material that is less good at conducting electricity than the main portion7. The portions7and9are designed so that both of them touch the sliding surface2together when the circuit breaker is closed, but at a short distance apart. The invention could be implemented with single-portion stationary contacts such as the contacts3and4shown inFIG. 1, perhaps with less effectiveness.

Reference is made toFIG. 3, which shows an embodiment of the invention in which the stationary contacts3and4have been replaced by stationary contacts11and12constructed in accordance withFIG. 2. The switching state that is shown is quite close to the opening, with the main portion7of the first stationary contact11being located not far from the end6, and with the switching portion9being a little farther away. The movable contact, now referenced13, includes an intermediate setback19having an end face20that constitutes another break in the sliding track2, and the main portion7of the second stationary contact12is close to the setback.

While the circuit breaker continues its opening stroke, this arrangement results in the portions7and9of the stationary contacts11and12separating from the sliding surface2, by crossing either the end6, or the end face20of the intermediate setback19. The order of separation could be as described below.

The first portion to separate is the main portion7of the second stationary contact12(FIG. 4); then the main portion7of the first stationary contact11(FIG. 5); then the switching portion9of the second stationary contact12(FIG. 6); and finally the switching portion9of the first stationary contact11(FIG. 7). However, some variants of operation can be accepted. Thus, the main portions7can separate in unspecified order, and the switching portions9can also separate in unspecified order; they can also separate simultaneously; and the states shown inFIGS. 5 and 6could occur in reverse order.

On separation of the portions7and9of the contacts11and12, current passes via the switching portions9when the main portions7separate from the movable contact1, and switching arcs14and15appear when the switching portions9separate from the movable contact1. In accordance with the invention, the switching arcs14and15exist simultaneously for an instant, as shown inFIG. 7, for reasons that are explained below with reference to the last figures. That is due to the simultaneous or almost simultaneous separation of the stationary contacts11and12that cross the end6and the end face20at a time interval that is advantageously shorter than one half-period of alternating current, by means of the movable contact1being moved at a suitable speed.

The curve16ofFIG. 8shows the current produced at the terminals of the circuit breaker. When theFIG. 6state is reached at an instant t0, if there were no second arc15, the switching arc14would be of magnitude that decreases in substantially linear manner along the segment17and that stops at the instant t+Δt1, the current then being then taken up by the arcing contacts5. But if, in accordance with the invention, the other switching arc15starts at t0or starts immediately after, at the instant t0+Δt2(FIG. 9), segment17, now given reference17′, is interrupted and it can be seen that the simultaneous arcs14and15then weaken much faster, along the following segment18, so that they are extinguished at the instant t0+Δt3, where Δt3is less than Δt1. The duration and the average magnitude of the arcs, as well as the damage inflicted on the contacts, are then significantly reduced. The same advantages remain present with the above-mentioned operating alternatives, and also for single-portion contacts, such as the conventional stationary contacts3and4.