DESCRIPTION

Field of Invention

The present invention relates to a disconnector and relative surge arrester, also called surge limiter, or in brief SPD (Surge Protective Device) ; in particular it re ^¬ lates to an arrester provided with disconnecting device or disconnector for the interruption of the short-circuit in the event of failure of the SPD.

BACKGROUND ART

By the term surge arrester is meant those electri ^¬ cal/electronic devices which, interposed between the active conductors of the electric system and the ground, provide for the discharging to the ground of the overcur- rent /overvoltage peaks - e.g., those generated by atmos ^¬ pheric lightning strikes and switching operations - that might otherwise produce serious damage to the electrical system and its apparatuses.

Indeed, the direct lightning phenomena are the main source of devastating destructive effects on electrical systems; indirect discharges and switching surges are also sources of many damages, the origin of which is not easy to identify, but which effects are equally devastating for the sensitive plants and where the operation continuity is es ^¬ sential. The duration of these phenomena varies from a few microseconds up to a few hundred milliseconds, but in this very short time they convey a very high energy content. These phenomena must be properly intercepted in order to protect the plants connected to the main and thus to ensure the integrity and function thereof.

In this context, reference is made to surge arresters of the most recent prior art, comprising a security element in the form of a varistor, which has an equivalent behav ^¬ iour to that of a variable (non-linear) resistance in term of voltage/current ratio. In the event of an overshot ref ^¬ erence voltage, for example when there is a short-term overvoltage/overcurrent peak, the varistor abruptly lowers its resistance, so that the peak can be easily discharged through it, towards the ground, and does not propagate to other parts of the plant with higher resistance. To the electrodes of the varistor the contacts of the connection terminals of the surge arrester are joined electrically, which are in turn connected respectively to a phase conduc ^¬ tor and to the protective conductor and/or the neutral con ^¬ ductor. In the internal circuit of the arrester, disposed in series with the protection element as a varistor, a "disconnector" is typically provided, which is a complex disconnecting device known per se, having protective func ^¬ tions in case of failure and/or degradation of the protec ^¬ tion element.

The thermal disconnector is substantially constituted by an electric conductor of various shape connected in se ^¬ ries with the electrode of the varistor. It consists of a complex unit, typically comprising an elastic metal plate attached to the electrode of the varistor by welding with a low melting solder dot, which is a material capable of melting at relatively low temperatures (120-180°C). The elastic plate is welded in an elastically flexed or spring- loaded condition, however placed in a resiliently loaded condition such as to define a bias, which tends to distance it from the electrode of the varistor. Thanks to this ar- rangement when, as a result of degradation, the varistor starts to discharge to the ground a significant current, which is not transient but continuous in nature, this tends to heat up by Joule effect. This temperature is transferred to the solder dot, and when the temperature of the low melting alloy is reached, the holding capacity of the sol ^¬ der dot is impaired, so as to free the metal plate from the contact with the electrode of the varistor, thus opening the electrical circuit and restoring the safety conditions.

Within certain range of short-circuit current, typi ^¬ cally a few tens of amperes, the disconnection system within the arrester is therefore able to perform this dis ^¬ connection in an autonomous way, i.e. without using other internal or external devices placed in series with the ar ^¬ rester itself.

However, when the internal impedance of the arrester suddenly reaches values close to zero and as a result a short-circuit is generated, it occurs a high-intensity cur- rent, which gives rise to an unacceptable condition within the electrical system.

Consequently, a disconnecting device must intervene in order to eliminate this condition. Note, however, that the disconnection obtained with a standard disconnector is not always sufficient. In fact, it should be considered that at the opening of an electric circuit where current is flow ^¬ ing, an electric arc could be created, which seeks to main ^¬ tain the continuity of the circuit itself. If the arc does not extinguishes by itself or the disconnector is not able to stop it, it creates a dangerous situation both in the arrester (overheating and possible fire and/or explosion) and in the relevant electric plant.

Typically, in the past, devices capable of interrupt ^¬ ing significant short-circuit currents, of the order of kArms, were constituted by a overcurrent protection, for example a fuse or a circuit breaker, placed in series with the arrester itself.

More recently, it has been provided a very effective solution, described in EP2790192 in the name of the same Applicant, in which one device includes the disconnection capacity to face slow degradation of the varistor, but also circuit opening means with relative self-extinguishing ca- pability to cope with important short-circuit currents.

This system turned out satisfactory, but the Applicant has noted that there is room for improvement of perform ^¬ ance .

In brief, the arrester described in EP2790192 com- prises a disconnector, consisting of a flexible metal plate made of conductive material with a geometry such that, in normal operating conditions, maintains an interception slider constrained thereon; the latter has the shape of a slider or mobile carriage with a suitable geometry to in- tercept and stop the electric arc that would be present during the short-circuit; in a suitable longitudinal recess of the slider a preloaded spring is inserted, suitable to provide the pushing energy to the slider during its opera ^¬ tion, which is maintained in compression by the presence of the disconnector itself, which acts as a constraint means.

When high short-circuit currents happen, interruption of the circuit takes place by the fact that the metal plate of the disconnector sublimates, so as to free the slider that in turn intercepts and stops the possible formed elec- trie arc.

However, it was found that the sublimation of the con ^¬ ductive plate generates two effects: on the one hand, the desired effect of elimination of the constrain means hold ^¬ ing the slider in its normal operating position, so that the slider is free to move due to the transformation of the potential elastic energy of the spring into kinetic energy; but, on the other hand, the non-desired effect of formation of a conductive gaseous mass, called plasma, which, along with the mains voltage, results in the triggering and the diffusion of the electric arc within the entire arc cham ^¬ ber, i.e., the cavity between the solder dot of the discon ^¬ nector and the residual root portion of the metallic plate.

In summary, the development of the plasma in the arc chamber causes an instantaneous rise in temperature and pressure .

At the same time, the release of the slider triggers the process that leads to the extinction of the electric arc (well described in EP2790192), but such operation must take place in a sufficiently fast manner so as to prevent the pressure and temperature from being excessively high within the device, up to create explosive effects.

It was found that as the short-circuit current in- creases, the only potential elastic energy of the spring may be insufficient to impart a thrust to the slider such as to reduce the actuation time and then extinguish the electrical arc in a time span compatible with the mechani ^¬ cal strength of the arrester housing.

In particular, it was noted that the high pressure of the plasma generated by the electric arc exerts on the front end surface of the slider a longitudinal counter- thrust, with a direction opposite that produced by the spring, which opposes the movement of the slider. So long as this pressure produces this counterthrust , the slider, although urged by the spring, is not able to move in a man ^¬ ner rapid enough to extinguish the arc within a time span compatible with the mechanical strength of the device hous ^¬ ing. The criticality of the phenomenon is inherent in the fact that the counterthrust generated by the plasma pres ^¬ sure increases with the square of the short-circuit cur ^¬ rent; vice versa, the thrust exerted by the spring is an invariant with respect to this current. This phenomenon is not mitigated adequately even by the provision of pressure evacuation holes pierced in the slider guide chamber on the back side of the slider itself.

Other arrangements of surge arresters are disclosed also in US20110170217, WO2007/093572, DE102006042028, US20120050935 and EP2725588, but none of them is supplying any useful suggestion to address the above cited technical problems .

Summary of the invention

The object of the invention is therefore to supply a disconnector that solves the problems of the prior art; namely, it is needed to provide a disconnector in a surge arrester that, without losing all the functional advantages of providing a sublimable internal lamina and a slider for the electric arc shut-off, allows to avoid that the pres ^¬ sure of the plasma, produced by the sublimation of part of the disconnector itself, approaches limits that are danger ^¬ ous for the life of the arrester.

This object is achieved through the features set out in essential terms in the appended claims.

Brief description of the drawings

Further features and advantages of the invention will anyhow be more evident from the following detailed descrip ^¬ tion of a preferred embodiment, given as a non-limiting ex- ample and illustrated in the accompanying drawings, wherein :

Fig. 1A is a schematic side elevation view, with parts cut away, of a surge arrester in an armed condition and with disconnector at rest;

Fig. IB is a cross-section view taken along line B-B of Fig. 1A; and,

Fig. 2 is a view similar to that of Fig. 1, of a surge arrester according to the invention, in a state in which the disconnector has reached the end of stroke and com ^¬ pleted the opening of the circuit.

Detailed description of a preferred embodiment

In Fig. 1 there is shown a configuration of a surge arrester known per se from EP2790192, which here is consid ^¬ ered included as reference.

A surge arrester is housed in a box-shaped body or housing, referenced as module C, with dimensions such as to be housed in a single standard module and wired inside a switchboard for electrical plants. In this housing C, in a per se known manner, two opposing terminals are accommo ^¬ dated - a first terminal 1 for the connection of the phase lead and a second terminal 2 for the connection of the pro ^¬ tective, or neutral, lead - between which a protection ele- ment (typically a varistor) is arranged, here schematized by a plate 3, on whose opposing surfaces the respective conducting electrodes are arranged (in the figures only an electrode 4 is illustrated, the other being on the opposite side is not visible in the drawing) .

The electrode 4 is electrically connected to the phase terminal 1, while the opposed electrode is connected to the ground or neutral terminal 2. The connection between the electrode 4 (Fig. 1A) and terminal 1 is realized by a con ^¬ ductor constituting an element of the disconnector. In par- ticular, this conductor of the disconnector is in the form of a flexible lamina or plate 5, which is elastically pre ^¬ loaded and joined to the electrode 4 by a suitable low- melting solder dot at the point marked with 5d.

The material used to make the low-melting solder and the exact configuration of the flexible plate is not rele ^¬ vant in this context and not be described here in further detail . The flexible plate 5 is preferably made of a low thickness (in the order of a few tenths of a millimetre, for example 0.2-0.3 mm) and a reduced section, with a me ^¬ tallic material having conductive properties equal or lower to that of copper.

In case it is used a material having lower conductive properties (=conductivity rate) , the thickness can be in ^¬ creased for example up to 0.5-1 mm. An exemplary conductiv ^¬ ity rate can be a IACS (International Annealed Copper Stan- dard) < 60; in this case, the material is preferably made from a copper alloy with elements such as to modify its conductivity (copper IACS < 90) and confer elastic proper ^¬ ties.

Such a plate is advantageously conceived to sublimate rapidly - namely passing from solid to gaseous state - when run by short-circuit currents above a preset amount of cur ^¬ rent, of the order of a few kArms, e.g. from 3 up to 16 (indicative but not binding values) .

Between the abutment rigid wall of the plate 5 (Fig. 1A) and an inner housing for accommodating the varistor 3, it is defined a guide 6 where a slider 7 is sliding accom ^¬ modated for the interception and compression of the arc. In particular, the slider 7 is longitudinally guided by two parallel containment walls 11a and lib. In addition, pref- erably, the slider 7 is provided with a pair of longitudi ^¬ nal grooves 7a, on the two opposite sides, intended to en ^¬ gage and slide on corresponding longitudinal ribs 9 ar ^¬ ranged within the guide 6.

The slider 7 is mounted to slide longitudinally within the guide 6 while being constrained in rest conditions (represented in Fig. 1A) , on one side against a bottom wall 13 of the guide 6 and, on the other side, on a part of the flexible plate 5. The slider 7 is mounted being biased to- ward (arrow F) the plate 5 by means of an elastic element, such as a spring 8 (visible in Fig. 2), which is mounted pre-compressed between the bottom wall 13 and the body of the slider 7.

In particular, to exploit the interior space of the device, the slider 7 has a longitudinal cavity, in which a major part of the spring 8 is inserted.

Preferably, the slider 7 is made as a tubular body, closed at one front end (the lower end in the drawing) and opened at the other back end.

With this construction, the slider 7 is retained by the plate portion 5, which is abutting on the front end and which is opposing to the thrust of the spring 8.

Instead, when the retaining action of the plate 5 is released, as a result of its sublimation resulting from the short-circuit current, the slider 7 is released and pushed by the spring 8 in the direction of the arrow F, so as to perform its electric arc extinguishing function and ending its run in abutment against an end wall.

Note that the slider 7 must have a significant length, for example of the order of some tens of mm, because it must ensure an adequate area of contact with the contain ^¬ ment and guide walls 11a and lib as well as a high creepage distance favourable to the arc extinguish function . Due to the necessary sliding clearance between the slider 7 and the walls 11a and lib, if the creepage distance is not suf ^¬ ficiently extended, there would be a high risk that the electric arc can remain switched on between the slider 7 and the guide walls 11a and lib, circulating around the slider which would be not more effective for the arc ex ^¬ tinction. Therefore, it is appropriate that the side walls of the slider, those perpendicular to the direction of propagation of the electric arc, are extended as much as possible .

This significant length of the slider causes in part the problems arising from the development of plasma, be ^¬ cause the pressure front of the plasma has to travel a long way before reaching the back side of the slider and rebal ^¬ ancing the thrust that is generated on the front side op ^¬ posing to the spring 8: as a result, the timing of the ac ^¬ tion of the slider are getting longer, and there is the risk of explosion of the device due to the greater energy developed inside the casing.

According to the invention, this problem is solved by providing in either or both opposite walls of the tubular body of the slider 7 one or more openings 10 (visible in FIG. 2), which put in communication the environment outside the slider with the environment within its longitudinal cavity housing the spring 8.

In this way, when the plasma diffuses into the arc chamber, the pressure front reaches and enters, through the openings 10, the longitudinal cavity of the slider 7 so as to automatically and immediately determine a re-balancing of the pressures acting on the front end surface of the slider 7: this is an advantageous condition in order that the potential elastic energy of the slider spring is no longer hampered by the plasma pressure and can perform in short time its effectiveness in moving the slider in the working direction F.

To avoid adverse effects on the creepage distance be ^¬ tween the slider and guide walls, which has the effect in ^¬ dicated above, preferably said openings 10 are located on the upper and lower sides (i.e. those parallel to the lying plane of the annexed drawings) of the slider having a quad ^¬ rilateral section. In other terms, the openings 10 are placed on the sides parallel to the extending path of the plate, which is the path on which the electric arc is propagated naturally.

Still more preferably, the openings 10 are in the form of narrow slits located within the opposed grooves 7a, as clearly shown in Fig. 2.

In this way, the plasma pressure front is directly channelled by the grooves 7a, enters within the cavity of the slider 7 through the openings 10 and, on one hand, it balances the pressure on the front surface of the slider 7 (allowing an effective action of the spring 8) and, on the other, by raising the pressure inside the slider 7, it cre ^¬ ates an reaction effect with a direction according to the arrow F, being able only to escape towards the rear side, which further assists the desired propulsion of the slider 7.

In fact, the hot gases generated by the electric arc conveyed through the feed channels into the inner chamber of the slider, tend to expand naturally according to a phe ^¬ nomenon similar to the expansion of the gases inside the cylinder of an internal combustion engine.

The thrust generated by the expansion of the gases is suitably exploited to speed up the slider by means of a construction that suitably guides the escape of the gases. According to a further preferred feature of the invention, it is in fact provided a valve body 11, similar to a check valve, placed on the back of the slider and forming a sup ^¬ port for the rear end of the spring 8. The valve 11 is maintained by the spring 8 in abutment on a nozzle 6a for venting the exhaust gases to the outside, formed on an abutment wall of the housing C, preventing the escape of the gases from the cavity of the slider 7 before they have completed their rebalancing and thrusting function on the slider itself. Preferably, the valve 11 is in the form of a poppet body, whose shank is inserted between the coils of the spring 8.

The system thus conceived is therefore able to ade- quately convey the plasma under pressure and transform part of the problem (i.e. the huge plasma pressure energy) in the solution of the same.

Compared to the spring 8, which is able to exert a force not variable with respect to the pressure, this adju- vant effect of the plasma - as long as it is within the limits of the mechanical strength of the whole system - is advantageously a function of the square of the current: the higher the short-circuit current and the resulting arc pressure, the greater the thrust exerted by hot gases on the slider in the direction that allows the arc extinction.

In conclusion, the internal disconnection system on the arrester allows realizing the extinction of the short- circuit current through the combination of the following three principles:

- plasma thermodynamics in the inner chamber of the slider: the plasma entering the inner cavity or chamber of the slider allows the rebalancing of the pressures by cre ^¬ ating the conditions for a timely intervention of the slider itself; furthermore, the hot gas expansion provides an additional thrust to that exerted by the preload spring;

- dynamics due to the shape of the slider: the slider movement stretches and compresses the electric arc, forcing it into a constrained path;

- electric arc electrodynamics: the elongation and the compression of the electric arc raise its electrical resis ^¬ tance and consequently its voltage up to match the voltage of the driving energy (i.e., mains voltage) resulting in a rapid decrease of the short-circuit current until its ex- tinction .

As is well understood from the above description, the configuration of the invention, despite its simplicity, is extremely effective for the safe shut-down of the arc by the disconnector apparatus, even in the presence of high short-circuit currents, which in turn develop an important amount of conductive plasma resulting from the sublimation of the conductive plate.

It is understood, however, that the invention is not to be considered as limited by the particular arrangement illustrated above, which represents only an exemplary implementation of the same, but different variants are possible either inside or outside of the arrester, all within the reach of a person skilled in the art, without departing from the scope of the invention itself, as defined by the following claims.

For example, the device above described is sized to be coordinated with any overcurrent limiters which should be required in the case the short-circuit current (Isc) of the plant is greater than the self-extinction capacity of the mains current (Ifi) of the disconnection device of the SPD .

In addition, the disconnection device (disconnector) as described above can also be implemented in a special enclosure (housing) and used as an independent short- circuit switching device, regardless of the presence of a surge arrester.