ES2649749T3 - Surge arrester for the protection of a power plant against transient surges - Google Patents

Abstract

A surge arrester comprising a first and a second electrical terminals (1, 2) to connect to the active wires of a power plant, between which a protective element (3) is inserted, provided with a pair of electrodes (4 ) electrically connected to said electrical terminals, between which said first electrical terminal (1) and an electrode (4) of the protection element (3) a disconnector is provided comprising a flexible, resilient, conductive sheet (5) having an end of the base (5a) electrically connected to said first terminal (1) and a more distant end (5d) kept electrically connected to said electrode (4) in an elastic preload condition, by welding with a low temperature material of fusion, characterized in that between said end of the base (5a) of the sheet (5) and said electrode (4) of the protection element (3) is provided with a sliding guide (6) for a cursor of interception (7) loaded according to a longitudinal direction of said slide guide (6) by preloaded elastic means (8), and by which at least an inclined portion (5c) of said sheet (5) travels through said guide of sliding (6) according to a certain angle with respect to the longitudinal sliding axis thereof, said inclined portion (5c) of the sheet acting as a support element for a head end (7b) of said cursor (7).

Description

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

DESCRIPTION

Surge arrester for the protection of a power plant against transient surges Scope of the invention

The present invention relates to an improved structure of a surge arrester, also defined as a surge arrester or, more concisely, SPD (Surge Protection Device). With these terms, reference is made to those electrical or electronic devices, connected between the active terminals of the power plant and ground, that discharge surge peaks to the ground - such as those generated by atmospheric rays and switching maneuvers - that could otherwise mode, cause serious damage to the power plant and its equipment.

Prior State of the Art

Direct atmospheric phenomena are the main cause of the devastating destructive effects on power plants; Indirect discharges and switching surges are also a source of great damage, whose origin is not easy to identify, but whose effects are equally harmful for sensitive plants and where it is essential that there is continuity in their operation. The duration of these phenomena varies between a few microseconds to a few hundred milliseconds, but in this short period of time they concentrate an extremely high energy content. These phenomena must be properly intercepted to protect the plants connected to the electricity grid and thus guarantee their integrity and operation. At present, with the increasing use of electrical and electronic equipment and with the exponential increase in the level of integration of semiconductors, it is necessary to pay increasing attention to this matter, compared to the past. The awareness that transient overvoltages represent a conditioning factor for the characteristic "mean time between failures" (MTBF) of a plant or equipment forces the need to adopt increasing sophisticated and more effective safety measures.

All this has led to the increasing attention of the sector, which has generated, on the one hand, that the industry participates in the development of increasingly effective devices and, on the other hand, that the standardizing bodies commit themselves to the definition of national requirements and more precise internationals that meet technological advances.

The present invention relates to the realization of devices for surge protection - devices referred to below as arresters - the application of which is regulated, for example, in IEC standards EN 62305-1 / 4, Ed. 2 (2011-02), IEC 60364-4-44-443 Ed. 2 (2007) and CLC / TS 50539-12 (2010-03) for protection against lightning and surges due to switching. The requirements and the methods of verification refer to the norm lEc 61643-11 Ed. 1 (2011-03) and to the norm CEI EN 50539-11 (2013-02).

In particular, the present invention relates to the arresters according to the most recent technique, which comprise a varistor-shaped protection element and specific to BT plants (low voltage, that is, nominal voltage of up to 1000 V AC and 1500 V DC) and photovoltaic plants (AC side and DC side).

The varistor used in the arresters is a well known component; its behavior is the same as that of a variable (non-linear) resistance in the voltage / current ratio. Once the reference voltage has been exceeded, for example when a short-term overvoltage or an extracurrent peak occurs, the varistor abruptly reduces its resistance, so that the peak can be easily discharged to ground and does not continue towards others. equipment or parts of greater resistance of the plant. A varistor usually consists of a mass of semiconductor material (for example, ZnO), arranged as a sandwich between two opposite metal surfaces, which make up the electrodes to which the terminal contacts are electrically connected for connection with the arrester . Normally, in these devices, the two terminals are connected to the phase wire and the protection wire and / or the neutral wire, respectively.

In the internal circuit of the arrester, in series with the varistor-shaped protection element, a "disconnector" is normally supplied, which forms the specific object of the invention and whose function will be described extensively below. The disconnect is a complex disconnection device, with protective functions in case of failure of the protection element.

Under normal conditions, that is, in the absence of surges / extracurrents, the protection element has such a high impedance that it represents an interruption of the ground circuit and the current flowing inside the arrester is negligible. Therefore, no current flows to the ground that can generate dangerous conditions for the safety of people. This system, widely known, works highly effectively as long as the protection element (for example, the varistor) is completely intact and functioning.

After several overloads due, for example, to large transient currents discharged to ground, to an excessive number of operating cycles or to abnormal conditions of the supply network, the protection element (normally, the varistor) undergoes a process of aging and degradation of form that gradually begins to reduce its impedance and, therefore, to discharge to ground, even in the absence of

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

surges / extracurrents, increasing and increasingly significantly the values of the current. If the impedance reduction is slight, and consequently the value of the grounding current, the operation of the arrester will still be acceptable, but above certain values, the arrester becomes unusable and dangerous for users. At this point it must necessarily be disconnected from the plant.

The disconnector is therefore entrusted with the task of interrupting the circuit, both in the case of weak currents (operation normally carried out with inherent functions), and in the case of high short-circuit currents (operation normally carried out with the fuse support). The activation of the disconnection takes place by thermal effect (Joule effect). The disconnector that normally performs this circuit opening function can be internal / external to the arrester.

The thermal disconnect consists substantially of a varied electric wire connected in series with the varistor electrode. It consists of a complex unit, which normally comprises a metal sheet attached to the varistor electrode by welding with a low melting point material, that is, capable of melting at a relatively low temperature (120 ° -180 ° C). The resilient sheet is welded according to an elastically flexible arrangement or loaded by a spring, in any case forced elastically to generate an effort tending to move it away from the varistor electrode. Due to this assembly, when - after degradation - the varistor begins to discharge a significant current to ground, not in the case of transients but continuously, it tends to heat due to the Joule effect. Such temperature is transferred to the weld and when the low melting temperature of the alloy is reached, the welding retention characteristic ceases, releasing the metal sheet from contacting the varistor electrode, thereby opening the electrical circuit and restoring the security conditions.

Some examples of this general scheme are described in US 2012/14028 and EP 2541577.

This system has been proven to be quite effective but not entirely devoid of inconveniences. In practice, after the opening of an electric circuit traveled by a current, an electric arc tends to be established, which tries to maintain the continuity of the circuit. If the arc does not extinguish naturally or if the disconnector fails to try to interrupt it, a dangerous situation arises both in the arrester (overheating with the possibility of fire and / or explosion) and in the affected power plant. In order to interrupt the high short-circuit currents, support fuses are often used outside or inside the arrester itself.

Possible solutions to this problem that involve the introduction into the disconnector of the arrester of the complete arc interruption mechanisms already exist in the market. In particular, the use of rotating discs or separators, arranged in a plane parallel to that of the varistor, or oscillating lever arms, arranged in the arc travel after opening the disconnector has already been described. However, these mechanisms according to the prior art are complicated and not always effective, since the arc tends to circumvent the interception device.

Most of these mechanisms and the support fuses represent a significant obstacle to the application of the arresters, since it is desirable to design them to allow the complete surge protection system to be accommodated in the typical “classic” modular panels of BT devices, with DIN rail mounts, modular configurations more than consolidated for electrical cabinets for civil and industrial uses.

Summary of the Invention

The technical problem to consider is therefore to propose a downloader structure that overcomes the aforementioned drawbacks. In particular, a arrester with a disconnector that allows: to open the circuit safely in the case of slow degradation of the protection element and open the circuit equally safely also in the case of very rapid degradation (short circuit) of the protection element, inherently performing The fuse function. At the same time it is desired, in both situations, that the disconnector be able to effectively extinguish any form of electric arc, also interrupting the high short-circuit currents. The proposed structure of the unloader, in addition to performing all these functions, must be able to have a small volume so that it can be housed in normal frames.

These objectives are achieved by means of the features set forth in the essential terms in the appended claims, that is, the present invention relates to a surge arrester according to claim 1, the embodiments of the invention being detailed in the dependent claims. attached. In particular, according to a first aspect of the invention, a surge arrester is provided consisting of a first and a second electrical terminals for connection to the active wires of a power plant, between which a protective element is inserted , provided with a pair of electrodes, said disconnector being provided between said first electrical terminal and the electrode of the protection element consisting of a conductive, resilient and flexible sheet having an electrically wired end of the base to said first terminal and a further end distantly kept electrically connected to said electrode by welding a material

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

low melting temperature conductor, in which said sheet is mounted elastically loaded in a direction such as to push said more distant end away from said electrode and having a thickness less than 0.5 mm and a conductive material with a conductivity much smaller than that of copper (IACS <60) such as to melt / sublimate by heating by Joule effect after the passage of a short-circuit current.

According to a further aspect, between said end of the base of the sheet and said electrode of the protection element there is provided a sliding guide for an interception cursor loaded according to a longitudinal direction of said sliding guide by elastic preload means, and wherein at least an inclined portion of said sheet travels through said sliding guide at a certain angle with respect to the longitudinal axis of travel thereof, said inclined portion of the sheet acting as a support and fastener for one head end of said cursor.

Preferably, the head end of said interception cursor has a longitudinal washer suitable for firmly engaging with an overhanging tooth of an end-of-travel wall of said slide guide.

More preferably, the head end of said interception cursor has a heel-shaped surface, inclined with respect to a sliding axis of said sliding guide according to the same angle of said inclined portion of said flexible, resilient sheet .

Said sliding guide is limited by a separation wall in which at least a portion of said sheet rests in said preload condition.

According to a different aspect, a surge arrester comprising a first and second electrical terminals is provided to be connected to the active wires of a power plant, between which a protective element, provided with a pair of electrodes, is inserted electrically wired to said electrical terminals, a disconnector being provided between said first electrical terminal and an electrode of the protection element consisting of a resilient, flexible conductive sheet having an end of the base electrically connected to said first electrical terminal and an end more distant electrically connected to said electrode, in an elastic preload condition, by welding with a low melting temperature material, wherein between said end of the base of the sheet and said electrode of the protection element is provided a slide guide for a loaded interception cursor in a longitudinal direction of said slide guide by elastic means of preloading, and in which at least an inclined portion of said slide travels through said slide guide at a certain angle with respect to the longitudinal axis of sliding thereof, said inclined portion of the sheet acting as a support and a fastener for a head end of said cursor.

The head end of said interception cursor may have a heel-shaped surface, inclined with respect to the sliding axis of said sliding guide according to the same angle of said inclined portion of the resilient, flexible sheet.

Preferably said cursor has a longitudinal washer suitable to fit with a corresponding longitudinal rib of the slide guide.

In addition, the sliding guide is limited by a separation wall in which at least a portion of said sheet is in said preload condition.

A further aspect is that said inclined portion of the sheet ends in a bent welded foot, said more distant end forming.

Preferably, said interception cursor is made of BMC material (Bulk Molding Compound) that complies with UL 94 with a class of flammability HB and preferably with degassing properties by means of CO2 release in the presence of an electric arc.

Brief description of the drawings

The additional features and advantages of the invention are in any case made more apparent from the following detailed description of the preferred embodiments, given purely as a non-limiting example and illustrated in the following drawings, in which:

Figure 1 is a schematic side elevation view, with certain parts removed, of the structure of the arrester according to the invention in an intact condition and with the disconnector at rest;

Figure 2 is a view similar to that of Figure 1, but in a condition of degradation and with the disconnect in an initial phase of open circuit; Y

Figure 3 is a view similar to that of Figure 1, in a degradation condition and with the disconnect that reached the final condition and that has completed the opening of the circuit.

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

Detailed description of a preferred embodiment

As the different side views show, a arrester is housed in a cabinet body, module C, of dimensions such that it can be housed in a single standardized module and wired inside an electrical cabinet for BT (low voltage) power plants. In this module C two opposite electrical terminals are housed, in a manner known per se - a first electrical terminal 1 for the connection of the phase wire and a second electrical terminal 2 for the connection of the protective wire or the neutral wire - between the which a protection element (typically a varistor), schematized here by means of a wafer 3, is mounted on the opposite surfaces of the respective conductive electrodes (only one electrode 4 is shown in the drawings, the other located on the opposite side not visible from the drawing).

The electrode 4 is directly connected to the phase 1 electrical terminal, while the opposite electrode is connected to ground or to the neutral electrical terminal 2. The connection between the electrode 4 and the phase 1 electrical terminal is made by means of a wire that It forms an element of the disconnector. In particular, said disconnector wire is in the form of a flexible sheet 5, which is elastically preloaded and connected to the electrode 4 by means of an appropriate welding of low melting temperature in the place marked 5d.

The material used in low melting temperature welding typically belongs to the group of alloys based on tin, lead, bismuth or, Indian in binary or ternary, eutectic and non-eutectic formulations, with melting temperature levels between 120 ° C and 180 ° C

In the exemplary embodiment illustrated in the drawings, the disconnector sheet 5 is generically folded in the form of a U-shaped So (for maximum flexibility), and has:

- a first portion 5a physically and electrically attached to the body of the electrical terminal 1, which continues in

- a second portion 5b that moves adjacent to a rigid wall 11 inside the body of the cabinet C, and in

- a third flexible portion 5c, projecting in a cantilever plan from the rigid support wall 11, forming a resilient flexural joint 5c 'with the second portion 5b, and in

- a portion of the fourth terminal in the form of a foot 5d, additionally bent (according to an angle of approximately 60 °) with respect to the third portion 5c, welded (at the normal operating point) with low melting temperature welding material at electrode 4 of varistor 3.

The disconnector wire, in the form of said metal foil, is resilient and remains offset (therefore elastically preloaded) in the working position illustrated in Figure 1, against the reaction of the fixed rigid wall 11, by welding on electrode 4.

During ordinary operation (sleep condition of the disconnector), this embodiment ensures electrical continuity between the electrical terminal 1 and the electrode 4 of the varistor, by means of the sheet 5.

According to one aspect of the invention, the sheet 5 is preferably of low thickness (of the order of a few tenths of a millimeter, for example 0.2-0.3 mm) and of a material of electrical conductivity lower than that of copper , for the reasons explained below. An example conductivity level may be an IACS (International Annealed Copper Standard) <60. The material preferably consists of a copper alloy of such elements as to change its conductivity (copper IACS <90) and confer resilient properties. to a material that is notoriously ductile and malleable but not elastic.

According to a further aspect of the invention, in addition, between the rigid support wall 11 of the sheet 5 and an internal compartment 12 housing the varistor 3, a cursor 7 is defined to intercept the arc and to compress it, whose cursor is guided along a sliding path by means of guide 6.

In particular - as seen in the drawings - the guide 6 has two parallel walls (the rigid wall 11 and a wall at the opposite perimeter of the compartment 12), which guide the longitudinal sliding movement of the cursor 7, and two end walls of stroke, one of start 13 and one of end 14. The cursor 7 is loaded and bound from the start wall 13 to the end wall 14 by elastic preload means, for example a pre-compressed spring 8 (it is best seen in the drawings 2 and 3). The cursor 7 is preferably provided with a longitudinal washer 7a, arranged to fit with and slide on a longitudinal rib 9 disposed within the guide 6.

Preferably, especially when the device is arranged to operate with alternating currents, the cursor 7 is made of BMC material (Bulk Molding Compound) that complies with UL 94 with a class of flammability HB and preferably with degassing properties by means of the release of CO2 in the presence of an electric arc.

The third inclined portion 5c of the resilient sheet 5 is arranged to intercept the sliding path of the cursor 7: as is visible in the drawings, the inclined portion 5c moves through the guide 6 with a certain angle of inclination with respect to to the longitudinal guide axis, usually an inclination angle of the order of 45 ° - 70 °, preferably around 60 °. Correspondingly, the head end of the cursor 7 has the

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

minus a front surface portion 7b that keeps a certain angle with respect to the longitudinal axis, defining a kind of heel-shaped surface 7b which, when coinciding with the same angle of inclination, rests uniformly on the inclined portion 5c of the resilient sheet 5.

With this construction, in the rest condition of the disconnector (when the foot 5d is welded to the electrode 4), the cursor 7 is limited between the start wall 13 and the inclined portion 5c of the sheet 5, overcoming the precompression effort of the pier 8.

In addition, a longitudinal washer 7c is also provided on the head end of the cursor 7, whose function will be illustrated below; the washer 7c is preferably obtained in correspondence with the longitudinal washer 7a. Correspondingly, on the end wall 14 there is provided a tooth 15, projected longitudinally, preferably in correspondence with the rib 9 and capable of firmly fitting with the washer 7c of the cursor 7.

In a condition of integrity of the arrester and therefore in a resting condition of the disconnector, the portion 5b of the sheet is adjacent to the inner wall 11 that travels longitudinally to the sliding path of the guide 6, while the inclined portion 5c is bent towards the varistor and intercepts the guide 6 at a certain angle and therefore the sliding path of the cursor 7, representing a stop and support element of the end of the cursor head 7 loaded by means of the spring 8. Accordingly , the cursor 7 is held in a significantly loaded position by means of a spring 8 (for example with a force of about 5-10 N) due to the head support 7b on the inclined portion 5c of the sheet 5, which is by its part retained in position by welding in 5d.

When, due to the slow degradation of the varistor-shaped protection element, the current (even of low intensity but continuously) begins to flow through the disconnector sheet 5, ends by heating the welding point located between the electrode 4 and the foot 5d, until it melts the welding material and thereby interrupting the restriction.

When, due to the very rapid degradation (short circuit) of the protection element (usually a varistor), a high current value - such as that of a possible short circuit - flows through the resilient sheet 5, it behaves as if it were a fuse. Certainly, when it is traversed by high current levels, a small portion of the sheet 5 tends to melt / sublimate in the vicinity of the weld point, the restriction being interrupted in this case as well. Therefore, it is not necessary to insert an additional auxiliary fuse in series with the arrester. This advantageously contributes to the reduction of the volume of the system provided with the arrester according to the invention.

In both modes of activation of the disconnector, under its own pre-loaded elastic tension, the sheet 5 is released and placed suddenly in the opening position shown in Figure 2. The separation of the sheet 5 from the electrode 4 is also favored by the thrust of the cursor 7 preloaded by the spring 8 which - due to the heel-shaped end surface thereof - tends to generate a force with a transverse component towards the weld surface of the foot 5d.

After this condition occurs, the cursor 7 is no longer retained and is fired towards the end wall 14 (the only one at the base of the drawing) taking the sheet 5 further away from the main area of the slide guide 6 and consequently from the electrode 4, as shown in figure 3.

With this movement, the slide cursor 7 performs multiple mutually synergistic actions in order to interrupt the electric arc:

a) Accelerate to a greater degree the movement away from the sheet 5 from the welding point on the electrode 4; for that reason the opportunity of formation of an arc is reduced and, it would be necessary to say the form of the arc), it reduces the energy that passes through it, thereby reducing the duration of the conduction phase;

b) Extends significantly, by means of mechanical elongation, any arc that must be formed between electrode 4 and sheet 5; this is a first effective action leading to increase the resistance of the electric arc;

c) The forward movement of the cursor 7 in the guide chamber 6 causes a reduction in the volume of the arc chamber, with the consequent compression of the column of the ionized gas, increasing its density; after the increase in density, the resistance increases: this is a second effective action leading to increase the resistance of the electric arc;

d) In the case (optional) that the cursor is made of BMC natural material with a flammability class HB in accordance with UL 94, due to the electric arc, CO2 is released into the guide chamber 6, in a proportional amount at the temperature of the electric arc that propagates inside. CO2 breaks the arc, determining an increase in the resulting resistance; since the temperature is a function of the energy developed, the breaking action is proportional to the square of the current, times the arc's duration; this intensifies the second effective action mentioned above leading to increase the resistance of the electric arc;

5

10

fifteen

twenty

25

e) The speed fit of the tooth 15 with the washer 7c (position of figure 3) in addition to generating an additional localized micro compression of the plasma, creates a final mechanical cutting effect tending to cut the residue of the ionized gas column : This is a third effective action leading to the interruption of the arc.

This configuration, obtained by means of the synergistic combination of a resilient sheet and an elastically loaded cursor, even in its simplicity, is highly effective for the safe cutting of the arc by means of the disconnecting apparatus. Advantageously, in addition, the compactness of the elements that form the disconnector and the short movements necessary for its operation, allow to offer extremely low volume levels, so that the body of the box containing it can be housed in a single standardized C module of the BT electric cabinets.

Although it is not considered necessary to describe it in detail, it should be considered that it is also advantageous to use an element that is sensitive to the movement of the interception cursor, which makes the condition of the out-of-service arrester visible from outside the device after the disconnection action. For example, in the drawings a secondary cursor 20 is shown which is pushed transversely towards the interception cursor 7, in correspondence with the start wall 13. When the interception cursor is activated and moves from the non-operative position thereof , frees up space for the secondary cursor 20, which can be moved from its resting position (figure 1) and makes a minimum displacement (position of figure 3) that can be detected from the outside by means of an optical signal easily perceived by Operator’s attention. It is clear that such signaling mechanism can also take other forms without significantly affecting the main description provided in this document.

Finally, it should be noted that the preferred embodiment described herein advantageously provides that the longitudinal sliding axis of the interception cursor can be mounted in accordance with a line constituting the T-shaped head of the container body C. It is not ruled out that various orientations can be provided, despite obtaining at least part of the inventive results described in this document.

However, it should be understood that the invention should not be considered limited to the particular arrangement illustrated above, which represents only one exemplary embodiment thereof, but that different variants, external or internal with respect to the STD, are all possible within within the scope of those skilled in the art, without departing from the scope of protection of the invention, as defined in the following claims.

Claims (8)

5 10 fifteen twenty 25 30 35 1. A surge arrester comprising a first and second electrical terminals (1, 2) to connect to the active wires of a power plant, between which a protective element (3) is inserted, provided with a pair of electrodes (4) electrically connected to said electrical terminals, between which said first electrical terminal (1) and an electrode (4) of the protection element (3) a disconnector is provided comprising a flexible, resilient, conductive sheet (5) having one end of the base (5a) electrically connected to said first terminal (1) and a more distant end (5d) kept electrically connected to said electrode (4) in an elastic preload condition, by welding with a material of low melting temperature, characterized in that between said end of the base (5a) of the sheet (5) and said electrode (4) of the protection element (3) is provided with a sliding guide (6) for a course or interception (7) loaded according to a longitudinal direction of said slide guide (6) by preloaded elastic means (8), and by which at least an inclined portion (5c) of said sheet (5) travels through said sliding guide (6) according to a certain angle with respect to the longitudinal sliding axis thereof, said inclined portion (5c) of the sheet acting as a support element for a head end (7b) of said cursor (7). 2. The surge arrester according to claim 1, wherein the head end (7b) of said interception cursor (7) has a longitudinal washer (7c) suitable to fit firmly with a tooth (15) protruding from a limit switch wall (14) of said slide guide (6). 3. The surge arrester according to claims 1 or 2, wherein the head end (7b) of said interception cursor (7) has a heel-shaped surface, inclined with respect to the sliding axis of said guide sliding (6) according to the same angle of said inclined portion (5c) of the flexible, resilient sheet (5). The surge arrester according to claims 1, 2 or 3, wherein said cursor (7) has a longitudinal washer (7a) suitable to fit with a corresponding longitudinal rib (9) of the slide guide (6). The surge arrester according to any one of claims 1 to 4, wherein said sliding guide (6) is limited by a separation wall (11) in which at least a portion (5b) of said sheet ( 5) rest in said preload condition. The surge arrester according to any one of claims 1 to 5, wherein said inclined portion (5c) of the sheet ends in a bent welded foot, said more distant end (5d) forming. 7. The surge arrester according to any one of claims 1 to 6, wherein said interception cursor (7) is made of BMC material (Bulk Molding Compound) that complies with UL 94 with a flammability class HB and preferably with degassing properties by means of the release of CO2 in the presence of an electric arc. The surge arrester according to any one of claims 1 to 7, characterized in that said sheet (5) is assembled elastically loaded in a direction such that it pushes said more distant end (5d) away from said electrode (4 ) and is made with a small thickness and with a material with lower electrical conductivity than that of copper so that it melts / sublimates by heating by Joule effect after the passage of a short-circuit current.