FIELD OF THE INVENTION
[0001] The present invention relates to an improved structure of a surge arrester, also
defined as surge limiter or more synthetically SPD (Surge Protection Device). By these
terms those electric/electronic devices are referred to which, arranged between the
active leads of the electric plant and the ground, provide to discharge to the ground
surge peaks - such as the ones generated by atmospheric lightning and by switching
manoeuvres - which could otherwise cause serious damages to the electric plant and
to the apparatuses thereof.
STATE OF THE PRIOR ART
[0002] Direct lightning phenomena are the main sources of devastating destructive effects
on electric plants; indirect discharges and switching surges are also the sources
of faults, the origin of which is not easy to identify, but the effects of which are
equally terrible for sensitive plants and in which running continuity is paramount.
The duration of these phenomena varies from few microseconds to a few hundreds of
milliseconds, but in this very short time they concentrate an extremely high energy
content. These phenomena must be suitably intercepted in order to protect the plants
connected to the network and thus guarantee the integrity thereof and the functions
thereof. in the modern era, with the growing use of electric and electronic equipment
and with the exponential increase of the integration level of semiconductors, ever
growing attention to this issue has become necessary, compared to the past. The awareness
that transient surges represent a relevant factor in the "mean time between failures"
(MTBF) feature of a plant or apparatus, has caused the need to adopt increasingly
greater and more effective safety measures.
[0003] All that has led to increasing attention in the sector which has caused, on the one
hand, an effort by the manufacturers in the development of ever more performing equipment
and, on the other hand, an effort by the rule-issuing bodies in the definition of
ever more accurate national and international requirements and standards which meet
technological developments.
[0004] The present invention relates to the manufacture of surge protection apparatuses
- referred to in the following as arresters - the application of which is regulated,
for example, by the CEI EN 62305-1/4 Ed. 2 (2011-02), IEC 60364-4-44-443 Ed.2 (2007)
and CLC/TS 50539-12 (2010-03) standards for the protection against lightning and switching
surges. The requirements and the proof methods refer to the IEC 61643-11 Ed. 1 (2011-03)
and to the CEI EN 50539-11 (2013-02) standards.
[0005] In particular, in this context it will be dealt with the arresters of the most recent
prior art, comprising a protection element in the form of a varistor and meant in
particular to LV plants (low voltage, that is, nominal tension up to 1000 V AC and
1500 V DC) and to photovoltaic plants (AC side and DC side).
[0006] The varistor employed in the arresters is an already largely known component; the
behaviour thereof matches that of a variable (non-linear) resistance in the voltage/current
ratio. Once the reference voltage has been exceeded, for example when a short-lived
surge peak occurs, the varistor abruptly lowers the resistance thereof, so that the
current peak can be easily discharged therethrough, to the ground, and does not continue
to other, higher-resistance parts of the plant. A varistor typically consists of a
mass of semiconductor material (for example, ZnO) enclosed sandwich-like between two
opposite metal surfaces, which make up the electrodes to which the contacts of the
terminals for connection to the arrester are electrically joined. Typically, in these
devices, the two terminals are then connected to a phase lead and to the protection
lead and/or to the neutral lead, respectively.
[0007] In the inner circuit of the arrester, in series to the protection element in form
of a varistor, a "disconnector' is typically provided, that is a complex releasing
device, with protective functions in case of failure of the protection element.
[0008] A particularly effective disconnector is described for example in
EP 14160969.3 in the name of the same Applicant, the contents of which is hereby considered recomprised
by reference.
[0009] In standard conditions, that is, in the absence of surges, the protection element
has such a high impedance as to represent a circuit interruption to the ground and
the current circulating within the arrester is not significant. Consequently, no current
circulates to the ground which may generate dangerous conditions for people's safety.
This system, widely known, operates in a highly effective way as long as the protection
element (for example the varistor) is fully operative.
[0010] Following various overloads due to large transient currents discharged to the ground,
to an excessive number of operations or to abnormal conditions of the supply network,
the protection element (typically the varistor) undergoes such an aging and degradation
process following which it gradually begins to reduce the impedance thereof and hence
to discharge to the ground, even in the absence of surges, growing and ever more significant
current values. As long as the impedance reduction is small, as is the value of the
groundbound current, the operation of the arrester is still acceptable, but beyond
certain values the arrester becomes unusable and dangerous for the users. At this
point it must necessarily be switched off (disconnected) from the plant.
[0011] Since the arrester degradation does not have a linear trend, a device must be provided
for quickly disconnecting it from the plant, should the values of current discharged
to the ground suddenly become high. Such a device is indeed the disconnector mentioned
above.
[0012] However, this type of arrangement operates regardless of the ability to identify
beforehand the actual state of degradation of the arrester. It must in any case be
considered that a prior signal of the degradation progress would instead be advantageous
and desired to be able to prepare in a timely manner the replacement of the arrester
still operating, before the functionality thereof is ultimately impaired. This prior
activity, as logical, is by all means desirable in order to be able to always have
the plant in superior operating conditions.
[0013] It must not be forgotten that an arrester operating in degraded conditions cannot
carry out effectively its protection function when it is stressed by the maximum impulsive
currents for which is designed and installed. This is a particularly insidious condition
because current law requirements provide a whole series of rules for safety installation,
but they state nothing on the sensitive matter of the ability to guarantee the surge
protection function in the presence of reduced-efficiency situations. In other words,
if the protection function is not performed effectively, the rules just provide, afterwards,
the obligation to replace the component, but as a precautionary measure nothing is
prescribed.
[0014] On the other hand, there is no doubt that the life of an arrester cannot be determined
beforehand, since it is affected by multiple factors, primarily by the number and
by the intensity of the current discharges which run through it and which progressively
cause the internal impedance to decay: as a matter of fact, it is known that the decay
of an arrester is always associated with a Joule-effect heating, increasing over time,
caused by small leakage currents which run through it (on a new one, they are below
1 mA, in degraded conditions the order of magnitude becomes of a few mA).
[0015] For a punctual verification, in theory such current could hence be measured, but
that is difficult in practice to be accomplished in technical and economic terms in
the allowed application and spaces. Also the temperature detection, as an index of
the degradation state, would be a viable and more direct solution, but it would be
necessary to measure the complex quantity "temperature over time". As a matter of
fact, in the impulsive discharge processes the varistor temperature rises abruptly,
but the subsequent cooling process begins at once: a strong thermal stress is hence
at work for a short period of time; on the contrary, a degradation process derives
from a less intense thermal stress, which, however, extends indefinitely over time.
The monitoring of such a quantity over time is a technically difficult and expensive
task.
[0016] In some installations it has been proposed to count the number of impulsive discharges,
as an approximate index of the potential degradation of the arrester, but the result
is not fully reliable and hence very early replacements must be provided with respect
to the functionality which the arrester could instead guarantee.
[0017] The Applicant has addressed this issue, considering that the correct indicator of
the degradation of an arrester cannot be but the leakage current and the related heating
lasting over time, while poor attention can be given to the number of the repeated
impulsive phenomena which trigger degradation.
SUMMARY OF THE INVENTION
[0018] The problem at the base of the invention is therefore to propose an arrester structure
which overcomes the difficulties mentioned above and which allows to provide a reliable
and realistic indication, in real time, of the actual degradation state of the arrester.
These objects are achieved through the features set forth in essential terms in the
attached claims.
[0019] In particular, according to a first aspect of the invention it is supplied a surge
arrester comprising a first and a second terminal for connection to the active leads
of an electric plant, between which a protection element is inserted, provided with
a pair of electrodes electrically connected to said connection terminals, said protection
element being temperature-degradable depending on time, and between said first terminal
and an electrode of the protection element a disconnector being provided, further
comprising a signalling device of the state of degradation of said protection element,
comprising at least movable means, having a pressure element being biased by elastic
means against a cushion made of temperature-variable viscosity material, signalling
means being connected with said movable means, which signalling means make evident
the level of degradation depending on their displacement caused by the sinking of
said pressure element of said movable means into said cushion. In particular, said
movable means is a moving slider. Preferably, said moving slider is in the shape of
a rigid rod, slidable along the longitudinal axis thereof under the preload of said
elastic means, and said signalling means are in the shape of a signalling strip visible
from outside a casing of the surge arrester through a window.
[0020] According to a preferred aspect of the invention, the arrester furthermore is provided
with a contrast plate hidden from view, through said window, by the end of said signalling
strip and progressively made visible upon the displacement of said signalling strip.
[0021] Preferably, said signalling means comprises a microswitch apt to trigger a warning
signal and which is actuated when said movable means has performed a pre-set travel.
[0022] According to another aspect, said cushion is laid in between said pressure element
of movable means and a fixed abutment integral with a casing of the surge arrester.
[0023] According to a preferred aspect, said cushion is made of a hot-melt resin. In particular,
said hot-melt resin has a softening temperature ranging between 90C° and 140C°.
[0024] According to a an aspect of the invention, said cushion (H) is obtained
in situ through the injection of a hot-melt resin mass in a fluid state and caused to solidify
while keeping said elastic means in a preloaded status.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] Further features and advantages of the invention are in any case more evident from
the following detailed description of a preferred embodiment, given purely as a nonlimiting
example and illustrated in the attached drawings, wherein:
fig. 1 is a schematic side elevation view, with parts removed, of the structure of
an arrester provided with a degradation signalling device according to the invention;
the arrester is shown in a properly operative condition and
with new degradation signalling device;
fig. 2 is a view similar to that of fig. 1, but with the degradation signalling device
in a medium condition; and
fig. 3 is a view similar to that of fig. 1, but with the degradation signalling device
in a final state (device to be replaced).
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0026] The different side views of the drawings herewith attached represent an arrester
which, in its general configuration, corresponds to the one described in the above-cited
application
EP14160969.3. It comprises a box-like body or module C, of such dimensions to be housed in a single
standard module and wired within a switchboard for electric plants in LV.
[0027] In this module C two opposite terminals are housed - a first terminal 1 for the connection
of the phase lead and a second terminal 2 for the connection of the protection or
neutral lead - among which a protection element is arranged (typically a varistor),
schematised by a plate 3, on the opposite surfaces of which respective conducive electrodes
are provided (in the drawings an electrode 4 only is shown, the other one being on
the opposite side is not visible in the drawing).
[0028] Electrode 4 is electrically connected to phase terminal 1, while the opposite electrode
is connected to the ground or neutral terminal 2. The connection between electrode
4 and phase terminal 1 is accomplished through conductor means making up an element
of the disconnector.
[0029] In particular, such conductor means of the disconnector is in the shape of a flexible
lamina, 5, which is elastically preloaded and joined to electrode 4 through a suitable
low-melt welding in the point marked as 5d.
[0030] The material used for carrying out the low-melt welding typically belongs to the
group of alloys with a tin, lead, bismuth, indium base in binary or ternary, euteptic
and non-euteptic formulations, with melting intervals ranging between 120 and 180
C°.
[0031] In the exemplifying configuration illustrated in the drawings, the lamina 5 of the
disconnector is generically folded into an S or a U shape.
[0032] During standard operation (rest condition of the disconnector), this configuration
ensures electric continuity between connection terminal 1 and the electrode 4 of the
varistor, through lamina 5.
[0033] In the module C a guide 6 is furthermore formed along which a slider can move longitudinally,
biased and pushed by preloaded elastic means, for example a precompressed spring,
for the interrupting function of the electric circuit better described in the cited
application
EP14160969.3.
[0034] With this arrangement, in the rest condition of the disconnector, when footpiece
5d is welded to electrode 4, the slider is retained in its home position, against
the precompression bias of the spring. This is the condition of integrity of the arrester
(shown also in fig. 1).
[0035] When, due to the slow degradation of the protection element in the form of varistor,
current begins to flow (even of low intensity, but continuously) through the lamina
5 of the disconnector, said current ends up heating the welding point between electrode
4 and footpiece 5d, until melting the welding material and interrupting hence the
constraint. That triggers the disconnector action, according to the mode widely described
in application
EP14160969.3.
[0036] According to the invention, close to the arrangement of the disconnector, a degradation
indicator is provided, based on materials with temperature-dependent viscosity.
[0037] In particular, the degradation indicator according to the invention consists of a
signalling, movable means, like a slider, which is biased by elastic means against
a cushion made of resin, with temperature-dependent viscosity, in particular a resin
of the family called "hot-melt resins".
[0038] These resins, already known and used (in specific formulations) in hot-gluing processes,
are solid at room temperature, but take up a progressively less viscous behaviour
as the temperature increases, until full fluidification.
[0039] Suitable hot-melt resins are thermoplastic polymers which consist of a polymer base
in addition to a variety of additives (pigments, stabilisers, plasticisers, ..). The
polymer base typically consists of polymers such as copolymers of ethyl vynyl-acetate
(EVA), ethylen-acrylate, polyolefins (PO) and amorphous polyolefins (APO, APAO), polyethylen
(LDPE, HOPE), polypropylen (PP, APP), polybuthene-1 and copolymers thereof, polyamides,
polyesters, polyurethane (PUR), thermoplastic polyurethane (TPU), styrenebased copolymers
(SBC, SBS, SIS, SEBS, SEP), polycarbonates and fluoropolymers.
[0040] Normally the thermoplastic bonds within these resins are reversible, unless they
undergo curing processes: hence, upon undergoing a sufficient thermal load, they tend
to soften and liquefy, losing cohesion and strongly reducing this viscosity, even
after a first solidification.
[0041] The application temperatures of hot-melt resins typically lie in the interval between
160°C and 220°C, in which the corresponding viscosity rapidly decays from 3000 mPa
to less than 2000 mPa. A fundamental feature of these materials, especially with reference
to the context of the present application, is the softening temperature (softening
point), at which the viscosity of the material begins to significantly decay.
[0042] Surge arresters are meant to work at maximum outer temperatures not above 120°C but,
at the same time, they must be able to be maintained at the temperature of about 80°C
without undergoing alterations: therefore, according to the invention, the variable-viscosity
materials (hot-melt resins) are chosen so that the softening temperature (that is,
the temperature above which the viscosity decays below predetermined, conventionally
established values) lies in the interval 90-140°C.
[0043] Returning to the specific configuration of interest for the invention, it must be
noticed that penetration resistance, which a hot-melt resin cushion offers to an elastically
biased movable means, like a slider, is hence inversely proportional to the temperature
which such cushion is exposed to: the slider progressively sinks into the cushion,
the more so the lesser the cushion viscosity and the longer the time elapsed, hence
performing a travel or displacement proportional to the temperature and to the exposal
time to such temperature.
[0044] Exploiting this behaviour, the solution proposed by the present invention is that
of arranging a hot-melt resin cushion between a part of the electric circuit subject
to being heated by the dispersion currents and a movable equipment subject to the
action of bias means which push it against said resin cushion.
[0045] Making now reference to the attached drawings, a preferred embodiment is illustrated,
in which the degradation signalling device consists of a slider 30, sliding in a guiding
ring 31, in turn integral with a conducive terminal 32 of the arrester. For such purpose,
terminal 32 has a hole through which slider 30 passes, the slider being preferably
in the shape of a circular-section rod.
[0046] On slider 30 a ring nut 34 or other stopping element is fastened, against which a
first end of a spiral spring 35 abuts, the other end of which rests against ring 31,
that is the terminal 32 which acts as fixed abutment element. Due to this mounting
of spring 35, slider 30 is elastically biased in the direction of the free end 33
thereof, that is, downwards in the drawing of fig. 1.
[0047] According to the invention, between a pressure element 30a integral with slider 30
and a fixed abutment part, for example the same terminal lead 32, a cushion H of temperature-related
viscosity resin, typically hot-melt resin, is arranged. Cushion H, when it is in its
solid state, maintains the pressure element 33a of slider 30 spaced apart from fixed
abutment 32, against the bias promoted by spring 35. This spacer function, against
the action of the spring 35, is performed as long as the hot-melt resin cushion has
a solidity or viscosity sufficient to prevent pressure element 30a from penetrating
into the resin.
[0048] Pressure element 30a, for example, is in the shape of a short arm which protrudes
transversally from slider 30.
[0049] The mass of cushion H preferably extends on a volume of at least 400 mm
3, so as to determine a slider intervention travel of at least 6 mm.
[0050] Cushion H may be manufactured separately and then installed on-board of body C in
the solid phase, after having brought spring 35 into preload. Alternatively, cushion
H may be manufactured in situ: initially, pressure element 30a of slider 30 is caused
to move backwards, bringing spring 35 into compression and then a molten hot-melt
resin mass is injected directly in its seat within the body C, for example through
a glue gun. Once the mass of cushion H has been injected, it is waited that the resin
cools down and solidifies and then slider 30 can be released, so that spring 35 brings
pressure element 30a into abutment on the solidified cushion.
[0051] The mass of cushion H may also be wound around part of slider 30, which should nevertheless
be free to slide through.
[0052] According to another embodiment, it can be provided that the mass of resin H be enclosed
in a suitable envelope the volume of which is caused to vary by the movement of pressure
element 30a, which hence acts as plunger. In such case, the envelope furthermore has
a calibrated outflow opening, from which the viscous mass of resin H can exit: thereby
the displacement of slider 30 can be furthermore controlled by acting both onto resin
composition, and on the dimension and conformation of the outflow opening.
[0053] Preferably, in a position opposite to the distal end 33 of slider 30 a fixed, travel-end
abutment 33a is provided: advantageously on travel end 33a a microswitch may be provided,
for the function which will be illustrated in detail further on.
[0054] At the proximal end of slider 30, opposite to the distal end, a signalling strip
36 is fastened, consisting for example of a thin flexible lamina, which runs along
a first segment, substantially oriented according to the axis of slider 30, and then
along a second segment, perpendicular to the first one, until in correspondence of
a signalling window 37.
[0055] Signalling strip 36 is preferably in the form of a polycarbonate lamina, for example
coloured in green.
[0056] Signalling window 37 is substantially an opening or a clear area in the casing of
body C, such as to make visible from the outside the underlying portion of the device.
[0057] The terminal portion of signalling strip 36 is located behind window 37 and hides
an underlying contrast plate or tape having a very different appearance from that
of signalling strip 36, for example being orange colour. In standard operation conditions,
through window 37 only signalling strip 36 is visible, for example green colour, which
entirely covers underlying contrast tape 38. When, following the displacement of slider
30, also the end of signalling strip 36 is dragged and displaced with respect to window
37, at least part of contrast tape/plate 38 is cleared by the strip and becomes visible,
in a possible orange colour.
[0058] The extension of contrast tape/plate 38 which is visible outside, through window
37, gives an indication of the extent of the displacement performed by slider 30 and,
in the last analysis, of the degradation which the device may have undergone.
[0059] During operation the signalling device according to the invention has the following
operating mode.
[0060] Whenever the arrester device experiences overheating, especially if of a significant
duration over time, the hot-melt resin mass of cushion H reduces its viscosity and
produces a certain penetration of pressure element 30a of the slider, which results
in a corresponding displacement of slider 30 and hence of travel of signalling strip
36. Repeated heating cycles of this type produce a travel of signalling strip 36 sufficient
to make the phenomenon visible from the outside, through the appearance of contrast
tape/plate 38 through window 37, in a way the more evident the more the heating has
caused cushion H to soften (and hence device degradation).
[0061] In an initial degradation phase, this travelling of strip is moderate and hence the
green part of strip 36 is still, at least partly visible through window 37. However,
the orange colour of plate 38 is - at least in part - already visible. That is clearly
shown in fig. 2 and indicates that degradation has commenced.
[0062] It is evident that the system user is able to intervene in the replacement of the
apparatus as soon as the orange coloured part takes on dominant relevance through
window 37.
[0063] Fig. 3 finally shows the final arrangement of the signalling device, that is, when
signalling strip 36 has been entirely displaced and has made fully visible the sole
orange contrast tape 38. In this condition the replacement of the apparatus is mandatory.
[0064] In this terminal phase of the signalling process, the end 33 of slider 30 ends its
travel against the fixed travel-end abutment 33a. Should abutment 33a be provided
with a microswitch, it is also possible to generate an electric signal which can drive
a full-degradation remote warning (in addition to the evidence locally obtained on
the window 37 of the arrester device).
[0065] As is evident, the advantage of the described arrangement is given by the fact that
the displacement of signalling strip 36 is progressive depending on the amount of
heat developed by the system over time, hence, in other words, not only depending
on the temperature reached or of the heating undergone in absolute value, but also
depending on the time elapsed in the various heating conditions.
[0066] This displacement is furthermore connected to visual signalling means, which indicate,
according to various ways, the degradation reached and hence the need to replace the
arrester before the natural end-of-life thereof. This signalling lies temporally between
the two signals provided by the rules, that is, that of regular operation and that
of failure.
[0067] However, it is understood that the invention must not be considered limited to the
special arrangement illustrated above, which makes up only an exemplifying embodiment
thereof, but that different variants are possible, be they either internal or external
with respect to the SPD, all within the reach of a person skilled in the field, without
departing from the scope of the invention, as defined by the following claims.
[0068] For example, though the movable means are disclosed in the drawings as a slider,
they may take up a different shape, like a pivoting lever or swinging flag, or other.
1. Surge arrester comprising a first and a second terminal (1, 2) for connection to the
active leads of an electric plant, between which a protection element (3) is inserted,
provided with a pair of electrodes (4) electrically connected to said connection terminals,
said protection element (3) being temperature-degradable depending on time, and between
said first terminal (1) and an electrode (4) of the protection element (3) a disconnector
being provided, characterised in that it further comprises a signalling device (30-39) of the state of degradation of said
protection element (3), comprising at least movable means (30, 33), having a pressure
element (30a), being biased by elastic means (35) against a cushion (H) made of temperature-variable
viscosity material, signalling means (36) being connected with said movable means,
which signalling means (36) make evident the level of degradation depending on displacement
thereof caused by the sinking of said pressure element (30a) of said movable means
(30) into said cushion (H).
2. Surge arrester as claimed in claim 1, wherein said movable means is a movable slider.
3. Surge arrester as claimed in claim 2, wherein said movable slider is in the shape
of a rigid rod (30), slidable along the longitudinal axis thereof under the preload
of said elastic means (35), and said signalling means are in the shape of a signalling
strip (36) visible from outside a casing (C) of the surge arrester through a window
(37).
4. Surge arrester as claimed in claim 2 or 3, wherein it is furthermore provided a contrast
plate (38) hidden from view, through said window (37), by the end of said signalling
strip (36) and progressively made visible upon the displacement of said signalling
strip (36).
5. Surge arrester as claimed in any one of the preceding claims, wherein said signalling
means (36) comprises a microswitch apt to trigger a warning signal and which is actuated
when said movable means (30) has performed a pre-set travel.
6. Surge arrester as claimed in any one of the preceding claims, wherein said cushion
(H) is laid in between said pressure element (30a) of movable means (30) and a fixed
abutment (32) integral with a casing (C) of the surge arrester.
7. Surge arrester as claimed in any one of the preceding claims, wherein said cushion
(H) is made of a hot-melt resin.
8. Surge arrester as claimed in claim 7, wherein said hot-melt resin has a softening
temperature ranging between 90C° and 140C°.
9. Surge arrester as claimed in claim 7 or 8, wherein said cushion (H) is obtained in situ through the injection of a hot-melt resin mass in a fluid state and caused to solidify
while keeping said elastic means (35) in a preloaded status.