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The switching of the configuration of an electrical system operating acircuit breaker is not generally a simple process.
The operation of a breaker in a circuit with a current flow, implies thenecessity of extinguishing the electric arc which appears betweenthe contacts.
The arc extinction, which must be done under very severe physicaland time conditions, may itself provoke transient phenomenaaccompanied with overvoltages.
Introduction
According to the standard IEC 265-1:1983, a circuit breaker is a
mechanical device of connection, capable of establish, withstand
and break the current under the normal operation of the circuit and
occasionally, under specific overload in-service conditions, as wellas endure specific abnormal circuit currents (e. g. short circuit
currents) during a given time (generally fractions of second).
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In order to turn gases into electric conductors their temperature shouldreach a certain limit.
Thus, their molecules and atoms begin to lose electrons and the gasesbecome conductors.
Metals have their own conductive properties due to the existence of free electrons in their inside.
In their surface there is a potential barrier produced by a layer of positiveions in the metal’s inside, which prevents electrons from escaping thesurface, unless their kinetic energy is greater than its charge multiplied bythe potential barrier.
When the temperature in the metal is raised, energy is transmitted to
electrons that may lead them to overcome the potential barrier, thuscausing the thermionic emission.
Other forms to extract electrons from a metal is to expose it to a strongelectric field or to a luminous radiation (photoionic emission)
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The curve in Fig. shows the temperatureof the medium surrounding the arc as afunction of the distance to its axis. Thetemperatures depend on the contactsmaterial.
The arcs shows up as an incandescentgas column with an almost straight-linetrajectory between the electrodes, whichcore reaches temperatures between6,000 and 10,0000 ºC. The surface of contact of the arc with the electrodesappears incandescent.
The collision of the molecules with theelectrons emitted by the cathodegenerates the ions of the arc column.
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curve shows a very considerabledecline at the beginning and then itdecreases more slowly.
This is because the arc subsists at aconstant current density, which
implies a section growth when thecurrent rises and an increase of theair conductivity.
If the current of the arc decreases under a given value, the points of thecurve do not match, but they are below the curve.
This phenomenon is due to the calorific inertia of the arc.
The surface of the cathodic stain, the arc diameter, the ionisation currentand the tª do not adapt instantly to the new values of current and theygive place to a lesser voltage of the arc.
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Characteristics of the arc The extinction voltage is smaller than the
restriking voltage, because when the
extinction takes place (following aninstant of strong dissipation of heat dueto thermal inertia), the column of the archas thermodynamic and conductiveconditions superior to those preceding
the restriking. The restriking occurs after a very short
time without arc, of about millionths of second, in which a cooling and an intensedeionisation of the arc take place.
The arc restriking, with the current in the opposite direction, is producedwhen the inverse voltage of recovery applied between the electrodes ishigher than the restriking voltage.
The value of the restriking voltage depends on the separation betweenelectrodes, the pressure of the medium and the concentration of chargecarriers, influenced by the refrigeration and thermal conductivity of the
medium and the electrodes.
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The arc in alternate current In an AC circuit the current passes through zero twice in a cycle. If the
voltage and current of the arc are registered by an oscillograph, the
obtained “voltage-current” curves present forms that depend on the kindof gas, the material of the electrodes, the arc length and the frequency of the current.
The difference of ordinates between the curve of rising current and thecurve of decreasing current is due to the thermal capacity of theelectrodes and of the gas of the arc and, in particular, of the calorificinertia of the arc (arc hysteresis).
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The arc in alternate current RESTRIKING VOLTAGE (Ur )
It is the voltage between electrodes needed to restrike the arc after it
extinguishes when the current naturally crosses zero. If the voltage betweenelectrodes is lower than the restriking voltage of the arc, the circuit staysdefinitely open.
EXTINCTION VOLTAGE (Ue) It is the peak voltage of the arc when the current reaches zero value.
The decreasing shape of the characteristic of the arc and the smaller concentration of charge carriers (resultant from the current decrease) justifythe rise of the arc voltage Ua, which peak is the extinction voltage Ue.
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Breaking an alternate current If the circuit breaker is capable to open its contacts at the instant when
the current crosses zero fast enough so that the voltage between
contacts does not reach the restrike voltage, the circuit remains openand, since the electromagnetic energy is null in that instant, noovervoltages are present between contacts.
For this to happen in 50 Hz networks, the circuit breaker should be able toopen in less than ten thousandth of second.
u
t
i
u a
u rd
i
u
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Breaking an alternate current The voltage drop in the arc brings the power required to keep the high
temperatures (due to the Joule effect), balancing the heat losses of the
arc due to conduction, convection and radiation. As the current decreases when it approaches to zero, the thermal power
of the Joule effect is lower than the thermal power given to theenvironment. This leads to the cooling of the arc and produces arecombination of ions and electrons, which diminish the conductance of
the path of the arc. If the curve Up is constantly above the curve Utr , the arc will not restrike
and the circuit opening will be definitive. Nevertheless, if the voltagecurve Utr crosses the curve Up, at that instant the dielectric puncture of the medium will occur and a new arc suddenly restrikes.
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Breaking an alternate current The final extinction of the arc will be possible in one of the instants when
the current crosses zero, providing that the voltage between contacts in
those instants is unable to restrike a new arc toward the remainder plasma (between contacts), more or less deionised.
The possibility of an arc restrike or its definitive extinction depends onthe rate of rise of the TRANSIENT RECOVERY VOLTAGE (TRV), andof the dielectric strength of the zone surrounding the arc at such time.
The dielectric strength is a function of tª and the fractional ionisation of the plasma in the instant of the zero crossing of the current.
The arc trajectory should acquire briefly a dielectric strength enough toresist the recovery voltage between electrodes.
The rate of rise of the transient recovery voltage (TRV) is very importantfor the value of the breaking capacity of a circuit breaker. In the highvoltage circuits, the TRV may reach initial values of around kV/Ts.
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the moment of contactparting, the thin layer of fluid(air, oil, SF6, etc) betweenthem is crossed by thecurrent, which implies a veryfast rising of the temperaturein the contacts originatingmetallic vapours.
The isolating mediumsurrounding the arc suffers a
violent heating whichoriginates its transformationinto conductor.
Arc extinction process
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The case of close-in faults is characterized by a high current, with ahigh phase angle close to 90º (load strongly inductive).
It can lead to several arc restrikings and the arc extinction does nottake place before the 2º or 3º zero cross of the current.
Em
u
t
i
u a
u rd
u rd
i
u
The arc is established by contact parting.The voltage drop is Ua.
At the first zero cross the TRV rises veryquickly trying to reach the grid voltage buta restrike occurs because the dielectricstrength is not high enough.
At this zero cross, the dielectric strength ishigher than the TRV and the arc is finallyextinguished.
Arc extinction process. Fault current interruption
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When the short circuit is at the far end of the line, a transientovervoltage may add to the TRV, which can cause restrikings eventhough the current is not very high.
The case of interrupting small inductive currents may also cause sometroubles since the arc may extinguish even before the zero cross of thecurrent, hence generating voltage peaks by induction effect, andconsequent restrikings.
Arc extinction process. Other cases
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Rated voltage The rated or nominal voltage of a network (Un) is the standard value of
voltage for which the network’s operation and insulation have been
designed. The limit values of a network’s voltage (excluding all transitory or
abnormal conditions) are the highest and lowest value of voltage thatmay be present in the network at a given instant or place under normaloperation conditions.
Generally, those limit values are around ± 10% from the nominal voltage of the network.
The highest voltage for a circuit breaker is the maximum specified for itrelated to:
Its insulation
Other attributes associated to this voltage
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The insulation level of a circuitbreaker is given by:
Nominal power-frequencywithstand voltage
Nominal lightning withstandvoltage
And eventually by: Nominal switching withstand
voltage
These values characterize the
device’s insulation regarding itsaptitude to withstandovervoltages at power frequency,lightning overvoltages andswitching overvoltages of steepwavefront.
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Lightning overvoltage waves in overhead lines may have severalforms, but those registered by a cathode-ray oscillograph during
storms had shown that they might be represented by a non-periodicunidirectional wave of steep front, attenuated afterwards.
In order to typify the insulation of a given device, this wave can bestandardized as a 1.2/50 waveform;
this is, a waveform which front has a conventional duration T1 = 1.2 s,
and the conventional duration of the waveform afterwards until itreaches half its amplitude in the tail is of 50 s, according to thestandard DIN VDE 0432.
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Insulation level. Switching wave In high and medium voltage lines, the breaking of the current in a
circuit provokes overvoltages, with an unidirectional wave of steep
front, attenuated afterwards, that be standardized as a 250/2500 shockwave, this is, a waveform which front has a conventional duration T1 =250 s and T2 = 2500 s.
These shock voltages are generally triggered by an arrangement inwhich a given number of capacitors are charged in parallel by a high-
voltage direct current source and then discharged in series over acircuit composed by the tested device in parallel with a pure resistanceR and a linear inductance L.
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Nominal or rated current It is the current assigned by the manufacturer that the device can
endure indefinitely (or for a given time) under normal operation
conditions, without suffering any heating higher than that fixed by thestandards, and without undergo any modification in its functionalfeatures.
The values in the former table are for circuit breakers operatingoutdoors. For circuit breakers operating indoors, these limits of temperature rise are related to the temperature indoors and should notexceed 40 ºC if the circuit breaker contacts are made of silver or silvery
copper.
PART LIMITS OF RISE OF TEMPERATURE IN ºC
Oil circuit breakers Other circuit breakers
Contactors in air 30 35
Contactors in oil 30 --
Oil 30 --
Voltage coils with insulation type 0* 35 35
Series coils with insulation type 0* 50 50
Series and voltage coils with insulation type A 50 50
Series and voltage coils with insulation type B 70 70
All the other parts of the circuit breaker 70 70
S t i l d t i l b ki t
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The asymmetrical current is the RMS value of the total currentcomposed by the AC and DC components in a pole in the instant of contact separation and its value is given by :
The symmetrical current is the effective value of the AC component inthe pole at the instant of contact separation and its value is given by:
2
x = I sim
)(Y +2
x = I
2
2
asim
S t i l d t i l b ki t
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The extent of the asymmetrical period and its importance of theasymmetry depend, for each phase, on the instantaneous value of the
electromotive force (e.m.f.) in the initial moment of the short-circuit andits maximum value when the initial instant corresponds with a zero of the e.m.f.
Usually the relationship between the symmetrical and asymmetricalshort-circuit currents is expressed by a factor of asymmetry K:
K depends on the relationship between the inductive reactance and theresistance of the circuit where the circuit breaker will be mounted. It isgenerally tabulated in tables.
The breaking capacity of a circuit breaker is calculated as:
I K = I simasim
R
x f = K
I U 3= ASIM P
I U 3= SIM P
ASIM ncc
SIM nccSIM P K = ASIM P cccc
Sh t i it ki t
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This value distinguishes the capacity of a circuit breaker to close itscontacts under short-circuit conditions in the system.
The making current of a breaker when its contacts close under short-circuit conditions is the value of the total current (including alternateand direct components) and which are measured from the envelope of the current waveform in its first peak value.
The making current of a breaker is that associated to its closing at
service voltage. If this value is not present in the nameplate, should becalculated as follows:
Making current = 1,8 Isim = 2,55 Isim
P i ibl t d h t ti t
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The rated sequence of operation of a circuit breaker consists in anumber of operations established in a certain succession and in given
ranges of time. According to the IEC standards, the sequence of operation of a circuit breaker not specified as a recloser can beexpressed as follows:
o - t - co - t' - co
o - t² - co
Where:
o = opening operation, c = closing operation
co = closing operation followed by an opening operation
t, t', t² time ranges, t y t' expressed in minutes, t² expressed in seconds
For example, a circuit breaker with a double operation sequence o -0,15seg – co, means than when the fault takes place, the circuitbreaker opens, waits 0,15seg, closes and, if the fault continues, itopens again.
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In order to select the appropriate breaking technology, the followingaspects should be taken into account:
The highest security for personnel and material The fewer requirements of maintenance
The best treatment of switching overvoltages in order to keep theminto secure levels (less risk for the material)
The best economical conditions, considering cost of acquisition andassembling, as well as maintenance yearly expenses, cost of renewal of damaged material (due to repeated arcs) and cost of indispensable auxiliary systems (like air compression systems inairblast circuit breakers)
P di t bl f t f h t h l
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El principio del soplado magnético consisteen producir, por la acción de un campo
magnético excitado por la propia corrientea cortar, un más rápido alargamiento delarco, el cual es canalizado hacia el interior de una cámara de extinción de materialaislante, refractario, de gran capacidadcalorífica.
En base a este principio, es posible lograr la ruptura de muy elevadas corrientes enbaja tensión y aún en media tensión,siempre y cuando se cuente con una
potencia de refrigeración suficiente en lazona del arco como para impedir elembalamiento térmico post-arco.
Interruptores de A..T. Interruptores para el Interior.
Magnetic blast circuit breakers
Magnetic blast circ it breakers
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Es condición fundamental, en uninterruptor de soplado magnético, que el
arco se extinga dentro de la cámara deextinción, sin salirse de ella. La misión deesta cámara es laminar el arco y enfriar enérgicamente el plasma de gasesionizados, al paso por cero de la corriente.
Conviene destacar que el sopladomagnético en los interruptores de corrientealterna, es nulo en el momento deextinguirse el arco (paso por cero de lacorriente), no ejerciéndose en estosinstantes acción electromagnética algunasobre los iones y electrones presentes enla columna del mismo, lo cual limita lautilización de este tipo de aparatos paratensiones muy altas.
Interruptores de A..T. Interruptores para el Interior.
Magnetic blast circuit breakers
Magnetic blast circuit breakers
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Ua =Tensión de arcoe = F.E.M. del circuitoi = Corriente
r a = Resistencia del aire
La figura ilustra este proceso, en el caso de un circuito de corriente alterna.La técnica utilizada en estos interruptores no pretende cortar bruscamenteel arco al paso por cero de la corriente, sino que aprovecha los instantesque preceden y suceden a ese instante para cambiar el régimen defuncionamiento del interruptor, pasando de un arco de pequeña resistencia
a un arco de elevada resistencia. La ruptura sobreviene a continuación, alincrementarse esta resistencia hasta el infinito, tal como se tiene en losinterruptores de corriente continua.
El éxito de esta técnica, inicialmente aplicada a los interruptores de baja tensión y muy especialmente en los interruptoresde corriente continua ultrarrápidos hasta 3 kV, llevó a los constructores a extrapolar su utilización a los aparatos de alternade media tensión, hasta tensiones de 24 kV.
Como sea que, para alcanzar una tensión de arco del orden de la tensión de la red, la longitud de aquel debe ser muy
importante; y una elevada tensión de arco con corrientes fuertes sería causa de un considerable desarrollo de energía (por defecto Joule), que además de inútil sería perjudicial. Es necesario que en tanto la corriente sea fuerte el arco sea corto,forzando su alargamiento únicamente al ir aproximándose la corriente a cero. Esto se ha conseguido jugando con lassecciones de paso ofrecidas al arco, por ejemplo, disponiendo en las pantallas de las cámaras de ruptura rendijas deanchura variable.
Interruptores de A..T. Interruptores para el Interior. Ruptura con solapado Magnético
Magnetic blast circuit breakers
Air blast circuit breakers
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Breaking an alternate current in oil In the instant of electrodes separation, a considerable resistance named
appears between them. Since the current cannot changeinstantaneously, a voltage takes place between electrodes, forming anarc.
This arc is constituted by a mixture of metallic particles and volatilised oil,hence forming a blend of gases partially dissociated and ionised andbecoming a conductor path of weak resistance when the arc is stable.
This resistance gets weaker when the current gets higher, and increaseswhen the arc is enlarged.
Breaking an alternate current in oil
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Breaking an alternate current in oil When the electrodes start to separate, as soon as the arc is established,
the resistance is very low and does not sensibly modify the condition of current I, which follows its normal variation and until it reaches zero.
The arc extinguishes at this instant, but the gaseous path does notdisappear and the arc restrikes when the voltage between electrodesreaches the appropriate value.
The phenomenon repeats with every change of sign of the current;
however, as the arc enlarges, the resistance of the arc grows, thecurrent amplitude diminishes slightly, and the restriking voltageincreases noticeably.
Finally, the restriking voltage gets to be higher than the voltage betweenelectrodes, hence the arc does not restrike anymore and the circuit is
opened.
Arc extinction using oil blast
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Arc extinction using oil blast To extinguish an arc it is required to deionise its
path in a very short time (µs). Then, the blast of turbulent gases should be thrown to the ionisedfiles that constitute the arc.
Consider two electrodes A and B inside aninsulating enclosure cross by a transversalchannel. If a given amount of oil is thrown
through the channel in the direction of the arrow,the oil will penetrate the arc. At the instant whenthe current passes through zero, the voltage willstay at its normal value, since an insulating layer had been introduced between electrodes, andthis layer would be able to stand that voltage.
For the extinction to be ultimate, it is alsorequired that the insulating layer would becapable to endure the recovery voltage Ur for aslong as it is present.
This is, the fluid speed must be proportional to theradient of the recover volta e.
Arc extinction using oil blast
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The dielectric strength Ur to beintroduced is that of the fluid relative tothe shock voltages. For oil, even if it ishighly contaminated, it is around 220kV/cm.
In the circuit breakers with transversalblast, the extinction of the arc is aided
by the fact that the dielectric strength of oil, under shock voltage, is greater thanthe dielectric strength of the arc columnat the instant of extinction, which isaround 7 kV/cm.
The speed of the oil is inevitably limited(20 to 40 m/s), however, when thegradient g gets too large, after thevoltage rise in a circuit of a givenfrequency f o, the artifice of multiplechannels in parallel is used.
Arc extinction using oil blast
Operation of low oil content CB
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For high breaking capacities, theblowout of gases towards the arc is
perpendicular to the contacts axis;meanwhile for low capacities, theblowout is axial.
The contacts of these circuit breakerscan stand, according to the statisticsprovided by the manufacturers, thefollowing number of operations withoutneed of replacement.
At rated current 4000 operations.
At half of the maximum short-circuitpower 8 operations.
At full short-circuit power 3 operations.
Gas
evacuation
Oil circuit breakers
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Under a few bars of pressure its dielectric strength reaches 5
times that of air . This is due to two reasons:
First, the dimension of its molecule, which effective section of collision withan accelerated electron inside an electric field is higher to that of thenitrogen or oxygen, for example. This means that the electron will endurestatistically a greater number of collisions in SF6 than in air.
But mainly, the second, which results from the property of the SF6 moleculeto capture an electron in an electron-molecule collision, thus forming a
negative ion.
This property of capturing electrons comes from the extraordinarilyelectronegative nature of the fluorine atom. When this atom lacks anelectron to complete its external layer, it generates an elevated level of attraction towards any electron inside its influence field. This provides this
element its well-known chemical reactivity. This point will be further developed when analysing the deionisation
phenomena..
Hence, in the field of current breaking, SF6 is the ideal gas, as it willbe analysed below.
The SF6 as the breaking gas. Thermal features
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When the current increases, it can beobserved in most gases the appearance of a thermal threshold and the developmentin the centre of the tube of a cylindricalzone in which the temperature rapidlyrises, called central core.
The thermal conductivity of SF6 presents apeak near the thermal threshold thattranslates in an important heat release.
In SF6 the temperature of the threshold(2.200ºK) is close to the temperature of recombination of the SF6 atoms inmolecules.
Therefore, there is an important absorptionof energy that causes a new descend intemperature, which goes below 2100ºK.
At this temperature, the SF6 is basically aninsulator and impedes the restrike after the
current crosses zero.
The SF6 as the breaking gas. Deionisation
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SF6 has another notable feature, related to the strongly
electronegative nature of fluorine.
In fact, the atom of fluorine lacks an electron to fulfil its external layer, whichcreates a high level of attraction over any electron inside its influence field.
There is a noticeable decrease in the number of free electrons (responsibleof the arc conductivity) below 6,000ºK.
Such electrons are captured by the fluorine atoms to form negative ions F-,
185 times slower. Hence, for every captured electron, the current isautomatically divided by 185.
Therefore, in the temperature range of 6,000 and 3,000ºK, in which
almost all free electrons had been captured, the conductance
decreases very faster than in gas without the electronegative
properties of fluorine. Summarizing, in SF6, even before the central core has completely
disappear while the cooling of the arc, its conductance is almost
null, due to the capture of the free electrons by the fluorine atoms,which become electrons traps below 6,000ºK.
Breaking technologies in sf6 . Self-compression
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The breaking technology of self-compression inSF6 was first used in high voltage circuitbreakers, and then it moved on medium voltage,following its own evolution.
The active elements are mounted inside thesealed terminal boxes that constitute the poles.
Breaking technologies in sf6 . Self-compression
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The mobile main contact has an almosttruncated-cone shape, is silvered and firmly
linked to the piston.
This contact technology has the doublepurpose to avoid corrosion in the maincontacts due to the arc and to keep the devicefeatures after a high number of breaks.
Breaking technologies in Sf6. Self-compression
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Once the arc contacts are separated, an electrical arc is establishedamong them with a temperature higher than 10,000ºK, which keeps thecurrent flow.
During this period, it is essential to evacuate the thermal energy of the arc, given by the network. The evacuated energy will be higher as the gas density and its specific heat increase.
The heat evacuation during the arc duration is obtained mainly by
convection, due to the replacement of a given quantity of hot gas bycold gas.
Due to the high temperatures, an important heat exchange byradiation could be expected.
Breaking technologies in Sf6. Self-compression
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The empty tubular contacts ease the fast circulation of hot gases andlead to instability in the arc extremes, which avoid the wearing out of arc contacts.
The breaking process by self-compression is especially effective, sinceit works with the injection of a small quantity of gas between contacts.
In the case of a break of 25 kA at 20 kV, the energy to be evacuated is of around 30,000 Jules, which is the energy provided by the arc to keep it at atemperature from 10,000 to 15,000ºK.
1 g of SF6 will be enough to achieve this.
The break by self-compression in SF6 is used in high voltage in outdoor circuit breakers up to 800 kV and in gas-insulated sealed substations(GIS), where the SF6 is used not only as the break medium but also as
the insulator in buses and switches. In medium voltage, the self-compression in SF6 is used in circuit
breakers up to 36 kV.
Extinction chambers with a low consumption of driving energy
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g gy Recent technological advances allow to obtain extinction chambers of
automatic circuit breakers in SF6 which require 40% less mechanicalenergy to disconnect than the former self-compression chambersbased in the pressure generated between a mobile cylinder and a fixedpiston.
The arc extinction is currently achieved by the following effects:
Self-compression.
Arc thermal effect.
Effect of assistance to disconnection by expansion gases, patentedby GEC ALSTHOM.
Extinction chambers with a low consumption of driving energy
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When the contact bar 8 exits the throat of nozzle 9, the thermaloverpressure present in volume V
tis released, which creates a blowout
just before the zero crossing of the current, ensuring the arc extinction.
At the same time, the rise of pressure originated close to the arcspreads towards piston 10, exerting a driving force over the mobilesystem, providing the required energy for the manoeuvre in the
disconnection springs.
Extinction chambers with a low consumption of driving energy
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g gy The arc extinguishes and the molecules of SF6 dissociated by the arc
are instantly recombined.
The secondary products of the breaking end up deposited in themolecular sieve 11 without affecting negatively the circuit breaker.
In the particular case of breaking weak currents, such as those presentin switching capacitor banks or unloaded lines or transformers, thethermal energy of the arc is too small to generate enough
overpressure. To obtain the adequate blowout of the arc, the classicaleffect of self-compression that takes place in volume Vp is used.
Consequently, these chambers have a blowout that depends on thecurrent to be opened, causing:
Maximum blowout in case of short-circuit currents.
Reduced blowout in case of small currents; hence, the extinction of such currents generates weak overvoltages.
Advantages of the use of Sf6
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g At pressures higher than 3.5 bars an temperatures lower than –40ºC
the gas becomes liquid.
Due to this, in the case of circuit breakers of two pressures, it is required toheat the gas of the extinction chamber to keep the equilibrium at roomtemperatures lower than 15ºC.
The gas is odourless, colourless and tasteless.
In closed places, care should be taken to avoid leaks, since it may provoke
suffocation in personnel by lack of oxygen (due to its higher density, thegas is displaced by air).
In some places it may be convenient to set up extractors that shouldoperate before the personnel entry.
The secondary products of the arc are toxic, and combined with
humidity produce hydrofluoric acid, which attacks porcelain and thecement that seals the nozzles.
Effect of the impurities in SF6
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This impurities come from the manufacturing technique or the depositspollution.
Test most be performed to detect the different impurities specifying thelimits of its content in the gas and the methods to control suchimpurities (see IEC 376).
Nature of impurities
Toxic impurities
Impurities that affect the apparatus security
Impurities that dilute the product
Impurities and odour
Impurities have an unnoticeable effect over the dielectric strength of the sulphur hexafluoride
Breakdown in SF6 circuit breakers
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The main breakdowns in this type of circuit breakers are the gas leaks,which require special devices to be detected.
In a well-installed apparatus, the gas losses should be less than 2% annualof the total volume of the gas inside the apparatus.
In case of total loss of the gas pressure and due to the high dielectricstrength of SF6 the voltage that the contacts can bear when opened isequal to double the phase-to-ground voltage.
Anyways, it is not convenient to operate an SF6 circuit breaker when itspressure has been reduced by a leak and the control circuit should beblocked to avoid an accident.
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The SF6 gas has rated conditions of 6 bars at 20ºC and each SF6chamber has a densimeter associated. The use of SF6 technology insealed chambers causes that this type of circuit breakers requires low
maintenance.
The compact and simple design of the dead-tank circuit breaker considerably reduces the support structure and the space required for itslocation in the installation. Besides, this design allows factoryassembling and testing, noticeably diminishing mounting time andcomplexity.
Another feature to consider is the low level of noise at normal operation,which added to the reduced required space, the low neededmaintenance and the utilisation of non-toxic materials, convert this typeof circuit breakers in an important alternative to diminish visual andenvironmental impact.
The dead tank circuit breakers can be used for rated voltages from 38kV up to 550 kV.
SF6 Dead tank circuit breakers
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The dead-tank circuit breakers utilise, in every breaking chamber, theprinciple of “autopufferTH”, which consists in a combined system of blowout, called “pufferTH”, and of self-blast (arc overpressure). Besides,
they add a zip gearing system of double speed. This design turns thebreaking chamber into a compact and relatively small device, whichpermits the breaking of capacitive currents.
There are main contacts and arc contacts, which execute the currentbreak. The main contacts, separated from the arc contacts, open firstand do not endure the arc erosion. By means of this system the dead-tank circuit breakers present the same maintenance requirements anddurability of the conventional circuit breakers.
SF6 Dead tank circuit breakers
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When breaking low currents, the circuit breaker utilizes the SF6 blowoutsystem; when breaking high currents, the nozzle design leads to a gasoverpressure in the arc zone when the contacts arc begin to separate.
This overpressure, added to the gearing system of double speed, makespossible the breaking of current using a small amount of energy and asimple mechanical system.
The nozzle design and the zip gearing system of double speed causethat the arc contacts move at double speed, using a small amount of energy. This reduces mechanical stresses, since the speed required tomove the connections and the switching mechanism are cut by half, incomparison with a conventional circuit breaker of the same ratedvoltage.
SF6 Dead tank circuit breakers
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The vacuum, meaning the air at a vacuum level of around 10-4 to 10-5 Pa (10-6 to 10-7 mmHg) reaches a dielectric strength superior to 199kV/cm.
Such exceptional dielectric strength, in addition to the fact that the arcat vacuum presents a quite low voltage (since the electrons releasedby the cathode find no obstacles in their path towards the anode) and
that the dielectric regeneration of the medium is almost instantaneous(since there are not ionised gas molecules between electrodes),motivated the research of the application of vacuum to circuit breakers.
Vacuum circuit breakers
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Research was first oriented to obtain isolating breaking chambers ableto permanently maintain the vacuum, in which inside the contactswould be located.
The contacts should be able to cross the chamber keeping an absolutetightness in the gaskets.
Vacuum circuit breakers
O l d th bl th h i t d t th
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The strike of a high-current arc in vacuum entails an unavoidable
vaporisation of the electrodes that rapidly leads to a dynamic pressurebetween contacts that can be equal to the atmospheric pressure.
Initially this arc is alike to that present in other devices, with the particularityof presenting a conductor column strongly concentrated and originating aunique and incandescent cathodic stain, which boiling surface emitsplentiful metallic vapours.
Vacuum circuit breakers
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When the current reaches zero, suchas in the case of the vacuum diode, theelectrons stop travelling through thespace between electrodes; hence, theresistance of this space becomesinfinite, facing a inverse voltage, whilethe anode, now cold, is incapable to
emit electrons when acting as acathode.
Vacuum circuit breakers
The automatic vacuum circuit
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The manufacturer must provide information related to the maintenancemeasures to be observed under normal service conditions.
It is desirable that the manufacturer indicates the number of switchings(or time) following which it is convenient to perform the maintenance of the different parts of the circuit breaker.
Besides, the manufacturer must provide the information related to thecircuit breakers inspection after:
A) Short-circuit operation
B) Normal service operation
This information must include the number of switchings according to A) and B) following which the circuit breaker should be checked.
Maintenance of circuit breakers
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Inspection, adjustment and renovation of contacts.
Instructions to measure the transition resistance of the main circuit.
Prescriptions of the admissible wear in the contacts.
Information about the tolerances of opening and closing times.
Oil (or any other liquid) and gas for insulation or extinction of the arc
Samples, tests, drying, filling and/or substitution of the liquid or gas.
Recommendations related to quality and absence of pollution. Indicationof the required quantity of oil or liquid.
Driving mechanism
Maintenance and adjustment Whenever possible during the inspection the circuit breaker must be
operated a few times with the assistance of the drives, to ensure that thedrive mechanism operates smoothly and that everything works correctlybefore the start off of the circuit breaker .
Maintenance of circuit breakers
Control circuits auxiliary circuits auxiliary equipment
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Control circuits, auxiliary circuits, auxiliary equipment
Verification of coils, relays, interlocking gears, adjustable electrical
devices, heating and drying devices. Bearings and similar pieces
Indication in the instructions of the parts to verify.
Connections
Indication in the instructions of the points to verify Compressed air and hydraulic systems
Verification of the pneumatic and hydraulic valves. Inspection andsubstitution of joints. Instructions to inspect the inner of the pressurizedcontainers regarding pollution, periodical inspection and substitution of the
air-drying devices and humidity absorption. It is advisable to periodically open the purge valve of the air containers to
eliminate condensed water .
Maintenance of circuit breakers
Resistances and capacitors
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Verification of resistances and capacitors. The allowed tolerances must beindicated.
Lubrication and greasing
Specification of the quantity of oil and grease
Cleaning
Instructions regarding the cleaning methods. It is recommended to indicate that the insulating parts should be treated
with special care and in case of abnormal conditions, such as salinedeposits, cement powder or acid vapours, it might be needed to cleanfrequently in order to avoid possible flashovers
Spare parts and materials List of spare parts and materials that should be stored (in warehouse).
Special tools
List of special tools required to assembly or inspection (when not providedwith the circuit breaker).
Maintenance of circuit breakers. Low oil content
Each delivery of circuit breakers includes detailed instructions
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Each delivery of circuit breakers includes detailed instructionsregarding their assembly, start off and maintenance.
Care should be taken that the circuit breaker, the driving mechanismand the steel structures fit perfectly among them, in order to reduce tominimum the on-site labour.
The assembly consist mainly in place in-site the different pieces and fitthem together with bolted joints.
Manufacturers recommend to perform maintenance in the circuitbreaker after 12 to 16 years of service.
Before that it is advisable to check the bolted joints and lubricate themobile parts every 2 to 4 years.
The conditions of the contacts should be checked after switching under loads up to 10 times the short-circuit current.
The maintenance of the circuit breaker must be easily performed on-site.
The instructions should carry complete information about inspection andmaintenance.
Maintenance of circuit breakers. SF6
For the circuit breaker to require slight maintenance it is
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For the circuit breaker to require slight maintenance, it isindispensable that its extinction chamber is very simple and has fewmobile pieces.
The secondary products of decomposition that do not completelyrecombine precipitate as metallic fluorides, or deposit in a static filter thatalso absorbs the residual humidity. This diminish the maintenance costs.
The inspection of circuit breakers must be performed considering the
accumulated value of the interrupted currents, the number of switchings executed and the time in operation.
The following criteria are valid as guiding values:
An inspection should be performed, at least:
Every ten years
After 2,000 switching cycles or
After breaking an accumulated short-circuit current of 600 kA.
Maintenance of circuit breakers. SF6
In the inspection a diagnosis measurement is
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In the inspection a diagnosis measurement isperformed with the extinction chamber closed,besides general works such as visual control,check of the high and low voltage joints andverification of all screws in the rack.
Thus, it should be checked:
Operation times.
Transition resistance of the main breakingspace.
Absorbed currents of the driving coils.
Humidity content and acids concentration inSF6.
Gastightness of the SF6 enclosures and thedriving system.
Maintenance of circuit breakers. Extinction chamber Chamber disassembly
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When the resistance between terminals is high it is time to repair thechamber. If there is a spare chamber stored, it can be completely replaced.
To disassembly the chamber the gas must be drained through the fillingvalve or to the atmosphere until the pressure is equal to 1 bar.
If the gas is drained to the atmosphere, it is required to use mask and rubber gloves, because the used gas might content harmful decompositionproducts.
Substitution of the fixed contact
It is performed with the pole in open position. The screws that joint thecontact to the terminal plate must be removed. It can be entirely replaced or,in case of slight deterioration, it can be cleaned with fine sandpaper.
Substitution of the mobile contact It requires the disassembly of nozzles and arc contacts, and then the
dismounting of the crown of the mobile contact.
Driving maintenance
It involves the substitution of the closing/opening valves of compressed air.
AGENDA
GENERAL
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y , g p y gy,by means of a manually tightened spring or an electrical motor.
This device consist in powerful springs that accumulate the energyrequired for the connection. With this purpose they are tightenedmanually with a lever or by an electrical motor.
This drive always operate with a constant closing force, since during theclosing manoeuvre is totally independent from any external source of
energy. Besides, the closing can not start until the springs are totally tightened.
Given that the energy is stored in the springs before the closing manoeuvre,the tightening mechanism requires a moderated power, even when theclosing force has to be high and the closing, fast.
After any closing manoeuvre, the springs are automatically tightened again;hence, the mechanism is always ready to operate immediately, after anopening manoeuvre.
The closing manoeuvre is rapidly started by a
Driving systems of CB. Energy accumulation
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p g gy plosses from the moment of storing it until it has to be released.
Consequently, the energy to connect and disconnect is always ready to be
used according to the demands of the operation or protections of theelectrical system.
It is evident that, if a system of energy storage has no losses, it is notrequired any surveillance system about the storage.
The loading of the energy required for the switchings is obtained byelectrical or mechanical means. In case of emergency, the mechanicaldrives allow to manually storage the springs energy.
The energy is transmitted towards the mobile contacts (between whichthe arc is established and extinguished) by means of secure mechanical
transmissions. During the circuit breaker assembly, there is no need to connect
pressurized fluids tubes, valves or any other element for the driveservice. All that is required are electrical connections.
The driving system can be manually
Driving systems of CB. Compressed air
ecur y anometer
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The disconnection command is electrically transmitted to the coils of thedriving valve "DES" (6). Trough the open valve "DES" the compressed air arrives to the driving of the circuit breaker pole. Simultaneously the outletfor air evacuation (5) is closed in the valve "DES" and the driving valve"CON" (4) is retained pneumatically-mechanically using a blockingsystem. The driving piston (7) moves from position "CON" (C) to position
"DES" (O) and the circuit breaker is disconnected.1. SF6 enclosure
2. Driving bar
3. Pressurized deposit
4. Driving valve "CON"
5. Outlets for air evacuation
6. Driving valve "DES"7. Driving piston
8. Driving cylinder
9. Auxiliary circuit breaker with position indicator
The pressurized liquid most commonly
Driving systems of CB. Pressurized liquid
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When the coil 15 opens the valve 6, the highpressure over piston 10 disappears, the valve
2 is opened and the valve 3 is closed,removing the high pressure in piping 7. Thevalve 5 is opened, emptying piston 20 to thedeposit 22 through the piping 8, by means of the spring M that activates the mobile contact17 to its disconnected position.
A surveillance pressure relay automaticallyconnects and disconnects the pump-and-engine set that keeps the pressure in theaccumulators between normal levels.
Another pressure relay blocks the operationof the circuit breaker when the pressuredescends below the admissible level. Theconnection interlocking works under apressure higher than the disconnectioninterlocking, so any connection manoeuvre
can be followed by a immediate
The following table shows a statistic analysis performed in the Mexicant k
Driving systems of CB. Reliability
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It can be clearly observed that the failure level for the circuit breakerswith spring-based drives is inferior to the failure level for circuit breakerswith other kinds of drives.
Heating of any of the parts does not exceed the specified limits.
Insulation is according to the specified limits.
Capability to establish and break the short-circuit currents.
Capability to endure its permissible rated short-time current.
Capability to break currents on unloaded cables.
Capability to break currents on capacitors banks.
Capability to break small inductive currents.
The results of all type tests are recorded in type tests registries that
contain all required data to demonstrate its compliance to standards.They also include the data needed to identify the essentialcharacteristics of the tested automatic circuit breaker.
Each of the type tests should be performed on a new and cleanautomatic circuit breaker, and the different type tests can be carried out
in diverse times and laces
The mechanical tests exclusively comprise the execution of f i / l i ith t lt t i th i
Mechanical Tests on circuit breakers
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Among all parts of the main circuit of each pole and the frame,successively. All parts of the main circuit of the rest of the poles (if any)have to be connected to the frame.
Open position:
Among all parts of the main circuit of all poles connected among
themselves and the frame. Between the terminals of each pole successively and the frame, being
all parts of the main circuit of the rest of the poles (if any) connected tothe frame.
Between the terminals of a side connected among themselves and the
terminals of the opposite side connected among themselves and theframe. The tests will be repeated inverting the connections that link theterminals with the source and the frame, unless the distribution of theterminals of a pole is symmetrical regarding the frame.
They lie on subjecting the circuit breakers to shock waves of 1.2/50 µs.
Dielectric Tests on circuit breakers. Shock waves
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During each test, five consecutive shock waves are applied.
It is considered that the automatic circuit breaker complies the test if duringwhich neither strikes nor punctures take place.
If some puncture or two or more strikes take place, it is considered that thecircuit breaker does not comply the test.
If only one strike takes place, ten additional shock waves will be applied,and it will be considered that the circuit breaker complies successfully thetest solely if during the additional applications neither strikes nor puncturestake place.
The circuit breaker must be capable to comply the specified tests withvoltages of positive and negative polarity, even when it is enough tocarry out the test with one polarity if it is evident that such polarityresults in a lower strike voltage.
The test voltages are usually obtained by means of a pulse generator,composed by a given number of capacitors all equal that are
Dielectric Tests on circuit breakers. Shock waves
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composed by a given number of capacitors, all equal, that aresimultaneously charged in parallel through some resistances, using a
source of direct voltage (DC), and are later discharged in series througha circuit that includes the tested device (Marx Principle).
The DC voltage is generally obtained from an alternate voltage source,at 50 Hz, by means of metal rectifiers, until a spark is generated in thespark-gaps “e”, all regulated at exactly the same distance, which is
related to the voltage to be applied to the device.
The test voltage will be raised to the given value and will be maintainedby one minute It is considered that the circuit breaker does not comply
Dielectric Tests on circuit breakers. Power freq.
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by one minute. It is considered that the circuit breaker does not complythe test if during which some strike or puncture takes place.
In the test, the voltage reached in the test circuit must be stable enoughso as not to be affected by the leakage current variations or by partialdischarges or pre-discharges.
This condition is complied if the total capacitance of the tested device(including the additional capacitances of the circuit) is not higher than1,000 pF, and the value of the current permanently delivered by thetransformer when the device is short-circuited at test voltage is notlower than 1 A (rms value).
In the resonant circuit, the stability of the resonance conditions and theconstancy of the value of the test voltages depend on the constancy of the circuit impedances and the frequency of the source.
It must be ensured that the device and its main circuits do not becomeexcessively hot when the rated current is circulating through them
Heating Tests on circuit breakers.
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excessively hot when the rated current is circulating through them.
The test must be performed over a new device, with clean contacts.
Before carrying out the test, the ohmic resistance of the main circuitsmust be measured.
The test must be performed causing the circulation through all poles(with the exception of high voltage switchgear higher to 72.5 kV, in
which only one pole is tested) of the rated current at steady state and atpower frequency if it is AC, during a time range enough for the heatingto be constant (when the variation does not exceed 1ºC by hour).
For other conductors than those of coils, the tª of the different parts willbe measured using thermometers or thermocouples located in the
Heating Tests on circuit breakers.
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be measured using thermometers or thermocouples located in theavailable hottest point.
For the opening and closing coils that are excited solely during theopening and closing manoeuvres, the heating test consists in feedingthese coils at their rated voltage ten successive times with a 2 s intervalbetween the excitation instants, supposing the circuit breaker has anautomatic device to open the control circuit at the end of the
manoeuvre, or feeding them ten successive times during 1 s being 2 sthe interval between excitations.
To perform the heating test an adjustable alternate current source isrequired (exceptionally a DC source) with a capacity equal to the ratedcurrent of the tested switchgear. The pertinent measurement devices
are also required: voltmeters, ammeters, millivoltmeters, double bridge(Thomson), thermometers, thermocouples, low voltage or very lowvoltage transformers, etc.
The rated transient recovery voltage (TRV) for terminals failure,associated to the rated short-circuit breaking capacity is the
Short circuit Tests on circuit breakers. TRV
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associated to the rated short-circuit breaking capacity, is thepredictable limit voltage of reference of the circuits that the circuit
breaker must be able to clear in case of a terminals short-circuit. The waveform of the TRV changes accordingly the configuration of the
real circuits.
In networks with rated voltages higher than 100 kV and for importantshort-circuit currents (with respect to the maximum short-circuitcurrent), the TRV presents an initial period during which the risingspeed is high and a subsequent period during which such speed isreduced.
This waveform is well-enough defined by means of an envelopeformed by three straight-line segments determined by four parameters.
Before accomplishing the tests of breaking capacity and makingcapacity it is required to carry out several switchings under no load
Short circuit Tests. Breaking and making cap.
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capacity, it is required to carry out several switchings under no load,during which the accurate operation of the automatic circuit breaker will
be verified and the travel speed, closing time and opening time will beregistered.
The generated overvoltages will not exceed the maximum admissibleand external flashover will not take place.
If the automatic circuit breaker has an electrical drive, these tests mustbe performed feeding the closing device at 105% and 85% of the ratedvoltage of operation of the drive.
In case of air-compressed or pressurized oil drives, the tests must beperformed at minimum pressure with the shunt triggers fed at 85% therated voltage of operation and repeated at nominal pressure at 100%the rated voltage of operation and at maximum specified pressure at110% the rated voltage of operation.
In case of energy accumulation drives (springs), the tests must beperformed with the shunt triggers fed at 110% and 85% the rated
voltage of operation
During the tests of breaking and opening inside the given limits of breaking capacity and making capacity the circuit breaker must not
Short circuit Tests. Breaking and making cap.
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breaking capacity and making capacity, the circuit breaker must notpresent exaggerated signs of wear-out neither risk the operator
integrity. After every sequence of tests, the mechanical parts and their insulators
will be practically in the same conditions that before the tests. After thesequence of the short-circuit test, the automatic circuit breaker will becapable to close and open its steady state rated current at rated
voltage, admitting that its possibilities to open and close the short-circuit current will be considerably reduced after the tests.
It is considered that a circuit breaker does not comply the sequence of short-circuit tests if the damages in the main insulation (that issubjected to electrical stresses under normal operation conditions)
alter its insulating condition at rated voltage. The fundamental short-circuit tests in the high voltage automatic circuit
breakers consist in a series of five basic sequences of short-circuittests.
They have the purpose of evidence the imperfections of the material or the manufacturing that would alter the properties and quality of the
Individual tests in circuit breakers
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