Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to a device for
protecting an electric equipmen~ against a power arc produced
along a bar system of the equipment, such a device comprising
an arc detecting member for sending, in case of arc fault,
a tripping order to a cut off apparatus located above the
fault.
The protection against the short-circuits of a Low
Voltage switchboard is generally carried out by one or several
circuit breakers placed at the switchboard input. These
apparatuses are equipped with instantaneous or short delay
releases which control the opening of the circuit breaker
in reIation- with two conditiorls:
- That a certain current threshold is exceeded,
~ That a certain time-lag is passed in terms of
15 tripping selectivity imperatives between the circuit breaker
and other circuit breakers electrically connected in series
therewith at lower levels. ~In some cases the time-lag can
be zero).
The protection with threshold and possible time-lag
is satisfactory for the faults said "straightforward", i.e.
for which the short-circuit current flows only in solid
conductors. Then, the bar systems of the switchboard have
only to tolerate precisely definite mechanical and thermal
stresses. The bar systems can therefore be easily designed
to support such stresses.
Unfortunately, this type of fault is rather
theoretical and the real short-circuits inside the switchboards
are almost always accompanied with an electric arc, even when
the fault is initialized by a metal element, the fusion or
repulsion of the latter leading very rapidly to the
production of an arcO These faults with arc have a double
characteristic:
- They are an important source of damages for the
equipment and danyer for the staff~ These risks are directly
~.
~2~a~8~
proportional to the time during which the fault is maintained
and they are already important for tim~ corresponding to
the time lags usually chosen for the shoxt delay releases.
They present a current intensity much lower
than a straightforward fault. As a matter of fact, the arc
voltage present between conductors represents an important
fraction of the net system voltage, thus introducing a limited
state which can reach a ratio of 2 to 1. This current
decrease is rather favorable as long as the stripping remains
assured, but it can have very serious consequences if the so
affected cuxrent reaches a value below the tripping threshold
of the concerned circuit breaker. Then, the elimination of
the Eault will require the tripping of another protection,
g~nqrally much slower.
~s a solution to this risk, the inst~ntaneous releases
can be set to a threshold as low as possible. However
such a solution is limited because transitory peaks of
current may cause untimely tripping. Furthermore, setting
modifications more or less controlled are always possible
under working conditions.
The result of this analysis can be expressed as
follows:
- The faults with an arc on the main conductors of the Lcw
Voltage switchboards constitute a major risk for the equipment
and for the staff.
- This risk is very limited if the fault is cancelled with~ut
time-lag by a circuit breaker.
- In case of time-lag the risk increases with the time of
this one and can become rather important for the values
usually employed (0.2 to 0.5 second).
- Independently of any time-lag notion, the risks become
very serious if the fault current intensity is maintained
below the tripping threshold of the circuit breaker which
ought normally to eliminate it.
~2~
The present invention aims to restore a high level
of security for the faults with arc, by introducing, in
addition to classical protections, an additional protection
through an arc detection relay. The basic principle of this
relay is to detect the presence of an arc on the bar system
of a switchboard and to send to the circuit breaker an
instantaneous tripping order in response to such an arc
detection. The above-mentioned risks are thus cleared:
- Any time-lag needs no longer to be introduced insofar as
the fault taken into account can only be found immediately
below the concerned circuit breakex and as any selectivity
notion disappears.
- The criterion of arc detection being not the current,
there is no risk of no tripping due to shortage of this
current.
It remains to define a measurable criterion
reliably revealing the presence of an arc inside the
switchboard. Some engineers proposed to use the intense
light produced by the arc to excite photosensitive sensors,
the change of state of which being used to generate a
tripping signal. This technology presents some disadvantages:
- On important switchboards,the necessity to provide several
sensors at different positions to eliminate the risk that an
arc is not "seen".
- Presence of cabled connectiorsunfavorably influencing the
reliability,
- Risk to mistake the gleam of a fault arc with the one of
the apparatus cut-off.
The criterion proposed by the present invention
permits to get rid of these disadvantages. It simply consists in a
continuous analysis of the voltage between for example the phases-of a
polyphase power circuit. In case of anarc fault on this
circuit, this voltage takes a very characteristic aspect,
which can be perfectly distinguished from that which is
-- 3 --
.~,.
. ~TI,
' '
~2~
usually met, even on a disturbed net system. The function
of the arc detection relay is tocarry Ollt this discrimination
within a period of time as short as possible and to send
the tripping order to the circuit break~r.
First, it can be noticed that the voltage collection
may be single-phase, except in a very special arrangPment of
the conductors. As a matter of Eact, in most of the cases,
these conductors are open mounted bars succeeding side by
side at distances of few centimeters. Under these conditions,
it is e~ident that an arc arising between two phases will
spread very rapidly and consequently such an arc will
become detect~ble through the voltage present between
any of the three conductors.
More particularly, according to the present
inventiQn, there is provided a device for protecting an
electric equipment against a power arc produced along a bar
system of the equipment, said protecting device comprising
an arc detecting member for sending, in case of arc fault,
a tripping order to a breaking device located above the
ault, wherein the arc detecting member is formed by an
electronic relay comprising:
- means for analysingan alternating voltage U present between
conductors of the bar system;
- analog means for discriminating the fault, said
X5 analOg means being sensitive to change in the waveform of
the voltage U and comprising means for generating a control
signal when a high temporary derivative of the voltage U
exceeds a predetermined value; and
- means for processing the control signal for building up
a tripping process following the appearance of an arc fault
wherein said analog means comprise a filtering circuit
associated with a threshold gate, which is connected to a
transformation circuit cooperating with a logic processing
circuit playing a role in the elaboration of the tripping
.
process by assuring a distinction between a temporary
incident and a real arc fault, said processing circuit
comprising:
- time-lag members constituted by a first and a second mono-
stable trigger circuit both connected to an output of thetransformation circuit, said first and sPcond monostable
trigger circuits remaining in the logic state 1 during a
first predetermined time-lag and a second predetermined time-
lag, respectively, when a first pulse is generated on the
output of the transformation circuit, the second time-lag of
the second monostable trigger circuit being above the first
time-lag of the first monostable trigger circuit;
- a first bistable trigger circuit controlled by the
trans~ormation circuit and by the second monostable
trigger circuit so that passage of an output of the
first bistable trigger circuit to the logic state l
happens when the tran~formation circuit emits on its output
a second pulse and so that the output of said first bistable
txigger circuit is automatically reset to the logic state O
at the end of the second time-lag; and
- a second bistable trigger circuit controlled by the
transformation circuit and by an output of a logic AND gate
receiving an output of the first monostable trigger circuit
and the output of the first bistable trigger circuit, the
processing circuit comprising means for delivering the
tripping order after the emission of three pulses on the
output of the transformation circuit including said first and
second pulses, the third pulse from the transformation circuit
being obligatorily emitted during the second time-lag of
the second monostable trigger circuit and after the end of
the first time-lag of the first monostable trigger circuit.
~ The experience shows that the so tapped voltage
between conductors of the bar system is very clearly
distinguished from the voltage under regular service:
- By its form which is very far from a pure sinusoid. In
particular wi~h regard to each passage by zero, a very rapid
increase is noticed up toa value of about 200 V, giving rise
to a derrivative dV/dt much above that which exists under
normal operation.
- By its value which is slightly below th~e voltage of the
net system.
A5 defined hereinabove~the first criterion used
in the arc detection relay is the voltage waveform
criterion.
A second criterion of operation used in the arc
detection relay may be the value of the voltage U~ For this
purpose, the relay may comprise a ~evice detecting the
value o~ this alternating voltage U, so as to deliver a
lS tripping order when the v~lue of the voltage U is comprised
between first and second limits of predetermined values.
The value of the first limit may be chos~n rather
low (a few tens of volts for example). Its role is essentially
to avoid working ofthe relay when the system is not energized.
The value of the second limit should be above the
highest arc voltage which can arise and below the minimum
possible voltage of the net system, what lets an adequate
margin in the usual equipments.
This detection mode should be associated with a
short time-lag during which the voltage value should be held
in the above defined zone. This time-lag should be adequate:
- To eliminate voltage fluctuations of very short dura-
tion (cut-off by a downstream apparatus, for example).
- To allow the filtering circuit to cancel the harmonics
insofar as the basic component is only kept for "U".
An additional criterion of the arc detection relay
may be the intensity of the current flowing along the bar
system. This criterion may be based on a current threshold
low enough so that any arc fault produces a current having
an intensity above this threshold, for example in the region
of the rated current. Preferably, the relay does not work
as long as the current is below the thres]hold.
The three working criterions of the relay using
the form and value of the voltage and the current intensity
may be associated through a logic AND circuit to avoid the
risks of untimely tripping.
Other advanta~es and technical data will more
clearly appear from the following non restrictive description
of a preferred embodiment of the invention, made for the
purpose of exemplification only with reference to the
accompanying drawings, in which:
. Figure 1 shows a Low Voltage electrical switch-
board equipped with a electronic arc detection relay according
to th~ invention;
. Figure 2 represents the synoptic scheme of the
electronic arc detection relay;
. Figure 3 shows the scheme of the logic proces-
sing circuit of the relay;
. Figure 4 illustrates the diagram of the circuit
logic states according to Figure 3;
. Figure 5, which is disposed on the same sheet of
formal drawings as Figure 3, represents a development of
the invention taking into account the waveform of the voltage,
the value of this voltage, as well as the intensity of the
current.
Figure 1 schematically shows a cabinet 10 of a Low
Voltage distribution switchboard including a bar system 12
provided with conductors Ll, L2 and L3 supplied through an
input circuit breaker 14. This circuitbreaker 14, located
at the input of the switchboard, is electrically connected
to the supply net system or to a transformer Medium Voltage/
Low Voltage. The cabinet 10 can receive any fixed or draw-out
electrical apparatus. As an example, a three-phase
branch 16 connected to the bar system 12 ,and protected by an
output circuit breaker 18 is represented in Figure 1.
The input circuit breaker 14 comprises a magneto-
thermal or electronic classical release for carrying out an
automatic opening of the contacts of this circuit breaker 14
when the current intensity exceeds a pred~etermined tripping
threshold, for example in the case of an Dverload or short-
circuit. Auxiliary tripping means can also be incorpoxated
inside the circuit breaker 14, in particular for the
dettection of a differential fault or of an under-voltage on
the net system.
According to the invention, an additionaJ protection
is assigned to the input circuit breaker 14, which additional
protection comprises an arc detection relay designated by the
ge~eral re~erence 20. This relay 20 is used to detect,
inside the cabinet ~0, the presence of an arc on the bar
system 12 and to send, after processing, an instantaneous
tripping order to a coil 22 which controls the opening of
the input circuit breaker 14.
A supply circuit AL is associated with the arc
detection relay 20 and with the circuit of the tripping coil
22. A first input of the relay 20 is connected through
conductors 24 to different phases of the net system above
the input circuit hreaker 14. ~ second input of the relay
20 is electriGally connected to current sensors 26 provided
to deliver an image of the current flowing along the bar
system 12 when the contacts of the circuit breaker 14 are
closed. The arc detection is carried out by the relay 20
through continuous analysis of the voltage U, for example,
between the conductors 24 connected to two phases of the net
syst~m above the circuit breaker 14. This voltage U is
identical to the one present between the conductors L1, L2,
and L3 of the bar system 12. The occurence of an arc on the
bar system 12 is revealed by a change in the waveform of the
-- 8 --
L7
voltage U, compared with the aspect of this voltage U under
normal conditions of operation. The function of the r~lay
20 consists in carrying out this distinction within a period
of time as short as possible and in sending a tripping order
to the input eircuit breaker 14.
Fig. 2 represents a synoptic scheme of the arc
detection relay 20. The main functions of this relay 20 are
the following:
- Detection of electric disturbances in the alternating
voltage U present between the conductors Ll, L2, and L3 of
the bar system 12;
- Distinction of a transitory incident and of an arc fault;
- Storage of the arc fault and tripping of the input circuit
breaker 14 above the fault.
The first function consisting in detecting electric
disturbances is carried out through an analog part of the
relay 20 which comprises a filtering circuit 28 provided
with a band-pass active filter associated with a threshold
gate 30, sensitive to the amplitude of variation of the
~o voltage U. As a matter of fact, the arc voltage wave is
characterized by a rise front having a very high dUt which
is detected through derivation of the voltage signal U. The
iltering circuit 28 allows only passage of frequencies
located within a range in which such sharp rise fronts are
located:
- The low cut-off frequency of the band-pass filter is about
300 Hz, so as to reject the fundamental frequency (50 Hz) and
its harmonics up to the order of 5.
- The high cut-off frequency is in the neighbourhood of
3~ 8 KHZ so as to suppress the high frequencies having no
correlation with an arc fault.
The threshold gate 30 allows to eliminate some
interfering phenomena existing under normal operation and
to keep exclusively the sharp fronts generating a sudden
g
~z~
variation of voltage of a predetermined amplitudeL The
threshold gate 30 utilizes a positive threshold and a negative
threshold of identical values, so as to take into account the
positive and negative variations of the arc voltage. The
threshold gate 30 generates an output signal which can
present two logic states 0 or 1 indicating, either the
presenc~ or the absence of an arc on the bar ~ystem.
Several pulses close in time ~less than 1 ms between two
successive pulses) are considered as a single pu~se owing to
a diode circuit and a capacitor circuit associated with the
output of the threshold gate 30.
The second function which consists in the distinc-
tion of a transitory incident and of a real arc fault is
carried out by the digital part of the relay 20 following
the threshold gate 30. ~s a matter of fact, it should be
ensured that the pulse delivered by the threshold gate 30
really corresponds to an arc fault on the bar system 12 and
not to transitory incidents originating, for example, from
the opening of a circuit breaker located downstream, from a
2~ transitory absence of voltage, from the phenomena of recovery
transitory voltage giving rise to voltage oscillations or to
the supply of the Low Voltage net system by switching-on a
circuit breaker, the contacts of which bounce. The output
of the threshold gate 30 is connected to a transformation
circuit 32 connected to a logic processing circuit 34 which
will be described in detail hereinafter with refererlce to
Fig. 3 of the drawings.
The logic processing circuit 34 (Fig. 3) provided
with sequential binary operators comprises, as an example,
two monostable trigger circuits 36, 38 and two bistable RS
trigger circuits 40, 42 energized by the supply circuit AL.
The inputs of the trigger circuits 36, 38, 40 are connected
to the output o the transformation circuit 32, and the out-
put of the second bistable trigger circuit 42 delivers a logic
-- 10 --
.
~ ~,
~2~
level 1 or 0 indicating the presence or the absence of an
arc fault. One of the inputs of the triqger circuit 42 is
connected to the transformation circuit 12 through a logic
gate 44. The other input of the trigger circuit 42 is
connected to the output of a logic AND gate 46, the inputs
of which are respectively connected to the outputs of the
trigger circuits 36 and 40. A reset circuit RAZ provided
with a manually operated switch 48 is associated with the
bistable trigger circuit 42.
The operation of the logic processing circuit 34,
according to Fig. 3, is illustrated on the diagram of Fig.4:
- The first pulse generated by the circuit 32 at
time tl produces tripping of the two monostable trigger
circuits 36 and 38. The outputs of these two trigger circuits
36 and 38 remain in logic state 1 during predetermined time-
lags T1 and T2, the time-lag T2 of the second monostable
triggex circuit 38 being above the time lag Tl of the first
monostable trigger circuit 36.
- The first bistable trigger circuit 40 generates
a second pulse at time t2 during the time-lag T1 by producing
on its output a transition logic state O - logic state 1.
This bistable trigger circuit 40 is automatically reset at
the end of the time-lag T2 of the second monostable trigyer
circuit 38O The logic state 1 of the first bistable trigger
circuit 40 indicates the emission of two pulses by the trans-
formation circuit 32.
A hole or transitory absence of voltage can also
generate two pulses with sharp fronts, one when the voltage
disappears and another when it comes back. To avoid an untimely
working of the arc detection relay 20, the logic processing
circuit 34 i8 designed to generate a tripping order only after
a counting of three pulses.
The third pulse from the circuit 32 to be taken
nto consideration arld which produces tripping of the relay
~;
20, should arise for example, at time t3 after the end of
the time-lag Tl gener2ted by the first monostable trigger
circuit 36 and obligatorily during the time-lag T2 generated
by the second monostable trigger circuit 38. The first bist-
able trigger circuit 40 is then in state 1 and the logic ANDgate 46 is released. The fault is thus stored by the
second bistable trigger circuit 42 which produces on its out-
put a logic state 0 logic state 1 transition.
The third function consisting in StOring the arc
fault and tripping the input circuit breaker 14 is actuated
by the output trigger circuit 42 of the logic processing
circuit 34. The change from the logic state 0 to the logic
state 1 of the second bistable trigger circuit 42 generates
a tripping pulse applied to a pulse generator 50 connected
lS to the release trigger electrode of a static switch 52, in
particular a triac, disposed in series with the tripping coil
22. Conduction of the triac 52 produce activatiOn of the
coil 22 followed by the opening of the contacts of the input
circuit breaker 14. The presence of the arc fault is
simultaneously indicated at a remote location by means of a
visualization member 54 connected tO the trigger circuit
42 of the circuit 34 through a control device 56.
Th~ detection of the arc fault by the circuit of
Fig. 2 is based on the chage in the form of the wave of
the arc voltage, owing to the continuous analysis of the
voltage U between the power conductors, thus constituting a
first working criterion of the relay 20. The logic circuit
34 should count three pulses before activating the tripping.
It was noticed that the presence of an arc inside
the switchboard gives rise to a voltage having a fun~ntal com~onent
which is slightly below the normal voltage of the net systemO
It is then easy to use it as another wor3cing criterion of
the relay 20, by designing this relay so that a tripping
order is sent if the tapped effective voltage "U" is comprised
- 12 -
between two limits "U1" and "U2".
The value "Ul t- should be chosen low enough (a
few tens of volts, for example). Its function is essentially
to prevent working of the relay 20 when the system is not
energi~ed.
The value "U2" should be above the highest arc
voltage which can be produced and below the minimum possible
voltage sf the net system. This leaves a sufficient margin
in the usual equipments.
This detection mode can be associated with a time-
lag during which the voltage can be maintained in the
required ~ead zone. This time-lag should be adeguate to
eliminate fluctuations of very short duration voltage (cut-
o~ by a downstream apparatus for example). This second
lS criterion is built up in a detecting device 60 (Fig. 5~.
To improve the reliability of the system, the two
criterions of ~he relay 20 based on the waveform and the effective
value of the voltage "U" are associated by means of a
logic AND gate 62 (Fig. 5). The two criterions should be
simultaneously met so as to permit delivering of the tripping
order to the tripping coil 22 of the relay 20.
The risks of untimely tripping are thus greatly
reduced. Indeed, if a probability P1 is assumed for ab-
normal appearance of the waveform cxiterion and P2 for abnormal
appearance of the voltage value criterion, the probability for an
untimely tripping of the relay is reduced to Pl x P2.
This reliability can still be improved by adding
a third criterion, in the circumstances the current. This
criterion is based on a threshold Is low enough 50 that the
arc current is obligatorily above this threshold. The current
criterion is built up in a comparing element 64 (Fig. 5)
having an input connected to the current sensors 26 and an
output connected to the third input of the logic AND gate 62.
The action of the comparing element 64 occurs only when the
~Z~8fl~7
current I flowing through the bar system 12 is above the
threshold Is.
According to Fig. 5, three conditions must be met
to obtain delivering by the logic gate 62 of a -tripping order
to the triac 52:
- Counting of three pulses by the logic processing curcuit
34, the third pulse occurring obligatorily during the time-
lag T2;
- Release of the circuit 60 when the effective value of
the voltage "U" is comprised between the lirnits "Ul" and "U2~;
- Release of the compaxing element 64 when the current I is
above the threshold Is.
In a first embodiment, the arc de~ection relay 20
according to Figs. 1 to 5 can be presented as an independant
lS component which can be set up inside the switchboard to be
protected and connected in a classical manner, on the one
hand to the bar system 12 and, on the other hand, to the
circuit breaker 14 to be actuated(and moreover if necessary,
to the auxiliary source if such a source is provided, and to
a current sensor 26, if the current criterion is used).
Consequently, this embodiment assumes wired con-
nections which should be minimized by positioning the relay
20 as close as possible to the circuit breaker 14, while
taking care that the same is not e~posed to a possible arc.
Such a type of relay is necessary to equip an
already existing equipment.
In a second embodiment, the relay 20 is incorporated
inside the circuit breaker 14 itself. This presents many
advantages : first, the wired connections are suppressed.
Then, if the circuit breaker is already equipped with a static
release, a certain number of functions are alxeady present
therein and the arc detection relay 20 is not presented anymore
as additional components in limited amount.
This last type of embodiment permits, in particular,
- 14 -
.
~Z~ 7
to take into consideration the current criterion very easily,
this information being always present on a level with the
release of the circuit breaker 14.
It can be found parts of equipments which are not
S protected by a circuit breaker. It is the case, for example,
of bars which are immediately below an alt:ernator. Then an
arc detection relay can still be used, but: the order given
by the latter will be used to de-activate the alternator.
The supply circuit AL is combined with the relay
20 and comprises a component capable of storing electric
energy, in particular a capacitor or an accumulator. The
tripping coil 22 should be supplied under an appropriate
voltage and with an adequate energy. Two cases should be
distinguished:
lS - An independant auxiliary source is available. The
function of the relay is then confined to connect this source
with the coil circuit.
- The coil 22 can be supplied only by the net system voltage.
At the time of the fault, this one is turned into an arc
voltage, the form and value of which can largely fluctuate.
Then it should be ensured that this voltage will be always
adequae to actuate the coil, even in minimal value, which
means that it will be very superabundant in maximum value.
Consequently, the coil should be designed to tolerate this
maximum voltage value only during a very short period of
time, suf~icient enough to ensure the tripping.
The present invention is not at all limited to the
embodiments more fully described hereinabove and/or shown in
the accompanying drawings, but on the contrary it extends to
any variant remaining in the limit of the electrical engineer-
ing and electronic equivalences, especially those in which
the arc detection relay would be used inside metal-clad
substations provided with metal housings or high or medium
voltage distribution panels.
- 15 -
