Field of invention
The present invention relates to an electric protection
device for a switchgear, the device including an arc eliminator.
The invention also relates to a switchgear or a distribution
system provided with such a device and to a use of the device.
Background of invention
In arcs, caused by large fault currents, very large amounts
of energy are released in the form of heat and radiation. In, for example, enclosed
switchgear, with its limited space, these amounts of energy give rise to increases
in pressure which may blast the enclosure. Such switchgear must therefore be equipped
with space-demanding relief openings through which the heated gases are given a
possibility to flow out. Further, the high arc temperatures cause the material in
conductors and in switching equipment to melt and even evaporate. Burnable organic
material may also be ignited when subjected to the high temperature and intense
radiation of the arc. By decomposition of air (NOx) and evaporation of metals, the
arc gives rise to poisonous gases. It is, therefore, common for such switchgear
to be provided with devices for pressure relief in the form of evacuating channels,
automatically openable hatches, etc. This means that such switchgear will be bulky
In industry, there has long been a great need to primarily
prevent arcs from arising. In the second place, there has been a need to minimize
the arc duration. In this way, material damage as a result of the heat and the pressure
increase, built up during the duration of the arc, can be reduced. Also, the risk
of personal injury and poisoning is reduced.
In case of a duration of the arc of about 30 ms, a switch-gear
unit may be completely blown out. The pressure wave caused by the arc usually reaches
its maximum even after 10-25 ms. To reduce the material damage, there is thus a
need to limit the arc duration to about 10 ms. To reduce the risk of personal injury,
still shorter arc durations should be aimed at. One common way of limiting the duration
of the arc is, therefore to rapidly switch the fault current to ground before the
actual breaking operation. The protecting device also re-distributes the fault currents
in any network with several fault current sources. By returning the fault currents
to each and respective source, the maximum fault current in each point in the network
An arc eliminator thus can be a fast closing contact, one
for each phase connecting to ground or an arrangement connecting all phases to ground.
Alternatively the arc eliminator can include a switch or semiconductor device of
any kind of rapid change of impedance between phase conductors and the ground. The
actuating energy for the contacts can be stored under continuous supervision. Tripping
signal can be provided by means of the light sensors.
An arc eliminator thus performs a certain kind of protection
in relation to a switchgear. An example of an arc eliminator is disclosed in
US 6518 865
Another kind of protection device is a surge arrester.
A surge arrester is normally capable of passing surge currents
without any arcing within the housing of the arrester. Under normal voltage conditions,
the metal-oxide varistor elements have a high resistance that essentially blocks
current flow therethrough; but should a voltage surge appear across the arrester,
the varistor elements will respond to the rising voltage of elements, thereby limiting
the voltage across the arrester and across any protected equipment that is connected
in parallel with the arrester. Normally, this current through the arrester will
be confined to the solid material of the varistor elements, and no arcing will occur
within the arrester.
A surge arrester can be installed between phases or phase
An example of a surge arrester is disclosed in
Each of the two kinds of protective device described above
performs a specific function. In many cases there is a need to provide an arc eliminator
as well as surge arrester in a switchgear.
Each of the two kinds of protective device also is exposed
to risks that can cause damage to the respective device.
Summary of invention
The object of the present invention is to achieve an electric
protective device including an arc eliminator, which device has a low risk for failure.
This object is met in that an electric protection device
for a switchgear and including an arc eliminator is provided with a surge arrester
connected in parallel with the arc eliminator.
By combining these two devices it is attained a mutual
protection between them. Arranged in this way the arc eliminator protects the surge
arrester from excess energy and explosion. Simultaneously the surge arrester will
protect the arc eliminator from excess voltages and flash-over risks.
According to a preferred embodiment the arc eliminator
includes a closing contact arranged to connect the switchgear to ground in case
of occurrence of an arc.
According to a further preferred embodiment the surge arrester
includes a metal oxide varistor, preferably a ZnO varistor.
In these types of arc eliminator and surge arrester, respectively,
the effect of the mutual protection between the two devices is optimized. The invention
is particularly suitable for these types, wherethrough the advantages of the invention
are made benefit of to full extent.
According to a further preferred embodiment the device
includes a signal connection between the arc eliminator and the surge arrester and
a measuring device arranged to measure an electric parameter related to the surge
arrester, the signal connection being arranged to transmit data from the measuring
device to the arc eliminator.
By this arrangement the arc eliminator automatically can
be trigged to operate in response to conditions in the surge arrester line that
require the arc eliminator to be activated. This does not exclude that the arc eliminator
can be activated also in response to other conditions such as light and pressure.
According to a further preferred embodiment said electrical
parameter is the current and/or the current time integral of the current flowing
through the surge arrester.
Although other electric parameters than these in some circumstances
might be useful for providing a trigging signal, the current or the current time
integral or the combination of these have been found to be the most relevant in
According to a further preferred embodiment, the device
includes a processing circuit arranged to control the interaction between the arc
eliminator and the surge arrester.
Providing surveillance by a processing circuit, e.g. a
microprocessor assures an adequate function of the device, such that the respective
component will automatically be activated to protect the other one should it be
required by the circumstances. By means of the processing circuit the device also
can be individually tailord for different applications and the device can be adapted
to changes in the operating conditions. The processing circuit may be active or
According to a further preferred embodiment the processing
circuit includes signal receiving means for receiving information about electric
and/or other parameters relating to the arc eliminator and/or to the surge arrester
and signal transmitting means for providing signals to the surge arrester and/or
the arc eliminator.
By including these functions in the processing circuit,
the processing circuit will be able to assure automatic operation of the ac eliminator
in response to need arising from the surge arrester and/or vice versa. The extent
to which all these functions are present in the processing circuit depends on the
level of sophistication that is required for a certain application. The defined
functions of course does not exclude other functions, to be built into the processing
circuit, such as inputs relating to other conditions in the switchgear or the surrounding
and programs for processing the input signals according to certain algorithms which
can be predetermined and/or be dependent on the combination of input signals.
According to a further preferred embodiment, the surge
arrester includes a plurality of varistor blocks connected in series and is provided
with a first terminal for connection to ground, a plurality of second terminals
for connection to a voltage and switching means arranged to connect one of said
second terminals to the voltage.
Thereby the characterics of the surge arrester can be varied
and thereby adapted to various operation conditions for the arc eliminator when
cooperating with the surge arrester.
According to a further preferred embodiment the arc eliminator
and the surge arrester are arranged in a common housing.
When the arc eliminator and the surge arrester are in the
same housing both functions are in the same switchgear compartment.
A switchgear or a distribution system provided with the
invented electric protective device and the preferred embodiment thereof have the
advantages corresponding to those described above.
Brief description of the drawings
- Fig 1
- is a schematic graph illustrating a first example of the invented device.
- Fig 2
- illustrates an arc eliminator suitable for use in the invented device.
- Fig 3
- illustrates a surge arrester suitable for use in the invented device
- Fig 4
- is a perspective view of a detail of fig 3
- Fig 5
- is a schematic graph illustrating an alternative example of the invented device
- Fig 6
- illustrates an alternative example of the surge arrester
Fig 1 is a graph illustrating a first example of the invention,
where 1 represents an arc eliminator, 2 a surge arrester and 3 a bustar, system,
capacitor bank or other or equipment to be protected.
The arc eliminator 1 and the surge arrester 2 are in this
example located in a common housing 4.
If persisting currents create excess energy or explosion,
the contacts of the arc eliminator will close such that negliable current will flow
through the surge arrester, whereby damage to the latter can be avoided.
If there is excess voltage or a flash-over risk, the surge
arrester 2 is trigged to a low-resistance stage such that excess current flows therethrough,
and the maximum voltage across the arc the arc eliminator 1 will be reduced.
In the example illustrated in fig 1 a measuring device
5 measures the current through the surge arrester 2. A signal device 6 transmits
data from the measuring device 5 to the arc eliminator, for activating the same.
Fig 2 is an example of an arc eliminator that can be used
in the device illustrated in fig 1.
The arc eliminator includes a switching device with a first
contact part 7, a second contact part 8 and a movable contact part 9. The movable
contact part is formed as a sleeve. The sleeve surrounds the first contact part
and is formed with a flange 10. Between the first and second contact parts, respectively,
and the movable contact part, contact members 11 are arranged. In the embodiment
shown, these contact members are designed in the form helically wound wire. The
wires surround the first and second par, respectively, and are arranged in grooves
in the respective contact part. The helically wound shape permits the wire to make
contact with a resilient force with the respective contact part. The contact members
are adapted to allow the movable contact part a longitudinal movement with a retained
low-ohmic contact with each of the first and the second contact part, respectively.
A first helical coil 12, a so-called Thomson coil, is arranged
adjacent to the flange 10. In the figure, the helical coil is arranged immediately
below the flange, the flange being adapted to make contact with the coil. When a
current pulse flow through the coil from a current source (not shown), a variable
magnetic filed arises, which induces eddy currents in the flange 10. The eddy currents,
in turn, give rise to formation of a magnetic field directed opposite the first
magnetic field. This gives rise to a strong repulsive force which throws away the
flanged sleeve 9 against the second part.
A plurality of arc fingers 13 of spring steel are arranged
around the second contact part. These are fingers are clamped into contact with
the second part, in the upper part of the figure, and have their free ends, the
fingertips, directed obliquely to the first contact part. The arc fingers are adapted
to exhibit a deflecttion with a high resonant frequency. When the fingers are hit
by the forward-moving movable contact part, a vibrating movement thereof arises.
The fingertips then bounce against the movable contact part. Each time the fingertip
leaves the surface of the movable contact part, a small arc arises. However, since
a plurality of fingers are arranged around the second contact part and all lie in
different phases and have different resonant frequencies, at all time always some
fingertip is in contact with the movable contact part. This causes these arcs to
An example of a surge arrester that can be used in the
device of the preferred invention is illustrated in fig 3.
The surge arrester shown in fig 3 includes two electrically
serie-connected surge arrester units 14 and 15. Each surge arrester unit comprises
one or more cylindrical zinc oxide varistor blocks arranged in a stack. The varistor
or the stack of varistors is arranged centrally in an elongated porcelain housing
17 having metallic end flanges 18 and 19. The two surge arrester units are mounted
together coaxially and oriented with the longitudinal axis in the vertical direction.
The surge arrester is provided with a top terminal 20 for connection to a live line
and a bottom terminal 21 for connection to ground. A grading ring 22 is suspended
from the upper end of the surge arrester. The metallic flanges at the joint 23 between
the surge arrester units 14, 15 form a galvanic connection between the varistor
stacks and the outer surfaces of the porcelain housing.
A ZnO block has an equivalent circuit consisting of a capacitance
24 connected in parallel with a greatly voltage-dependent resistance 24. The capacitance
24 is dependent on the composition and dimension of the block and may, for example
be between 300 and 1200 pF for each block At normal operating voltage, the capacitive
part of the leakage current is predominant, and the equivalent capacitances 24,
if they were allowed to act alone, would provide a purely linear voltage distribution
along the surge arrester.
Each varistor block is of the conventional construction
depicted in fig 4, comprising a circular cylinder 16 of sintered metal-oxide material,
a thin glass or ceramic collar 26 bonded to the circular outer periphery of the
cylinder 16, and flat metal electrodes 27 and 28 bonded to the upper and lower faces
of the cylinder. Each cylinder 16 is of conventional metal oxide varistor formulation,
preferably one containing as its principal constituent zinc oxide, and the electrodes
27 and 28 are of good conductive material, preferably arc or flame sprayed aluminium.
The glass or ceramic collar 26 at its axially-opposed ends terminates short of the
electrodes 27 and 28, leaving the electrodes free to make good contact with the
juxtaposed electrodes of adjacent varistors when the varistors are stacked and pressed
axially together in the assembled arrester.
Fig 5 schematically illustrates a further example of the
protection device, which includes a processing circuit 29. A first signal line A
connects the processing circuit to the arc arrester 1 and a second signal line B
connects the processing circuit 29 to the surge arrester 2. The signal lines A,
B are capable of transferring signals representing parameters relating to the arc
eliminator 1 and the surge arrester 2, respectively. Correspondingly the signal
lines A, B can give actuating orders to the two components.
The actuating orders are given in response to the received
signals after being processed in the processing circuit 29. A third signal line
C might be arranged to provide additional information from other sources for the
Fig 6 illustrates an alternative embodiment of the surge
arrester that is suitable to be used in the invented device. The location of the
terminal at the voltage side of the surge arrester 2 can be varied by bypassing
some of the varistor blocks. Thus the connection between the live line and the surge
arrester 2 can be made through either of the switches 30, 31, 32 or 33 thereby including
a varying number of varistor blocks in the surge arrester.
Which one of the switches 30-33 that is connected to the
surge arrester is controlled by the processing circuit 29 (se figure 5) and the
signal line A also provides the processing circuit with information about the status
regarding the location of the terminal.