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The present invention refers to a compact apparatus for
picking up current in a medium-voltage line suitable
for being connected to a protection relay, which drives
a medium-voltage automatic switch.
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Medium-voltage switches for indoors are currently known
which use air or gas (usually sulphur hexafluoride, SF6)
to extinguish the electric arc and as insulating means
between fixed and mobile main contacts.
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Medium-voltage automatic tri-polar switches, with a
nominal voltage of up to 24 kV, are used in all
applications of the secondary distribution of medium
voltage and in medium-voltage/low-voltage
transformation substations in buildings, offices in the
industrial sector in general and in tertiary industry.
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In particular, sulphur hexafluoride (SF6) switches are
particularly suitable for service in primary and
secondary distribution stations, where there is the
need to carry out fast shut down cycles in overhead and
cable distribution lines, for manoeuvring industrial
power factor correction units with heavy-duty operation
and limited maintenance, for the command of motors and,
in general, for high reliability and long lifetime
applications.
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The interruption of the arc is normally realised
through self-pneumatic blow out of SF6 gas combined with
the contribution of self-generation and, therefore,
with limited absorption of mechanical energy.
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The system operates normally sealed under pressure and
is suitable for repeated fast shut downs, operating
with limited overvoltages also in the manoeuvre of
small inductive and capacity voltages.
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The poles, placed in cylinders of casting resins with
gas sealing systems, and the support and command
structures of the tri-polar switch can realise fixed or
removable front and/or side operations.
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Moreover, the aforementioned automatic switches are
equipped with current sensors and/or circuit-breakers
or electronic maximum current relays, powered by
themselves, which allow use in unmanned mediumvoltage/low-voltage
transformation substations without
auxiliary power supply.
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Currently, electronic relays for protecting the maximum
current power themselves with current reducers
(transformer, 1, 2, 3, 4), which are applied on board
the switch, so as to guarantee the maximum homopolar
and directional safety protection and protection from
overcurrents in relation to all types of data which can
be detected inherent to the protection problems.
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The two or three transformers applied to the switch
supply the energy necessary for the operation of the
relay and for the switching off of the switch through a
demagnetisation actuator group applied to the command,
which allows overloads (at long reverse time and at
adjustable fixed time) and short-circuits (at
adjustable independent time) to be detected.
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Alternatively, the maximum power circuit-breakers can
be realised with a microprocessor and are suitable for
taking care of the function of protection against
overload and against instantaneous and delayed short-circuit,
as well as the function of protection against
grounding failure.
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In general, however, the different forms of protection
devices for automatic tri-polar switches have some
drawbacks.
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First of all, in the case in which electronic
protection relays are used, it is necessary to wind a
series of toroids on board the switch, with the
inevitable consequences due to problems of
electromagnetic interference, which translate into the
need to foresee a specific insulation between each
toroid and the respective pole, adding to the
encumbrance, connection time, connection procedures and
causing the substantial lack of practicality and
reliability of the entire structure. Then, in the case
of use of microprocessor circuit-breakers, in which the
problem of electromagnetic interference is partially
solved, one of the drawbacks encountered most is that
of the substantial production, installation and
operating costs.
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The purpose of the present invention is therefore that
of avoiding the aforementioned drawbacks and, in
particular, that of realising a compact measurement
apparatus for the protection of medium-voltage lines
with medium-voltage automatic switches, which allows
extreme reliability in general to be obtained, capable
of being used without substantial interventions or
complex treatments on board the switchboard, at the
same time keeping the possibility of effectively
measuring the data relative to the problems of
protection of the electrical lines.
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Another purpose of the present invention is that of
realising a compact measurement apparatus for the
protection of medium-voltage lines, which allows the
carrying out of a measurement of the functions or
malfunctions of the current circulating in the medium-voltage
lines, of a reading and detection of the
grounding currents, of a pinpointing of short-circuits
and of possible unbalances between the phases, without
using particularly expensive components and without
foreseeing particular or extremely specific insulation
devices.
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A further purpose of the present invention is that of
indicating a compact measurement apparatus for the
protection of medium-voltage lines which is highly
precise, stable, effective and safe.
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The last but not least purpose of the invention is that
of realising a compact measurement apparatus for the
protection of medium-voltage lines at a low cost and
without using complex or expensive technologies.
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Such purposes are achieved by a compact measurement
apparatus for the protection of medium-voltage lines,
according to claim 1, to which we refer for the sake of
brevity.
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Advantageously, the protection apparatus according to
the invention includes within a single epoxy resin
block, once enclosed in a container made from
thermoplastic material, a combined series of toroidal
windings, having a sufficiently long core to obtain
good electrical performance, and a further winding,
suitable for collecting the phase current, separated
from the other windings by the resin insulation.
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The apparatus allows a series of samples of electric
current to be taken, in order to obtain both homopolar
and directional protection, thus allowing the function
of protection against overload, against instantaneous
and delayed short-circuit and against grounding failure
to be taken care of.
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The detection of the line and grounding currents, both
in nominal conditions and in the presence of
disturbances, can be carried out by means of a
protection relay, combined with the apparatus object of
the present invention which, thanks to suitable
procedures controlled by an internal microprocessor,
allows adequate active current protection, defined by
the prevailing national and international
electrotechnical regulations, to be realised.
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Further purposes and advantages of the present
invention shall become clearer from the following
description and from the attached drawings, provided as
an example and not for limiting purposes, in which:
- figure 1 is a schematic front view of a measurement
apparatus for the protection of medium-voltage lines,
connected to a medium-voltage automatic tri-polar
switch and realised according to traditional
techniques;
- figure 2 is a first perspective view of a compact
measurement apparatus for the protection of medium-voltage
lines, realised according to the present
invention;
- figure 3 is a second perspective view of a compact
measurement apparatus for the protection of medium-voltage
lines, realised according to the present
invention;
- figure 4 represents a schematic perspective view of
an example of application of the compact measurement
apparatus for the protection of medium-voltage lines,
according to the invention.
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With particular reference to the quoted figure 1, a
conventional embodiment of a medium-voltage automatic
tri-polar switch is generically indicated with 10, in
which each apparatus for the protection of medium-voltage
lines (in particular, a transformer), which is
generically indicated with 15, is applied directly to
each pole 13 of the switch 10.
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The switch 10 comprises a switchboard 11 fixed to a
support base 12 for the poles 13, each of which
essentially consists of an insulating cylinder 17, made
from casting resin with gas sealing systems, whereas
the protection apparatus 15 in general takes care of
the function of protection against current overload,
instantaneous and delayed short-circuit and grounding
failure.
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Figures 2, 3 and 4, respectively, refer to an
embodiment of the compact measurement apparatus for the
protection of medium-voltage lines, according to the
present invention (generically indicated with 16 in the
figures), and to an example of application of such an
apparatus 16 in a compartment with a bar switch and
disconnecting switch.
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In particular, the apparatus 16, object of the
invention, foresees the use of three toroidal
transformers 40, 44, 47, one for each phase, which are
installed above an attachment support 46 and inside an
insulating casing 20, as illustrated in detail in
figures 2 and 3.
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The toroidal transformers 40, 44, 47 are combined
together and have a core with a substantially largediameter.
Moreover, according to the invention, it is
foreseen to couple a homopolar transformer,
schematically indicated with 39, for the collection of
the three phases.
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The homopolar transformer 39 is separated from the
other three toroidal transformers 40, 44, 47 by means
of a resin insulation and allows the grounding failure
current to be detected through the execution of a
vector sum of the phase currents.
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Referring, in particular, to figure 4, the compact
protection apparatus 16 can be applied with undoubted
advantages in a compartment 21 inside of which is
housed a bar switch and disconnecting switch, of the
tripolar type, according to a preferred but not
limiting embodiment of the present invention,
generically indicated with 22 in figure 4.
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In such a case, the cables 23 of the medium-voltage
line, which are arranged inside an underground passage
24, arrive near to the compartment 21 and pass inside
the toroidal transformers 40, 44, 47 of the apparatus
16, which is fixed, inside the compartment 21, to the
front of the switch 22.
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One of the medium-voltage cables, specifically
indicated with 28 in figure 4, is used as a cable
intended for general use and for input for the general
power supply of the switchboard 27 attached to the
compartment 21 of the bar switch and disconnecting
switch 22.
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As can be seen in figure 4, each cable 23, after having
crossed the respective toroid, is connected directly to
the switch 22, through a specific lower connector 25,
whereas a series of signals in output from the toroidal
transformers 40, 44, 47 (proportional to the current
circulating in the phases) are conducted directly to a
protection relay 26 for the switchboard 27. In
particular, the three connectors 36, 37, 38, through
the respective outputs 41, 42, 43 of the apparatus 16,
send the respective phase signals of the toroidal
transformers 40, 44, 47 to the protection relay 26 in
order to detect the current of the separate phases.
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Moreover, the connector 45, by means of the output 44A,
sends a homopolar output signal, taken directly from
the homopolar transformer 39, to the protection relay
26 so as to allow the detection of grounding failures.
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The self-powered relay 26, realised in microprocessor
digital technology, is placed to protect the maximum
current values (biphase or triphase) and operates the
opening or switching off of the switch 22 directly on
the relative command, in the case of intervention of
the protection functions.
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Thanks to the installation of the protection apparatus
16, the correct operation of the entire switching off
unit of the switch 22 is guaranteed in the presence of
a current greater than or equal to 20% of the nominal
value on at least one of the phases.
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Finally, as can be seen clearly in figures 2 and 3, to
maximise the compactness of the structure, the
protection apparatus 16 is contained entirely inside a
single container casing 20 made from thermoplastic
material, which rests upon the attachment surface 46,
whereas, inside the casing 20, an epoxy resin suitable
for the necessary insulation is used.
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In such a way, as well as the possibility of
substantially reducing the encumbrance, with respect to
known protection devices (regarding which we emphasise
the possibility of inserting the entire apparatus 16
object of the invention inside the compartments 21 of
bar switches and disconnecting switches, as illustrated
in figure 4), there is also greater precision and
stability of detected current values and, therefore, a
safer intervention, mainly due to the fact that, in
such a way, the protection apparatus is no longer
installed, like in the prior art, on board the switch.
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In fact this particular arrangement of the apparatus
inside compartments allows the problems of interference
determined by the electromagnetic flows between toroid
and pole to be eliminated, maintaining high electrical
characteristics, differentiated uses (for variable
current intensities between 40 and 630 A), a compact
structure and a substantially low overall production
and operating cost, with respect to the prior art.
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Furthermore, the apparatus according to the invention
is suitable for detecting current signals to realise
overload protection at long reverse time and at
adjustable fixed time, short-circuit protection at
adjustable independent time and, in general, all
maximum phase current protection (instantaneous, with
adjustable delay as well as with dependent and fixed
time), as well as, possibly, maximum grounding failure
current protection (with adjustable delay), calculated
as a vector sum of the three primary phase currents and
realised by means of the homopolar toroid 39, which
processes the phase currents.
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From the description which has been made the
characteristics of the compact measurement apparatus
for the protection of medium-voltage lines, object of
the present invention, are clear, just as the
advantages are also clear.
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In particular, they are represented by:
- precision of interventions;
- wide adjustment ranges;
- ensured operation even with monophase power supply;
- no limitation of the nominal interruption power of
the current for a short duration of the switch;
- single and simultaneous adjustment of the three
phases;
- constancy and stability of the characteristics,
precision and reliability of operation;
- minimal encumbrance, maximum compactness;
- limited cost.
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Finally, it is clear that numerous variants can be
brought to the apparatus in question, without for this
reason leaving the novelty principles inherent to the
inventive idea, just as it is clear that, in the
practical embodiment of the invention, the materials,
the shapes and the sizes of the illustrated details can
be whatever according to the requirements and they can
be replaced with others which are technically
equivalent.
