Note: Descriptions are shown in the official language in which they were submitted.
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Switch gear station
FIELD OF THE INVENTION AND PRIOR ART
This invention is related to a switch gear station compris-
ing at least one switch gear and at least one transfor-
mer/reactor comprising at least one winding including at
least one electric conductor.
For all transmission and distribution of electrical energy,
transformers are used and their task is to allow exchange of
electrical energy between two or more electric systems and
for this, electromagnetic induction is utilized in a well-
known manner. The transformers primarily intended with the
present invention belong to the so-called power transformers
with a rated power of from a few hundred kVA up to more than
1000 MVA with a rated voltage of from 3-4 kV and up to very
high transmission voltages, 400 kV to 800 kV or higher.
Although the following description mainly refers to power
transformers, the present invention is also applicable to re-
actors, both single-phase and multi-phase reactors. As re-
gards insulation and cooling there are, in principle, the
same embodiments as for transformers. Thus, air-insulated and
oil-insulated, self-cooled, pressure-oil cooled, etc., reac-
tors are available. Although reactors have one winding (par
phase) and may be designed both with and without a magnetic
core, the description of the background art is to a large ex-
tent relevant also to reactors.
The winding may in some embodiments be air-wound but com-
prises as a rule a magnetic core of laminated, normal or ori-
ented, sheet or other, for example amorphous or powder-based,
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2
material, or any other action for the purpose of allowing an
alternating flux, and a winding. The circuit often comprises
some kind of cooling system etc.
5 To be able to place a power transformer/reactor according to
the invention in its proper context and hence be able to de-
scribe the new approach which the invention means as well as
the advantages afforded by the invention in relation to the
prior art, a relatively complete description of a power
10 transformer as it is currently designed will first be given
below as well as of the limitations and problems which exist
when it comes to calculation, design, insulation, grounding,
manufacture, use, testing, transport, etc., of these trans-
formers.
A conventional power transformer comprises a transformer
core, in the following referred to as a core, often of lami-
nated oriented sheet, usually of silicon iron. The core com-
prises a number of core limbs, connected by yokes which to-
20 gether form one or more core windows. Transformers with such
a core are often referred to as core transformers. Around the
core limbs there are a number of windings which are normally
referred to as primary, secondary and control windings. As
far as power transformers are concerned, these windings are
25 practically always concentrically arranged and distributed
along the length of the core limbs. The core transformer nor-
mally has circular coils as well as a tapering core limb sec-
tion in order to fill up the coils as closely as possible.
30 Also other types of core designs are known, for example those
~rrhich are included in so-called shell-type transformers.
These are often designed with rectangular coils and a rectan-
gular core limb section.
35 Conventional power transformers; in the lower part of the
above-mentioned power range, are sometimes designed with air
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cooling to carry away the unavoidable inherent losses. For
protection against contact, and possibly for reducing the ex
ternal magnetic field of the transformer, it is then often
provided with an outer casing provided with ventilating open
s ings.
Most of the conventional power transformers, however, are
oil-cooled. One of the reasons therefor is that the oil has
the additional very important function as insulating medium.
10 An oil-cooled and oil-insulated power transformer is there-
fore surrounded by an external tank on which, as will be
clear from the description below, very high demands are
placed.
15 The following part of the description will for the most part
refer to oil-filled power transformers.
The windings of the transformer are formed from one or sev-
eral series-connected coils built up of a number of series-
20 connected turns. In addition, the coils are provided with a
special device to allow switching with the aid of screw
joints or more often with the aid of a special changeover
switch which is operable in the vicinity of the tank. In the
event that changeover can take place for a transformer under
25 voltage, the changeover switch is referred to as an on-load
tap changer whereas otherwise it is referred to as a de-ener-
gized tap changer.
Regarding oil-cooled and oil-insulated power transformers in
30 the upper power range, the breaking elements of the on-load
tap changers are placed in special oil-filled containers with
direct connection to the transformer tank. The breaking ele-
ments are operated purely mechanically via a motor-driven ro-
tating shaft and are arranged so as to obtain a fast movement
35 during the switching when the contact is open and a slower
movement when the contact is to be closed. The on-load tap
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changers as such, however, are placed in the actual trans-
former tank. During the operation, arcing and sparking arise.
This leads to degradation of the oil in the containers. To
obtain less arcs and hence also less formation of soot and
5 less wear on the contacts, the on-load tap changers are nor-
mally connected to the high-voltage side of the transformer.
This is due to the fact that the currents which need to be
broken and connected, respectively, are smaller on the high-
voltage side than if the on-load tap changers were to be con-
10 nected to the low-voltage side. Failure statistics of conven-
tional oil-filled power transformers show that it is often
the on-load tap changers which give rise to faults.
In the lower power range of oil-cooled and oil-insulated
15 power transformers, both the on-load tap changers and their
breaking elements are placed inside the tank. This means that
the above-mentioned problems with degradation of the oil be-
cause of arcs during operation, etc., affect the whole oil
system.
From the point of view of applied or induced voltage, it can
broadly be said that a voltage which is stationary across a
winding is distributed equally onto each turn of the winding,
that is, the turn voltage is equal on all the turns.
From the point of view of electric potential, however, the
situation is completely different. One end of a winding is
normally connected to ground. This means, however, that the
electric potential of each turn increases linearly from prac-
30 tically zero in the turn which is nearest the ground poten-
tial up to a potential in the turns which are at the other
end of the winding which correspond to the applied voltage.
This potential distribution determines the composition of the
insulation system since it is necessary to have sufficient
35 insulation both between adjacent turns of the winding and be-
tween each turn and ground.
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The turns in an individual coil are normally brought together
into a geometrically coherent unit, physically delimited from
the other coils. The distance between the coils is also de-
5 termined by the dielectric stress which may be allowed to oc-
cur between the coils. This thus means that a certain given
insulation distance is also required between the coils. Ac-
cording to the above, sufficient insulation distances are
also required to the other electrically conducting objects
10 which are within the electric field from the electric poten-
tial locally occurring in the coils.
It is thus clear fram the above description that for the in-
dividual coils, the voltage difference internally between
15 physically adjacent conductor elements is relatively low
whereas the voltage difference externally in relation to
other metal objects - the other coils being included - may be
relatively high. The voltage difference is determined by the
voltage induced by magnetic induction as well as by the ca-
20 pacitively distributed voltages that may arise from a con-
nected external electric system on the external connections
of the transformer. The voltage types that may enter exter-
nally comprise, in addition to operating voltage, lightning
overvoltages and switching overvoltages.
In the current leads of the coils,.additional losses arise as
a result of the magnetic leakage field around the conductor.
To keep these losses as low as possible, especially for power
transformers in the upper power range, the conductors are
30 normally divided into a number of conductor elements, often
referred to as strands, which are parallel-connected during
operation. These strands must be transposed according to such
a pattern that the induced voltage in each strand becomes as
identical as possible and so that the difference in induced
voltage between each pair of strands becomes as small as pos-
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sible for internally circulating current components to be
kept down at a reasonable level from the loss point of view.
When designing transformers according to the prior art, the
general aim is to have as large a quantity of conductor mate-
rial as possible within a given area limited by the so-called
transformer window, generally described as having as high a
fill factor as possible. The available space shall comprise,
in addition to the conductor material, also the insulating
10 material associated with the coils, partly internally between
the coils and partly to other metallic components including
the magnetic core.
The insulation system, partly within a coil/winding and
partly between coils/windings and other metal parts, is nor-
mally designed as a solid cellulose- or varnish-based insula-
tion nearest the individual conductor element, and outside of
this as solid cellulose and liquid, possibly also gaseous,
insulation. Windings with insulation and possible bracing
20 parts in this way represent large volumes that will be sub-
jected to high electric field strengths, which arise in and
around the active electromagnetic parts of the transformer.
To be able to predetermine the dielectric stresses, which
arise and achieve a dimensioning with a minimum risk of
25 breakdown, good knowledge of the properties of insulating ma-
terials is required. It is also important to achieve such a
surrounding environment that it does not change or reduce the
insulating properties.
30 The currently predominant insulation system for high-voltage
power transformers comprises cellulose material as the solid
insulation and transformer oil as the liquid insulation. The
transformer oil is based on so-called mineral oil.
35 The transformer oil has a dual function since, in addition to
the insulating function, it actively contributes to cooling
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of the core, the winding, etc., by removal of the loss heat
of the transformer. Oil cooling requires an external cooling
element, an expansion coupling, etc.
5 The electric connection between the external connections of
the transformer and the immediately connected coils/windings
is referred to as a bushing aiming at a conductive connection
through the tank which, in the case of oil-filled power
transformers, surrounds the actual transformer. The bushing
10 is often a separate component fixed to the tank and is de-
signed to withstand the insulation requirements being made,
both on the outside and the inside of the tank, while at the
same time it should withstand the current loads occurring and
the ensuing current forces. It should be pointed out that the
15 same requirements for the insulation system as described
above regarding the windings also apply to the necessary in-
ternal connections between the coils, between bushings and
coils, different types of changeover switches and the bush-
ings as such.
All the metallic components inside a power transformer are
normally connected to a given ground potential with the ex-
ception of the current-carrying conductors. In this way, the
risk of an unwanted, and difficult-to-control, potential in-
25 crease as a result of capacitive voltage distribution between
current leads at high potential and ground is avoided. Such
an unwanted potential increase may give rise to partial dis-
charges, so-called corona. Corona may be revealed during the
normal acceptance tests, which partially occur, compared with
30 rated data, at increased voltage and frequency. Corona may
give rise to damage during normal operation.
The individual coils in a transformer must have such a mecha-
nical dimensioning that they may withstand any stresses oc-
35 curring as a consequence of currents arising and the resul-
tant current forces during a short-circuit process. Normally,
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the coils are designed such that the forces arising are ab-
sorbed within each individual coil, which in turn may mean
that the coil cannot be dimensioned optimally for its normal
function during normal operation.
5
Within a narrow voltage and power range of oil-filled power
transformers, the windings are designed as so-called sheet
windings. This means that the individual conductors mentioned
above are replaced by thin sheets. Sheet-wound power trans-
10 formers are manufactured for voltages of up to 20-30 kV and
powers of up to 20-30 MW.
The insulation system of power transformers within the upper
power range requires, in addition to a relatively complicated
15 design, also special manufacturing measures to utilise the
properties of the insulation system in the best way. For a
good insulation to be obtained, the insulation system shall
have a low moisture content, the solid part of the insulation
shall be well impregnated with the surrounding oil and the
20 risk of remaining "gas" pockets in the solid part must be
minimal. To ensure this, a special drying and impregnating
process is carried out on a complete core with windings be-
fore it is lowered into a tank. After this drying and impreg-
nating process, the transformer is lowered into the tank,
25 which is then sealed. Before filling of oil, the tank with
the immersed transformer must be emptied of all air. This is
done in connection with a special vacuum treatment. When this
has been carried out, filling of oil takes place.
30 To be able to obtain the promised service life, etc., pumping
out to almost absolute vacuum is required in connection with
the vacuum treatment. This thus presupposes that the tank
which surrounds the transformer is designed for full vacuum,
which entails a considerable consumption of material and
35 manufacturing time.
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If electric discharges occur in an oil-filled power trans-
former, or if a local considerable increase of the tempera-
ture in any part of the transformer occurs, the oil is disin-
tegrated and gaseous products are dissolved in the oil. The
5 transformers are therefore normally provided with monitoring
devices for detection of gas dissolved in the oil.
For weight reasons large power transformers are transported
without oil. In-situ installation of the transformer at a
10 customer requires, in turn, renewed vacuum treatment. In ad-
dition, this is a process which, furthermore, has to be re-
peated each time the tank is opened for some action or in-
spection.
15 It is obvious that these processes are very time-consuming
and cost demanding and constitute a considerable part of the
total time for manufacture and repair while at the same time
requiring access to extensive resources.
20 The insulating material in conventional power transformers
constitutes a large part of the total volume of the transfor-
mer. For a power transformer in the upper power range, oil
quantities in the order of magnitude of hundreds of cubic me-
tres of transformer oil may occur. The oil which exhibits a
25 certain similarity to diesel oil is thinly fluid and exhibits
a relatively low flash point. It is thus obvious that oil to-
gether with the cellulose constitutes a non-negligible fire
hazard in the case of unintentional heating, for example at
an internal flashover and a resultant oil spillage.
It is also obvious that, especially in oil-filled power
transformers, there is a very large transport problem. Such a
power transformer in the upper power range may have a total
weight of up to 30-40 tons. It is realised that the external
design of the transformer must sometimes be adapted to the
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current transport profile, that is, for possible passage of
bridges, tunnels, etc.
Here follows a short summary of the prior art with respect to
5 oil-filled power transformers and what may be described as
limitation and problem areas therefor:
An oil-filled power transformer
10 - comprises an outer tank which is to house a transformer
comprising a transformer core with coils, oil for insulation
and cooling, mechanical bracing devices of various kinds,
etc. Very large mechanical demands are placed on the tank,
since, without oil but with a transformer, it shall be capa-
15 ble of being vacuum-treated to practically full vacuum. The
tank requires very extensive manufacturing and testing pro-
cesses and the large external dimensions of the tank also
normally entail considerable transport problems;
20 - comprises for larger power ranges a so-called pressure-oil
cooling. This cooling method requires the provision of an oil
pump, an external cooling element, an expansion vessel and an
expansion coupling, etc.;
25 - comprises an electrical connection between the external
connections of the transformer and the immediately connected
coils/windings in the form of a bushing fixed to the tank.
The bushing is designed to withstand any insulation require
ments made, both regarding the outside and the inside of the
30 tank;
- comprises coils/windings whose conductors are divided into
a number of conductor elements, strands, which have to be
transposed in such a way that the voltage induced in each
35 strand becomes as identical as possible and such that the
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difference in induced voltage between each pair of strands
becomes as small as possible;
- comprises an insulation system, partly within a
coil/winding and partly between coils/windings and other
metal parts which is designed as a solid cellulose- or var-
nish-based insulation nearest the individual conductor ele-
ment and, outside of this, solid cellulose and a liquid, pos-
sibly also gaseous, insulation. In addition, it is extremely
10 important that the insulation system exhibits a very low
moisture content;
- comprises as an integrated part an on-load tap changer,
surrounded by oil and normally connected to the high-voltage
winding of the transformer for voltage control;
- comprises oil which may entail a non-negligible fire hazard
in connection with internal partial discharges, so-called co
rona, sparking in on-load tap changers and other fault condi
20 tions;
- comprises normally a monitoring device for monitoring gas
dissolved in the oil, which occurs in case of electrical dis-
charges therein or in case of local increases of the tempera-
ture;
- comprises oil which, in the event of damage or accident,
may result in oil spillage leading to extensive environmental
damage.
The presentation above shows, accordingly, that the switch
gear stations of today leave much to be desired as a conse-
quence of the design of the transformers/reactors. It is also
pointed out that the switch gear station as concerns the de-
35 sign of the switch gear thereof is very bulky and, accord-
ingly, costly in at least some embodiments.
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It is pointed out that the term switch gear station here re-
fers to a station, which is intended for collection and/or
distribution of electrical energy and comprises, for this
task, required equipment for a.o. switching and supervising.
SUMMARY OF THE INVENTION
The object of the present invention is primarily to provide a
switch gear station, in which at~least one or some of the
disadvantages discussed above and impairing prior art
has/have been eliminated.
The primary object is obtained by means of a device of the
kind defined in the enclosed claims, and then first of all in
claim 1.
In a wide sense, it is established that the design according
to the invention reduces the occurring losses such that the
device, accordingly, may operate with a higher efficiency as
a consequence of the fact that the invention makes it possi-
ble to substantially enclose the electric field occurring due
to said electric conductor in the insulation system. The re-
duction of losses results, in turn, in a lower temperature in
the device, which reduces the need for cooling and allows
possibly occurring cooling devices to be designed in a more
simple way than without the invention.
The conductor/insuiation system according to the invention
may be realised as a flexible cable, which means substantial
advantages with respect to production and mounting as com
pared to the prefabricated, rigid windings which have been
conventional up to now. The insulation system used according
to the invention results in absence of gaseous and liquid in
sulation materials.
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As to the aspect of the invention as a power transformer/ re-
actor, the invention, first of all, eliminates the need for
oil filling of the power transformers and the problems and
disadvantages associated thereto.
The design of the winding so that it comprises, along at
least a part of its length, an insulation formed by a solid
insulating material, inwardly of this insulation an inner
layer and outwardly of the insulation an outer layer with
10 these layers made of a semiconducting material makes it pos-
sible to enclose the electric field in the entire device
within the winding. The term "solid insulating material" used
herein means that the winding is to lack liquid or gaseous
insulation, for instance in the form of oil. Instead the in-
15 sulation is intended to be formed by a polymeric material.
Also the inner and outer layers are formed by a polymeric ma-
terial, though a semiconducting such.
The inner layer and the solid insulation are rigidly con-
20 nected to each other over substantially the entire interface.
Also the outer layer and the solid insulation are rigidly
connected to each other over substantially the entire inter-
face therebetween. The inner layer operates equalising with
respect to potential and, accordingly, equalising with re-
25 spect to the electric field outwardly of the inner layer as a
consequence of the semiconducting properties thereof. The
outer layer is also intended to be made of a semiconducting
material and it has at least an electrical conductivity being
higher than that of the insulation so as to cause the outer
30 layer, by connection to earth or otherwise a relatively low
potential, to function equalising with regard to potential
and to substantially enclose the electric field resulting due
to said electric conductor inwardly of the outer layer. On
the other hand, the outer layer should have a resistivety
35 which is sufficient to minimise electric losses in said outer
layer.
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The rigid interconnection between the insulating material and
the inner and outer semiconducting layers should be uniform
over substantially the entire interface such that no cavi-
5 ties, pores or similar occur. With the high voltage levels
contemplated according to the invention, the electric and
thermal loads which may arise will impose extreme demands on
the insulation material. It is known that so-called partial
discharges, PD, generally constitute a serious problem for
10 the insulating material in high-voltage installations. If
cavities, pores or the like arise at high electric voltages,
internal corona discharges may arise, whereby the insulating
material is gradually degraded and the result could be elec-
tric breakdown through the insulation. This may lead to seri-
15 ous breakdown of the electromagnetic device. Thus, the insu-
lation should be homogenous.
The inner layer inwardly of the insulation should have an
electric conductivity which is lower than that of the elec-
20 tric conductor but sufficient for the inner layer to function
equalising with regard to potential and, accordingly, equal-
ising with respect to the electrical field externally of the
inner layer. This in combination with the rigid interconnec-
tion of the inner layer and the electric insulation over sub-
25 stantially the entire interface, i.e. the absence of cavities
etc, means a substantially uniform electrical field exter-
nally of the inner layer and a minimum of risk for PD.
It is preferred that the inner layer and the solid electrical
30 insulation are formed by materials having substantially equal
thermal coefficients of expansion. The same is preferred as
far as the outer layer and the solid insulation are con-
cerned. This means that the inner and outer layers and the
solid electrical insulation will form an insulation system
35 which on temperature changes expands and contracts uniformly
as a monolithic part without those temperature changes giving
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rise to any destruction or disintegration in the interfaces.
Thus, intimacy in the contact surface between the inner and
outer layers and the solid insulation is ensured and condi
tions are created to maintain this intimacy during prolonged
5 operation periods.
Furthermore, it is pointed out that it is essential that the
materials in the inner and outer layers and in the solid in-
sulation has a high elasticity so that the materials may en-
10 dure the strains occurring when the cable is bent and when
the cable during operation is subjected to thermal strains. A
good adhesion between the solid insulation and the inner and
outer layers and a high elasticity of these layers and the
solid insulation respectively are particularly important in
15 case the materials in the layers and the solid insulation
would not have substantially equal thermal coefficients of
expansion. Furthermore, it is preferable that the materials
in the inner and outer layers and in the solid insulation
have substantially equal elasticity (E-modulus), which will
20 counteract occurrence of shear stresses in the boarder zone
between the layers and the solid insulation. It is preferred
that the materials in the inner and outer layers and in the
solid insulation have an E-modulus which is less than 500
MPA, preferably less than 200 MPA. In order to be able to
25 form windings by means of the cable, it is essential that the
flexibility thereof is good. It is preferred that the cable
should be capable of being subjected to bending, without
negative influence on the function, with a radius of cunra-
ture.which is 20 times the cable diameter or less, suitably
30 15 times the cable diameter or less. It is preferred that the
cable should be possible to bend to a radius of curvature of
four or five times the cable diameter or even less.
The electric load on the insulation system decreases as a
35 consequence of the fact that the inner and the outer layers
of semiconducting material around the insulation will tend to
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form substantially equipotential surfaces and in this way the
electric field in the insulation properly will be. distributed
relatively uniformly over the thickness of the insulation.
5 It is known that high voltage cables for transmission of
electrical energy may be constructed of conductors with an
insulation of a solid insulation material with inner and
outer layers of semiconducting material. In transmission of
electrical energy, it has since long been realised that the
10 insulation should be free from defects. However, in high
voltage cables for transmission, the electric potential does
not change along the length of the cable but the potential is
basically at the same level. However, also in high voltage
cables for transmission purposes, instantaneous potential
15 differences may occur due to transient occurrences, such as
lightning. According to the present invention a flexible ca-
ble according to the enclosed claims is used as a winding in
the electromagnetic device.
20 An additional improvement may be achieved by constructing the
electric conductor in the winding from smaller, so-called
strands, at least some of which are insulated from each
other. By making these strands to have a relatively small
cross section, preferably approximately circular, the mag-
25 netic field across the strands will exhibit a constant geome-
try in relation to the field and the occurrence of eddy cur-
rents is minimised.
According to the invention, the winding is thus preferably
30 made in the form of a cable comprising the electric conductor
and the previously described insulation system, the inner
layer of which extends about the conductor. Outside of this
inner semiconducting layer is the main insulation in the form
of a solid insulation material.
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The outer semiconducting layer shall according to the inven-
tion exhibit such electric properties that a potential
equalisation along the conductor is ensured. The -outer layer
may, however, not exhibit such conductivity properties that
5 an induced current will flow along the surface, which could
cause losses which in turn may create an unwanted thermal
load. For the inner and outer layers the resistance state-
ments (at 20°C) defined in the enclosed claims 5 and 6 are
valid. With respect to the inner semiconducting layer, it
10 must have a sufficient electric conductivity to ensure poten-
tial equalisation for the electrical field but at the same
time this layer must have such a resistivety that~the enclos-
ing of the electric field is ensured.
15 It is important that the inner layer equalises irregularities
in the surface of the conductor and forms an equipotential
surface with a high surface finish at the interface with the
solid insulation. The inner layer may be formed with a vary-
ing thickness but to ensure an even surface with respect to
20 the conductor and the solid insulation, the thickness is
suitably between 0.5 and 1 mm.
Such a flexible winding cable which is used according to the
invention in the electromagnetic device thereof is an im-
25 provement of a XLPE (cross-linked polyethylene) cable used
per se for transmission purposes or a cable with EP (ethyl-
ene-propylene) rubber insulation. The improvement comprises,
inter alia, a new design both as regards the strands of the
conductors and in that the cable, at least in some embodi-
30 ments, has no outer casing for mechanical protection of the
cable. However, it is possible according to the invention to
arrange a conducting metal shield and an outer mantle exter-
nally of the outer semiconducting layer. The metal shield
will then have the character of an outer mechanical and elec-
35 tric protection, for instance to lightning. It is preferred
that the inner semiconducting layer will lie on the potential
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18
of the electric conductor. For this purpose at least one of
the strands of the electric conductor will be uninsulated and
arranged so that a good electric contact is obtained to the
inner semiconducting layer. Alternatively, different strands
5 may be alternatingly brought into~electric contact with the
inner semiconducting layer. Manufacturing transformer or re-
actor windings of a flexible cable according to the above en-
tails drastic differences as regards the electric field dis-
tribution between conventional power transformers/reactors
10 and a power transformer/reactor according to the invention.
The decisive advantage with a cable-formed winding according
to the invention is that the electric field is enclosed in
the winding and that there is thus no electric field outside
the outer semiconducting layer. The electric field caused by
15 the current-carrying conductor occurs only in the solid main
insulation. Both from the design point of view and the manu-
facturing point of view this means~considerable advantages:
- the windings of the transformer may be formed without hav-
20 ing to consider any electric field distribution and the
transposition of strands, mentioned under the background art,
is omitted;
- the core design of the transformer may be formed witho~.z~
25 having to consider any electric field distribution;
- no oil is needed for electrical insulation of the winding,
that is, the medium surrounding the winding may bs air;
30 - no special connections are required for electric connecticn
between the outer connections of the transformer and the im-
mediately connected coils/windings, since the electric con-
nection, contrary to conventional plants, is integrated with
the winding;
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- the manufacturing and testing technology which is needed
for a power transformer according to the invention is consid-
erably simpler than for a conventional power trans-
former/reactor since the impregnation, drying and vacuum
5 treatments described under the description of the background
art are not needed.
Above it has already been described that the outer semicon-
ducting layer of the winding cable is intended to be con-
10 nected to ground potential. The purpose is that the layer
should be kept substantially on ground potential along the
entire length of the winding cable. It is possible to divide
the outer semiconducting layer by cutting the same into a
number of parts distributed along the length of the winding
15 cable, each individual layer part. being connectable directly
to ground potential. In this way a better uniformity along
the length of the winding cable is achieved.
Above it has been mentioned that the solid insulation and the
20 inner and outer layers may be achieved by, for instance, ex-
trusion. Other techniques are, however, also well possible,
for instance formation of these inner and outer layers and
the insulation respectively by means of spraying of the mate-
rial in question.
25
It is preferred that the winding cable is designed with a
circular cross section. However, also other cross sections
may be used in cases where it is desired to achieve a better
packing density.
30
BRIEF DESCRIPTION OF THE DRAWINGS
With reference to the enclosed drawings, a more specific de
scription of embodiment examples of the invention will follow
35 hereinafter.
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WO 99/33074 PCT/SE98/02148
In the drawings:
Fig 1 is a view showing the electric field distribution
around a winding of a conventional power trans
5 former/reactor;
Fig 2 is a partly cut view showing the parts comprised in
the modified standard cable in question;
10 Fig 3 is a perspective view showing an embodiment of a
power transformer according to the invention;
Fig 4 is a plan view of a switch gear;
15 Fig 5 is a side view of the switch gear according to Fig
4;
Fig 6 is a cut side view illustrating an oil filled power
transformer according to. prior art;
Fig 7 is a side view illustrating a power transformer ac-
cording to the invention;
Fig 8 is a plan view illustrating a switch gear insula-
tion in a building;
Fig 9 is a section through the building;
Fig 10 is a plan view similar to the one in Fig 8 but il-
lustrating an alternative possibility for a trans-
former designed according to the invention;
Fig 11 is a vertical section through the building;
Fig 12 is a plan view of a building containing a switch
gear station conceived to be gas insulated;
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21
Figs 13
and 14 are a front view and a side view respectively il-
lustrating switching equipment comprised in a
switch gear;
5
Fig 15 is a perspective and diagrammatical view illustrat-
ing how a plurality of box like metal casings have
been co-ordinated for. formation of a switch gear
station;
10
Fig 16 is a partly cut view illustrating electric conduc-
tors being covered by means of an electrically in-
sulating layer; and
15 Fig 17 is a diagrammatical view illustrating in section
how windings are disposed in the core window of a
transformer.
DESCRIPTION OF PREFERRED EMBODIMENTS
20
An important condition for being able to manufacture a mag-
netic circuit in accordance with the invention, is to use for
the winding a conductor cable with a solid electrical insula-
tion with an inner semiconducting layer between the insula-
25 tion and one or more electrical conductors located inwardly
thereof and with an outer semiconducting layer located out-
wardly of the insulation. Such cables are available as stan-
dard cables for other power engineering fields of use, namely
power transmission. To be able to describe an embodiment,
30 initially a short description of a standard cable will be
made. The inner current-carrying conductor comprises a number
of strands. Around the strands there is a semiconducting in-
ner layer or casing. Around this semiconducting inner layer,
there is an insulating layer of solid insulation. The solid
35 insulation is formed by a polymeric material with low elec-
trical losses and a high breakthrough strength. As concrete
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22
examples polyethylene (PE) and then particularly cross-linked
polyethylene (XLPE) and ethylene-propylene (EP) may be men-
tioned. Around the outer semiconducting layer a metal shield
and an outer insulation casing may be provided. The semicon-
5 ducting layers consist of a polymeric material, for example
ethylene-copolymer, with an electrically conducting constitu-
ent, e. g. conductive soot or carbon black. Such a cable will
be referred to hereunder as a power cable.
10 A preferred embodiment of a cable intended for a winding ap-
pears from Fig 1. The cable 1 is described in the figure as
comprising a current-carrying conductor 2 which comprises
transposed both non-insulated and insulated strands. Electro-
mechanically transposed, solidly insulated strands are also
15 possible. These strands may be stranded/transposed in a plu-
rality of layers. Around the conductor there is an inner
semiconducting layer 3 which, in turn, is surrounded by a ho-
mogenous layer 4 of a solid insulation material. The insula-
tion 4 is entirely without insulation material of liquid or
20 gaseous type. This layer 4 is surrounded by an outer semicon-
ducting layer 5. The cable used as a winding in the preferred
embodiment may be provided with metal shield and external
sheath but must not be so. To avoid induced currents and
losses associated therewith in the outer semiconducting layer
25 5, this is cut off, preferably in the coil end, that is, in
the transitions from the sheet stack to the end windings. The
cut-off is carried out such that the outer semiconducting
layer 5 will be divided into several parts distributed along
the cable and being electrically entirely or partly separated
30 from each other. Each cut-off part is then connected to
ground, whereby the outer semiconducting layer 5 will be
maintained at, or near, ground potential in the whole cable
length. This means that, around the solidly insulated winding
at the coil ends, the contactable surfaces, and the surfaces
35 which are dirty after some time of use, only have negligible
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23
potentials to ground, and they also cause negligible electric
fields.
In an alternative embodiment, the cable which is used as a
winding may be a conventional power cable as the one de-
scribed above. The grounding of the outer semiconducting
layer then takes place by stripping the metal shield and the
sheath of the cable at suitable locations.
10 Figure 1 shows a simplified and fundamental view of the elec-
tric field distribution around awinding of a conventional
power transformer/reactor, where 17 is a winding and 18 a
core and 19 illustrates equipotential lines, that is, lines
where the electric field has the same magnitude. The lower
part of the winding is assumed to be at ground potential.
The potential distribution determines the composition of the
insulation system since it is necessary to have sufficient
insulation both between adjacent turns of the winding and be-
20 tween each turn and ground. The figure thus shows that the
upper part of the winding is subjected to the highest insula-
tion loads. The design and location of a winding.relative to
the core are in this way determined substantially by the
electric field distribution in the core window.
The cable which can be used in the windings contained in the
dry power transformers/reactors according to the invention
have been described with assistance of Fig 2. The cable may,
as stated before, be provided with other, additional outer
30 layers for special purposes, for instance to prevent exces-
sive electric strains on other areas of the trans-
former/reactor. From the point of view of geometrical dimen-
sion, the cables in question will as a rule have a conductor
area which is between 2 and 3000 mm2 and an outer cable dia-
meter which is between 20 and 250 mm.
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WO 99/33074 PCT/SE98/02148
24
The windings of a power transformer/reactor manufactured from
the cable described in the disclosure of the invention may be
used both for single-phase, three-phase and polyphase trans-
formers/reactors independently of how the core is shaped. One
5 embodiment is illustrated in Figure 3 which shows a three-
phase laminated core transformer. The core comprises, in con-
ventional manner, three core limbs 20, 21 and 22 and the re-
taining yokes 23 and 24. In the embodiment shown, both the
core limbs and the yokes have a tapering cross section.
Concentrically around the core limbs, the windings formed
with the cable are disposed. As~ is clear, the embodiment
shown in Figure 3 has three concentric winding turns 25, 26
and 27. The innermost winding turn 25 may represent the pri-
15 mart' winding and the other two winding turns 26 and 27 may
represent secondary windings. In order not to overload the
figure with too many details, the connections of the windings
are not shown. Otherwise the figure shows that, in the em-
bodiment shown, spacing bars 28 and 29 with several different
20 functions are disposed at certain points around the windings.
The spacing bars may be formed of insulating material in-
tended to provide a certain space between the concentric
winding turns for cooling, bracing, etc. They may also be
formed of electrically conducting~material in order to form
25 part of the grounding system of the windings.
A conventional air insulated switch gear station is il-
lustrated in Fig 4 and 5. It comprises a high voltage switch
gear 42 (which is the one which is air insulated), two trans-
30 formers 43 and a medium voltage switch gear 44, which is
built into a building 45. 7 denotes simply a fence around the
area in question.
The transformers 43 are of such a type which is oil insu-
35 lated. As already has been discussed above, oil insulation of
a transformer involves a considerable risk for fire and the
CA 02311748 2000-OS-25
WO 99/33074 PC'f/SE98/02148
environment. Besides, the transformers become more heavy and
bulky than what is possible according to the present inven-
tion in case the winding is formed by means of flexible ca-
bles having a solid insulation. More specifically, oil filled
5 transformers require concrete walls 46 around the trans-
formers. Furthermore, there must be, under the transformers,
a comparatively costly oil collection pit in case there would
be a leakage.
10 Fig 6 illustrates more clearly that the transformer requires
a costly foundation 47, on which the transformer 43 and the
previously mentioned firewalls are placed. As appears from
Fig 6 there is also under the transformer a collection space
for oil in case transformer leakage would occur. Oil flows
15 down into the space 48 through a distinguishing layer, which
for instance consists of a basket filled with stones.
Fig 7 illustrates a transformer 49 according to the inven-
tion. Thus, this transformer has its windings achieved by
20 means of the flexible cable described. As a consequence
thereof, the transformer is not filled with oil and therefore
it does not need any oil collection space under itself. The
transformer according to the invention becomes considerably
lighter and is also more easy to connect by means of output
25 cables 50 since these cables do not have to pass through any
sealing devices etc for oil. Considerable advantages are thus
obtained by replacing the conventional oil filled transformer
with the transformer according to the invention in a switch
gear station of the kind illustrated in Fig 4 and 5.
Figs 8 and 9 illustrate in plan view and side view a switch
gear station built into a building generally denoted 51. The
building 51 comprises a first section 52 housing the medium
voltage switch gear and a control unit, a second section 53
35 housing the high voltage switch gear and a third section 54
located therebetween and housing the transformer.part of the
CA 02311748 2000-OS-25
WO 99/33074 PCT/SE98/02148
26
station, here in the form of two transformers 55. The differ-
ent sections are separated from each other by means of fire-
proof walls 56. Also the two transformers are separated by
means of a f fireproof wall .
The switch gear station according to Figs 8 and 9 intended to
be built into a building involves substantial advantages
relative to the station illustrated with the aid of Figs 4
and 5. First of all an air insulated station as the one in
10 Figs 4 and 5 is negative from the point of view of appear-
ance. Such stations are not well suited for location near
populated areas. On the contrary, the station illustrated in
Figs 8 and 9 may very well be located close to built-up areas
as a consequence of the total encasing of the station without
15 cause to anticipate disturbances. Even if the switch gear
station according to Figs 8 and 9 is enclosed, it is con-
ceived to be air insulated. Such a station could per se also
have its devices encased in hermetical containers with the
components surrounded by SF6-gas. Such gas causes the re-
20 sistance to break through to increase considerably, for what
reason the requirements for a safety distance between compo-
nents at different voltage level may be decreased so that a
more compact building mode is made possible. However, such
encasing is very costly and a rigorous supervision is also
25 required with regard to the risk of leakage. .
Use of convention oil filled transformers in the station ac-
cording to Figs 8 and 9 involves the practical disadvantages
already described with regard to oil leakage and fire rise.
30 Furthermore, limitations are in practice involved as to the
location of the transformers. Besides it is established that
it per se would be possible to provide conventional oil
filled transformers with cable bushings to the adjacent high
voltage and medium voltage switch gears but such cable
35 bushings become complicated and expensive since the oils of
the cable termination and the transformer may not be mixed
CA 02311748 2000-OS-25
WO 99/33074 PCT/SE98/02148
27
with each other. In oil filled transformers one therefore
often selects conventional porcelain bushings on the
transformer and in wall bushings ~s connections to the high
voltage switch gear. Pressure relieving channels must pass
5 under the ceiling in the transformer space since outlets in
the roof of the high voltage switch gear part involve a
higher risk of water leakage. This in combination with
porcelain bushings on the transformer and the oil container
under the transformer lead to a large height of the
l0 transformer part in the building, which clearly appears from
Fig 9. Since minimal distances are desirable between the
transformers and the high voltage and medium voltage switch
gears respectively, the transformers are placed between the
switch gears. When more than two power transformers stand in
15 a row the problem arises that the intermediate transformer
can not be lifted with a reasonable size of a mobile crane.
It is, namely, normal procedure to place and withdraw
respectively the transformers into/out of the building
through the roof thereof. By replacing the conventional oil
20 filled transformers in Figs 8 and 9 with the transformer
according to the invention illustrated in Fig 7 the
advantages already expressed are obtained, including more
easy handling with respect to lifting.
25 In addition the transformer according to the invention in-
volves a larger amount of freedom with respect to the design
of the switch gear station, which'is explained with the aid
of Figs 10 and 11. Again, the high voltage switch gear is de-
noted 53 whereas the medium voltage switch gear is denoted
30 52. The transformer section is denoted 54. As is apparent,
the transformer section is here placed at one side of the
building so that accordingly the medium voltage section is
located between the high voltage section and the transformer
section. This involves very easy access to the transformers
35 49 in case any of them would have to be replaced or trans-
ported away for service. The reason why the transformers 49
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28
according to Fig 10 may be placed in a section at one side of
the building without appreciable disadvantages is due to the
circumstance that the windings in the transformer are formed
by means of flexible cables 50 and provide freedom to have
5 comparatively large distances between the transformers and in
this case the high voltage switch gear without disadvantages
worth mentioning; the cables may simply be drawn in the most
suitable manner between the transformers into the high volt-
age switch gear.
When comparing Figs 8 and 9 with Figs 10 and 11 it can be
seen that resorting to a transformer designed in accordance
with the invention with cable technology means that the
building volume may be considerably decreased. In particular,
15 the intermediate medium voltage section can be made lower,
which simplifies pressure relieving of the high voltage
switch gear. The lower ceiling height is a consequence of the
fact that a transformer with cable technology as a rule be-
comes smaller and lighter and the connections to the switch
gears become more convenient by means of the cables.
Fig 12 illustrates in plan view a switch gear station con-
ceived to be of a gas insulated type. The gas may for in-
stance be SF6. As a consequence of the improved insulation by
25 means of gas, such gas insulated switch gears may be con-
structed extremely compact. Transformers occupy a large part
of these stations. For this reason transformers according to
the present invention fit extremely well into these switch
gear stations in view of their compact design and the absence
30 of need for extra space requiring measures caused by oil. In
Fig 12, 55 denotes a section for the medium voltage switch
gear and control and power systems. The section 56 denotes a
relay and control room. The gas insulated high voltage switch
gear is placed in the section 57. On both sides of this sec-
35 tion there are sections 58 for two transformers. If these
transformers instead of being conventionally oil filled are
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29
constructed according to the invention substantially smaller
space and investments in the form of buildings, oil spillage
spaces etc are required.
5 Figs 13 and 14 illustrate parts of a switching equipment for
a switch gear station intended to function with air lines.
More specifically, two or more switching elements, for in-
stance two circuit breakers 59 and two disconnectors 60, com-
prised in the switch gear station are intended to be arranged
10 on a single common column-like carrier 61, which is intended
to have one single point of support against the underlayer
and a height which is less than the horizontal extent of the
switching equipment located thereabove. As appears from Fig
14, the disconnectors 60 may for instance have the character
15 of pantograph constructions.
The arrangement of several switching elements on one and the
same carrier in the manner just described involves occupation
of a considerably reduced land area as compared to the prior
20 art according to which one always have carried switching ele-
ments of the kind in question with individual carrying struc-
tures. By combining such compact carrying structures with
transformers likewise designed in a compact manner in accor-
dance with the present invention one obtains an optimisation
25 of the switch gear station in its entirety.
Fig 15 illustrates diagrammatically a switch gear station, a
basic idea of which is that electric devices belonging to the
switch gear station are provided in air insulated and
30 grounded metal casings 62. Each such casing has basically the
character of a box and individual casings are carried out
such that a switch gear station in its entirety may be built
up by a combination of a larger number of such casings placed
side by side and above each other as indicated in Fig 15.
35 More specifically, three layers of casings stacked above each
other are shown. The principle to enclose the switch gear
CA 02311748 2000-OS-25
WO 99/33074 PCT/SE98/02148
equipment in such grounded metal casings involves a more com-
pact building mode due to the reduced need for insulation
distance. To this compactness, the use of transformers in ac-
cordance with the cable technology~in accordance with the in-
s vention contributes. The compactness is further increased in
case insulation by means of another gas than air for instance
SF6 is resorted to.
Fig 16 illustrates a further embodiment directed towards re-
10 ducing the insulation distances and, accordingly, creating
and increasing compactness in switch gear stations. More spe-
cifically, it is illustrated in Fig 16 how an electric con-
ductor 63 in a branch point 75 is branched-of f to a further
conductor 64. 65 denotes a high voltage device. To improve
15 insulation of the conductors 63, 64 these are coated with an
electrically insulating coating 65. This coating should pre-
ferably have such a thickness and be of such a material that
it may withstand electric puncture by means of voltage which
is at least 30~ of the total rated voltage applied on a gap
20 between two conductors or between a conductor and another
conducting part, for instance ground in the switch gear.
Thus, a considerable increase of the insulation is achieved
with a consequent possibility to arrange the switch gear
equipment more closely in the switch gear.
Fig 16 also illustrates that means in the form of shields 66
rnay be present to prevent propagation of creep charges. These
shields may either be integrated with the insulating coating
65 or placed in contact with the coating.
Fig 17 illustrates diagrammatically a transformer, the core
of which is denoted 71. Windings present in the core are
shown to be cut. The windings are formed by means of flexible
cables of the kind previously described. The transformer com-
35 prises at least one high voltage winding and at least one low
voltage winding. Two layers of high voltage windings are de-
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31
voltage winding. Two layers of high voltage windings are de-
noted 72. Between these layers there is in the example one
layer of a low voltage winding. This mixing of high voltage
and low voltage windings is possible in the transformer ac-
s cording to the invention as a consequence of the fact that
the flexible cable forming the windings has an insulation
system ensuring the electric field around the conductor of
the cable is substantially enclosed in the cable and thus can
not operate disturbing on adjacent winding turns. As a conse-
10 quence of this considerably smaller losses occur in the
transformer according to the invention. Furthermore, the
windings may easily be arranged such that current induced
forces at least partially balance each other.
15 It is per se known to cast the windings of transformers and
reactors in resins. However, this causes problems due to the
low thermal conductivity of resins. According to the present
invention it is proposed to embed the winding/windings of the
transformer/reactor in a substantially inorganic material.
20 The embedding proper occurs as a rule by casting. The mate-
rial in question is suitabhy concrete. This material has the
advantage that it is non-expensive and easy to handle. Be-
sides, it has a higher thermal conductivity than materials
previously used for this purpose and, furthermore, a higher
25 specific heat capacity. This is important with regard to
overload conditions. According to a particularly preferred
embodiment, a constituent comprising a material increasing
the heat conductivity of the concrete is mixed into the same.
The material in question may for instance be an arbitrary
30 metal powder, for instance aluminium.
POSSIBLE MODIFICATIONS
It is evident that the invention is not only restricted to
35 the embodiments presented hereinabove. Thus, men skilled
within this art will realise that numerous detail modifica-
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32
tions are possible when knowledge about the basic inventive
concept has been obtained without for this reason deviating
from the inventive concept as it is defined in the enclosed
claims. As an example, it is pointed out that the invention
5 is not restricted to the specific selections of materials ex-
emplified above. Functionally equivalent materials may ac-
cordingly be used instead. With respect to the manufacturing
of the insulation system according to the invention, it is
pointed out that also other techniques than extrusion and
10 spraying are possible as long as intimacy between the differ-
ent layers is achieved. Furthermore, it is pointed out that a
larger number of equipotential layers could be arranged. For
example, one or more equipotential layers of semiconducting
material could be provided in the insulation between the lay-
15 ers designated " inner" and " outer" above. Although some
specific switch gear stations have been illustrated above it
is emphasised that the present invention should not necessar-
ily be considered as restricted to any of the embodiments now
described. On the contrary, the ideas according to the inven-
20 tion are generally applicable to switch gear stations.
