U.S. patent application number 08/973307 was filed with the patent office on 2001-09-06 for dc transformer/reactor.
Invention is credited to FROMM, UDO, JAKSTS, ALBERT, LEIJON, MATS, LI, MING, SASSE, CHRISTIAN.
Application Number | 20010019494 08/973307 |
Document ID | / |
Family ID | 20402760 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010019494 |
Kind Code |
A1 |
LEIJON, MATS ; et
al. |
September 6, 2001 |
DC TRANSFORMER/REACTOR
Abstract
The present invention relates to a DC transformer/reactor
comprising a magnetic circuit, wherein the magnetic circuit
comprises a magnetic core and at least one winding. The winding
comprises at least one current-carrying conductor. Each winding
comprises also an insulation system, which comprises on the one
hand at least two semiconducting layers, wherein each layer
constitutes substantially an equipotential surface, and on the
other hand between them is arranged a solid insulation.
Inventors: |
LEIJON, MATS; (VASTERAS,
SE) ; FROMM, UDO; (VASTERAS, SE) ; JAKSTS,
ALBERT; (VASTERAS, SE) ; LI, MING; (VASTERAS,
SE) ; SASSE, CHRISTIAN; (VASTERAS, SE) |
Correspondence
Address: |
JOHN P DELUCA
WATSON COLE GRINDLE WATSON
1400 K STREET NW
SUITE 1000
WASHINGTON
DC
200052477
|
Family ID: |
20402760 |
Appl. No.: |
08/973307 |
Filed: |
November 28, 1997 |
PCT Filed: |
May 27, 1997 |
PCT NO: |
PCT/SE97/00889 |
Current U.S.
Class: |
363/125 |
Current CPC
Class: |
H02K 9/19 20130101; H02K
3/28 20130101; Y10S 174/26 20130101; H01F 27/288 20130101; H01F
2027/2833 20130101; H02K 3/14 20130101; Y10S 174/28 20130101; H02K
3/40 20130101; H01F 27/34 20130101; Y10S 174/14 20130101; Y10S
174/22 20130101; H01F 3/14 20130101; H01F 2027/329 20130101; H01F
27/323 20130101; H02H 3/025 20130101; Y10S 174/25 20130101; H01F
2029/143 20130101; H02K 1/165 20130101; Y10S 174/13 20130101; Y10S
174/17 20130101; H02K 2203/15 20130101; Y10S 174/24 20130101; Y10S
174/15 20130101; H02K 15/12 20130101; H02K 15/00 20130101; H01F
29/14 20130101; Y10S 174/29 20130101; H02K 11/048 20130101; Y10S
174/20 20130101; H02K 3/48 20130101; H01F 3/10 20130101; Y10S
174/19 20130101 |
Class at
Publication: |
363/125 |
International
Class: |
H02M 005/45 |
Foreign Application Data
Date |
Code |
Application Number |
May 29, 1996 |
SE |
9602079-7 |
Claims
1. A DC transformer/reactor comprising a magnetic circuit, wherein
the magnetic circuit comprises a magnetic core and at least one
winding, characterized in that the winding comprises at least one
current-carrying conductor and each winding comprises an insulation
system, which comprises on the one hand at least two semiconducting
layers, wherein each layer constitutes substantially an
equipotential surface, and on the other hand between them is
arranged a solid insulation.
2. A DC transformer/reactor according to claim 1, characterized in
that at least one of said semiconducting layers has in the main
equal thermal expansion coefficient as said solid insulation.
3. A DC transformer/reactor according to claim 2, characterized in
that the potential of the inner one of said layers is essentially
equal to the potential of the conductor.
4. A DC transformer/reactor according to claim 2 or 3,
characterized in that an outer one of said layers is arranged in
such a way that it constitutes an equipotential surface surrounding
the conductor.
5. A DC transformer/reactor according to claim 4, characterized in
that said outer layer is connected to a specific potential.
6. A DC transformer/reactor according to claim 5, characterized in
that said specific potential is ground potential.
7. A DC transformer/reactor according to any one of claim 1, 2, 3,
4, 5, or 6, characterized in that at least two of said layers have
substantially equal thermal expansion coefficients.
8. A DC transformer/reactor according to any of the previous
claims, characterized in that the current-carrying conductor
comprises a number of strands, only a minority of the strands being
non-isolated from each other.
9. A DC transformer/reactor according to any one of the preceding
claims, characterized in that each of said two layers and said
solid insulation is fixed connected to adjacent layer or solid
insulation along substantially the whole connecting surface.
10. A DC transformer/reactor comprising a magnetic circuit, wherein
the magnetic circuit comprises a magnetic core and at least one
winding, characterized in that the winding comprises a cable (1)
comprising at least one current-carrying conductor (2), each
conductor (2) comprises a umber of strands, around said conductor
(2) is arranged an inner semiconducting layer (3), around said
conductor (2) is arranged an inner semiconducting layer (3), around
said inner semiconducting layer (3) is arranged an insulating layer
(4) of solid insulation, and around said insulating layer (4) is
arranged an outer semiconducting layer (5).
11. A DC transformer/reactor according to claim 10, characterized
in that said cable also comprises a metal shield and a sheath.
12. A DC transformer/reactor according to claim 11, characterized
in that the cable has a diameter comprised in the approximate
interval 20-250 mm and a conductor are comprised in the approximate
interval 80-3000 mm.sup.2.
13. A DC transformer/reactor according to any of claims 1-12,
characterized in that windings on different potentials are wound in
direct contact to each other.
14. A DC transformer/reactor according to any of claims 1-13,
characterized in that a winding on DC potential versus the core is
wound directly on, or very near the core.
15. A DC transformer/reactor according to any of claims 1-14,
characterized in that the magnetic circuits exposed for DC
magnetisation comprise at least one zone with lowered permittivity
in the magnetic main flux.
16. A DC transformer/reactor according to claim 15, characterized
in that the zone with lowered permittivity is accomplished with an
air gap arranged in said core.
17. A DC transformer/reactor according to claim 15, characterized
in that the zone with lowered permittivity is accomplished with a
series of small air gaps arranged in said core.
18. A DC transformer/reactor according to claim 15, characterized
in the zone with lowered permittivity is accomplished with a gap
arranged in said core, wherein said gap is made of a material with
a relative permeability, .mu..sub.r, which satisfies the expression
1.ltoreq..mu..sub.r.ltoreq..mu..sub.core.
19. A DC transformer/reactor according to any of claims 15-18,
characterized in that said core comprises an essentially
rectangular outer member having opposing side sections and
connecting end sections and further comprises a cylindrical center
member that is continuous in the longitudinal direction, said core
end sections having a pair of aligned circular apertures extending
completely there through into which opposite ends of said
cylindrical center member extend to define between at least one end
section and the respective end of said center member a radial
magnetic air gap of constant predetermined length for controlling
the permeability of said core.
20. A DC transformer/reactor according to any of claim 16, 17, or
19, characterized in tat each air gap is compensated by a
capacitively loaded compensation winding.
21. A DC transformer/reactor according to any of claims 1-20,
characterized in that said transformer/reactor also comprises a
housing including at least thyristor valve.
22. A DC transformer/reactor according to claim 21, characterized
in that all of said thyristor valves are of the integrated
type.
23. A DC transformer/reactor comprising a magnetic circuit, wherein
the magnetic circuit comprises a magnetic core and at least one
winding, characterized in that the winding comprises at least one
current-carrying conductor, and also comprising an insulation
system, which in respect of its thermal and electrical properties
permits a voltage level in said HVDC transformer/reactor exceeding
36 kV.
24. An integrated back-to-back station, characterized in that said
station comprises two transformers/groups of DC transformers
according to any of claims 1-23.
25. An integrated arrangement for transformation of high electric
power from one DC voltage level to another DC voltage level,
characterized in that said arrangement comprising a DC transformer
according to any of claims 1-23, wherein the DC transformer
comprising first valve windings and second valve windings, wherein
the first valve windings are connected to a first valve bridge and
the second valve windings are connected to a second valve bridge,
whereby the first valve bridge is operated as an inverter and the
second valve bridge is operated as a rectifier.
26. An integrated arrangement according to claim 25, characterized
in that said first valve bridge comprising at least one six-pulse
inverter bridge which includes a plurality of self-commutated
thyristors and further comprising a plurality of diodes, each diode
being connected antiparallel to a self-commutated thyristor, and
said second valve bridge comprising a six-pulse rectifier bridge
which includes a plurality of diode valves.
27. A DC transformer/reactor according to claim 13 or 14,
characterized in that the cable is specially adapted for mixed
voltage.
28. A reactor for DC plants according to claim 20, characterized in
that said compensation winding is loaded with a variable
capacitance so as to be able to vary the inductance of the
reactor.
29. A DC transformer/reactor according to any of claims 10-22, 27,
characterized in that said outer semiconducting layer (5) is cut in
a number of parts, each of which is connected to ground
potential.
30. A DC transformer/reactor according to any of claims 1-22, 27,
and 29, characterized in that the DC transformer/reactor also
comprising at least one sensor/transducer for monitoring and
diagnostics.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to a DC transformer/reactor in
accordance with the introductory part of claim 1.
[0002] The DC transformers/reactors have a rating power ranging
from several hundred kVA to 1000 MVA and above with a rating
voltage from 3-4 kV to very high transmission voltages, up to the
order of 1 MV.
BACKGROUND OF THE INVENTION
[0003] In order to describe a DC transformer/reactor according to
the invention a description of a conventional power transformer
will be given.
[0004] Reference may be made, for example, to the following well
known literature.
[0005] The J & P Transformer Book, A Practical Technology of
the Power Transformer, by A. C. Franklin and D. P. Franklin,
published by Butterworths, edition 11, 1990.
[0006] Regarding the internal electrical insulation of windings,
etc., the following can be mentioned:
[0007] Transformerboard, Die Verwendung von Transformerboard in
Grossleistungstransformatoren by H. P. Moser, published by H.
Weidman AG, CH-8640 Rapperswil.
[0008] The latter publication has been published by the insulation
manufacturer Weidman in Switzerland and belongs to the basic
literature of every transformer manufacturer.
[0009] From a purely general point of view, the primary task of a
power transformer is to allow exchange of electric energy between
two or more electrical systems of, normally, different voltages
with the same frequency.
[0010] A conventional power transformer comprises a transformer
core, in the following referred to as a core, often of laminated
oriented sheet, usually of silicon iron. The core comprises a
number of core limbs, connected by yokes which together 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 practically always concentrically arranged and
distributed along the length of the core limbs. As a rule the core
transformer has circular coils and a tapered leg section to utilize
the window space as well as possible.
[0011] In addition to core type transformers there are shell type
transformers. These are often designed with rectangular coils and a
rectangular core limb section.
[0012] Conventional power transformers, in the lower part of the
above-mentioned power range, are sometimes designed with air
cooling to carry away the unavoidable inherent losses. For
protection against contact, and possibly for reducing the external
magnetic field of the transformer, it is then often provided with
an outer casing provided with ventilating openings.
[0013] 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. An
oil-cooled and oil-insulated power transformer is therefore
surrounded by an external tank on which, as will be clear from the
description below, very high demands are placed.
[0014] Normally, means for water-cooling of the coil are
provided.
[0015] The following part of the description will for the most part
refer to oil-filled power transformers.
[0016] The windings of the transformer are formed from one or
several series-connected coils built up of a number of
series-connected turns. In addition, the coils are provided with a
special device to allow switching between the terminals of the
coils. Such a device may be designed for changeover 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 voltage, the
changeover switch is referred to as an on-load tap changer whereas
otherwise it is referred to as a de-energized tap changer.
[0017] Regarding oil-cooled and oil-insulated power transformers in
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 elements are
operated purely mechanically via a motor-driven rotating shaft and
are arranged so as to obtain a fast movement during the switching
when the contact is open and a slower movement when the contact is
to be closed. The on-load tap changers as such, however, are placed
in the actual transformer 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 less wear on the contacts, the on-load tap changers are
normally 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 connected to the
low-voltage side. Failure statistics of conventional oil-filled
power transformers show that it is often the on-load tap changers
which give rise to faults.
[0018] In the lower power range of oil-cooled and oil-insulated
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 because of
arcs during operation, etc., effect the whole oil system.
[0019] 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.
[0020] 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 practically zero in
the turn which is nearest the ground potential up to a potential in
the turns which are at the other end of the winding which
correspond to the applied voltage.
[0021] This 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 between
each turn and ground.
[0022] The turns in an individual coil are normally brought
together into a geometrical coherent unit, physically delimited
from the other coils. The distance between the coils is also
determined by the dielectric stress which may be allowed to occur
between the coils. This thus means that a certain given insulation
distance is also required between the coils. According to the
above, sufficient insulation distances are also required to the
other electrically conducting objects which are within the electric
field from the electric potential locally occurring in the
coils.
[0023] It is thus clear from the above description that for the
individual coils, the voltage difference internally between
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 capacitively distributed
voltages which may arise from a connected external electrical
system on the external connections of the transformer. The voltage
types which may enter externally comprise, in addition to operating
voltage, lightning overvoltages and switching overvoltages.
[0024] 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 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 possible for internally circulating
current components to be kept down at a reasonable level from the
loss point of view.
[0025] When designing transformers according to the prior art, the
general aim is to have as large a quantity of conductor material 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 material associated with
the coils, partly internally between the coils and partly to other
metallic components including the magnetic core.
[0026] The insulation system, partly within a coil/winding and
partly between coils/windings and other metal parts, is normally
designed as a solid cellulose- or varnish-based insulation nearest
the individual conductor element, and outside of this as solid
cellulose and liquid, possibly also gaseous, insulation. Windings
with insulation and possible bracing parts in this way represent
large volumes which will be subjected 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 breakdown, good knowledge of the properties of insulating
materials is required. It is also important to achieve such as
surrounding environment that it does not change or reduce the
insulating properties.
[0027] 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.
[0028] The transformer oil has a dual function since, in addition
to the insulating function, it actively contributes to cooling of
the core, the winding, etc., by removal of the loss heat of the
transformer. Oil cooling requires an oil pump, an external cooling
element, an expansion coupling, etc.
[0029] The electrical 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 is often a separate
component fixed to the tank and is designed 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.
[0030] It should be pointed out that the same requirements for the
insulation system as described above regarding the windings also
apply to the necessary internal connections between the coils,
between bushings and coils, different types of changeover switches
and the bushings as such.
[0031] All the metallic components inside a power transformer are
normally connected to a given ground potential with the exception
of the current-carrying conductors. In this way, the risk of an
unwanted, and difficult-to-control, potential increase as a result
of capacitve voltage distribution between current leads at high
potential and ground is avoided. Such an unwanted potential
increase may give rise to partial discharges, so-called corona,
Corona may be revealed during the normal acceptance tests, which
partially occurs, compared with rated data, increased voltage and
frequency. Corona may give rise to damage during operation.
[0032] The individual coils in a transformer must have such a
mechanical dimensioning that they may withstand any stresses
occurring as a consequence of currents arising and the resultant
current forces during a short-circuit process. Normally, the coils
are designed such that the forces arising are absorbed within each
individual coil, which in turn may mean that the coil cannot be
dimensioned optimally for its normal function during normal
operation.
[0033] 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 transformers are
manufactured for voltages of up to 20-30 kV and powers of up to
20-30 MW.
[0034] The insulation system of power transformers within the upper
power range requires, in addition to a relatively complicated
design, also special manufacturing measures to utilize 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 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 before it is lowered into a tank. After
this drying and impregnating process, the transformer is lowered
into the tank 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.
[0035] 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 manufacturing
time.
[0036] If electric discharges occur in an oil-filled power
transformer, or if a local considerable increase of the temperature
in any part of the transformer occurs, the oil is disintegrated and
gaseous products are dissolved in the oil. The transformers are
therefore normally provided with monitoring devices for detection
of gas dissolved in the oil.
[0037] For weight reasons large power transformers are transported
without oil. In-situ installation of the transformer at a customer
requires, in turn, renewed vacuum treatment. In addition, this is a
process which, furthermore, has to be repeated each time the tank
is opened for some action or inspection.
[0038] 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.
[0039] The insulating material in conventional power transformers
constitutes a large part of the total volume of the transformer.
For a power transformer in the upper power range, oil quantities in
the order of magnitude of several tens of cubic meters of
transformer oil are not unusual. The oil which exhibits a certain
similarity to diesel oil is thinly fluid and exhibits a relatively
low flash point. It is thus obvious that oil together 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.
[0040] 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 oil volume of
several decades of cubic meters and may have a weight of up to
several hundred tons. It is realized that the external design of
the transformer must sometimes be adapted to the current transport
profile, that is, for any passage of bridges, tunnels, etc.
[0041] The following problem areas regarding conventional
oil-filled power transformers will briefly be summarized:
[0042] A conventional oil-filled power transformer
[0043] 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 laced on the tank, since, without oil
but with a transformer, it shall be capable of being vacuum-treated
to practically full vacuum. The tank requires very extensive
manufacturing and testing processes and the large external
dimensions of the tank also normally entail considerable transport
problems;
[0044] normally comprises 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.;
[0045] 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 requirements made,
both regarding the outside and the inside of the tank;
[0046] 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 strand becomes as
equal as possible to neighbouring strands in order to minimize
induced voltage between neighbouring strands;
[0047] 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 varnish-based insulation nearest
the individual conductor element and, outside of this, solid
cellulose and a liquid, possibly also gaseous, insulation. In
addition, it is extremely important that the insulation system
exhibits a very low moisture content;
[0048] 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;
[0049] comprises oil which may entail a non-negligible fire hazard
in connection with internal partial discharges, so-called corona,
sparking in on-load tap changers and other fault conditions;
[0050] comprises normally a monitoring device for monitoring gas
dissolved in the oil, which occurs in case of electrical discharges
therein or in case of local increases of the temperature;
[0051] comprises oil which, in the event of damage or accident, may
result in oil spillage leading to extensive environmental
damage.
[0052] The DC transformers/reactors also have the additional
problem, that the electric field is a superposition of an A field
and of a DC field. The magnetic flux through the core may further
contain a dc component leading to quite large designs.
SUMMARY OF THE INVENTION
[0053] The object of the present invention is to solve the above
mentioned problems and to provide a DC transformer/reactor wherein
all space outside the cable screens are essentially potential free.
This object is achieved by providing the DC transformer/reactor,
defined in the introductory part of claim 1, with the advantageous
features of the characterizing part of said claim.
[0054] Accordingly, the winding of the DC transformer/reactor
comprises at least one current-carrying conductor, and each winding
comprises an insulation system, which comprises on the one hand at
least two semiconducting layers, wherein each layer constitutes
substantially an equipotential surface, and on the other hand
between them is arranged a solid insulation.
[0055] A very important advantage of the present invention, as
defined in claim 1, is that the use of the special winding makes it
possible to obtain a DC transformer/reactor with optimal design
regarding the thermal, electric and mechanic design. This results
in space reduction between windings with different DC potential,
space reduction between windings with DC potential and the core and
possible compensation of the air gaps at 50/60 Hz by a compensation
winding loaded with a capacitor. This will reduce the size,
especially of reactors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] Embodiments of the invention are described hereafter, in
association with the accompanying drawings in which:
[0057] FIG. 1 shows the parts included in the current modified
standard cable;
[0058] FIG. 2 shows a transmission scheme;
[0059] FIG. 3 shows the electrical field distribution around a
winding in a conventional transformer/reactor;
[0060] FIG. 4 shows a DC transformer/reactor in accordance with the
present invention;
[0061] FIG. 5 shows a reactor with a compensated air gap according
to the present invention;
[0062] FIG. 6 shows an isometric view of another embodiment of a DC
transformer incorporating an improved air gap arrangement; and
[0063] FIG. 7 shows the circuit diagram of an integrated
arrangement for transformation of high electric power from one DC
voltage level to another DC voltage level in accordance with the
present invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0064] An important condition for being able to manufacture a
transformer/reactor in accordance with the description of the
invention is to use for the winding a conductor cable with an
extruded electrical insulation comprising a semiconducting layer
both at the conductor and at the casing. Such cables are available
as standard cables for other power engineering fields of use. As
described under the summary of the invention, however, an improved
embodiment of such a standard cable will be used as a winding. To
be able to describe an embodiment, a short description of a
standard cable will first be made. The internal current-carrying
conductor comprises a number of strands. Around the strands there
is a semiconducting inner casing. Around this semiconducting inner
casing, there is an insulating layer of extruded insulation. An
example of such an extruded insulation is PEX or, alternatively,
so-called EP rubber. This insulating layer is surrounded by an
external semiconducting layer which, in turn, is surrounded by a
metal shield and a sheath. Such a cable will be referred to below
as a power cable.
[0065] A preferred embodiment of the improved cable is shown in
FIG. 1. Accordingly cable 1 is described in the figure as
comprising a current-carrying conductor 2 which comprises both
transposed non-insulated and insulated strands. Electromechanically
transposed, extruded insulated strands are also possible. There is
an inner semiconducting layer 3 around the conductor which, in
turn, is surrounded by an extruded insulation layer 4. This layer
is surrounded by an external semiconducting layer 5. The cable used
as a winding in the preferred embodiment has no metal shield and no
external sheath.
[0066] FIG. 2 shows a transmission scheme. In this figure there is
disclosed a transformer including two serie connected,
phase-shifted valve bridges 6, 7, wherein the valves are diode
valves.
[0067] FIG. 3 shows the electrical field distribution around a
winding in a conventional DC transformer/reactor. In FIG. 3 there
is disclosed a winding 8 wound around a core 9. The reference
numeral 10 represents equipotential lines of the electrical field
distribution of a conventional winding when the lower part of the
winding is on earth potential. The design and the placement of a
winding in relation to the core is mainly determined by the
electrical field distribution in the core window.
[0068] FIG. 4 shows a DC transformer/reactor in accordance with the
present invention. In FIG. 4 there is disclosed a three-phase
laminated core transformer. The core comprises in a conventional
manner, three core legs 20, 22, and 24 and the connecting yokes 26
and 28. In the disclosed embodiment both the core legs and the
yokes have tapered sections. The DC transformer comprises three
concentrical winding turns 30, 32, and 34. The innermost winding
turn 30 can represent the primary winding and the two other winding
turns 32 and 34 can represent the secondary winding. The DC
transformer also comprises spacing rails 36 and 38 with some
different functions. The spacing rails 36 and 38 can be made by
isolating material, which functions as a certain space between the
concentric winding turns for cooling, bracing. It shall be pointed
out that the DC transformer disclosed in FIG. 4, as opposed to the
transformer disclosed in FIG. 3, will not present any electrical
fields outside the cables of the windings.
[0069] FIG. 5 shows a principle sketch of a reactor for DC plants
with a compensated air gap in accordance with the present
invention. The reactor comprises a magnetic core 60 and a winding
62 comprising a cable in accordance with FIG. 1. The reactor also
comprises an air gap 64 in the core 60. Air gap means in this
context any way to achieve a zone with lowered permittivity in the
magnetic main flux. The reactor also comprises a compensation
winding 66 which is capacitively loaded with a capacitor 68.
[0070] Another way of achieving this zone with lowered permittivity
in the magnetic main flux is by reducing the air gap length, e.g.
by dividing the air gap into a series of smaller air gaps in order
to limit the radial components of the magnetic flux. Yet another
way of achieving this zone with lowered permittivity in the
magnetic main flux is to use another material than air, wherein the
material has a relative permeability .mu..sub.r which satisfies the
expression 1<.mu..sub.r<.mu..sub.core.
[0071] FIG. 6 shows an isometric view of another embodiment of a DC
transformer incorporating an improved air gap arrangement. The
magnetic core structure with an orthogonal or radial air gap for
low acoustic noise is shown in FIG. 3. This three-legged magnetic
core is comprised by an essentially rectangular one-piece outer
member 30 having opposing side sections 30s and end sections 30e,
and by a cylindrical center member 31 that is continuous in the
longitudinal direction A pair of aligned circular apertures or
openings 32 are machined into the two outer member end sections
30e, or alternatively the rectangular outer member is pressed into
this shape. The opposite ends of cylindrical center member 31
respectively extend into aligned circular apertures 32 to thereby
define at either end a radial magnetic air gap of constant
predetermined length that is further circumferentially continuous
and uninterrupted. A non-magnetic spacer 33 is mounted in the
magnetic air gap space between each end section 30e and the
respective end of center member 31 to maintain the radial air gap
and support the center member. A winding assembly wound directly on
the center member or on a toothed split bobbin, is placed about the
center member in the window areas 35 provided between outer and
center members 30 and 31. If desired, spacer 33 at one end can be
eliminated and a zero air gap provided at that end.
[0072] FIG. 7 shows the circuit diagram of an integrated
arrangement for transformation of high electric power from one DC
voltage level to another DC voltage level.
[0073] A DC/DC power transformer is provided which is an
arrangement for direct transformation of high electric power from
one DC voltage level to another DC voltage level without an
intermediate AC voltage network, The DC voltage is today basically
used for transmission of high electric power at long distances. The
DC voltage level for these transmissions are of the order of
several 100 kV. The DC/DC power transformer allows several DC
voltage levels to be used by connecting DC networks with different
voltages. The principle for this arrangement is that the valve
windings (43, 45) from one or several converter transformers (47)
are connected to two valve bridges, which generate opposing
cyclically variating magnetic flows in the transformer cores (44).
One of the valve bridges is operated as an inverter (42) and the
other as a rectifier (46) and in this manner the power is
transformed from one DC voltage level (U.sub.d1) to another
(U.sub.d2). At high voltage levels the leakage inductances in the
transformers will be high for conventional transformers as a
consequence of the large insulation distances and therefore special
arrangements must be made in order to commutate the magnetic energy
from one phase of the transformer to another without creating great
losses.
[0074] A DC transformer in accordance with the present invention
can be made with very low leakage inductances.
[0075] The DC transformer/reactor in accordance with the present
invention can e.g. be a HVDC or MVDC transformer/reactor.
[0076] The invention is not limited to the embodiments described in
the foregoing. It will be obvious that many different modifications
are possible within the scope of the following claims.
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