U.S. patent number 3,593,243 [Application Number 04/840,090] was granted by the patent office on 1971-07-13 for electrical induction apparatus.
This patent grant is currently assigned to High Voltage Power Corporation. Invention is credited to Bryon Lee Johnson, Brian Skillicorn, John George Trump.
United States Patent |
3,593,243 |
Trump , et al. |
July 13, 1971 |
ELECTRICAL INDUCTION APPARATUS
Abstract
An electrical induction apparatus, adaptable for use as a
transformer or a reactor which contains a magnetic circuit and an
electrical circuit. The active portions of the magnetic circuit are
formed of electrically isolated segments, each segment being
electrically connected to a distinct portion of the electrical
circuit such that when a voltage is impressed on the electrical
circuit a systematic and uniform progression of voltage is imposed
on the magnetic circuit. The electrical isolation appearing between
each segment of the magnetic circuit is coated with a material
adapted to establish electric field boundaries between the segments
and prevent the creation of high electrical stress points while
also preventing excessive eddy current loses.
Inventors: |
Trump; John George (Winchester,
MA), Skillicorn; Brian (Topsfield, MA), Johnson; Bryon
Lee (Chelmsford, MA) |
Assignee: |
High Voltage Power Corporation
(Burlington, MA)
|
Family
ID: |
25281421 |
Appl.
No.: |
04/840,090 |
Filed: |
June 2, 1969 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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567641 |
Jul 25, 1966 |
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Current U.S.
Class: |
336/60; 336/84R;
336/212; 336/184; 336/219 |
Current CPC
Class: |
H01F
30/12 (20130101); H01F 27/346 (20130101); H01F
27/324 (20130101); H01F 30/02 (20130101); H01F
27/343 (20130101); H01F 27/33 (20130101) |
Current International
Class: |
H01F
27/34 (20060101); H01F 30/06 (20060101); H01F
30/00 (20060101); H01F 30/02 (20060101); H01F
27/32 (20060101); H01F 27/33 (20060101); H01F
30/12 (20060101); H01f 015/04 (); H01f
015/14 () |
Field of
Search: |
;336/212,219,69,70,84,197,184,185,180,60,210 ;174/73,35.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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315,372 |
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1930 |
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GB |
|
390,500 |
|
1933 |
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GB |
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732,281 |
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Feb 1943 |
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DT |
|
Primary Examiner: Kozma; Thomas J.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of our present pending
application Ser. No. 567,641 filed on July 25, 1966, now abandoned.
Claims
We claim:
1. An electrical induction apparatus for connection to a
high-voltage AC power system comprising a plurality of magnetic
legs, each of said legs having a first and a second end, a magnetic
return interconnecting at least one end of said legs, at least one
of said plurality of legs being encompassed by at least one coil
means for connecting at least one coil between a high-voltage AC
power system and ground, means for placing a portion of each leg at
a potential related to the potential of the high-voltage AC power
system, means for maintaining at least one of said returns at
ground potential, at least one of said legs being comprised of a
plurality of magnetic segments electrically insulated from each
other by insulation means, each of said magnetic segments being
electrically coupled to the interior winding of a respective coil
to distribute any voltage developed across said winding, a portion
of said insulation means being subjected to both a resultant
electric field and active magnetic field that may be created in
said apparatus, first voltage surge distribution means positioned
on those portions of the insulation means abutting magnetic
segments and electrically coupled to respective magnetic segments,
second voltage surge distribution means positioned within said
insulation means in a plane parallel to said insulation means and
extending throughout said second voltage surge distribution being
electrically coupled to the exterior winding of the immediate coils
on either side of said insulation means, voltage surge distribution
means cooperating with said insulation means to create a dual
series capacitance between any two magnetic segments and said first
and second voltage surge distribution means having low eddy current
losses and distributing any created electrostatic potential
uniformly across said insulation means with controlled reduction of
electric stress.
2. The apparatus of claim 1 wherein each magnetic segment is
surrounded by at least one set of parallel connected coils and
wherein each of said sets is connected in series, said coil sets
being electrically connected at their interior winding to the
respective magnetic segment.
3. The apparatus of claim 2 wherein said insulation means comprises
sheet material and bar spacers having open spaces therebetween.
4. The apparatus of claim 1 wherein said insulation means comprises
insulators adapted to conform to said segments.
5. The apparatus of claim 1 wherein said insulation means has
electrically lengthened edges to increase the flashover, surface
creepage strength.
6. The apparatus of claim 1 wherein the voltage surge distribution
means comprises semiconducting layers.
7. The apparatus of claim 1 wherein said magnetic segments are
chamfered to approximate a Roqowski profile such that
intensification of magnetic gradients at the edges thereof is
substantially avoided.
8. The apparatus of claim 1 wherein there is provided on one of
said legs two windings one of said windings having an emf induced
therein when the other of said windings is coupled to said
high-voltage AC system.
9. The apparatus of claim 1 wherein a portion of said winding is
connected between one line of the power consuming portion of the AC
power system and ground.
10. The apparatus of claim 1 wherein each of said magnetic returns
extends at least to cover the planar area of said coils.
11. The apparatus of claim 1 wherein there is further provided
means for electrically insulating said coils independently of the
means isolating said segments to provide lateral freedom of
movement between said segments and said coils.
12. The apparatus of claim 1 wherein said coils are supported by
insulating means physically independent of the insulating means
supporting said segments.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to devices having time-varying
magnetic flux and, more particularly, to transformers and reactors
which use an insulated core.
Today, the pressing requirement for more and cheaper electrical
power faces growing technical and esthetic problems including the
national desire to maintain the attractiveness of populated areas.
To meet this continuing need for more electric power without adding
more generating and transmission systems within cities and suburban
areas, electric utilities are now building power plants in remote
regions close to the source of either large amounts of hydropower
or large coal deposits. This power can be transmitted to load
centers most economically by overhead transmission lines. However,
because of increased population density and pressure to preserve
the esthetic and economic values of the countryside, transmission
rights-of-way are increasingly difficult to obtain. The utilities
are thus compelled to increase several-fold the power transmitting
capacity of their existing lines, and to plan on still further
increases in power in the future. For these and other reasons, the
electric power industry is rapidly converting to extra high
voltages (EHV) for the transmission of electric power having
line-to-line voltages in excess of 345 kv. Already 500 kv. systems
have recently been built, and plans for 750 kv. systems have been
announced. Such high voltages permit the transfer of larger blocks
of power over extensive geographic areas. EHV interconnections will
also be used to even out peak demands over large regions and to
improve the reliability of the total system. Indeed, the trend
toward higher voltages is fundamental to meeting the predictable
power needs of the next two decades.
Although there are important technologic and economic reasons for
using EHV, serious difficulties have been encountered in designing
reliable terminal and line equipment for use at these high-voltage
levels. The simple extension of prior art concepts to EHV equipment
is proving inadequate and new concepts in power handling equipment
at EHV are clearly required. Especially needed are novel
transformers and reactors capable of reliable insulating
performance at these extra high voltages and characterized by more
efficient utilization of their materials and volume.
In its simplest form, a transformer consists of two conducting
coils having high mutual inductance. The primary winding is that
coil which receives electric power and the secondary winding is
that coil which delivers the power induced therein by currents
flowing through the primary winding. In normal practice these coils
are wound on a core of magnetic material. In EHV transformers, the
necessity of increasing the insulation between the high-voltage
windings and the grounded core adversely affects the operating
characteristics, the cost, and the insulation reliability of the
apparatus.
Additionally, it is essential that transformers built for EHV duty
be designed to avoid or withstand the greatly increased forces
associated with short circuits, voltage impulses, switching surges
and the like.
Attempts were made to solve these problems with prior art
transformer designs by increasing the required insulation and the
strike distances. One idea, for example, was to use a transformer
or reactor core of several sections wherein each section was
potentially graded by the surrounding winding and insulated one
from the other by sandwiching insulation between the core sections.
It was intended that the common insulation between the cores and
the windings provide an exact linear voltage distribution along the
winding column while the voltage on the insulation increases from
the edge towards the center and is constant beneath the core. In
this manner, a good capacitive division of pulse voltages and a
relative sensitiveness of the apparatus against overvoltages is
achieved. Furthermore, metal foils were placed on both sides of the
insulation layer in order to reduce the capacitive effect between
the windings. Although this apparatus combination is suitable at
lower voltages, at the extra high voltage levels contemplated by
the present invention, the intended result could not be achieved
because the creation of severe electrical stress at the corners of
the core sections would subsequently cause a breakdown in the
insulation. Even by providing an insulation layer thick enough to
withstand the extra high voltage, the reluctance of the magnetic
path would be so altered as to defeat all the benefits derived by
separating the core segments in the first place. Moreover, although
the metal foil on both sides of the insulation layer would tend to
enchance the capacitive effect between the windings, it would also
deteriorate the unit inasmuch as it fails to prevent the excessive
eddy current losses and to distribute the electrostatic potential
uniformly across the surface of the insulation. The resultant
heating would soon cause thermal failure of the unit. Suffice it to
say that conventional extensions of prior art apparatus have been
unacceptable in high voltage applications and leave serious
unanswered problems in the control of the electric field
distribution, the tendency toward excessive bulk and uncertain
insulation integrity, and proper heat and noise dissipation.
The dilemma remained without a solution until the present invention
was conceived which not only solves these problems but also
provides a novel EHV transformer of high electrical and spatial
(volumetric) efficiency. Moreover, the design can easily be
extended to handle even higher voltages than are presently
contemplated.
Large AC magnetic core transformers, as known to the prior art, are
highly efficient and practical power transforming devices whose
availability has made possible the modern AC power system. However,
when a conventional design is operated at the extra high voltage
contemplated by this invention, the voltage insulation problem
which can be readily managed at low voltages becomes difficult and
capable of culminating in catastrophic breakdown.
Similarly, a reactor for electric power systems is primarily a
high-voltage-high-power inductance coil used primarily to
constitute a high lagging power factor load. For the most part such
devices comprise a coil and a magnetic circuit so related as to
exhibit high reactance with low resistance. Reactors are usually
used as shunt reactors on long lines to supply or compensate for
line charging current. With the advent of EHV, shunt reactors carry
an increased importance. For example, in EHV systems, leading
currents can cause excessive voltages at the end of a long,
lightly-loaded line. Unless prevented, these excessive voltages can
create instabilities and subsequent failure in the terminal
apparatus. Shunt reactors connected as required on the line end
would have the desirable effect of preventing these instabilities
and failures.
The reactors known to the prior art generally were either of the
so-called shell design or the gapped-core design. The shell design
reactor consists of an air-core coil having a magnetic shell
surrounding it, while the gapped design comprised a modification of
this which included an iron core within the coil which was
intercepted by segments of stiff nonmagnetic material.
In the shell design, the coils of large diameter and radial buildup
are subjected to a high leakage flux with resultant severe eddy
current loss. In the gapped-core design, the lower reluctance of
the magnetic circuit generally results in lower winding losses but
the voltage insulation between winding and core is rendered far
more difficult. Saturation of core iron must be avoided both to
avoid losses and to insure constant inductance over the entire
operating voltage range.
Thus, until the present invention was conceived, the design of EHV
reactors was progressively more difficult and uncertain as to
insulating strength, reliability, freedom from corona and radio
noise. No clearly adequate solution for these problems was
discernable, especially when operating at their high voltage
limits.
SUMMARY OF THE INVENTION
Application of the principles of the present invention provides not
only excellent distribution and control of the normal AC operating
potentials but also excellent impulse voltage distribution. The
invention leads to relatively compact windings with fewer turns and
with relatively low losses due to leakage flux. Other features of
the present invention tend to reduce both acoustical and
magnetostrictive noise.
The principles of this invention also contribute to the avoidance
of saturation of elements of the magnetic core and thus make
possible a constant reactance over the full voltage range and the
nonintroduction of harmonic components in the reactor-current.
Additionally, low radio frequency noise levels can be achieved
because the electrical stress distribution is properly controlled
by the present invention and all parts of the electrified system
can be held below corona onset levels. Moreover, the present design
permits the return portion of the magnetic circuit to be at
maintained ground potential, thus simplifying the insulation and
physical support problem.
These and other advantages are accomplished in the present
invention while simultaneously reducing the overall size of the
required unit and its cost. All of these benefits are realized in
both transformer and reactors constructed under the present
invention by utilizing an insulating core concept in which the
active portions of the magnetic circuit are formed of electrically
isolated segments, each electrically connected to its surrounding
coil, to provide systematic and uniform progression of imposed
voltage on both the active portions of the magnetic core and its
associated electric circuitry.
DESCRIPTION OF THE DRAWINGS
The invention will be best understood and appreciated from a
perusal of the following description taken in conjunction with the
figures wherein:
FIG. 1 shows in diagrammatic section view a conventional
transformer.
FIG. 2 shows a diagrammatical section view of a transformer built
in accordance with the principles of the present invention.
FIG. 3 shows in detail one segment of the insulated core.
FIG. 4 shows in detail a lamination of the core of FIG. 3.
FIG. 5 shows detail of the arrangement of the transformer coils and
cores together with the insulating discs.
FIG. 6A shows in section an insulating disc suitable for use in the
invention.
FIG. 6B shows in section an insulating disc suitable for use in the
invention.
FIG. 7 shows the invention used as a three phase transformer.
FIG. 8 shows in section, one type of prior art reactor.
FIG. 9 shows in section, a different type of prior art reactor.
FIG. 10 shows a cutaway view of a reactor built in accordance with
the principles of the present invention.
FIG. 11 shows a partially broken away view of the operational
element of FIG. 10.
FIG. 12 is a view of the device of FIG. 10 taken along the lines
12-12.
FIG. 13 is a detail view of the operation element of FIG. 12 taken
along the lines 13-13.
FIG. 14 shows further additional detail of the reactor of FIG.
10.
FIG. 15 shows an additional view of the operational element of FIG.
10.
FIG. 16 shows in schematic form wiring connection suitable for use
in the invention.
FIG. 17 shows the detail interconnection of the pattern shown in
FIG. 16.
FIG. 18 shows in schematic form, a different wiring connection
suitable for use in the invention.
FIG. 19 shows the detail interconnection of the wiring pattern of
FIG. 8.
FIG. 20 shows a possible modification of the equipotential hoops
used in the invention.
FIG. 21 illustrates a possible improvement that can be used with
the present invention.
FIG. 22 shows a cutaway view of another reactor built in accordance
with the principles of the present invention.
FIG. 23a is a detailed view of three core elements of the reactor
of FIG. 22 with the surrounding coils.
FIG. 23b shows graphically the potential decrease through the coils
surrounding each core element.
FIG. 24 shows another geometric form of insulation layer which
could advantageously be used in the reactor of FIG. 22.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning first to FIG. 1 which shows in section a conventional
transformer, it can be seen that such a transformer basically
comprises a magnetic circuit 20 and a pair of current carrying
coils 22 and 23 contained in a housing 24. In such conventional
transformers the magnetic circuit 20 usually consists of a closed
laminated core of hollow rectangular shape in which transverse
nonmagnetic gaps are carefully avoided.
Such transformers operate as follows. A time-varying magnetizing
current in the primary coil 22, connected to a source of AC power,
produces a synchronously changing magnetic flux in the magnetic
circuit which in turn induces an electromagnetic force in the
secondary coil 23. Load current and power delivered by coil 23 is
matched by the input to primary coil 22 of a corresponding input
current and power plus associated losses. In order to minimize
I.sup.2 R losses the coils should be wound close to the magnetic
circuit. However, the electrical conductivity and great mass of the
core requires that the magnetic circuit be at ground potential;
thus adequate voltage insulation is required between the core and
the coils. As the operating voltages and transient overvoltages of
this type of apparatus are increased, the required insulation must
also be reliably increased. The coils are spaced progressively
further from the core with an attendant increase in winding losses.
Insulation barriers are introduced to control field distribution
and the migration of space charge and electrified particulates. The
insulation distances must be increased more rapidly than the rated
voltage. These factors cause the physical size of both coils and
core to increase thus adding to the size, weight and losses of the
unit.
The present invention restores the insulation harmony between core
and coil by maintaining them at close to the same potential at all
times irrespective of the voltage rating of the apparatus. This is
done by separating the active, or winding-bearing portion of the
magnetic circuit into core elements, mounting them in a stack or
column with each core element electrically insulated from its
neighbor by an adequate but relatively thin layer of high quality
dielectric. Each of these insulated magnetic elements has in close
proximity around it a proportional share of the total winding, with
the midpoint or some other point in this local winding being
electrically connected to its associated core section and firmly
establishing its potential at all times. In this way the stack of
insulated cores follows closely the potential distribution of the
associated total winding and the electrical incompatibility of
winding and core which characterizes conventional transformer and
reactor designs is almost totally avoided.
One embodiment of the present invention will now be described in
conjunction with FIG. 2 which shows a stepdown single-phase,
autotransformer, constituting one phase of a three-phase
wire-connected system, capable of handling extra high voltages and
built in accordance with the principles of the present invention.
In this figure the magnetic circuit 30 comprises a pair of magnetic
returns 31 and 32 which couple a pair of segmented legs 33 and 34
formed of a stack of magnetic core sections 35 and 38 electrically
isolated from one another by insulating discs 36.
Surrounding each leg is a pair of current carrying windings. The
autotransformer shown consists of four series windings 41, 42, 43
and 44 and four common windings 45, 46, 47 and 48. Series windings
41 and 42 and common windings 45 and 46 are mounted on segmented
leg 34 while the remaining windings are mounted on the other
segmented leg 33. Each winding is composed of a plurality of
current carrying coils 37 which surround a core section 35 and are
electrically connected thereto.
A central core 38, without a coil, my be provided in each segmented
leg to separate the series windings and to serve as a means of
introducing the high voltage to the series windings. High voltage
is supplied to the four parallel series windings and to cores 38 by
high-voltage lead 39 which is connected to a suitable AC power
source (not shown).
The output high-voltage tap 40 is connected to the junction of the
series windings and the common windings. The other lead from each
common winding is, in turn, connected to the closest magnetic
return 31 or 32, and to ground.
Turning now to FIGS. 3 to 5 the details of one form of construction
will be described. In this embodiment each core section is made up
from a multiplicity of rectangular silicon-steel laminations 50
formed with a plurality of holes 51 whereby the entire series of
laminations making up the core section may be united together. To
avoid magnetic saturation at the core section edges and to improve
the electric field distribution the narrow ends 52 and 53 of each
lamination may be formed with a Rogowski shaped profile. After the
stampings 50 are assembled, the corners of each core segment may be
chamfered and the edges 55 and 56 ground to assume a profile
similar to ends 52 and 53. Alternatively the core sections 36 could
be formed from a single strip of material spirally wound. The
actual dimensions of each core section are dependent on the power
to be handled and can readily be determined by one skilled in the
art.
Each coil, as shown in FIG. 5, is wound close to its respective
core segment.
The coils can be formed in two parts 57 and 58 from insulated
conducting strips spirally wound. In this embodiment coil half 57
is wound in one direction while coil half 58 is wound in the other
direction. The two halves are then electrically connected to each
other and to the adjacent core section by an appropriate conductor
59. Adjacent coils are joined by a suitable connector 60 to form
the total winding.
An insulating disc 36 is provided between adjacent coil-core pairs
so as to electrically isolate and insulate each coil-core pair from
the coil-core pair adjacent to it. Interposed between each coil
half is a second insulating disc 61 which has affixed thereto a
support 62 for maintaining an equipotential ring 63 around the
coils. Both the discs and the support 62 can be made of any
suitable insulating material. Examples of suitable materials are
laminated resin-impregnated paper and cross-linked polyethylene. To
prevent irregularities in the coils from creating high electrical
stresses across the discs 61 and mechanical damage to the discs 61,
a semiflexible coil spacer 65 can profitably be fitted around each
coil pair 57 and 58.
As indicated previously most of the difficulties encountered by the
prior art in attempting to produce reliable EHV transformers
centered around the necessity of separating and insulating the
high-voltage winding from the grounded iron core through which the
magnetic working flux flows.
The segmenting and electrical isolation of each core section from
adjoining sections solves the voltage insulation problem because
each core section and its surrounding coil are at the same
potential and no conflict in insulation strength exists between
them. Thus the need of large spacings and heavy insulation between
the coils and the cores is avoided. However, introduction of
insulation into the gap tends to increase the reluctance of the
magnetic circuit which tends to increase the requirement for high
magnetizing current and power and simultaneously permits increased
leakage of the magnetic flux. Increased leakage flux results in
increased reactance drop under load.
The reactance drop can be reduced by providing on each leg two
complete windings in parallel so that for a given total power
output per leg the load current per winding is halved. With this
arrangement, as shown in FIG. 2 the highest voltage level is
reached at the midpoint of each leg and only a very small portion
of the entire magnetic circuit is at this high potential. Although
this arrangement approximately doubles the active height of each
transformer leg, our studies show that the overall core utilization
is actually improved because a larger proportion of the core is
actively used for winding purposes. Moreover, each magnetic return
is now at ground potential and can be designed and supported more
economically and effectively. The insulated core and winding
construction defined by this invention has further important
advantages. By this series capacitance system better division of
surge potential is provided across the core and winding columns.
This improved surge division occurs because the insulated,
segmented coil-core arrangement constitutes a series chain of high
capacitance of nearly equal value. It is practical for the first
time to design this series capacitance system so that transient
overvoltages will be distributed with nearly exact uniformity along
the entire stack. It is therefore expected that higher BIL (Basis
Insulation Level) and increased insulation integrity can be
achieved under the use of the invention.
By controlling to greater uniformity the normal and transient
electrical stresses, the apparatus can be designed for subcorona
operation with the elimination of radio noise. It is to be noted
that preferably each coil, core and insulator is mechanically and
physically identical in shape and function to every other coil,
core and insulator. The repeated fabrication of identical subunits
is expected to further reduce costs and improve quality.
Preferably the insulator 36 will be molded in the form of a disc
with antitracking configurations of its outer periphery. Two such
designs are shown in section of FIGS. 6A and 6B. The disc of FIG.
6A comprises a planar disc of insulating material 70 which is
molded with a flared periphery 71 and coated on both sides with a
thin conductive layer of intermediate resistivity material 72.
Typically this coating 72 will have a resistance of between 5,000
to 50,000 ohms per square, thus preventing excessive eddy currents
in this coating. A beading 73 is then molded over the flare 71 and
the extremities of the coating 72.
The embodiment shown in FIG. 6B also comprises a planar disc of
insulating material. In this configuration however, the flare 76,
around its periphery, is provided with a plurality of corrugations
78 such that the surface of the flare presents a zigzag pattern and
thereby increases the electrical path length between opposite sides
of the disc. This disc is also coated with high resistivity
material 72.
This smooth conductive coating 72 in intimate contact with the
solid dielectric establishes the electric field boundary and thus
prevents irregularities in the laminations of the cores from
creating points of high electrical stress. The conductive coating
is designed to distribute the electrostatic potential uniformly
across the entire surface of the insulator 36 with controlled
reduction of electric stress at the edges.
In certain circumstances it would be desirable that the insulator
36 be made in two parts. One part would be a central disc having a
configuration as shown in FIG. 6A or 6B. The core segments would
abut this part. The other portion would comprise a ring concentric
with this disc which would isolate the coils from one another. This
ring would also have a sectional configuration such as shown in
FIG. 6A or 6B. This splitting of the insulator 36 would have the
beneficial result of permitting the cores and the coils to move
independently of one another thus providing a more flexible
design.
It should now become obvious to one skilled in the art that this
arrangement will also adapt itself for use with three-phase
circuits. In such three-phase applications the structure might well
appear as shown in FIG. 7. In this application three segmented core
legs 80, 81 and 82 are provided. EAch leg carries the necessary
number of coils and is magnetically interconnected to the other two
leg top and bottom delta shaped magnetic returns 83.
The present invention can also be advantageously used as a
reactor.
Shunt reactor needs are determined by the length of the
transmission line, its loading, and the general problem of var
control. They are also quite effective in helping to limit
transient overvoltages. On many EHV systems, generator reserve
requirements or pool reserve requirements will be such as to cause
the lines to be on standby service much of the time. In such cases,
shunt reactors will be essential to control system voltages.
The daily loading cycle will also affect the placement of this
reactive compensation. EVen during full load, many lines will
require permanently connected EHV reactors. At light loads,
additional reactive compensation will be needed as the terminal
voltages rises.
The better prior art reactor designs all utilize a so-called shell
or frame yoke. The basic elements of such a prior art reactor are
shown in FIG. 8. A coil 86 is made in the form of a hollow cylinder
surrounded by a laminated frame yoke 87. This coil is provided with
a high-voltage lead 84 at one end thereof and with a low-voltate
lead 85 at the other end thereof. Since good design dictates that
the yoke 87 be at ground potential it is necessary that the
high-voltage end of the coil be adequately insulated from the yoke
87 both at he end and the side. Simultaneously, this spacing must
be kept small if the flux lines are to be maintained in the
preferred orientation; that is, parallel to the coil axis. Such
requirements generally impede the use of this design for extra high
voltages. Furthermore, in designing reactors, it is found that the
rated volt-amperes (EI) are proportional to the product
.beta..sup.2 V where:
.beta. is the magnetic flux density in the coil
V is the volume of the magnetic field.
It is obvious that for a compact design it is preferable to sue
large values of magnetic flux density, .beta..
The air core, shell type reactor of FIG. 8 precludes the use of
high values of the magnetic flux density because of the high
reluctance of the air core of the windings. Consequently the
volume, V, is large and the flux density, .beta., is relatively
low. Also, the coil is physically large and is exposed to
practically the full value of magnetic field in some areas. These
factors result in a reactor which will have high resistive and
eddy-current losses in the coil.
In an attempt to avoid these problems, the gapped-core shell type
and the gapped-core core type reactors were tried. The gapped-core
shell type variation is illustrated in FIG. 9 and consisted of the
addition of an interrupted iron core 88 inserted in the center of
the coil 86. This core 88 is magnetically coupled to the yoke 87.
Pieces of a vary stiff nonmagnetic filler material 89 are inserted
in the core interruptions to maintain the core segments 79 in
spaced relationship. This design permits the use of values of
.beta. approaching those that will cause the iron to saturate.
Hence, the volume, V, is smaller. This design does not solve the
high voltage insulation problem between the coil and yoke nor does
it achieve good distribution of transient overvoltages. As a result
the overall unit becomes inordinately long when designed for EHV
duty. This increase in length of course adds considerable size and
weight to the unit.
The present invention specifically avoids the insulation problem
especially when the device is used for EHV duty and while operating
at high values of .beta. which permits a reduction in the weight
and physical size of the reactor. The consequent reduction in size
not only reduces the cost but also reduces electrical losses and
the inherent magnetostrictive noise. The efficiency is such that
electrical losses are only about one-half of those in conventional
reactors. The novel design employed in utilizing the present
invention further permits the application of powerful compressive
forces in the direction of the induced magnetic field to insure the
mechanical stability of the assembly and to reduce acoustical
noise.
Additionally the invention reduces the magnetic leakage flux while
providing improved and uniform voltage distribution of undersired
surges or impulses, thereby eliminating local areas of high voltage
stress.
Broadly speaking these and other advantages and features are
achieved in a reactor by providing a pair of parallel windings
around a magnetic circuit comprising a pair of magnetic returns
coupled by insulating magnetic core legs, and electrically and
progressively coupling the windings to the insulated core legs to
provide systematic and controlled distribution of the impressed
voltage along the legs.
A reactor built in accordance with the present invention is shown
as a cut away view in FIG. 10. This reactor comprises a housing 90,
of generally cylindrical form, mounted by legs 94 on a pad 95.
Passing through the top of tank 90 to the interior thereof is a
high-voltage bushing 91 and a low-voltage bushing 92. These
bushings may be of a conventional condenser type and are matched
mechanically, thermally and electrically to the operating element
97 contained within the housing 90. Affixed to the sides of tank 90
and having passageways connecting with the interior thereof are a
plurality of radially extending hollow core radiators 93. A
suitable insulating fluid 96 is provided within the tank 90 in
sufficient amount to cover element 97 and to circulate by
convection through radiators 93. These convection currents are
established in the fluid by heating of element 97 when power is
applied thereto; they may be further assisted by forced fluid
pumping. In addition the tank is provided with the usual associated
equipment (not shown) normally found on reactors. This equipment
includes thermometers, alarm circuits, pressure relief devices,
entrance and inspection ports, drain valves and the like.
One novel aspect of any reactor built in accordance with the
present invention resides primarily in the operating element 97
which is shown in greater detail in FIGS. 11, 12, 13, 14 and
15.
This element 97 basically comprises a magnetic circuit as shown in
FIGS. 11, 12, 13, 14 which is composed of a pair of laminated
magnetic returns 100 and 101 coupled together by a pair of
insulated core legs 102 and 103. Each insulated core leg comprises
a plurality of core segments 104 electrically isolated from one
another by discs 106 and spacers 107. Each core segment is built up
of strips as discussed in conjunction with FIGS. 3 and 4. Each core
segment 104 used in this reactor may have Rogowski shaped profiles
on all edges to prevent saturation in the cores and to permit
operation of the device at optimum flux density.
Each core 104, except for the central core 104A on each leg, is
surrounded by a current-carrying coil 108. These coils are
electrically connected to each other and to core segment which they
surround. Each coil-core pair is electrically isolated from one
another by discs 106 and spacers 107. These decks and spacers
further aid in positioning the cores and coils in spatial
relationship. These decks and spacers may be made of any suitable
insulating material such as laminated epoxy-impregnated paper
products or pressboard. Each deck 106 may, in turn, be surrounded
by an equipotential hoop 110 which when properly electrically
connected to the coils and cores on either side thereof will assist
in the distribution of surge or impulse voltage across the element
97.
The entire assembly is held together, in compression, by a
plurality of tension members such as tie bolts 98 which pass
through suitable brackets 99 affixed on each magnetic return 100,
101. In order to equalize the compressive forces applied to the
assembly by these bolts 98 a dummy coil 111 made up of suitable
insulating material may be provided around the central core 104A.
In most embodiments this central core 104A and the dummy coil 111
can be advantageously eliminated. Additionally equalizing may be
provided by making the spacers 107 of a resilient material capable
of permitting some lateral movement between each coil and its
respective core.
When the assembly 97 is placed in the housing 90 such that the
magnetic circuit is parallel to the base of the tank the spacers
107 are perpendicular thereto and a free vertical path 118 is
provided between these spacers 107. This path is such as to permit
free flow of the insulating fluid 96. Passage of the fluid along
these paths 118 cools and insulates the coils and cores. Because of
unavoidable losses in the assembly heating of the assembly occurs.
This heat is transferred to the fluid 96 by conduction. When the
fluid in path 118 becomes sufficiently warm convection currents
will be set up in the fluid such that it rises along paths 118 to
the top of radiators 93, downward through the radiators as it cools
and into the tank again at the bottom. This establishment of these
convection currents cool the assembly and keep it at a
predeterminable temperature.
It has been discovered however that with the intense electric field
encountered when the unit is used with extra high voltages that
long conducting hydrocarbon chains can be created in the fluid. The
existence of such chains is deleterious to the equipment and can
cause electrical breakdown between portions of the assembly and the
tank 90. To prevent this, insulating baffles 125 of pressboard or
other material, are provided around the exterior of the assembly as
shown in FIG. 15.
As noted previously each coil 108 is electrically connected not
only to each adjacent coil but also to its respective core 103.
Since these connections can be series, series parallel or parallel
a brief discussion should now be given in conjunction with FIGS.
16, 17, 18, 19 and 20. FIGS. 16 and 17 show the coils 108 on each
deck electrically coupled to adjacent coils so as to provide two
parallel windings on each insulating core leg to result in a total
of four parallel windings. As shown, the high voltage is introduced
to the central point of the combination via lead 109. By
introducing the high voltage into the center of the assembly,
interconnecting each coil to its adjacent core as shown in FIG. 17
while insulating each core segment from the next segment in the
stack, the frame yoke may be eliminated since the magnetic flux is
confined to the magnetic returns 100 and 101 and the insulating
core legs 102 and 103. By eliminating the yoke and by connecting
the coils to the cores the voltage insulation problem existing in
the prior art between the winding and the yoke is also eliminated.
The low-voltage power is extracted via lead 105. We have found that
a high-voltage reactor built in accordance with the present
invention can be lighter in weight and considerably smaller in
overall size than a conventional reactor of the same voltage and
half the power rating. Even more important, its insulation
reliability is inherently higher.
FIGS. 18 and 19 show the coils 105 on each deck serially connected.
This wiring arrangement is not preferred however because it
produces only two parallel windings for the entire unit and under
transient impulse voltage conditions does not have desirable
characteristics.
In either case the difficulties associated with prior art devices
when used for high-voltage duty are avoided and a progressive,
systematic, and preferably uniform distribution of voltage is
achieved across each insulating core leg from the midpoint thereof
to each magnetic return. This systematic voltage distribution is
also achieved under surge conditions due to the excellent voltage
division accorded by the large intercore capacities.
It should of course be understood that in either event the winding
sense of coils on each disc must be such as to cause the magnetic
field to travel in a closed loop as indicated by arrows 114.
FIG. 20 shows a modification of the coils 108, their
interconnection and the deck. Here the coil 108 surrounding each
core segment 104 is divided into two halves. One-half 119 is wound
in one direction and the other half 120 wound in the opposite
direction. The core 104 is then connected to the center point
between the two coil halves. It is, of course, necessary that each
coil half be insulated from the other half. Additionally the deck
106 can be a laminated structure built up of two sheets of suitable
insulating material 130, and 131 having a conducting grid 116
sandwiched therebetween. This grid 116 is configured to minimize
eddy currents. Such a sandwiched disc could be used with each of
the configurations described above including the transformer.
Insertion of this conduction grid 116 acts to capacitively couple
each core segment to its adjacent core segments to further improve
the voltage surge and impulse response of the unit. This view
illustrates still a further modification that could be utilized in
any of the above-described devices. In this modification, the
equipotential hoop 110 is replaced with an encompassing
hemispherical ring formed of an insulating material having a
conducting coating deposited thereon.
Returning momentarily to FIG. 15, additional features of the
baffles 125 will be discussed. These baffles 125 are shown shaped
to approximately conform to the electric equipotential field lines
existing between equipotential hoops of like voltage on each
insulating core leg. To assure that any generated conducting
hydrocarbon chains are broken the baffles are filled with a
suitable number of randomly placed foils 126. To provide adequate
flow, a multiplicity of orifices 127 are provided in each baffle.
This combination of orifice, foils and baffles creates a great deal
of turbulence in the fluid and this prevents the formation of
deleterious conducting chains.
It should be noted that a single large opening 124 is provided for
the high-voltage lead 109.
FIG. 15 also illustrates a modification that may be found
desirable. This modification comprises the addition of compression
springs 128 on the end of each tie rod 98 to assure that a constant
tension is applied across the insulating core legs at all times. A
further modification, not shown, is the additional cylindrical
insulation over the tension rods 98.
It should now be obvious to those skilled in the art that the
invention not only provides an improved reactor capable of EHV duty
but does so with significant savings in weight and cost. The
present invention thus permits the design of a reactor of
significantly smaller size since the full operating voltage is
applied so that each portion of the winding bears only its
proportional part of the total voltage under both normal and
transient conditions. Furthermore, by connecting each segment of
the insulated core to the windings the need for extensive
insulation between the coils and the core segments is eliminated.
This coupling of core segment with surrounding coils uniformly
varies the voltage down each leg such that the potential gradient
is constant down each leg thus making maximum use of the leg length
for insulating purposes.
Since core segment, deck structure and coils are identical the unit
lends itself to mass production methods and economy of
manufacture.
Since the deck material provided the function of electrical
insulation as well as mechanical support there is a large value of
interdeck capacitance which contributes to the uniformity of the
voltage distribution along each leg under impulse conditions. This
diminishes the impact of high-voltage transient stress in the unit.
Utilization of an insulating core provides additional advantages in
that it provides accurate control of the reactor inductance.
Additionally, this concept, by permitting uniform application of
the magnetizing ampere turns over the whole of the insulating
cores, reduces magnetic leakage flux.
The described mechanical configuration provides advantages in that
the unit, when mounted horizontally, can be cooled by natural
convection currents while simultaneously applying large compressive
forces to the unit thereby reducing both acoustical noise and
magnetostrictive noise.
It should be obvious that other modifications and adaptations can
now be made to described reactor. For example, the reactor could be
adapted to three-phase operation by using three legs each of which
has a high-voltage input lead in the center.
Still further as shown in FIG. 21, the shaped core 104 can be
encased in a molded solid dielectric 117 which is shaped in the
form of a spool. The coils are then wound on the spool and the
spools stacked to form the insulating core legs of the unit. If
desired the conductive grid 116 of FIG. 20 could be inserted
between each spool and connected to an equipotential hoop
surrounding the spool interface.
Referring now to FIGS. 22, 23a, a preferred embodiment of an
insulated core reactor built in accordance with the principles of
the present invention is shown and designated generally by the
numeral 180. This reactor comprises a magnetic circuit which is
developed through end yokes 182 and 184, and legs 183 and 185 each
of which comprise a plurality of core segments such as 186. Each
core segment 186 is surrounded by coils 189. Separation of the core
segments is achieved through insulating layer 191. The entire
assembly is enclosed in a tank 193 which holds the end yokes 182
and 184 and the legs 183 and 185 in compression. Connection is made
to the reactor by a high-voltage terminal 195 containing a
high-voltage line 220. For ease of movement, the reactor 180 is
mounted on a skid 197 which in itself may exert some compressive
force. In FIG. 23a three core segments 188, 190 and 192 are shown
in detail. Each core segment such as 190 is surrounded by a set of
four coils 194, 196, 198 and 200 electrically connected such that
the coils 194 and 196, and 198 and 200 each form a parallel set by
connections 202, 204, 206 and 208. The resulting parallel coil
combinations 194, 196 and 198, 200 are then connected in series by
connector 210 which also provides a connection to the core segment
190. By this parallel arrangement no potential difference exists
between corresponding turns in the space 212 between cores 194 and
196 and space 213 between cores 198 and 200. Accordingly, oil or
any other suitable cooling medium can be readily circulated
throughout the plurality of spaces such as 212 and 213 between
parallel core sets without any risk of insulation failure in these
areas.
Unlike the previously described induction apparatus, no dummy
center core is required. Moreover, as shown in FIG. 22, the core
segments 186 are of an even number and thus the high-voltage line
220 is made between the two center core sections 188 and 190. Thus
as previously described, the voltage would then decrease from the
high-voltage connection in both directions to the grounded end
yokes 182 and 184. It should be noted that the end yokes 182 and
184 are extended over the coils in order to carry the total
magnetic flux existing in the coil winding area. This feature has
the desirable effect of keeping the magnetic field from curving at
the end coils and thus reduces significantly any current loss which
would otherwise be present. In addition, the end yokes themselves
can now be used for clamping the coil assemblies together as well
as minimizing vibration and eliminating extra clamping devices.
For purposes of explanation, it may be assumed that the voltage
drop across each set of four coils is equal to
V=V.sub.2 -V.sub.1 where
V.sub.2 = the potential at connector 202
V.sub.1 = the potential at connector 206
and thus the voltage drop across any parallel set would be equal
to
(V.sub.2 -V.sub.1 /2)
Fig. 23b shows in graph form the voltage level decrease
corresponding to the coil sets 194, 196 and 198, 200, The voltage
decrease through the parallel coil set 194, 196 decreases from a
maximum value of V.sub.2 at the outer extremity to a value of
(V.sub.2 -V.sub.1 /2)
at the inner extremity which is likewise the constant potential at
which the core segment 190 is maintained. From the inner extremity
of the parallel coil set 198, 200 to the outer extremity the
voltage continues to decrease from a value of
(V.sub.2 -V.sub.1 /2)
to V.sub.1. As shown in FIG. 23a, the outer extremity of coil set
198, 200 is then connected to the outer extremity of the next coil
set 216, 218 through connector 209 and the same voltage decrease
then takes place throughout the remaining coil sections and core
segments until eventually the respective grounded yoke 184 is
reached. Similarly, the high-voltage line 220 is connected to the
coils surrounding core segment 188 and the voltage distribution
decreases similarly to its respective grounded end yoke 182.
Since the voltage decrease throughout each parallel set of coils is
equal to
(V.sub.2 -V.sub.1 /2)
the difference in potential between the inner extremity of parallel
coils set 198, 200 and parallel coil set 216, 218 is V.sub.2
-V.sub.1. Likewise, the potential difference between the outer
extremities of parallel coil sets 194, 196 and parallel coil set
198, 200 is also equal to V.sub.2 -V.sub.1. These potential gaps
are necessarily maintained by separating insulation layers which
will inhibit breakdown and assure this potential difference at all
times. Accordingly, insulation 191 is provided between adjacent
core segments which extend beyond that of the coil extremities.
Similarly, insulation 224 is interposed between coil sets in the
form of a washer-disc.
The insulation 191 is comprised of two layers 226, 228 each of
which has applied on the portion of the surface abutting the
respective core segment a conducting layer 230 which has an area
substantially equivalent to that of the surface area of the
respective abutting core segment. This conducting layer 230 takes
the form of a conducting grid and has applied thereon a protective
coating 232 which is in intimate contact with both respective
conducting grids 230 and the respective core segments and acts to
prevent shorting of the grid pattern with the respective conducting
core segments. Suffice it to say that without such a protective
coating the laminations of a core segment would be shorted and
cause a severe eddy current loss. This conducting grid 230 nearest
to core segment 190 is electrically connected to connector 210 and
core segment 190 such that the potential of both at all times is
equivalent.
Another conducting grid 234 is applied between the two insulation
layers 226, 228. This conducting grid extends substantially over
the entire surface area of the insulation layers 226, 228 such that
it not only covers the corresponding area between core segments but
also extends out and under the respective core sets to the
insulation edges where electrical contact thereto can be made such
as with the high-voltage line 220. This conducting grid between
insulation layers is electrically connected by connector 209 to the
outer extremities of the parallel coil set on the other side of the
insulation 191 and is at a potential equal to half of the potential
difference between adjacent core segments. Thus each dual
insulation layer 191 acts as a series capacitor and must withstand
a potential difference of V/2.
Under surge or transient conditions when a sudden voltage increase
appears on the high voltage line 220, the increase in potential is
quickly spread throughout the conducting grids 234. Inasmuch as the
capacitance of the coils is significantly greater than that
existing between adjacent core sections, the potential takes the
path of least resistance and will distribute itself capacitively
throughout the core sections to the grounded end yokes rather than
distributing itself through the surrounding coils with the
necessarily attendant damage that would follow due to the inability
of the voltage to distribute itself evenly in a sufficient time
interval.
The insulation layers with applied grids which separate various
core segments and core sections may take other forms than a flat
sheet. In FIG. 24, insulation layer 221 is shown which could
advantageously be used in the reactor of FIG. 22. The core segment
186 would fit within the center depressed area 223 and the coil
sets 189 would be adjacent to the outer extensions 225. The curving
up 227 of the insulation layer in the center eliminates areas of
intensification of the electric field. The actual shape of the
curve can be calculated and ideally would be an expotential curve
of infinite length, however, for practical purposes this can be
approximated by an arc of a circle of infinite length. This
configuration is substantially superior to a parallel or flat
insulation layer which would normally have a high electric field
intensification at the core segment edges. This type of field
intensification is well known in classical studies of electrostatic
field configurations at electrode edges and corners. Continual
field intensification will cause the surrounding insulating medium
to become ionized and will eventually lead to an accumulation of
surface charge on the insulation layer in the vicinity of the core
segment edges which would eventually cause a failure or puncture of
the insulation layer.
The grid pattern which is applied to the surface of the insulation
layer need not be restricted to any particular geometry form and
any number of grid patterns could be employed to produce the same
result. As previously stated, the function of the conducting grid
is simply to provide an electrostatic plane under surge or
transient conditions. It is essential that the grid distribute
evenly the electrical stress throughout the grid surface area and
that no voids occur which would initiate a breakdown. Accordingly,
the conductivity of the grid must be sufficiently low to permit
charge distribution across the equipotential plane and get the high
enough to avoid high eddy currents in the high intensity varying
magnetic fields. The resistance of the conducting grid must have a
resistivity such that the time constant is adequate for surge
distribution while at the same time being able to withstand a high
temperature environment of up to 120.degree. C. as well as being
nondeteriorating in an oil surrounding. The conducting grid has
advantageously been applied to the insulation layer by electrolytic
deposition but printed circuits, painting or other equivalent
methods may be employed.
Again, where the word coil is used in this description of the
invention, each coil can be composed of a plurality of smaller
coils, e.g. pancake coils, with or without their conductors
transposed in some predetermined sequence to further reduce the
losses caused by eddy currents flowing in the conductors.
Having now described a specific embodiment of the present invention
it is desired that it be limited only by the following claims.
* * * * *