Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
ELECTRICAL TRANSFORMER HAVING
CORONA SHIELDING MEANS
BACKGROUND OF THY INVENTION
Field of the Invention:
_
This invention relates generally to electrical
inductive apparatus and, more particularly, to trays-
former having corona shielding means for shielding edges of coil windings of electrical inductive apparatus.
Description of the Prior Art:
Corona is an ionization phenomena and occurs as
a result of an emission of electrons from the surface of
lo electrical conductors at high potentials, and is dependent
upon the curvature of a conductor surface with most ems-
sons occurring from sharp points when the electric field
strength is high enough to cause a breakdown of the sun-
rounding air. Such a breakdown is a discharge of current
and is particularly undesirable due to its deteriorative
effect upon surrounding electrical insulation. The disk
charges commonly cause current pulses which, in turn,
result in power losses and radio interference.
A transformer core normally consists of famine-
lions or thin sheets of electrical steel. Though such adore provides optimum magnetic properties, the many edges
of the laminations have the detrimental effect of product
in voltage stress concentrations.
Moreover, many prior coils were constructed of
an epoxy spool which provided for winding the coil in four
r I 3
discrete sections. One inherent problem with such a con-
struction was the lack of an orderly Jinxing pat-tern,
i.e., random winding. Another problem was the absence of
any high-voltage shielding.
SUMMARY O THE INVENTION
It has been found in accordance with this invent
lion that the foregoing problems may be overcome by pro-
voiding an electrical transformer comprising a phase wind-
in disposed in inductive relation with a magnetic core
having at least one leg portion; the phase winding having
low-voltage and high-voltage coils on the core; the coils
being concentrically wound on and around the core leg
portion; each core having inner and outer layers wound
between opposite edges; the outer coil having opposite
edges indented within corresponding edges of the inner
coil; a plurality of layers of electrically insulating
material between the low- and high-voltage coils; a first
corona shield formed of at least one layer of electrically
conductive, non-magnetic material disposed within the
layers of electrically insulating material; a second
corona shield formed of at least one layer of electrically
conductive, non-magnetic material disposed around the
outer coil; and a semi-conductive tape enclosing the core
so as to provide a resilient ground plane and tug enhance
voltage radiant between high-voltage winding and ground.
The advantage of the device of this invention is
the elimination of sectionalized electrical failure by
using an avalanche effect and rapidly increasing the fault
current to thereby open an associated fuse quickly and0 avoiding gas generation and its usual explosive effect.
GRIEF DESCRIPTION OF TOE DRAWINGS
figure 1 is a perspective view of electrical
inductive apparatus;
Fig 2 is a vertical sectional view, taken on
I the inn II-II of Fig. 1, and showing in broken line a
surrounding envelope of a thermosetting resin;
Fig. 3 is a vertical sectional view, taken on
the line III-III of Fig. 2;
Fig. 4 is an enlarged sectional view of a port
lion of the high-voltage winding shown in Fig. 3;
Fig. 5 is a circuit diagram;
Fig. 6 is a graph of applied voltage vs. number
of layers of insulation for an unshielded coil; and
Fig. 7 is a graph of applied voltage vs. number
of layers of insulation for a shielded coil structure.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Electrical inductive apparatus, such as trays-
former, include electrical windings disposed in inductive
relation with a magnetic core. The magnetic core includes
at least one opening for receiving electrical windings,
lo and is formed of a plurality of stacks of metallic famine-
lions, such as grain oriented silicon steel, with the
stacks being arranged to form a complete magnetic circuit.
The edges of the magnetic core which face the windings
normally include many sharp edges as a result of prior
shearing of the laminations from stock, which edges form
sharp electrodes that are a possible source of corona
discharge or streamers. Corona may seriously erode and
degrade the solid insulation on the adjacent windings,
accelerating the breakdown process.
A transformer generally indicated at lo (Figs.
l, I) includes first and second magnetic core sections 12,
14 disposed in side-by-side relation. Each magnetic core
section includes a plurality of stacks of. superposed
magnetic laminations in the conventional manner. The
stacks are disposed to form complete magnetic circuits
about a plurality of openings 16, 18 for receiving elect
tribal windings 20, 22. Although the transformer 10 is
disclosed and described as a single-phase electrical
transformer having two high-voltage coils for line-to-line
applications, it is understood that the transformer may
comprise one high-voltage coil for line Toronado applique-
lions.
I
In accordance with this invention a winding
assembly (Fig. 3) includes a low-voltage coil I insular
-lion layers 26, a corona shield 28, the high voltage
windings 20, 22, and outer corona shields 30, 32. In
addition, each of the core sections 12, 14 is covered with
an insulative coating 34, 36, respectively (Fig. 2).
Inasmuch as the cores 12, 14 are comprised of stacks of
laminating thin sheets that are stamped from electrical
steel stock, they have a detrimental effect of many sharp
edges which result in voltage stress concentrations for
which reason the cores are wrapped with insulative coat-
ins 34, 36 which are comprised of a butyl-backed semi-
conductive tape. Thus, a smooth and resilient ground
plane is provided to enhance the voltage gradient between
live and grounded components.
The low-voltage coil 24 is comprised of at least
-two layers of wire which is spirally wound with each layer
being a continuation of an adjacent layer. Leads or
cables 38, 40 (Fig. 3) extend from the coil 24 in a con-
ventional manner. The wire comprising the coil may have either round or rectangular cross section, the latter
being preferred for a more even low-voltage ground plane.
The insulation layers 26 are wound around the low-voltage
coil 24 and more directly upon a layer or coating 42 of
dielectric material such as one layer of a polyester
composite. Opposite sides 44, 46 are coextensive with
opposite sides of the low-voltage coil 24. The spirally
- wound insulating layers 26, preferably polyester film,
~-~. include opposite sides which are indented to provide a
margin between said sides and the opposite sides 44, 46 of
the layer 42, thereby avoiding breakdown of a medium over
an intervenirlg solid insulating surface which is known as
creep age breakdown.
A second layer 48 of insulating arterial similar
to the layer or coating 42 is applied around the outer
surface of the spirally wound insulating layer 26. The
insulative layer 48 includes similar opposite side port
~99~
lions 50 which extend beyond opposite sides of the insulating layers 26 but within the extensions of the sides
44, 46 of the layer 42. 'the opposite edges of the layer
48 are indented to eliminate internal electrical tracking.
Like the layer 42, the layer 48 is comprised of a polyp
ester composite and is disposed between the slayer 26 and
the corona shield 28.
The shield 28 is a cylindrical or sleeve-like
member around the outer surface of the layer 48 and prey-
drably has similar edges 52 which are indented within the side portions 50 of the layer 48. The shield 28 is metal-
fig and is composed of either copper or aluminum and
preferably the latter. Opposite ends of the shield 28 are
connected to the midpoint of high voltage golfs 20 and 22
lo as shown in Fig. 5.
The layer 54 of semi-conductive material, such
as crepe tape, is wound around the outer surface of the
shield 28. The layer 54 covers the shield completely from
edge to edge to grade off any sharp edges in the shield.
Parts 24, 26, 28, 42, 48 and 54 are collectively indicated
at 55 in Fig. 2.
The high-voltage windings 20, 22 are wound
around the outer surface of the semi-conductive layer 54
and spaced from each other as shown in Fig. 3. Each
winding 20, 22 is comprised of uniformly wound layers of a
metal wire having a high coefficient of electrical conduct
tivity, such as copper, for many multiple turns per layer.
For example, 200 turns per layer may be provided with a
No. 29 gauge wire (0.113 inch) for, say, thirty layers.
Each layer is spirally and helically wound upon a precede
in layer and is a continuation thereof in a conventional
manner. Between each layer insulation is provided (Fig.
4) in the form of layers 56 of insulative material, such as
friction-coated polyester film to prevent wire slippage
during winding of each consecutive layer. Thus, kinesic-
live layers 58 of wire winding are insulated by the layers
56.
Several outer layers of each winding 20, 22
comprise only one turn 60 per layer with insulative layers
56 there between. The outer one-turn-per-layer structure
improves the voltage surges and impulse strength of the
overall transformer structure.
The corona shield 30 which is comprised of a
strip of metal such as aluminum and cased in polyester
film is wound around the outer single turn layer 60 and is
connected at one end to the end of the single turn 60 and
at the other end to a large diameter bus wire 62, thereby
providing a uniform high-voltage plane-to-ground and good
mechanical connection with the coil.
A conductive layer 64 surrounds each outer
corona shield 30 and is preferably comprised of a semi-
conductive tape to grade off any non-uniformity in the
shield construction. The insulative layers 26, 54, 56 are
layers of a resinous film, such as polyester film without
fibers. On the other hand, the layers 42, 48 are fibrous
composites for hi-lo insulation. Also, layers 54 and 60
are fibrous composites, semi-conductive in nature.
Inasmuch as the core sections 12, 14 are come
prosed of stacks of laminated steel having a thickness of approximately 11 miss, there may be sharp edges that are
detrimental to providing a uniform ground plane which
results in voltage stress concentrations. For that reason
the butyl-backed semi-conductive tape 34, 36 is wrapped
around the cores. This construction provides a smooth and
resilient round plane as well as enhancing voltage grad-
tent between the high-voltage windings and ground. More-
over, the buty].-backed semi conductive tape 34, 36 allows
mechanically for differentiation in coefficients of expand
soon between the core and an outer epoxy encapsulant 56.
Finally, in order to control inter-turn and
inter-la~er voltage stresses during impulse surges, start
and finish shields are incorporated within the windings.
Without the shields the voltage distribution would be
non linear with most of the voltage being across the first
few turns of the first layer such as shown in Fig. 6. To
produce a uniform response and eliminate the voltage
concentrations across the layers, the impressed outage
must be uniformly graded radially from line to ground and
constrained axially to be parallel with the windincJ
layers. This is accomplished by positioning concentric-
cylindrical shields at the start and finish of the wind-
ins. As a result a substantially uniform radial voltage
gradient (Fig. 7) is produced because the field is con-
trolled between two cylinders and approximates the grad-
tent between the parallel plates. Moreover, equipoten-
trials (voltage) are constrained in an axial direction lobe parallel with the major portion of each layer so that
little voltage appears between turns and the normal inter-
turn insulation is adequate.
By using the outer shields 30, 32 as turns on
the windings 20, 22 a back EM created by an ungrounded
(floating) turn is eliminated. The shield itself is
comprised of a polyester-aluminum-polyester laminate
thereby providing insulation and stress control. Terminal
lion of each shield 30, 32 is accomplished with a self-
piercing crimp which insures a mechanical as well as
- electrical connection.
In conclusion, toe shielded layer winding is
characterized by an excellent surge response, and an
insulated structure which is well suited to withstand the
directional stresses developed during both steady-state
and transient conditions. The electrostatic fields pro-
duped are such that the winding is always stressed in the
direction of its maximum strength, normally to the round
plane of the core.
