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‘sept- 10, 1946-
Filed Sept. 9, 1943
Patented Sept-1Q, 1943
Hans Hartmann, Baden, Switzerland, assignor to
Aktiengesellschaft Brown, Boveri & Cie, Baden,
Application September 9, 1943, Serial No. 501,696
In Switzerland August 4, 1942
'7 Claims.
(C1. 175--356)
In plants where very high voltages occur it is
preferable to use voltage instrument transformers
present not be ?xed with regard to the allowable
breakdown voltage but was determined by the
leakage discharge voltage and this therefore ne
which are connected between a single phase and
cessitated a much stronger insulation.
earth. Several suitable arrangements for this
The difficulties described in connection with a,
purpose are known Where the voltage of the high Cl
voltage Winding increases steadily from Zero up
to the high voltage potential. If the active part
of such a transformer is located in an insulating
transformer with a closed U-shaped core and a
U-shaped insulation cover closely ?tted to the
high and low voltage winding are avoided ac
cording to the present invention by every kind
casing containing a liquid or gaesous insulating
medium, the surface of this part has a potential 10 of edge effect being prevented by means of a
controlled potential distribution, so that it is
distribution controlled by the winding.
possible to dimension the insulation solely out
It is also known that inside such a transformer
of consideration for its breakdown strength.
the potential gradient between the windings can
constructional examples of the invention. are
be taken up by a U-shaped insulating cover
which encloses the iron yoke on the high voltage
illustrated in the acompanying drawing in which
side and is open on the earth side, whereby the
Fig. 1 is a vertical sectional view of a high volt
potential increases continuously from the open
age transformer embodying the invention; and
Fig. 2 is a fragmentary vertical section through
an upper portion of the core and insulating cov
ends of the cover up to the high voltage end of '
the high voltage winding.
This insulating cover
can be located on a conductive support, for in 20 ering of a modi?cation.
In Fig. 1 of the drawing, the U~shaped iron
stance on the iron core which is also U-shaped.
core a is encased in a correspondingly U-shaped
With such transformers it is also known that it is
sleeve of insulation a’ and is closed by the yoke
an advantage if the windings lie in close contact
12. On both core limbs there is the cylindrical
with the insulating material. Furthermore, it
low voltage winding 0. The insulating cover d is
is also known to cover the low voltage inner side
a winding of fabric or paper type such as em
of the insulating cover, particularly over the sec
ployed in cable constructions, being wound di
tion which encloses the bent yoke and is free of
rectly upon the winding 0 and extending without
windings, with a metal layer.
interruption over the yoke arch of the iron core,
With these known constructions of high volt
age instrument transformers it is, however, not 30 the insulating cover being open only at both the
ends which abut against the lower yoke b.
yet possible to make the best possible use of the
The iron core and also the beginning of the
high break-down value Of the solid insulation be
high voltage winding e at the lower end are at
tween the low and high Voltage. If there are oil
earth potential. The winding e is a cylindrical
gaps between both windings or between one wind
ing and the insulating cover, the insulating ; winding composed of disc coils, the potential of
this winding increasing from the bottom to the
strength of these gaps is much lower than that
top where the highest potential prevails, this
of the paper insulation; nevertheless these oil
being connected to the high voltage terminal 1‘.
gaps are subjected to a higher ?eld strength than
In order to control the potential at the bend of
the paper insulation. Furthermore the insula
tion problem is also considerably complicated 40 the insulating cover where there is no winding
and in order to avoid edge effects the insulating
by the instability of the electrical ?eld caused
tube 11 is provided not only on the inside but
by the winding edge at the high voltage end of
also on the outside of this bend with conductive
the high voltage winding cylinder. Because even
layers g, h, the outer layer g being maintained
if the oil gaps are avoided due to the windings
being in close contact with the insulating cover ' at high voltage potential by means of a con
ductive ‘connection whilst the inner layer It is
corona and leakage discharges occur at this edge
at earth potential. The transformer rests on a
which are very di?icult to control and cannot be
base plate is and is enclosed in a casing of ce
prevented by merely providing the inside of the
ramic insulating material 2'. The hollow space in
insulating cover with a metal layer. The usual
arrangement of rounded protective rings on the 50 side this casing can be ?lled with insulating liquid
or gas.
winding edges can increase the initial voltage
The conductive layer it can of course cover
at which corona occurs but does not, however,
prevent dangerous leakage discharges. The dis
the entire inner surface of the insulating cover
tance and the insulation strength between both
07.. The layers can consist of metal or some con
voltage carrying parts could therefore up to the 55 ductive material, for instance the layers 9', h’ of
Fig‘. 2 comprise varnish, mixed with metallic
powder. They can for instance be made in such
a manner that at least for the bent part of the
cover it a conductive band is employed for the
innermost and outermost layer, this band being
either a metal band, a metallized band or a band
which is impregnated or painted with a conduc
tive material. The layers can also be wound
wound upon the legs of said core, a tubular in
sulating cover closely ?tting said secondary sec
tions and the portion of the core between the
same, a high voltage primary winding on the legs
of said core and closely ?tting said insulating
cover, said primary winding comprising a plural
ity of serially connected disc coils, with a low
voltage terminal adjacent an end of a leg of
the core and a high voltage terminal adjacent
The layers 10 the bend of the U-shaped core, and means sup
on or into the insulating cover 12 or pressed on,
sprayed or painted on to the cover.
g, h can also be made so as to allow liquid or
gas to pass through, for instance in the form of
pressing edge eliects at the high voltage ends
wire netting and the like. It is furthermore also
possible to make the layer on the outside (high
ing layers at and closely adjacent the portion of
the insulating cover enclosing the bend of the
of said windings, said means comprising conduct
voltage side) in the form of a bare or covered 15 core and not covered by the primary.
winding which in the latter case also serves as
the ?rst turn of the high voltage winding.
With the arrangement according to the‘inven
2. A high voltage transformer as recited in
claim 1 wherein the outer conducting layer is
electrically connected to the adjacent high po
tion all parts of the insulating cover are stressed
tential end of the primary winding and the
perpendicularly to the wall and solely as regards 20 inner conducting layer is grounded.
breakdown. The U-shaped form of the iron core
3. A high voltage transformer as recited in
and cable-like insulating cover d surrounding the
claim 1 wherein said conducting layers are me
core enables the ‘potential distribution along its
surface to be arranged in such a manner that
4. A high voltage transformer as recited in
the voltage increases from zero value at the lower 25 claim 1 wherein said conducting layers comprise
open ends of the cover to a maximum value at
varnish containing conductive particles on the
the closed upper end. Since the outer surface of
the insulating cover d has high voltage potential
at its upper end, the core a which is at earth
inner and outer surfaces of said insulating cover.
5. A high voltage transformer as‘ recited in
claim 1 wherein said conducting layers are fluid
potential is completely screened from the upper 30 permeable layers of conductive material at the
metal cap which closes the insulating casing z‘
inner and outer surfaces of said insulating cover.
and carries the high voltage terminal f.
6. A high voltage transformer as recited in
A further advantage of the arrangement de
claim 1 wherein said potential gradient means
scribed is that due to the short distance between
comprises a conductive winding at a surface of
the windings the short-circuit voltage becomes 35 the
'bend portion of said tubular insulating cover
so small that even with an iron core having a
and connected electrically to the adjacent ter
comparatively small cross-sectional area a very
minals of the primary transformer windings on
large measuring power can be obtained. At the
the respective legs of the core.
'7. A high voltage transformer as recited in
1 wherein said outer conducting layer com
tained as regards the voltage.
prises a wire wound upon the bend portion of
I claim:
said tubular insulating cover and connected elec
1. A high voltage transformer comprising a
to the adjacent terminals of the primary
grounded U-shaped core closed by a yoke, a low
same time the radial dimensions become reduced
despite the large degree of ‘safety which is ob
voltage secondary winding including sections
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