High-voltage voltage transformer with shields

A high-voltage voltage transformer with a core at high voltage which is surrounded coaxially by a high voltage winding provided with a slotted metal shield and with a low voltage winding also coaxially surrounding the same, as well as with a coaxial shielding electrode at ground potential and surrounding the high and low voltage winding is to be so constructed that coupling effects of high potentials for high frequency transient switching operations onto the secondary side, above all the secondary lead-out connecting lines cannot occur to any disturbing extent. This is achieved in that spaced at a slight distance up to a maximum of about 5 mm. from the metal shield a discharge electrode concentrically surrounding the same is provided insulated with respect to the metal shield. The discharge electrode is electrically conductively connected by way of one or several connecting lines of low inductance with the shielding electrode. The low voltage winding is provided with a further slotted metallic shielding which is also electrically conductively connected in a manner low in inductance with the shielding electrode (FIG. 2).

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to a high-voltage voltage transformer, 
especially to a combined high-voltage current- and voltage-transformer of 
head-type construction with a core at high voltage that is surrounded 
coaxially by a high-voltage winding having a slotted metal shield and with 
a low-voltage winding also coaxially surrounding the high-voltage winding 
as well as with a coaxial shielding electrode at ground potential and 
surrounding the high- and low-voltage winding. 
Such high-voltage voltage transformers are known as such. With combined 
current- and voltage transformers of head-type construction, it is 
customary to mount the active parts of the transformer consisting of a 
core and of a high- and low-voltage winding on an insulating column and to 
surround the same with a head housing. The insulating column is secured at 
a base part which carries the terminal box with the terminal panel for the 
lead-out connecting lines of the transformer. The secondary lead-out 
connecting lines from the head housing of the transformer to the terminal 
panel are therefore relatively long. This is true in particular for 
combined high-voltage current- and voltage-transformers of the type of 
construction according to the older, non-prepublished patent application P 
36 08 390.9, which corresponds to U.S. Pat. No. 4,731,599, of the assignee 
of this application in which the voltage transformer is arranged above the 
current transformer. 
As a result of the large distance conditioned by reason of the necessary 
fastening elements for the low-voltage winding between the metal shield of 
the high-voltage winding and of the low voltage winding, the capacity 
between the metallic shields of the two windings is relatively small. 
It has now been discovered with such arrangements that during transient, 
especially high frequency occurrences, in particular triggered by 
switching operations, the metal shield of the high voltage winding of the 
voltage transformer which as such is at low voltage potential, can be 
raised to a high potential of some 10 kV up to 100 kV or more. 
The consequence is that during the occurence of high potentials by reason 
of high frequency transient surge or over-voltages at the metal shield of 
the high-voltage winding, these high frequency voltages may be coupled 
into the low voltage winding. This may even lead to the destruction of 
such high-voltage voltage transformers if the insulation between the 
metallic shielding and the low-voltage winding is destroyed. 
Accordingly, it is the object of the present invention to so improve a 
high-voltage voltage transformer, especially a combined high-voltage 
current- and voltage-transformer of head-type construction of the 
aforementioned type, that coupling effects of high potentials, especially 
triggered by high frequency transient switching operations, onto the 
secondary side of the transformer, especially onto the secondary 
connecting terminals can no longer occur to any troubling extent. 
The underlying problems are solved according to the present invention in 
that the metal shield is connected with the connecting terminal for the 
high-voltage winding, a discharge electrode concentrically surrounding the 
metal shield is provided insulated with respect to the metal shield at a 
small distance of up to 5 mm. from the metal shield, the discharge 
electrode is electrically conductively connected low in inductance with 
shield electrode by way of one or several connecting lines. 
The present invention assures that a very large capacity is formed between 
the metal shield of the high voltage winding and the discharge electrode 
of the voltage transformer. The discharge electrode is electrically 
connected with the outer shielding electrode along the shortest path and 
therewith extraordinarily low in inductance. A rise of the potential of 
the metal shield of the high voltage winding to non-permissively high 
values is thereby reliably prevented and a coupling of non-permissively 
high transient over-voltages into the low voltage winding is prevented.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring now to the drawing wherein like reference numerals are used to 
designate corresponding parts in the various views, a combined high 
voltage current- and voltage-transformer is illustrated in FIG. 1. It 
consists of a base part 1 with a terminal box 3 containing a terminal 
panel 2. A support insulator 4 is sealingly and securely arranged on the 
base part 1, which carries at the top a metallic closure plate 5. A 
U-shaped primary conductor 6 with the legs 7 and 8 and the base 9 is 
secured on the plate 5, whereby the one leg 8 is electrically conductively 
connected with the closure plate 5 and the other leg 7 is electrically 
insulatingly lead out and adapted to be contacted from the outside. 
The base 9 of the primary conductor 6 is concentrically surrounded by a 
secondary winding 10 having several cores. The primary conductor 6 and the 
secondary winding 10 form a high voltage current transformer whose 
secondary lead-out connecting lines are extended to the terminal panel 2. 
The connections 11 of the legs 7 and 8 and therewith the closure plate 5 
are at high voltage potential. 
A high-voltage voltage transformer 12 is arranged above the high-voltage 
current transformer 6, 10. The high-voltage voltage transformer 12 is 
electrically connected with the high-voltage current transfomer 6, 10, for 
example, by means of a connecting member 13 whereby the lead-out 
connecting lines of the voltage transformer 12 are arranged in the 
connecting member 13 and are then extended to the terminal panel 2, like 
the secondary lead-out connecting lines of the current transformer 6, 10. 
The voltage transformer 12 consists in a known manner of an iron core 15 of 
magnetizable material. The high voltage winding 16 is applied about the 
one leg thereof as cylinder-, banked- or trapezoidal winding. The same is 
surrounded concentrically by a low-voltage winding 17. These active parts 
of the entire voltage transformer 12 are surrounded concentrically by a 
ring-shaped, respectively, cylindrical shielding electrode 18 disposed at 
ground potential. 
A pot-shaped single- or multi-partite housing 19 is sealingly and securely 
attached on the closure plate 5 which surrounds in common both 
transformers 6, 10 and 12 and is at high voltage potential. 
A section of the voltage transformer of a combined high-voltage current- 
and voltage-transformer of head-type construction according to FIG. 1 is 
illustrated in FIG. 2. Of course, this voltage transformer can also find 
application without being combined with a current transformer. The last 
winding or winding layers of the two-step high-voltage winding 16 
surrounding the iron core is provided with a slotted metal shield 20 
extending over the winding length thereof. The metal shield 20 consists of 
a layer of electrically good conducting material, for example, copper, 
silver or zinc, or of a metal layer in the form of a metal film or of a 
metal cylinder. The metal shield 20 is slotted in a known manner in the 
direction of the longitudinal axis W of the winding in order to prevent 
the occurence of short-circuit currents. An outlet line 21 which is 
electrically connected with the metal shield 20, is extended to the 
connecting terminal X for the high-voltage winding 16 in the terminal 
panel 2. 
At a small distance A from the metal shield 20 a discharge electrode 22 
concentrically surrounding the same is provided which forms a large 
capacity with the metal shield 20. The intermediate space present as a 
result of the distance A is partly or completely filled with insulating 
bands or insulating films whereby the same are impregnated with the 
insulating gas or with a liquid insulating medium of the existing gas- or 
oil-insulated combined high-voltage current- and voltage-transformer. The 
provision of only a few insulating layers, for example, from four to six 
insulating layers with an insulating band- or insulating film-thickness of 
40 to 60.mu. suffices which produces an overall distance A of 160 to 
360.mu.. However, also several insulating layers can be provided up to a 
maximum distance A of about 5 mm. 
In order that no short-circuit current is induced also in the discharge 
electrode 22, the same is also provided with at least one slot extending 
in the direction of the winding axis W. The discharge electrode 22 
consists of an electrically conductive layer, especially metal layer or of 
a metallic cylinder or of a metal film or sections thereof. With the use 
of metallic cylinder- or metal film-electrodes, the same can be adhesively 
bonded and/or bandaged-in, for example, by means of an insulating bandage 
winding. 
The coaxial low voltage winding 17 is arranged above the discharge 
electrode 22 at a relatively large distance B which is determined above 
all by necessary fastening and support elements 24. The low voltage 
winding 17 is provided with a grounded metallic shielding 25 which 
preferably surrounds the low voltage winding 17 on all sides; however, its 
cylindrical parts are also slotted in the direction of the longitudinal 
axis W of the low voltage winding 17 and in the circumferential direction 
in order to prevent short-circuit currents. The shielding 25 of the low 
voltage winding 17 is electrically conductively connected by a connection 
low in inductance with the shielding electrode 18 placed at ground 
potential by way of one, preferably several, connecting lugs 26 
distributed over the circumference. 
The discharge electrode 22 is also connected with the shielding electrode 
18 by way of at least one, however, preferably by way of a large number of 
short and as wide as possible connecting lines 27 distributed over the 
circumference. The connection is to be as low in inductance as possible, 
i.e., must be constructed as short as possible. 
The secondary lead-out connecting lines 28, 29 of the low voltage winding 
17, like the lead-out connecting line 21 for the metal shield 20, are 
extended to the terminal panel 2. This can take place in common under 
maintenance of an insulating level of 3 kV inside of the connecting-line 
pipe. The danger of breakdowns at transient high frequency occurences as a 
result of a coupling effect to high potentials onto the connection 
terminals in the terminal box 3 is effectively precluded by the measures 
according to the invention. 
In the embodiment according to FIG. 3 which illustrates a modification of 
the high-voltage voltage transformer according to FIG. 2, the same parts 
are designated with the same reference numerals as in FIG. 2. 
In modification of the high-voltage voltage transformer according to FIG. 
2, the metal shield 20 of the high-voltage winding 16 is electrically 
conductively in contact in this embodiment by way of electrically 
conducting connecting elements 20b distributed over the circumference of 
the high-voltage winding 16 and the metal blocks 24 with at least one 
further metallic layer 20a on the support body 30 of the low-voltage 
winding 17 and is connected by means of the lead-out connecting line 21 
with the connecting terminal X for the high-voltage winding 16. Details 
therefor can be seen in particular from FIGS. 4 and 5. 
Spaced at a slight distance A up to maximum 5 mm. from at least one of the 
further metallic layers 20a on the support body 30 of the low-voltage 
winding 17, at least one discharge electrode 22a concentrically 
surrounding this further metallic layer 20a is electrically insulatingly 
provided. The electric insulation between the metallic layers 20a and the 
discharge electrodes 22a corresponds preferably to the embodiment, as has 
been described more fully in FIG. 2. Also in the embodiment according to 
FIG. 3 the discharge electrode, respectively, discharge electrodes 22a are 
electrically conductively connected with the shielding electrode 18 by way 
of one or several connecting lines 27a along the shortest path, i.e., by a 
connection low in inductance. 
The use of several metallic layers 20a and correspondingly several 
discharge electrodes 22a according to FIG. 5 offers the advantage of a 
larger capacity between these layers, respectively, electrodes. 
While we have shown and described several embodiments in accordance with 
the present invention, it is understood that the same is not limited 
thereto but is susceptible numerous changes and modifications as known to 
those skilled in the art, and we therefore do not wish to be limited to 
the details shown and described herein, but intend to cover all such 
changes and modifications as are encompassed by the scope of the appended 
claims.