The subject of the invention is a multirange current instrument transformer for high voltage application applicable especially in combined high voltage instrument transformers comprised of a voltage instrument transformer and a current instrument transformer, both placed in one enclosure.

There are known designs of multirange current instrument transformers in which the primary winding is located in a metal tank filled with an insulating/cooling liquid or gas, while the ends and beginnings of primary windings used to supply current, change the range, and conduct current out are brought out of the tank through bushings located in the tank enclosure to two opposite terminal strips located on the tank enclosure on oppositely situated walls of the tank. The current range is switched by connecting, by means of cramps, the coil beginnings and coil ends corresponding to the given measuring range of the current of the windings. This connection must be done on both terminal strips, which due to the number of the cramps and their combinations is burdensome and offers opportunities for making wrong connections.

There are also known designs of multirange current instrument transformers, for example the EJOF 145 type of instrument transformer presented in Pfiffner catalog, in which the primary windings are brought outside the tank through bushings, and the conducting enclosure of the tank is used as the return winding of the primary current path. The ends and beginnings of primary windings used to supply current, change the range, and conduct current out are connected, in a way corresponding to the given current measuring range, on the terminal strip through cramps connected to the terminal strip or to the terminal strip and the tank enclosure. In these designs the incoming and outgoing terminals that conduct primary current from the feeder line are placed in bushings located in the tank enclosure. Insulation of current terminals through bushings is a cumbersome solution due to the necessity to maintain tightness of each bushing separately. In addition, due to the use of the conducting enclosure of the tank as the return winding of the primary current path, there is an ambiguity of determination of the place of connection of screen potentialization with reference to the current path. In the case of combined transformers such design is the cause of the unstable point of connection of the voltage coil, which depends on the currently selected current range in which the instrument transformer works.

There are also known designs of one- two- and three-range current instrument transformers, for example the type JUK 123 presented in ABB catalog, in which current windings are situated in a metal tank filled with insulating/cooling liquid or gas, and the appropriate ends of the current primary windings, used to change the ranges, are connected internally in the metal tank on the terminals of the individual ranges brought out through bushings individually for each current range on both sides on the outside of the tank enclosure. In this case, when the current range is changed, the possibility of incorrect connection of the outside feeder is avoided, but a change of the current range requires switching the external feeder to an external terminal suitable for the given primary current measuring range, which necessitates disconnection of the instrument transformer from the feeder. This operation is difficult as the terminals are located in different parts of the tank and this switching often requires making an additional segment or modifying a segment of the external feeder in order to accomplish the switching, especially where the connection is made in the form of so called inflexible connections, i.e. ones made by means of a flat bar or a connection pipe.

From Japanese patent description JP8115836 there is known a three-range current instrument transformer in which the primary and secondary windings are located in the metal bottom tank filled with insulating/cooling liquid or gas. The ends and beginnings of the primary windings are connected to the primary terminals of the instrument transformer, situated in the insulating cover of the top tank of the instrument transformer. Two incoming and outgoing terminals, connected to the external feeder, are arranged evenly on the circumference of the insulating cover of the top tank of the instrument transformer and they are brought out of the instrument transformer through bushings. Inside the instrument transformer there are connections of the instrument transformer outer terminals to the extreme output leads of the whole primary winding. The ends and beginnings of all primary windings are marked and brought out of the instrument transformer in pairs through the insulating cover of the upper tank. The current range of the instrument transformer is switched by making series, parallel or series-parallel connections of the ends and beginnings of the windings brought out through the insulating cover of the upper tank, through cramps allowing the equipment operation on the following ranges, respectively: the highest rated current, medium current or the lowest rated current, for which the instrument transformer has been designed.

From patent specification GB183480 there is known a multiple range electric current transformer in which the primary winding is provided with tappings which are led out to a connecting device. In accordance with this invention a switching device, by means of which it is possible to pass from one position to another without interrupting the current, is arranged on the part closing the case of the transformer. A switching device has a rotating massive conducting piece which is an electric bridge between the external current supply terminal and the ends of tappings of the current path of the transformer. The external current supply terminal has a plate with particularly circular shape and is located on the diametrically opposite side of a circle relation to outgoing terminals of tappings . Changing the range of the instrument transformer is realized by turning the rotating conductive piece, by means of an external insulating hand wheel, fixed to the rotating conductive piece. During the rotation one and of the rotating conductive piece is sliding on the circular terminal whereas the second end is changing its location and contacts the outgoing terminals of tappings. This invention is applicable only to low voltage current transformers and must not be used for high voltage current transformers for obvious personnel safety reasons: The terminals for the primary circuit are at high voltage potential and the measurement range must only be changed with the primary circuit de-energized and properly grounded. There is also a risk for dangerous electric arc in case of oxidized contacts during sliding the switching device on the circular plate of the current supply terminal.

The instrument transformer according to the invention comprises a current path placed in a metal enclosure, formed by at least two current circuits electrically interconnected and connected to an external current supply terminal and an external current outgoing terminal. The external current supply terminal is a metal plate to which anyone of the current path terminals is connected, one at a time with one single cramp and it is situated in a bushing fixed tightly in the wall of the metal enclosure of the instrument transformer tank. In the bushing there are at least two metal elements in the form of external terminals K1 , K2...Kn used to electrically connect one end of the current path with the external current supply terminal by means of only one single cramp. The single cramp for connection of current supply terminal with anyone of the terminals (K1 , K2...Kn) is located on the outer side of the bushing externally on the metal enclosure. The other end of the current path is permanently connected with an internal terminal attached to the wall of the metal enclosure inside the instrument transformer. The internal terminal being a contact connection with the external current outgoing terminal is an integral part of the instrument transformer enclosure.

Preferably the current supply terminal is a rectangular metal plate and the external terminals K1 and K2 are placed in a bushing near one outside edge of the metal current-supply terminal-plate, within the reach of the span of the bolts that fasten the cramp.

Preferably the current supply terminal is a rectangular metal plate and the external terminals K1 and K2 are placed in a bushing near one outside edge of the metal current-supply terminal-plate, within the reach of the span of the bolts that fasten the cramp, and a terminal K3 is placed in a bushing near another outer edge of the metal current-supply terminal-plate, within the reach of the span of the bolts that fasten the cramp.

Preferably the cramp has the shape of a quadratic prism.

Preferably the cramp fixed to the external terminals (K1...Kn) by means of bolts.

Alternately the cramp is fixed to the external terminals (KL..Kn) by means of screws.

Preferably the internal terminal is attached to the instrument transformer enclosure wall on the side opposite to the situation of the bushing. The instrument transformer according to the invention has a number of advantages as compared to known devices.

• The permanent invariable position of the external feeder connection terminal does not require disconnecting the feeder when changing the current range of the instrument transformer.

• Simple switching between measuring ranges only on one side of the instrument transformer practically eliminates possibility of wrong switching.

• Switching of measuring ranges is done by means of only one and the same cramp.

• A limited number of bushings in the top tank reduces the risk of leakage from the tank.

The subject of the invention is presented as an embodiment in the drawing where fig. 1 shows a combined instrument transformer in longitudinal section, fig. 2 - a fragment of the instrument transformer from fig. 1 including the instrument transformer head in cross-section along line A-A, fig. 3 - the front view of the bushing, fig. 4 - the wiring diagram of the primary current path of the current instrument transformer, and fig. 5 - the cramp in a perspective view.

The head-type combined instrument transformer comprises a current part in the form of a current instrument transformer 1 and a voltage part in the form of a voltage instrument transformer 2. The current instrument transformer 1 is placed in a metal enclosure 3 situated in the upper part of the combined instrument transformer. The enclosure 3 is a cast aluminum top tank which is the head of the combined instrument transformer. The voltage instrument transformer 2 is situated in the bottom part of the combined instrument transformer, in a cast-aluminum bottom tank 4. The metal enclosure 3 is electrically separated from the bottom tank 4 by means of a high-voltage insulator 5 made of porcelain or silicone. The inside of the enclosure 3 and of the bottom tank 4 as well as of the insulator 5 is filled with insulating and cooling liquid 6. The output leads of the secondary winding 7 of the current instrument transformer 1 and of the secondary winding 8 of the voltage instrument transformer 2 are brought to the terminals of a terminal box 9 connected to the outer part of the bottom tank 4. One end of the primary winding of the voltage instrument transformer 2 is directly connected to the wall of the metal enclosure 3, whereas the other end of the primary winding of this instrument transformer is connected through a terminal box 9 to an earth electrode terminal 10 connected to the bottom tank 4. The current path 11 of the current instrument transformer 1 is formed by primary windings interconnected in series, and the beginning of the current path 11 is connected with an external current supply terminal P1 , and the end is connected with an external current outgoing terminal P2 externally connected to the conducting wall of the enclosure 3 to which, on the inside of the tank, an internal terminal 12 is attached. Placing of the internal terminal 12 inside the enclosure 3 does not require unsealing the tank, because it does not require using an additional bushing for bringing the current path leads out of the instrument transformer. The internal terminal 12 is electrically connected through the enclosure 3 with the external current outgoing terminal P2, to which the external feeder is connected.

Current is supplied to the current path 11 of the current instrument transformer 1 from the feeder through the external current supply terminal P1 and external terminals K1 , K2, K3. The current supply terminal P1 is a metal plate which is situated in a hollow 13 of a bushing 14 made of resin and tightly fixed in the wall of the upper tank 2. The metal plate of the terminal P1 is sealed in the bushing 14 and it has seven threaded holes of which four holes 15 are centrally arranged on the plate of the terminal P1 and they are used to attach the external feeder. The other threaded holes 16 are used to connect a single cramp 17, used to change the current range, to one of the external terminals K1 , K2 and K3. The external terminals K1 , K2 and K3 are metal elements with a threaded hole 18. The external terminals K1 , K2, K3 can be made, for example, as aluminum quadratic prisms or cylinders containing the threaded hole 18. These terminals can be made as threaded segments of metal pipes and they can also have other shapes, which is not shown in the drawing. The external terminals K1 , K2 and K3 are placed port- like in the bushing 14 in such way that none of these elements touch themselves or the metal plate terminal P1 , so that there is no electric contact between these elements. The distance between the holes 18 of the terminals K1 , K2, K3 and the holes 16 of the terminal plate P1 is identical for each terminal pair K1-P1 , K2-P2 and K3-P1. The cramp 17 is a metal plate with two holes 19 which are arranged on the face of the plate at a distance ,,L" corresponding to the distances of the holes of each pair of terminals K1-P1 , K2-P2 i K3-P1. By means of the cramp 17 the winding of the current path with a specific range of rated current is connected. The instrument transformer according to the invention is suitable for operation within three ranges of rated current whose value is in an interrelation of 1 : 2 : 4. Marking for each terminal K1 , K2 and K3 its output as L1 , L2 and L3 respectively, the following current paths are obtained: a) a current path within the range 1 - P1(K1-L1) P2, b) a current path within the range 1 :2 - P1 (K2- L2)(K1-L1 ) P2, c) a current path within the range 1 :4 - P1 (K3- L3)(K2-L2)(K1 -L1 ) P2, Changing the measuring range of the instrument transformer is done in a simple way by unscrewing two fixing bolts 20, switching the cramp 17 to a different position and re-screwing the two fixing bolts, which does not require disconnecting the external feeder.

For a two-range instrument transformer terminal K3 is not used. In order to obtain a bigger range than 1 :2:4, more external terminals K4..Kn are installed in the bushing 14, which is not shown in the drawing, which terminals are connected to the end of the respective current path winding needed to obtain the given current range. The cramp 17 is fixed to the external terminals K1....Kn by means of the fixing bolts 20 or screws, not shown in the picture.