The present invention relates to an overvol¬ tage protection system for a transformer, especially a supply transformer, as defined in the preamble of claim 1.

It is known that during a thunderstorm a lar¬ ge voltage difference exists between the earth and the cloud. In most cases the cloud has a more positive charge than the earth, so a discharge occurs from cloud to earth more often than the other way round. However, the overvoltage protectors used also function in the case of lightning strokes from earth to cloud.

As to the surge generated by a lightning and its behaviour, the following is known. The surge di- verges into branches partly in accordance with Kirch- hoff's and Ohm's laws, in other words it is discharged to earth along several paths and principally along paths that offer the least resistance, i.e. where the specific resistance of the medium is as low as possi- ble. When a surge advancing at 1/3 of the velocity of light along a straight conductor encounters an abrupt bend in the conductor, it cannot proceed along the conductor but instead there occurs at the bend a phenomenon resembling an explosion (arc, corona) , whereupon the discharge has to find a new path, being induced into nearby metallic structures on its way to¬ wards the earth. Moreover, the surge behaves in an un¬ predictable manner. For instance, having jumped over a distance of several kilometres from cloud to earth, a lightning may also leap on a transformer from one structure to the next across a few metres.

The best way to subdue the surge and protect the transformer is to offer the surge a path that is advantageous for the transformer and a spark gap that is mounted in the right place and has a lower electro¬ de with a strong earth potential. When an overvoltage

is discharged, an arc is set up and a ground contact occurs. To extinguish the arc, automatic relays switch off electricity from the network for about one second. This power break is termed quick reconnection. In currently used known technology, the facts stated above have not been given close enough conside¬ ration. Mistakes are made in small details, thereby even inviting the lightning to damage the transformer. The commonest types of damage in known transformer constructions are as follows: the transformer windings are burnt out, a feedthrough insulator of a transfor¬ mer full of oil is sooted and cracked, the outer shell of an oil-filled transformer is burst due to the mel¬ ting action of an arc or the shell is cracked due to the pressure created by an internal explosion. In ot¬ her words, the problem with known technology is that the surge cannot be safely and securely conducted past the transformer and its various parts, but instead the surge is often allowed to damage different parts of the transformer.

The object of the present invention is to eliminate the drawbacks mentioned above. A specific object of the invention is to present a new type of transformer overvoltage protection system that effec- tively protects the structures of the transformer against a surge and directs the surge past the trans¬ former to the earth.

As to the features characterizing the inven¬ tion, reference is made to the claims. According to the invention, the overvoltage protection system for a supply transformer comprises a by-pass rail extending substantially horizontally from the top end of the feedthrough insulator of the trans¬ former, one end of s'aid rail being connected to a downlead leading down from the disconnector above the transformer. Also connected to this same end of the by-pass rail is a first protective horn pointing down-

wards and forming an essentially straight extension to the downlead. Mounted on the transformer frame is a second protective horn placed below the first protec¬ tive horn and connected from the transformer frame to earth by means of a separate earth conductor. Like the first protective horn and the downlead, the second protective horn and the earth conductor connected to it also form an essentially straight downward conduc¬ tor link having no significant bends or curvatures where the surge could not follow the conductor.

Thus, the essential point in the structure of the invention described above is that a surge advan¬ cing towards the transformer is directed past the who¬ le transformer structure along a substantially straight path that may only have smooth or round bends so that the surge, regardless of its magnitude, is ab¬ le to follow the conductor without significant arcs or corona discharges, naturally excluding the arc occur¬ ring across the spark gap. The second protective horn is preferably at¬ tached to a supporting rail mounted on the transformer frame. In this case, the corresponding second protec¬ tive horns of different phases can all be connected to one and the same supporting rail, allowing the earth conductors to be joined into one earth lead common to all phases with conductors running in a curvilinear fashion from the protective horns and supporting rail to a common earth juncture.

As compared with previously known technology, the overvoltage protection system of the invention provides the advantage that the surge can be safely directed past the whole transformer structure inclu¬ ding the frame, thus ensuring that the transformer will suffer no damage whatsoever, but after quick re- connection the transformer is always in full working order.

In the following, the invention is described in detail by referring to the attached drawings, in which

Fig. 1 presents a previously known overvoltage protec- tion system for a supply transformer,

Fig. 2 presents another previously known solution, and Fig. 3 presents the overvoltage protetion system of the invention.

Fig. 1 shows a typical distribution substati- on of a type which was commonly used until the end of the 1970 's, in which lightning protection is implemen¬ ted using an air gap 7 formed by protective horns 5 and 6 mounted on the feedthrough insulator 2. The lo¬ wer protective horn 6 has earth potential via the transformer frame 1, while the upper protective horn 5 is at the voltage potential (usually 20kV) . The trans¬ former is filled with oil, and the feedthrough insula¬ tor 2 also contains oil and a conductor passing the electricity to the windings inside the transformer. As for protection, the protective horns are in the right place but in the wrong position.

When a lightning strikes the electric line 3 and a travelling wave or a surge advances towards the transformer, a first explosion A takes place in the terminal holder 17 (termination of the straight eelct- ric line 3) . From the terminal holder, the conductor generally continues upwards at an angle of 90°. For a lightning, this is the wrong direction, because a lightning always seeks the shortest path to earth. In explosion A, the surge branches upwards and downwards directly to the high-voltage connection lead or down¬ lead. Traces found on such equipment indicate that a second explosion B occurs at the bend at the upper end of the distance rail. From here, part of the surge may proceed to the transformer windings, damaging these (explosion C) . Another part of the surge jumps from explosion point B, as can often be stated on the basis

of fusion traces, via the protective horns 5 and 6 to the transformer frame 1, which is connected 18 to earth. If, protective horn 6 has not been properly fixed at the juncture (oxidated, paint between the parts) , an explosion D takes place at the loose junction and the arc set up burns a hole in the trans¬ former cover.

In such cases, the highly inflammable trans¬ former oil leaks out, almost invariably causing a fire in the transformer and its environment. The outer shell or frame 1 of an oil-filled transformer should not be used as part of the earth lead. Another draw¬ back with this manner of protection is that animals getting into the spark gap 7 can cause electric dis- turbances. For this reason, it has been necessary to use an excessively wide spark gap, although there would have been a need to reduce the spark gap width 7 to improve overvoltage protection.

A frequent mounting fault in the situation depicted by Fig. 1 is that the downlead 4 feeding the transformer is connected directly from the disconnec¬ tor 9 to the connectors of the feedthrough insulator 2, without a lateral bend in the downlead. In this ca¬ se, explosion B occurs right at the upper or lower end (explosion C) of the feedthrough insulator 2, with the result that the insulator is smashed to pieces. In ad¬ dition to the situation in Fig. 1, the explosion B creates a third discharge path for the surge, and in this case the arc causes the outer surface of the feedthrough insulator 2 to be blackened with soot and burns a hole in the transformer cover at the foot of the feedthrough insulator 2.

Fig. 2 presents another known transformer station type, in which the protective horns 20 and 21 are placed high up above the rest of the equipment, even above the electric line 22. In this case, the air gap 23 (spark gap) is in a horizontal direction. The-

refore, the fact that a lightning discharged from a cloud is always attracted towards the earth is ignored in this solution. Moreover, upon reaching the first explosion point A, the travelling wave generated by a lightning stroke may proceed almost linearly downwards via the downlead 24 to the transformer 25. The surge advancing along the electric line 22 has to make two 90° bends before encountering the earth-potential pro¬ tective horn 21 after the arc. Before a disruptive discharge occurs in the spark gap 23, a parallel surge can travel a long way down along the downleads 24. This downward "suction" could be reduced by removing the earth connection 26 from between the transformer shell and the ground, but there are other more impor- tant reasons which forbid this disconnection. When the protective horns are mounted on the highest point on the transformer station, the earthing functions as an excellent lightning arrester in the case of direct lightning strokes, but the surge discharged through the earth conductor and the travelling wave induce in the parallel downleads 24 an overvoltage which is discharged unabated into the transformer windings. In this case, the protective horns are in the wrong place and in the wrong position with respect to the trans- former.

Fig. 3 presents an overvoltage protection system for a transformer as provided by the invention, which eliminates the problems described above. For im¬ mediate protection of the transformer, it is important that the protective horns 5 and 6 be mounted in the vicinity of the feedthrough insulator 2. A new feature is that the downlead 4 from the overhead disconnector is passed in as straight a form as possible to the end of a by-pass rail 8 extending essentially horizontally from the top end of the feedthrough insulator 2, the rail 8 being additionally provided with a spark horn 5 forming an essentially straight downward extension. A

protector 12 of insulating material around ^' the by-pass rail 8 prevents animals from getting into contact with live metallic parts. Another new and inventive feature is that the earth conductor 10 starts directly from the supporting rail 11 below the spark horn 6, forming an essentially straight extension to the horn, so that no explosion points are created for the surge. Thus, the easily damaged transformer frame 1 is not used as a discharge path for the surge. By means of the by-pass rail 8 and the supporting rail 11, the spark gap 7 can be adjusted to a sufficient horizontal dis¬ tance from the feedthrough insulator 2. In this arran¬ gement, an arc struck in the spark gap 5-6 will not cause sooting of the feedthrough insulator 2. The protective horns 5 and 6 may have a cur¬ ved shape. In this case, as seen from one side, the upper horn 5 may form a triangle whose function is to throw e.g. twigs off the spark gap electrodes. The up¬ per horn 5 consisting of the triangle may also act as an excellent connection loop for temporary earth clamps. As the tendency is nowadays to provide all li¬ ve parts with a plastic coating, this loop is about the only possible place for earth connection. The pro¬ tective horns 5 and 6 may also be implemented in some other form, e.g. as pointed electrodes. It is likewise possible to connect to the by-pass rail 8 some other kind of temporary earth loop suitably shaped.

The by-pass rail 8 is mounted in a slightly slanting position with a downward and outward slope. This ensures that rainwater will flow outwards from the rail, thus making it more difficult for the surge to move towards the feedthrough insulator 2.

The embodiment in Fig. 3 also comprises a previously known protective bracket 13 placed in the air gap 7 to prevent animals (usually birds and squir¬ rels) from perching between the protective horns 5 and 6, where they could cause a ground contact (quick re-

connection) . The bracket is of a platelike shape and has a slight downward slope in the outward direction. When an arc is initiated from protective horn 5 to¬ wards horn 6, it encounters the platelike obliquely downward sloping bracket 13. Upon this encounter the arc is broken up and, due to the position of the bracket, thrown aside in a direction away from the feedthrough insulator 2, so it will not cause the feedthrough insulator 2 to become sooty. Due to this break-up, the arc is also quickly extinguished, which means that no quick reconnection takes place. The pla¬ telike bracket also prevents animals from simultane¬ ously approaching both arc electrodes 5 and 6 from a lateral direction, so no arcs caused by animals will occur. For this reason, the shape of the bracket per¬ mits a narrower spark gap 7 to be used, thus improving the overvoltage protection of the transformer.

The platelike bracket (bird prong) described can also be mounted in a place different from the feedthrough insulator as described above, e.g. on the disconnector of the transformer.

In addition, the overvoltage protection sys¬ tem in Fig. 3 comprises a protective element 16 which is connected to the earth conductor 15 linking the frame of the disconnector 9 to ground and which ex¬ tends to a height above the electric line 3 and the disconnector 9. This by-pass arrangement guides direct lightning strokes from above past the transformer sta¬ tion structures directly to earth. Thus, the overvoltage protection system desc¬ ribed above effectively directs the surge generated by lightning which has strayed into electrical parts past the transformer without damaging it. At the same time, it also considerably reduces the number of short-time power failures.

The invention has been described above in de¬ tail by the aid of the attached drawings, but diffe-

rent embodiments of the invention are possible within the scope of the inventive idea defined by the claims.