WO2013178447A1

WO2013178447A1 - Safety device for electrical transformers and related electrical transformer

Info

Publication number: WO2013178447A1
Application number: PCT/EP2013/059671
Authority: WO
Inventors: Giuseppe Leonardo MANCO; Carlo Carollo; Andrea Tonin
Original assignee: Abb Technology Ag
Priority date: 2012-05-31
Filing date: 2013-05-08
Publication date: 2013-12-05
Also published as: EP2669910B1; BR112014030054A8; CN104380400A; EP2669910A1; BR112014030054A2; PL2669910T3; ZA201407960B; CN104380400B

Abstract

Safety device for electrical transformers, comprising: - a casing provided with an internal chamber suitable to be connected in hydraulic communication with an oil-containing vessel and an oil-containing conservator of an associated electrical transformer, for oil flowing through said internal chamber between the vessel and the conservator; - a first oil outflow detecting vane which is movably connected to the casing and is arranged along the path of the oil flow, the first oil outflow detecting vane being suitable to move from a rest position when oil outflows from the vessel with an oil speed greater than an alarm oil speed threshold; - a second oil outflow detecting vane which is movably connected to the casing and is arranged along the path of the oil flow, the second oil outflow detecting vane being suitable to move from a rest position when oil outflows from the vessel with an oil speed greater than a safety oil speed threshold, wherein the safety oil speed threshold being greater the said alarm oil speed threshold; - first detection means for detecting a movement of the first oil outflow detecting vane from its rest position, the first detection means being suitable to be connected to alarm means for signalling when the detected movement of the first oil outflow detecting vane from its rest position exceeds the alarm oil speed threshold; - second detection means for detecting a movement of the second oil outflow detecting vane from its rest position, the second detection means being suitable to cause the intervention of a circuit breaker device suitable to disconnect the electrical transformer from an associated power grid when the detected movement of the second oil outflow detecting vane from its rest position exceeds the safety oil speed threshold.

Description

SAFETY DEVICE FOR ELECTRICAL TRANSFORMERS AND RELATED

ELECTRICAL TRANSFORMER

The present disclosure refers to a safety device for electrical transformers, particularly of the type of Buchholz relays, and to a related electrical transformer using such a safety device. The safety device of the present disclosure is suitable to be used in the field of electrical power transmission and distribution to detect electrical transformers' failures or internal faults.

In particular the present safety device is suitable to be mounted on an oil filled electrical transformer to detect fast or slow gas accumulation and oil leaks and oil flow variation.

Nowadays, known safety devices, typically Bucholz relays, are mounted on oil-filled electrical transformers equipped with an external overhead oil reservoir, called "conservator". These traditional safety devices comprise a casing with an internal chamber which is in communication with the vessel and the conservator of the electrical transformer.

Depending on the model, such known safety devices have multiple systems to detect a failure or malfunctioning of an electrical transformer.

A first model of known safety devices is provided with a float switch disposed in the internal chamber.

The float switch is electrically connected to a circuit breaker suitable to disconnect the electrical transformer from the power grid in case of failure.

In the event of a slow accumulation of gas in the electrical transformer, for example due to a slight overload which causes a decomposition of insulating oil, produced gas accumulates in the top of the safety device's internal chamber and forces the oil level down.

In that case, if the oil level lowers beyond a certain threshold thus moving down the floating switch, the latter disconnects the electrical transformer from the power grid by means of a circuit breaker.

A second traditional model of known safety devices is provided not only with said first floating switch but also with a second floating switch and with an alarm device.

The second floating switch serves to detect, in general, slow oil lowering in the transformer' s oil vessel, and is suitable to activate the alarm device, to which is connected if the oil level in the internal chamber of the safety device lowers beyond an alarm oil level threshold.

Traditionally, the above described safety devices are further equipped with a flow activated safety switch, suitable to actuate the circuit breaker in the event of an oil flow.

More in detail, for example if an arc forms in the electrical transformer, gas accumulation is rapid, and oil quickly flows into the conservator passing through the safety device. The flow activated safety switch detects if the oil speed exceeds a safety oil speed threshold and, if so, it disconnects the electrical transformer from the power grid by actuating the associated circuit breaker.

In particular, the flow activated safety switch comprises a frame fixed into the casing of the safety device and an oil outflow detecting vane arranged along the path of the moving oil and movably supported by the frame.

The oil outflow detecting vane is suitable to be moved from a rest position by the flowing oil, so that, if the speed of the oil exceeds said prefixed oil speed threshold, the oil outflow detecting vane is moved from its rest position and activates the circuit breaker, which disconnects the electrical transformer from the power grid.

Although such known safety devices adequately perform their functions, there are still some aspects which can be improved. For example, at the moment, in case of a failure of the transformer, it is impossible to determine, the origin of the failure.

In fact, in the above described safety devices, if a failure occurs so that the circuit breaker is activated and thus the electrical transformer is put out of service, and the oil level is beyond the oil level threshold, it would be almost impossible, in particular from a remote location, to determine whether the circuit breaker activation was triggered by the float switch, as a result of a slow oil lowering as in the case of an oil leak, or by the oil outflow detecting vane, as a result of a rapid oil movement produced by a rapid gas accumulation. Accordingly, it is not possible to understand the origin and so the gravity of the electrical transformer's failure. Another drawback of the above described safety devices consists in that the activation of the circuit breaker may derive from an accidental movement of the oil outflow detecting vane due, for example, to strong vibrations or to a magnetic field action.

In that situation, there is no possibility to distinguish the case of an accidental movement of the oil outflow detecting vane from the case in which the deactivation derives from a movement of the oil outflow detecting vane due to the oil quick outflowing from the vessel of an electrical transformer.

A further drawback of known safety devices described above, consists in that it is impossible to test the mobility of the oil outflow detecting vane, so that it is impossible to determine whether the vane is able to activate the circuit breaker in case of rapid oil outflow from the electrical transformer vessel, or not.

Finally, once a device is calibrated, it is impossible to adjust the oil speed threshold, so that the safety device cannot be adapted to specific operating needs.

The present disclosure is aimed at facing at least some of the above indicated issues. Such aim is fulfilled by a safety device for electrical transformers, comprising:

- a casing provided with an internal chamber suitable to be connected in hydraulic communication with an oil-containing vessel and an oil-containing conservator of an associated electrical transformer, for oil flowing through said internal chamber from/to said vessel to/from said conservator;

characterized in that it further comprises:

- a first oil outflow detecting vane which is movably connected to said casing and is arranged along the path of the oil flow, said first oil outflow detecting vane being suitable to move from a rest position when oil outflows from said vessel with an oil speed greater than an alarm oil speed threshold;

- a second oil outflow detecting vane which is movably connected to said casing and is arranged along the path of the oil flow, said second oil outflow detecting vane being suitable to move from a rest position when oil outflows from said vessel with an oil speed greater than a safety oil speed threshold, said safety oil speed threshold being greater than said alarm oil speed threshold;

- first detection means for detecting a movement of said first oil outflow detecting vane from its rest position, said first detection means being suitable to be connected to alarm means for signalling when the detected movement of said first oil outflow detecting vane from its rest position exceeds said alarm oil speed threshold;

- second detection means for detecting a movement of said second oil outflow detecting vane from its rest position, said second detection means being suitable to cause the intervention of a circuit breaker device suitable to disconnect said electrical transformer from an associated power grid when the detected movement of said second oil outflow detecting vane from its rest position exceeds said safety oil speed threshold.

Further characteristics and advantages will emerge from the following description of preferred, but not exclusive embodiments of a safety device according to the present disclosure, non-limiting examples of which are provided in the attached drawings, wherein:

- figure 1 schematically shows a safety device according to the present disclosure;

- figure 2a, 2b and 2c are perspective views of the safety device of figure 1 depicted in three different operative configurations;

- figure 3 is a perspective view of a particular of the safety device of figure 1;

- figure 4 is another perspective view of the particular of the safety device of figure 3;

- figure 5 illustrates an exemplary oil-filled electrical transformer comprising a safety device according to the present disclosure. With reference to the aforesaid figures, the overall reference numeral 10 indicates a safety device for electrical transformers, particularly of the type of Buchholz relays, suitable to be used in the field of electrical power transmission and distribution to detect some failures of an associated electrical transformer.

In particular, the present safety device 10 is suitable to be mounted on an oil filled electrical transformer 11 to detect fast oil flow, and/or slow gas accumulation and/or oil leaks.

The safety device 10 comprises a casing 14 provided with an internal chamber 15 suitable to be connected in hydraulic communication with an oil-containing vessel or tank 12 and an oil- containing conservator 13 of the electrical transformer 11 into which the safety device 10 is used; in practice, when installed, the safety device 10 is positioned along the flowing path of the transformer oil 103 from/to the vessel 12 to/from the conservator 13 with oil flowing through the internal chamber 15.

According to solutions well-known in the art or readily available to those skilled in the art, the safety device 10 is operatively connected to a circuit breaker device 16 which is suitable to be electrically connected to the electrical connection between the electrical transformer 11 and a power distribution or transmission grid 17, associated to the transformer 11 itself. As it will be described in more detail hereinafter, the circuit breaker device 16 is suitable to disconnect, on command, the electrical transformer 11 from the power distribution or transmission grid 17. Furthermore, the safety device 10 comprises at least one floating switch, preferably two floating switches 18a and 18b, disposed in said internal chamber 15 and provided with connection means, for example electric wires, for operatively connecting the first floating switch 18a to the circuit breaker device 16 and the second floating switch 18b, to first alarm means, schematically indicated in figure 1 by the reference number 40. Such first alarm means can be of any suitable type, e.g. visual, such as a lamp, acoustic, local or remote; for example they can comprise an alarm light provided in a control room or consists in an electronic device suitable to send an electronic alarm message to a receiving device monitored by an operator. In particular, the first floating switch 18a is connected to the circuit breaker device 16 to command a disconnection of the electrical transformer 11 from the power distribution or transmission grid 17 if the oil level in the internal chamber 15 lowers beyond a safety oil level threshold.

The second floating switch 18b is suitable to activate said first alarm means 40 in case the oil level in the internal chamber 15 lowers beyond an alarm oil level threshold; the alarm oil level threshold is different from the safety oil level threshold and in particular corresponds to an oil level in the internal chamber 15 higher than the oil level corresponding to the safety oil level threshold.

According to the present disclosure, the safety device 10 comprises at least two oil outflow detecting vanes 19 and 20, which are movably connected to the casing 14, preferably by means of a frame 21, and arranged along the path of the oil flow between the vessel 12 and the conservator 13.

The oil outflow detecting vanes 19 and 20 are suitable to move from a rest position, due to a hydrodynamic action of an oil flow directed from the oil vessel 12 to the conservator 13, to detect if a rapid oil flow occurs in the vessel 12 of the electrical transformer.

In more detail, a first oil outflow detecting vane 19 is movably connected to the casing 14 and is adapted to move from its rest position in case of oil outflowing from said oil vessel 12 at a speed greater than an alarm oil speed threshold.

So that, if oil outflows from the oil vessel 12 through the safety device 10 at a speed lower than said alarm oil speed threshold, the first oil outflow detecting vane 19 does not perceptibly move and rests substantially in its rest position; otherwise if oil outflows at a speed greater than said alarm oil speed threshold, the first oil outflow detecting vane 19 moves leaving its rest position.

A second oil outflow detecting vane 20 is also movably connected to the casing 14, and is also arranged along the path of the oil flow and is adapted to move from a rest position in case of oil outflowing from said oil vessel 12 at a speed greater than a safety oil speed threshold. The safety oil speed threshold is greater than the alarm oil speed threshold.

Hence, likewise the first oil outflow detecting vane 19, also the second oil outflow detecting vane 20 is adapted to remain substantially still in its rest position if oil outflows from the oil vessel 12 through the safety device 10 at a speed lower than said alarm oil speed threshold, but it also remains substantially still in its rest position if oil speed exceeds said alarm oil speed threshold without exceeding said safety oil speed threshold.

Furthermore, the safety device 10 comprises first detection means 22 and second detection means 23 for detecting a movement of said first and second oil outflow detecting vanes 19, 20 from their respective rest positions, respectively.

The first detection means 22 are suitable to be connected to second alarm means, schematically indicated in figure 1 by the reference 50 for signalling a movement of the first oil outflow detecting vane 19 from its rest position, which means that the speed of oil flowing in the safety device 10 from the oil vessel 12 to the conservator 13 exceeded said alarm oil speed threshold.

Said second alarm means 50 can be of any suitable type, e.g. visual, such as a lamp, acoustic, local or remote; for example they can comprise an alarm light provided in a control room or consists in an electronic device suitable to send an electronic alarm message to a receiving device monitored by an operator.

The second detection means 23 are connected to the circuit breaker device 16 to command the disconnection of the electrical transformer 11 from the power distribution or transmission grid 17 in case of a movement of the second oil outflow detecting vane 20 from its rest position, which means that the speed of oil flowing in the safety device 10 from the oil vessel 12 to the conservator 13 exceeds said safety oil speed threshold.

For example, the safety oil speed threshold is set to an oil speed value corresponding to (or, conservatively, lesser than) the speed of the oil flowing in the safety device 10 from the oil vessel 12 in case an arc forms in the electrical transformer 11.

In an exemplary embodiment of the present disclosure, as illustrated in the attached figures, the safety device 10 comprises the frame 21 which is connected to the casing 14 in said internal chamber 15.

The internal chamber 15 has a first opening 25, which in operation is connected in hydraulic communication with the oil vessel 12 of the electrical transformer 11, and a second opening 26, which in operation is connected in hydraulic communication with the conservator 13. The first and second openings 25, 26 are for example coaxially facing each other and are provided on opposite walls of the internal chamber 15.

In the exemplary embodiments illustrated, the first and second oil outflow detecting vanes 19, 20 are provided each one with a frontal plane face 19a, 20a which is transversal, e.g. substantially perpendicular, to the common axial direction of the first and second openings 25, 26, when the first 19 and the second oil outflow detecting vane 20 are in the rest position. Said axial direction of the first and second openings 25, 26 defines the oil flow direction B through the internal chamber 15.

The frontal plane faces 19a, 20a are oriented so as to face the first opening 25 to be invested by oil flowing from the oil vessel 12 to the conservator 13.

More in detail, the first and the second oil outflow detecting vanes 19, 20 are movably connected to the frame 21, preferably with respect to rotation axes C, D which are substantially perpendicular to said oil flow direction B.

Furthermore, at least one, preferably each of said first and second detection means 22, 23 comprises a reed switch 22a, 23a, connected to the casing 14, and a magnetic actuator 22b, 23b connected to a corresponding oil outflow detecting vane 19, 20.

In particular, the first detection means 22 comprise a first magnetic actuator 22b fixed on the first oil outflow detecting vane 19, and a first reed switch 22a fixed to the frame 21 adjacent to the first oil outflow detecting vane 19, so that if the first oil outflow detecting vane 19 moves from its rest position the magnetic actuator passes in the proximity of the first reed switch 22a, thus activating the latter.

In case the first reed switch 22a is activated, the first detection means 22 operate the above said alarm means 50 to signal that oil is flowing through the safety device, thus signalling a status of abnormal operation for the electrical transformer 11 which may correspond to an electrical overload.

The second detection means 23 comprise a second magnetic actuator 23b, connected on the second oil outflow detecting vane 20, and a first reed switch 23a, fixed to the frame 21 adjacent to the second oil outflow detecting vane 20 so that if the second oil outflow detecting vane 20 moves from its rest position the second magnetic actuator 23a passes into the proximity of the second reed switch 23a, and activates the latter.

If the second reed switch 23a is activated, the second detection means 23 commands the circuit breaker device 16 to disconnect the electrical transformer 11 from the power distribution or transmission grid 17.

Furthermore, the safety device 10 advantageously comprises first retention means 25 mechanically connected to said casing 14 in said internal chamber 15 for exerting a first retention force on the second oil outflow detecting vane 20 to prevent or at least contrast movement of the latter from its rest position until the hydrodynamic force exerted on said second oil outflow detecting vane 20 by the oil flowing from the oil vessel 12 to the conservator 13, is lesser than said first retention force.

In other words, the retention means 25 are designed to maintain the second oil outflow detecting vane 20 substantially in its rest position unless the hydrodynamic force exerted on it by the oil flowing from the oil vessel 12 to the conservator 13, is greater than said first retention force.

For instance, the first retention force is a magnetic force. To this scope, the second oil outflow detecting vane 20 for example comprises or is completely made of a magnetically responsive material, and the first retention means 25 advantageously comprise a first magnet 28 fixed on the frame 21 vis a vis with the second oil outflow detecting vane 20, and preferably facing the frontal plane face 20a of the second oil outflow detecting vane 20, to exert on the latter said first retention force.

Clearly, as retention force and related retention means it is possible to use alternative solutions, e.g. mechanical, et cetera. According to the present disclosure, the safety device 10 advantageously further comprises adjusting means 60 for adjusting said first retention force. These adjusting means can be of any suitable type relative to the type of retention force and means used; for example, it is possible to use a screw-based mechanism which allows to adjust the distance between the vane and the retention means, or a torsion spring suitably sized, et cetera.

Preferably, the safety device 10 also comprises second retention means, not shown, mechanically connected to said casing 14 in said internal chamber 15 for exerting a second retention force on said first oil outflow detecting vane 19 to prevent movement of the latter from its rest position in case the hydrodynamic force exerted on said second oil outflow detecting vane 20, by oil flowing from the oil vessel 12 to the conservator 13, is lesser than said second retention force.

Said second retention means advantageously are, mutatis mutandis, of the same type of the above described first retention means 25 to which reference is made; accordingly also the first oil outflowing detecting vane 19 for example comprises or is completely made of a magnetically responsive material. Clearly also for the second retention means and the related first vane 19 is possible to use different types of retention means as above indicated for the first retention means 25. Likewise, also further adjusting means for adjusting the second retention force can be used.

Furthermore, the casing 14 preferably has an access opening 30 communicating with the internal chamber 15 and comprises a stopper 31 suitable to close the access opening 30.

The frame 21 is mechanically fixed to the stopper 31 and is suitable to be extracted from/inserted in the internal chamber 15 through said access opening 30.

Advantageously, the safety device 10 further comprises blocking and unblocking means, schematically represented in figure 1 by the reference number 70, for blocking at least the second oil outflow detecting vane 20 in a moved position different from said rest position. Such blocking and unblocking means 70 may consist for example in one or more elastic teeth protruding from the frame 21 to which are fixed, and suitable to engage the second oil outflow detecting vane 20 in said moved position, to maintain the latter in said moved position.

Preferably, the safety device 10 is also provided with means, e.g. mechanical, or electromechanical, or magnetic suitable to operate said blocking and unblocking means, on command, so as to allow the return of the second oil outflow detecting vane 20 in its rest position.

Thus, the safety device 10 allows to identify if the activation of the circuit breaker device 16 is due to the movement of the second oil outflow detecting vane 20 from its rest position, in which case the second oil outflow detecting vane 20 is blocked in said moved position by the blocking and unblocking means, until the operator operates the latter to allow the second oil outflow detecting vane 20 to return in its rest position.

To test the effectiveness of said second detection means 23, the safety device 10 preferably comprises means for actuating the movement of the second oil outflow detecting vane 20 from its rest position, which for example consists of a rod 34 mechanically connected to the second oil outflow detecting vane 20, axially sliding through a hole provided in the casing 14, and having an end 35 projecting outside the casing 14 to be operated to move the second oil outflow detecting vane 20.

Advantageously, also the first oil outflow detecting vane 20 is mechanically connected to the rod 34 to be moved.

Functionally, if an overload of the electrical transformer 11 occurs or an arc forms in the electrical transformer 11, gas is produced which forces the oil to flow from the oil vessel 12 to the conservator 13, and forces down the oil level in the internal chamber 15 of the safety device 10.

In that case, if the oil speed exceeds the alarm oil speed threshold without exceeding the safety oil speed threshold, the first oil outflow detecting vane 19 moves from its rest position thus activating the related alarm means, as for example, is illustrated in figure 2b.

If the oil speed exceeds also the safety oil speed threshold, the second detection means 23 activates the electrical circuit breaker 16 , as, for example, illustrated in figure 2c.

If the oil level in the internal chamber 15 lowers beyond the alarm oil level threshold, the second floating switch 18b activates the related alarm means; if the oil level in the internal chamber 15 lowers beyond the safety oil level threshold, the first floating switch 18a activates the electrical circuit breaker 16 which in turn disconnects the electrical transformer 11. In particular, in order to identify whether the intervention of the circuit breaker 16 has been triggered by the floating switch 18a ("safety oil level threshold exceeded")or by the second detection means ("safety oil speed threshold exceeded"), it is possible to use two different circuit breakers, for example connected in series to each other, or to have one signal output locally or remotely by the second detection means (or alternatively by the floating switch 18a) at the same time when the circuit breaker 16 is activated by them.

On the basis of what set forth above, it is therefore evident that the safety device for electrical transformers, particularly of the type of Buchholz relays of the disclosure allows to face at least some issues of known prior art devices.

In particular, the safety device 10 as described and defined in the appended claims allows an easy and more effective identification of the origin of failures in an oil filled electrical transformer. Indeed, the safety device 10 of the present disclosure allows to easily distinguish whether a safety disconnection of an electrical transformer from the associated power grid is due to a failure caused by a rapid oil flow or otherwise caused by a slow gas accumulation or by an oil leak, or whether it is even accidental.

Further, the structure of the safety device 10 allows also adjusting one or more of the related thresholds and offers even the possibility to test the oil outflow detecting vane mobility, so as to easily reveal failures of the oil outflow detecting vane.

Thanks to its improved functionalities and simple structure, the safety device 10 of the present disclosure can be used substantially in any type of oil-filled electrical power transformer. Hence, the present disclosure also encompasses an oil-filled electrical power transformer characterized in that it comprises a safety device as defined in the relevant appended claims and before described.

The safety device thus conceived is susceptible of modifications and variations, all of which are within the scope of the inventive concept including any combination of the above described embodiments which have to be considered as encompassed by the above description; all details may further be replaced with other technically equivalent elements. For example, the device 10 can have a different number of components assembled together or the components may be differently shaped provided they are suitable for the scope they were conceived. In practice, the materials, so long as they are compatible with the specific use, as well as the individual components, may be any according to the requirements and the state of the art.

Claims

1. Safety device (10) for electrical transformers, comprising:

- a casing (14) provided with an internal chamber (15) suitable to be connected in hydraulic communication with an oil-containing vessel (12) and an oil-containing conservator (13) of an associated electrical transformer (11), for oil flowing through said internal chamber (15) from/to said oil vessel (12) to/from said conservator (13);

characterized in that said safety device (10) also comprises:

- a first oil outflow detecting vane (19) which is movably connected to said casing (14) and is arranged along the path of the oil flow, said first oil outflow detecting vane (19) being suitable to move from a rest position when oil outflows from said vessel (12) with an oil speed greater than an alarm oil speed threshold;

- a second oil outflow detecting vane (20) which is movably connected to said casing (14) and is arranged along the path of the oil flow, said second oil outflow detecting vane (20) being suitable to move from a rest position when oil outflows from said vessel (12) with an oil speed greater than a safety oil speed threshold, said safety oil speed threshold being greater than said alarm oil speed threshold;

- first detection means (22) for detecting a movement of said first oil outflow detecting vane (19) from its rest position, said first detection means (22) being suitable to be connected to alarm means (50) for signalling when the detected movement of said first oil outflow detecting vane (19) from its rest position, exceeds said alarm oil speed threshold;

- second detection means (23) for detecting a movement of said second oil outflow detecting vane (20) from its rest position, said second detection means (23) being suitable to cause the intervention of a circuit breaker device (16) suitable to disconnect said electrical transformer (11) from an associated power grid (17) when the detected movement of said second oil outflow detecting vane (20) from its rest position exceeds said safety oil speed threshold.

2. Safety device (10) as in claim 1, characterized in that it comprises a frame (21) connected to said casing (14) in said internal chamber (15), and wherein said internal chamber (15) comprises a first opening suitable to be connected in hydraulic communication with the vessel (12) of said electrical transformer, and a second opening, suitable to be connected in hydraulic communication with the conservator (13) of said transformer, said first and second oil outflow detecting vanes (19, 20) being movably connected to said frame (21).

3. Safety device (10) as in one or more of the preceding claims, characterized in that it comprises first retention means operatively connected to said casing (14) in said internal chamber (15) for exerting a preset first retention force on and contrasting movement from its rest position of said second oil outflow detecting vane in case the force exerted on said second oil outflow detecting vane (20) by oil flowing from said oil vessel (12) to said conservator (13) is lesser than said first retention force.

4. Safety device (10) as in claim 3, characterized in that said second oil outflow detecting vane (20) comprises a magnetically responsive material, and in that said first retention means comprises a first magnet (23b) connected on said frame (21) to exert on said second oil outflow detecting vane (20) a magnetic force, said preset first retention force consisting in said magnetic force.

5. Safety device (10) as in one or more of claims 3-4, characterized in that it comprises adjusting means (60) for adjusting said preset first retention force.

6. Safety device (10) as in one or more of the previous claims, characterized in that it comprises second retention means operatively connected to said casing (14) in said internal chamber (15) for exerting a second retention force on and preventing movement from its rest position of said first oil outflow detecting vane (19) in case the force exerted on said second oil outflow detecting vane (20), by oil flowing from said oil vessel (12) to said conservator (13), is lesser than said second retention force.

7. Safety device (10) as in one or more of the preceding claims, characterized in that said casing (14) has an access opening (30) communicating with said internal chamber (15) and comprises a stopper (31) suitable to close said access opening (30), said frame (21) being mechanically connected to said stopper (31) and suitable to be extracted from/inserted in said internal chamber (15) through said access opening (30).

8. Safety device (10) as in one or more of the preceding claims, characterized in that it comprises at least a first floating switch (18a) disposed in said internal chamber (15) and operatively connected to said circuit breaker device (16) so as to cause a disconnection of said electrical transformer (11) from the associated power grid (17) when the oil level in said internal chamber (15) lowers beyond a safety oil level threshold.

9. Safety device (10) as in claim 8, characterized in that it comprises a second floating switch (18b) disposed in said internal chamber (15) and operatively connected to associated alarm means (40) for signalling when the oil level in said internal chamber (15) lowers beyond an alarm oil level threshold.

10. Safety device (10) as in one or more of the preceding claims, characterized in that it comprises blocking and unblocking means (70) for blocking at least said second oil outflow detecting vane (20) in a moved position different from said rest position.

11. Safety device (10) as in one or more of the preceding claims, characterized in that it comprises means for actuating the movement of said second oil outflow detecting vane (20) from said rest position to test the effectiveness of said second detecting means (23).

12. Safety device (10) as in one or more of the preceding claims, characterized in that each of said first and second detection means (22, 23) comprises:

- a reed switch connected to said casing (14) in the proximity of a corresponding one of said first and second oil outflow detecting vanes (19, 20) for detecting a movement from its corresponding rest position;

- a magnetic actuator of said reed switch connected to said corresponding one oil outflow detecting vane, to activate said reed switch in the event said corresponding one oil outflow detecting vane moves from its rest position.

13. Safety device (10) as in one or more of the preceding claims characterized in that it is a Buchholz relay.

14. An oil-filled electrical power transformer (11) characterized in that it comprises a safety device (10) according to one or more of the previous claims.