WO2014203108A1

WO2014203108A1 - Oil corrosiveness testing device and method

Info

Publication number: WO2014203108A1
Application number: PCT/IB2014/061972
Authority: WO
Inventors: Gian Carlo Montanari; Matteo FATTORI; Stefano Serra
Original assignee: Techimp Technologies S.R.L.
Priority date: 2013-06-21
Filing date: 2014-06-05
Publication date: 2014-12-24
Also published as: ITBO20130317A1

Abstract

An oil corrosiveness testing device (1 ) comprises: a box-shaped body (2) containing an electronic board (3) designed to receive and process data; a stem (4) having a first end (5) insertable into an opening of a vessel containing the oil to be tested and a second end (6) connected to the box- shaped body (2); a probe (7) equipped with at least one active element (8) made of copper or silver and electrically connected to the electronic board (3), the probe (7) being connected to the first end (5) of the stem (4) so as to be operatively in contact with the oil; a heater configured to heat the oil in the proximity of the active element (8), the probe (7) being removably connected to the stem (4) to allow the probe (7) to be substituted for another probe.

Description

DESCRIPTION

OIL CORROSIVENESS TESTING DEVICE AND METHOD

Technical field

This invention relates to a device and a method for testing oil corrosiveness. Background art

The invention addresses the field of diagnostic and detection devices, with particular regard to oil corrosiveness. More specifically, but not exclusively, the invention relates to the diagnostic assessment of electrical equipment insulated in oil (and paper) such as transformers or cables.

As is known, oil is frequently used in the casings of electrical equipment as an insulator or also as a lubricant. In light of this, the corrosiveness of the oil must be tested in order to be able to plan and correctly perform maintenance on the equipment the oil is used in, because corrosiveness is a potential cause of damage to the equipment.

For example, in the context of medium- or high-voltage transformers, systems for monitoring oil corrosiveness are used because the acids contained in corrosive oil react with the copper of the windings, producing conductive deposits on the paper and thereby spoiling the seal.

Systems for detecting oil corrosiveness are known, for example, from patent document WO2013/076623A1 in the name of the same Applicant as this invention, or from patent documents JP57207309 and US4675662. The technical solutions known from JP57207309 and US4675662 are inconvenient because they do not guarantee promptness in the diagnostic assessment of the effect of the corrosiveness. Furthermore, these prior art systems are characterized by relatively rapid wear.

The device described in WO2013/076623A1 allows speeding up system response times because it entails heating the oil near the corrosiveness detection probe (in effect, the higher the oil temperature, the higher its corrosiveness).

The solution known from WO2013/076623A1 also has some limitations. First of all, the device described in WO2013/076623A1 is not durable and must be substituted frequently, when the active part of the sensor deteriorates by the very effect of corrosion. Substitution is a relatively time- consuming and costly operation.

Further, the speed of system operation (that is, the time needed for the device to give a reliable response) is limited by the fact that the temperature which the heater can be brought to must be low enough to avoid spoiling the oil.

From patent document US4675662 it is known a device for measuring the corrosiveness of oil in vehicles. From patent document US7581434 it is known a device for measuring the corrosiveness of oil in dynamo-electric machines, such as for example pumps or compressors. However, such devices are not particularly efficient and their time response is particularly long.

Disclosure of the invention

The aim of this invention is to provide an oil corrosiveness testing device which overcomes the above mentioned disadvantages of the prior art.

More specifically, the aim of this invention is to provide an oil corrosiveness testing device which can be used for particularly long periods, involving quick and easy maintenance operations.

A further aim of the invention is to provide an oil corrosiveness testing device which can provide a particularly rapid response without the risk of spoiling the oil.

A further aim of the invention is to provide an oil corrosiveness testing device which is particularly dependable and accurate.

A further aim of the invention is to provide an oil corrosiveness testing device which is particularly flexible, allowing it to be quickly and easily adapted for installation on equipment of different kinds and sizes. These aims are fully achieved by the device according to the invention as characterized in the appended claims.

More specifically, the invention relates to an oil corrosiveness testing device comprising: a box-shaped body containing an electronic board designed to receive and process data; a stem having a first end which is insertable into an opening of a vessel containing the oil to be tested and a second end which is connected to the box-shaped body; a probe equipped with at least one active element made of copper or silver (or other electrical conductor which is a good conductor of electricity and which is subject to corrosion when in contact with corrosive oil) and electrically connected to the electronic board, the probe being connected to the first end of the stem so as to be operatively in contact with the oil; a heater configured to heat the oil in the proximity of the active element.

According to the invention, the probe is removably connected to the stem to allow the probe to be substituted for another probe.

This allows the device to be used for particularly long periods of time, while maintaining a high level of dependability and necessitating only periodic substitution of the probe, which contains the only wear parts of the device. Thanks to a quick connect/disconnect system, the probe can be removed from the stem and substituted in a particularly quick and easy manner.

Preferably, the device is provided with a cap which is removably coupled to the first end of the stem to surround the probe. The cap is provided with openings to allow the passage of oil through a space which is defined by the cap and inside which the probe is positioned.

This speeds up the time needed for the device to provide a reliable response and allows energy saving. In effect, the cap allows the oil in the immediate vicinity of the probe to be effectively heated, although the actual temperature of the heater used can be limited to relatively low values (so as to avoid spoiling the oil).

Preferably, the probe comprises two active elements, defining two sensors operating in parallel with each other. These sensors (and the electronic board acting in conjunction therewith) are configured in such a way as to operate based on different physical principles, that is to say, different modes of detecting and deriving the value of oil corrosiveness.

This makes the device particularly precise (and accurate) and robust, since it allows striking a medium between the results of the two sensors and to compensate for any errors or limitations of the sensors.

More specifically, of the two sensors (operating preferably in parallel), one comprises a wire made of copper (or silver, or other electrical conductor subject to corrosion by the oil), defining a resistor which is operatively in contact with the oil. In this case, measuring involves finding a variation of resistance in the resistor by comparing measurements taken at defined time intervals (in effect, corrosion reduces the working section of the wire, thus increasing its resistance).

The other (of the two sensors) comprises two (or more) copper armatures to form a capacitor on a surface which is operatively in contact with the oil. This capacitor is supplied with (preferably constant) alternating voltage (for example at a frequency from a few Hz to several hundred kHz).

In this case, measuring involves finding a variation of tan delta (that is, of the loss factor) of the capacitor by applying a technique known as Electrochemical Impedance Spectroscopy. More specifically, measurements of the impedance and phase of a test current injected into the capacitor are taken. These measurements are repeated at defined time intervals and the results are compared.

In light of this, it should be noted that the two sensors (the resistive one and the capacitive one) provide different and complementary diagnostic information, which means the effects of the two sensors are not simply summed but interact synergistically.

In effect, the resistive sensor provides information on the actual corrosiveness of the oil. In the electrical application (diagnostic assessment of the insulation of a transformer in paper and oil, for example, or of another insulating system) the corrosiveness of the oil is itself an indicator of the likelihood (rate) of generating conductive particles in the oil (which form conductive deposits on the solid insulator). The capacitive sensor, on the other hand, provides information on the actual presence of conductive substances in the oil.

According to another aspect of the invention, the device is preferably configured to be coupled to the oil vessel (of the appliance to be monitored), with the possibility of adjusting the depth at which the probe is positioned inside the vessel. In light of this, for example, the device comprises a sealing lock ring (designed to couple the device stably to the opening of the vessel), slidably coupled to the stem (externally) to be moved and locked along the stem (with the possibility of adjusting its position along the stem).

This makes the device flexible because it allows the device to be adapted for use on vessels of different kinds and sizes.

Brief description of drawings

This and other features of the invention will become more apparent from the following description of a preferred, non-limiting example embodiment of it, with reference to the accompanying drawings, in which:

- Figures 1 and 2 are perspective views of the testing device according to the invention;

- Figures 3 and 4 illustrate the testing device according to the invention in a side view and a sectional view, respectively;

- figure 5 is a sectional view of the device of Figure 3 through the section plane B-B;

- figure 6 is a sectional view of the device of Figure 3 through the section plane l-l;

- Figures 7 to 9 are three views of a detail of the testing device of the preceding figures;

- Figures 10 to 1 1 are schematic views of a detail of the testing device of the preceding figures;

- Figures 12 to 13 are two schematic views of a spares kit for the testing device of the preceding figures;

- Figure 14 shows a general electric diagram of the device of Figure 1 .

Detailed description of preferred embodiments of the invention

The numeral 1 in the accompanying drawings denotes an oil corrosiveness testing device according to the invention.

It should be noted that the oil corrosiveness testing device 1 can be applied to a plurality of machines or appliances to test the oil used in the machine or appliance for corrosiveness (for example, the oil may be used for cooling or lubrication).

More specifically, the device 1 may be utilized advantageously to measure the corrosiveness of the oil in electric transformers.

As is known, the electrical windings of electric transformers are immersed in an oil bath in order to be cooled by the oil.

Over time, the properties of the oil deteriorate and the oil becomes highly corrosive.

In this context, therefore, it is very important to test the oil for corrosiveness in order to be able to plan maintenance operations correctly. The device 1 can also be used to measure the state of deterioration, that is, the corrosiveness, of the oil in internal combustion engines.

The device 1 comprises a box-shaped body 2 containing an electronic board 3 designed to receive and process data.

It should be noted that the term "box-shaped body" 2 is used to mean any body or enclosure defining a housing for the electronic board 3.

The electronic board 3 (not visible in the accompanying drawings) comprises electronic hardware components capable of performing the functions described below.

The device 1 also comprises a stem 4 having a first end 5 which is insertable into an opening of a vessel containing the oil to be tested and a second end 6 connected to the box-shaped body 2.

It should also be noted that the stem 4 is preferably rigidly connected to the box-shaped body 2.

Preferably, the stem 4 is hollow so as to define a passage for the electrical wiring and/or other accessory elements.

The device 1 comprises a probe 7 equipped with at least one active element 8.

The probe 7 is connected to the first end 5 of the stem 4 so as to be, in use, operatively in contact with the oil.

Preferably, the stem 4 is made of steel and is internally resin-coated to protect the connections, that is to say, to electrically insulate the active element 8 of the probe 7 from the electronic part contained in the box- shaped body 2.

The active element 8 is made of a metal subject to corrosion in oil.

For example, the active element 8 is made preferably of copper or silver. It should be noted that the active element 8 is, in use, operatively in contact with the oil inside the vessel.

The active element 8 is electrically connected to the electronic board 3. The element 8 is made of a metallic wear material, that is to say, an element which is subject to wear when it is placed in contact with oil (which is corrosive).

Thus, wear rate of the active element 8 - by the effect of corrosion due to contact with oil - makes it possible to derive, or obtain, an indication of the degree of corrosiveness of the oil.

The device 1 comprises a heater configured to heat the oil in the proximity of the active element 8.

It should be noted that, in use, the heater heats the oil locally, in the proximity of the active element 8.

Thus, it is advantageously possible to speed up the wear rate of the active element 8 - locally, that is, only in the proximity of the active element 8 -, without excessively heating the rest of the oil in the vessel. Indeed, as is known, heating deteriorates oil performance: for this purpose, the heater makes it possible to heat only a minimal quantity of it in the proximity of the active element 8 in order to facilitate, or reduce the amount of time needed for, testing the corrosiveness of the oil, without heating all the oil in the container.

That way, by speeding up the wear rate of the active element 8, it is therefore possible to obtain in a particularly short time, an indication of the level of corrosiveness of the oil.

Preferably, the heater is an electrical resistance element which is fully incorporated in the supporting element 15 and which is connectable to an electrical power source.

It should be noted that in the device 1 , the probe 7 is removably connected to the stem 4 to allow the probe 7 to be substituted for another probe.

In other words, the probe 7 may be coupled to / uncoupled from the stem 4 in such a way as to allow quick and easy substitution.

The fact that the probe 7 can be substituted very easily means that the device 1 provided is particularly simple and effective and allows a plurality of measurements to be taken and is at the same time capable of operating for an indefinite length of time - requiring only the probe 7 to be substituted.

It should also be noted that the stem 4 is equipped with means 25 for fastening the probe 7 to the stem and configured to allow quick connection / disconnection.

According to this aspect, the device 1 is equipped with a retaining element 25 configured to keep the probe 7 mechanically connected to the stem 4 and electrically connected to the electronic board 3 and configured to also allow the probe 7 to be quickly disconnected from the stem 4.

Preferably, the probe 7 has at least two pairs of terminals, or electrical contacts. Therefore, preferably, the retaining element is configured to hold the probe 7 in a position of mechanical connection in which said at least two pairs of electrical contacts are aligned with and in contact with corresponding pairs of terminals of the stem 4.

Preferably, the probe 7 is movable relative to the stem 4 between a locking position, in which a disconnection of the probe 7 from the stem 4 (with a movement of the probe 7 away from the stem 4) and a release position, in which a disconnection of the probe 7 from the stem 4 (with a movement of the probe 7 away from the stem 4) is allowed. Preferably, the probe 7 is rotatable, that is, movable by rotation, with respect to the stem 4 between the locking position and the release position; preferably, this rotation takes place around an axis along which the stem 4 is elongated.

According to another aspect, the device 1 comprises a cap 9 which is removably coupled to the first end 5 of the stem 4 and which is shaped to surround the probe 7.

The cap 9 is provided with at least one opening 10 to allow the passage of oil into and out of a space which is internally defined by the cap 9.

In the preferred embodiment, the cap 9 is provided with a plurality of openings 10 giving onto its side wall.

Preferably, therefore, the cap 9 is provided with a plurality of openings 10 giving onto its lateral surface.

In the example illustrated, the cap 9 is provided with a plurality of openings 10 formed on the lateral surface and bottom of the cap 9 itself.

Still more preferably, the cap 9 has a cylindrical shape.

Further, the cap 9 has a cylindrical shape and comprises a cavity which defines an internal housing configured to receive, in use, the probe 7. It should be noted that the cap 9 is configured to be fitted, or coupled, to the end 5 of the stem 4.

More specifically, the end 5 of the stem 4 is provided with a cavity which defines a housing for the probe 7.

The cap 9 is also provided with a cavity which, after coupling the cap to the stem 4, defines together with the cavity at the end 5 of the stem 4, the housing for the probe 7, that is, for the active element 8.

With reference to the functionality of the cap 9, it should be noted that, in use (that is, when the cap 9 is applied to the stem 4 and the probe 7 is operatively connected to the stem 4), the opening / openings 10 allow oil to enter the housing in which the probe 7 is positioned, in such a way that the active element 8 can be in contact with the oil, that is to say, can be corroded by the oil itself.

It should be noted that the cap 9 allows the oil to remain immobile inside the housing in which the probe 7 is positioned, defining a space where the oil circulates at reduced or substantially zero speed, since it is not affected by convective motions.

The effect of the cap 9 combined with the heater is therefore to allow local heating of the oil, substantially limited to the oil inside the housing in which the probe 7 is positioned.

That means the time scales needed for testing to obtain an indication of the corrosiveness of the oil are reduced to relatively short spaces of time. As to the heater, it should be noticed that, preferably, the heater is operatively connected to the probe 7, and/or to the cap 9.

According to another aspect, the device 1 comprises a sealing lock ring 12 configured to be stably connected to the opening of the vessel containing the oil and externally coupled slidably to the stem 4 to be moved and locked along the stem 4.

As may be observed in Figure 3, the sealing lock ring 12 comprises a first portion 12a and a second portion 12b, which can be coupled to each other and which are movable along the stem 4.

More specifically, the first and second portions (1 2a,12b) are provided with threaded locking means and can be connected to each other through the agency of the threaded locking means in such a way as to engage each other and lock the ring portions to one another.

The fact that the lock ring 12 can be slidably moved along the stem 4 means that the user, based on the application, can adjust the length of the part of the stem 4 which is inserted in the vessel containing the oil.

With reference to another aspect, the probe 7 defines a first surface F3 and a second surface F4 and comprises a first active element 8a and a second active element 8b connected, respectively, to the first surface F3 and to the second surface F4 (clearly visible in Figures 10 and 1 1 , whereas the two active elements are deliberately not illustrated in Figures 7 and 9).

The first active element 8a comprises a wire 19 defining a resistor 20 and the second active element 8b comprises at least two armatures (A1 ,A2) of a capacitor 21 .

Preferably, the first active element 8a comprises a plurality of resistance elements connected to each other in such a way as to define a Wheatstone bridge or, more preferably, a "half Wheatstone bridge", (it should be noted that a "half Wheatstone bridge" differs from a traditional Wheatstone bridge in that it has only two legs, compared to the four legs of a Wheatstone bridge, and has the advantage, in its application to this invention, of being more stable in temperature).

As is known, the Wheatstone bridge allows precise measurement of an electrical resistance value.

It should also be noted that for this purpose, some resistance elements are coated by an insulating layer 31 which prevents them from coming into contact with the oil during use.

That means some of the resistance elements of the Wheatstone bridge are not subject to corrosion and, as such, under equal external conditions (temperature, etc.) keep their resistance value substantially unchanged over time.

It should be noted that according to this aspect, the device 1 allows taking a measurement with two different active elements (8a,8b), measuring two different electrical quantities (a value of resistance and a value of capacitance, respectively).

Preferably, the electronic board 3 is configured to combine the resistance signal from the resistor and the capacitance signal from the capacitor to derive an indication of oil corrosiveness. Preferably, the signals are analyzed in the electronic board 3 using a spectroscopic method.

It is thus possible to derive a particularly reliable and accurate indication of oil corrosiveness because two signals, namely resistance and capacitance, are used in combination.

Also defined is a spares kit 23 for the oil corrosiveness testing device 1 (described in the foregoing).

It should be noted that, as described above, the kit 23 comprises a probe 7 having a supporting element 15 and at least one active element 8 made of copper or silver (or, more generally, of metal) and connected to a surface of the supporting element 15.

Preferably, the supporting element 15 is made of a heat conducting material; for example the supporting element 15 is made of a metallic material (e.g. alluminium), or of a ceramic material; in case the supporting element 15 is made of a metallic material, the active element 8 is electrically insulated with respect to the supporting element 15.

The kit 23 also comprises a protective capsule 16 internally defining an oxygen-free empty space in which the probe 7 is totally enclosed.

Advantageously, the fact that the probe 7 is enclosed in an oxygen-free space substantially prevents corrosion, in particular oxidation, of the active element 8.

In effect, the fact that the space is free of oxygen protects the active element 8 against oxidation during storage, so that when the probe 7 is mounted in the testing device 1 , the active element 8 is in perfect condition.

Advantageously, in an embodiment illustrated in Figure 12, the protective capsule 18 is a box 29 provided with a body 13 and a lid 14 which is removably and sealingly coupled to the body 13.

The box 29 is filled with an oxygen-free storage fluid or it may internally define a vacuum (that is, a negative pressure).

It should be noted that the fluid may be a liquid or a gas. More specifically, the storage fluid is preferably an acid based liquid or an inert gas (for example, nitrogen).

According to another aspect not illustrated in the accompanying drawings, the lid 14 of the box 29 is provided with means 30 for retaining the probe 7 and configured to allow the probe 7 to be quickly coupled to / uncoupled from the lid.

It should be noted that according to this aspect, it is easier to pull the probe 7 out of the body 13, that is to say, to grip the probe 7, because the probe 7 is coupled to the lid 14.

According to one embodiment, the capsule 18 is shaped to define internally a closed, oxygen-free empty space.

In the embodiment illustrated in Figure 13, the capsule 18 is shaped to fit snugly against an outside profile of the probe 7 so that the closed, oxygen- free empty space is fully occupied by the probe 7.

According to this embodiment, the probe 7 is positioned inside the protective capsule 18 in a vacuum.

In this embodiment therefore, the closed space is totally free of any fluid. It should also be noted, more in general, that the probe 7 may be positioned inside the capsule 18 in a vacuum without the capsule 18 necessarily fitting snugly against an outside profile of the probe 7.

It should be noted that if the probe 7 is positioned inside the capsule 18 in a vacuum and the capsule 18 does not fit snugly against an outside profile of the probe 7, the capsule 18 comprises an air extraction valve which can be opened and closed to extract air from the internal storage space of the probe 7.

It should also be noted, more in general, that the capsule 18 may house inside it one or more probes 7.

Preferably the capsule 18 houses only one probe 7 inside it.

Also defined is an oil corrosiveness testing method comprising the following steps:

- preparing a stem 4 having a first end 5 connected to a probe 7 equipped with at least one active element 8 made of copper or silver and a second end 6 connected to a box-shaped body 2 containing an electronic board 3 designed to receive and process data and electrically connected to the probe 7;

- inserting the first end 5 of the stem 4 into an opening of a vessel containing the oil to be tested, so that the probe 7 is operatively in contact with the oil;

- heating the oil in the proximity of the active element 8;

- acquiring the values of one or more parameters measured by the at least one active element 8 and representing the corrosiveness of the oil;

- substituting the probe 7 for another probe 7 by pulling the stem 4 out of the vessel opening, disconnecting the probe 7 from the stem 4, connecting the new probe 7 to the stem 4 and re-inserting the stem 4 into the vessel opening.

According to another aspect, the method comprises a step of applying to the first end 5 of the stem 4 a cap 9 shaped to surround the probe 7 and provided with at least one opening to allow the passage of oil into and out of a space which is internally defined by the cap 9, the cap 9 being temporarily removed to allow substitution of the probe 7.

According to yet another aspect, the method comprises the step of acquiring the values of

- a first parameter, representing a resistance of a first active element 8a defining a test winding;

- a second parameter, representing a phase angle of a second active element 8b defining a test capacitor.

According to yet another aspect, the method comprises an acquisition step which is repeated over time to obtain a sequence of measured values at predetermined time intervals for the aforementioned one or more parameters.

Claims

1 . An oil corrosiveness testing device (1 ), comprising:

- a box-shaped body (2) containing an electronic board (3) designed to receive and process data;

- a stem (4) having a first end (5) insertable into an opening of a vessel containing the oil to be tested and a second end (6) connected to the box- shaped body (2);

- a probe (7) equipped with at least one active element (8) made of copper or silver and electrically connected to the electronic board (3), the probe

(7) being connected to the first end (5) of the stem (4) so as to be operatively in contact with the oil;

- a heater configured to heat the oil in the proximity of the active element

(8) ,

characterized in that the probe (7) is removably connected to the stem (4) to allow the probe (7) to be substituted with another probe.

2. The device according to claim 1 , comprising a retaining element (25) configured to keep the probe (7) mechanically connected to the stem (4) and electrically connected to the electronic board (3) and configured to allow the probe (7) to be quickly disconnected from the stem (4).

3. The device according to claim 1 or 2, comprising a cap (9), removably coupled to the first end (5) of the stem (4) to surround the probe

(7) and provided with at least one opening (10) to allow the passage of oil into and out of a space which is internally defined by the cap (9).

4. The device according to claim 3, wherein the cap (9) is provided with a plurality of openings (10) formed on the lateral surface of the cap to allow the passage of oil into and out of a space which is internally defined by the cap (9).

5. The device according to any of the preceding claims, wherein the heater is operatively connected

- to the probe (7) in order to heat it, and/or - to a cap (9) in order to heat it, the device comprising a cap (9), removably coupled to the first end (5) of the stem (4), shaped in such a way as to surround the probe (7) and provided with at least one opening (10) to allow the passage of oil into and out of a space which is internally defined by the cap.

6. The device according to any of the preceding claims, wherein the probe (7) comprises a plate-shaped supporting element (15) having a first face (F1 ) and a second face (F2), and wherein the active element (8) has a planar extension and is connected to the surface of at least one of the faces (F1 .F2).

7. The device according to any of the preceding claims, wherein the probe (7) defines a first surface (F3) and a second surface (F4) and comprises a first active element (8a) and a second active element (8b) connected, respectively, to the first surface (F3) and to the second surface (F4), and wherein the first active element (8a) comprises a wire defining a resistor and the second active element (8b) comprises at least two armatures of a capacitor.

8. The device according to any of the preceding claims, comprising a sealing lock ring (12) configured to be stably connected to the vessel opening and externally coupled slidably to the stem (4) to be moved and locked along the stem (4).

9. An oil corrosiveness testing method comprising the following steps:

- preparing a stem (4) having a first end (5) connected to a probe (7) equipped with at least one active element (8) made of copper or silver and a second end (6) connected to a box-shaped body (2) containing an electronic board (3) designed to receive and process data and electrically connected to the probe (7);

- inserting the first end (5) of the stem (4) into an opening of a vessel containing the oil to be tested, so that the probe (7) is operatively in contact with the oil;

- heating the oil in the proximity of the active element (8); - acquiring the values of one or more parameters measured by the at least one active element (8) and representing the corrosiveness of the oil;

characterized in that it comprises a step of substituting the probe (7) for another probe (7), by pulling the stem (4) out of the vessel opening, disconnecting the probe (7) from the stem (4), connecting the new probe (7) to the stem (4) and re-inserting the stem (4) into the vessel opening.

10. The method according to claim 9, comprising a step of applying to the first end (5) of the stem (7) a cap (9) shaped to surround the probe (7) and provided with at least one opening to allow the passage of oil into and out of a space which is internally defined by the cap (9), the cap (9) being temporarily removed to allow substitution of the probe (7).

1 1 . The method according to claim 9 or 10, comprising a step of acquiring the values of

- a first parameter, representing a resistance of a first active element (8a) defining a test winding;

- a second parameter, representing a phase angle of a second active element (8b) defining a test capacitor.

12. The method according to any of the claims from 9 to 1 1 , wherein the acquisition step is repeated over time to obtain a sequence of measured values at predetermined time intervals for said one or more parameters.