Device for analysing an insulating medium

Abstract

The invention relates to a device (100) for analysing an insulating medium (101), comprising: an on-load tap-changer (2) with a diverter switch (10); a first line (40) and a second line (50); an access (13); wherein the diverter switch (10), the first line (40) and the second line (50) form an insulating medium circuit (5) in which the access (13) is arranged; the insulating medium (101) circulates in the insulating medium circuit (5) by means of thermal convection; the insulating medium (101) is analysed locally by a sensor device (16) arranged in the access (13) or externally after a removal of the insulating medium (101) via the access (13).

Description

[0001] Device for analyzing an insulating medium

[0002] The invention relates to a device for analyzing an insulating medium.

[0003] Both transformers and the on-load tap-changers installed within them have always been insulated and cooled with oil. Monitoring the oil in the transformer is relatively simple, whereas monitoring the oil in the on-load tap-changer presents enormous challenges.

[0004] The object of the invention is therefore to provide a device for analyzing an insulating medium in an on-load tap-changer, which is of simple and cost-effective construction, enables local monitoring, can be retrofitted cost-effectively or provides a simple option for removing the insulating medium for external analysis.

[0005] This object is achieved by the device for analyzing an insulating medium according to claim 1. The subclaims form advantageous embodiments of the invention.

[0006] The invention proposes a device for analyzing an insulating medium, comprising: an on-load tap-changer with a diverter switch vessel; a first line and a second line; an access; wherein the diverter switch vessel, the first line and the second line form an insulating medium circuit in which the access is arranged; the insulating medium circulates in the insulating medium circuit by means of thermal convection; the analysis of the insulating medium is carried out locally by a sensor device arranged in the access or externally after removal of the insulating medium via the access.

[0007] The device enables a particularly straightforward and simple analysis of the insulating medium located in the diverter switch vessel of an on-load tap-changer. On-load tap-changers generally have two standard lines, with a first line connecting the on-load tap-changer to an oil conservator, and a second line allowing the insulating medium to be extracted from the diverter switch vessel during maintenance. Connecting the two lines outside the diverter switch vessel creates an insulating medium circuit equipped with an access point for extraction and / or a sensor device. Furthermore, connecting the two lines causes the insulating medium to circulate within the resulting circuit.Since the insulating medium in the diverter switch vessel, which is located in the transformer, is warmer than the air surrounding the transformer and the two lines, the insulating medium begins to flow from the first line, via the second line, and past the inlet with the sensor device, back into the on-load tap-changer. Due to thermal convection caused by the temperature differences in the insulating medium circuit, the insulating medium circulates independently from the diverter switch vessel through the circuit, past the inlet, and back into the diverter switch vessel of the on-load tap-changer. Because the two lines leading into and out of the diverter switch vessel are usually already attached or installed on existing transformers and on-load tap-changers,are present, the connection can be designed relatively simply and inexpensively, making it possible to set up online monitoring of the insulating medium even on existing on-load tap-changers. The access enables a condition analysis of the insulating medium by continuously removing the insulating medium from the insulating medium circuit. Furthermore, a sensor device arranged in the access can record various data and parameters of the insulating medium in real time and also enables a condition analysis of the insulating medium. After the insulating medium has been removed from the insulating medium circuit, it is used for external analysis.

[0008] The on-load tap-changer can be designed in any desired manner and can have a diverter switch in the diverter switch housing, which corresponds to that of a simple diverter switch or that of a load selector. Thus, the on-load tap-changer can be designed as a load selector or as an on-load tap-changer with a diverter switch and a selector.

[0009] The access can be configured in any way, for example, as a tap, ball valve, or another type of access point to the insulating medium. The access is designed in such a way that a sample of the insulating medium can be easily taken, or a sensor device can be produced through or by means of the access to the insulating medium. The access can have a sampling device, which can also be designed as a tap or ball valve.

[0010] The first and second lines can, for example, be connected to each other directly outside the on-load tap-changer.

[0011] The device can be designed in any desired manner, wherein a first end of the first line is arranged in the diverter switch vessel and forms an inlet; a second end of the second line is arranged in the diverter switch vessel and forms an outlet.

[0012] The insulating medium enters the insulating medium circuit via the inlet. The insulating medium returns from the insulating medium circuit to the diverter switch vessel via the outlet.

[0013] The device can be designed in any desired manner, wherein an oil expansion vessel is arranged between the first line and the second line in the insulating medium circuit.

[0014] As an alternative to a direct connection between the first and second lines, an oil conservator can be arranged between the lines. This then becomes part of the insulating medium circuit. The oil conservator is designed and constructed to absorb excess insulating medium from the diverter switch vessel when the volume of the insulating medium changes due to a temperature increase / decrease in the on-load tap-changer.

[0015] By connecting the two lines, an insulating medium circuit is created in which the insulating medium can circulate.

[0016] The device can be designed in any desired manner, for example, the inlet being arranged in an upper half of the diverter switch vessel; the outlet being arranged in a lower half of the diverter switch vessel.

[0017] The device can be designed in any desired manner, for example the access is arranged in the second line between the drain and the oil expansion tank.

[0018] The device can be designed in any desired manner, for example a first temperature sensor is arranged in the first line between the inlet and the oil expansion tank.

[0019] The device can be designed in any desired manner, for example a second temperature sensor is arranged in the second line between the access and the oil expansion tank.

[0020] The device can be designed in any desired manner, for example, with a heating element arranged in the second line between the inlet and the outlet. The use of the heating element can heat the insulating medium in the insulating medium circuit, thus increasing its flow rate.

[0021] The device can be designed in any desired manner, wherein the sensor device in the access, the first temperature sensor, the second temperature sensor and the heating element are connected to an evaluation device; the evaluation device receives signals from the sensor device, the first temperature sensor and the second temperature sensor and processes and evaluates them; the evaluation device sends signals to the heating element and controls and regulates it.

[0022] The evaluation device can receive, evaluate, and process the signals from the temperature sensors, the heating element, and the sensor device located in the access point, and, if necessary, issue control commands based on the received signals and data. Thus, based on the data from the temperature sensors, it can be determined whether and how quickly the insulating medium is circulating. If the flow rate is slow, the evaluation device can control the heating element and switch it on to increase the temperature and thus the flow rate of the insulating medium. The evaluation device can also evaluate the data received from the sensor device, make it available to a user in a cloud, or store and output it locally on the evaluation device.

[0023] The invention and its advantages are described in more detail below with reference to the accompanying drawings. They show:

[0024] Fig. 1 shows a device for analyzing an insulating medium;

[0025] Fig. 2 shows a device for analyzing an insulating medium with a sensor device;

[0026] Fig. 3 shows a further embodiment of the insulating medium circuit with components for detecting and increasing the flow rate of the insulating medium;

[0027] Fig. 4 shows the design of the diverter switch vessel with the arrangement of the inlet and outlet.

[0028] Figure 1 shows a device 100 for analyzing an insulating medium 101, which comprises an on-load tap-changer 2 arranged in a transformer 1. The on-load tap-changer 2 has a selector 8 and a diverter switch 7. The diverter switch 7 is filled with an insulating medium 101. A diverter switch insert 7.1, which takes over switching in the on-load tap-changer 2, i.e., contacting and separating the tap contacts under load, is arranged in the insulating medium 101. Each time the diverter switch contacts of the diverter switch 7 filled with insulating medium 101 are actuated, brief arcs arise in the insulating medium 101, which over time can lead to its decomposition and soot formation. The diverter switch 7 of the on-load tap-changer 2 therefore has a diverter switch vessel 10, which is filled with the insulating medium 101 and in which the diverter switch insert 7.1 is arranged.The insulating medium 101 can, for example, be a natural or artificial insulating medium such as insulating oil or ester. The transformer 1 has a lower side which is designed as a base part 1.2. This lower side, i.e. the base part 1.2, is connected to the substrate or ground when assembled. The transformer 1 therefore stands on the substrate. The transformer 1 also has a plurality of walls 1.3 and a transformer cover 1.1. The transformer cover 1.1 is arranged opposite the base part 1.2 and thus opposite the lower side. When assembled, the on-load tap-changer 2 is fastened to the transformer cover 1.1 of the transformer 1. An upper half 10.1 of the diverter switch vessel 10 is thus located or arranged closer to the transformer cover 1.1 of the transformer 1, and a lower half 10.2 of the diverter switch vessel 10 is located or arranged closer to the base part 1.2 of the transformer 1 and thus to the lower side.arranged.

[0029] In the assembled state, the diverter switch vessel 10 has a head part 9 which closes the diverter switch vessel 10. A first line 40 and a second line 50 each lead through the head part 9. A first end 41 of the first line 40 forms an inlet 30 for the insulating medium 101. A second end 52 of the second line 50 forms an outlet 20 through which the insulating medium 101 enters the diverter switch vessel 10. The first line 40 is connected to an oil expansion tank 12 by its second end 42. The second line 50 is also connected to an oil expansion tank 12 by its first end 51. An access 13 which can be used to take a sample of the insulating medium is arranged in the second line 50 between the second end 52 and the first end 51 of the second line 50.The access 13 then has a withdrawal device 15, which can be designed as a tap, ball valve, or another access point to the insulating medium 101, and enables a sample of the insulating medium 101 from the second line 50. After taking a sample of the insulating medium 101, it can be analyzed either on-site or in a laboratory. In this embodiment, the on-load tap-changer 2, in particular its diverter switch vessel 10, the first and second lines 40, 50, and the oil conservator 12 form an insulating medium circuit 5, in which the access 13 is arranged, the insulating medium circuit 5 being part of the device 100 for analyzing the insulating medium 101. The precise arrangement of the inlet 30 and the outlet 20 in the diverter switch vessel 10 is described further in Figure 4.

[0030] The insulating medium 101, which is located in the diverter switch vessel 10, can circulate in the insulating medium circuit 5. Due to the temperature difference between the interior of the on-load tap-changer 2 and the ambient air, the insulating medium 101 leaves the on-load tap-changer 2 via the inlet 30 through the head part 9 and enters the first line 40 of the insulating medium circuit 5. The first and second lines 40, 50 as well as the inlet 30 and outlet 20 are pipelines or tubes with a diameter of approximately one inch. The insulating medium 101 then flows through the first line 40 into the oil conservator 12, which is located above the on-load tap-changer 2 and the transformer 1. The oil conservator 12 is preferably located several meters above the transformer cover 1.1. This oil conservator

[0031] 12 absorbs volume changes of the insulating medium 101 in the diverter switch vessel 10 when the temperature increases or decreases, so that the pressure in the diverter switch vessel 10 can be kept largely constant.

[0032] The insulating medium 101 falls due to the height difference between oil expansion tank 12 and access

[0033] 13 from the oil conservator 12 through the second line 50 to the inlet 13. Once the insulating medium 101 has passed through the inlet 13, it returns to the diverter switch vessel 10 of the on-load tap-changer 2 via the outlet 20 and the second end 52. The insulating medium 101 circulates in the insulating medium circuit 5 due to thermal convection. This is because, ideally, the insulating medium 101 has different temperatures at different points in the insulating medium circuit 5. Thus, the insulating medium 101 in the diverter switch vessel 10 is always warmer than, for example, in the oil conservator 12, since the oil conservator 12 is usually exposed to the ambient air. The diverter switch vessel 10 and thus also the on-load tap-changer 2 are arranged inside the transformer 1. The transformer 1, which is filled with an additional insulating medium, has, in its interior, in addition to the on-load tap changer 2, also windings on an iron core which carry a current.As a result, heat energy is generated during operation, which causes the temperatures of the insulating medium 101 in the on-load tap-changer 2 and transformer 1 to be higher than the temperature of the ambient air. The flow direction or the direction of circulation of the insulating medium 101 is shown in the figures by arrows along the first and second lines 40, 50. Because the insulating medium 101 in the diverter switch vessel 10 is warmer than in the first line 40, which is also exposed to the ambient air, a temperature difference arises and thus density differences occur in the insulating medium 101. This creates static buoyancy, which causes the insulating medium 101 to begin circulating. The insulating medium 101 thus rises from the diverter switch vessel 10 through the inlet 30 into the first line 40 into the oil conservator 12 and then flows further into the second line 50 and to the inlet 13.Because warm insulating medium 101 constantly flows from the diverter switch vessel 10 into the insulating medium circuit 5 and because gravity pressure acts on the insulating medium 101 due to the height difference between the oil expansion vessel 12 and the inlet, the insulating medium 101 flows through the second line 50 from the inlet 13 back into the diverter switch vessel 10 via the inlet 20. Thus, the insulating medium circuit 5 is closed and the insulating medium 101 can circulate independently.

[0034] In a further embodiment, the oil expansion tank 12 between the first line 40 and the second line 50 of the insulating medium circuit 5 can be omitted. The insulating medium then flows directly from the first line 40 into the second line 50. Instead of the oil expansion tank 12, there is then a pipe connection that directly connects the second end 42 of the first line 40 to the first end 51 of the second line 50. The insulating medium 101 then flows directly from the first line 40 via the pipe connection into the second line 50. Circulation also occurs without an oil expansion tank 12, as long as the lines 40, 50 are at least partially exposed to the ambient air.

[0035] In a further embodiment, the on-load tap-changer 2 can also be a load selector. The load selector combines the functions of a diverter switch and a selector in one switching device. The load selector also has a diverter switch vessel 10 in which a corresponding diverter switch is arranged. This diverter switch vessel 10 is also filled with an insulating medium 101 and is part of the insulating medium circuit 5.

[0036] Figure 2 shows a further embodiment of the insulating medium circuit 5. In this embodiment, a sensor device 16 is arranged in the inlet 13, wherein the remaining structure of the insulating medium circuit 5 corresponds to the structure described in Figure 1. It is also possible for both a sensor device 16 and a removal device 15 to be located in the inlet 13. In this embodiment, the inlet 13 can, for example, also be designed as a tap, ball valve, or the like, into which the sensor device 16 can be inserted or connected. The sensor device 16 can be inserted into the inlet 13 by pushing, snapping, or plugging. The inlet preferably has a receptacle that corresponds to the sensor device 16 and enables the sensor device 16 to be inserted into the insulating medium circuit 5 or to be connected to it.

[0037] The sensor device 16 is designed to detect, process, and / or evaluate or output characteristic values ​​of the insulating medium 101. The sensor device 16 can, for example, be a sensor for analyzing dissolved gases (dissolved gas analysis), which can detect certain dissolved gases or contaminants in the insulating medium 101. Furthermore, the sensor device 16 can detect a temperature or the humidity of the insulating medium 101. The sensor device 16 is connected to an evaluation device 14 via a third connection 14.3. This can process, evaluate, and / or output the results and values ​​determined by the sensor device 16. The third connection 14.3, or the connection of the sensor device 16 to / with the evaluation device 14, can be wireless or wired.

[0038] Figure 3 shows a further embodiment of the device 100 for analyzing the insulating medium 101 with additional elements arranged in the insulating medium circuit 5 or on the first and / or second lines 40, 50. In this embodiment, the first line 40 of the insulating medium circuit 5 has a first temperature sensor 21 and a protective relay 24, or a temperature sensor 21 and a protective relay 24 are arranged in the first line 40. The first temperature sensor 21 is designed to detect a temperature of the insulating medium 101 flowing through the first line 40 of the insulating medium circuit 5. The protective relay 24 serves to protect the on-load tap-changer 2 and the transformer 1 in the event of a fault. It triggers when the predetermined oil flow rate in the first line 40 between the on-load tap-changer 2 and the oil conservator 12 is exceeded. In this embodiment, the second line 50 is divided into a first section 50.1 and divided into a second section 50.2. The first section 50.1 runs from the oil expansion tank 12 to the inlet 13, and the second section 50.2 runs from the inlet 13 to the outlet 20 into the diverter switch tank 10. In this embodiment, the second line 50 has a cooling element 19 in the first section 50.1, which extends along the lines in these sections and is designed to cool the insulating medium 101 coming from the oil expansion tank 12. The cooling element 19, which cools the insulating medium 101 of the second line 50, can be designed, for example, in the form of cooling fins or as a cover plate that prevents direct sunlight.

[0039] The cooling of the insulating medium 101 results in heat or thermal energy being extracted from the insulating medium 101, which thus reaches the oil expansion tank 12 and from the oil expansion tank 12 to the inlet 13 more quickly, since the cooler insulating medium 101 sinks to the inlet 13. A flow meter 32 and a second temperature sensor 31 are also arranged in the first section 50.1 of the second line 50. The flow meter 32 is designed and configured to detect the flow velocity of the insulating medium 101 in the first section 50.1 in order to determine whether convection or circulation is occurring. The second

[0040] Temperature sensor 31 is designed and configured to detect a temperature of the insulating medium 101 flowing through the first section 50.1 of the second line 50 of the insulating medium circuit 5. In this embodiment, a heating element 18 is arranged downstream of the inlet 13 in the second section 50.2 of the second line 50. The heating element 18 is designed to heat the insulating medium 101 in the second section 50.2 of the second line 50. This accelerates the flow velocity and thus the circulation of the insulating medium 101, and it returns more quickly through the outlet 20 into the diverter switch vessel 10. In this embodiment, the second section 50.2 of the second line 50 of the insulating medium circuit 5 can also have insulation 17 on the second line 50. This prevents the insulating medium 101 in the second section 50.2 from cooling down and losing flow velocity after being heated by the heating element 18.The first temperature sensor 21, the second temperature sensor 31, the flow meter 32, and the heating element 18, in addition to the sensor device 16 arranged in the access 13, are connected in this embodiment to an evaluation device 14. For this purpose, the evaluation device 14 has a first connection 14.1 between the first temperature sensor 21 and the evaluation device 14, as well as a second connection 14.2 between the second temperature sensor 31 and the evaluation device 14. The heating element 18 is also connected to the evaluation device 14 via a fourth connection 14.4. The flow meter 32 is connected to the evaluation device 14 via a fifth connection 14.5. These connections can also be wireless or wired connections, via which data and information can be exchanged, or the individual components can be controlled, switched on, or off.The evaluation device 14 is designed and configured to detect and compare the temperatures of the insulating medium 101 detected by the first temperature sensor 21 and the second temperature sensor 31, and to determine the temperature difference between the two measured values. From this, a flow velocity of the insulating medium 101 can be derived, or it can be determined whether the insulating medium 101 is circulating. In the case of a slow flow velocity, which is determined by the flow meter 32 or the temperature difference detected and derived by the temperature sensors 21, 31, the evaluation device 14 can control the heating element 18 and switch it on to accelerate the flow or circulation by heating the insulating medium 101 in the second section 50.2 of the second line 50.

[0041] Figure 4 shows a schematic representation of the on-load tap-changer 2 with the diverter switch vessel 10 and the inlet 30 and outlet 20, or the first end 41 of the first line 40 and the second end 52 of the second line 50. The on-load tap-changer 2 is connected to or fastened to the transformer cover 1.1 of the transformer 1. The selector 8 is arranged below the diverter switch vessel 10 in the transformer 1. The diverter switch insert 7.1 is located in the diverter switch vessel 10 and is arranged in an insulating medium 101. The diverter switch vessel 10 is closed by a head part 9, which is designed like a cover. The head part 9 has openings for the first end 41 of the first line 40 and the second end 52 of the second line 50 and thus the inlet 30 and the outlet 20. The first end 41 of the first line 40 forms the inlet 30 and the second end 52 of the second line 50 forms the outlet 20.The outlet 20 and inlet 30, each configured as a single line, extend from the outside, i.e., from outside the on-load tap-changer 2 and thus outside the transformer 1, into its interior. The outlet 20 is arranged in the lower half 10.2 of the diverter switch vessel 10 in a base region 10.3 of the diverter switch vessel 10. This means that the second end 52 of the second line 50 is closer to the base region 10.3 of the diverter switch vessel 10 and the base part 1.2 of the transformer 1 than to the transformer cover 1.1 and thus also to the head part 9 of the on-load tap-changer 2. The inlet 30 is arranged in the upper half 10.1 of the diverter switch vessel 10. This means that the first end 41 of the first line 40 is closer to the upper part 10.1 of the transformer 1, i.e. the transformer cover 1.1, than to the bottom part 1.2 of the transformer 1 and the bottom area 10.3 of the diverter switch vessel 10. The inlet 30 orThe first end 41 of the first line 40 thus extends less deeply into the diverter switch vessel 10 than the outlet 20 or the second end 52 of the second line 50. This results in hot insulating medium 101 being withdrawn from the upper part or upper half 10.1 of the diverter switch vessel 10 and entering the insulating medium circuit 5, and in the colder insulating medium 101 at the end of the insulating medium circuit 5 being directed into the lower part or lower half 10.2 of the diverter switch vessel 10. Thus, over time, the entire insulating medium 101 of the on-load tap-changer 2 enters the insulating medium circuit 5 and passes through the inlet 13 with or without the sensor device 16.

[0042] List of reference symbols

[0043] 1 transformer

[0044] 1.1 Transformer cover

[0045] 1.2 Base part

[0046] 2 on-load tap-changers

[0047] 5 Insulating medium circuit

[0048] 7 load changeover switches

[0049] 7.1 Load diverter switch insert

[0050] 8 voters

[0051] 9 head part

[0052] 10 diverter switch vessel

[0053] 10.1 upper half of the diverter switch vessel

[0054] 10.2 lower half of the diverter switch vessel

[0055] 10.3 Bottom area of ​​diverter switch vessel

[0056] 12 Oil expansion tank

[0057] 13 Access

[0058] 14 Evaluation device

[0059] 14.1 first connection

[0060] 14.2 second connection

[0061] 14.3 third connection

[0062] 14.4 fourth connection 14.5 fifth connection

[0063] 15 Removal device

[0064] 16 Sensor device

[0065] 17 Insulation

[0066] 18 Heating element

[0067] 19 Cooling element

[0068] 20 Procedure

[0069] 21 first temperature sensor

[0070] 24 protective relays

[0071] 30 Admission

[0072] 31 second temperature sensor

[0073] 32 flow meters

[0074] 40 first line

[0075] 41 first end of the first line

[0076] 42 second end of the first line

[0077] 50 second line

[0078] 50.1 first section of the second line

[0079] 50.2 second section of the second line

[0080] 51 first end of the second line

[0081] 52 second end of the second line

[0082] 100 Device for analyzing the insulating medium 101 Insulating medium

Claims

Patent claims 1. A device (100) for analyzing an insulating medium (101), comprising: an on-load tap-changer (2) with a diverter switch vessel (10); a first line (40) and a second line (50) forming an inlet (13); wherein the diverter switch vessel (10), the first line (40), and the second line (50) form an insulating medium circuit (5) in which the inlet (13) is arranged; the insulating medium (101) circulates in the insulating medium circuit (5) by means of thermal convection; the analysis of the insulating medium (101) is carried out locally by a sensor device (16) arranged in the inlet (13) or externally after the insulating medium (101) has been removed via the inlet (13).

2. Device (100) for analyzing an insulating medium (101) according to claim 1, wherein a first end (41) of the first line (40) is arranged in the diverter switch vessel (10) and forms an inlet (30); a second end (52) of the second line (50) is arranged in the diverter switch vessel (10) and forms an outlet (20).

3. Device (100) for analyzing an insulating medium (101) according to one of claims 1 to 2, wherein an oil expansion vessel (12) is arranged between the first line (40) and the second line (50) in the insulating medium circuit (5).

4. The device (100) for analyzing an insulating medium (101) according to claim 3, wherein a second end (42) of the first line (50) is connected to the oil expansion vessel (12) and a first end (51) of the second line (50) is connected to the oil expansion vessel (12).

5. Device (100) for analyzing an insulating medium (101) according to one of claims 2 to 4, wherein the inlet (30) is arranged in an upper half (10.1) of the diverter switch vessel (10); the outlet (20) is arranged in a lower half (10.2) of the diverter switch vessel (10).

6. Device (100) for analyzing an insulating medium (101) according to one of claims 1 to 5, wherein the access (13) is arranged in the second line (50).

7. Device (100) for analyzing an insulating medium (101) according to one of claims 1 to 6, wherein a first temperature sensor (21) is provided and the first temperature sensor (21) is arranged in the first line (40).

8. Device (100) for analyzing an insulating medium (101) according to one of claims 1-7, wherein a second temperature sensor (31) is provided; the second temperature sensor (31) is arranged in the second line (50).

9. A device (100) for analyzing an insulating medium (101) according to any one of claims 1-8, wherein a heating element (18) is provided; the heating element (18) is arranged in the second line (50).

10. Device (100) for analyzing an insulating medium (101) according to one of claims 1 - 9, wherein an evaluation device (14) is provided; the sensor device (16) in the access (16), the first temperature sensor (21), the second temperature sensor (31) and the heating element (18) are connected to the evaluation device (14); the evaluation device (14) receives signals from the sensor device (16), the first temperature sensor (21) and the second temperature sensor (31) and processes and evaluates them; the evaluation device (14) sends the signals after processing and evaluation to the heating element (18) and controls and regulates the latter.

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