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(11) | EP 0 150 130 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | On-load tap changer |
| (57) An on-load tap changer comprises a switch 18 such as a diverter switch or a selector
switch including an current interrupting unit 20, a vessel 16 containing the switch
together with an electrically insulating oil, a closed oil passage loop 43 including
an oil conduit 38 having an inlet opening at the bottom portion of the vessel, the
conduit extending through the upper portion of the vessel, passing through a level
lower than the vessel, and connecting to the top portion of the vessel. The conduit
has formed therein a small hole 44 at a position higher than the arc interrupting
unit. The diameter of the small hole is sufficiently smaller than the inner diameter
of the conduit to permit most of the insulating oil supplied into the oil filter unit
36 to be supplied from the inlet opening of the conduit at the bottom portion of the
vessel rather than from the upper portion of the vessel. |
BACKGROUND OF THE INVENTION
[0001] This invention relates to an on-load tap changer, and more particularly, to an oil-filtering system for an oil-filled on-load tap changer.
[0002] A diverter switch of a conventional on-load tap changer is basically a switch for switching a load current of a transformer to a preselected tap by the tap selector at no current. In order to facilitate the extinguishing of an electric arc generated at the time of current commutation, the diverter switch is enclosed within a vessel filled with an electrically insulating oil. The insulating oil is gradually polluted and degraded as it is decomposed to form foreign matters such as carbon and sludge by the electric arc upon each tap change, thereby degrading the electrically insulating properties. Therefore, the on-load tap changer for use with a high-voltage transformer is provided with an oil filter unit, which is a kind of a filter with a pump in it, and the insulating oil is filtered through the oil filter unit once a day as is well known in the art.
[0003] The conduit connecting the vessel for the diverter switch and the oil filter unit is arranged such that polluted oil at the bottom of the vessel of the tap-changer is suctioned into the conduit and pumped through the filter element of the oil filter unit to return to the upper portion of the tap changer vessel. Since the diverter switch is typically suspended from the top of the transformer tank, and the oil filter unit is positioned outside the transformer tank and placed on the same level as the transformer tank. Also, since the suction conduit from which the oil enters for filtering is typically constructed and incorporated as one of the parts of the tap changer vessel, the conduit rises from the bottom portion of the vessel along its length and extends outwardly through the upper portion of the vessel which is secured to the top of the transformer tank. Furthermore, since the vessel is suspended from the top plate of the transformer tank, the vessel is usually positioned at a higher level than is the oil filter unit.
[0005] When insulating oil leaks from the filter element or the pump of the oil filter unit, the insulating oil within the conduits and the vessel leaks out and the oil level within the vessel falls. This lowering of the insulating oil continues until the oil level reaches the level at the inlet opening of the piping because the conduit has an inverted U-shape which causes the U-shaped conduit to function as a siphon conduit. Therefore, the oil leakage from the oil filter unit may cause most of the insulating oil within the tap-changer vessel to flow outside of the system. If the oil level becomes lower than the current interrupting unit of the diverter switch, then the arc will not be extinguished, leading to a short-circuit between the taps.
SUMMARY OF THE INVENTION
[0006] Accordingly, an object of the present invention is to provide an on-load tap changer which is free from the above-mentioned drawback.
[0007] Another object of the present invention is to provide an on-load tap changer which is free from the risk that most of the electrically insulating oil can flow out from the tap changer system.
[0008] A further object of the present invention is to provide an on-load tap changer which is free from the risk that the level of insulating oil can fall due to an oil leak, thus exposing the interrupting unit of the tap changer.
[0009] With the above objects in view, an on-load tap changer of the present invention comprises a switch such as a diverter switch or a selector switch including an arc interrupting unit, a vessel containing the switch, an electrically insulating oil disposed within said vessel, and a closed oil passage loop. The oil passage loop includes an oil conduit having an opening at the bottom portion of the interior of the vessel, extending through the upper portion of the vessel, passing through a level lower than the vessel and connected to the top portion of the vessel. The oil conduit has formed therein a small hole at a position higher than the interrupting unit, the hole being inside of the vessel. The hole has a diameter sufficiently smaller than the inner diameter of the conduit for permitting most of the insulating oil supplied into the oil filter unit to be supplied from the inlet opening of the conduit at the bottom portion of the vessel rather than the upper portion of the vessel.
BRIEF DESCRIPTION OF THE DRAWING
[0010] The invention will become more readily apparent from the following detailed description of the preferred embodiment of the present invention and the sole figure, which is a diagramatic view of an on-load tap changer constructed in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0011] An on-load tap changer 10 of the present invention is illustrated in the figure as being incorporated into an electrical power transformer 12 having a hermetic housing 14. The tap changer 10 comprises a hermetic vessel 16 within which a changer-over switch 18 and an arc interrupter unit 20 are enclosed. The vessel 16 has an upper portion 22 and a lower portion 24 in communication with each other. The upper portion 22 of the vessel 16 is placed on the top plate 15 of the transformer housing 14 and the lower portion 24 of the vessel 16 is suspended into the interior of the transformer housing 14. The diverter switch 18 and the current interrupting unit 20 are disposed within the lower portion 24 of the vessel 16. Therefore, although they are positioned within the transformer housing 14, they are isolated by the wall of the vessel 16 from the interior of the transformer housing 14.
[0012] The tap-changer vessel 16 is substantially filled with an electrically insulating oil 26 in order to promote the quick extinction of the arc generated during the tap changing. In order to absorb volumetric expansion of the insulating oil 26 due to temperature changes, a conservater 28 connected to a breezer 30 by a conduit 32 is connected to the upper portion 22 of the tap-changer vessel 16 through a conduit 34. The tap changer 10 also comprises an oil filter unit 36 which includes a pump and a filter element. It is seen that the bottom wall of the oil filter unit 36 is positioned at the same level as the bottom wall of the transformer housing 14 and therefore the oil filter unit 36 is positioned at a lower level than the tap changer l0 which is positioned at the upper portion of the transformer housing 14.
[0013] The inlet of the oil filter unit 36 is connected to the bottom portion of the tap changer vessel 16 by a conduit 38 having an inlet 40 at the bottom portion inside the tap changer vessel 16. The conduit 38 has the shape of an inverted "U". This is because the tap changer 10 is typically designed as an assembled unit with at least part of the conduit 38 incorporated into the vessel 16 and it is not desirable to design the walls of the vessel 16 and the transformer housing 14 to be penetrated by the conduit 38. The outlet of the oil filter unit 36 is connected to the upper portion 22 of the tap changer vessel 16 through a second conduit 42. Thus, a closed oil passage loop composed of the vessel 16, the inverted-U shaped conduit 38, the oil filter unit 36, and the second conduit 42 is formed.
[0014] According to the present invention, the inverted U-shaped conduit 38 has formed therein a small air breezing hole 44 at its top portion. The small hole 44 communicates the inside of the conduit 38 to the interior of the upper portion 22 of the tap changer vessel 16. The small hole 44 is designed to exhibit a very high flow resistance against the insulating oil 26 flowing therethrough as compared to the flow resistance in the section of the conduit 38 between the inlet opening 40 and the small hole 44. In the illustrated embodiment, the small hole 44 has a very small diameter compared to the inside diameter of the conduit 38.
[0015] In order to clean the insulating oil 26 of the tap changer 10, the pump (not shown) in the oil filter unit 36 is energized to suction the insulating oil 26 in the bottom portion of the tap changer vessel 16 from the inlet opening 40. It is to be noted that, since the small hole 44 formed in the wall of the conduit 38 is sufficiently small compared to the inside diameter of the conduit 38, the flow resistance of the small hole 44 is very high compared to the flow resistance of the flow path in the conduit 38 through the inlet opening 40. Therefore, most of the insulating oil 26 supplied into the oil filter unit 36 is supplied from the inlet opening 40 at the bottom of the tap changer vessel 16 rather than from the upper portion 22 of the vessel 16. The oil 26 supplied through the conduit 38 to the oil filter unit 36 passes through the filter element (not shown) of the oil filter unit 36 to be cleaned. The filtered oil 26 is returned to the upper portion 22 of the vessel 16 and descend to the bottom portion of the vessel 16. Thus, the insulating oil 26 recirculates through the closed oil passage loop composed of the vessel 16, the first conduit 38, the oil filter unit 36, and the second conduit 42. When the insulating oil 26 has been cleaned, the pump is deenergized.
[0016] If an oil leak occurs in the oil filter unit 36 which is lower than the bottom of the tap changer vessel 16, the insulating oil 26 within the tap changer vessel 16 flows out from the inlet opening 40 of the conduit 38 through the conduit 38 and from the leak in the oil filter unit 36. As the oil continues to leak, the insulating oil 26 is emptied first from the conservator 28, and then the level of the insulating oil 26 within the upper portion 22 of the tap changer vessel 16 gradually decreases. When the oil level in the upper portion 22 of the tap changer vessel 16 reaches the small hole 44 in the conduit 36, air introduced within the vessel 16 from the breezer 30 flows into the small hole 44, preventing further lowering of the oil level in the tap changer vessel 16 because the insulating oil 26 in the upper portion 22 and the lower portion 24 of the vessel 16 is trapped in the vessel 16. The oil 26 within the section of the conduit 38 to the left of the small hole 44 and the oil in the oil filter unit 36 flow out through the leak in the filter unit 36. Therefore, even when an oil leak occurs in the oil filter unit 36, the insulating oil 26 around the arc interrupting unit 20 in the vessel is maintained.
[0017] As has been described, according to the present invention, the oil conduit has formed therein a small hole at a position higher than the interrupting unit, and the hole has a diameter sufficiently smaller than the inner diameter of the conduit. Therefore, the level of the insulating oil cannot become lower than the arc interrupting unit, preventing accidents such as short-circuit between taps.
1. An on-load tap changer comprising:
a switch including an arc interrupting unit;
a vessel containing said switch;
an electrically insulating oil disposed within said vessel for electrically insulating said switch; and
a. closed oil passage loop for circulating said insulating oil therealong, said loop including a conduit which has an inlet at the bottom portion of the interior of said vessel and which upwardly extends inside of said vessel and exits from the upper portion of said vessel and extends through a position lower than said vessel and is connected to an upper portion of said vessel;
. said conduit having formed therein a small hole at a position higher than said arc interrupting unit, said small hole being inside of said vessel and having a diameter sufficiently smaller than the inner diameter of said conduit to permit most of said insulating oil supplied into said oil filter unit to be supplied from said inlet opening of said conduit at the bottom portion of said vessel rather than from said upper portion of said vessel.
2. An on-load tap changer as claimed in claim 1, wherein said switch comprises a diverter
switch.
3. An on-load tap changer as claimed in claim 1, wherein said switch comprises a tap
selector switch.
4. An on-load tap changer as claimed in claim 1, wherein said oil conduit includes
an oil filter unit.