THREE-PHASE ON-LOAD TAP CHANGER

Abstract: A three-phase on-load tap changer according to this invention is so constructed that change-over switches (D1-3) for three phases and tap selectors (S1-3) individually coresponding thereto are divided into at least two groups, that on-load tap changer units (LTC1, 2) which individually receive the groups are mechanically coupled to each other, and that a driver (3) which drives all the on-load tap changer units is coupled to a specified one of them. To be chosen as the specified on-load tap changer unit is the unit (1) in which, in the set of the change-over switch (D2) and the tap selector (S2) corresponding to a certain predetermined one of the three phases, only the change-over switch (D2) is separated and is received.

Description: Background of the Invention This invention relates to a three-phase on-load tap changer principally featuring the combination of change-over switches and tap selectors. More particularly, it relates to an on-load tap changer capable Of preventing the occurrence of the worst situation, in which current is interrupted by only a tap selector, even in case of any abnormality. As an on-load tap changer for three phases, there has been known one which is furnished with a change-over switch and a tap selector for each phase. It is desirable for the on-load tap changer of this type that the change-over switches and the tap selectors corresponding to the three phases are received in a single LTC (on-load tap changer unit). However, when the on-load tap changer is to be applied to the star connection, it requires the insulation between the phases, and hence, it becomes very large in size for a high voltage class. For example, in a case where the change-over switches and the tap selectors corresponding to the three phases are arrayed coaxially and with predetermined interphase insulation distances set in the axial direction, the full length of the LTC becomes conspicuously great to spoil practicability. As a measure of solution against this drawback, it has been proposed to combine the change-over switches of two phases with the tap selector of one phase and combine the change-over switch of the remaining one phase with the tap selectors of the remaining two phases, to construct one LTC for each of the combinations, and to connect the two LTC's in parallel thereby to form the single three-phase LTC as a whole. Fig. 1 is a schematic arrangement diagram of such a prior-art device. Referring to the figure, numeral 1 designates a first LTC, which includes therein change-over switches D1 and D2 for first and second phases respectively and a tap selector S1 for the first phase, and numeral 2 designates a second LTC, which includes therein a change-over switch D3 for a third phase and tap selectors S2 and S3 for the second and third phases respectively. The respectively corresponding change-over switches and tap selectors are electrically connected by connection leads L1, L2 and L3. Symbol P1 denotes a first coupling shaft by which the first LTC 1 and the second LTC 2 are directly coupled, while symbol P2 denotes a second coupling shaft by which the second LTC 2 and a driver 3 are coupled. In the prior-art device of such arrangement, when the driver 3 operates, the second LTC 2 operates through the second coupling shaft P2, and simultaneously, the first LTC 1 operates through the first coupling shaft P1, whereby tap change operations corresponding to the three phases are performed. Patent Specification GB-A-1 496 184 describes tap changing equipment for three-phase regulating transformers, which makes use of a first sub-assembly housing change-over switches of first and second phases and a tap selector switch of the first phase, and a second sub-assembly housing a change-over switch of a third phase and tap selector switches of the second and third phases. The switches of the first sub-assembly are arranged coaxially with a first drive shaft; and the switches of the second sub-assembly are arranged co-axially with a second drive shaft. Both of the drive shafts are connected directly to a common drive mechanism. In both of the on-load tap changers constructed of the change-over switches and the tap selectors as described before, when a change-over switch has become inoperative for any reason into the state in which only the tap selector can operate, current can be interrupted by the tap selector which originally has no current interrupting function. This involves the danger of leading to the occurence of a serious accident. In the prior-art device explained with reference to Figure 1, when the first coupling shaft P1 has lost its coupling function for any reason, the first LTC 1 fails to operate, and only the second LTC 2 operates upon being driven by the driver 3. In consequence, the change-over switch D2 for the second phase becomes inoperative , and only the tap selector S2 for the second phase operates. This signifies the problem that current interruption in the tap selector S2 for the second phase arises to incur the aforementioned dangerous state leading to the occurrence of a serious accident. Similarly, if the drive shaft of the first sub-assembly of the device disclosed in GB-A-1 496 184 fails, the same problem arises. Summary of the Invention This invention has been made in order to solve the problem of the prior-art device described above, and has for its object to provide an on-load tap changer wherein the operation of only a tap selector without its corresponding change-over switch is avoided even when the abnormal situation in which a coupling shaft coupling a plurality of LTC's comes off has arisen. According to the present invention there is provided a three-phase on-load tap changer according to the appended claim, to which reference is hereby directed. According to this invention, when a coupling shaft which mechanically couples tap changers to each other has come off for any reason during the driving operation thereof, the tap changer including a tap selector which might be dangerous if the drive continues under this state is separated from a driver, whereby the occurrence of a serious accident is prevented. Brief Description of the Drawings Fig. 1 is a schematic arrangement diagram of a three-phase on-load tap changer in a prior art; and Fig. 2 is a schematic arrangement of a device according to this invention. In the drawings, the same symbols indicate identical or corresponding portions. Description of the Preferred Embodiment Fig. 2 is a schematic arrangement diagram of an on-load tap changer which is an embodiment of this invention. Referring to the figure, a first LTC 1 includes therein change-over switches D1 and D2 for first and second phases respectively and a tap selector S1 for the first phase, while a second LTC 2 includes therein a change-over switch D3 for a third phase D3 for a third phase and tap selectors S2 and S3 for the second and third phases respectively. The respectively corresponding change-over switches and tap selectors are electrically connected by connection leads L1, L2 and L3. The first LTC 1 and the second LTC 2 are interconnected through a first coupling shaft P1, and a driver 3 is connected to the first LTC 1 through a second coupling shaft P2. In the device of the embodiment of this invention having the above arrangement, it is now assumed that the first coupling shaft P1 have come off for any reason. At this time, the second LTC 2 becomes inoperative, and only the first LTC 1 continues to operate. The operating states of the change-over switches of the respective phases and the tap selectors corresponding thereto on this occasion are as listed in Table 1 below. As understood from the table, when there has occurred the abnormal situation that the first coupling shaft P1 comes off for any reason, the worst state in which the change-over switch becomes inoperative and only the tap selector corresponding thereto continues to operate does not arise in any of the phases. As is well known, even when the tap selector becomes inoperative and only the change-over switch corresponding thereto continues to operate, no dangerous state is involved. Accordingly, the state as in the second phase in Table 1 does not lead to any accident. As apparent from the above description, according to this invention, an LTC on a side to which a driver is coupled is put into a specified arrangement, whereby an on-load tap changer whose reliability is greatly enhanced can be realized without requiring any new additional components.

Claims: A three-phase on-load changer comprising: a first on-load tap changer unit (LTC 1) which receives therein a change-over switch (D1) for a first phase, a change-over switch (D2) for a second phase and a tap selector (S1) for the first phase so as to operate them, a second on-load tap changer unit (LTC 2) which receives therein a change-over switch (D3) for a third phase, a tap selector (S2) for the second phase and a tap selector (S3) for the third phase so as to operate them, connection leads (L1-L3) each of which connects said change-over switch and said tap selector of the same phase, a change-over driver (3) which drives said first and second on-load tap changer units; a first coupling (P1) shaft which couples said first and second on-load tap changer units, characterised in that said three-phase on-load tap changer further comprises: a second coupling shaft (P2) which couples said driver and said first on-load tap changer unit.

Inventors: YOSHIDA, KAZUO ITAMI WORKS MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1985
Main classification: H01H9
IPC codes: H01F29, H01H9