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| Figure 6-6 Operation of tap-changer under load. |

| The sequence of operation is as follows: With selector switch 1 and transfer switches A and C closed the voltage at the middle of the preventive autotransformer is the same as the voltage at the end of the winding at position 1. To step to the next tap, switches are operated as shown in the sequence: Switch C is opened; Switch 2 is closed; Switch B is closed; Switch A is opened; Switch C is closed; Switch 1 is opened. Now the middle point of the autotransformer and tap 2 are at the same voltage, and the part of the winding from 1 to 2 is unused temporarily. The sequence of switches for other tap changes is shown in Table 6-1. The selector switches are always closed before the corresponding transfer switches; they are always opened after the transfer switch has been opened. In this scheme, three transfer switches and one selector switch for each tap are required. Other schemes and modifications are designed to serve the same purpose. |

| When transfer switch C is closed, both ends of the preventive autotransformer are at the same voltage and one-half the current flows through each half. Since these currents flow around the core in opposite directions, only a small voltage drop occurs from the reactance of the circuit under these |

TABLE 6-1 Sequence of Operation of Switches

Position | 1 |  | 2 |  | 3 |  | 4 |  | 5 |  | 6 |  | 7 |  | 8 |  | 9
Transfer Switch A | X | X |  | X | X | X |  | X | X | X |  | X | X | X |  | X | X
Transfer Switch B |  | X | X | X |  | X | X | X |  | X | X | X |  | X | X | X |
Transfer Switch C | X |  | X |  | X |  | X |  | X |  | X |  | X |  | X |  | X
Selector Switch No. 1 | X | X |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Selector Switch No. 2 |  | X | X | X |  |  |  |  |  |  |  |  |  |  |  |  |
Selector Switch No. 3 |  |  |  | X | X | X |  |  |  |  |  |  |  |  |  |  |
Selector Switch No. 4 |  |  |  |  |  | X | X | X |  |  |  |  |  |  |  |  |
Selector Switch No. 5 |  |  |  |  |  |  |  | X | X | X |  |  |  |  |  |  |
Selector Switch No. 6 |  |  |  |  |  |  |  |  |  | X | X | X |  |  |  |  |
Selector Switch No. 7 |  |  |  |  |  |  |  |  |  |  |  | X | X | X |  |  |
Selector Switch No. 8 |  |  |  |  |  |  |  |  |  |  |  |  |  | X | X | X |
Selector Switch No. 9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | X | X

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