On-load Tap-changer
This tap-changer is used for daily or short period voltage alterations. The output voltage can be regulated with the changer, without any supply interruptions. During the operation of an on-load tap changer;
(1) the main cireuit should not be opened otherwise dangerous sparking will occur and
(2) no part of the tapped winding should get short-circuited.
One form of elementary on load tap-changer is illustrated in Fig. 1.56 (a). The centre tapped reactor C prevents the tapped winding from getting short-circuited. The transformer tappings are connected to the correspondingly marked segments 1 to 5. Two movable lingers A and B. connected to centre-tapped reactor uia switches x and y, make contact with any one o the segments under normal operation.
In Fig. 1.56 (a), both the fingers are in contact with segment 1 and full winding is in circuit. Switches x, y are closed. One half of the total current flows through x,lower half of the reactor and then to the external circuit. The other half of the total current flows through y,
upper half of the reactor and then to the external circuit. It is seen that currents in the upper and lower halves of the reactor flow in opposite directions. Since the whole reactor is wound in the same direction, the m.m.f. produced by one-half is opposite to the m.m.f. produced by the second half. These m.m.fs. are equal and the net m.m.f. is practically zero ; therefore, the reactor is almost non-inductive and the impedance offered by it is very small. Consequently the voltage drop in the centre-tapped reactor is negligible.
When a change in voltage is required, the fingers A and B can be brought to segment 2 by adopting the following sequence of operations:
(1) Open switch y, Fig. 1.56 (b-i). The entire current must now flow through the lower half of the reactor. It, therefore, becomes highly inductive and there is a large voltage drop. It
should be noted that the reactor must be designed to handle full load current, momentarily.
(2) The finger B carries no current and can, therefore, be moved to segment 2, without any sparking [Fig. 1.56 (b-ii)]
(3) Close switch y, Fig. 1.56 (b-iii). The Transformer winding between taps 1 and 2 gets connected across the reactor. Since the impedance offered by the reactor is high for a
current flowing in only one direction, the local circulating current flowing through the reactor and tapped winding is quite small. In this manner, the reactor prevents the tapped winding from getting short-circuited. The terminal voltage will be mid-way between the potentials of
tappings 1 and 2.
(4)Open switch x. The entire current starts flowing through the upper half of the reactor, manifested by a large voltage drop, Fig. 1.56 (b-iv).
(5) Move the finger A from segment 1 to segment 2 and then close switch x. The winding between taps 1 and 2 is, therefore, completely out of circuit, Fig. 1.56 (b-v). If further change in voltage is required, the above sequence of operations is repeated.
For large power transformers, the switches x and y may be circuit-breakers.
Another form of on-load tap-changer, also provided with a centre-tapped reactor, is illustrated in Fig.1.57. The function of the reactor is again to prevent the short-circuit of the tapped winding. The switches 1,2……5 are connected to the correspondingly marked taps. The switch S in Fig. 1.57 is closed during normal operation. With switches 2, 3, 4, 5 opened and switch 1 closed, the entire winding is in circuit. Here again the two halves of the reactor, carry half of the total current in opposite directions. In changing from tap 1 to tap 2, the following sequence of operations is carried out:
(1) Open switch S. Now total current flows through the upper half of the reactor and there is more voltage drop.
(2) Close switch 2. Winding between taps 1 and 2 is connected across the reactor.
(3) Open switch 1. The entire current now flows through the lower half of the reactor and there is more voltage drop.
(4) Close switch S. The total current now flows equally between the upper and lower halves of the reactor.
For changing from tap 2 to 3,the above sequence of operations is repeated.
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