ON LOAD TAP CHANGER
1. On load tap changer arrangement is employed for changing the turn ratio of the transformer to regulate the system voltage while the transformer is delivering the load.
2. Nowadays almost all the power large power transformers are provided with an on-load tap changer.
3. It is the invariable practice to provide the tapping at the neutral end of HV winding of a generator transformer.
4. The tapping on the winding is brought out through the terminal board to a separate oil-filled compartment in which the on-load tap changer switch is housed.
5. The tap changer is in the form of a selector switch.
6. The changer is operated by the motor-operated driving mechanism by local or remote control and a handle is also fitted for manual operation in case of an emergency.
7. The Most needed feature of an on-load tap changer are that during its operation the main circuit can not be opened to prevent heavy sparking and no any part of the tapped transformer winding can get short-circuited.
8. All types of on-load tap changing circuits are provided with an impedance, which is introduced to limit the short circuit current during the operation of tap changing.
9. The impedance may be either a resistor or center tapped reactor on the basis of which the on-load tap changer can, in general, classified as resistor or reactor type.
10. In modern designs the current limiting is almost invariably carried out by a pair of resistors.
OFFLOAD TAP CHANGER
1. This is the cheapest method of changing the turn ratio of a transformer.
2. As the name indicates it needs disconnection of the transformer from the supply before changing the tap.
3. This adjustment is made by tapping the respective winding as required and taking connections of the tappings to some position near the top of the transformer.
4. Tap changing is affected manually through hand holes provided in the core.
5. Alternatively the reconnection can be made by carrying the tapping leads through the cover for changing either by hand or by manually operated switches.
6. The commonly used switches are (i) verticle tapping switches and (ii) faceplate switches.
7. Vertical tapping switch compresses an insulated bar, one for each phase, carrying knife contacts, to be raised or lowered by hand wheel and gearing to make a connection to the tapping contacts.
8. In the faceplate switch the spindle of movable contact arms is geared to a shaft with a handwheel external to the tank.
9. Such switches may be mounted on the upper yoke, or located nearest to the tapped positions on the windings.
10. A mechanical lock is provided to prevent unauthorized operation of the off-load tap changer.
11. Further, for preventing inadvertent operation, an electromagnetic latching device or micro switch is provided to operate the circuit breaker so as to disconnect the transformer from the supply as soon as the movable arm is moved; well before the contact of the arm with the stud, with which it was in contact , opens.