ON Load Tap Changer
There is always requirement of variation of voltage at secondary side even when load is connected then on load tap changer is used for the same. In case of on load tap changer tapping’s on the winding are brought out through a terminal board to separate oil filled compartment in which the on-load tap changer switch is housed. This tap changer is in the form of switch with motorized control. Handle is also provided in case of emergency.
The essential feature of an on-load tap changing gear is the maintenance of circuit continuity throughout the tap changing operation. The circuit must not be broken otherwise there will be discontinuity of supply to load. Selector switch must not break current an additional separate oil filled compartment is used to house the diverter switch which breaks the load current by an interrupted arc. During that process carbon will be formed therefore oil in the diverter switch compartment must be prevented from mixing with oil in the main tank. Oil in the selector switch tank might be connected to main transformer tank oil through conservator.
In case of on load tap changer continuity with next tap must be made before one tapping is left opened. Therefore, selector switch is on-load tap changers is a make before a break switch and during the period of transition from one tap to another, momentary connection must be made between adjacent taps. This will lead to short circuiting of turns between the adjacent tappings. Therefore, the short circuit current must be limited by including resistors or reactors.
On Load tap changer winding connection are shown below where there is one Selector switch S1 is at Tap no.1 and other S2 is on Tap 2. The Diverter switch S3 is shown connecting tap 1 to the neutral point of the transformer winding and the Switching for the changeover to Tap2 is as follows:-
1. Contacts “a”and “b” are closed Resistance R1 shorted as shown. The load current flows from tap through contact b. This is the running position at Tap1.
2. An External mechanism moves the diverter switch S3, contact b opens. The load current from Tap1 now flows through resistor R1 and contact a.
3. As the moving contact of S3 continues it travels to the left, contact d closes and resistance R1 is open circuited. Both the resistances R1 and R2 are connected across taps 1 and 2 and the load current flows through these resistances to their mid point.
4. When diverter switch S3 moves still further to left contact a is opened. The load current flows from Tap2 through resistance R2 and contact d.
5. Finally as the Switch S3 reaches to extreme left position, contact c Closes and resistance R2 is short circuited. The load current from tap2 flows through contact a. This is running position for tap 2.
From above we have seen that there isn’t any movement of S1 and S2 and only there was movement of S3.
However , a further tap change is the same direction i.e. from tap 2 to tap 3 , is required, the selector switch S1 is moved to tap 3 before selector switch S1 is moved to Tap 3 before the diverter Switch S3 Moves. The diverter switch then follows the sequence as described above but in the reverse order.
To limit energy loss due to resistance, resistance should be kept for as minimum time in line as possible.