On Load Tap Changing such an arrangement is employed for changing the turn-ratio of the transformer to regulate the system voltage while the transformer is delivering load.
Nowadays almost all the large power transformers are provided with on-load tap clanger. It is invariable practice to provide the tappings at the neutral end of hv windings of a generator transformer.
The tappings on the windings are brought out through a terminal board to a separate oil-filled compartment in which the on lond tap changer switch is housed.
The tap changer is in the form of a selector switch The changer is operated by a motor-operated driving mechanism by local or remote control and a handle is also fitted for manual operation in case of an emergency.
The essential features of an on-load tap-changer are that during its operation the main circuit should not be opened to prevent sparking and no part of the tapped winding should get short-circuited.
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.
The impedance may be either a resistor or centre-tapped reactor on the basis of which the on-load tap-changers can, in general, be classified as resistor or reactor type. In modern designs, the current limiting is almost invariably carried out by a pair of resistors.
On-load tap-changing gear with resistor transition, in which one winding tap is changed over for each operating position is shown in figs.
The sequence of operations during the transition from one tap to the next (adjoing) (in this case from tap 2 to tap 3).
Back-up main contactors are provided which short-circuit the resistors for normal operation. In order to limit the loss of energy, it is essential that the resistors are in the circuit for a minimum
of time durations possible.
The resistors are designed for short-time rating for economical consideration and therefore, it is desirable to reduce to a minimum their duty time duration.
So it becomes essential to incorporate some form of energy storage in the driving mechanism to ensure that a tap change, once initiated, shall be completed irrespective of failure of auxiliary control supply.
All modern on-load tap-changers use springs energy as storage elements. They reduce the time that a resistor is in the circuit to a few seconds. The current break is eased by the fact that the short-circuit resistor current has unity power factor.
On-load tap-changer control gear may be from simple push-button initiation to complex automatic control of several transformers operating in parallel.
The aim of control is to maintain a given voltage level within a specified tolerance, or to raise it with load to compensate for the voltage drop in the transmission line.
The main components are an automatic voltage regulator, a time-delay relay, and compounding elements. The time delay prevents the initiation of a tap change due to minor transient Voltage Fluctuation. It may be set for a delay up to 60 seconds.
Another form of on-load tap-changer, provided with a centre tapped reactor is illustrated in fig.
The function of the reactor is again to prevent the short-circuiting of the tapped windings. During normal operation the short-circuiting switch, S remains closed.
The reactor used is suitably designed to prevent large values of current flowing in any section of the primary winding when two tapping switches are closed simultaneously.
For understanding applications of this type of on-load tap changer assume that the tapping switch 1 is closed and the output voltage is minimum.
For raising the output voltage, the short-circuiting switch S is opened, the second tapping switch is closed, the first tapping switch is opened and finally, the short-circuiting switch S is closed.
When the short-circuiting switch S is in the open position and the two tapping switches are in the closed position, a part of the transformer winding between the two tappings is shunted by the reactor, but a large circulating current is not established on account of its high reactance.
The line current remains unaffected by this reactance, as the current equally divides and flows in opposite directions in the two halves of the reactor.
When only one tapping switch is closed the reactor carries the full primary current. Instead of switches sliding contacts may be used.
Sliding contacts are so attached to the ends of the reactor that one makes before the other breaks and in normal operating conditions both contacts touch the same tapping stud.
If the reactor is designed to carry the full-load current continuously then a working position is possible with two consecutive tapping switches closed.
This provides twice as many turn ratios as there are taps, minus one. Usually, the tappings are located midway between the end-turns of the winding in order to prevent the surge voltages from getting into the load-ratio control elements.