Mehran University Of Engineering & Technology, SZAB Khairpur Mirs Campus ENGR. AHSANULLAH MEMON LECTURER DEPARTMENT OF ELECTRICAL ENGINEERING MUCET KHAIRPUR MIRS TAPPINGS

 When a transformer carries load current there is a variation in output voltage which is known as regulation.  In order to compensate for this, additional turns are often made available so that the voltage ratio can be changed using a switch mechanism known as a tap changer.  Voltage supplied by transformers can be varied by changing it s transformation ratio.  This can be achieved by tappings which are provided on transformers.  The tappings are the leads which are connected to various points on a transformer winding.

 A tap changer is an connection point selection mechanism along a power transformer winding that allows a variable number of turns to be selected in discrete steps.  A transformer with a variable turns ratio is produced, enabling stepped voltage regulation of the output.  The tap selection may be made via an automatic or manual tap changer mechanism.  tap points are usually made on the high voltage (primary) or lower current winding of the transformer, to minimize the current handling requirements of the contacts.

Why Tappings are on High Voltage Side  A fine voltage regulation is possible with high voltage winding as it carries large number of turns.  The low voltage winding of the transformer carries large current. So if tappings are provided on low voltage side then then there are difficulties encountered in the interruption of high currents which makes its impracticable.  For the reasons of requirement of insulation, the low voltage (l.v.) winding is placed near the core while the HV winding is placed outside. Hence practically it is easier and simpler to provide tappings on high voltage winding.

 In case of step down transformers, it is an added advantage to provide tappings on h.v. side. At light loads, the l.v. side side voltage increases. It is required to decrease this voltage by adjusting the tapping on h.v. side to a position where number of turns are large. With large number of turns, the flux and flux density decreases. This results in reduction of core loss which increases transformer efficiency at light loads.  If the tappings are provided on the l.v. side then the exact voltage regulation may not be provided.

 An on-load tap-changer must not break the supply to the transformer's windings while the transformer is on load.  Large network transformers which are provided with on-load tap changing normally have a much larger number of taps in smaller steps.  The principle used is ‘make-before-break’: this means that the new tap must be connected before the old tap is broken, otherwise there would be a break in supply and an interruption of full-load current by the tapping switch.  The difficulty with this simple idea is that, during the transition period while both taps are made, a small number of turns of the transformer’s HV winding are short-circuited by the two taps, and a heavy current will flow through them. Arrangements are therefore made to insert resistance temporarily into this short-circuited loop to limit the current until the tap change is complete and the short-circuit removed.

 A, B and C are adjacent taps on an HV winding. In (a) the tapping is on A, and it is desired to move it, on load, to B.  The moving member consists of a main contact M and two ‘transition’ contacts P and Q which are connected to M each through a resistance. In position (a) M carries the full load, and P and Q are not in contact.  In the first part (b) of the transition the main contact M is still on tap A. Contact Q moves to B and contact P is still on A. Q and M now short-circuit the HV turns between A and B, but the short-circuit current is limited by the lower half of the resistance. Meanwhile M is still carrying the load current from tap A.  At the next stage (c) the moving member has travelled on, and the main contact M leaves tap A. P and Q now share the load current which passes through both halves of the resistance. These two halves also limit the current in the shorted turns between A and B.  At the next stage (d) the main contact M has moved to tap B, so that it is once again carrying the load current, but now from the new tap. P however is still on tap A, so that the current from the shorted turns is limited by the upper half of the resistance.  Finally the moving member is at position (e), where the main contact M is on B and carrying the load, while P and Q are out of contact, as they were in position (a), but now on the new tap.  During these transition stages the load current has never been interrupted, nor has the main contact ever been called upon to break any large current. Moreover the current in the short-circuited turns is always limited by one or both halves of the resistance

 An off-load tap-changer does not require a make-before-break arrangement, because it is designed to be operated when the transformer is disconnected.