 myself, Mrs. Vina Sunil Patki, Assistant Professor, Department of Electronics Engineering, Valchan Institute of Technology, Solapur. Welcome you all for this session. At the end of this session, students can describe connections of three-phase transformer. Now, first let us discuss about the construction of three-phase transformer. So, in this diagram, you can see the core-type transformer and shell-type transformer. So, three-phase winding is placed on three limbs. You can see in this diagram, the three limbs are provided with the three windings. The primary winding and secondary winding is connected on the same limb to provide the maximum linking of the magnetic fluxes from primary to secondary. If we connect the AC voltage to the primary winding that will create the fluctuating fluxes in this transformer, that fluctuating fluxes are linked to the secondary winding. If we want to explain the working of transformer in one word, we can say the mutual induction. Then what is the mutual induction? Mutual induction is nothing but when we vary the current in primary or first coil that induces the EMF in second coil. So, this is the working principle of transformer. So, there are types of three-phase transformer connections. So, when we say the three-phase winding is there, we have to connect that three-phase winding in two manner, star or delta. So, according to that, there are number of types of connection, delta-delta connection, delta-star connection, star-delta connection, then star-star connection, then interconnected star or delta connection or interconnected star or star connection means the primary winding is connected in delta and secondary winding is connected in delta like that. So, according to that connection of primary and secondary, these are the types of the transformer connection. So, we are going to discuss about these connections in detail. Now, pause the video and discuss about this. What is the working principle of transformer in one word? In only one word, we can say the mutual induction is the working principle of transformer. Now, let us discuss about the delta-delta connection. Primary is connected in delta and secondary is connected in delta. These are the three phases and these are the three lines. Three line voltages are equal and three-phase voltages are equal for balanced system. And the line voltage and phase voltage for this delta is same. But the current, line current and the phase current has the relation that is the line current is I L and if the phase current is I P H, then the phase current into root 3 is nothing but the line current. So, here the relation between line voltage and phase voltage is same and line current is root 3 times the phase current. And you can see from this phasor diagram, both voltages are in phase. If one transformer is disabled, system can continue to operate in open delta connection but with reduced available capacity. And this connection is used for moderate voltages also. No distortion of flux in third harmonic current and not flowing to the line wire. Here in delta connection, the star point is not there or neutral point is not there. So, if the fault may occur, that fault may be severe. Primary is connected in delta. Again the delta connection line voltage and phase voltages are equal. Line current is given by root 3 times phase current. But in star, line voltage and phase voltage has the relation. But here, line voltage both are not in phase for primary and secondary. There is phase difference of 30 degree. From phasor diagram you can see that. So, secondary phase voltage is 1 by root 3 times the line voltage. And this type of connection is mainly used in step up transformer at the beginning of transmission line. So, neutral in secondary can be grounded for 3 phase 4 wire system. And the ratio of secondary to primary line voltage is root 3 times the transformation ratio. And there is 30 degree shift between primary and secondary line voltages. And neutral shifting third harmonic are present in this system. Now, let us discuss about the star delta connection. So, primary is connected in star. In star, line voltage and phase voltage has the relation. That is, phase voltage into root 3 times is the line voltage. And in secondary, the delta connection is there. Line voltage and phase voltages are equal. But in star connection, line current and phase current both are equal. And here also, the phase difference is there for primary and secondary. You can see that from that phasor diagram. The ratio of secondary to primary line voltage is 1 by root 3 times the transformation ratio. There is 30 degree shift between the primary and secondary line voltages. And large unbalanced loads can be handled satisfactorily by using this star delta connection. And we can use this connection at the end of transmission line for step down the voltage. Star star connection in this primary and secondary both are connected in star. You can see from the phasor diagram, primary and secondary voltages are in phase with each other. We can handle the heavy load by using these connections. Line voltages on both sides are in phase with each other. This connection works satisfactorily only in balanced load. And it requires less amount of winding as compared to delta connection. And due to that, the star star connection is cheaper because here the number of winding turns are less. As the phase voltage is less, dielectric strength in insulating material is required. Less material is required because as we discuss about this, the number of turns are less for this connection. So, the dielectric strength should be less for this. And transformer core always leads to generation of third harmonic for this connection. In delta connection, if any one phase is not working, we can say that that connection is the VV connection. You can see here in the second diagram, one phase or one winding is not working. So, only two phases are present so that that is why this connection is called as the VV connection. So, three phase load that can be carried without exceeding the rating of the transformers is 57.5% of the original load. Means this VV connection works with 57.5% of the original load that is related to delta connection. So, we can work delta connection with less load if one phase is not working. And some voltage unbalanced due to the non-symmetry of the voltage regulation effects under this load. And the average power factor at which VV bank is operating is less than that with the load. And this power factor is 86.6% of the balanced load power factor. And generally we can use this VV connection to start the AC motors. So, this is the Scott connection. So, teaser transformer is there and main transformer is there. The teaser transformer has the primary voltage rating that is root 3 by 2 or 0.866 of the voltage rating of the main transformer. We can use this TT connection or Scott connection for three phase to two phase conversion. Also used to link the three phase system with two phase system with flow of power in either direction. And it is used to supply a single phase load such as electric train and electric furnace installation. You can refer these books. Thank you.