Ahmedabad Institute of Technology Branch : Electrical Subject : DC Machine & Transformer (2130904) Semester : 3rd Enrollment no Name 130020109014 Halpati Brijal 130020109015 Hedau Nikita 130020109016 Jadav Yash 130020109017 Jadia Sachin 130020109018 Kamaliya Dipak 130020109019 Koladiya Prakash 130020109020 Kothadiya Umang 130020109021 Kumawat Nirav 130020109022 Ladumor Mayur

Three Phase Transformer Connection

Introduction There are different method for connecting 3-phase transformers 3- phase voltage to higher or lower value. The HV &LV winding of a 3-phase transformer can be connected in star or delta. The choice of a particular connection depends upon the condition under which the transformer is to be operated. The most common & widely used transformer connection are Star/Star (2) Delta/Delta (3) Delta/Star or Star/Delta (4) Zig Zag

Star/Star This is the most economical connection for small, high voltage transformer. As the phase voltage Vph = VL / 31/2, it needs lesser turns per phase & insulation required is minimum. Possibility to provide neutral connection. The star point can be used in case of 3-phase, four wire system. This connection is more suitable for core type transformers.

Delta/Delta This connection is more suitable for large, low voltage transformers. It increases the number of turns per phase & reduces the necessary cross section area of conductors, thus insulation problem is not present It gives satisfactory performance even if there is large unbalanced in load. The third harmonic voltages are absent due to closed delta. The absence of star or neutral point is the disadvantages of this connection.

Delta/Star or Star/Delta This connection is most commonly used for power supply transformer. The neutral point allows both types of loads to be met. The delta connected winding carries third harmonic current due to which potential of neutral point is stabilized. The HV side is normally connected in delta so that the three phase loads & single phase loads can be supplied by LV side using three phase, four wire system.

Zig Zag connection This connection reduces the third harmonic voltages. It allows unbalance loading. This connection is mainly employed for LV winding. For a given total voltage per phase, the Zig Zag side requires 15% more turns as compared to normal phase connection. Zig Zag connection is used where delta connections are mechanically weak due to large number of turns & small cross section of conductor in case of delta in high voltage transformers. It also used for rectifiers.

Terminal marking According to standards, the terminals of transformer are brought out in rows with HV winding on one side while LV winding on the other side. The HV terminals are represented with capital letters. Generally A, B & C are used for three phases. The LV terminals are represented with small letters. Generally a, b & c are used for three phases. The neutral terminal are represented after line terminals. The capital letter N is used to represent the HV side neutral terminal & small letter n is used for representing the LV side neutral. The two ends of each winding are designated by subscript numbers 1,2. The phase displacement between HV & LV winding id indicated by the angle in terms of clock face.

The vector for the HV winding is taken as reference phasor The vector for the HV winding is taken as reference phasor. The phase displacement of the phasor of LV winding from the reference phasor is represented by the use of clock hour method. The HV vector being at 12 o’clock & the corresponding LV vector at hour hand number. For example, if the connection is written as ‘Dy11’ then it represent ,HV side delta, LV side star connected transformer & LV emf vector is at 11o’clock position i.e. =30o in advance of the 12 o'clock position of HV emf. It represented by clock face method as shown in fig

Phase Groups &Vector diagrams For transformer Connection In three phase transformer, the polarity alone is insufficient to represent the relation between HV & LV windings. In addition to the terminal markings on HV & LV windings, the vector diagram of induced voltages are required to show the angular displacement between HV & LV windings. According to Indian Standards, there are four vector or phase groups of standard connection. Group(1) : Zero phase displacement (Yy0, Dd0, Dz0) Group(2) : 180o phase displacement ( Yy6, Dd6, Dz6) Group(3) : 30o lag phase displacement (Dy1, Yd1, Yz1) Group(4) : 30o lead phase displacement (Dy11, Yd11, Yz11)

Group 1 : Zero phase displacement

Group 2 : 180o phase displacement

Group 3 : 30o lag phase displacement

Group 4 : 30o lead phase displacement

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