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Electrical Machine – I Aim: To Study the Basic connections of 3- Transformer. Specific Objective : After having study the experiment, one should be able to : Identity the H.V and L.V winding. Understand the basic fundamentals of 3- transformer connections. Machine Specification: 3- Transformer. Rationale : Three phase operation can be either by means of a single three phase transformer or three numbers of identical single phase transformers. So altogether 3 sets of windings are used for this operation. Each of the three phases will have one HV winding and one LV winding according to construction or one primary winding and one secondary winding according to the operation. Let A1 A2 , B1 B2 and C1 C2 be terminals of the three primary winding and a1a2 , b1 b2 and c1 c2 are the terminals of respective secondary windings. The high –voltage and low-voltage winding terminals of a three-phase transformer are connected either in star or in delta for connections to a three –phase system. A bank of three single-phase transformers can also be connected similarly. When the primary high-voltage windings terminals are connected in, say, star and the secondary low-voltage winding terminals are connected in, say, delta, it is said that the transformer windings are connected in star-delta .Similarly, the following are some of the more popular connection used. Star–Star (Y-Y), Delta –Delta (∆-∆) Star -Delta (Y-∆), Delta –Star (∆-Y) There is a define time-phase relationship between the terminals voltages of the high-voltage side and low-voltage side for three connections. The time-phase relationship between the voltages of high-voltage and lowvoltage side will depend upon the manner in which the windings are connected . Star-Star Connection: Sankersinh Vaghela Bapu Institute of Technology Page 1 of 8 Electrical Machine – I This connection is symbolically denoted as Y-Y. Fig 1(a) shows the connection diagram and the Vector Diagram for star-star connection. Here, the finishing end of H.V windings A1, B1 and C1 are connected together to form the primary neutral point N. The three phase input supply points R, Y, and B are connected to the starting ends A1, B1 and C1 respectively. In star-star connection line-to-line voltage ratio is same as the phase –tophase voltage ratio which is nothing but the secondary to primary turns ratio K. Further, the secondary line-to-line voltage is in phase with the primary line-to-line voltage i.e phase displacement is zero as shown in vector diagram. If the high-voltage side and low-voltage side windings are connected in star-star as shown in fig (a), the phase displacement will be zero. If ,however, the low-voltage windings connections are reversed, the timephase displacement in induced voltages between the high-voltage and lowvoltage winding will be 180 deg as shown in fig (b). Advantages of The number of turns/phase and the amount of insulation is minimum as Vphase = VL /√ 3 . Hence , Y-Y connection is the most economical connection for small capacity, HV transformer. The possibility of utilizing both the star points N and n for a fourth wire may be useful. Disadvantages of Even if a sinusoidal voltage wave is impressed on the primary side, the exciting current is not a pure sine wave, but contains third harmonic component. Consequently , the three exciting current do not add up to zero and neutral is needed to a carry the resultant current. If the neutral is not present, the exciting current in the three phases will be sine wave, leading to the flux and hence the secondary phase voltage will have a third harmonic component. If the loads are unbalanced, the secondary phase voltage becomes unbalanced unless the secondary star point is earthed. For a satisfactory operation, another set of delta connected secondary winding called tertiary winding may be required. This delta connected winding suppresses the third harmonic components as well as supply power to the local loads in the generator station or sub-station. Delta-Delta Connection Symbolic representation for this connection is ∆-∆ Fig 1(c) Shows the connection diagram and 4(b) shows the Vector Diagram for delta-delta connection. In ∆-∆ connection ,the line-to-line voltage ratio is same as the transformation ratio K. similar to the condition seen in Y-Y Connection. Sankersinh Vaghela Bapu Institute of Technology Page 2 of 8 Electrical Machine – I Further , the output line-to-line voltages VRY , VYB and VBR are in phase with input line-to-line voltages VRY , VYB and VBR respectively. If the high-voltage side and low-voltage side windings are connected in delta-delta as shown in fig1 (c), the phase displacement will be zero. If ,however, the low-voltage windings connections are reversed, the timephase displacement in induced voltages between the high-voltage and lowvoltage winding will be 180 deg as shown in fig (d). Advantages The cross-sectional area of conductor is reduced because the current through the winding or phase current is 1/√ 3 times the line current. Third harmonic component of magnetizing current can flow within the delta connected primary windings without flowing in the line wires. Large unbalance of load can be met without difficulty. If the operation is done with three units of single-phase transformers , it is possible to operate the transformer on 58 percent of its normal rating in V-V connection, should one of the transformer develop a fault. Disadvantages The absence of star point be the only disadvantage. Star-Delta Connection Symbolic representation for this connection is Y-∆ Fig 2(a) Shows the connection diagram and the Vector Diagram for star-delta connection. In Y-∆ connection , the line-to-line ratio becomes K/√ 3 and normally used for the step-down operation. Further the secondary line-to-line voltage lags the input or primary line-to-line voltage by 300. If the primary high-voltage windings are connected in star and the secondary low-voltage windings are connected in delta, the phase displacement will be -300as shown in fig 2(a) If the winding connections of the low-voltage side are reversed the phase displacement will +300 as shown in fig 2(b) . Advantages Delta connected secondary causes the third harmonics currents to get circulated within the winding and not to appear on the lines. The connection can withstand the unbalanced operation in a stable manner. Sankersinh Vaghela Bapu Institute of Technology Page 3 of 8 Electrical Machine – I If the HV winding is in star-connected side, there is some saving in the cost of insulation as the phase voltage is 1/√ 3 times line voltage. Delta-Star Connection Symbolic representation for this connection is ∆-Y Fig 2(c) Shows the connection diagram and the Vector Diagram for delta-star connection. In ∆-Y connection, the line-to-line voltage ratio becomes √ 3 √3 K or times the phase voltage ratio and hence used for step-up operation. Further the output line-to-line voltage VRY , VYB and VBR leads the supply input line-toline voltages VRY , VYB and VBR be 300 respectively. If the primary high-voltage windings are connected in delta and the secondary low-voltage windings are connected in star, the phase displacement will be +300 as shown of fig.2(c). If the winding connections of the high-voltage side are reversed the phase displacement will -300 as shown in fig 2(d). Advantages of ∆-Y Delta connected primary causes the third harmonics currents to get circulated within the winding and not to appear on the lines. The connection can withstand the unbalanced operation in a stable manner. If the HV winding is in star-connected side, there is some saving in the cost of insulation as the phase voltage is 1/√ 3 times line voltage. The phase difference between the high voltage and low-voltage windings for different types of connections can be represented by comparing it with the hour hand of a clock. When the hour hand of clock is at the 12 o’clock position it is considered zero displacement. When it is at the 11 clock position the displacement is +300 (Anticlockwise is positive). When the hand is at the 1 o’clock position the displacement is -300 and at the 6 o’clock position it is 1800, as shown in fig.3 Thus the connections of fig (a), (b), (c) and (d) can respectively be represented Yyo , Yy6 ,Dd0 and Dd6 . The connections of fig (a), (b), (c) and (d) are often represented as Yd1 , Yd11 , Dy1 and Dy11 respectively. Sankersinh Vaghela Bapu Institute of Technology Page 4 of 8 Electrical Machine – I Fig-1 Sankersinh Vaghela Bapu Institute of Technology Page 5 of 8 Electrical Machine – I Fig-2 Sankersinh Vaghela Bapu Institute of Technology Page 6 of 8 Electrical Machine – I Y Date: Z 1 1 YZ 1 Sankalchand Patel College of Engineering Page 7 of 8 Electrical Machine – I Sankalchand Patel College of Engineering Page 8 of 8 Date: