Electrical Engineering Principle DNT 2353 CHAPTER 10 THREE

Electrical Engineering Principle DNT 235/3 CHAPTER 10 THREE PHASE TRANSFORMER baizura@unimap. edu. my

Three Phase Transformer 2

The knowledge that students can obtain from this chapter are: • Basic principle and structure of a three phase transformer. • Description of winding connections, delta and star configurations and autotransformer. 3

Construction • A three-phase transformer is constructed by winding three single-phase transformers on a single core. • Three-phase transformers are connected in either wye or delta configurations. 4

Three-phase transformer construction 5

The basic principle of a 3 phase transformer. • • The figure shows which primary windings have been shown inter connected in star output across 3 phase supply. The three cores are 120 o apart and their empty legs are shown in contact with each other. The centre leg, formed by these three, carries flux produced by the three-phase currents IR, IY and IB. As any instant IR+ IY + IB = 0, hence the sum of three flux is also zero. Therefore, it will make no difference if the common legs will act as the return for the third just as in a three phase of system any two conductors act as the return fore the current in the third conductor. 6

Basic Structure • Three similar limbs are connected by the top and bottom yokes, and each limb having primary and secondary windings are arranged concentrically. • The primary is shown by star-connected and the secondary by delta connected. 7

Based on STRUCTURE OF COILS 1. air core -- inside area of coil is empty -- inefficient induction 2. open core --- ferromagnetic material inside coils B = induction 8

(cont. ) 3. closed core -- ferromagnetic ends connected -- B =induction 4. shell -- lazy square eight -- overlap of coils on same ferromagnetic core 9

Transformer Type

Three phase transformer connections. • Labeling of transformer terminal – Terminal on the HV side of each phase will be labeled as capital letters A, B, C. – Terminal on the LV side will be labeled as small letters a, b, c. – Terminal polarities are indicated by suffixes 1 and 2 with 1’s indicating similar polarity ends and so do 2’s. – Assuming the transformer to be ideal, VA 2 A 1 (voltage of terminal A 2 with respect to A 1) is in phase with Va 2 a 1 and IA is in phase with Ia. 11

• There are only 4 possible transformer combinations: • Delta to Delta - use: industrial applications • Delta to Wye - use : most common; commercial and industrial • Wye to Delta - use : high voltage transmissions • Wye to Wye - use : rare, don't use causes harmonics and balancing problems. 12

Star/ Delta or Y- ∆ Connection. • The equation for Y - ∆ connection are ; – Primary side – I 1 phase = I 1 line. – V 1 phase = – Secondary side – I 2 phase = – V 2 phase = V 2 line Output Power = √ 3. V 2 Line. I 2 Line. cos Ф Transformation ratio. K = 13

• Star/delta -30 o connection – From the phasor diagram that phase a to neutral on the delta side lags by -30 o to the phase to neutral voltage on the star side; this is also the phase relationship between the respective line to line voltages. – This connection, therefore, is known as – 30 o connection. 14

• Star/delta +30 o connection 15

Star/ Star or Y – Y Connection • This connection is most economical for small, high voltage transformers because the number of turns/phase and the amount of insulation are minimum (as phase voltage is only 1/√ 3 of line voltage). 16

Star/star 0 o-connection 17

Delta / Delta or ∆ - ∆ Connection. • This connection is economical for large, low voltage transformers in which insulation problem is not so urgent, because it increases the number of turns/phase. 18

Delta/delta 0 o-connection • • The sum of voltages around the secondary delta must be zero; otherwise delta, being a closed circuit, means a short circuit. With polarities indicated on primary and secondary side, voltages Va 2 a 1, Vb 2 b 1 and Vc 2 c 1 add to zero as per phasor diagram if the delta is formed by connecting a 1 b 2, b 1 c 2 and c 1 a 2. It is easily seen from the phasor diagram that the primary and secondary line voltages are in phase so it is the 0 o – connection. The phase transformation ratio is x: 1, the transformation ratio for line quantities is also x: 1. 19

Auto transformer • An auto transformer is an electrical transformer with only one winding. • This single coil has one or more extra taps or electrical connections in various positions along the winding. • Each tap corresponds to a different voltage so that effectively a portion of the same inductor acts as part of both the primary and secondary winding. • I 1 and I 2 = Primary and secondary currents respectively 20

Step-down auto transformer • A single phase auto transformer having N 1 turns primary with N 2 turns tapped for a lower voltage secondary. The winding section BC of N 2 turns is common to both primary and secondary circuits. • In fact it is nothing but a conventional two winding transformer is connected in a special way. The winding section AC must be provided with extra insulation, being a higher voltage. • Current flows in the winding section BC is Ic = I 2 – I 1 =K= 21

Step-up auto transformer • A single phase autotransformer having N 1 turns primary with N 2 turns tapped for a higher voltage secondary. • Current flows in the winding section AB is Ic = I 1 – I 2 22

Example 4. 1 • A Three phase transformer bank consisting of three 1 phase transformers is used to step down the voltage of a three phase, 6, 600 V transmission line. If the primary line current is 10 A, calculate the secondary line voltage, line current and output k. VA for the following connections: (a) Y/∆ and (b) ∆/Y. The turn ratio is 12. Neglect ratio. 23

Solution 24

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Example 4. 2 • Determine the core area, the number of turns and the position of the tapping point for a 500 k. VA, 50 Hz, single phase, 6, 600/5, 000 V autotransformer, assuming the following approximate values : e. m. f. per turn 8 B. Maximum flux density 1. 3 Wb/m 2. 26

Solution 27

Example 4. 3 • The 2000/200 -V, 20 -k. VA transformer is connected as a step-up auto transformer as in Figure 4. 17 in which AB is 200 V winding and BC is 2000 -V winding. The 200 -V winding has enough insulation to withstand 2200 -V to ground. If Core loss of transformer is 120 W and full load copper loss is 300 W. Calculate (a) The LV and HV side voltage ratings of the autotransformer; • (b) Its KVA rating; • (c) Its efficiency at full-load 0. 8 pf. 28

Solution 29

Cont’ 30