U-III_Three_Phase_Transformer enhance.pptx

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1. 1 Teaching Innovation - Entrepreneurial - Global The Centre for Technology enabled Teaching & Learning , N Y S S , India DTEL (Department for Technology Enhanced Learning)

2. DEPARTMENT OF ELECTRICAL ENGINEERING Program : III Sem. Reg. 2 UNIT -III THREE PHASE TRANSFORMERS Course Name: ELECTRICAL MACHINES Course code: EL 2203 Lectures per week: 04

3. Unit - IIi : SPECIFIC Objective DTEL . Comparison between a 3-ph. unit & a bank of 3 single phase transformer 1 2 3 3 5 Objectives: The student will be able to understand: . 6 . 7 Open delta connection Types of Three Phase Transformers Construction of different types of 3-phase transformers Different connections of the transformer 4 Polarity marking, its need & tests to mark polarities Vector grouping of the transformer & clockface method

4. Unit -III: SPECIFIC Objective DTEL . Concept of 3- winding transformer & its application 8 On load & off load tap changing transformers , 9 10 4 12 Objectives: The student will be able to understand: O.C. & S.C. Tests Parallel Operation of 3-phase transformers All day Efficiency 11

5. Unit –III: SYLLABUS DTEL 5 UNIT-II: THREE PHASE TRANSFORMER: Types of 3-phase transformers, Construction, Labeling of terminals, Vector Groups, Polarity Test, Concept of three winding transformer and its application for stabilizing, On load and off load tap changing transformers, OC & SC tests, Parallel Operation, All day efficiency, Type and Routine tests on three phase transformers, cooling and maintenance of 3-phase transformers, Open delta connection.

6. LECTURE 1 DTEL 6 INTRODUCTION :3-PH. TRANSFORMER INTRODUCTION: Three phase system is used to generate and transmit large amounts of power because it is economical over single phase system. For facilitating transformation of power at different levels of power system, three phase step up or step down transformers are required. Objective of this unit is to understand types, construction, connections, operation and different tests of three phase transformers. 2.1:COMPARISON BET’N A 3-PH.UNIT & A BANK : 3-ph. transformation can be achieved in either way: 1. A bank of three 1-phase transformers suitably connected for 3-ph.operation 2. A single three phase unit with cores and windings for three phases combined in one single structure.

7. LECTURE 1 DTEL 7 COMPARISION :3-PH. UNIT & BANK 3-Ph. Unit Transformer: Advantages over a bank: It is lighter in weight, cheaper in cost and small in size occupying less space. It has more efficiency. High voltage terminals to be brought out of the transformer tank are three as against two from each 1-phase transformer forming the bank. Busbar structure, wiring and switchgear for a single 3-phase unit are simple. However, if any one phase winding breaks down, the whole transformer is to be removed from service for repairs & thus continuity of power supply gets disturbed. Also a whole unit is to be maintained as a reserve unit increasing capital cost and interest thereupon.

8. LECTURE 1 DTEL 8 Bank of Three 1-Ph. Transformers: Advantages over a unit 3-ph. transformer: Transport of single phase transformers forming bank is convenient. Three phase unbalanced load can easily supplied by taking one single phase transformer of higher KVA rating than that of others. Only one single phase transformer is to be maintained as reserve unit. When any one 1-ph. transformer develops a fault, it can be removed from service and power can still be supplied with remaining two transformers connected in open delta, but at reduced capacity of 57.735% COMPARISION :3-PH. UNIT & BANK

9. LECTURE 1 DTEL 10 CONSTRUCTION:3-PH.TRANSFORMER: 2.2.1: 3-PH. CORE TYPE T’FORMER: Construction of magnetic core of 3-ph. core type transformer can be understood easily by assuming cores of three 1-phase core type transformers positioned at 120° to each other as shown in Fig. 2.2.1.1, as three phase voltage phasors are displaced from each other by 120°. If balanced 3-ph. sinusoidal voltages are applied to primary windings of three 1-ph. transformers, they produce sinusoidal & balanced alternating fluxes. Fig.2.2.1.1 Cores of three single ph. transformers https://youtu.be/H2hYUu8lPY0

10. LECTURE 1 DTEL 11 CONSTRUCTION:3-PH.T’MER: (Contd..) If the central limb is formed by merging the 3 limbs of the three single phase transformers, it will carry flux φ = φ a + φ b + φ c which is equal to zero. Hence the central limb can be completely removed as it carries no flux. After removal of the central limb, magnetic core will be like that shown in fig. 2.2.1.2. Fig.2.2.1.2 Core with central limb removed This configuration is neither easy to construct nor economical. Hence structure with three limbs in the same plane as shown in fig.2.2.1.3 is used. https://youtu.be/H2hYUu8lPY0 Fig.2.2.1.3 Practical Core arrangement with flux distribution

11. LECTURE 1 DTEL 12 This structure can be build from E-I or T-U sections of laminations. In this structure, the magnetic paths for fluxes produced by windings on the three limbs are not identical. Paths for the outer limbs are greater than that of the central limb, however the resulting imbalance in the magnetizing current magnitudes is not much significant. WINDING ARRANGEMENT: Both primary and secondary windings of a phase are provided on the same limb. LV winding is provided next to the core with insulation between the core & LV winding HV winding is provided over the LV winding with suitable insulation between them. CONSTRUCTION:3-PH.T’MER: (Contd..)

12. LECTURE 1 DTEL 13 CONSTRUCTION:3-PH.T’MER: (Contd..) 2.2.2 THREE PHASE SHELL TYPE TRANSFORMER: 3-phase shell type transformer can be made up of magnetic core having shape as if cores of 3 single phase shell type transformers are stacked one above another, as shown in fig. 2.2.2.1. HV & LV windings of a phase are provided on same central limb of a section. The direction of winding on the middle section is opposite of those on the top & bottom sections. Fig. 2.2.2.1 Core arrangement & Flux distribution in 3-Phase Shell Type Transformer

13. LECTURE 1 DTEL 14 CONSTRUCTION:3-PH.T’MER: (Contd..) For balanced system with phase seq. abc , the fluxes are balanced i.e. ф a ∟0°, ф b ∟-120° = α 2 ф a , ф c ∟120° = α ф a where α = 1 ∟120°. Flux in the adjacent yoke of sections a & b is ½ φ a + ½ φ b = ½ φ a ( 1∟0°+ 1∟240°) = ½ φ a ∟-60° Thus the combined flux being equal to magnitude of each component flux, the cross sectional area of adjacent yokes, outer legs and top & bottom yokes are all equal to that of middle legs. Slight imbalance in mag. Paths for 3 phases has negligible effect on performance. Windings on the HV & LV sides may be connected in Star or Delta as desired.

14. LECTURE 1 DTEL 15 If the winding on the middle section is in same direction as those in top & bottom sections, flux in the adjacent yokes of sections a & b is ½ φ a - ½ φ b = ½ φ a (1 ∟ 0°-1 ∟ 240 °) = 0.866 φ a ∟30° Thus core cross sectional area in the yoke portions adjacent to sections a & b and b & c are 173 % of that in the top & bottom yokes, the outer legs. CONSTRUCTION:3-PH.T’MER: (Contd..)

15. DTEL 16 THANK YOU

16. LECTURE :2 DTEL 17 POLARITY MARKING, ITS NEED & TESTS Polarity marking of a transformer is done for identifying the relative directions of induced voltages in the two windings. The polarities result from the relative directions in which the two windings are wound on the core. In order to connect windings of same transformer in series or parallel, to connect two or more transformers in parallel or to connect single phase transformers in to a bank for polyphase transformation of voltages, it is necessary to know the polarity of different terminals.

17. LECTURE :2 DTEL 18 Fig.2.3.1 Polarity marking Polarity marking designates relative instantaneous directions of current and voltage in and at the transformer leads. Directions of instantaneous currents are shown by arrows and induced voltages by ± sign in fig.. 2.3.1 for power supplied to HV winding and load connected to LV w’ding . Induced emfs EH 1 H 2 & Ex 1 x 2 are in phase and current I 1 enters terminal H1 while current I 2 leaves terminal X1 i.e currents I 1 & I 2 have approx. 180° phase difference. These instantaneous currents are magnetically opposite to each other. POLARITY MARKING, ITS NEED & TESTS

18. LECTURE :2 DTEL 19 POLARITY MARKING, ITS NEED & TESTS HV terminals, LV terminals and Tertiary winding terminals, if any, are labeled with letters H, X & Y respectively, voltage magnitude in H-X-Y being in decreasing order. Subscripts 1,2,3, etc depending upon number of leads. As per Indian Standards, in single phase transformers the terminals are labeled as: 1.1 for H 1 , 1.2 for H 2 , 2.1 for X 1 , 2.2 for X 2 , 3.1 for Y 1 and 3.2 for Y 2. When primary winding H 1 -H 2 and secondary winding X 1 -X 2 are wound in the same direction, the transformer is said to have subtractive polarity. Ref. Fig.2.3.2 Fig. 2.3.2 Subtractive Polarity

19. LECTURE :2 DTEL 20 POLARITY MARKING, ITS NEED & TESTS Fig. 2.3.3 Additive Polarity When primary winding H 1 -H 2 and secondary winding X 1 -X 2 are wound in the opposite directions, the transformer is said to have additive polarity. Ref. Fig.2.3.3

20. LECTURE :2 DTEL 21 POLARITY MARKING, ITS NEED & TESTS A.C. Polarity Test: To determe polarity of a t’former with unmarked terminals, two of the adjacent HV & LV windings are connected together and a voltmeter is connected between other two adjacent terminals. A small AC voltage is impressed on the high voltage winding. If the voltmeter reading is less than the impressed voltage on the high voltage winding, the transformer has subtractive polarity. Instead, if voltmeter reading is greater, then the transformer has additive polarity. Fig.2.3.4 AC Polarity Test

21. LECTURE :2 DTEL 23 Fig.2.3.5 DC Polarity Test D.C. Polarity Test: In this test, a battery is connected across any one winding with a switch in series. A voltmeter is placed across this winding to read upscale. The voltmeter terminals are then transferred directly to the adjacent terminals of other winding. The switch is then suddenly opened and voltmeter deflection is observed. If the deflection is upscale, then the polarity is additive whereas the polarity is subtractive when deflection is downscale. POLARITY MARKING, ITS NEED & TESTS

22. LECTURE :2 DTEL 24 The HV & LV windings of three phase t’formers can be connected in different ways giving diff. phase differences between the corresponding line voltages on the HV & LV sides. 3-phase t’formers are divided into 04 main vector groups depending upon this phase difference. These groups are : Group 1 – 0° or No phase displacement group Group 2 – 180° phase displacement group Group 3 – (-30°) or 30° lag phase displacement group Group 4 – (+30°) or 30° lead phase displacement group VECTOR GROUPS OF 3-PH. T’FORMERS:

23. LECTURE 1 DTEL 25 LECTURE ;2 DTEL 25 The windings on the primary and secondary sides of a 3-phase transformer can be connected in different ways so that the phase angle between corresponding primary and secondary voltages are different. Instead of expressing the phase difference between the voltages in degrees, it is more convenient to use clockface method of angle designation. HV side line voltage is considered as the minute hand and it is set at 12 O’clock. The LV side line voltage phasor is represented by the hour hand and is set on the dial of the clock according to its position of phasor of the line voltage on LV side. VECTOR GROUPS:CLOCKFACE METHOD

24. LECTURE 1 LECTURE :2 DTEL 26 VECTOR GROUPS:CLOCKFACE METHOD Vector Group Hour Hand Settting Minute Hand Setting Example 1: 0° 12 12 Dd0, Yy0, Yz0 2: 180° 6 12 Dd6, Yy6, Yz6 3: 30° Lag 1 12 Dy1, Yd1, Dz1 4: 30° Lead 11 12 Dy11, Yd11, Dz11

25. LECTURE :3 DTEL 27 A 3-phase transformer may be a single three phase unit or a bank of three 1-phase transformers. The primary and secondary sides will have 3 windings on each side. These three windings can be connected in a Star 0r a Delta depending upon the application. Thus there are 4-possible connections for a 3-phase t’former . ∆ - ∆ or D – d ( Delta primary – Delta secondary) Y – Y or Y – y ( Star primary – Star secondary) ∆ - Y or D – y ( Delta primary - Star secondary) Y – ∆ or Y – d ( Star primary – Delta secondary) 3-PH. TRANSFORMER CONNECTIONS: https://youtu.be/CUpoOHMlMdY

26. LECTURE :3 DTEL 28 CONNECTION PLAN: For different connections, the phase windings on each side are connected either in star or delta. Following are some important points of winding connections and vector diagrams: Primary and secondary windings of a phase are provided on the same limb of the core, hence they are drawn parallel to each other & the voltage across them i.e. the phase voltages are drawn with no phase difference between them. 3-PH. TRANSFORMER CONNECTIONS: Secondary winding for phase ‘a’ i.e. a 1 a 2 corresponds to primary winding A 1 A 2 such that the terminals A 1 & a 1 have same polarity. Delta terminal ‘A’ formed by connecting winding terminals A 1 & C 2 will have same polarity as the delta terminal ‘a’ formed by connecting winding terminals a 1 & c 2.

27. LECTURE :3 DTEL 29 Phasor diagrams are drawn for balance load at lagging power factor cos ф , neglecting magnetizing current and impedance voltage drops. v) Phase currents on secondary side are 180° out of phase of the respective primary side phase currents. 3-PH. TRANSFORMER CONNECTIONS:

28. LECTURE :3 DTEL 30 3-phase windings on both primary and secondary sides are connected in delta as discussed above. Line currents on the primary side being drawn from the mains, they are towards the delta terminals but the line currents on the secondary side are away from the delta terminals . DELTA-DELTA (∆- ∆) CONNECTION:-

29. LECTURE :3 DTEL 31 DELTA-DELTA (∆- ∆) CONNECTION:- Under balanced load condition, the line currents are √3 times the phase or winding currents and are displaced behind phase currents by 30°. Line & phase voltages are equal in magnitude in delta connection. Secondary line voltages V ab , V bc & V ca are in phase with respective primary line voltages V AB , V BC and V CA with voltage ratio ratios equal to the turns ratio: V AB / V ab = a.

30. LECTURE :3 DTEL 32 DELTA-DELTA (∆- ∆) CONNECTION:- Current ratio with magnetizing current neglected is I AB / I ab = 1/a. In this connection, the primary and secondary side corresponding line voltages being in same phase, this connection falls in Vector group 1 i.e. 0° phase displacement group and is known as Dd0 connection. Advantages of D-d connection: Satisfactory for balanced & Unbalanced loads, Third harmonic circulates in the mesh & hence output voltage is free of third harmonic, Can operate as open delta connection in case of failure of any 1-phase transformer forming bank.

31. LECTURE :3 DTEL 33 DELTA-DELTA (∆- ∆) CONNECTION:- Dd6 Connection: If the connections of the ph. windings are reversed on either side, a phase difference of 180° is obtained bet’n respective primary and secondary line voltages. Such a connection falls in the vector group 2 i.e. 180° lag group. This connection is known as Dd6 connection. The winding and vector diagram for Dd6 connection

32. DTEL 34 THANK YOU

33. LECTURE :4 DTEL 35 STAR-STAR CONNECTION: Star-star connection is formed by connecting identical terminals of the three windings on the primary as well the secondary sides. The common terminal thus formed is termed as star point.

34. LECTURE :4 DTEL 36 Salient features of Y-y Connection: Phase and Line currents are equal and in phase, Line voltage is √3 times the phase voltage, Line voltage leads phase voltage by 30°, Primary to secondary phase voltages have voltage ratio equal to turns ratio i.e. (V AN / V an )=a The current ratio (I AN / I an )=1/a Line voltages on both sides being in phase, this connection falls in V.G. 1 & connection is Yy0 . STAR-STAR CONNECTION: Demerits of Star-star connection: This connection suffers two serious problems. 1. If the neutral is not provided, the phase voltages tend to tend to be severely unbalanced on unbalanced loads and the neutral position shifts. Hence Y-Y connection, without neutral connection, is not suitable for unbalanced loads.

35. LECTURE :4 DTEL 37 Magnetizing current of a transformer is non-sinusoidal and contains 3 rd harmonic current. The third harmonic is necessary to overcome saturation in order to produce sinusoidal flux. In balanced 3-ph. system, 3 rd harmonic components in the magnetizing currents are equal & cophasal i.e. they are additive. Without neutral connection, the total third harmonic current, not finding path, distorts the flux wave and, in turn, the generated voltages on both primary & secondary sides will have third harmonic components. STAR-STAR CONNECTION:

36. LECTURE :4 DTEL 38 Remedies: Grounding of Neutrals: The star point on the primary side of the transformer is grounded either solidly or through a low impedance. This provides path to the third harmonic current to flow to the neutral of the alternator through ground. The neutral grounding also provides a return path for unbalanced currents due to unbalanced loads. Providing tertiary windings: In Y-Y connection without neutral grounding, each transformer is provided with a tertiary winding connected in delta. The mesh winding allows circulation of third harmonic current. STAR-STAR CONNECTION:

37. LECTURE :4 DTEL 39 STAR-STAR CONNECTION: Yy6 Connection: Phase voltages on secondary side are out of phase of the corresponding primary phase voltages i.e. they have phase diff. of 180°. Hence the connection falls in Group 2 & connection is k/as Yy6 connection.

38. One of the major effects of power system  harmonics  is to increase the current in the system. This is particularly the case for the  third harmonic , which causes a sharp increase in the zero sequence current, and therefore increases the current in the neutral conductor.

39. LECTURE :4 DTEL 41 DELTA-STAR (∆- Y) CONNECTION:- Primary side windings are connected in delta but the secondary side windings are connected in star. Primary Side: V L = V PH & I L = √3 I PH Secondary Side: V L = √3 V PH & I L = I PH Ratio of line to line voltage: V LP /V LS = a/√3 Secondary phase voltages lead the resp. primary ph. voltages by 30°. Secondary line volt. also lead the corres’g prim. line voltages by 30°.

40. LECTURE :4 DTEL 42 DELTA-STAR (∆- Y) CONNECTION:- Hence this connection falls in V. Group 4 i.e. 30° lead vector group & the connection is known as Dy11 connection. connection is employed for Distribution t’formers . Dy1 Connection: By reversing connections on either side, the secondary system voltages are made to lag the respective voltages on the primary side by 30°. The connection falls in Group 3 i.e. 30° Lag V. Group & It is k/as Dy1 connection.

41. LECTURE :6 DTEL 43 Parallel Operation of 1-ph. T’formers Transformers are said to be connected in parallel when their primary windings are connected to a common voltage supply and their secondary windings are connected to a common load. Need of Parallel Operation: For large loads it may be impracticable & uneconomical to use a single large transformer. For reducing the cost on spare capacity of transformers, it is advisable to supply large loads by connecting appropriate no. of transformers in parallel. For future expansion of the substation to supply a load beyond the installed capacity. For maintaining continuity of supply in case of breakdown of a transformer in the system. For preventive maintenance, transformer can be removed from service.

42. Conditions for satisfactory parallel operation i ) Transformers should be properly connected with regard to their polarities. ii) The voltage ratings and votage ratios of the transformers should be same. iii) Percentage impedance of transformer should be equal. iv) X/R ratios should be same.

43. LECTURE :5 DTEL 45 OPEN DELTA(V-v) CONNECTION:- From D-d connected bank of three 1-phase transformers, if any one is removed due to fault on it, the remaining two continue to supply 3-phase load as Open Delta Connection. Prim. Side line volt. V AB , V BC & V CA are equal in magn . On secondary side, line voltages are V ab = V bc = V ca , but V bc = - ( V ca + V ab ) Also V an = V bn = V cn . Hence 3-ph. load can be supplied. For a bal. load at p.f . cos ф lag, bal. load currents are I a , I b & I c .

44. LECTURE :5 DTEL 46 Secondary winding currents are I ba = - I b & I ac = I c . On Prim. side the line & winding currents are related as: I B = - I AB , I C = I CA and I A = - (I B + I C ) . One winding operates at power factor cos (30° - φ ) & other one operates at p.f . of cos (30°+ φ ). Max.loading =√3V L (I L /√3) = V L I L Combined capacity of 2t’formers = (2/3)(√3V L I L ) = (2/√3)V L I L Thus Max. Loading is 86.6% of the combined capacity of 2 Transformers. OPEN DELTA(V-v) CONNECTION:-

45. LECTURE :5 DTEL 47 THREE WINDING TRANSFORMER: Three winding transformers have a third winding, in addition to the primary & secondary windings. The third or Tertiary wdg has lowest voltage rating amongst the 3 windings. In 2-wdg transformers, KVA ratings of both HV & LV windings are same, but in 3-wdg transformer KVA ratings of all three windings are usually unequal. Tertiary winding is connected in Delta, as the delta connection suppresses harmonic voltages generated in primary and secondary windings. Thus the primary and secondary phase voltages are restored to their normal value. This reduction in load imbalance on secondary side is reflected on primary side as reduction in primary ph. voltage magnitudes and phase angles.

46. LECTURE :5 DTEL 48 In the adjoining figure, subscripts 1,2 &3 are respectively used for primary, secondary and tertiary windings. For an ideal 3-wdg transformer: (V 2 /V 1 ) = (T 2 /T 1 ) (V 3 /V 1 ) = (T 3 /T 1 ) Also I 1 T 1 = I 2 T 2 + I 3 T 3 THREE WINDING TRANSFORMER: EQ. CIRCUIT OF 3-WINDING TRANSFORMER: Equivalent circuit of 3-wdg transformer is represented on per phase basis. Each winding is represented by its resistance and leakage reactance referred to primary side. V1, V2,V3 be the voltages and I1,I2and I3 be the currents of primary, secondary and tertiary windings.

47. LECTURE :5 DTEL 49 Excitation branch has resistance R & leakage reactance X and draws current I . This branch may be neglected for ease in evaluation of performance. PARAMETERS OF 3-WINDING TRANSFORMER: Parameters are found from O.C. & S.C. tests. O.C.Test : The test is performed in the same manner as that for a two wdg transformer, exciting primary wdg & leaving secondary and tertiary wdgs open. The test gives core loss, turns ratios and the magnetizing branch impedance. a 12 = (V 1 /V 2 ), a 13 = (V 1 /V 3 ) and a 23 = (V 2 /V 3 ) = (a 13 /a 12 ) Z = (V /I ) R = W /I 2 X = √(Z 2 – R 2 ) THREE WINDING TRANSFORMER:

48. DTEL 50 REFERENCES Books: - Electrical Machines, 2 nd -1993, Dr. P. K. Mukherjee and S. Chakravarti , DhanpatRai Publications (P) Ltd 2.Electric Machines , Ashfaq Husain , DhanpatRai Publications (P) Ltd. 3. Electrical Machines , 3 rd -2010 , I.J.Nagrath and Dr. D.P.Kothari ,Tata McGraw Hill 4. Electrical Machinery , P.S.Bhimbra