CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM Line H: Install Electrical Equipment H-2 LEARNING GUIDE H-2 INSTALL TRANSFORMERS

Transcription

1 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM Level 3 Line H: Install Electrical Equipment H-2 LEARNING GUIDE H-2 INSTALL TRANSFORMERS

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3 Foreword The Industry Training Authority (ITA) is pleased to release this major update of learning resources to support the delivery of the BC Electrician Apprenticeship Program. It was made possible by the dedicated efforts of the Electrical Articulation Committee of BC (EAC). The EAC is a working group of electrical instructors from institutions across the province and is one of the key stakeholder groups that supports and strengthens industry training in BC. It was the driving force behind the update of the Electrician Apprenticeship Program Learning Guides, supplying the specialized expertise required to incorporate technological, procedural and industry-driven changes. The EAC plays an important role in the province s post-secondary public institutions. As discipline specialists the committee s members share information and engage in discussions of curriculum matters, particularly those affecting student mobility. ITA would also like to acknowledge the Construction Industry Training Organization (CITO) which provides direction for improving industry training in the construction sector. CITO is responsible for organizing industry and instructor representatives within BC to consult and provide changes related to the BC Construction Electrician Training Program. We are grateful to EAC for their contributions to the ongoing development of BC Construction Electrician Training Program Learning Guides (materials whose ownership and copyright are maintained by the Province of British Columbia through ITA). Industry Training Authority January 2011 Disclaimer The materials in these Learning Guides are for use by students and instructional staff and have been compiled from sources believed to be reliable and to represent best current opinions on these subjects. These manuals are intended to serve as a starting point for good practices and may not specify all minimum legal standards. No warranty, guarantee or representation is made by the British Columbia Electrical Articulation Committee, the British Columbia Industry Training Authority or the Queen s Printer of British Columbia as to the accuracy or sufficiency of the information contained in these publications. These manuals are intended to provide basic guidelines for electrical trade practices. Do not assume, therefore, that all necessary warnings and safety precautionary measures are contained in this module and that other or additional measures may not be required.

4 Acknowledgements and Copyright Copyright 2011, 2014 Industry Training Authority All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or digital, without written permission from Industry Training Authority (ITA). Reproducing passages from this publication by photographic, electrostatic, mechanical, or digital means without permission is an infringement of copyright law. The issuing/publishing body is: Crown Publications, Queen s Printer, Ministry of Citizens Services The Industry Training Authority of British Columbia would like to acknowledge the Electrical Articulation Committee and Open School BC, the Ministry of Education, as well as the following individuals and organizations for their contributions in updating the Electrician Apprenticeship Program Learning Guides: Electrical Articulation Committee (EAC) Curriculum Subcommittee Peter Poeschek (Thompson Rivers University) Ken Holland (Camosun College) Alain Lavoie (College of New Caledonia) Don Gillingham (North Island University) Jim Gamble (Okanagan College) John Todrick (University of the Fraser Valley) Ted Simmons (British Columbia Institute of Technology) Members of the Curriculum Subcommittee have assumed roles as writers, reviewers, and subject matter experts throughout the development and revision of materials for the Electrician Apprenticeship Program. Open School BC Open School BC provided project management and design expertise in updating the Electrician Apprenticeship Program print materials: Adrian Hill, Project Manager Eleanor Liddy, Director/Supervisor Beverly Carstensen, Dennis Evans, Laurie Lozoway, Production Technician (print layout, graphics) Christine Ramkeesoon, Graphics Media Coordinator Keith Learmonth, Editor Margaret Kernaghan, Graphic Artist Publishing Services, Queen s Printer Sherry Brown, Director of QP Publishing Services Intellectual Property Program Ilona Ugro, Copyright Officer, Ministry of Citizens Services, Province of British Columbia To order copies of any of the Electrician Apprenticeship Program Learning Guide, please contact us: Crown Publications, Queen s Printer PO Box 9452 Stn Prov Govt 563 Superior Street 2nd Flr Victoria, BC V8W 9V7 Phone: Toll Free: Fax: crownpub@gov.bc.ca Website: Version 1 Corrected, January 2017 Corrected, March 2016 Corrected, September 2015 Revised, April 2014 Corrected, January 2014 New, October 2012

5 LEVEL 3, LEARNING GUIDE H-2: INSTALL TRANSFORMERS Learning Objectives Learning Task 1: Describe the construction and features of three-phase transformers Self-Test Learning Task 2: Describe the connections of three-phase transformer banks Self-Test Learning Task 3: Calculate voltage, current and kva values for three-phase transformer banks Self-Test Learning Task 4: Describe common connections for autotransformers in three-phase circuits Self-Test Learning Task 5: Calculate voltage, current and kva values for three-phase autotransformer circuits Self-Test Learning Task 6: Describe instrument transformer connections in three-phase circuits Self-Test Learning Task 7: Calculate instrument-transformer ratings and meter readings in three-phase circuits Self-Test Answer Key CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 5

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7 Learning Objectives H-2 Learning Objectives The learner will be able to connect and maintain three-phase transformers. The learner will be able to describe three-phase applications of autotransformers. The learner will be able to describe three-phase applications of instruments transformers. The learner will be able to determine installation requirements for three-phase transformers. Activities Read and study the topics of Learning Guide I-2: Install Transformers. Complete Self-Tests 1 through 7. Check your answers with the Answer Key provided at the end of this Learning Guide. Resources You are encouraged to obtain the following text to provide supplemental learning information: Alternating Current Fundamentals by John R. Duff and Stephen L. Herman; Delmar Publishers Inc. Delmar s Standard Textbook of Electricity, 5 th Revised Edition by Stephen Herman. Cengage Learning. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 7

8 BC Trades Modules We want your feedback! Please go the BC Trades Modules website to enter comments about specific section(s) that require correction or modification. All submissions will be reviewed and considered for inclusion in the next revision. SAFETY ADVISORY Be advised that references to the Workers Compensation Board of British Columbia safety regulations contained within these materials do not/may not reflect the most recent Occupational Health and Safety Regulation. The current Standards and Regulation in BC can be obtained at the following website: Please note that it is always the responsibility of any person using these materials to inform him/herself about the Occupational Health and Safety Regulation pertaining to his/her area of work. Industry Training Authority January CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

9 Learning Task 1: Describe the construction and features of three-phase transformers Three-phase power is transformed from one voltage level to another using a three-phase transformer or a bank of single-phase transformers. When the transformer bank is used, the windings of each phase are on separate cores. See Figure 1. Figure 1 A three-phase transformer bank Figure 2 A three-phase transformer In a three-phase transformer, the high-voltage and low-voltage windings for each phase are wound on the same leg of the transformer. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 9

10 Learning Task 1 H-2 Construction of three-phase transformers Figure 3 shows the core of a simple three-phase transformer. Notice that the core has three legs (the vertical sections). The primary and secondary windings of any one phase are wound on each leg of the core. Figure 3 Three-phase transformer core At any instant, the flux in any one leg of the transformer equals the phasor sum of the flux in the other two legs. Remember that for any given load on the transformer, the resultant flux in any leg is determined by the counter emf it must produce in the primary winding. It is not determined by the load current. Figure 4 shows the three phase voltages at four instants during a cycle. It also shows the core flux that results from these voltages. Three-phase transformer connections The windings of a three-phase transformer or transformer bank are generally connected in one of two configurations: wye or delta. A wye connection is made as shown in Figure 5 by connecting the three similarly labelled ends of the windings to form a common point called the star point or neutral. A delta connection is made as shown in Figure 6 by connecting: The oppositely labelled winding ends of the first and second transformer The oppositely labelled winding ends of the second and third transformer The oppositely labelled winding ends of the third and first transformers Since it is possible to connect both the high- and low-side windings in either of these two configurations, there are four different transformer configurations: Wye-to-wye Delta-to-delta Wye-to-delta Delta-to-wye 10 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

11 Learning Task 1 H-2 In these names, the first word represents the configuration of the primary winding and the second word represents the configuration of the secondary winding. Figure 4 Core flux in a three-phase transformer core CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 11

12 Learning Task 1 H-2 Figure 5 Wye-connected transformer windings Figure 6 Delta-connected transformer windings Advantages of a single three-phase transformer A single three-phase transformer has the following advantages: A three-phase transformer has slightly higher operating efficiency than three singlephase transformers connected in a bank. 12 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

13 Learning Task 1 H-2 A three-phase transformer weighs less and requires less space than three single-phase transformers. A single three-phase transformer costs less than the three single-phase transformers. A three-phase transformer is easier to install because the transformer is less complex. The interconnections between the phases are already complete when the unit is shipped. Advantages of a bank of three single-phase transformers The advantages that three single-phase transformers have over one three-phase transformer all relate to breakdown. Since modern transformers are very reliable, these advantages are somewhat limited. A bank of three single-phase transformers may be operated at a reduced kva capacity with one unit down for repair in some cases. Since all phases of a three-phase transformer are wound on a single core, this is not possible with a three-phase unit. A bank of three single-phase transformers is simpler than a three-phase transformer. Therefore, if one phase needs repair, the single-phase transformer is less expensive to repair than the three-phase unit. The cost of a spare single-phase transformer is less than that of a spare three-phase transformer. Now do Self-Test 1 and check your answers. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 13

14 Learning Task 1 H-2 Self-Test 1 1. List four advantages that a single, three-phase transformer has over a bank of single-phase transformers. 2. List three advantages that a bank of single-phase transformers has over a single three-phase transformer. 3. List the two common configurations in which the windings of a three-phase transformer may be connected. Go to the Answer Key at the end of the Learning Guide to check your answers. 14 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

15 Learning Task 2: Describe the connections of three-phase transformer banks There are four fundamental connections used for three-phase transformers (or banks). These are: Wye-wye connection Delta-delta connection Wye-delta connection Delta-wye connection In this Learning Task, you will also look at some special delta connections. These are: Four-wire delta connection Open-delta connection Wye-wye connection The wye-to-wye transformer connection is used mainly when transforming from one high voltage to another. The ability to ground the neutral point reduces the potential stress on the insulation. This means that less insulation can be used in the transformer. Constructing voltage diagrams for three-phase transformers Figure 1 shows the four standard voltage diagrams for three-phase transformers as specified by the Canadian Standards Association (CSA). Note that the diagrams are not marked with primary and secondary but rather with high side and low side. These standards ensure that all CSAapproved three-phase transformers have a consistent phase displacement between the high and low sides. The phase displacement is defined as the angle between the voltage V AN of the high side and V AN of the low side, where N is the neutral point of the diagram. Neutral refers to a point such that the voltages between it and each of the phase conductors are equally spaced in magnitude and phase angle. On the delta diagram, V AN is indicated by the dashed line. Wye-to-wye connections and delta-to-delta connections must always have a phase displacement of 0. It is physically possible to connect single-phase transformers in a three-phase bank so that the phase displacement is 180. However, avoid this, as it will not permit the bank to be connected in parallel to a standard wye-to-wye, or delta-todelta, three-phase transformer. This is covered in detail later in this Learning Task. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 15

16 Learning Task 2 H-2 º Figure 1 Standard voltage diagrams for three-phase transformers Wye-to-delta connections and delta-to-wye connections must always have a phase displacement of 30, with the high side leading the low side for a standard ABC phase rotation. If the phase rotation is reversed to ACB, then the high side lags the low side by 30. Again, it is possible to connect single-phase transformers in a three-phase bank so that they produce other phase displacements, but you should avoid this, as it will not permit the bank to be connected in parallel to a standard wye-to-delta or delta-to-wye three-phase transformer. There are only three rules for reproducing standard transformer diagrams. You can use the diagrams produced to ensure that proper connections are made on the transformers. Rule 1 The phasors represent instantaneous directions of the induced voltages in the windings. This means the primary phasor represents the counter emf, which is 180 out of phase with the primary applied voltage. Numbering is used in place of arrowheads. It is conventional to show induced voltage phasors for transformers with the tail of the arrow at the lower subscripted terminal and the head of the arrow at the higher subscripted terminal. For example, H 1 and X 1 represent the tail of the phasors and H 2 and X 2 represent the head of the phasors. Rule 2 The high-voltage phasor V AN is always drawn at 30 to the horizontal and the high-voltage phasor V AB is always drawn at 60 to the horizontal. This corresponds to the 30 phase displacement that exists between the line and phase voltages in a three-phase system. When making the diagrams, always draw the high-side diagram first, whether it is for a step-up or step-down application. º 16 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

17 Learning Task 2 H-2 Figure 2 Phasors representing induced voltages Rule 3 For wye-to-wye and delta-to-delta connections the phase displacement from the high to low side must be 0. For wye-to-delta and delta-to-wye connections the phase displacement from high side to low side must be 30 with the high side leading. Constructing a voltage diagram for a wye-to-wye connected transformer bank 1. Draw the first phasor (V AN ) of the high side at 30 to the horizontal with H 1 at the lower end and H 2 at the upper end, as in Figure 3. Figure 3 First high-side phasor drawn at 30 to the horizontal 2. Draw the other two high-side phasors equally spaced 120 apart from each other and from the first phasor. Connect the H 2 points together to form the wye point as shown in Figure 4. Figure 4 Complete voltage diagram of high-side wye CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 17

18 Learning Task 2 H-2 3. Since the low-side induced voltages are in phase with the high-side induced voltages, draw the low-side phasors parallel to the high-side phasors as in Figure 5. Notice that the X 2 points are all connected together to form a wye point. Figure 5 Voltage diagram of wye-to-wye connection 4. Connect the H 1 terminal of the first high-side phasor drawn to phase A; connect the next lagging one to phase B; and connect the other to phase C as shown in Figure 6. Label the X 1 terminals of the phasors on the low side A, B or C according to which of the high-side phasors they are in phase with. 5. The phasors represent the induced voltages in the transformer windings. The phasor with its H 1 terminal connected to phase A is called the A phase transformer. Likewise for the other two phases. Figure 6 Phase connections shown on a wye-to-wye voltage diagram Drawing a wiring diagram from the voltage diagram Normally a voltage diagram does not show all the individual H 1, H 2, X 1 and X 2 terminals of each transformer. Also, the phase terminals are not identified as A, B and C. Instead: On the high side, the phase A terminal is labelled H 1, the phase B terminal, H 2, and the phase C terminal, H 3 as shown in Figure 1. On the low side, the phase A terminal is labelled X 1, the phase B terminal, X 2, and the phase C terminal, X CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

19 Learning Task 2 H-2 This is also the way the terminals are labelled on a unit three-phase transformer. If present, the neutral points are labelled H 0 for the high side and X 0 for the low side. With the voltage diagram completed properly and the terminal information added, you can use the transformer voltage diagram to create a wiring diagram for the transformers. The voltage diagrams are independent of the individual transformer polarities. That is, it does not matter whether the transformers used in the bank are additive polarity, subtractive polarity, or some mixture of the two. The terminals that connect to a given point depend only on their subscript identification. 1. Usually you arrange the transformer diagrams so that the phase A transformer is on the righthand side when viewed from its high side and the phase B transformer is next to it, followed by the phase C transformer on the left. See Figure 7. Figure 7 Three-phase bank of additive-polarity transformers 2. Whether the transformer has additive or subtractive polarity, always put the H 1 terminal of each transformer on the right-hand side when viewed from its high side. 3. Now transfer information from the voltage diagram in Figure 6: Each transformer s H 2 terminals connect to form the high-side wye point. The H 1 terminal of each phase s transformer connects to its corresponding phase line. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 19

20 Learning Task 2 H-2 Figure 8 shows this information transferred. Figure 8 Wye, high-side connections The X 2 terminals of each transformer connect together to form the low-side wye point. The X 1 terminal of each phase s transformer connects to its corresponding phase line. Figure 9 shows the completed wiring diagram. Figure 9 Three-phase, wye-to-wye bank of additive-polarity transformers Figure 10 shows a completed wiring diagram for a three-phase bank of subtractive-polarity transformers. It does not matter which polarity the transformers are, they will still have the same connections with respect to the terminal markings. All the H 2 terminals still connect together and all the X 2 terminals still connect together. As well, all the H 1 and X 1 terminals of each phase s transformers still connect to their corresponding phases. 20 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

21 Learning Task 2 H-2 A B C N A B C H 1 H 2 H 1 H 2 H 1 H 2 X 1 X 2 X X 1 X 2 1 X 2 N A B C Figure 10 Three-phase, wye-to-wye bank of subtractive-polarity transformers Figure 11 shows a three-phase bank of transformers with mixed polarities. A B C N A B C H 1 H 2 H 1 H 2 H 1 H 2 X 1 X 2 X X X X 2 N A B C Subtractive Additive Subtractive Figure 11 Three-phase, wye-to-wye bank of mixed-polarity transformers For a step-down application from a wye high side to a wye low side: Connect the transformers so that the primary circuit connects to the high windings and the secondary circuit connects to the low windings. For a step-up application from a wye low side to a wye high side: Connect the transformers so that the primary circuit connects to the low windings and the secondary circuit connects to the high windings. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 21

22 Learning Task 2 H-2 Voltage relationships for the wye-to-wye transformer bank The line-to-line voltage-transformation ratio is exactly the same as the individual transformer s turns ratio for a wye-to-wye connection. The voltage ratings of the transformer s primary and secondaries must be equal to the corresponding line voltages divided by 3. If both high-side and low-side connections are made correctly, the line-to-line voltages on the secondary side should read 3 times the secondary phase voltage. Wrong connections Consider what happens if the connections to the X 1 and X 2 terminals of transformer B in Figure 9 are accidentally reversed as shown in Figure 12. Figure 12 Incorrect low-side connection on the B-phase transformer 22 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

23 Learning Task 2 H-2 The voltage diagram for this incorrect bank connection would look like the one shown in Figure 13. B H 1 B phase X 2 H H 2 2 H 2 C phase X 1 X 2 X 1 X 1 A C A H 1 H 1 A phase C X 2 B Figure 13 Voltage diagram for the incorrect circuit shown in Figure 12 For an incorrect low-side connection on the B-phase transformer: The phase angle between the two phase voltages V AN and V CN is the proper 120. The line-to-line voltage measured between lines A and C is still voltage. 3 times the phase However, the phase angle between the two phase voltages V AN and V BN (or between phase voltages V BN and V CN ) is only 60. Therefore, the line voltages between lines A and B and between B and C equal the phase voltage. If the incorrect connections were made on transformer A or C instead of B, a similar imbalance of phase angles and line voltages would occur. The same incorrect line voltages and phase angles occur if the same mistake happens on the high side of one of the transformers rather than on the low side. Single-phase loads are unaffected by this incorrect connection. Three-phase loads such as motors are adversely affected. Also, the neutral wire might be overloaded. After the wye-to-wye transformer connections are made and before the load is applied, test the line voltages for balance. All line-to-line voltages should be equal to 3 times the phase voltage. 180 phase shift As mentioned earlier, you can connect the transformers so that the voltage and phase relationships on the secondary are truly three-phase but there is a primary to secondary phase shift of 180. Figure 14 shows one way of connecting the transformers to do this. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 23

24 Learning Task 2 H-2 Figure 14 Transformer bank connected for a 180 phase shift Figure 15 shows the voltage diagram for this three-phase bank connection. Notice the voltage V AN on the low side is 180 out of phase with V AN on the high side. Figure 15 Voltage diagram for a wye-to-wye 180 phase shift Loads connected to this transformer bank would work perfectly well and the phase sequence would be the same as that for the 0 phase shift. However, you should avoid this connection because it is not possible to parallel another three-phase transformer with this bank. The CSA standard specifies that three-phase transformers connected wye-to-wye have a 0 phase shift. Delta-delta connection The delta-to-delta connection is used mainly in the medium voltage range where a neutral point is not required for the operation of loads. 24 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

25 Learning Task 2 H-2 Constructing a voltage diagram To construct a voltage diagram for a delta-to-delta connected transformer, do the following: 1. Draw the first phasor of the high side (V AB ) at 60 to the horizontal with H 1 at the lower end and H 2 at the upper end. See Figure 16(a). Figure 16 First high-side phasor drawn at 60 to the horizontal Notice the dotted line. This represents the voltage V AN. As mentioned earlier, the phase displacement is the angle between V AN of the high side and V AN of the low side. Since there is no physical point on the delta connection that meets the definition of neutral, this voltage V AN is shown as a dotted line. Its sole purpose is to show the phase displacement between the high and low side of the three-phase transformer. On the high side it is drawn at 30 to the horizontal. 2. Draw the second high-side phasor 120 behind the first phasor with its H 1 terminal connected to the H 2 terminal of the first phasor as in Figure 16(b). 3. Draw the third high-side phasor 120 behind the second phasor with its H 1 terminal connected to the H 2 terminal of the second phasor and its H 2 terminal connected to the H 1 terminal of the first phasor. See Figure 16(c). This completes the high-side delta connection. 4. Since the low-side induced voltages are in phase with the high-side induced voltages, draw the low-side phasors parallel to the high-side phasors of the same transformer. See Figure 17. Figure 17 Voltage diagram of a delta-to-delta connection CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 25

26 Learning Task 2 H-2 Notice that the X 1 terminals are at the same relative ends of the low-voltage phasors as the H 1 terminals are on the high-voltage phasors. 5. The H 1 terminal of the first high-side phasor drawn connects to line A, the next lagging one connects to line B, and the final one connects to line C, as shown in Figure 18. Identify the low-side phasor that is in phase with the high-side phasor connected to line A. Label its X 1 terminal as line A. Similarly, label the low-side terminals for lines B and C. º º Figure 18 Phase connections shown on a delta-to-delta voltage diagram 6. The phasors represent induced voltages in the transformer windings. The phasor with its H 1 terminal connected to line A is called the A-phase transformer. Likewise for the other two phases. Drawing a wiring diagram from the voltage diagram The voltage diagram does not normally show the individual H 1, H 2, X 1 and X 2 terminals of each transformer and the phase terminals are not identified as A, B and C. These markings are to help you draw a proper wiring diagram for the transformers. The voltage diagrams are independent of the individual transformer polarities. Again, it does not matter whether the transformers used in the bank are additive polarity, subtractive polarity or some mixture of the two. The terminals that connect to a given point depend only on their subscript identification. 1. As usual, arrange the individual transformers in the bank with the A-phase transformer on the right and the C-phase transformer on the left when viewed from the high side. Put the H 1 terminal of each transformer on the right-hand side when viewed from its high side. See Figure CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

27 Learning Task 2 H-2 A B C H 1 H 2 H 1 H 2 H 1 H 2 X 2 X 1 X 2 X 1 X 2 X 1 Figure 19 Three-phase bank of additive-polarity transformers 2. Now transfer information from the high side of the voltage diagram in Figure 18: The H 2 terminal of the A-phase transformer connects to the H 1 terminal of the B-phase transformer. The H 2 terminal of the B-phase transformer connects to the H 1 terminal of the C-phase transformer. To complete the high-side delta, the H 2 terminal of the C-phase transformer connects to the H 1 terminal of the A-phase transformer. Connect each of the H 1 terminals to its corresponding line as shown in Figure 20. Figure 20 Delta high-side connections CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 27

28 Learning Task 2 H-2 3. Next transfer the information from the low side of Figure 18 to the wiring diagram: The X 2 terminal of the A-phase transformer and the X 1 terminal of the B-phase transformer connect together and to line B. The X 2 terminal of the B-phase transformer and the X 1 terminal of the C-phase transformer connect together and to line C. To complete the low side delta, the X 2 terminal of the C-phase transformer and the X 1 terminal of the A-phase transformer connect together and to line A. Connect each of the X 1 terminals to its corresponding line as shown in Figure 21. Figure 21 Three-phase, delta-to-delta bank of additive-polarity transformers Figure 22 shows a wiring diagram for a three-phase bank of subtractive-polarity transformers connected delta-to-delta. Figure 23 shows a three-phase bank of transformers with mixed polarities. No matter which polarity the transformers are, they have the same connections with respect to the terminal markings. For a step-down application from a delta high side to a delta low side, the transformers have the primary circuit connected to the high windings and the secondary circuit connected to the low windings. For a step-up application from a delta low side to a delta high side, the transformers have the primary connected to the low windings and the secondary connected to the high windings. 28 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

29 Learning Task 2 H-2 Figure 22 Three-phase, delta-to-delta bank of subtractive-polarity transformers Figure 23 Three-phase, delta-to-delta bank of mixed-polarity transformers Voltage relationships for the delta-to-delta transformer bank For a delta-to-delta connection, the line-to-line voltage-transformation ratio is exactly the same as the individual transformers ratios. The voltage ratings of the transformer s primary and secondaries must be equal to the corresponding line voltages. In a delta connection the two ends of any phase winding are connected directly across the lines. Therefore: The line voltage in the delta connection equals the phase voltage. The voltage rating of each transformer winding is equal to the line voltage. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 29

30 Learning Task 2 H-2 For this reason, a different type of voltmeter test must be performed on a secondary delta connection to determine if all the connections are properly made. This test is referred to as the mesh or delta-closure test. To perform a mesh or delta-closure test, install a voltmeter in place of the final secondary delta connection as shown in Figure 24. Always do this test on the secondary side. It detects improper connections in either the primary or the secondary circuit. Figure 24 Mesh or delta-closure test In theory, if the connections are all made correctly, the closure voltage should be zero volts. In practice, this is not the case. Third harmonic voltages created by the transformer core cause the voltmeter to read a voltage higher than zero, but less than line voltage. If a mistake is made in the transformer connections, the voltmeter reads double the line-voltage value. To see why these two voltages occur in the two different situations, examine the secondary voltage diagram of a correctly connected transformer and then of an incorrectly connected transformer. Figure 25 shows the diagram of a correctly connected transformer with a voltmeter connected across the final connection. Follow the voltage rises around the delta from one side of the voltmeter to the other. Note that the last phasor returns to the starting point. This indicates a difference between start and finish of zero volts. Calculate the voltage between the tail of the first phasor and the head of the last phasor by adding the three voltage rises: V = V + V + V T A phase Bphase Cphase = 240 V 60º V 60º V 180º = 0 V 0º 30 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

31 Learning Task 2 H-2 Figure 25 Secondary voltage diagram of a correctly connected transformer bank Figure 26 is the diagram of an incorrectly connected transformer with secondary leads of the C phase transformer connected backward. This means that the secondary voltage of this phase is 180 out of phase from what it should be. Once again a voltmeter is placed across the final connection. Follow the voltage rises around the delta from one side of the voltmeter to the other. Note that the last phasor does not return to the starting point. This indicates a difference between start and finish of twice the phase voltage. Once again, the voltage between the tail of the first phasor and the head of the last phasor can be calculated by adding the three voltage rises: V = V + V + V T A phase Bphase Cphase = 240 V 60º V 60º V 0º = 480 V 0º º Figure 26 Secondary voltage diagram of an incorrectly connected transformer bank CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 31

32 Learning Task 2 H-2 If the incorrect connections are made on the primary rather than the secondary, the same 180 phase shift occurs and the voltmeter still reads 480 V. If there is an incorrect connection and you make the last connection before doing a voltmeter test, it can result in a disastrous short circuit across 480 V. Always do this voltmeter test before making the secondary closure connection. Whether the incorrect connection is made on the primary or the secondary, always correct it before you replace the voltmeter by the final connection. 180 phase shift It is possible to connect the transformers so that the voltage and phase relationships on the secondary are truly three-phase but there is a primary to secondary phase shift of 180. Figure 27 shows one way of doing this. Figure 28 shows the voltage diagram for this three-phase connection. Notice that the voltage V AN on the low side is 180 out of phase with V AN on the high side. Loads connected to this transformer bank would work perfectly well and the phase sequence would be the same as that for the 0 phase shift. Avoid this connection, because you must not parallel a three-phase transformer with this bank. The CSA specifies that three-phase transformers connected delta-to-delta must have a 0 phase shift. Figure 27 Transformer bank connected for a 180 phase shift 32 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

33 Learning Task 2 H-2 º Figure 28 Voltage diagram for a delta-to-delta 180 phase shift Wye-delta connection The wye-to-delta transformer connection is commonly used to transform electrical energy from high voltages to medium voltages where a neutral is not required to supply single-phase loads. The voltage diagram to use for a wye-to-delta, step-down transformer bank is the one shown in Figure 29. This ensures a phase displacement of 30 with the high side leading the low side. This diagram is also used for delta-to-wye, step-up connections. º Figure 29 Voltage diagram for a wye-to-delta, step-down transformer bank and for a delta-to-wye, step-up transformer bank The voltage diagram to use for a wye-to-delta, step-up transformer bank is the one shown in Figure 30. Again, this ensures a phase displacement of 30 with the high side leading the low side. This diagram is also used for delta-to-wye, step-down connections. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 33

34 Learning Task 2 H-2 Figure 30 Voltage diagram for a wye-to-delta step-up transformer bank and for a delta-to-wye, step-down transformer bank Using the proper diagram for the required application ensures that the banks have the proper phase displacement to be paralleled with other wye-to-delta or delta-to-wye, three-phase transformers. Constructing these diagrams is not difficult if you remember to construct the high-voltage diagram first. This is true whether it is step-up or step-down. Then, apply the three rules discussed previously. Drawing voltage diagrams Wye-to-delta, step-down transformer bank 1. Draw the first phasor (V AN ) of the high-side wye at 30 to the horizontal with H 1 at the lower end and H 2 at the upper end as in Figure 31(a). Figure 31 Voltage diagram of a wye high-side connection 2. Draw the other two high-side phasors so that all three are equally spaced 120 apart. Connect the H 2 points together to form the wye point as shown in Figure 31(b). 34 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

35 Learning Task 2 H-2 3. Since the low-side induced voltages are in phase with the high-side induced voltages, draw the low-side phasors parallel to the high-side phasors of the same transformer: Connect the X 2 terminal of the A-phase, low-voltage phasor to the X 1 terminal of the B-phase, low-voltage phasor. Connect the X 2 terminal of the B-phase, low-voltage phasor to the X 1 terminal of the C-phase, low-voltage phasor. To complete the delta, connect the X 2 terminal of the C-phase, low-voltage phasor to the X 1 terminal of the A-phase, low-voltage phasor. See Figure 32. Figure 32 Voltage diagram of a wye-to-delta, step-down connection 4. The phasor with its H 1 terminal connected to line A is called the A-phase transformer. Likewise for the other two phases. Label the X 1 terminal of the low-side phasor from the A-phase transformer as line A on the low side. Do the same for phases B and C. Wye-to-delta, step-up transformer 1. It is important to draw the high side first even if the high side is the secondary. Since the high side is a delta connection, draw the first phasor V AB at 60 to the horizontal with H 1 at the lower end and H 2 at the upper end as in Figure 33(a). Figure 33 Voltage diagram of delta high-side connection CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 35

36 Learning Task 2 H-2 2. Draw the second high-side phasor 120 behind the first phasor, with its H 1 terminal connected to the H 2 terminal of the first phasor as in Figure 33(b). 3. Draw the third high-side phasor 120 behind the second phasor, with its H 1 terminal connected to the H 2 terminal of the second phasor. To complete the high-side delta connection, connect its H 2 terminal to the H 1 terminal of the first phasor as shown in Figure 33(c). 4. Since the low-side induced voltages are in phase with the high-side induced voltages, draw the low-side phasors parallel to the high-side phasors of the same transformer. To ensure the proper phase displacement of 30, you will have to connect the X 1 terminals of the low-side phasors to form the neutral or wye point. See Figure Connect the H 1 terminal of the first high-side phasor drawn to line A. Connect the next lagging one to line B and the third one to line C as shown in Figure For this diagram, the way you select which X 2 terminal to label as line A on the secondary is a little different. To ensure the correct phase displacement of 30º between the high side and low side, with the high side leading, connect: Line A to the X 2 terminal of the C-phase transformer Line B to the X 2 terminal of the A-phase transformer Line C to the X 2 terminal of the B-phase transformer Figure 34 Voltage diagram of a wye-to-delta, step-up connection Wye-to-delta, step-down transformer bank The voltage diagram does not normally show the individual H 1, H 2, X 1 and X 2 terminals of each transformer. Also, the line terminals are not usually identified as A, B and C. These markings are just to help you draw a proper wiring diagram for the transformers. Voltage diagrams do not depend on individual transformer polarities. It does not matter whether the transformers used in the bank are additive polarity, subtractive polarity or some mixture of the two. The terminals that connect to a given point depend only on their subscript identification. Figure 35 shows a three-phase bank of additive-polarity transformers. 36 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

37 Learning Task 2 H-2 1. As usual, arrange individual transformers in the bank with the A-phase transformer on the right and the C-phase transformer on the left when viewed from the high side. 2. Always put the H 1 terminal of each transformer on the right-hand side when viewed from its high side. A B C H 1 H 2 H 1 H 2 H 1 H 2 X 2 X 1 X 2 X 1 X 2 X 1 Figure 35 Three-phase bank of additive-polarity transformers 3. Now, transfer information from the voltage diagram in Figure 32. In Figure 32, the H 2 terminals of the transformers connect together to form the high-side wye point. The remaining H 1 terminals connect to their corresponding phase lines. Figure 36 shows this information transferred. Figure 36 Wye high-side connections 4. Next, transfer the information from the low side of Figure 32 to the wiring diagram. In Figure 32, the X 2 terminal of the A-phase transformer and the X 1 terminal of the B-phase transformer connect together and to line B. The X 2 terminal of the B-phase transformer and the X 1 terminal of the C-phase transformer connect together and to line C. To complete the delta connection, the X 2 terminal of the C-phase transformer and the X 1 terminal of the A phase transformer connect together and to line A. Figure 37 shows this information transferred and the completed transformer bank connections. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 37

38 Learning Task 2 H-2 A B C N A B C H 1 H 2 H 1 H 2 H 1 H 2 X 2 X 1 X 2 X 1 X 2 X 1 A B C Figure 37 Three-phase, wye-to-delta, step-down transformer bank using additive-polarity transformers Figure 38 shows a completed wiring diagram for a wye-to-delta, step-down transformer bank using subtractive-polarity transformers. A B C N A B C H 1 H 2 H 1 H 2 H 1 H 2 X 1 X 2 X 1 X 2 X X 1 2 A B C Figure 38 Three-phase, step-down bank of subtractive-polarity transformers Figure 39 shows a completed wiring diagram for a wye-to-delta, step-down transformer bank using mixed-polarity transformers. Again, it does not matter which polarity the transformers are, they will still have the same connections with respect to the terminal markings. 38 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

39 Learning Task 2 H-2 A B C N A B C H 1 H 2 H 1 H 2 H 1 H 2 X 1 X 2 X X X X 2 A B C Subtractive Additive Subtractive Figure 39 Three-phase, step-down bank of mixed-polarity transformers Wye-to-delta, step-up transformer bank 1. Transfer information from the voltage diagram in Figure 34. In Figure 34, the X 1 terminals of each of the transformers are connected together to form the low side neutral or wye point. The X 2 terminal of the C-phase transformer connects to line A. The X 2 terminal of the A-phase transformer connects to line B. The X 2 terminal of the B-phase transformer connects to line C. Figure 40 Wye low-side connections 2. Next, transfer the information from the high side of Figure 34 to the wiring diagram. In Figure 34, the H 2 terminal of the A-phase transformer and the H 1 terminal of the B-phase transformer connect together and to line B. The H 2 terminal of the B-phase transformer and the H 1 terminal of the C-phase transformer connect together and to line C. To complete the delta connection, the H 2 terminal of the C-phase transformer and the H 1 terminal of the A-phase transformer connect together and to line A. See Figure 41 for the completed transformer-bank connections. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 39

40 Learning Task 2 H-2 Figure 41 Three-phase, wye-to-delta, step-up transformer bank using additive-polarity transformers Figure 42 shows a completed wiring diagram for a wye-to-delta, step-up transformer bank using subtractive-polarity transformers. A B C N X 2 X 1 X 2 X 1 X 2 X 1 H 2 C H 1 H 2 H 1 H 2 H 1 B A A B C Figure 42 Wye-to-delta, step-up transformer bank using subtractive-polarity transformers Figure 43 shows a completed wiring diagram for a wye-to-delta, step-up transformer bank using mixed transformer polarities. It does not matter which polarity the transformers are, they still have the same connections with respect to the terminal markings. 40 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

41 Learning Task 2 H-2 A B C N X 2 X 1 X 2 X 1 X 1 X 2 H 2 C H 1 H 2 H 1 H 2 H 1 B A A B C Figure 43 Wye-to-delta, step-up transformer bank using transformers of mixed polarities Voltage relationships for the wye-to-delta transformer bank For a wye-to-delta connection: The line-to-line voltage ratio is equal to the individual transformer turn-ratio times 3. The voltage ratings of the transformer s primary windings must be equal to the primary line voltage divided by 3. The voltage ratings of the transformer s secondary windings must be equal to the secondary line voltage. To determine if both the primary and secondary connections are correct, do the mesh or delta-closure test on the secondary delta before making the final connection. Figure 44 shows the voltmeter placed in the secondary circuit to make this test on a wye-to-delta, stepdown application. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 41

42 Learning Task 2 H-2 Figure 44 Delta closure test on a wye-to-delta, step-down application Delta-to-wye connection The delta-to-wye, three-phase transformer connection is the most popular connection used to supply low-voltage distribution systems. The reasons are: It can supply both single-phase and three-phase loads. It is easily grounded at the neutral point and this limits the voltage to ground available during a ground fault on any line conductor. In the past, many low-voltage, three-phase loads were supplied by ungrounded, deltaconnected secondaries. In those systems, induced transient overvoltages often caused insulations to break down. Today, new installations are usually supplied by grounded, wyeconnected secondaries. Delta-to-wye, step-down bank The voltage diagram to use for a delta-to-wye, step-down transformer bank is shown in Figure 45. This ensures a phase displacement of 30, with the high side leading the low side. Figure 45 Voltage diagram for a delta-to-wye, step-down transformer bank 42 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3

43 Learning Task 2 H-2 Delta-to-wye, step-up bank The voltage diagram to use for a delta-to-wye, step-up transformer bank is the one shown in Figure 46. Again this ensures a phase displacement of 30 where the high side leads the low side. Figure 46 Voltage diagram for a delta-to-wye, step-up transformer bank Using the proper diagram for the required application ensures that the banks will have the proper phase displacement to be paralleled with other wye-to-delta and delta-to-wye, threephase transformers. Constructing voltage diagrams For a delta-to-wye, step-down transformer bank 1. Since the high side is a delta connection, draw the first phasor V AB at 60 to the horizontal. Have H 1 at the lower end and H 2 at the upper end as in Figure 47(a). Figure 47 Voltage diagram for a delta high side 2. Draw the second high-side phasor 120 behind the first phasor with its H 1 terminal connected to the H 2 terminal of the first phasor as in Figure 47(b). 3. Draw the third high-side phasor 120 behind the second phasor. Connect its H 1 terminal to the H 2 terminal of the second phasor. Now, connect its H 2 terminal to the H 1 terminal of the first phasor to complete the high-side delta connection. See Figure 47(c). CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3 43

44 Learning Task 2 H-2 4. Since the low-side induced voltages are in phase with the high-side induced voltages, draw the low-side phasors parallel to the high-side phasors of the same transformer. To ensure the proper phase displacement of 30, connect the X 1 terminals of the low-side phasors together to form the neutral or wye point. See Figure 48. Figure 48 Voltage diagram of a delta-to-wye, step-down connection 5. Connect the H 1 terminal of the first high-side phasor drawn to line A. Connect the next lagging one to line B, and the third one to line C as shown in Figure The way you select which X 2 terminal will be labelled as line A on the secondary is a little different for this diagram. Choose it so that there is a 30 phase shift between the high and low side with the high side leading. 7. Connect line A to the X 2 terminal of the C-phase transformer; connect line B to the X 2 terminal of the A-phase transformer; connect line C to the X 2 terminal of the B-phase transformer. For a delta-to-wye, step-up transformer bank 1. Draw the first phasor (V AN ) of the high-side wye at 30 to the horizontal. Have H 1 at the lower end and H 2 at the upper end as in Figure 49(a). 2. Draw the other two high-side phasors equally spaced 120 apart from each other and from the first phasor. Connect the H 2 points to form the wye point as shown in Figure 49(b). 44 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 3