Transformer Connections

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  1. it is important to note that three-phase transformers with Delta-connected primaries when connected to a 30, 4-wire supply system do not utilize? (NC set transformer connections) source: http://www.federalpacific.com/university/transbasics/chapter3.html
    • The 4th wire or neutral.
    • Image Upload 1
  2. A wye system is also known as what? Transformer Principle and Applications, 2006: Transformer Connections, WB PP 23
    A wye system is also known as a star system.
  3. A wye to delta connection may be made with what in leu of one delta transformer to increase reliability and maintain voltage in case of one phase loss? Transformer Principle and Applications, 2006: Transformer Connections, WB PP 23
    think of an alternative to a delta transformer in a situation where 3 phase undergrounded delta power is needed.
    Three single phase transformers to create a delta bank.
  4. Wye to delta connected transformers, with properly marked leads, always have an angular phase displacement of _________? Transformer Principle and Applications, 2006: Transformer Connections, WB PP 23
    This does not imply use of a nuetral.
    • 30 degrees voltage vector in a Delta ALWAYS; whereas, a 120 degree phase displacement (voltage vector and not current) only occurs on a wye or a high leg delta to ground.Image Upload 2
    • Image Upload 3
  5. A _______ connection may also be called a V connection because only two transformers on a delta bank are included. Transformer Principle and Applications, 2006: Transformer Connections, WB PP 23
    Open Delta.
  6. The voltage tolerance on most motors is plus minus what voltage in a percentage? Transformer Principle and Applications, 2006: Transformer Connections, WB PP 23
    10 percent.
  7. Today, the majority of dwellings are wired with a how many wire service? Transformer Principle and Applications, 2006: Transformer Connections, WB PP 23
    the number of wires is just that, wires: not current carrying conductors necessarily.
    Three wires: 2 wire and a messenger; where two phase conductors create a single phase service.
  8. Transformer polarity refers to what? Transformer Principle and Applications, 2006: Transformer Connections, WB PP 24
    The voltage-vector relationship of the transformer leads as brought outside the case.  voltage vector means degrees of phase shift.  Delta is 30° voltage vector displacement.  Unless Line to Nuetral in a high leg then it is 120° .Image Upload 4
  9. Single phase transformer polarity is additive and subtractive when? Transformer Principle and Applications, 2006: Transformer Connections, PP 73
    subtractive when h1 and x1 are adjacent and additive when diagonal.

    • h1 h2 h1 h2
    • x1 x2 x2 x1
    • subt add
  10. What must be done when the transformer secondary taps are not positively identified? Transformer Principle and Applications, 2006: Transformer Connections, PP 73
    The transformer must be tested to determine the correct polarity. Tech Fact.  A transformer comparison to another identical transformer can be done as well.
  11. In a Delta what is the winding voltage from phase A,B, & C with a standard Delta and no high leg center tap? Electrical field reference manual, PP 414
    480V if it is a standard Delta.

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  12. What simplifies the task of connecting transformers in parallel? Transformer Principle and Applications, 2006: Transformer Connections, PP 74
    Image Upload 6
    • The H1 lead is normally brought out on the right-hand side of the transformer case when facing the high voltage side.  Notice this is the case on the Delta/ Wye transformer below regarding the data plate.
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  13. Methods used to test for polarity and check transformer lead markings include what? Transformer Principle and Applications, 2006: Transformer Connections, PP 74
    • Transformer comparison,
    • DC use, and...
    • AC use.
  14. Instrument Transformers (PT's and CT's) and large substation transformers are ________Polarity, so the H1 and X1 Bushings will be on the what side of the transformer?
    • Instrument Transformers (PT's and CT's) and large substation transformers are Subtractive Polarity, so the H1 and X1 Bushings will be on the same side of the transformer. 
    • It is worth noting that on a 37.5MVA substation transformer where the primary and secondary are on opposite sides.  the primary is 161KV and the secondary is 13,800V.  Subtractive is not adjacent.
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  15. What is the difference between phase and line voltage?
    Image Upload 9
    Line voltage is the voltage measured between any two lines in a three-phase circuit. Phase voltage is the voltage measured across a single component in a three-phase source or load. https://www.quora.com/What-is-difference-between-line-to-line-voltage-and-line-to-neutral-voltage
  16. What is a chopper circuit?
    In electronics, a chopper circuit is used to refer to numerous types of electronic switching devices and circuits used in power control and signal applications. A chopper is a device that converts fixed DC input to a variable DC output voltage directly.
    A DC transformer is nicknamed the Chopper circuit. Image Upload 10
  17. In a Delta system, the winding (phase) current is actually _________ percent of the line current; in other words, the line current is equal to phase current times ________. Transformer Principle and Applications, 2006: Transformer Connections, PP 77
    Phase voltage is the voltage measured across a single component in a three-phase source or load. Line current is the current through any one line between a three-phase source and load. Phase current is the current through any one component comprising a three-phase source or load
    57.7 percent line current.  and 1.73 x phase current.  Phase current by definition is phase to nuetral.  

    • (.577x208V=120V. 208 is 2 phases and a N.) Where 1.73x120=208V)Image Upload 11
    • 1.73
  18. The square √3 is also 1.73.  this is used in a 3 phase power system for what reason?
    Image Upload 12
    The line voltage is best though of as "line to line"
    The square root of three is a magnitude multiplier that accompanies a 30 degree phase shift between line voltages and their respective phase (phase is to nuetral as opposed to a line voltage) voltages for a wye system, and line currents and their respective phase currents for a delta system that is a result of adding (or subtracting) two vectors that are equal in magnitude and 120 degrees apart.
  19. what is different from a transformer primary and a stator in a polyphase motor? Transformer Principle and Applications, 2006: Transformer Connections, PP 78
    • Nothing.
    • note: except that the secondary does not rotate in a transformer.
  20. What type of transformer describes the following statement: "The three ends can be safely connected at the neutral point because no voltage difference exists between them." Transformer Principle and Applications, 2006: Transformer Connections, PP 78
    A Wye transformer.  Also known as a star transformer.Image Upload 13
  21. In a WYE / Star:
    the phase current (line to nuetral and not line to line) is equal to what?  How far out of phase are the voltages? the primary and secondary are how far out of phase in a WYE?
    • The phase current is equal to the line current, and they are in phase. There is a phase separation of 30º between the line and phase voltage.
    • The 180º phase shift between the primary and secondary of the transformer is shown in the figure above.  (applied voltage is 180 out of phase of the induced voltage!)
  22. A Delta to Wye connected 3 phase system has how many volts on the primary and on the secondary phase to phase? Electrical Field Reference Manual, PP 415
    Remember a phase voltage is phase to nuetral and line voltage is line to line.
    • The primary has 2400V or 480V line to line Secondary has 277V/480 or 120/208V with a nuetral across any phase (phase voltage and not line voltage). Used where a 4 wire secondary distribution system is desired.
    • Image Upload 14
  23. Why do generators always provide a WYE output with larger generators?
    Because the phase voltage (phase to nuetral) is 57.7% line voltage (line to line) and the current is KVAx1000/√3 (1.73)x E.  this means the voltage for the stator would be very high, and so would current.   this would mean a much more expensive build for winding insulation, etc.
  24. In 3 phase power, a total of _____line leads are required if the three circuits are kept separate,  from the generator to the apparatus using the current (remember generators are WYE output). This is impractical, so what is used? Transformer Principle and Applications, 2006: Transformer Connections, PP 76.
    6, so a common wire are used. Aka grounded wires.  Image Upload 15
  25. What provides a 120V/208V feeder for a panelboard? Transformer Principle and Applications, 2006: Transformer Connections, PP 81
    regarding a 480v bus. An advantage of the Delta connection is higher reliability. If one of the three primary windings fails, the secondary will still produce full voltage on all three phases. The only requirement is that the remaining two phases must be able to carry the load
    A dual-wound delta primary connected onto the 480V line to line bus (not phase to phase as phase is phase to nuetral by definition). No primary neutral, but the secondary neutral can be connected to a center tap to ground.
  26. What is the advantage of a Delta power system?  Especially for a primary delta in an installation where sustainability is required?
    Remember: phase voltage is phase to nuetral. Think line to line; which is not line to nuetral.
    An advantage of the Delta connection is higher reliability. If one of the three primary windings fails, the secondary will still produce full voltage on all three phases. The only requirement is that the remaining two phases must be able to carry the load
  27. What is a synchronous condenser?
    • synchronous capacitor or synchronous compensator) is a device, identical to a synchronous motor, whose shaft is not connected to anything but spins freely.[1] Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid's voltage, or to improve power factor.
    • https://en.wikipedia.org/wiki/Synchronous_condenser
  28. A third winding that is often used in power transformers to provide station power requirements or a tie with synchronous condensers is what? Transformer Principle and Applications, 2006: Transformer Connections, PP 82
    Remember: there is an advantage to a tertiary winding. See next card.
    A tertiary winding.Image Upload 16
  29. What is the advantage of a tertiary winding?  A 3rd transformer (delta) between two wye transformers.
    When the fault or short circuit occurs, high circulating current will flow through the primary and secondary winding. Because of that current , overheating of transformer is produced, but by using a tertiary winding  we can avoid that overheating. Inductive reactance of tertiary winding can limit over current.
  30. When is a Wye to Wye connection used? Transformer Principle and Applications, 2006: Transformer Connections, PP 82

    A tertiary winding is used in a Wye to Wye connection for what purpose? Transformer Principle and Applications, 2006: Transformer Connections, PP 82
    When tying together two high-voltage transmission systems of unequal voltage.

    A wye to wye is used To suppress 3rd harmonics. (An odd numbered harmonic which adds to each other, although triplen harmonics, odd multiples of the third harmonic: 3, 9, 15th are only single phase load related) May cause transformer overheating, overloaded neutrals, telephone interference.
  31. Disadvantages of a wye-to-wye connection include what? Transformer Principle and Applications, 2006: Transformer Connections, PP 82.

    what is used to limit this issue?
    Harmonics.

    A K series isolation transformer (rated K4-K20) and is simply a series transformer.Image Upload 17
  32. In a Wye to Wye connection the voltage of each single-phase unit connected to form a wye-to-wye connection is _____percent of the line phase (to nuetral) voltage: an advantage. Transformer Principle and Applications, 2006: Transformer Connections, PP 83
    Remember: a WYE is either 120/208 or 277/480. The lower voltage always has a N.
    • 57.7 percent
    • example 120 is 57.7% of 208.  And 277 is 57.7% of 480V.  
    • Image Upload 18
  33. Delta to Delta systems are used where? Transformer Principle and Applications, 2006: Transformer Connections, PP 85
    In industry where old equipment is used on new high-voltage 480 V bus duct systems.  To answer in industrial settings is sufficient.
  34. What is more common in an industrial setting.... a three phase system or three single phase units? Transformer Principle and Applications, 2006: Transformer Connections, PP 85
    Three single phase units in an industrial delta to delta system.  This allows the full voltage on the secondary with only 2/3 line voltages available.
  35. One of the disadvantages of a Delta to Delta connected system is what? Transformer Principle and Applications, 2006: Transformer Connections, PP 86

    To obtain a grounded system, what must be done? PP 87
    The absence of an intentional connection to ground on the secondary of the transformer.

    One of the corner of the delta secondary must be grounded.
  36. what is the voltage on a corner grounded Delta transformer?
    • Always 480V.
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  37. When a Delta to Delta system has no neutral, and one is derived to give a reference to ground, this system is also referred to as what type of transformer? Transformer Principle and Applications, 2006: Transformer Connections, PP 87
    • grounded B phase. Which is also a hi leg. Wild leg.  A 3 phase 4 wire Delta is another answer. 
    • Which is a center grounded delta, but also derives a neutral. Versus corner grounded. The big difference from a WYE is a grounded b phase provides 208volts, and also a 480 option. 
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    • Image Upload 21
  38. In a Delta to Delta grounded B phase system the center conductor is identified as what? What must be done with fusing. Transformer Principle and Applications, 2006: Transformer Connections, PP 87
    Is identified AS what, not how is it identified.
    As a neutral (not a grounded because it carries unbalanced current) conductor, and fusible clips must have a shunt, usually a copper tube, installed if used as a serviceable disconnect.
  39. the voltage vector relationship of an open delta is what?
    Image Upload 22
    • Line voltage displacement is 120°.  With a 30° displacement for current.
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    •  the vectors form a closed loop with the sum of the three voltages adding up to zero. This is basic mechanism of ‘deriving’ the third phase by using only two transformers.
  40. The KVA required for an open delta on the primary versus the secondary is what?
    The combined transformer kVA has to be 115% of the load kVA.

    • This means an open delta transformer will only be able to deliver 57.7% of the capacity of an equivalent three phase delta transformer with similar voltage and load current.
    • http://voltage-disturbance.com/power-engineering/open-delta-transformer/
  41. A transformer configuration that allows for low cost install for future expansion is what? Transformer Principle and Applications, 2006: Transformer Connections, PP 88

    The primary on an Open Delta is _____volts and the secondary is ______volts. With a voltage vector relationship of 60 degrees.  Transformer Principle and Applications, 2006: Transformer Connections, PP 88
    An open delta, or two single phase transformers.

    Primary voltage is 2400V and the secondary voltage is 240 volts with a voltage vector relationship at 60 degrees.
  42. What type of connection makes it possible to maintain operation in an emergency? Transformer Principle and Applications, 2006: Transformer Connections, PP 89
    An open delta connection.
  43. The load on an open delta must be reduced by how much from that of a closed delta. Transformer Principle and Applications, 2006: Transformer Connections, PP 89
    42.3 percent.  Since an open delta has to limit the load by 57.7%.  Remember that a closed delta has a line and phase current difference but the open does not.  You can remember this by picturing the phase angle displacement.  Two windings are equal and three are 120 degrees apart.
  44. One of the more popular Delta to Delta connections is the ________. Transformer Principle and Applications, 2006: Transformer Connections, PP 89

    The advantage of a hi-leg to ground is what?
    120/240V 4 wire system with high leg to ground.

    120/240V versus 120/208V which is not adequate for 240 volt motors having a voltage tolerance of +/- 10 percent. Low voltage will result in overheating.
  45. A wye to wye connection may have an angular-phase displacement of ______ or _____ degrees. Transformer Principle and Applications, 2006: Transformer Connections, PP 91
    0 or 180 degrees.
  46. A Wye to Delta connection always has an angular phase displacement of _____ degrees, with properly marked leads. Transformer Principle and Applications, 2006: Transformer Connections, PP 91
    30 degrees.
  47. A disadvantage of a delta connection is? Transformer Principle and Applications, 2006: Transformer Connections, Chapter 4 summary
    Remember a Wye to Wye has the advantage of transforming to higher and unequal voltages.
    that the coil is rated for the line voltage and thus makes the high voltage required for transmission difficult.
  48. Wye connection offer what? Transformer Principle and Applications, 2006: Transformer Connections, Chapter 4 summary
    lower voltage-to-ground combined with the higher phase-to-phase voltage for long-distance power distribution.
  49. What does the 30 degree difference in phase displacement mean regarding a wye to delta / delta to wye transformers on the same feeder? Transformer Principle and Applications, 2006: Transformer Connections, Chapter 4 summary
    phase is not to nuetral. phase is phase to phase. Line is line to nuetral.
    • It means it is impossible to parallel a wye and delta transformer on the same feeder.
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  50. What makes it difficult to maintain good balance from phase-to-phase in a wye connection? Transformer Principle and Applications, 2006: Transformer Connections, Chapter 4 summary.
    The ability to take single phase loads from line to neutral in a wye connection make it difficult to maintain good balance from phase-to-phase.  Note: this is why a WYE panelboard will have all 3 phases balanced in a panel schedule.
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122720
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Transformer Connections
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Transformer connections per the 2008 code cycle Electrical Field Reference Handbook and Transformers Workbook
Updated
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