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JL Exam > Grey Book > Flashcards

Grey Book Flashcards

1
Q

What is the core of a transformer made of?

A

laminated iron or steel core about which are wound two or more coils.

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2
Q

what are the 11 transformer components consist of:

A

core, coils, ratio, tank, bushings, cooling, losses, taps, impedance, polarity, nameplate

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3
Q

what is the purpose of the laminate sheet insulating material coating the core steel sheets?

A

to reduce the core loss and the heating produced in the core by insulating each sheet from the other.

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4
Q

define - permeability

A

the ease which soft iron conducts magnetic lines of force; the softer the iron, the greater the permeability

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5
Q

In coils, in comparison to the primary and secondary. What are the size, number of turns, and insulation of the coil wires?

A

The primary coil has more turns, smaller wire, higher insulation; while the secondary coil has the lesser turns, larger wire and the lesser insulation.

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6
Q

In coils what is the shape of the primary and secondary wires?

A

Primary wire may be round or rectangular or sheet. Secondary wire is usually rectangular or sheet.

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7
Q

Askarel is also know as what?

A

PCB - Polychlorinated Biphenyl

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8
Q

Losses: What are the two main losses of the transformer?

A

iron and copper losses, iron (core loss) consists of eddy current and hysteresis losses; copper losses consists of heating loss of the conductor I2R loss.

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9
Q

what does the iron loss consist of?

A

eddy current and hysteresis losses; that is why the core is laminated instead of being a solid bar.

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10
Q

the amount of current to fully magnetize the iron core is sometimes known as the?

A

exciting current or charging current

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11
Q

What is the formula for the copper losses or heating loss?

A

I2R

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12
Q

what are the standard taps for a transformer?

A

4-2 1/2% below or 2- (plus/minus) 2 1/2% taps

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13
Q

when changing taps, what happens to the secondary voltage when you raise and lower the taps?

A

when you raise the taps it lowers the customers voltage, when you lower the tap it raises the customers voltage

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14
Q

symbol for impedance is

A

%Z

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15
Q

capacitance is measured in ______ and its symbol is __

A

farads; C

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16
Q

inductance is measured in _______ and its symbol is __

A

henerys; L

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17
Q

inductive reactance is measured in ____ and its symbol is __

A

ohms; XL

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18
Q

capacitive reactance is measured in _____ and its symbol is __

A

ohms; XC

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19
Q

impedence is measured in ____ and its symbol is__

A

ohms; Z

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20
Q

resistance is measured in ____ and its symbol is __

A

ohms; R

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21
Q

the total opposition to current flow in and AC circuit is called _________ and is measured in ____.

A

impedance; ohms

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22
Q

impedance is made of

A

resistance, inductive reactance, capacitive reactance

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23
Q

in AC circuits with reactance the voltage can lead or lag the current by as much as __ degrees

A

90

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24
Q

in parallel operation, the transformer with the lower impedance will carry the?

A

greater load

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25
Q

can transformers of different KVA be paralleled?

A

yes if the impedances are nearly the same, the load will split proportionally

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26
Q

define polarity

A

a designation of the relative direction of current flow in the primary and secondary windings.

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27
Q

in the box, out of the box rule for transformers

A

transformers rated more than 200KVA or having a primary voltage over 8600 volts are subtractive. All others transformers are additive polarity.

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28
Q

in order to properly parallel two transformers, what factors have to be checked?

A

same voltage ratios, nearly the same impedance (within plus/minus 10%), must be connected to the same phase, polarity must be connected the same

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29
Q

The highest tap is called _, _, or _

The lowest tap is called _, _, or _

A

1, I, or A

5, V, or E

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30
Q

what are the two main enemies of transformers?

A

dangerously high voltage surges caused by lightning, and heating caused by overloading

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31
Q

what are the two basic lightning arresters used on distribution systems

A

expulsion (open gap), and the valve type

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32
Q

what protection does the CSP trans former have built in?

A

external tank mounted lightning arrester to protect it from high voltage surges, a secondary circuit breaker to protect it from external short circuits and overloads, an internal high voltage fuse is mounted in the primary bushing to protect the system from internal faults in the transformer.

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33
Q

the primary booster is a conventional transformer connected how?

A

as an auto transformer; it is not grounded and has no fuse protection

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34
Q

primary boosters raise the voltage by how much?

A

5% or 10%

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35
Q

what does a ground consist of?

A

an artificial electrical connection to the earth, having a very low resistance to the flow of electric current.

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36
Q

what does it mean to ground a transformer?

A

to connect it to the earth

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37
Q

what are the six possible transformer connections

A 
open-delta/open-delta
open-wye/open-delta
delta/wye
wye/wye
delta/delta
wye/delta
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38
Q

in the wye system the voltage from the tip to the tip represents the ____ voltage

A

line

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39
Q

in the wye system the voltage from the tip to the center of the wye represents the _____ voltage

A

phase

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40
Q

what is the most popular distribution voltage today?

A

7200/12,470Y volt system

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41
Q

how much more efficient is the wye primary system as compared to the delta primary system?

A

73%

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42
Q

phasing and rotation rule

A

phasing insures rotation; rotation does not insure phasing

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43
Q

what unexplainable voltage condition exists in the wye-wye and wye-delta transformer connections?

A

third harmonic (180 hertz), an unwanted voltage on the system

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44
Q

how much larger can the third harmonic voltage be than the phase voltage?

A

150%

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45
Q

on a wye-delta connection where will there be a third harmonic voltage measured?

A

where the neutral bus floats

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46
Q

if jumpers are placed between the same identified phases (A to A, B to B, C to C) of two three phase circuits, the circuits would phase together under what conditions?

A

1) that the circuits are electrically in phase (angular displacement). if a circuit is not 30 degrees out of phase due to a wye-wye and delta-wye transformation
2) on a grounded system, the voltage across the gap is near zero.
3) on and ungrounded delta system:
(a) make sure all foreign grounds are removed from the system.
(b) connect any two leads together, one lead from each circuit
(c) measure voltage across both gaps. They are in phase when the voltage is near zero.

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47
Q

if a customer request 120/240 volts, three phase, four wire what transformers can be used?

A

transformers rated 120/240 volts can be used
a transformer rated 120/240 volts must be used as a “lighter”, but transformers rated 240 x 480 volts can be used as the power pots by paralleling the secondary coils

48
Q

if a customer requests 240 volts,three phase, three wire, what transformers can be used?

A

transformers rated 120/240 volt or 240 x 480 volts can be used

49
Q

if a customer requests 120/208 volts, three phase, four wire, what transformers can be used?

A

only 120/240 volts transformers can be used

50
Q

if a customer requests 277/480 volts, three phase, four wire, what transformers can be used?

A

only 277 volt transformers can be used

51
Q

if a customer requests 480 volts, three phase, three wire, what transformers can be used?

A

transformers rated 277 volt or 240 x 480 volts can be used

52
Q

when using the open-wye/open-delta connection or an open-delta/open-delta, in an emergency situation when one transformer in a closed wye-delta or closed-delta/closed-delta bank fails, the KVA rating of the bank is reduced by what factor

A

86.6% of the two remaining transformers

or 57.7% of the initial three transformers

53
Q

what disadvantage of the open-wye/open-delta connection can be a hazard to a lineman working on the bank?

A

the primary neutral also carries the full load current even though the load is balanced. if the primary neutral connection is disconnected when the bank is energized, one-half of the phase to ground will be present

54
Q

can CSP transformers be used on open-wye/open-delta banks?

A

Yes. only if future plans do not call for closing the delta

55
Q

on a open-wye/open-delta bank what would happen if one of the primary phases were to be switched to the other phase?

A

rotation would be reversed, all secondary voltages would stay the same.

56
Q

can a closed delta and an open delta bank be paralleled?

A

yes when properly phased

57
Q

what condition will exist if one primary fuse is blown on an open-wye/open-delta or open-delta/open-delta bank, if the three phase load is still connected?

A

a slight backfeed voltage will exist on the primary lead of the blown fuse transformer.

58
Q

Rule of Thumb: in order to size the two transformers in an open-wye/open-delta or open-delta/open-delta to carry a three phase balanced load, multiply the load be __%.

A

57%, the single phase load is added only to the lighter transformer.

59
Q

can CSP transformers be used in a open-delta/open-delta connection?

A

yes. only if future plans do not call for the closing of the delta. must be two bushing transformers

60
Q

can an open-wye open-delta bank be paralleled?

A

No. they would be 30 degrees out of phase

61
Q

in a wye-delta or delta-delta bank how is the single phase and three phase load split between the cans?

A

the lighter carries two-thirds of the single phase load, and each power pot carries one-third of the single phase load. the three phase load splits one-third of the load on each transformer.

62
Q

when banking transformers the impedance of the transformers should be matched or no more than __% out of balance

A

10%

63
Q

what will happen if a CSP transformer is used as the lighter transformer on a wye-delta or delta-delta bank and the secondary breaker opens (or trips off)?

A

the neutral will become isolated, causing unequal voltages on the single phase loads (it would act as an open neutral)

64
Q

can CSP transformers be used on a wye-delta bank?

A

yes, only on 240 volt and 480 volt three phase, three wire systems

65
Q

when the primary neutral on a wye-delta bank is floated, and the secondary wild leg is opened to the load and the voltage is measured across this gap, it will read approximately 11/2 times the phase voltage. what is this phenomenon called?

A

third harmonic. when the primary neutral is grounded this voltage will return to zero.

66
Q

in the wye-delta bank connection what should be done with the primary neutral?

A

it should be “floated” or insulated from ground

67
Q

in a wye-delta bank what two things happen if the neutral bus becomes grounded, and a primary transformer fuse is blown?

A

1) the voltage condition remains the same as before, except that the bank capacity is reduced to 57% of the bank rating
2) the transformer on the blown fuse line acts as a step-up transformer and provides power to the so called dead line (this is known as back feed)

68
Q

on a wye delta bank what is the voltage on the floated neutral bus when a transformer fuse is blown?

A

one-half of the phase to ground voltage.

69
Q

on a wye-delta bank what is the voltage on the floated neutral bus if two transformer fuses are blown?

A

full phase to ground voltage

70
Q

what is the voltage on the floated neutral bus on a wye-delta bank every time it is energized or de-energized or a primary line is either energized or de-energized

A

one-half to full phase to ground voltage will be present on the neutral bus

71
Q

what needs to happen everytime a wye-delta bank is energized or de-energized?

A

the floated neutral bus has to be tied down.

72
Q

is there a third harmonic voltage in the delta-delta connection?

A

No.

73
Q

what are the two common transformer secondary voltage ratings and connections used if a customer requires 480 volt, three phase, three wire service?

A

1) 240 x 480 volt delta (open or closed)

2) 277/480 volt closed wye connected by not grounding the neutral connection

74
Q

what are the two common transformer secondary voltage ratings and connections if a customer requires 240 volt, three phase, three wire service?

A

1) 120/240 volt delta series secondary coils

2) 240 x 480 volt delta parallel secondary coils

75
Q

in a wye-wye 120/208 volt bank how should the transformers KVA ratings be sized

A

all should be the same size

76
Q

in a wye-wye 120/208 volt bank can single bushing cans and CSP cans be used?

A

yes

77
Q

in a wye-wye 120/208 volt bank the primary neutral and secondary neutral connections between the three transformers must have the same what?

A

ampacity as the phase conductors

78
Q

in a wye-wye 120/208 volt bank the primary and secondary line neutrals can be what?

A

of smaller size, as the wire carries only the unbalance in the three phase lines.

79
Q

in a wye-wye 120/208 volt bank, what must be done to the secondary coils?

A

must be paralleled

80
Q

in a wye-wye 120/208 volt bank what must be tied, if not what may develop on the secondary side?

A

primary neutral must be firmly tied to the system neutral; if not, excessive voltage may develop on the secondary side.

81
Q

in a wye-wye 120/208 volt bank if the primary neutral is disconnected from the system neutral, what condition will exist?

A

a third harmonic voltage exists between the primary neutral and the system neutral and ground. this excessive voltage could be hazardous.

82
Q

in a delta-wye 120/208 volt closed bank the KVA rating of the three cans should be?

A

the same size

83
Q

in a delta-wye 120/208 volt closed bank, the secondary neutral connections between the three transformers must have the same ________ as the phase conductors

A

ampacity

84
Q

in a delta-wye 120/208 volt closed bank, the secondary line neutral can be of?

A

smaller size, as the wire carries only the unbalance in three phase lines.

85
Q

in a delta-wye 120/208 volt closed 120/208 volt bank what must be done to the secondary coils?

A

must be paralleled

86
Q

in a delta-wye 120/208 volt closed bank, if it is a four wire wye system, the primary neutral should be?

A

connected to the secondary neutral

87
Q

in a delta-wye 120/208 volt closed bank what should be done with the grounding straps and why?

A

remove all the transformer grounding straps from all the transformers and ground the secondary neutral only at one location. this keeps tanks and tank ground connections from being in parallel with the neutral bus, and eliminates overheating in the grounding circuit.

88
Q

in a wye-wye 277/480 volt closed bank the three transformers KVA rating should be sized how?

A

the same.

89
Q

in a wye-wye 277/480 volt closed bank the primary and secondary neutral connections between the three transformers must have what?

A

the same ampacity as the phase conductors

90
Q

in a wye-wye 277/480 volt closed bank the primary and secondary line neutrals can be of what?

A

smaller size, as this wire carries only the unbalance in the three phase lines.

91
Q

in a wye-wye 277/480 volt closed bank, the primary neutral must be? if not?

A

tied firmly to the system neutral; if not, excessive voltages may develop on the secondary side.

92
Q

in a wye-wye 277/480 volt closed bank, if the primary neutral is disconnected from the system neutral what exists?

A

a third harmonic voltage exists between the primary neutral and the system neutral to ground. this excessive voltage could be hazardous.

93
Q

in a wye-wye 277/480 volt closed bank what should be done with the grounding straps and why?

A

remove all the transformer grounding straps from all the transformers and ground the secondary neutral only at one location. this keeps tanks and tank ground connections from being in parallel with the neutral bus, and eliminates overheating in the grounding circuit.

94
Q

in a wye-wye 277/480 volt closed bank whats up with the secondary coils?

A

this transformer has only one secondary coil

95
Q

in a delta-wye 277/480 volt closed bank the three transformers KVA rating should be sized how?

A

the same.

96
Q

in a delta-wye 277/480 volt closed bank how many primary bushing are needed per can?

A

two

97
Q

in a delta-wye 277/480 volt closed bank the primary and secondary neutral connections between the three transformers must have what?

A

the same ampacity as the phase conductors

98
Q

in a delta-wye 277/480 volt closed bank the primary and secondary line neutrals can be of what?

A

smaller size, as this wire carries only the unbalance in the three phase lines.

99
Q

in a delta-wye 277/480 volt closed bank if it is a four wire wye system, the primary neutral should be?

A

connected to the secondary neutral

100
Q

in a delta-wye 277/480 volt closed bank what should be done with the grounding straps and why?

A

remove all the transformer grounding straps from all the transformers and ground the secondary neutral only at one location. this keeps tanks and tank ground connections from being in parallel with the neutral bus, and eliminates overheating in the grounding circuit.

101
Q

in a delta-wye 277/480 volt closed bank whats up with the secondary coils?

A

the is only one secondary coil

102
Q

in a wye-wye 277/480 volt closed bank how should the transformers be sized?

A

all three transformers should be the same size

103
Q

in a wye-wye 277/480 volt closed bank the primary and secondary neutral connections between the three transformers must have what?

A

the same ampacity as the phase conductors

104
Q

in a wye-wye 277/480 volt closed bank the primary and secondary line neutrals can be of what?

A

smaller size, as this wire carries only the unbalance in the three phase lines.

105
Q

in a wye-wye 277/480 volt closed bank the primary neutral must be? if not?

A

tied firmly to the system neutral; if not, excessive voltages may develop on the secondary side.

106
Q

in a wye-wye 277/480 volt closed bank if the primary neutral is disconnected from the system neutral what exists?

A

a third harmonic voltage exists between the primary neutral and the system neutral to ground. this excessive voltage could be hazardous.

107
Q

in a wye-wye 277/480 volt closed bank what should be done with the grounding straps and why?

A

remove all the transformer grounding straps from all the transformers and ground the secondary neutral only at one location. this keeps tanks and tank ground connections from being in parallel with the neutral bus, and eliminates overheating in the grounding circuit.

108
Q

in a wye-wye 277/480 volt closed bank whats up with the secondary coils?

A

this transformer has only one secondary coil

109
Q

in a wye-wye 277/480 volt closed bank for 480 volt 3 phase 3 wire operation what voltage would you get between the phases? why is this against the delta secondary rules?

A

480 volts phase to phase, normally 480 volts would be a delta secondary hook up, however in this situation with the 277/480 volt cans it can be a wye secondary hook up, by omitting the secondary neutral connection.

110
Q

define eddy current

A

when the magnetic field induces a voltage into a transformer coil, a voltage is also induced into the iron core. this induced voltage causes current, known as eddy current, to flow in the iron core. eddy currents cause heating and are a waste of energy.

111
Q

define permeability of the core

A

the capability of the core material to accommodate flux: accept charge

112
Q

define magnetic reluctance

A

the resistance to the formation of flux in a magnetic circuit

113
Q

define hysteresis losses

A

also known as excitation loss, energy consumed when the iron core is magnetized and demagnetized by AC power at 120 times a second

114
Q

define copper losses

A

the losses in a transformer due to the resistance of the wires making up the turns of the primary winding

115
Q

how is hysteresis losses reduced

A

the core is made of more permeable iron, which means the core is made with various steel alloys that are easier to magnetize and demagnetize

JL Exam (2 decks)

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