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Transformers > Chapter 6 Week 3 > Flashcards

Chapter 6 Week 3 Flashcards

Three-Phase Connections

1
Q

How can a three-phase service be supplied?

A

One Three-Phase Transformer Unit

Three Single-Phase Units

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2
Q

Why is it easier to install one three-phase unit?

A

The proper polarity and interconnections between the phases are fixed inside the tank.

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3
Q

What is the most common method used by utility companies?

A

Interconnecting (banking) three single-phase units

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4
Q

Why is it most common to bank transformers?

A

Because it is more economical, being less expensive to replace one transformer of a bank.

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5
Q

What do single-phase transformers offer?

A

Flexibility because they can be installed as individual units for single-phase services, or banked for three phase services

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6
Q

Can Three-Phase Transformers be used in single-phase service?

A

No

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7
Q

What should the lineworker focus on when selecting single-phase transformers that are to be banked or paralleled together?

A
  1. ) Circuit Voltage
  2. ) Service Voltage
  3. ) Transformer Voltage Rating
  4. ) kVA Ratings of the Transformers
  5. ) Polarity of the Transformers
  6. ) Impedance of the Transformer
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8
Q

What is a good idea before selecting any transformer?

A

Testing it is, especially for used transformers

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9
Q

Where should circuit and service voltages be marked clearly?

A

On the job print

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10
Q

How should the job print indicate transformer sizes?

A

In kilo-volt amperes (kVA)

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11
Q

Who usually determines transformer size prior to installation?

A

Engineering Dept.

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12
Q

How are transformer banks of WYE connected secondaries composed?

A

With transformers of equal size because all three transformers share the single-phase load equally

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13
Q

How are transformer banks of DELTA connected secondaries composed?

A

With transformers where one is greater in size than the other two because one transformer in the bank serves two-thirds of the single-phase load.

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14
Q

What should you make certain of when banking transformers?

A

Make sure they’re of the same polarity

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15
Q

What happens when banking transformers of different polarities?

A

They will work electrically but can confuse lineworkers who may need to work on transformer bank in the future and can also create serious safety concerns.

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16
Q

Every effort should be made to select transformers with the same _________ when paralleling or banking?

A

Impedances

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17
Q

What happens when mismatching impedances?

A

It requires a de-rating because the load does not the divide in proportion to the kVA Ratings.
The lowest impedance transformer will supply a greater share of the load and possibly end up being overloaded.

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18
Q

What does each coil in a transformer have?

A

A positive and negative end

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19
Q

What are the positive connections on a single-phase transformer?

A

H-1 and X-1

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20
Q

What are the negative connections on a single-phase transformer?

A

H-2 and X-3 (Sometimes X-2)

21
Q

What will a reversed polarity cause?

A

An extremely high current flow when the transformer is energized.

22
Q

How are the connections made on a WYE connection?

A

The negative terminals are all connected together and the positive terminals are connected to each individual phase

23
Q

How are the connections made on a DELTA connection?

A

A positive terminal is always connected to a negative terminal and then connected to the phase.

24
Q

How are DELTA banks presented regardless of polarity?

A

With the same appearance

25
Q

What are many banking and paralleling errors linked to?

A

Improper transformer selection

26
Q

In order to supply customers with three-phase power, how do transformers need to be configured?

A

Transformer coils need to be interconnected into a configuration that will involve all three-phases with the proper phase sequence.

27
Q

What are the two connections to create a potential difference across a transformer coil?

A
  1. ) Phase-to-Phase Connection (Delta)

2. ) Phase-to-Neutral Connection (WYE)

28
Q

When the three coils are each connected phase-to-phase, how are they connected?

A

In Series

29
Q

In a Delta System, how does the coil voltage compare?

A

Same as the System Voltage

30
Q

In a Delta System, how does the coil current compare?

A

1.73x higher

31
Q

How do you complete a circuit?

A

Electrical current must return back to the source.

32
Q

In a Delta System how does the circuit return flow to the source?

A

Through the opposing phase conductors.

33
Q

When the three coils are each connected as phase-to-neutral, how are they connected?

A

In Parallel

34
Q

On a WYE System how should the load be if there was no current flow in the neutral?

A

The load on each of the three phases is balanced.

35
Q

In a WYE System, how does coil voltage Compare?

A

1.73x higher

36
Q

In a WYE System, how does the coil current compare?

A

Same as the Phase Current

37
Q

How can Three-phase power be supplied?

A

Either two single-phase transformers in a bank, or three single-phase transformers in a bank

38
Q

What does the term Open Bank mean?

A

When two transformers are banked together, they do not complete the entire Delta or WYE configuration

39
Q

What is the general rule when a customer’s load consists of both three-phase and single-phase?

A

If the single-phase load is larger than the three-phase load, an open bank may be used.

40
Q

What is a “lighter” Transformer?

A

It supplies the bulk of the “lighting” load.

41
Q

What is the smaller transformer referred to as?

A

“Power” Transformer

42
Q

What is the only exception when using open banks?

A

Three-phase power cannot be derived from an open bank if the secondary is connected open WYE

43
Q

What can be done if one transformer in an open bank with a Delta secondary goes bad?

A

The bank can be temporarily re-wired to an open bank.

44
Q

When are Open-Delta - Open-Delta banks used?

A

When the single-phase load is predominate over the three-phase load.

45
Q

Approximately how efficient are open banks when compared to closed banks?

A

57.7%

46
Q

What do Open-WYE - Open-Delta banks require?

A

Only two primary phases and a system neutral and can be connected in one of two ways.

47
Q

How was the same-opposite connection given its name?

A

The “same” primary bushings are connected together (H2-H2) and grounded, while the “opposite” secondary bushings are connected together (X1-X3)

48
Q

What are open banks almost exclusively?

A

120/240-volt three-phase loads

49
Q

How was the opposite-same connection given its name?

A

The “opposite” primary bushings are connected together (H2-H1) and grounded, while the “same” secondary bushings are connected together (X1-X1)

Transformers (7 decks)

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