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Transformers > Chapter 5 > Flashcards

Chapter 5 Flashcards

Winding Designations

1
Q

What do transformer manufacturer’s most commonly comply with?

A

ANSI Standard C57.12.00 when designating winding voltage ratings

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2
Q

What is the most common transformer-related question asked by lineworkers?

A

“If the primary coil rating of a transformer is 7200 volts, what is the second number (12470Y) for, such as 7200/12470Y?”

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3
Q

Which number is the voltage that needs to be impressed on the primary coil for proper operation?

A

The smaller number

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4
Q

What does the larger number refer to?

A

The voltage on the system that the transformer should be installed (assuming that system voltage is always read phase-to-phase)

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5
Q

What are the three choices engineers have to choose from?

A
  1. ) Phase-to-Phase Connection only
  2. ) Phase-to-Phase or Phase-to-Ground Connections
  3. ) Phase-to-Ground Connections only
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6
Q

Phase-to-Phase connections have a primary voltage rating that depicts how many numbers?

A

Only one number

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7
Q

What does this one number serve as?

A

Both the primary voltage rating and the system voltage

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8
Q

How are Phase-to-Phase Transformers configured?

A

Two primary bushings connected between two primary phases that will provide the rated voltage.

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9
Q

What would happen if you connected a Phase-to-Phase Transformer Phase-to-Ground?

A

Creates radio interference and corona problems

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10
Q

Phase-to-Phase or Phase-to-Ground connections have a primary voltage rating that depicts how many numbers?

A

Two separated by a slash(/)

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11
Q

What is the first number?

A

-The first number is the primary coil rating (the smaller number)

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12
Q

What is the second number?

A

System Voltage

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13
Q

How are Phase-to-Phase or Phase-to-Ground Transformers configured?

A

Two primary bushings and can be connected between two phases or between a phase and ground that will provide the rated voltage.

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14
Q

Why are Phase-to-Phase or Phase-to-Ground transformers the most versatile?

A

Because they can be used for both types of connections

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15
Q

What does the letter “Y” entail?

A

Should a transformer be installed on a “Y” Volt System it should be connected Phase-to-Ground

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16
Q

Phase-to-Ground connections have a primary voltage rating that depicts how many numbers?

A

Two separated by a slash (/)

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17
Q

What is the first number in a Phase-to-Ground Connection?

A

System voltage

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18
Q

What is the second number in a Phase-to-Ground Connection?

A

Primary Coil Rating (the smaller number)

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19
Q

How are Phase-to-Ground Transformers configured?

A

One primary bushing and can only be connected between a phase and ground that will provide the rated voltage

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20
Q

What happens to the insulation?

A

The insulation of the primary coil of these transformers is reduced at the neutral end.

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21
Q

What do the letters “Grd” provide?

A

Indication to the lineworker that the neutral end of the primary coil must be grounded for the transformer to operate properly

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22
Q

What transformers might some companies purchase and why?

A

Dual-Rated Transformers because they have two coil ratings and can be installed on systems of differing voltages.

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23
Q

How do Dual-Rated Transformers work?

A

They have an external no-load switch on the side of the transformer case.

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24
Q

What does the switch on a Dual-Rated Transformer do?

A

Changes the primary coils turns ratio

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25
Q

What is important to understand about Dual-Voltage Switch?

A

They should only be operated when the transformer is de-energized

26
Q

What are some distribution transformers supplied with?

A

Taps

27
Q

Why are some distribution transformers installed with taps?

A

To provide the same secondary voltage at different locations on the system where the primary voltage may not be the same, and the utility does not want to add another level of high-cost voltage regulation

28
Q

What might the voltage be closer to the substation?

A

Higher than the line is rated

29
Q

What might the voltage be at the far end of the circuit?

A

Lower than the line is rated

30
Q

Why would fluctuating voltage levels on a distribution circuit be unwanted?

A

Because transformers located along the circuit must supply the same 120/240-volt service to the customer.

31
Q

What would be the result of a standard distribution transformer not equipped with a tap?

A

Significantly different secondary voltages supplied to customers along the same circuit.

32
Q

In order to provide consistent service voltage to customer what can be utilized?

A

Transformers with taps

33
Q

Where can the tap settings be found?

A

On the nameplate

34
Q

How should taps be adjusted on transformers?

A

With the transformer De-Energized

35
Q

What is a tap changer?

A

No-Load Manual Switch that is connected to the primary coil and increases or decreases the turns ratio of the transformer with each tap setting.

36
Q

Depending on the transformer, each tap raises or lowers the secondary voltage by how much?

A

2.5% or 4.5%

37
Q

Where can the tap-changing handle be located?

A

Either inside or outside the tank

38
Q

What happens if a tap is raised?

A

The number of turns of the primary coil is reduced, which maintains the output voltage as the primary voltage drops

39
Q

What is always recommended prior to installation of a transformer?

A

Testing the integrity

40
Q

What kinds of tests can be performed on transformers

A
  • Continuity Testing
  • Insulation Testing
  • Turns-Ratio Testing
41
Q

How is Continuity Testing achieved?

A

Easily with a multi-tester

42
Q

What does a multi-tester do?

A

Gives off an audible sound when the two probes contact one another, either directly or through a conductive object

43
Q

When is this feature especially useful?

A

When proving the integrity of transformer coils

44
Q

What should happen when touching the two probes to Bushings H1 and H2?

A

An audible sound should be heard when the multi-meter is set to continuity

45
Q

Where else should an audible sound be heard?

A

When touching any of the Secondary Bushings with both probes

46
Q

When should an audible sound not be heard?

A

When touching the probes between Primary and Secondary Bushings

47
Q

What is a “TLC” Meter?

A

Transformer Last Check.

48
Q

What is a “TLC” Meter used for?

A

To verify that distribution transformers do not have shorted or open windings prior to energizing.

49
Q

What is an Insulation Tester referred to as?

A

Megger

50
Q

Why is a Megger used?

A

Troubleshooting and preventative maintenance programs to test the insulation in electrical apparatus, cables, wire, or networks.

51
Q

What can a Megger be used for?

A

To check for transformer coil insulation damage (and coil continuity).

52
Q

What must be done prior to using a Megger?

A

All connections, including the ground on the X2 terminal, must be removed

53
Q

What should be read between any bushings and the case when using a Megger?

A

INFINITY

54
Q

What does infinity indicate when using a Megger?

A

The insulation is not damaged

55
Q

What else does a Turns-Ratio test do besides the obvious?

A

Gives a good indication that the transformer is operational and that there are not shorts between the turns in the windings.

56
Q

What will a Turns-Ratio Test not show?

A

Weakness in the insulation

57
Q

How is a Turns-Ratio Test carried out?

A

By energizing the High-Voltage coil with a Low-Voltage. The device can then compare the output voltage with the known input voltage.

58
Q

What should the measured ratio be equal to?

A

The ratio on the nameplate.

59
Q

How is the turns ratio calculated?

A

By dividing the input voltage by the output voltage.

60
Q

When doing a Turns-Ratio test what should you remember?

A

Transformers work both ways.

61
Q

How does one configure this test?

A

The low-input voltage must be connected to the high-voltage winding.

62
Q

What happens if the input voltage is applied to the low-voltage winding?

A

A high voltage will be present on the primary bushings.

Transformers (7 decks)

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