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Power transformers > Final exam > Flashcards

Final exam Flashcards

1
Q

What are the three main insulation class systems for large dry type distribution and power transformers mentioned in the document?
A. 100°C, 150°C, 185°C
B. 150°C, 185°C, 220°C
C. 120°C, 160°C, 200°C
D. 185°C, 200°C, 220°C

A

B. 150°C, 185°C, 220°C (Page 1)

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2
Q

What are the typical types of interlayer insulation mentioned for the 150°C insulation class?
A. Nomex, Mylar
B. HPTZ, Nomex
C. Nomex, Polyester
D. Heavy film, Nomex

A

A. Nomex, Mylar (Page 1)

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3
Q

What is the temperature rise for the average winding temperature in the 3φ dry type transformer?
A. 80°C, 115°C, 150°C
B. 100°C, 130°C, 180°C
C. 120°C, 150°C, 200°C
D. 90°C, 140°C, 170°C

A

A. 80°C, 115°C, 150°C

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4
Q

Which insulation class is typically used for dry type transformers above 5kVA?
A. 150°C
B. 185°C
C. 220°C
D. 250°C

A

C. 220°C

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5
Q

What is the consequence of operating a transformer at a temperature higher than the insulation rating?
A. Higher efficiency
B. Lower cost
C. Decreased transformer life
D. Increased transformer capacity

A

C. Decreased transformer life

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6
Q

What is the relationship between temperature rise above the insulation rating and transformer life?
A. Every 5°C reduces life by 1/3
B. Every 10°C reduces life by 1/2
C. Every 15°C reduces life by 2/3
D. Every 20°C reduces life by 3/4

A

B. Every 10°C reduces life by 1/2

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7
Q

What is the equilibrium point for a transformer with a load, and why is it important for the insulation?
A. The point where heat dissipated equals heat generated; important to maintain insulation life.
B. The point where heat generated exceeds heat dissipated; important for transformer efficiency.
C. The point where the transformer stops functioning; important for safety.
D. The point where the transformer overheats; important for capacity.

A

A. The point where heat dissipated equals heat generated; important to maintain insulation life.

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8
Q

How does the choice of Average Winding Temperature Rise impact the capacity and efficiency of a transformer using 220°C class insulation?
A. Higher rise allows for increased capacity and efficiency.
B. Lower rise allows for increased capacity and efficiency.
C. Higher rise decreases capacity and efficiency.
D. Lower rise decreases capacity and efficiency.

A

C. Higher rise decreases capacity and efficiency.

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9
Q

What are the implications of using 80°C Average Winding Temperature Rise versus 150°C Average Winding Temperature Rise in terms of capacity, cost, losses, and efficiency?
A. Increased capacity, higher cost, lower losses, increased efficiency (150°C)
B. No increased capacity, lower cost, higher losses, decreased efficiency (80°C)
C. No change in capacity, cost, losses, or efficiency
D. Decreased capacity, higher cost, lower losses, decreased efficiency (80°C)

A

B. No increased capacity, lower cost, higher losses, decreased efficiency (80°C)

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10
Q

How does the insulation class of an oil-filled transformer differ from that of a dry type transformer in terms of temperature rise and ambient temperature?
A. Oil-filled transformers have a higher average winding temperature rise and lower ambient temperature.
B. Oil-filled transformers have a lower average winding temperature rise and higher ambient temperature.
C. Oil-filled transformers have the same temperature rise but a lower ambient temperature.
D. Oil-filled transformers have the same temperature rise but a higher ambient temperature.

A

B. Oil-filled transformers have a lower average winding temperature rise and higher ambient temperature.

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11
Q

What is the purpose of correcting resistance to a different temperature in transformers?
A) To increase the efficiency of the transformer
B) To ensure accurate measurement of load losses
C) To prevent overheating of the transformer
D) To reduce the risk of electrical faults

A

B) To ensure accurate measurement of load losses

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12
Q

What are Load Losses in transformers also known as?
A) Eddy Current Losses
B) No Load Losses
C) Copper Losses
D) Stray Losses

A

C) Copper Losses

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13
Q

How are Load Losses specified in transformers?
A) At rated voltage only
B) At rated current or rated apparent power
C) Based on the insulation system
D) At a constant temperature of 20°C

A

B) At rated current or rated apparent power

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14
Q

In the context of transformers, what is the significance of the standard Average Winding Temperature Rises?
A) To determine the transformer’s voltage capacity
B) To calculate the transformer’s efficiency
C) To specify the load losses temperature correction
D) To regulate the oil temperature in oil-filled transformers

A

C) To specify the load losses temperature correction

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15
Q

What is the relationship between I^2R Loss and the material used in transformers?
A) The resistance varies with the transformer’s size
B) The current flow affects the transformer’s stability
C) The resistivity of the conductor is key
D) The temperature rise determines the load losses

A

C) The resistivity of the conductor is key

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16
Q

How do the Load Losses differ from No Load Losses in transformers?
A) No Load Losses are due to stray fluxes in steel parts
B) Load Losses involve current flow to the load
C) No Load Losses are present when voltage is applied
D) Load Losses are related to the insulation system

A

B) Load Losses involve current flow to the load

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17
Q

What is the standard Average Winding Temperature Rise for Oil-Filled Transformers?
A) 80°C
B) 115°C
C) 150°C
D) 55°C

A

D) 55°C

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18
Q

What is the role of the Temperature Correction formula in transformers?
A) To prevent short circuits
B) To adjust resistance to a specified temperature
C) To regulate the oil temperature in transformers
D) To determine the load losses in transformers

A

B) To adjust resistance to a specified temperature

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19
Q

What are the primary causes of eddy losses in conductors according to the document?
A) Voltage and resistance
B) Axial and radial fluxes
C) Frequency and electrical length
D) Number of turns and cross-sectional area

A

B) Axial and radial fluxes

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20
Q

How does the document suggest reducing eddy losses for radial flux in conductors?
A) Increase the thickness of the conductor
B) Increase the width of the conductor
C) Decrease the thickness of the conductor
D) Decrease the width of the conductor

A

D) Decrease the width of the conductor

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21
Q

What is the relationship between the axial and radial fluxes for most winding sections?
A) Axial flux is always greater than radial flux
B) Radial flux is always greater than axial flux
C) Axial flux is greater in the central part, while radial flux is greater at the edges
D) Both axial and radial fluxes are equal

A

C) Axial flux is greater in the central part, while radial flux is greater at the edges

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22
Q

According to the document, what is the most effective way to reduce eddy losses?
A) Increase the electrical length of the winding
B) Increase the percentage of eddy losses
C) Keep the percentage of eddy losses below 3%
D) Increase the number of turns in the winding

A

C) Keep the percentage of eddy losses below 3%

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23
Q

How does the frequency of the power supply affect eddy losses?
A) Higher frequency leads to lower eddy losses
B) Higher frequency leads to higher eddy losses
C) Frequency does not impact eddy losses
D) Eddy losses are independent of power supply frequency

A

B) Higher frequency leads to higher eddy losses

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24
Q

What are strays in a transformer primarily caused by?
A) Copper losses
B) Eddy losses in structural steel
C) Hysteresis losses
D) Dielectric losses

A

B) Eddy losses in structural steel

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25
Q

What factors contribute to the calculation of stray losses in a transformer?
A) Type of insulation material used
B) Type of winding (layer, foil, disc)
C) Voltage rating of the transformer
D) Efficiency rating of the transformer

A

B) Type of winding (layer, foil, disc)

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26
Q

How do higher currents affect stray flux in a transformer?
A) Increase it
B) Decrease it
C) Have no effect
D) Reverse it

A

A) Increase it

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27
Q

What is the typical range of stray losses as a percentage of load losses in a transformer?
A) Up to 5%
B) Up to 7%
C) Up to 10%
D) Up to 15%

A

B) Up to 7%

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28
Q

Impedance (%Z) is expressed as a percentage and consists of which two components?
A. Resistive and inductive
B. Resistive and capacitive
C. Inductive and capacitive
D. Inductive and reactive

A

A. Resistive and inductive

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29
Q

What is the nature of reactance (%X) in transformers?
A. It varies with temperature
B. It is dependent on the frequency of the supply
C. It is the capacitive component
D. It is the inductive component

A

D. It is the inductive component

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30
Q

How does increasing the build radial factor (DR) impact reactance (%X)?
A. Increases %X
B. Decreases %X
C. Has no effect on %X
D. Changes %X to a capacitive component

A

A. Increases %X

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31
Q

In the context of increasing the build radial factor (DR), which part of the transformer has the largest effect on increasing DR?
A. LV winding
B. Main gap
C. HV winding
D. MG winding

A

D. MG winding

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32
Q

What effect does increasing the number of turns (N) have on the reactance (%X) of a transformer?
A. Increases %X
B. Decreases %X
C. Has no effect on %X
D. Changes %X to a capacitive component

A

A. Increases %X

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33
Q

What is the purpose of calculating the Line and Phase Voltages for each winding in transformer design?
A) To determine the physical dimensions of the transformer
B) To understand the losses and efficiency of the transformer
C) To calculate the percentage impedance of the transformer
D) To estimate the load loss at a specific temperature

A

B) To understand the losses and efficiency of the transformer

34
Q

What is the significance of calculating the number of turns for each winding in transformer design?
A) To ensure the transformer meets the guarantees for power loss
B) To determine the efficiency of the transformer
C) To estimate the voltage drop across the windings
D) To calculate the eddy current losses in the windings

A

A) To ensure the transformer meets the guarantees for power loss

35
Q

Why is it important to calculate the resistance and %Eddies for each winding in transformer design?
A) To determine the physical length of the windings
B) To calculate the temperature rise in the windings
C) To estimate the losses in the windings
D) To understand the magnetic field strength in the windings

A

C) To estimate the losses in the windings

36
Q

How does calculating the %X and %R help in transformer design?
A) It helps in determining the load loss and impedance of the transformer
B) It provides information on the physical dimensions of the core
C) It assists in estimating the number of turns required
D) It helps in calculating the efficiency of the transformer

A

A) It helps in determining the load loss and impedance of the transformer

37
Q

What is the purpose of calculating the Load Loss and Impedance in transformer design?
A) To determine the color-coding of the windings
B) To understand the cooling system requirements
C) To estimate the losses and performance of the transformer
D) To calculate the weight of the transformer

A

C) To estimate the losses and performance of the transformer

38
Q

What is the purpose of measuring the impedance of a transformer during testing?
A) To determine the efficiency of the transformer
B) To understand the internal resistance of the transformer
C) To establish the voltage needed to circulate rated current in one winding while the other is shorted
D) To calculate the power loss in the transformer

A

C) To establish the voltage needed to circulate rated current in one winding while the other is shorted

39
Q

How does the impedance voltage affect the measurement of the transformer’s impedance?
A) It determines the efficiency of the transformer
B) It creates different ohmic values depending on which winding is tested
C) It indicates the total power loss in the transformer
D) It influences the temperature rise of the transformer

A

B) It creates different ohmic values depending on which winding is tested

40
Q

How does the temperature affect the resistance (%R) of the transformer?
A) The resistance decreases with temperature rise
B) The resistance increases with temperature rise
C) The resistance remains constant regardless of temperature
D) The resistance fluctuates randomly with temperature changes

A

B) The resistance increases with temperature rise

41
Q

What is the tolerance on impedance as per CSA standards?
A) ±5%
B) ±10%
C) ±7.5%
D) ±15%

A

±7.5%

42
Q

Why is it important to specify the temperature when stating the resistance percentage (%R) of a transformer?
A) Because the resistance fluctuates randomly with temperature changes
B) Because the resistance decreases with temperature rise
C) Because the resistance increases with temperature rise
D) Because the resistance remains constant regardless of temperature

A

C) Because the resistance increases with temperature rise

43
Q

What is the main benefit of having transformers with matching impedances when they are paralleled?
A) Circulating currents will flow between the transformers
B) Transformers will be unequally loaded
C) Voltage regulation will be easier to manage
D) Transformers will be equally loaded

A

D) Transformers will be equally loaded

44
Q

How does a higher impedance affect the maximum fault current in a system?
A) It increases the maximum fault current
B) It has no effect on the maximum fault current
C) It decreases the maximum fault current
D) It causes a short circuit

A

C) It decreases the maximum fault current

45
Q

What is the relationship between impedance and voltage regulation in a transformer?
A) Higher impedance results in better voltage regulation
B) Lower impedance results in better voltage regulation
C) Impedance has no impact on voltage regulation
D) Impedance causes voltage fluctuations

A

B) Lower impedance results in better voltage regulation

46
Q

Why is having a lower %X beneficial in terms of stray losses in transformers?
A) It increases stray losses
B) It has no effect on stray losses
C) It decreases stray losses
D) It causes short circuits in the system

A

It decreases stray losses

47
Q

What is the main advantage of auto transformers compared to standard transformers?
A. Lower cost
B. Higher efficiency
C. Smaller physical size
D. Greater impedance

A

C. Smaller physical size

48
Q

In the event that the common winding opens in an auto transformer, what is the primary concern?
A. Excessive current flow
B. Loss of isolation
C. Reduced voltage output
D. Increased impedance

A

B. Loss of isolation

49
Q

How is the series winding connected in relation to the common winding in an auto transformer?
A. In parallel
B. In series
C. Independently
D. In reverse

A

B. In series

50
Q

What is the primary use of auto transformers?
A. Voltage regulation
B. Current transformation
C. Motor starting
D. Heat dissipation

A

C. Motor starting

51
Q

When compared to standard transformers, auto transformers are known for:
A. Higher kVA ratings
B. Equal physical size
C. Lesser efficiency
D. Lower cost

A

D. Lower cost

52
Q

What is the purpose of using a Motor Starting Auto Transformer? A) To reduce the inrush current when starting a motor
B) To increase the voltage across the motor
C) To limit the counter EMF produced by the motor
D) To de-energize the auto-transformer

A

A) To reduce the inrush current when starting a motor

53
Q

How does a 3φ autotransformer differ from a standard transformer?
A) It has only one winding per phase
B) It has windings that are not electrically connected
C) It only works with single-phase systems
D) It has two windings that are electrically connected per phase

A

D) It has two windings that are electrically connected per phase

54
Q

What is the common winding in a 3φ autotransformer?
A) The winding that is unique to the LV per phase
B) The winding that is unique to the HV per phase
C) The winding common to both the LV and HV per phase
D) The winding that is connected to the series winding

A

C) The winding common to both the LV and HV per phase

55
Q

What is the role of the series winding in a 3φ autotransformer?
A) To increase the current flow from the LV to the HV
B) To reduce the inrush current when starting a motor
C) To limit the counter EMF produced by the motor
D) To add extra turns for the voltage of the HV winding per phase

A

D) To add extra turns for the voltage of the HV winding per phase

56
Q

How does a Motor Starting Auto Transformer help in reducing inrush current?
A) By decreasing the voltage across the motor during start-up
B) By increasing the voltage across the motor during start-up
C) By de-energizing the auto-transformer at the start
D) By allowing the counter EMF to build up before starting the motor

A

A) By decreasing the voltage across the motor during start-up

57
Q

What are the two types of losses in a transformer that affect its efficiency?
A) Voltage Losses and Current Losses
B) Load Losses and No Load Losses
C) Inductive Losses and Capacitive Losses
D) Primary Losses and Secondary Losses

A

B) Load Losses and No Load Losses

58
Q

What are the two types of taps found on transformers described in the PDF?
A. Taps with variable voltage
B. Taps at the end of winding and taps in the middle of the winding
C. Taps connected to the ground
D. Taps with adjustable current

A

B. Taps at the end of winding and taps in the middle of the winding

59
Q

How many extra voltages are available on the primary of smaller transformers, such as those used in factories or schools?
A. Two
B. Three
C. Four
D. Five

A

C. Four

60
Q

What does FCBN stand for in the context of transformer taps?
A. Full Capacity Below Nominal
B. Fully Charged Battery Needed
C. Full Capacity Beyond Nominal
D. Final Circuit Breaker Notification

A

A. Full Capacity Below Nominal

61
Q

What is another term for transformer taps that require the transformer to be de-energized before adjustment?
A) On Circuit Tap Changers
B) Off Circuit Tap Changers
C) Voltage Tap Changers
D) Load Tap Changers

A

B) Off Circuit Tap Changers

62
Q

What is used to adjust taps on transformers without the need for de-energizing?
A) Nuts and Bolts
B) Load Tap Changers
C) Oil Filled Transformers
D) Mini Blackouts

A

B) Load Tap Changers

63
Q

Which type of tap changer allows for tap adjustments while the transformer is under full load?
A) Off Circuit Tap Changers
B) On Circuit Tap Changers
C) Voltage Tap Changers
D) Load Tap Changers

A

B) On Circuit Tap Changers

64
Q

What is the main reason for using Load Tap Changers (LTC) on larger oil-filled transformers?
A) To minimize voltage fluctuations
B) To increase the number of taps
C) To decrease the kVA rating
D) To reduce the number of phases

A

A) To minimize voltage fluctuations

65
Q

What is the primary function of a Control Transformer?
A) Transforming current and voltage at power frequencies
B) Operating at high efficiency for metering and relaying
C) Providing control voltage for various systems
D) Isolating one electrical system from another

A

C) Providing control voltage for various systems

66
Q

Where are Distribution Transformers typically located?
A) In a small distribution electrical room indoors
B) In a city distribution substation outdoors
C) In a transmission substation outdoors
D) Both A and B

A

D) Both A and B

67
Q

What is the primary purpose of an Auto-transformer?
A) Isolating electrical systems
B) Adjusting voltages through a single winding
C) Stepping current or voltage down for standard meters
D) Providing control voltage for relays

A

B) Adjusting voltages through a single winding

68
Q

What is the recommended practice when a Current Transformer (CT) is not in use?
A) Keep it connected to the line
B) Disconnect it from the line and leave it open
C) Short the terminals
D) Remove it from the system entirely

A

C) Short the terminals

69
Q

What distinguishes a Power Transformer from a Control Transformer?
A) Operating at low efficiency
B) Providing control voltage for relays
C) Isolating one electrical system from another
D) Transforming current and voltage at power frequencies

A

D) Transforming current and voltage at power frequencies

70
Q

What is the main difference between Air Insulated (Dry Type) and Oil Insulated (Liquid Filled) transformers according to the text?
A) Size and weight
B) Voltage capacity
C) Insulation material
D) Maintenance cost

A

C) Insulation material

71
Q

Where are Air Insulated (Dry Type) transformers mostly used according to the information provided?
A) Outdoors in special enclosures
B) Schools and shopping plazas
C) Large industrial complexes
D) Residential areas

A

B) Schools and shopping plazas

72
Q

What is a key advantage of Oil Insulated (Liquid Filled) transformers over Air Insulated (Dry Type) transformers?
A) Lower maintenance cost
B) Faster delivery time
C) Better heat capacity
D) Lighter weight

A

C) Better heat capacity

73
Q

Which type of transformer is most likely to be smaller for similar ratings?
A) Air Insulated (Dry Type)
B) Oil Insulated (Liquid Filled)
C) Gas Insulated
D) Cannot be determined

A

B) Oil Insulated (Liquid Filled)

74
Q

In what type of applications are Oil Insulated (Liquid Filled) transformers mostly used?
A) Indoor applications with special vaults
B) Residential homes
C) Outdoor applications
D) Factories and schools

A

C) Outdoor applications

75
Q

Which type of oil is the most cost-effective?
A) Silicon Oil
B) Biodegradable Oil
C) PCB Polychlorinated bi-phenol
D) Standard Mineral Oil

A

D) Standard Mineral Oil

76
Q

Multiple Choice Question: Which type of oil has the highest operating temperature?
A) Biodegradable Oil
B) PCB Polychlorinated bi-phenol
C) Silicon Oil
D) Standard Mineral Oil

A

C) Silicon Oil

77
Q

Which type of oil is fire resistant and made from soy bean and other grain seed oils?
A) Biodegradable Oil
B) PCB Polychlorinated bi-phenol
C) Silicon Oil
D) Standard Mineral Oil

A

A) Biodegradable Oil

78
Q

Which type of oil was discontinued in 1977 due to health problems and is fire retardant?
A) Biodegradable Oil
B) PCB Polychlorinated bi-phenol
C) Silicon Oil
D) Standard Mineral Oil

A

B) PCB Polychlorinated bi-phenol

79
Q

What is the purpose of a pin insulator in a power system?
A) To arrest lightning
B) To provide support for conductors
C) To measure voltage
D) To regulate current

A

B) To provide support for conductors

80
Q

What is the function of a stress cone in a power system?
A) To regulate current flow
B) To prevent lightning strikes
C) To distribute voltage evenly
D) To reduce electric field stress

A

D) To reduce electric field stress

81
Q

What is the difference between strike distance and creepage distance in a power system?
A) Strike distance is longer than creepage distance
B) Strike distance involves breakdown in air, while creepage distance involves breakdown along a surface
C) Creepage distance is dependent on the dielectric strength of air
D) Strike distance is affected by the breakdown strength of insulating material

A

B) Strike distance involves breakdown in air, while creepage distance involves breakdown along a surface

Power transformers (1 decks)

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