Transformer Connections (Three-Phase Transformers) Flashcards

1
Q

What is the chief disadvantage of a wye/wye transformer?

A

Triplen harmonic frequencies which cause excessive heating in the neutral conductor.

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2
Q

Why do wye:wye transformers contain a neutral?

A

No neutral would create voltage balancing issues on the secondary voltages.

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3
Q

What frequency is the third harmonic in North America?

A

180 Hz.

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4
Q

Could one use slightly mismatched transformers in a Wye:Wye connection?

A

Yes, if carefully analyzed.

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5
Q

Why is transformer matching critical when connecting in delta?

A

To eliminate circulating currents due to voltage imbalance.

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6
Q

What is the primary advantage of delta:delta connections?

A

The transformer’s phase coils do not have to be as large as they only carry 57.7 percent of the load line current.

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7
Q

What is an advantage of a wye:delta connection?

Where might it be used?

A

The primary windings do not need to be fully insulated for incoming line voltage.

High voltage distribution.

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8
Q

Why is there a 30 degree phase displacement between VAB when connecting a wye:delta or delta:wye transformer bank?

A

Because the same flux, which is produced by the exciting current is responsible for inducing secondary voltage. Therefore, the phase voltages of both connections are “in-phase”.

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9
Q

What is the “common conductor” when talking about delta transformers?

A

The center tapped conductor on B phase of a 4 wire Delta. (Not a neutral)

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10
Q

What is the A phase conductor called in a 4 wire delta system?

A

The high leg;

The stinger;

The bastard leg.

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11
Q

Can a 4 wire delta transformer feed a 3 Φ 4 wire panelboard?

A

No.

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12
Q

What are the advantages of a delta:wye connection?

A
  • Secondary can supply both 1Φ and 3Φ loads.
  • Can feed a 3Φ 4 wire panelboard.
  • If single phase loads are balanced the system will be.
  • A common grounding point on the secondary to limit potential to ground.
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13
Q

Where is a delta:wye connection typically used?

A

In 3 Φ distribution applications.

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14
Q

Why would an open delta connection be an advantage?

A

To facilitate a controlled shutdown for coil replacement.

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15
Q

In an open delta connection, does line current phase shift from a closed delta connection?

A

No, phase current will shift and become line current, at a reduced magnitude but the same timing.

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16
Q

What is the main change from delta to open delta connections?

A

The line current no longer increases by 31/2;

It simply equals the phase current.

17
Q

By how much is the capacity of a 3Φ delta transformer bank reduced by if it becomes open?

A

Reduced to 57.7%.

18
Q

What is a neutral grounding resistor used for?

A

For impedance grounding a system.

19
Q

Why is an impedance grounding system chosen over ungrounded or solidly grounded?

A
  • Ease of locating fault;
  • Non damaging ground fault currents;
  • Reduced frequency of arc blasts.
20
Q

How many faults will it take to shut down an ungrounded system?

A

2 faults.

21
Q

What is required in an ungrounded system?

Where might you encounter one?

A

A ground fault detector;

In industrial applications, (it is uncommon due to overvoltages after a fault).

22
Q

What is the primary protection in solidly grounded systems?

A

Overcurrent devices.

23
Q

What are some advantages of solidly grounded over ungrounded?

A
  • Reduced overvoltages;
  • Simple ground fault localizing;
  • Improved system protection;
  • Reduced maintenance time;
  • Greater safety;
  • Improved lightning protection;
  • Reduced fault frequency.
24
Q

What will trip the power circuit breaker in an impedance grounded system?

A

Over current on the grounding conductor, measured with a CT.

25
Q

Does the bonding path of an impedance grounded system travel through the resistor bank?

A

No, it is straight to ground.

26
Q

Where only 3 Φ loads are served in an impedance grounded system, up to how much fault current is allowed, with one phase faulted?

A

10 A fault current max.

27
Q

What type of fault is still damaging in an impedance grounded system?

A

Line to line faults.

28
Q

Why are NGRs similar to fuses?

A

They limit magnitude and duration of fault current.

29
Q

What must the element material of an NGR be made of?

A

An electrical alloy with a low temperature coefficient of resistance.

30
Q

Why is low resistance grounding used?

A

To allow higher current values, which cleanly operate protective devices;

This fast trip time:

  • limits equipment damage;
  • Prevents additional faults;
  • Provides safety;
  • Localizes the fault.
31
Q

What is the typical rating of low resistance grounding and where is it found?

A

400A for 10 s;

High voltage systems.

32
Q

When is high resistance grounding used?

And why?

A

In low voltage systems (<750V);

It allows production to continue as long as a second fault does not occur.

33
Q

What system is relatively inexpensive to upgrade to an impedance grounding high R system?

A

An ungrounded system.

34
Q

What is the major advantage of High R grounding?

A

Elimination of dangerous flashovers to ground.

35
Q

What are the high resistance neutral grounding system’s disadvantages?

A
  • Increased voltage stress on the system with one phase faulted;
  • After a fault, the neutral cannot be used as it raises to line voltage. (only 3Φ);
  • If a second ground fault occurs a line to line short is created.