Transformer Connections (Three-Phase Transformers)

Question 1

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

Answer 1

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

Question 2

Why do wye:wye transformers contain a neutral?

Answer 2

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

Question 3

What frequency is the third harmonic in North America?

Answer 3

180 Hz.

Question 4

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

Answer 4

Yes, if carefully analyzed.

Question 5

Why is transformer matching critical when connecting in delta?

Answer 5

To eliminate circulating currents due to voltage imbalance.

Question 6

What is the primary advantage of delta:delta connections?

Answer 6

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

Question 7

What is an advantage of a wye:delta connection?

Where might it be used?

Answer 7

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

High voltage distribution.

Question 8

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

Answer 8

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”.

Question 9

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

Answer 9

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

Question 10

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

Answer 10

The high leg;

The stinger;

The bastard leg.

Question 11

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

Answer 11

No.

Question 12

What are the advantages of a delta:wye connection?

Answer 12

• 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.

Question 13

Where is a delta:wye connection typically used?

Answer 13

In 3 Φ distribution applications.

Question 14

Why would an open delta connection be an advantage?

Answer 14

To facilitate a controlled shutdown for coil replacement.

Question 15

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

Answer 15

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

Question 16

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

Answer 16

The line current no longer increases by 3^1/2;

It simply equals the phase current.

Question 17

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

Answer 17

Reduced to 57.7%.

Question 18

What is a neutral grounding resistor used for?

Answer 18

For impedance grounding a system.

Question 19

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

Answer 19

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

Question 20

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

Answer 20

2 faults.

Question 21

What is required in an ungrounded system?

Where might you encounter one?

Answer 21

A ground fault detector;

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

Question 22

What is the primary protection in solidly grounded systems?

Answer 22

Overcurrent devices.

Question 23

What are some advantages of solidly grounded over ungrounded?

Answer 23

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

Question 24

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

Answer 24

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

Question 25

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

Answer 25

No, it is straight to ground.

Question 26

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

Answer 26

10 A fault current max.

Question 27

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

Answer 27

Line to line faults.

Question 28

Why are NGRs similar to fuses?

Answer 28

They limit magnitude and duration of fault current.

Question 29

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

Answer 29

An electrical alloy with a low temperature coefficient of resistance.

Question 30

Why is low resistance grounding used?

Answer 30

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.

Question 31

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

Answer 31

400A for 10 s;

High voltage systems.

Question 32

When is high resistance grounding used?

And why?

Answer 32

In low voltage systems (<750V);

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

Question 33

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

Answer 33

An ungrounded system.

Question 34

What is the major advantage of High R grounding?

Answer 34

Elimination of dangerous flashovers to ground.

Question 35

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

Answer 35

• 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.