Three Phase Transformer Connections

Question 1

Describe a wye/wye transformer connection with a primary neutral

Answer 1

1. Eliminates voltage balancing issues, so the transformer bank can supply a wye connected secondary with a neutral
2. Even with a balanced load, the primary neutral contains a third harmonic (180hz) which can induce a current that causes excessive heating in the neutral

Question 2

Describe a wye/wye transformer connection without a primary neutral

Answer 2

1. Rare
2. Difficult to maintain balanced output voltages

Question 3

Advantages of a delta/delta

Answer 3

1. The phase current is only 57.8% of the line current, so the conductors of the individual single phase transformers do not have to be as large as the the line conductors supplying a three phase load

Question 4

Describe a wye/delta connection

Answer 4

1. 30 degree phase displacement between primary voltages and their secondary voltages
2. The conductors of the primary do not need to be insulated for the full three phase
   Line voltage

Question 5

Describe a delta/4 wire delta connection

Answer 5

1. The fourth wire impact the secondary is a common conductor that is connected to the centre point of one of the windings
2. Can feed a three phase, three wire panel board and a separate single phase three wire panel board
   3.CANNOT feed a three phase, four wire panel board

Question 6

Describe a delta/wye connection

Answer 6

1. The common connection for three phase distribution
2. 30 degrees phase displacement between the primary voltages and their respective secondary voltages
3. The secondary can supply both single and three phase loads
4. A four wire wye connection can feed a three phase, four wire panel board

Question 7

Describe an open delta/open delta connection

Answer 7

1. Same as a delta/delta connection but one transformer is removed

Question 8

What is the purpose of a neutral grounding resistor

Answer 8

1. They are meant for impedance grounding of industrial power systems in low and high voltage distribution networks
2. They protect power transformers and generators from damaging currents in the event of a ground fault

Question 9

Describe an ungrounded system

Answer 9

1. A system that requires a ground fault detection device that can notify people when there is a ground fault situation
2. These are used less because on inherent over voltages when the first fault intermittent

Question 10

Describe a solidly grounded system

Answer 10

1. The grounding electrode and the equipment to the circuit conductor are connected using a low impedance conductor
2. It is an improvement over ungrounded systems because it can speed up the process of locating faults.
3. Only used in low voltage applications at lower power levels

Question 11

Describe an impedance grounded system

Answer 11

1. A system that has resistance connected between the neutral of the system and the grounding electrode.
2. The primary is connected in series to the neutral and the resistor is connected on the secondary.
3. A CT is used to measure the ground fault current

Question 12

How does a neutral grounding resistor protect personnel and equipment from damage

Answer 12

1. Limiting how long the fault lasts
2. Limiting the faults magnitude

Question 13

If a ground fault condition exists, what does the NGR limit the current to in a low resistance system

Answer 13

200-400A

Question 14

What does a high resistance grounding neutral limit the current to in a ground fault condition

Answer 14

5-10A