Three Phase Transformer Connections Flashcards
Describe a wye/wye transformer connection with a primary neutral
- Eliminates voltage balancing issues, so the transformer bank can supply a wye connected secondary with a neutral
- 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
Describe a wye/wye transformer connection without a primary neutral
- Rare
- Difficult to maintain balanced output voltages
Advantages of a delta/delta
- 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
Describe a wye/delta connection
- 30 degree phase displacement between primary voltages and their secondary voltages
- The conductors of the primary do not need to be insulated for the full three phase
Line voltage
Describe a delta/4 wire delta connection
- The fourth wire impact the secondary is a common conductor that is connected to the centre point of one of the windings
- 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
Describe a delta/wye connection
- The common connection for three phase distribution
- 30 degrees phase displacement between the primary voltages and their respective secondary voltages
- The secondary can supply both single and three phase loads
- A four wire wye connection can feed a three phase, four wire panel board
Describe an open delta/open delta connection
- Same as a delta/delta connection but one transformer is removed
What is the purpose of a neutral grounding resistor
- They are meant for impedance grounding of industrial power systems in low and high voltage distribution networks
- They protect power transformers and generators from damaging currents in the event of a ground fault
Describe an ungrounded system
- A system that requires a ground fault detection device that can notify people when there is a ground fault situation
- These are used less because on inherent over voltages when the first fault intermittent
Describe a solidly grounded system
- The grounding electrode and the equipment to the circuit conductor are connected using a low impedance conductor
- It is an improvement over ungrounded systems because it can speed up the process of locating faults.
- Only used in low voltage applications at lower power levels
Describe an impedance grounded system
- A system that has resistance connected between the neutral of the system and the grounding electrode.
- The primary is connected in series to the neutral and the resistor is connected on the secondary.
- A CT is used to measure the ground fault current
How does a neutral grounding resistor protect personnel and equipment from damage
- Limiting how long the fault lasts
- Limiting the faults magnitude
If a ground fault condition exists, what does the NGR limit the current to in a low resistance system
200-400A
What does a high resistance grounding neutral limit the current to in a ground fault condition
5-10A