Transformer Connections (Three-Phase Transformers) Flashcards
What is the chief disadvantage of a wye/wye transformer?
Triplen harmonic frequencies which cause excessive heating in the neutral conductor.
Why do wye:wye transformers contain a neutral?
No neutral would create voltage balancing issues on the secondary voltages.
What frequency is the third harmonic in North America?
180 Hz.
Could one use slightly mismatched transformers in a Wye:Wye connection?
Yes, if carefully analyzed.
Why is transformer matching critical when connecting in delta?
To eliminate circulating currents due to voltage imbalance.
What is the primary advantage of delta:delta connections?
The transformer’s phase coils do not have to be as large as they only carry 57.7 percent of the load line current.
What is an advantage of a wye:delta connection?
Where might it be used?
The primary windings do not need to be fully insulated for incoming line voltage.
High voltage distribution.
Why is there a 30 degree phase displacement between VAB when connecting a wye:delta or delta:wye transformer bank?
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”.
What is the “common conductor” when talking about delta transformers?
The center tapped conductor on B phase of a 4 wire Delta. (Not a neutral)
What is the A phase conductor called in a 4 wire delta system?
The high leg;
The stinger;
The bastard leg.
Can a 4 wire delta transformer feed a 3 Φ 4 wire panelboard?
No.
What are the advantages of a delta:wye connection?
- 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.
Where is a delta:wye connection typically used?
In 3 Φ distribution applications.
Why would an open delta connection be an advantage?
To facilitate a controlled shutdown for coil replacement.
In an open delta connection, does line current phase shift from a closed delta connection?
No, phase current will shift and become line current, at a reduced magnitude but the same timing.
What is the main change from delta to open delta connections?
The line current no longer increases by 31/2;
It simply equals the phase current.
By how much is the capacity of a 3Φ delta transformer bank reduced by if it becomes open?
Reduced to 57.7%.
What is a neutral grounding resistor used for?
For impedance grounding a system.
Why is an impedance grounding system chosen over ungrounded or solidly grounded?
- Ease of locating fault;
- Non damaging ground fault currents;
- Reduced frequency of arc blasts.
How many faults will it take to shut down an ungrounded system?
2 faults.
What is required in an ungrounded system?
Where might you encounter one?
A ground fault detector;
In industrial applications, (it is uncommon due to overvoltages after a fault).
What is the primary protection in solidly grounded systems?
Overcurrent devices.
What are some advantages of solidly grounded over ungrounded?
- Reduced overvoltages;
- Simple ground fault localizing;
- Improved system protection;
- Reduced maintenance time;
- Greater safety;
- Improved lightning protection;
- Reduced fault frequency.
What will trip the power circuit breaker in an impedance grounded system?
Over current on the grounding conductor, measured with a CT.
Does the bonding path of an impedance grounded system travel through the resistor bank?
No, it is straight to ground.
Where only 3 Φ loads are served in an impedance grounded system, up to how much fault current is allowed, with one phase faulted?
10 A fault current max.
What type of fault is still damaging in an impedance grounded system?
Line to line faults.
Why are NGRs similar to fuses?
They limit magnitude and duration of fault current.
What must the element material of an NGR be made of?
An electrical alloy with a low temperature coefficient of resistance.
Why is low resistance grounding used?
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.
What is the typical rating of low resistance grounding and where is it found?
400A for 10 s;
High voltage systems.
When is high resistance grounding used?
And why?
In low voltage systems (<750V);
It allows production to continue as long as a second fault does not occur.
What system is relatively inexpensive to upgrade to an impedance grounding high R system?
An ungrounded system.
What is the major advantage of High R grounding?
Elimination of dangerous flashovers to ground.
What are the high resistance neutral grounding system’s disadvantages?
- 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.
