Negative sequence currents in an alternator

In summary, the alternator was star connected and we shorted Y and B phases together, leaving R phase open. This arrangement generates negative sequence currents. The direction of rotation of the magnetic field is reversed.
  • #1

cnh1995

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We recently performed one experiment regarding measurement of negative sequence impedance of an alternator(unloaded) in our lab session. The alternator was star connected and we shorted Y and B phases together, leaving R phase open(an L-L fault). How does this arrangement generate negative sequence currents? How is the direction of rotation of magnetic field reversed? Please help. Many thanks in advance!
 
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  • #2
https://en.m.wikipedia.org/wiki/Symmetrical_components

In electrical engineering, the method of symmetrical components is used to simplify analysis of unbalanced three-phase power systems under both normal and abnormal conditions. The basic idea is that an asymmetrical set of Nphasors can be expressed as a linear combination of N symmetrical sets of phasors by means of a complexlinear transformation.[1] In the most common case of three-phase system, the resulting "symmetrical" components are referred to as direct (or positive), inverse (or negative) and zero (or homopolar). The analysis of power system is much simpler in the domain of symmetrical components, because the resulting equations are mutually linearly independent if the circuit itself is balanced.
 
  • #3
We measured the open circuit voltage VRY and short circuit current IYB. The ratio VRY/IYB is the negative sequence impedance. How?
 
  • #4
Well, the very definition of impedance is the ratio of voltage to current. The only question is which impedance it is it?

I suggest that you study symmetrical components enough to solve the homework problem of making an equivalent circuit for a L-L fault. Once you have that, it should be obvious which impedance you are measuring.
 
  • #5
cnh1995 said:
We recently performed one experiment regarding measurement of negative sequence impedance of an alternator(unloaded) in our lab session. The alternator was star connected and we shorted Y and B phases together, leaving R phase open(an L-L fault). How does this arrangement generate negative sequence currents? How is the direction of rotation of magnetic field reversed? Please help. Many thanks in advance!
You should be able to plug and chug, but I'm not able to use a program to give me a quick and dirty answer, and I don't have time to do the calculation by hand.

See if this can shed any light onto your problem...
https://cdn.selinc.com//assets/Literature/Publications/White%20Papers/LWP0010-01_TutorialSymmetrical-Pt1_AR_20130422.pdf?v=20150812-081134
 

1. What are negative sequence currents in an alternator?

Negative sequence currents are currents that flow in the opposite direction of the normal sequence of currents in an alternator. They are caused by an imbalance in the three phases of the alternator, resulting in a negative sequence voltage.

2. What causes negative sequence currents in an alternator?

Negative sequence currents are caused by imbalances in the three phases of the alternator, such as unequal phase impedances or unequal phase voltages. They can also be caused by a fault in one of the phases or a sudden change in load.

3. What are the effects of negative sequence currents on an alternator?

Negative sequence currents can cause overheating and increased mechanical stress on the alternator, leading to potential damage and decreased efficiency. They can also cause voltage fluctuations and affect the stability of the power system.

4. How can negative sequence currents be detected in an alternator?

Negative sequence currents can be detected by using special relays or protection devices that measure the current in each phase and compare them to each other. They can also be detected through monitoring the voltage and frequency of the power system.

5. How can negative sequence currents in an alternator be prevented?

Negative sequence currents can be prevented by ensuring that the phases of the alternator are balanced and have equal impedances and voltages. Regular maintenance and monitoring of the alternator can also help identify and address any potential imbalances before they cause significant negative sequence currents.

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