Negative sequence currents in an alternator

cnh1995 · May 14, 2016

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!

anorlunda · May 14, 2016

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 complex linear 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.

cnh1995 · May 14, 2016

We measured the open circuit voltage V_RY and short circuit current I_YB. The ratio V_RY/I_YB is the negative sequence impedance. How?

anorlunda · May 15, 2016

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.

EverGreen1231 · May 17, 2016

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/Lite...etrical-Pt1_AR_20130422.pdf?v=20150812-081134

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Negative sequence currents in an alternator

cnh1995

anorlunda

Staff: Mentor

cnh1995

anorlunda

Staff: Mentor

EverGreen1231