Rotor and negative sequence impedance of alternator

[cnh1995]

When an alternator is supplied with a 3-phase voltage and its rotor is rotated in opposite direction to that of the stator rmf, the ratio phase voltage/phase current is the negative sequence impedance of the alternator. My understanding of this is as follows:
When rotor is rotated in opposite direction to that of the stator rmf, emf is induced in the rotor with a frequency twice that of the stator voltage. Due to transformer action, rotor magnetic flux links with the stator which increases the stator current. So, is it correct to say that rotor inductance affects the negative sequence impedance and more the rotor inductance, less is the negative sequence impedance of the alternator?

 

[Babadag]

I am sorry. Could you please ask your question in English?

 

[cnh1995]

I am sorry. Could you please ask your question in English?

I don't know what you mean by that.

 

[Babadag]

In my opinion, at first you have to synchronize the alternator with the supply grid in order to connect it. Now you have to reduce the speed in order to reverse the velocity. This has to be done by reducing the primary driver. However you cannot change the speed if your generator stay connected. The power will be supplied by the grid and the generator turns into a motor.
If you'll try to connect the generator while it will not be synchronized you’ll produce a short-circuit. So your question refers to an imaginary situation.

 

[cnh1995]

I was talking about the first paragraph of "Negative Sequence Impedance" in this manual.

 

[Babadag]

Sorry,cnh1995. You are speaking about a virtual negative sequence. It does not actually exist.
Using symmetrical components it does not mean this components exist. It is only a calculation artifice. Like imaginary sqrt(-1). It is very useful indeed, but as you know, actually it does not exist.
However, you don't need to rotate the rotor inversely. It is enough to unbalance the currents.

 

[cnh1995]

It does not actually exist.

Well, I know it's a calculation artifice and it does not occur separately but is the procedure mentioned in that paragraph valid? If yes, does the negative sequence impedance depend on the rotor inductance? If field winding turns are increased, will it change the value of negative sequence impedance?

 

[Babadag]

Sorry for the delayed answer. I am very busy in these days.
I have to recognize I never saw the phenomenon from this point of view.
But, since this negative sequence components induce double frequency currents in the surface of the rotor, the slot wedges of the rotor, the retaining rings, the field and the damping windings of the rotor of the machines that means it does exist. Like harmonics in a distorted wave current [not sinusoidal form wave] which they also produce losses [ and heating].
There are- at least as I know- 4 component systems-usually:
RST,αβ0[E.Clarke],120[C.L.Fortesque],dq0[R.H.Park).One could employ any of these systems to define components.
We are speaking about 120[or V1,V2,Vo,I1,I2,Io,Z1,Z2,Zo].
The negative sequence impedance depends in the same time on the stator windings and the rotor[ and since the rotor rotates forward with synchronous speed this frequency will be doubled].
However, as the stator is not moving the part of the stator negative reactance will be the same as positive sequence impedance.
The magnetic flow [the source it is the stator current negative components] rotating inversely will induce currents in rotor circuits and theses-in turn-will induce currents in stator. The frequency of theses currents will be double of synchronous also and will depend on rotor circuit impedance too. It is difficult to appreciate this since both emf and reactance depend directly on frequency. However the resistance depends on frequency-skin and proximity effect-and on temperature too.
I know the second method indicated in the above article for Z2 measurement.
This impedance-like Z1 and Zo-is referred to the stator [armature] winding only, never-the -less the influence of rotor impedance is involved here.