I don't understand the term "load angle" of the syncrhonous generator

[josean256]

I am studying the behavior of the magnetic fields between the rotor field and stator field(armature reaction) in a synchronous generator . I understand that the stator magnetic flux will vary depending on the power factor of the load (leading, lagging or resistive). If the load is resistive, the magnetic field of the stator will be at 90 degrees of separation, while the load is lagging(inductor) the magnetic field will be 180 degree respect the rotor field and finally, if the load is leading(capacitor) the magnetic field will be zero degree respect the rotor field.

Ok, this part I am clear but when I am reading the definition of "Load Angle", it's said that is the angle between the rotor field and stator field and the maximum degree of separation is 90 . If I think in the behavior of the magnetic field before mentioned ,I saw that I can to have until 180 degree of separation. Can someone explain me this to me with images or easy way to see? .
Furthermore, at generator Capability curves one can appreciate the load angle is 90 degrees when in the area of UNDER EXCITATION (leading area). I can't see How the load angle sets the stabilization limit of the machine when in a leading load the magnetic field of the stator is en the same axis of the rotor field.

 

[montoyas7940]

Furthermore, at generator Capability curves one can appreciate the load angle is 90 degrees when in the area of UNDER EXCITATION (leading area). I can't see How the load angle sets the stabilization limit of the machine when in a leading load the magnetic field of the stator is en the same axis of the rotor field.

I just have a minute but I can get you started. If a generator is underexcited the magnetic coupling (field) between the rotor and the stator is not as strong. A little underexcitation is okay if grid voltage is high but if the field is too weak the generator may "slip a pole".

I'll explain more in a little while if no one else has finished.

 

[anorlunda]

I can't be 100% sure what they. Mean, but I believe that they are talking about the electrical angle of the rotor with respect to,the power grid.

Consider the following greatly simplified model of the grid as seen by the generator. It is an infinite bus (AC source at 100% voltage and nominal frequency) . Choose the phase angle of the infinite source zero degrees as the reference. Now, the generator is connected to the infinite bus through a single inductive reactance X. Now, the power transmitted from the generator to the grid will be approximately:

P=(V1*V2)*sin Θ. Where V1 is the generator voltage, V2 is the bus voltage, and Θ is the angular difference between the generator and the bus. I think Θ is what they mean by load angle.

Further dΘ/dt is proportional to the frequency difference ω between generator and grid. Further dω/dt is proportional to the mechanical shaft power spinning the generator minus the electrical power transmitted.

Good luck with your studies.


Note that when Θ is < 90 degrees, when the generator runs faster, Θ increases, P increases and the net feedback is negative and everything is stable. If Θ is more than 90 degrees, the feedback becomes positive and the system becomes unstable.

The reactive power transmitted Q is mostly proportional to the voltage magnitude difference (V1-V2)*cos Θ, which is approximately (V1-V2) for small Θ.

This model of a power grid seems tremendously simple, but good power engineers memorize it because it is the fundamental relation that governs AC grids of arbitrary complexity. If you understand this thoroughly, you understand everything.