1. Does IEC require negative sequence harmonic impedances to be considered while calculating worst case voltage harmonic distortion in a HV network (for harmonics that exist only in the negative sequence, 5th, 8th, 11th etc). Else does calculating voltage distortion only for postive sequence impedances suffice?
2. If -ve seq impedances are not considered then the 5th, 8th (etc..) orders will not cause any harmonic voltage in the grid. What could be the consequences of neglecting them?
3. If IEC indeed suggests considering only +ve sequence impedances, does that mean harmonic phasors rotating opposite to the fundamental do not contribute greatly to voltage distortions? (perhaps only to be considered in the case of loads with rotating magnetic fields?)
Dear Divya,
as you may know I am working mainly on wind turbine aerodynamics.
please ask my "electrical" colleagues, prof jauch and/or hinrichs for that
best wishes
peter schaffarczyk
Dear Prof. Schaffarczyk,
Thank you for your reply. I am first of all very excited to hear from you and thank you again for your concern. :)
I thought I posted this question in the general section but will check that again.
Kind regards,
Divya.
Well if somone is following this question:
According to IEC harmonic source phase angles are not to be considered and all harmonics are to be assumed to be rotating in the direction of the fundamental. That way -ve sequence harmonics are considered but as +ve seq parameters and therefore the same +ve seq impedances are used to calculate them. This is a simplified method of calculation.
Usually it is assumed that at harmonic frequencies the positive and negative sequence impedances are the same. The reason is that for electric machines the slip frequency for both positive and negative sequence harmonics is so high that the impedance is defined by their sub transient short circuit impedance.
I agree with Jouko Niiranen, that the impedances are similar in positive an negative sequence harmonics. In induction motors, in principle they could be influenced by the frequency dependance of the rotor impedance (and a different frequency in the rotor). In practice, my own experience in induction motors is that emfs caused by non-linear effects are also present and are not negligible at all, they might have an origin in closed slots in the rotor or saturation in stator teeth and yoke. So the practical precision is anyhow much lower than one could expect from a linear model (even a frequency dependent model) from a simulation.
If you know very little about the motor, it might be useful to consider the starting torque. It corresponds with a rotor frequency of 50 of 60Hz. It gives at least some impression of the increase of the rotor resistance at a rotor frequency which is still much higher than the slip frequency.
Synchronous machines are still different, but for harmonics the damper cage is important, which might resemble to induction motors. But also D-Q effects may appear. A real grid contains also inverters, this makes it even more different compared to simple models.
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