Unbalanced load/negative sequence currents.
Unbalanced load/negative sequence currents.
(OP)
Hi folks.
I have tried to find what recomondations there are regarding negative sequence currents in generators.
Standards state max 10% of rated current, but Aggreko told me their rental gen.sets could handle 33% negative sequence currents. When I asked why they differ from the standards, they told me that they use permanent magnet rotor for induction of field current, and not stator tapping.
But how does that help ? The field current/voltage is DC and thus not affected by the stator unbalance anyway. Or is the thyristor rectifier making trouble because of the unbalanced feeding voltages ?
Audun
I have tried to find what recomondations there are regarding negative sequence currents in generators.
Standards state max 10% of rated current, but Aggreko told me their rental gen.sets could handle 33% negative sequence currents. When I asked why they differ from the standards, they told me that they use permanent magnet rotor for induction of field current, and not stator tapping.
But how does that help ? The field current/voltage is DC and thus not affected by the stator unbalance anyway. Or is the thyristor rectifier making trouble because of the unbalanced feeding voltages ?
Audun






RE: Unbalanced load/negative sequence currents.
Rotor geometry and construction would seem to be a key feature in the ability to withstand this heating of rotor surface. Perhaps heating of the rotor of a permanent magnet generator is not as much of a concern since there are no embedded field windings whose insulation can deteriorate thermally (as is the case for most large syncronous generators).
RE: Unbalanced load/negative sequence currents.
RE: Unbalanced load/negative sequence currents.
damping windings. That would at least explain it...
Audun
RE: Unbalanced load/negative sequence currents.
I have a reference to portion of IEEEC50.13-1977 which statest that the permissible continuous negative sequence current is higher for salient pole machine with amortiseur windings (10%) limit than without (5%). I'm not sure if this supports or contradicts your idea since I'm not sure exactly what configuration you have.
RE: Unbalanced load/negative sequence currents.
Maybe the current in the damping windings don't cause as much heat as the eddy currents would do.
Audun
RE: Unbalanced load/negative sequence currents.
Using magnets can contribute to drastic eddy currents reduction, since they can be manufactured by using the ferrite synterization procedures.
Julian
RE: Unbalanced load/negative sequence currents.