Unbalanced load/negative sequence currents.

[Audun]

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

 

[electricpete]

Negative phase sequence in the stator current creates a reverse-rotating component of the stator field. This rotates at twice machine speed with respect to the rotor. This induces eddy currents in the rotor which due to high frequency will flow predominantly on the inner surface causing heating.

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).

 

[electrigail]

That's correct that you get eddy currents in the rotor causing heating at twice the machine speed. The negative sequence component is caused by unbalanced loads or faults, and it does flow in the surface iron, slots and rings. High levels can cause failure, lower levels reduce the life due to insulation deterioration. I'm not sure what standards you checked, but ANSI C5013-1977 and IEEE C37.102-1987 have guidelines for generators, usually the standards are recommended if the information is not available from the manufacturer. It's best to set the value as low as possible, which will be determined by your system loads, but as long as the manufacturer will document the 33% value, I'd go with that.

 

[Audun]

The permanent magnet rotor is to my understanding just a separate feeder for the main magneticing circuit.(or else there would not be possible to have voltage regulation) How this helps on the heating problem is what I don't understand. The main rotor still have to have field windings. Maybe the small (1.5 MVA) generators don't have
damping windings. That would at least explain it...

Audun

 

[electricpete]

You're right I misunderstood the permanent magnet part.

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.

 

[Audun]

That is probably right. The damping windings will induce current and work against the contrarotating field ensuring less induction/eddy currents in the rotor metal.
Maybe the current in the damping windings don't cause as much heat as the eddy currents would do.

Audun

 

[230842]

Besides eddy current heatings, I think there is other negative impact of reverse-rotating field produced for negative sequence currents: That reverse-rotating field induces a double frequence voltage on excitation dc windings that can damage excitation circuit components. That do not occur when permanent magnets are used.

Using magnets can contribute to drastic eddy currents reduction, since they can be manufactured by using the ferrite synterization procedures.
Julian

 

[Graeme38]

I have come across gen sets with stated NPS withstand of 30%. These were Ruston 1.5 MVA.