third armonics on asincronous generator 1

[181273]

Two months ago a asincronous machine 1 MW come to my office and get the star cable burn, only this cable burns as a fuse...
Now is on the power plant a little hydro site, and I measure armonics and I find that theres is a high level of armonics thrird one, but I don´t know where they are coming from.. cause I sincronize with the grid the generator and without capacitors I got the same level of current through the neutrl point... and there´s nothing else, a transformer with delta conection on primary, cause current is not coming from the net. I think. does somebody know where this armonics grouws up

 

[waross]

Your problem may not be harmonics. With the wye point connected you may experience high neutral currents in the event of a primary voltage unbalance. You may experience excessive neutral currents in the event of a primary phase loss or fault.
Try disconnecting the neutral.
respectfully

 

[181273]

no warros, I record the current with a line analyzer, and the current got 3 times the main frequency, if you got a problem with umbalanced currents you will get currents on the same frequency as fundamental.

 

[aolalde]

181273

The source of third harmonic is intrinsic to the induction generator. Since the air gap is constant around the rotor and stator bore, the Poles flux density distribution around the air gap is close to a square or rectangular wave. That type of wave generates a high content of 3rd harmonic (see Fourier Series for non-sinusoidal waves

http://www.kettering.edu/~drussell/Demos/Fourier/Fourier.html

). Some changes in the winding like coil span 2/3 of full pitch can reduce that particular harmonic but redesign of the machine is required. Some of the problems with 3rd harmonic are that it adds in phase for the three phases and circulates through the neutral.

 

[181273]

could you give me some more adress to study the efects of this poles flux density? thank a lot

 

[aolalde]

Not in the web but see "Alternating-Current Machines by M. Liwschitz & C. Whipple"

 

[rbulsara]

aolalde is right. Any winding pitch other than 2/3 will cause third harmonic circulating currents, which add up in the neutral, when paralleling with a utility source. This is common in the USA. You may want to check with proper sources in your area.

Solutions may include, rewind/replace the generator with 2/3 pitch windings or add reactors in the neutral circuit. Reactor in neutral can be added only if all the loads are 3-wire, not requiring line to neutral connection.

 

[rbulsara]

This may be a dumb question, I do not have much experience with asynchornous (induction) generators, but is it necessary to even interconnect its neutral to the utility neutral?

 

[waross]

Thanks for asking, Rafiq. I have been wondering also.
respectfully

 

[181273]

what will occur if you let the neutral without connection to ground? I think is possible to work in this way, only problem will occur in a fault on the windings of the generator (asincronous one) could somebody tell me something about it.. thanks a lot from the hot spain.

 

[rbulsara]

I would think it would work just fine without neutral connection to either the utility neutral or the ground.