Generator Impedance and Stator rewind 1

[dpc]

Electricpete's question on motor rewinds jogged me to ask this question to the group:

For an ac turbo-generator (approx 20 MVA) that has been rewound (stator)and given a higher kVA rating, what should be used for generator reactances?

We have the reactance data and time constants for the original generator from the manufacturer.

In the absence of any other available data, is it reasonable to assume that the ohmic values of reactance will not change too much and just recalculate the generator reactances in percent based on the new kVA rating? This seems reasonable to me since the stator core has not been changed, slot geometry is unchanged, and increase in kVA rating is likely due more to better winding insulation than anything else.

We are trying to get an answer on this from the manufacturer (who also did the rewind, but just trying to think through it logically.

 

[motorspert]

dpc
You are correct, if they have retained the generator's electro-mechanical parameters, number of turns, pitch and coil dimensions etc, then the ohmic value of reactance will remain the same and the per-unit values increase pro-rata with the new kVA.

The problem may be in determining how they have achieved more output - if its as simple as using less insulation (can be done with modern insulation systems) and more copper then the reactances will change slighly due to the position of the copper in the slot and the effect will be negligable in real terms.

If they have changed the number of turns (extremely unikely on a turbo-gen) or the coil pitch then there will be a difference in reactance - but if they have done this then they should have informed you of the performance changes.

Have they guaranteed the rotor at the new rating - or has the power factor changed?

 

[dpc]

Thanks motorspert.

Yes the power factor was changed from 0.8 to 0.95.

I should have mentioned that there was also a rewind of the field as well as a stator rewind.

Dave

 

[motorspert]

dpc,
The rotor rewind will have no effect on reactances, not much they can do there!

Richard

 

[jplinn]

Generator reactances and time constants are a function of the stator geometry. Rewinding the generator with a different winding configuration will not change the reactance of the machine. Increased copper sizes will decrease the resistance.

 

[dpc]

Thanks for the responses.

 

[motorspert]

Not quite right jplinn.........
The time constants generally do not vary much, but reactance is dramatically effected by the number of turns. So as long as the turns are the same then the reactances should be the same. However, we have effected repairs where we have increased the reactance (at customers request!) by designing the coils to push the copper deeper into the stator slot and putting pack in the top of the slot.

With a turbogen there isny much they can do to the runs in any case.

 

[jplinn]

motorspert

You sent me back to my books (and associates) to review the calculation of machine impedances and time constants (at Design).

Here is what I condensed from that review.

Reactance and time constants will be the same for a machine with the same parallel to coil turn ratio. A change in the parallel to coil ratio is not possible unless the field is changed to provide a different value of flux to generate the required machine voltage. Most rewinds do not change this ratio and keep the same voltage rating.

Provided the coil pitch, circuit resistance, air gap, slot and other dimensions are not changed the reactances and time constants will be the same.

The resistance does vary between rewinds. But my experience is that even with a change in the number of turns/parallels and copper dimensions, the change is small.

The values are unsaturated values.

jplinn