Harmonics calculation due to electronic motor drive

[JPetersen]

Hi fellows
,
You helped me before, here I go again ;-)

Need to calculate/simulate how much voltage harmonics, a motor drive introduce in rest of supply system.

The motordrive is placed near the supplying generator.

Should I use the generator synchronous reactance or subtransient reactance as the supply networks impedance?

 

[Skogsgurra]

Hej Petersen,

Subtransient reactance (x'') is the one to use. Remember that it is given at 50 (or 60) Hz. So you need to multiply with five, seven etc for the different harmonics.

 

[mpparent]

If this is running steady state, wouldn't you want to use xd?

Mike

 

[Skogsgurra]

No. Because every current "bump" is over within less than a quarter period and contains frequencies up to 1000 Hz and more.

Steady state, yes. But not 50 or 60 Hz. You have to think about that to see what it means.

 

[JPetersen]

I think I got it:

If the current "bumps" are over within the time used for xd'' to change to xd' then I have to use xd'' ??

Will it be reasonable to use xd' or even xd as a worst case calculation?

 

[davidbeach]

Wouldn't something close to xd'' apply any harmonics (other than the fundamental) for any duration for reasons similar to x2 being essentially xd''? Isn't it the air gap flux that makes xd different from xd' different from xd''? So even in steady state, a harmonic (just like neg. seq.) can exist without its impedance increasing away from the xd'' value.

 

[mpparent]

I thought that it depends on how x2 is defined...the Westinghouse T&D book has a long section regarding x2. I don't have it at my fingertips just now....

Mike