generator - sequence impedances

[tonyflair]

The software that I'm using to perform a fault study requires me to enter the maximum 3 phase fault current and the X/R ratio (of a substation/generator) and from this, it produces the positive and negative sequence impedances. It leaves the zero sequence impedance blank for me to fill in.

How does one determine this zero sequence impedance ?

All I know is what I have from the generator data sheets (i.e. Xd, X'd, X''d, ..... etc.).

Thanks

 

[alehman]

The generator manufacturer should be able to provide that. I don't know of any other way to get it except to measure directly. Also if at all possible you should get X/R data from the mfg.

 

[Bung]

Am I right in thinking that there is insufficient info in the thre-phase fault level and X/R to get neg seq impedances?


Bung
Life is non-linear...

 

[alehman]

Bung, I think you're right about that.

 

[CJFlatters]

You dont mention the rating / voltage etc, are you UK, USA - the actual code requirements vary slighly ??

Also are you islanded or operating in parallel with the utility, if so and you are in the the UK, you need to comply with G5/9 for Embedded Generation. Presumably similar codes exist in the US and other areas.

However - I digress

The zero sequence impedance of the network which you need needed to calculate the earth fault currents will depend not only on the machine electromagnetics, but also upon the method of earthing (is it solidly earthed, liquid resistance earthed, earthing transformer .....)

It is normal on high / medium voltage machines to use impedance in the star point earth connection to limit the earth fault current to less than 1PU amps.

On low voltage machines it is more usual to solidly earth.

_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager

 

[cuky2000]

....all I know is what I have from the generator data sheets (i.e. Xd, X'd, X''d, ..... etc.). How does one determine this zero sequence impedance ?

In absence of manufacturer data, the sequence impedances could be estimates as follow:

SEQUENCE REACTANCES :
1- Positive: X1= X”d (For hydro-gen ~ 0.75X”d).
2- Negative: X2 = ( X”d +X”q)/2.
3- Zero : Xo = k. X”d Where 0.16< k <0.60 ( k =0.5 Approx.)

Resistance for MV large units may be neglected.