Synchronous generator negative sequence reactance

[JPetersen]

Hi,
I have a small portable powergenerator:
350kVA, 280kW, 400V, 505A, 50Hz, PF=0,8 and 1500 rpm.

From data sheet
xd''=0.140 pu, saturated
x2 =0.140 pu (neg. seq.)

My question is now this x2, is it subtransient or what?
Can I use x2 (and xo, zero. seq.) for synchrounous generators for short circuit calculations?

Any comments or hints for books/

http://www links

are really needed!

Best Regards
John

 

[mpparent]

xd'' is your subtransient reactance. If you're looking to perform a short circuit study, and want the short circuit current for the first 5 cycles, then use xd'' for your positive sequence on your generator. Use your x2 for your negative sequence. xo is less than xd'' typically, and you might have to get ahold of your manufacturer to get this quantity.

Mike

 

[JPetersen]

So is x2 only valid for the first few ms, like xd''?
An x2'' ??
Best regards

 

[davidbeach]

x2 is x2 as long as the rotor is moving in positive sequence rotation at synchronous speed.

 

[Bung]

A rule of thumb I've used in the past for neg phase sequence X is X2 =(Xd"+Xq")/2



Bung
Life is non-linear...

 

[mc5w]

You also need to look at how much current it takes to saturate the steel core in the generator. Short circuit current cannot exceed the saturation current because any current flow beyond saturation will cause the voltage to drop to zero.

Source: IEEE Orange Book - Recommended Practice for Emergency and Standby Power Systems

Mike Cole, mc5w@earthlink.net

 

[mpparent]

x2 is x2 over the entire range. The same thing goes with xo. Your positive sequence reactance is the only value that changes w/ time.

1st 5 cycles - use xd''
5 - 200 cycles - use xd'
above 200 cycles - use xd

Mike

 

[JPetersen]

mpparent & davidbeach wrote:

x2 is x2.. (no matter time)

I do not doubt you, but can you give me a short lecture why this is so, or maybe a good book to study these issues?

Best Regards
John

 

[JPetersen]

A little more, hope someone will take his/her time:

Is it correct that the machine parameters (xd'', xd', xd Xq'' etc. and time constants) are unaffected by generator voltage and excitation level?

If I would like to do calculation on the generator transient respond to nearby motor starting, can I then use the machine parameters (xd'', xd', xd Xq'' etc. and time constants)? Or are the parameters only related to short circuits?

When the generator voltage is controlled by an AVR, can I calculate the Ik (steady state)?

Best Regards
John

 

[cuky2000]

Direct Axis Synchronous Reactance, Xd(i):
Direct Axis Transient Reactance, X'd(i):
Direct Axis Sub-transient Reactance, X"d(i):
Quadrature Axis Synchronous Reactance, Xq(i):
Quadrature Axis Transient Reactance, X'q(i):
Quadrature Axis Sub-transient Reactance, X"q(i):
Stator Leakage Reactance, Xl:
Negative Sequence Reactance, X2(i):
Zero Sequence Reactance, X0:
Saturated Sub-transient Reactance, X"d(v) (on MVA Base):
Armature Resistance, Ra (on MVA Base):

 

[cuky2000]

The above post is incomplete. This shows some of the typical parameters require to do generator transient analysis and a few of them can also be used for Short Circuit analysis. For transient analysis, the governor, EVR model and the mechanical characteristic of the generator need to be considering in the simulation.

I am not sure if for small units this exercise can be justified because is time consuming and high cost associated with softwares to model the system.

Here are some of the answers for the original post.


QUESTION: … Is x2, is it subtransient or what?
ANSWER: X2 is the negative sequence reactance. Its value can be estimated as follow:
a) Turbine Generator with 2 or 4 poles
& salient pole generators w/o damper………………….. X2 ~ X”d.

b) Salient pole generator with damper …………..1.1X”d<X2<1.5X”d

QUESTION: Can I use x2 (and xo, zero. seq.) for synchrounous generators for short circuit calculations?
ANSWER:Yes, X2, Xo and X”d can be used in conjunction with the connection configuration to calculate the short circuit. As mpparent states above, Xo is smaller than X”d. This value could be expected between the following

c) Range expected for Xo:……………………….. 0.1X”d<Xo<0.7X”d

 

[mpparent]

My advice to you is to get a copy of the Westinghouse T & D book (now sold by ABB), ...it goes through a lengthy discussion on x2.

That should explain everything you would ever want to know..



Mike