Generator Synchronous Reactance

[mityeltu]

I'm not sure how to ask this question. I'll try and maybe have to clarify later.

I'll be referring to Stevenson's Elements of Power System Analysis, 4th ed, Ch 10, pg 256. The following is given: generator with internal voltage Eg, synchronous reactance Xs, external impadance Zext, and load impedance ZL all in series. Current IL flows through the circuit before the fault. The companion to this is the faulted version using subtransient internal voltage Eg", subtransient reactance Xd", Zext and ZL. With open fault located across the load imopedance. IL is shown to flow through this circuit as well.

The book states that for a different value of IL, Eg will remain unchanged, but a new vaule of Eg" would be required.

This is where I lose it. If the current changes (assuming load and external impedance remain unchanged - this is not stated explicitly in the book), then one of 2 things must be true: 1-the internal voltage of the generator must change, or 2-the synchronous reactance must change.

The book states that the generator internal voltage remains unchanged. So how does the synchronous reactance change in a generator?

 

[dkarst]

I'll be referring to Stevenson's Elements of Power System Analysis, 4th ed, Ch 10, pg 256. The following is given: generator with internal voltage Eg, synchronous reactance Xs, external impadance Zext, and load impedance ZL all in series. Current IL flows through the circuit before the fault. The companion to this is the faulted version using subtransient internal voltage Eg", subtransient reactance Xd", Zext and ZL. With open fault located across the load imopedance. IL is shown to flow through this circuit as well.

The book states that for a different value of IL, Eg will remain unchanged, but a new vaule of Eg" would be required.

This is where I lose it. If the current changes (assuming load and external impedance remain unchanged - this is not stated explicitly in the book), then one of 2 things must be true: 1-the internal voltage of the generator must change, or 2-the synchronous reactance must change.The book states that the generator internal voltage remains unchanged. So how does the synchronous reactance change in a generator?

Since nobody has posted yet, I'll give it a try.... I think the part I've underlined above is the confusion. What the text is stating is if the pre-fault current changes, you have to calculate a new Eg". The book states "for a new value of Iload, Eg remains the same (but that only applies to steady state), but a new value of Eg" would be required (based on the pre-fault Iload). The thing that will likely change the pre-fault load current is a different load.

Here is an idea, try out problem 10.4 as given in text (don't peek at solution below...). It is pretty straightforward and shows you need the pre-fault current to determine Eg" and hence Ig". Now just imagine the load resistance is 25% less and calculate the new fault current. Hope this helped, if not please post more and I'll try again.

 

[mityeltu]

I followed the example and got the same answer, though I didn't do it like your posted answer. I don't think that really helped me though.

I understand that the generator internal voltage must be different from the terminal voltage because of the machines reactance. I also understand that Eg" will be different from Eg because of the difference between Xs and Xd".

I'll admit that changing the load impedance makes much more sense, but the book has been really good about being thurough. I would have assumed that had they meant to change the impedance, they would have stated it that way. That being said, after looking at it again, I have to admit, that it is really the ONLY things that makes sense.

Unless there is a way to change synchronous reactance, I'll have to admit that the only way to change IL is by changing one of the external impedances.

So, I guess what I'm saying (with ALOT of words) is.... I agree.