Static excitation system performances

[pbeigbeder]

Hello
I'm working on a power plant in France (around 500MVA). On this PP the generator is excited by a static excitation system and in the french rules it must be capable to whistand a short-circuit just after step-up transformer during 300ms without loss of synchronism.
Due to this excitation system, in my point of vue the excitation won't be able to perform this requirement. The voltage will decrease and the excitation source also, the excitation will then also decrease.
Do any of you have experiences on such system in terms of stability?
Thanks in advance

 

[Modula2]

So, when the fault is cleared within 300 ms, the unit comes back to synchronism. It's not too far out at that time that it can't be restored. Is that the objective?

 

[Bahram7]

What type of short-circuit is involved here? What is the inertia value (H) of the turbine-Generator? How strong is the network connected at the HV ide? 300 ms three-phase-to-ground fault at the HV side is quite severe to save the network synchronism!
Concerning the voltage at the low side where the excitation system draw its current, depending of the impedance of the GSU transformer some 30 to 40% voltage may still be available so the exciter will try to do the job by increasing the DC voltage up to its limit.

Bahram7

http://www.Simtech-Intl.com

 

[pbeigbeder]

Hello
The short-circuit involved on this case is a three phases short-circuit. The transformer short-circuit voltage is 23%.
I don't know the inertia of the complete shaft (generator + gaz turbine + steam turbine). The inertia of the generator alone is around 0.85kWs/kVA. ( I think between 3 and 4 is an correct value)
The nework short-circuit power is 2500MVA.
In this case the network is represented by 4 lines in parallel. Each line has a impedance of 25ohms in 225kV. The fault appear at 1% lenght on 1 line.
I agree with you that some voltage will remain on the LV side of the transformer, but it won't be sufficient to reach the ceiling excitation voltage. So during the short-circuit I won't overexcite the generator. Then in my point of vue this case is unfavourable for loss of synchronism.

 

[Bahram7]

Hello,

I agree with you about the ceiling. Also I do not believe taht the excitation system can do much in this very severe case. The most important parameter is the generating system inertia which limits the machine acceleration during the fault duration.

Bahram7

 

[Schwatzernov]

You can check whether the system is stable or not by doing Transient Stability Study, you can simulate such kind of fault with CB tripping time set at 300 ms as required. In fact it depends on various factors, however please do not forget that the AVR have field forcing function. You can check how much times of field forcing of your excitation system can boost the field current and corresponding generator terminal voltage.

 

[bacon4life]

Is the power source for the static exciter a potential transformer at the generator terminals? And therefore if the voltage drops too much during the fault, then the exciter will not output ceiling voltage.

In 'Power System Stability and control' Prabha Kundur discusses that the fast time response and high after fault ceiling voltage can offset the poor fault performance. Other systems are available that have current transformers to provide excitation power during faults. He references:

"Static systems of excitation with and without compounding (Static exciters with, and without, compounding)" by PENEDER F.; BERTSCHI R.;

01_1986 -- Advanced excitation controls for power system stability enhancement - P. Kundar and D.C. Lee

http://www.e-cigre.org/Search/

 

[davidbeach]

bacon4life, interesting you should mention the Kundur book. Are you deep into it right now?

 

[TurbineGen]

From where does the power source of your exciter come from? Is it possible to use a different power source, such as one not so severly affected by a GSU transformer fault?

Quote: Maybe if I try, I can get both feet in my mouth.

 

[pbeigbeder]

Hello
Thanks a lot for your answers. In order to clarify a little the purpose I'll give you some more explanation :
1) The transformer supplying the excitation is directly connected to the generator terminals
2) Concerning compounding, nothing is forseen in this way, probably due to the excitation power needed (compounding CTs must be saturable and then very hard to define, and also huge).Nominal excitation current is around 4000A
3) Stability studies will be performed to check the global stability of the system
4) To Bahram : I agree with you concerning the severity of the fault, the requirement (3 phases short-circuit fault, 300 ms) comes from the network manager (RTE in France) and I think that they want to check if the supplier has taken all possible care to ensure the best availability of the power plant. It means that they probably accept the new power plant even if this requirement isn't completely performed. But I'm a little worry because they can argue that with other excitation system the results could be better (with brushless and independant excitation power supply or compound).

 

[bacon4life]

David,

I used Kundur's book in a U. of Idaho Power System Stability class.

 

[davidbeach]

bacon4life, well, it is being used again. All done now except for the final I take on Wednesday.