High impedance generation 1

[bhalfweeg]

I'm investigating methods of managing fault level on an offshore installation with approximately 3 x 40 MW gas turbine generators. One option I am considering is increasing the sub-transient reactance of the turbo-generator.

In speaking to turbo-alternator manufacturers, their basic response is "Tell us the impedance you want, and we'll build it". What I am more interested in is the effects on a power system performance with higher impedance generation, it's effect of motor starting, stability and transient performance.

Brush advised me that I can expect stability problems with Xd" in excess of 20% (based on a p.f. of 0.8). For our initial studies, I've been using Xd" in the order of 15%, by raising this impedance to around 18%, my fault levels will be more manageable.

Has anyone faced similar issues and what have been their experiences been?

 

[RRaghunath]

Bhalfweeg,

With higher Xd'' for the turboalternator, the transient voltage dip at the alternator terminals (at the time of fault in the system) goes up. This could affect the stability of the running motors, the contactors in motor control systems may drop off, the drives may trip on undervoltage. Apart from this, the excitation system needs to respond fast to restore the terminal voltage or else the generator itself may experience pole slip due to the weak field.

It is advisable to tackle the problem of system fault levels by providing unit ratio transformers and such other means, I think.

 

[jOmega]

If you have any non-linear loads (i.e., rectification, lighting, etc) you will increase the Harmonic Distortion and cause additional problems that you haven't even consider yet.

Is leaving the gen. impedance alone, and adding impedance in the branches where the faults occur a viable consideration ?

 

[jbartos]

Suggestion to the original posting: Please, would you describe nature of loads, load startings, load max HP sizes, IC rating of circuit breakers, switchgear, switchboards, power panels, system voltages, transformers, motor drives, system grounding type, etc. The higher Xd" is a good solution in suitable power distribution systems and applications.

 

[jOmega]

Suggestion to jbartos:
please re-read rraghunath's post above

Do you still believe that increasing xd is a good solution ?

Would you still say that raising xd is a good solution if the distribution system has significant non-linear loads?