Generator Comparison 1

[kbrum2]

What kind of information can be gleaned from a generator spec sheet to determine performance of one machine over another? I have two 3-phase 4-pole 1800rpm 60hz diesel-engine driven salient-pole synchronous generators with the following data;

Gen 1 Gen 2
kW Rating 100 80
kVA Rating 125 100
Voltage (PH-N) 277 139
Power Factor 0.8 lagging 0.8 lagging
X''d 0.13 pu 0.118 pu
X'd 0.21 pu 0.397 pu
Xd 2.73 pu 3.694 pu
Xo 0.03 pu 0.010 pu
X2 0.31 pu 0.125 pu
T'd 75 ms 120 ms
Short Circuit Ratio 0.46 0.304

The application is for a non-linear load and the engine is only capable of producing 80kw out of the generator end. I don't want to needlessly upsize the generator and I do not know what the actual load will be; assume 60kw with 60% non-linear load. The manufacturer of generator #1 has a history of poor quality (IMHO) and I was hoping the smaller generator, #2, would demonstrate comparable performance. Generator #2 is capable of being reconnected for 277/480 volts, to match the rating of generator #1.

(As a tangent, if synchronous reactance is present during stead-state loads, how can it exceed 1.0 pu and not limit steady-state full-load current?)

I appreciate any and all respones.

Thanks

 

[jbartos]

Suggestion: It would be good to obtain the generator capability curves.

 

[SooryaShrestha]

First of all it is always better to purchase the required size. So the preliminary evaluation will always be for the appropriate size. So the preliminary selection will be 80 kW. But the process of evaluation will not be so.
It seems you are satisfied with the prime mover side without any reservation. However the evaluation will always be in the overall context. So the first start is to check whether the prime mover size is of adequate size having minimum input, rather minimum weighted input like fuel, lubricants etc. In the above case if both of the prime mover is suitable for only 80 kW output, then the extra power that can be generated with 100 kW becomes of no use. So the rating advantage of 100 kW can not be utilized for evaluation.
The second thing to evaluate is the losses of the two generating sets, which you must have obtained from the suppliers. I think the no load and the mechanical losses of the bigger generator will have bigger losses. The load losses at the required output should be calculated. This can also be obtained from the efficiency figures, which should be evaluated on the basis of certain capitalised figures normally depending upon the generating cost of the system in general or the avoided cost of the generating cost in particular or some other method applied by the corporate regulations.
With above fgures whichever is cheaper should be chosen as financial decision.
In the technical criteria as given by your posting, I think the phase voltage for 80 kW is not correct. If it is correct then fulfilling your requirement with 139 V will require stepping it up to 277 V.
The second thing to look at is the subtransient and the transient and the zero sequence reactance values, which will have effect on the fault current. Lower fault current the better, thus higher the reactances on the same base the better. Then comes the synchronous reactance which is inversly proportional to the short circuit ratio. Since in your case the scr is already given, which are quite lower than unity. This shows both machines are not suitable for supplying capacitive loads. I think you do not have such loads.
The other criteria like #1 generator to have poor quality, I think you are biased, because if somebody having proven poor quality, then you should reject it in the first screening.
Your other commets like #2 generator giving comparable performance, IMHO and the last captioned sentence are not very clear.

 

[jbartos]

Small encore: The nonlinear load may need a special attention in form of its analysis for the harmonic content by a harmonic analyzer. In case that the harmonic content is high, i.e. higher than the generators' manufacturer(s) permit and IEEE Std 519-1992 (and its IEC version), then some form of harmonic mitigation will be needed depending on the severity of the harmonic content, e.g. passive or active or hybrid harmonic filters, power line conditioners, phase shifting transformers, etc.