Generator source side interrupting rating
Generator source side interrupting rating
(OP)
I have some question about generator circuit breaker according to IEEE C37.013 standard.
GCB manufacturer indicates two symmetrical interrupting capability for example Siemens Type 3AH38 as follows;
Symmetrical interrupting capability;
-system source side = 50kA (IEEE C37.013 section 5.8.1)
-Generator source side = 25kA(IEEE C37.013 section 5.8.2.3)
DC Component = 75%(=0.75*sqrt(2)*50kA= 53kA)
If we calculated the short circuit current contributed from generator = 26kA, which rated symmetrical interrupting capability of GCB we should use to compare with this calculated value(26kA, 50kA(system source side rating) or 25kA (gen source side rating) ?
I have got clarification from Siemens gas turbine manufacturer for SGT 800 which they compare the calculated value 26kA with system source side rating(50kA) despite the fact that GCB rated gen source side rating is only 25kA.
Which rated symmetrical interrupting capability should be used to compare with the calculated generator source side symmetrical short circuit current ?
If this should be system source side rating, why do GCB manufacturer need to indicate generator source side symmetrical interrupting rating, for what purpose ?
Thank you very much,






RE: Generator source side interrupting rating
1. A quick internet search throws up a brochure indicating the current model of 3AH38 to have 50 kA rating for gen switching (at 17.5 or 24 kV). This is what Siemens may be referring to. Is your breaker the current model or older one? (I have no personal experience with this particular breaker model or mfr).
2. The generator switching duty for close in faults is more severe because of different X/R ratio used for testing, higher DC component and taking longer to reach zero crossing, etc. So I would stick with mfr's published gen duty when applying for gen switching.
Stay tuned for posts from more experienced and knowledgeable ones.
Rafiq Bulsara
http://www.srengineersct.com
RE: Generator source side interrupting rating
The short circuit current contributed from generator side should be compared with GCB rated value of generator source side and short circuit current contributed from generator transformer which is the system source side should be compared with GCB rated value for system source side.
But Siemens did the opposite thin, they compared generator short circuit current with the GCB rated value for system source side.
However It is to be noted that, I asked ABB and they answered that generator fault current should be compared with GCB rated symm current for generator source side. This is the same as my understanding.
It is to be noted that the Siemens's guy that I contacted is not Siemens circuit breaker but the Electrical and I&C guy who work in Siemens gas turbine segment.
RE: Generator source side interrupting rating
I agree what you have said.But wish to highlight two points.
1)Your calculated gen contribution is 26 kA.The GCB has only 25 kA rating as the Gen fed fault rating.Only 4% margin.Is it enough?I prefer to go for the next GCB rating of 31.5 kA allowing a better safety margin for calculation errors and future growth.
2)Your calculated gen contribution is 26 kA at what DC component (X/R ratio)?You say that your GCB has only 75% DC component.Is it for Gen fed fault or System fed fault?If your calculated DC component (X/R ratio) for a Gen fed fault is higher than 75% then your GCB seems to be out of spec.I know there is a CH GCB model rated for 135% DC component for a gen fed fault.
Hope this helps.
RE: Generator source side interrupting rating
Thanks for your advice.
The calculated 3-phase fault contributed from generator are as follows;
-Ik" = 26.959 kA
-Ib sym = 33.514 kA
-Idc = 27.197 kA
-Gen X/R = 55.8
Siemens GCB technical data are as attached file.
1.I think so but I am confused that why Siemens used GCB rated sym capacity for system source side instead of gen source side rating.
2.The published % DC component shown in Siemens 3AH38 GCB datasheet does not mention what side it is referred to but seems to be system source side as I have seen indication at rated sym interrupting capability as 100%I. Siemens also calculate the their GCB DC interrupting rating as = 0.75x sqrt(2)x 50kA= 53.033kA. Is their calculation for GCB Idc rating correct ?
I have seen
RE: Generator source side interrupting rating
Sorry, I donot have any idea of the equation calculating Idc.