Fault level ISc


Thread Starter



I was selecting a main incomer VCB for a turbo generator TG of 16000KW. I assumed ISc 31.5KA but an expert provided me calculation which I am sending you. He calculated total fault level 23.62KA and advised to select VCB of 25KA. I am also sending single line diagram of power house.

In calculation

* Considered sub transient reactance Xd" while I think should be considered transient reactance Xd'

* 1200KW diesel generator DG set Xd" is assumed 14%. I am not clear for this selected value is it assumed correctly?

* 20MVA transformer Z% is assumed 9%. I think it should not be assumed more then 7%. Am I right?

The expert told that DG set and grid transformer will contribute the fault current and the fault level as whole will increase than individual levels.I think that incomer VCB will have to bear just contributed part of ISc KA that is 8.33KA he suggested then why not select VCB of fault level 8.33KA?

In the single line diagram here are some more outgoing transformers of 2500 and 2000KVA I would like to know whether these out going other transformers at same bus bar would not considered in fault level calculation and these transformers will not contribute in fault level?

Your guidance is requested.
Shaheed... to start:

1) 'Expert' is correct about using Xd" for calculating CB's Interrupting-Duty!

2) What is 16MW Generator's MVA rating?

3) What is 16MW Generator's PF rating?

3) What is 16MW Generator's Xd"?

4) Define Ur and U!

5) Was Wapda Utility's SC-duty, 31.5kA, given y Utility or calculated by 'Expert'?

6) Is 11kV-bus adequate to withstand first-cyle peak-current, including contribution from all sources, even induction and synchronous motors (if any)?

7) Is there a substantial distance between 16MW generator's terminals, and CB terminals?

8) An Xd" value of 14% for the emergency gen-set, is not abnormal. But, you should have mfg confirm the value!

Phil Corso

Thank you for reply. I had emailed you the expert provided sheet but not received you! again coping here. Please check whether these are considerable. Here at this proposed 11KV same bus would be one 2.5MW 11KV induction motor and 2000KVA 11KV of 10 transformers.

One of my question is also that expert is showing TG set fault level 8.3KA then why selecting incomer VCB of 25KA for total SC of 23.63KA? Is this 23.63KA will have to bear the incomer VCB during SC?.
Expert provided calculations and answers to your questions are mentioned below.

TG set capacity: 16000 KW
System voltage: 11000 Volt
Considering 10% , - tolerance voltage: 9.9KV
(Ans-2) MVA rating: 20 MVA
(Ans-3) 0.8 cos
(Ans-4)U 12KV- Ur 17.5Kv
(Ans-5)Utility SC 31.5 is assumed by me not sure.
(ans-6)11KV bus is proposed 100*10mm CU copied from same other bus without calc.
(ans-7)16MW TG and CB are at distance of 10Mtr.
Xd”: 20%
Considering 30%, - tolerance Xd”: 0.14
Fault contribution from generator: MVA/Xd”=142.86MVA
Fault current KA: MVA/(1.732*KV)= 8.3KA
DG set capacity: 12KW
System voltage: 11KV
Considering 10%, -ve tolerance voltage: 9.9KV
MVA rating: 1.5MVA
Xd” (Assumed): 14%
Considering 30%, -ve tolerance Xd”: 0.098
Fault contribution from D generator: MVA/Xd” = 15.31MVA
Fault current: 15.31/(1.732*KV)=0.89KA
Grid Xfmr rating: 20MVA
System voltage: 11KV
Considering 10%, -ve tolerance voltage: 9.9KV
Xfmr impedance (Assumed): 9%
Considering 10%, -ve tolerance voltage: 0.08 or 8%
Fault contribution from Xfmr: MVA/Xd”= 246.91MVA
Fault current KA: MVA/(1.732*KV) = 246.91/(1.732*9.9)=14.40KA
Now Total fault level: 8.33+0.89+14.4=23.62KA. He suggested.


I beg your pardon, frankly speaking, the question is like admkhan. The actual scenario is! the TG set is coming to a site without incomer VCB and we intend to buy it from local market with bus data mentioned above. On ground there is no existing metal clad switchgear but we have to decide the rating as per fault level.

Best Regards,
Shaheed... your size-calc of the GTG's VCB Isc is based on your presumption that the Gen's contribution to a fault anywhere will not exceed 8.33kA.

Expert's size-calc, ~25kA, is based on contribution from all sources, and he is right!

Consider as a fault location, the Gen's VCB cubicle. The busbar, its joints, insulators, and bus-stubs, must withstand mechanical forces equal to Isc^2, which are 9 times times greater for Expert's 25kA value than for your 8.33kA value!

Note, all the rest of the Swgear will be exposed to the same 25kA value as well. That is why I asked the question about the withstand rating of the existing Swgr!

Shaheed... correction to earlier post:

The busbar, its joints, insulators, and bus-stubs, must withstand mechanical forces proportional to Isc^2, which will be about 9 times greater for Expert's 25kA value than for your 8.33kA value!


Expert's calc'n shows the Grid contribution as 14.4kA! Which of the Swgr cubicles shown on the SLD is used for the 20MVA Xfmr's incomer VCB?


further to my post on grounding. Ground-Fault level has little to do with 3-phase Momentary or Interrupting fault-duty.

Max-current allowable is selected to minimize iron-damage to major Electrical Apparatus, such as Xfmrs, Alternator, and Large Motors!