What short circuit level can I assume to motor starting analysis if I have no information? 1

[Dvhez]

The only information I have is the line voltage level and power transformers capacities from the substation that will feed this 69 kV line, from where I want to simulate in ETAP a tap-off substation of 30 MVA (69/23 kV) with its corresponding power demands. Is there a standard or any recommendation for studies like this when there's no information about the short circuit level at point of connection (1-ph and 3-ph Isc)? I suppose that it is better to assume the worst case scenario, but without any information I don't know what to do.

 

[7anoter4]

See:
IEC 60076-5/2000 Power transformers –Part 5:Ability to withstand short circuit
Table 1 – Recognized minimum values of short-circuit impedance for transformers with two separate windings:
For 25000 to 40000 kVA uk=10% [minimum]
Table 2 – Short-circuit apparent power of the system
Highest voltage for equipment, Um[kV]:52 and 72,5 Sk=3000 MVA [ for Europe] ; 5000 MVA [for North America]

 

[RRaghunath]

It is easier if it were for equipment sizing inside the plant - you could have considered Max. Short circuit fault level from the existing 69kV bus rating and worked-out.
For motor starting, you need to consider min fault levels as you need to ensure under all grid conditions, you are able to start the motor.
Having said that, practically, the fault level at 69kV level will have very little bearing on the fault levels at your motor bus. This you will be able to verify in ETAP by calculating the motor bus minimum fault levels at different assumed 69kV bus fault levels.
I suppose motor bus in your case is 13.2kV.
So, even if you assume infinite bus at 69kV and work-out taking positive tolerance on the %Z of 30MVA, 69/23kV transformers and those further down below, you should be on safer side.