Peak Unti Generators Fault Relative to Utility

[tmaly1]

Dear folks,

Scenario:
We have 3 peak-only generation units, 1000 kw, 1250 kVA, 4160-2400 volts each. All generate to a common bus then to a 3750 kVA step-up transformer to 12.47 kV dist. level. They tie to utility through NO breaker on the 12.47 kv TX side. The TX is Star on gen. side and Delta on 12.47 dist. side with Z=5.5%.

Questions:
Do you believe that this configuration might contribute in overheat bec. the TX delta winding might draw and trap all harmoinc and imbalance currents on the utility ckt as long as TX gen side (grounded star) is open when the generators
are not running while TX delta side remains energized and tied with the utility ckt (still at no-load)??

Also, how do I calculate the available SC current at gen. breaker and TX breaker. I used the 10 Ifl assuming 10-12% Generator Subtransient Impedance to get the genertor contribution. But, do I have to add the SC coming from gen. and those coming from utility as well??

I appreciate your opinion and feedback in advance

best regards,

 

[alehman]

The delta connection on the utility side is common practice and will appear as an open cicuit to triplen harmonics and unbalanced (zero sequence) voltage from the utility side.

If the generators operate in parallel with one another and with the utility, you must consider all as sources of fault current simultaneously.