Impedance in Parallel

[Harshal Tidke]

Hi,

I need to calculate Short Circuit current on HT side of Bus elevel for 4 number 1000 KVA Step Up transformers operating in parallel feeding a bus. The impedance of each transforner is 5.1% and FL current on HT side is 87.5 Amps. For one transformer the calculation would be 100/5.1*87.5=1715.68Amps.But for parallel operation the impedance would be 4/5.1=.78.But do I also multiply the currents by 4?

Please advice

 

[winnie]

Percentage impedance is a 'per unit' quantity, and remains essentially constant when you parallel transformers.

The incremental impedance will clearly have to change, meaning that when you place transformers in parallel you will see lower voltage drop for the same current drawn. But percentage impedance is expressed as voltage drop at _full load_ current. When you parallel transformers, the voltage drop per amp goes down, but the current at full load goes up, net result being that you get the _same_ voltage drop at full load, but with higher current flow.

So ignoring any secondary effects of the interaction _between_ transformers, the Isc in your case would be calculated using an impedance of 5.1% and a FLA of 350.

-Jon

 

[bob]

Hi,

I need to calculate Short Circuit current on HT side of Bus elevel for 4 number 1000 KVA Step Up transformers operating in parallel feeding a bus. The impedance of each transformer is 5.1% and FL current on HT side is 87.5 Amps. For one transformer the calculation would be 100/5.1*87.5=1715.68Amps.But for parallel operation the impedance would be 4/5.1=.78.But do I also multiply the currents by 4?

Please advice

From your information, I get a hi side voltage of 6.6 kv. Your calculation results of 1715 amps is a theoretical max. What will influence the fault current is the source impedance i.e. the primary cable impedance, the utility transformer and transmission lines, which will reduce the fault to a lower value. Each transformer has the capacity to deliver a given fault current. At the time of the fault each transformer will deliver its capacity to the fault point. It can be a magnitude of 4 times the value of an individual transformer.

 

[Harshal Tidke]

Thank you fellows.