400 V LV System - Short circuit calculations - Impedances Method

[Carlos Melim]

ood afternoon everybody.

I am involved in a project and I want to calculate the fault currents on several locations to validate the short-circuit power needed for each panel and the upstream trip switches nominal currents and its types A, B, C or D.

I use the impedances method and determine all the impedances ( resistances and reactances ), and add them up from the power transformer of the secondary substation, to the selected location.

To chose the protection devices short-circuit power, I consider a bolted three-phase short circuit;
For the the validation of the upstream trip switches nominal currents and types, I consider a single phase short-circuit, the fault current;

Please check an attached draft.

If the feeder cable of a panel is long I get a low fault current.
But if the feeder of a downstream panel is short the fault current is higher, even if the distance to the secondary substation is higher.

All the calculations that I´ve come across consider the three phase feeders of the panels as totally balanced. No neutral currents.
So the single phase short-circuit fault loop starts and ends on the upstream panel.

Is this calculation method valid?
What really happens to the neutral current during a single-phase short-circuit?
It should close on the neutral bar of the power transformer.

All help and enlightening will be much appreciated.

Best regards.

Carlos Melim

 

[mbrooke]

Do you have any motor contributions, solar or generation down stream from the substation? This might explain the higher short circuit current in your drawing. Otherwise I'd say there is an error.

 

[Julius Right]

The return conductor is different: up to panel 2 the return it is the neutral wire [If1= 207 A]. For the second [If2] the return, probably, is the grounding.