Three phase Fault

[Siyou]

Dear All,

If based from ETAP software and manual calculation shows that the single line to ground fault is higher compared to three phase fault what will I used
in the Evaluation of the short circuit rating of breaker.

Thank You.

 

[dpc]

In the US, breakers are rated based on three-phase faults, per ANSI standards I believe there is a statement that SLG faults can exceed the three-phase fault by 10% without any de-rating.

I would also evaluate your model. It's possible, in theory, for the SLG current to exceed the three-phase fault current at the wye terminals of a delta-wye transformer. But with the added bus/cable impedance, this difference will disappear pretty quickly as you get away from the transformer. It can also happen at generator terminals, but most generators are not solidly-grounded.

I don't know about IEC. I suspect they make it more complicated than ANSI, since they generally do.

Cheers,

Dave

 

[Siyou]

Thanks Dave.

I did the short circuit calculation in ANSI standard the Three phase fault is 23.5kA while the SLG is 28.2kA
With a 72.5kV CB rated in IEC standard, with 25kA breaking current and 65kA making current for the 69kV line. and 15kV Vaccum CB at 13.8kV line
rated in ANSI Standard with 25kA rated short circuit current. Correct me if im wrong, if I only did ANSI calculation for the whole system..
But I checked using ETAP the ANSI and IEC result of the short circuit calculation.

Thank You.

 

[7anoter4]

According to IEC 60909-0 the maximum current depends on Z2/Z1 ratio as shown in the attached image.

 

[7anoter4]

The symbols are as follows:
k1-the area in which SLG is the highest
k3-the area in which three-phase short-circuit is the highest
k2E=the area where two phase short-circuit is the highest
kE2E=the area where two phase to ground short-circuit is the highest

 

[Kiribanda]

Depending on the Z0 value of the transformer (for a Dyn1 the value of Z0<Z1) the bolted 1-ph to ground fault
current right at the transformer low side terminals is much higher than its 3-ph bolted fault current.But what ever the cable
or O/H line impedance offered between the low side bushing terminals and the bkr location will lower it.Therefore select the bkr
accordingly. Use your good engineering judgement because ETAP/SKM/EASYPOWER are all just "TOOLS" only.