Fault level on Transformer

[Cerkit]

Hi,

If have a star/delta transformer, 132/33kV, 90mva, and I know the total fault current for a phase to phase fault on the 33kV side, how can I calculate the three phase fault level?

Thanks

 

[cuky2000]

A simple approximation can be obtained dividing the total self-cooled full load current by the transformer impedance

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I_SC~I_FLC/Z_pu.

 

[7anoter4]

See IEC 60909-0 4.2.2 Line-to-line short circuit :
During the initial stage of the short circuit, the negative impedance is approximately equal to the positive-sequence impedance, independent of whether the short circuit is a near-to-generator or a far-from-generator short circuit. Therefore in equation (45) it is possible to introduce Z1=Z2.
I"k2=c*Un/[Z1+Z2]=I"k3*√3/2
I”k3=I”k2*2/√3

 

[waross]

ISC~IFLC/Zpu. will give a value known as the "Available Short Circuit Current"
This is the symmetrical, RMS, three phase fault current after the asymmetrical component has decayed to zero.
The peak current is root √2 or 1.414 times the RMS value.
The instantaneous fault current of a fully offset asymmetrical current will be 2 x √2 or 2.828 times the Available Short Circuit Current (ASCC).
ASCCis a three phase fault value.
Switchgear is rated in ASCC (with a correction factor for a typical X/R ratio).
Initial fully offset fault current will approach 2.828 times the ASCC.
How close the initial fault current approaches the limit will depend on the X/R ratio of the transformer, the point on wave at which the fault occurs and the strength of the supply grid.
The instantaneous fault current is used to calculate the maximum mechanical forces that will occur during a worst case fault.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamburgerHelper]

I think you need your thevinin impedance or short circuit values at the point of interconnect. If you are close or far away from a generator or contributing motor loads, you could get different values. If you are far away, Z1 should be very close to Z2.

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