Realistic max earth fault current for transformer 2

[TS Liew]

Dear friends,
For calculation of max fault current of 3 phase transformer. The usual method of using the % positive seq Z & the rated MVA is common. How should I calculate the max single phase to ground fault current? Say I have a 220KV/6.9KV, 70MVA, 12% zero seq Z assume +,-,0 seq Z to be the same.
Shuen Liew

TS Liew (Electrical testing, Commissioning, Power system studies)

 

[davidbeach]

Assuming infinite source and Z0=Z1 they will be the same. Assuming more real world conditions the max single phase will be higher. Unless you’re working with a bank of three single phase transformers, assume Z0 will be less than Z1, 85% is often a good guess absent better data.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

Are you looking for maximum current or Available Short Circuit Current?
In the worst case point on wave switching, as the X:R ratio increases, the maximum peak current (and the maximum magnetic forces) approach 2 x √2 x ASCC RMS.
Or, Peak Offset current approaches 2.828 times RMS Available Short Circuit Current.
Rated current divided by %imp yields ASCC, not peak.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TS Liew]

Thank you guys for your reply, David for your take on the steady state condition of fault, Waross for your take on the transient condition of the fault. Please correct me if I am wrong and show the right path. I believe the off-set @ 2.828 time ASCC is approached at really really high X/R ratio and the duration of decay of the off-set will depend on the damping time constant L/R right? And guys, what about the voltage? We learn from texts that the voltage will fall during a solid earth fault that carries very high fault current but this is also a post transient condition. I am curious about the sub-transient & transient voltage. Do you think the voltage will rise with the offset current to deliver the store energy in the inductance of the circuit(during fault)? Eager to hear from fellow electric fans.

TS Liew (Electrical testing, Commissioning, Power system studies)

 

[TS Liew]

Also with regards to David's proposal of %Z(o)=0.85 %Z(+), this is for core type transformer right? As for limb type, we can safely assume %Z(o)=%Z(+)=%Z(-) if only %Z(+) is given right?

TS Liew (Electrical testing, Commissioning, Power system studies)