zero sequence impedance

[bethbc]

Attempting to calculate the zero sequence impedance for a delta-wye transformer. Basically, typical transformer has a zero sequence impendance value of 85%-95% of impedance. I need to design for 120%-170% of impendance. What variable will allow for a larger ratio of zero sequence impendance to impedance? Thanks.

 

[gordonl]

I'm no design expert, but I would think the easiest way to acheive this would be a neutral grounding resistor. I think the only other way is to play with a zero sequence flux path. (4 limb core)

 

[dpc]

Your best bet is to add the impedance between the transformer neutral and ground. I'm not sure it is possible for the inherent zero sequence impedance to be greater than the positive sequence, at least in any practical transformer. The relationship between zero sequence impedance and positive sequence impedance in a delta-wye transforme is a by-product of the core design.

In a core-type transformer, the zero-sequence flux path typically must leave the iron. This reduces the inductance slightly.

To make the zero seq impedance higher, you would have to figure out a way to make the leakage reactance seen by zero sequence current greater than the leakage reactance seen by the positive seq current. I don't see how this can be done, but I'm not a transformer designer.

It should be quite simple to add a grounding resistor or reactor to the transformer to restrict ground fault current to whatever you desire. This is quite commonly done on medium voltage systems.

Hope this helps.

dpc

 

[230842]

Zero sequence impedance (ZSI) is almost infinity from delta side. From wye side ZSI value is normally 85% to 95%. When core transformer performs three legs, then its value is smaller, 20% to 50% of short circuit impedance.
The only way to increase ZSI value up to 120 to 170% is to add a reactance or resistor between neutral and ground. You must take into account that neutral impedances increase ZSI value in an amount equivalent to three times their own value.
Hope to help you Julian