GROUNDING TRANSFORMER ZERO SEQUENCE IMPEDANCE

[tszpun]

Hello Experts,

Does anybody know how to calculate the grounding transformer zero sequence impedance ?
I would like to order grounding transformer. And the manufacturer asking me Ohm/Phase value, but no Z% value.

Here are the details
1ry side : 66kV
Neutral Earthing Resistor Fault current : 400A
Capacity of the grounding transformer : 16 MVA.

The problem is that i have approached 4 Vendors.
2 Vendors gave me following calculation :Zero sequence impedance = 1.732 x (kV)^2 / Fault Current => 1.732 x 66 / 400 = 285.78 Ohm/phase
The Other 2 vendors came with following calculation : Impedance = (KV.base)^2 / MVA.base => 272.25 Ohm.base. As the specified value is 8% at 16MVA, they're coming with 272.95 x 0.08 = 21.836 ohm.

So i do not know which one is the good calculation, and the difference between the above 2 calculations.
Could somebody explain ?

 

[7anoter4]

If you have the rated voltage Vn[kV], the apparent power rated Sn[MVA] and the short-circuit voltage[uk%] then the impedance per phase will be:
Z=Vn^2/Sn*uk%/100=66^2/16*8/100=21.78 ohm

 

[7anoter4]

Considering Zxfr≈Xxfr then there is a grounding resistance in series Rg so total fault impedance will be Zg=Rg+Xxfr.i Zg= Vn/√3/Ifault Zg=66000/sqrt(3)/400 =95.26 ohm.
Rg=sqrt(95.26^2-21.78^2)=92.74 ohm

 

[7anoter4]

According to ABB Switchgear Manual 11 edition Table 3-8 Xo/X1 if the High voltage star neutral is grounded then Xo/X1=1

 

[7anoter4]

In my opinion your transformer it is as per attached link:

http://m.scotech-electrical.com/tra...6-mva-three-phase-power-transformer-66kv.html