Zero Sequence Impedance of YNzn11 Transformer

[EddyWirbelstrom]

An 11kV Utility with a 10 amp neutral earth resistor supplies an 11kV bus ( M1C-Pole1 ).
The 11kV bus supplies a 5 MVA, 11/66kV, YNzn11, 3 limb core transformer ( XF2-0023 ) with a positive sequence impedance voltage of 7.18%.
The transformer dataplate does not include zero sequence impedance.
The attached spreadsheet includes the results of a PTW32 Ver 1001 software short-circuit study for an earth fault on the 66kV side of the transformer using the PTW Comprehensive, the IEC60909 and the ANSI methods.
Only the results of the PTW IEC60909 method agreed with my manual calculations for an earth fault on the 66kV (YN) side of the YNzn11 transformer.

I have used the ‘ABB Handbook’ Xo/X1 ratios for a YNzn11 transformer with the interconnected star point (zn) not earthed and the star point of the YN point earthed.
For an earth fault on the 66kV bus I manually calculated an earth fault current of 216A, based on the ABB Handbook Xo/X1 ratios.

Can anyone provide equivalent circuits for the positive phase sequence impedance and zero phase sequence impedance for a 66kV (YN) side line-to-earth fault , or suggest how earth fault current on the 66kV bus can be calculated ?

See attached spreadsheet for manual manual calculations, PTW software resutls and YNzn11 transformer dataplate.

 

[7anoter4]

According to ABB Switchgear Manual 11 edition vol.3 Table 3-8, for high voltage YN and low voltage Z Xo/X =3..10 for 3 limb transformer voltage applied to upper winding.

 

[7anoter4]

Sorry. It seems to me the Table 3-8 of ABB Switchgear Manual is identical with Table 3-13 of ABB Handbook. I read only the title of the thread initially.

 

[prc]

1) What you were referring to was ABB Switchgear Manual Ed12.0-2012.In earlier editions, it was Table 3-13

2)For Ynzn connected 3 limb transformer, zero sequence impedance will be < 3times of positive sequence impedance when measured from Yn side and 0.1 times of X when measured from zn side.

3) Also see clause 4.7 of IEC 60076-8 Transformer Application Guide

 

[7anoter4]

In my opinion, the Utility system presents an impedance of 66^2/500 [500 MVA short-circuit apparent power as per IEC 60076-5 Table 2], and the transformer resistance has to be only 1/6.5 [not 1/12.76] as the copper losses of 5 MVA transformer has to be not less than 19 kW-in my opinion.