LG Current Calculation for XFMR Secondary Fault for Yyn0 Case considering the zero-seq magnetizing..

[Wfg42438]

I have seen some discussions about considering the zero-seq magnetizing current for secondary side fault of a XFMR with Wye grounding on the primary and y solid grounding on the secondary.

I am curious if anyone knows any good references that goes into an example calculation for the LG fault current for such as case?


For example, if i have the simple system below.
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I took a stab at it and came up with the following assuming all quantities below are per unit.

Positive, Negative and Zero Seq impedances for the Utility as : Z1_U, Z2_U, and Z0_U

Positive, Negative and Zero Seq impedances for the Cable as : Z1_C, Z2_C, and Z0_C

Positive, Negative and Zero Seq impedances for the ZFMR as : Z1_XFMR, Z2_XFMR, and Z0_XFMR, then i also have the magnetizing XFMR zero seq impedance as ZMag_XFMR

Zpos=Z1_U + Z1_C + Z1_XFMR

Zneg=Z2_U + Z2_C + Z2_XFMR

The zero seq impedance is the part im not 100% sure on

From my understanding there is no zero seq flow on the primary

So the Utility and cable wont be in the picture just the XFMR zero seq impedance and the magnetizing zero seq impedance.

Therefore: Zzero= Z0_XFMR in parallel with ZMag_XFMR

If i want the LG zero Seq current at the secondary of XFMR T1 would i simply say:

3I0= (Vpf)/ (Zpos + Zneg + Zzero)


If someone can please help point me in the right direction i would really appreciate it.

 

[7anoter4]

See[for informations]:

https://www.kau.edu.sa/GetFile.aspx?id=253144&fn=Ch.9 Unsymmetrical Faults.pdf

 

[wcaseyharman]

Generally you need a delta in the transformer to ignore the system zero sequence impedance. In your case you would include the equivalent system zero sequence impedance in your z_0 calculation.
I have never seen the magnetizing impedance considered for fault calculations, generally it is several orders of magnitude smaller than the series reactance of a transformer unless the iron is saturated (high voltage), which is unlikely during fault events. Realistically even the uncertainty in the system parameters is likely larger than the magnetizing current.

 

[jghrist]

I have seen some discussions about considering the zero-seq magnetizing current for secondary side fault of a XFMR with Wye grounding on the primary and y solid grounding on the secondary.

Is the primary grounded or ungrounded? If it is ungrounded, then there is no zero-seq connection from primary to secondary and no Ø-grd secondary fault current.

 

[dfdt]

With hv side ungrounded(star ungrounded) and LV side star grounded arrangement- the ground fault current on lv side is seen as line to line fault on hv side due to the effect of tank tertiary (aka phantom tertiary). Zero sequence impedance of such transformer is quite high in order of 50-100%

 

[7anoter4]

Usually Zm is 600-700 times Zsc, then may be considered ∞ and not need to consider it parallel.
For a Delta/Yo transformer Zo≈Zsc