Through fault current calculation for track transformer

[sneja]

I need to set an overall differential protection for track transformer which is fed from three phase system with rated voltage of 400kV. There is two secondary winding with rated voltage 25kV. For a single-phase transformer in isolation, there is no zero sequence impedance, as the zero phase sequence component represents the residual voltage or current which is present in a three phase circuit under fault conditions when a fourth wire is present either as a direct metallic connection or as a double earth on the system. For a single phase transformer will depend on the other components in the system. Where zero sequence impedance can not flow in the lines on both sides the zero phase sequence impedance is open circuited and it is thus equal to infinity in the relation to the series network. Since the LV side of the track transformer is single phase there can be no zero sequence current flowing in LV side and the transformer zero phase sequence impedance is infinity. To set the protection I need to calculate the through fault current at fault on the LV side. Which method shall I use to determine single phase fault current at LV side? I know the fault level of the source and the transformer impedance. Is it simply ignoring the zero sequence component and using the known method for unbalance fault calculation? Or there is something else I have to consider or assume when I do the calculations?

 

[jbartos]

Suggestion: Three winding transformer single phase can be treated similarly as three winding transformer three phase since it is a special case of three winding transformer three phase. Visit

http://thunderbird.kuee.kyoto-u.ac.jp/hseps/sect05/sub3/text.htm

http://www.dromeydesign.com/articles/Conductor_and_Transformer_Modeling.htm

(for impedances)
etc. for more info
Please, notice that the symmetrical components applied to a single phase system will result in their degenerated form.

 

[jghrist]

Can you be more descriptive of the transformer winding connections? It sounds like you are describing a transformer with 3ø 400 kV primary windings (connected delta or wye?) but 2-1ø 25 kV secondary windings (coupled to which primary windings?).

 

[rbulsara]

In addition to what jghrist asked, what is the primary voltage of your transfomer? 400kV or 400/SQRT(3) ?

The diferential protection is for you transformer only, right?

If this is a single phase transformer, it should be simple single phase short circut calcs for you Isc= V/Z, all values in p.u.

Do you have the impedance which takes in to account the primaty and any one of the secondary winding.

Or you can have a case for both secondary circuits shorting out at the same time, which will the worst case scenario.

 

[sneja]

Thank you all for you replies.
In addition, the transformer primary winding is single phase and is connected between two phases (rated voltage 400kV not 400kV/1.73)I have got the impedance of the transformer measured at each of the secondary winding when the primary one is short circuited. I will then do just simple single-phase short circuit calculation. I still don't know how I can transfer the given earth fault level (MVA) for three phase system to the secondary side.

 

[jghrist]

I'd calculate the system pos-sequence impedance from the 3ø fault MVA. Z_{ohms per ø} = 400²/MVA_fault. Then use that impedance in the short circuit calculations.

 

[rbulsara]

I beleive the primary side bus's earth fault MVA does not make any contribution to fault rating of the secondary side bus. It is only intended to check the fault rating of the primary bus during a single line to earth fault on the primary side.

You could treat this as the 3 phase system, with Single Line to earth fault on the secondary side. And you should be checking only for the SLE duty on your the secondary bus. ( I think this is what 'jbartos' also intended) Your secondary system can be simulated as 43300Y/25000V system on a program that takes only 3 phase system components as inputs.

The % impedances are always per phase values. I wish someone else with much more insight and experience will opine on this.

 

[jghrist]

rbulsara,

The primary side earth fault MVA includes the effect of pos-, neg-, and zero-sequence system impedances. The zero-sequence impedance does not restrict the secondary side earth fault, but the pos- and neg-sequence impedance does. That's why I suggested using the 3ø fault MVA.

It's a little confusing thinking of the primary side fault MVA "contributing" to the secondary side fault. If you ignore the primary side system impedance, you are assuming an infinite primary side fault MVA. It's easier for me to understand using impedances, not fault MVA's.

 

[rbulsara]

jghrist:

I am with you. I am not talking about ignoring primary impedance or the 3-phase fault MVA. I beleive what 'sneja' is referring to is the ground (earth) fault MVA (or current ) information furnished by the utility companies in addition to the 3 phase fault MVA. (My utiltiy company does that too in the USA, and people think what do in do with it?)

Even program like SKM has a place to enter the ground fault MVA (or current) data which means little to the secondary side evaluation. It is intended only to check primary side fault curent durint a primary side Single line to ground fault. SLG current duty can be higher than the 3 phase fault current duty.

 

[jbartos]

Suggestion: Please, notice that the short circuit MVAsys of the system are those MVAs on the primary (e.g. high side) of the transformer coming from upstream system source that has Vsys, i.e. system voltage, Isc,sys and Zsys, i.e. system impedance.
MVAsys=(Vsys)**2/Zsys, all in per unit

Now, on the transformer TR1 secondary, the short circuit MVAtr1 will be smaller since the transformer impedance, Ztr1 will reduce the short circuit current Isc to Isc,tr1.
MVAtr1=(Vsys)**2/(Zsys + Ztr1) all in per unit.

Another transformer downstream, TR2, will follow the above pattern, i.e.
MVAtr2=(Vsys)**2/(Zsys + Ztr1 + Ztr2) all in per unit

Additions of Isc contributions from energy sources, e.g. motors, capacitors, flywheels, etc. are trivial to add.

If one phase is used, the above equations should be expressed for one phase validity.

 

[sneja]

Thank you all for your suggestion. If I have been given two fault levels - there phase fault MVA and earth phase fault MVA which one I could use in my calculation since the only fault possible on the secondary side is single phase to earth fault. Based on your information, the zero sequence impedance does not restrict the earth fault level on the secondary side (considering a normal three phase system). I assume then that the three-phase fault MVA has to be used in the calculation presented as system fault impedance. Am I right here?

 

[rbulsara]

sneja:
In my opinion you are right.

(I even tried in SKM program with and without primary side earth fault level and the calculated secondary side SLG (single line to ground) duty remains the same.

 

[jghrist]

sneja,
I agree. The earth phase fault MVA is not required for your calculations.

Your calculations would be essentially the same as for a single phase fault on the low side of a three phase delta-wye transformer.

 

[stevenal]

This is a problem for the use of symmetrical components. A bolted fault on the secondary side of a single phase transformer connected line to line on a three phase system looks like a line to line fault with fault impedance on that system. The transformer impedance referred to the high side becomes the fault impdedance. Three phase fault MVA and X/R ratio can be used to find the impedances for your sequence networks.