Sequence Network for Wye Delta Transformer 1

[Elec81]

Hi,

I'd be grateful if someone could provide me with some help. I'm trying assess a wye (primary), delta (secondary) transformer under a secondary line to line fault. I have determined the secondary fault current in phase B and C using symmetrical components. However, I'm unsure how to obtain the the star (primary) phase current. The primary has a neutral connection. Therefore, I know there will be current flow in one phase and neutral of the primary (only). This is really my problem, as know the primary neutral Zn should be modelled as 3 times its value in the sequence diagram, but I'm unsure how the zero sequence should be connected to obtain the primary currents.

Any references or examples would be appreciated. I can't find any symmetrical component examples/references covering YNd transformers with a phase to phase secondary fault.

 

[WHiPCPL]

Hello

I did a project on this back in 2017, although I fear I only passed this because our teacher and examinator forgot we were doing work on Ynd transformers over Dyn.

The best I can give you is one reference, I would link you one that deals with this specifically, the problem is it's in danish and costs about 116USD and is at best a recommendation/analysis of (maybe) only the Dyn transformer. I'm not aware of an IEC standard that deals with this.

Electric Power Distribution Handbook by T.A. Short: Shows the current relationship in a wye-delta transformer where a phase-phase fault happens on the delta side on page 323. Granted that's for a floating wye side. In the book he references an IEEE transactions paper titled: "Modeling and analysis of unsymmetrical transformer banks serving unbalanced loads". However what's written in that is unknown to me since I no longer have unrestricted university access to IEEE.

Sadly that's the best help I can provide.

Short circuits, protection, arc flash assessment. That's what I know best

 

[Kiribanda]

Elec81,
On the WYE side of the transformer you will find ZERO currents on the three phases.
When you draw the three seq. networks and made them series connected, and apply
the pre-fault source voltage at the fault on the delta side, there cannot be any currents
because the network is broken by delta.
Therefore, all currents are zero on the lines on both sides.

 

[davidbeach]

There will absolutely be phase currents on the wye side.

Calculate your positive and negative sequence currents from the phase-phase fault on the delta side. Move those sequences currents to the wye side. Assuming that the delta leads the wye by 30 degrees, the positive sequence needs to be shifted by positive 30 degrees and the negative sequence by a negative sequence. Then calculate the phase currents.

 

[Kiribanda]

Elec81,
My apologies. I thought you have mentioned a single phase to ground fault.
For a phase to phase fault (say B to C) on the secondary,when pos seq network is connected in parallel with the neg seq network,
1)On the sec: Ia=0A, Ib=xpu/angle 180 & Ic=xpu/angle zero means Ib & Ic are out of phase
2)On the pri: There will be three line currents on each line (reflection of sec side currents)
Hope this helps.

 

[Elec81]

Thanks for everyone's replies and help so far.

Would it be possible for someone to sketch the sequence network? As discussed, I can calculate the secondary current. However, I'm not understanding how I then determine the primary currents. This is not just due to the Ia1 & Ia2 -/+ 30deg. phase shift through the TX alone, as there is obviously zero sequence current in the primary (only). My problem is really a lack of examples with this type of connection in most literature. Also, the fact that I don't play with symmetrical components that often

Many thanks in advance.

 

[davidbeach]

No zero sequence current in the primary, no current in the neutral.

 

[Elec81]

@davidbeach I'm sure this is only correct if the Wye primary is not connected to the system neutral. If the neutral is connected, current will flow in the line and neutral for a line to line secondary fault.

Thanks,

 

[Kiribanda]

Elec81,
Pl. see the attached sheet. Hope you will find the answer there.