Delta side current for a delta wye grounded transformer with faulty wye 4

The wye-side ground fault shows up as a phase-phase fault on the delta-side. The ground fault has positive, negative, and zero sequence components. The zero sequence doesn't make it out past the delta but the other two do. The phase shift for the negative sequence is opposite that of the positive sequence; for an ANSI standard transformer, assuming the delta is the high-side, the delta positive sequence leads the wye positive sequence and the delta negative sequence lags the wye negative sequence, both by 30 degrees.

It's possible to take a relay event record that has the delta high-side currents and the neutral current (SEL-501 anybody?), assume a voltage ratio (DETC and LTC may be unknowns), and produce the wye low-side currents.

and thus the line current at the delta side will no be affected by faults on the wye side

Click to expand...

This isn't quite correct. As David says, a L-G fault on the wye side will look like a phase to phase fault on the delta side My simple-minded way of thinking about it is that whatever current is flowing on the secondary windings, a corresponding current must be flowing on the primary windings. So for a L-G fault on the wye, all of the current flows in one winding of the wye. This same current (adjusted for turns ratio) must flow in the same winding on the primary side. Primary protection sees phase fault current but no ground fault current.

Looks weird to me! I have not seen a grounding transformer wired delta-wye!

Delta wye; not a grounding transformer. A common power transformer configuration.
When you look a the currents feeding a delta load or transformer, The current in the A phase line is the vector sum of the currents in a phase and b phase and is not at the same angle as either A phase or B phase load currents.
A fault on the A phase of the secondary will reflect on the primary as a fault current in phase with A phase.
The Resulting A phase current will be the vector sum of the C phase current and the reflected A phase fault current.
The C phase current may be affected by the voltage drops resulting from the fault.


Bill
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"Why not the best?"
Jimmy Carter

Thanks everyone.

I just came across an interesting article here :
The article mentions the wye-ground-wye-ground as not representing a ground source, as I understand that the fault current returning to the transformer secondary, with the source at the primary, will be opposed by and equal amount divided by the turns ratio going out of the primary neutral back to source. This is confusing enough for grounding design with respect to the actual fault current going to the grounding grid since most of the wye-ground-wye-ground transformers have only one neutral point.




Interested to see how the earth fault protection works by then!

Yg Yg transformer could be a ground source if the transformer is a 3 legged core type construction.

This effect is called phantom tertiary winding with zero sequence circuit looking like this:


This is not an issue for 5 legged core or shell type transformers.

Like folks here said, transformer passing through zero sequence current is not the same thing as transformer being a ground source. The former means if there is ground fault on secondary side, the primary side will also see the fault as ground fault. The latter means if there is a ground fault on the primary side, having this ground source tx will contribute to the ground fault.