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Star Star Auto-transformer with Delta tertiary 1

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Cerkit

Electrical
Jan 18, 2016
100
Hi,

Can someone comment on if a star-star auto-transformer which has a direct earth connection, and which has a delta tertiary where the tertiary is earthed via a reactor to ground, if there is a line to ground fault further down the network will both the auto-transformer neutral and the tertiary neutral carry fault current?

Thanks
 
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Not if the tertiary isn't connected to any outgoing lines. They are mostly used to reduce impedance for triplenes when there is non-linear loads on the secondary. Doing so by creating a path for the circulating current caused by triplenes.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
Yes, that is what I see in the diagram. The delta is isolated and grounded via a reactor. Not sure if it needs grounding. Someone may have more to say on that.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
That reactor does nothing, if zero sequence current needs limiting it needs to be on the star point.

More info is needed.
 
Hi,

To clarify, I am not aiming to limit the zero sequence, I am just trying to verify given the configuration shown if both neutrals will carry fault current for a line to ground fault downstream?

Thanks again
 
The territary neutral will not carry fault current.
 
With a grounded wye and a delta tertiary, the single phase impedance may be much less than the three phase impedance.
Currents on the unfaulted phases will will increase as the delta transfers energy from the healthy phases to the faulted phase.
Do not use the three phase impedance to calculate fault currents, determine the single phase impedance.
Corner grounding the delta will not make any difference.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
Is the tertiary grounded via a reactor, or is that a shunt reactor connected in the tertiary (the '4' next to it makes me think three phases plus a fourth on the star point)?

Cheers,
mgtrp
 
Hi,

The tertiary is grounded via a reactor. The '4' is just part of the labeling of the wider diagram as there are other reactors.

Thanks
 
Cerkit said:
To clarify, I am not aiming to limit the zero sequence, I am just trying to verify given the configuration shown if both neutrals will carry fault current for a line to ground fault downstream?
Then what is the purpose of the reactor?
 
Hi,

I do not know the purpose of the reactor, I am trying to understand what effects it has under fault conditions.

Thanks
 
OP said:
if there is a line to ground fault further down the network will both the auto-transformer neutral and the tertiary neutral carry fault current?
A delta tertiary seldom has a neutral.
When a delta winding has a neutral it is neutral in regards to only two phases. The third phase is the "wild" phase.
Is the wye point grounded?
A delta tertiary may be used to stabilize the voltages in an ungrounded wye/wye transformer and in cases where the ground impedance back to the system neutral is too high to be effective.
With a grounded star, a line to ground fault will cause a fault current to circulate in the delta. There will be no fault current in the grounding reactor.
Depending on the physical arrangement of the windings, capacitive coupling may induce a common mode voltage in an ungrounded winding.
The common mode voltage may exceed the voltage rating of the ungrounded winding.
Corner grounding the delta will discharge any common mode coupled voltages.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
I think there is some issue with the indication shown. I believe there are three reactors ( either inside the tank or external to tertiary bushing terminals) in series with tertiary delta terminals. These are provided to get a higher tertiary impedance to other circuits. Of course this will be effective only in case of a 3L fault on tertiary terminals. During LG fault on HV or LV side ,there will be circulating current inside delta. If that situation is critical,then these reactors will be inside delta, one in each phase ( if tertiary is loaded ) or a single reactor at corner of delta if tertiary is only a stabilizing winding. During unloaded tertiary operation, usually one terminal of the tertiary along with reactor ( not all three) will be grounded to avoid power frequency over voltages by neutral shifting.

The above is frequent in Europe but Indian customers do not prefer it and insist to build reactance in to winding by providing more number of turns in each phase or increasing the inter winding gaps.
 
Thank you all for your input.

Is it correct that a zigzag transformer will only pass zero sequence current?

Also in the diagram attached will there be fault current in the either neutral of this transformer if there is a line-ground fault somewhere on the network? My understanding is that the zero sequence network of a star star transformer with both neutrals earthed (whether with impedance or direct) does not have a connection with the zero potential bar, is this correct, therefore no fault current will flow in these neutrals for a L-G fault?

Thanks
 
 https://files.engineering.com/getfile.aspx?folder=98114b51-a1f8-492c-bd23-6513088e2cc4&file=Network.JPG
Referring to original post, the reactors are shunt reactors connected to the tertiary winding of the auto transformer and the purpose is to compensate some of the OHL capacitances on auto transformer primary/secondary (thus, control overvoltages in the grid).
The tertiary provides circulating path of for third harmonics in the transformer magnetising current and thus prevent the same from appearing in the transmission system.
The tertiary also help reduce the zero sequence impedance for faults earth faults in the transmission system.
Thus, when there is earth fault in transmission system (either on primary or secondary of the auto transformer), the currents appear in the tertiary delta winding but not in the shunt reactors.

Rompicherla Raghunath
 
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