Auto-transformer vs Wye-Wye-Delta

[MontanaEric]

Are auto-transformers the same thing as a wye-wye transformer with a buried tertiary (wye-wye-delta)? If not, how is the tertiary connected in a wye-wye with a buried tertiary?

 

[davidbeach]

In both cases the tertiary is similar. For the auto-transformer there's an electrical connection between the main windings. In the wye-wye 3-winding transformer there's no electrical connection between the main windings.

 

[Mbrooke]

The tertiary is its own winding. Two of the leads from the delta are brought out via bushing and shorted externally- the short can be removed for testing. Wye-Wye-Delta gives you the option of running your transformer with the primary ungrounded and the secondary grounded. Often done where no phase shift is required yet you need a neutral or ground reference. An auto transformer requires that the neutral be grounded because the secondary winding is a chunk of the primary winding.

 

[HamburgerHelper]

The auto is going to be cheaper and more efficient. You get more capacity from you windings because they only carry a split of the current.
You can have auto transformers connected in delta,too.
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If you can't explain it to a six year old, you don't understand it yourself.

 

[prc]

Mbrooke, is it common to use Yyn connection in transmission step down transformers? What is the advantage of it? In my part of the world, this is never used. Always Ynyn connection with out stabilizing tertiary winding.

 

[Mbrooke]

If you are referring to Y ungrounded -wye grounded -delta (3 winding trafo), they are slightly common in the US when stepping transmission down to distribution. It is one of three ways of gaining a 0 phase displacement between 115kv and 13.8kv, the other one being delta-zig zag and the second delta-delta with a separate grounding trafo but the latter is rare and stuck to legacy subs. The delta winding provides a place for zero sequence currents.

 

[prc]

Noted. But what is the disadvantage for grounded Y/grounded y arrangement? This also gives zero phase displacement and stabilizing tertiary can be avoided.

 

[Mbrooke]

Good question- it passes both operating current and short circuit current onto the transmission system. Big enough of a concern that an ungrounded primary is sought. Remember that in the US the neutral is run with the phases and carries load current on distribution circuit. Where as sub-tranmission is 3 wire and transmission is technically also 3 wire when you remove the static wires. While in theory you could get away with it, the earth is not allowed to be a stand alone neutral conductor.

 

[cranky108]

The delta can be brought out on either one, and may be used to serve station or other (rarely) loads.

In some cases, and rarely, on a Y-Y-D transformer the high and low sides can be at 60 or 120 degrees out of phase.
180 degrees is also possible.

Just some advantages of the Y-Y-D transformers. Down side is a higher cost.

 

[cuky2000]

Question 1: Are auto-transformers the same thing as a wye-wye transformer with a buried tertiary (wye-wye-delta)? No. Although the YY connection is the most simple economical and popular, there are other connections as showing in the table below. Buried delta (or stabilizing) buried winding may not be required if neutral loop for zero sequence current is closed & zero sequence impedances of the supplying network are low enough.

Question 2: If not, how is the tertiary connected in a wye-wye with a buried tertiary? For typical winding connections and diagrams, see the enclosed link

 

[davidbeach]

I'd be really interested is seeing diagrams for any of those other than the wye-wye. I can somewhat picture a delta-delta autotransformer but I'm not sure it would have no phase shift. I simply can't picture how a winding could be both wye and delta at the same time; the common part of the winding can be delta or it can be wye, but it can't be both simultaneously and I'm certainly not seeing how part of a delta winding could be in wye or how part of a wye winding could be in delta.

Must have drawings. Maybe I'll slap my forehead and say "duh" when I see them, but without drawings I don't see it happening.

 

[prc]

Let me raise a query-may be basic: In case of a YNyn transformer (neutral through earth and assume effective grounding) will there be any zero sequence current during an LG fault?

 

[cuky2000]

davidbeach

Below is an example of delta connected autotransformer.
Although with limited used in power systems, there are other known connections auto-transformer connections are: Open delta, extended delta, zigzag, T-connection, etc.

 

[davidbeach]

That's basically how I pictured a delta autotransformer, but it certainly doesn't have a zero degree phase shift.

 

[Mbrooke]

That is one step away from being a phase angle regulator.


Question- why do the phases need to swap to the other core?

 

[RRaghunath]

prc, YNyn transformer means secondary side L-G fault will be reflected on to the primary side. This is not desirable as it complicates earth fault coordination.
Often, we see the grid company insisting one Delta connected winding in the step-down transformer meant to supply an industry.
Auto-transformers are common as grid interconnecting transformers due to their low cost when compared to other types. These transformers are provided with a delta tertiary to allow third harmonics as well as zero sequence currents to circulate. The tertiary could be unloaded or loaded with shunt reactors for compensation of capacitive VARs in the transmission system.

Rompicherla Raghunath

 

[cuky2000]

Davidbeach,

One of the applications of D-D autotransformer is to match an existing phase angle.
For illustration, below is an example of the phase angle displacement for a D-D autotransformer

 

[cuky2000]

PRC,


I believe that ground current involves zero sequence during phase-ground faults is always present in solidly grounded neutrals autotransformer. The type of core design (3 to 5 limb) has a significant influence on the zero sequence magnitude as shown in the enclosed excerpt. The magnitude of the zero sequence varies and should be evaluated.

SIEMENS:
•When stabilizing winding used, the transformer’s zero sequence impedance will be linear and of low value.
•Wye-wye connections w/o stabilization can be used if multiple paths to ground are available via station’s other transformer’s neutrals

I hope this help to illustrate the inquire regarding zero sequence.

 

[prc]

Cuky, The delta auto connection as explained by you cannot be used for a ratio more than 2:1. In such condition, extended delta or open delta connection is used.When it is 2:1, rating required for delta connection is 16 % more than star connection. V1-V2 vectorially is much more than the arithmetic value due to the considerable phase shift. Delta auto connection is recently used in GSUs of pumped storage hydro plant. Delta LV of GT is provided as auto connection, motor starting at V2 and then later changed to higher voltage of V1. More details and theoretical analysis of these auto connections are covered in Chapter 8 ( written by A Boyajian of GE) of Transformer Engineering By L F Blume (1951)

Raghu,in India, almost all of step down transformers are YNyn with out any stabilizing delta winding. Auto transformers (below 100 MVA with 3 phase 3 limbed core type) are also without any delta winding. Major transmission company has 100 & 160 MVA 220/132 kV auto-transformers without any delta tertiary. This is the situation for the last 30-40 years. I understand from reliable sources that both US and France has many large auto-transformers ( 3 phase 5 limbed and single phase banks) with out stabilizing delta tertiary since 1950's.But US has delta tertiary in some of the step down units as Mbrook confirmed, may be due to Yyn connection used. So my query was, will there be zero sequence flux heating of tank in YNyn transformers during LG fault or unbalanced loading?

 

[cuky2000]

PRC,

We are working with several utilities in the US with a significant number of large power autotransformers with loaded MV tertiary windings 12.5 kV to 13 kV for use as station service power and some with 34,5 kV. We did perform a reliability analysis and do not find a history of tank heating do to zero sequence flux in YNyn transformers during LG fault or unbalanced loading.
Some of the aging transformers are in process to be replaced after 30 to 50 years in service.
A sample is shown in the enclosed link.