The connection is not the core
A quotation that says YNyn0 has told you about the winding connection and nothing about the physical core. Those are two separate things and factories routinely answer the first when you asked about the second. A three phase core can be built on three limbs, which is cheaper and correct for many connections, or on five legs, meaning three wound limbs plus two unwound side yokes that give flux somewhere else to go. Ask which one you are getting, in writing, because the nameplate will eventually say.
Why grounded wye to grounded wye is the case that needs thought
On a grounded wye primary feeding a grounded wye secondary, unbalanced load and single phase to neutral load drive zero sequence current, and that current needs a zero sequence flux path. On a plain three limb core there is no return path inside the steel, so the flux closes through the tank and the oil instead. The effect scales with how unbalanced the load is. With a heavily unbalanced load the tank heats, zero sequence impedance becomes high and erratic, and neutral voltage shifts. With a well balanced load the effect is small and the unit runs normally.
Three ways designers deal with it
The conventional North American answer for pad mounted units is a five legged core, where the side yokes carry the zero sequence flux and the unit behaves predictably under any load. Manufacturers such as Eaton and Prolec GE use five legged or triplex designs for grounded wye to grounded wye units, and many utility specifications require four or five legged construction. The second answer is a three limb core with a buried delta tertiary winding, which circulates the zero sequence current internally and avoids tank heating at lower cost than a larger core. The third is a plain three limb core used where the load is reliably balanced, such as large motor loads, HVAC plant or data centre equipment, and where the job specification allows it. All three are sold and installed in the United States.
Where a three limb core is the obvious choice
If the primary is delta, there is no zero sequence path required on the primary side at all, so a three limb core is correct and asking for five legs only adds cost. That covers most 34.5 kV delta primary units, commonly connected Dyn1. The distinction works in both directions. Specifying five legged construction where it is not needed is an easy way to pay more for nothing.
What it does to the price
A five legged core uses more electrical steel and a wider tank for the same rating, so it costs more to build. That is one reason two quotations for the same kVA can differ noticeably, and why a price well below the market deserves one question before a purchase order. A three limb unit is not a lesser product, it is a different one, and comparing a three limb price against a five legged price is not comparing like with like.
Check the job, then ask the factory
Start with the job specification or the utility's requirement, because that decides which construction is acceptable. If it calls for five legged, ask the factory whether the core is five legged or three limb, as a yes or no, and require the answer in the email body, on the nameplate and in the factory acceptance test report. If the specification allows three limb, ask whether the unit has a delta tertiary. Either way, the measured zero sequence impedance in the test report is the number that proves the construction.
Common questions
Is a five legged core the same as YNyn0?
No. YNyn0 describes the winding connection and the phase displacement. Five legged describes the physical core. A unit can be YNyn0 on either a three limb or a five legged core.
Is a three limb core allowed on a grounded wye to grounded wye pad mount?
It depends on the job. Many utility and engineer specifications require four or five legged construction for this connection, and five legged is the usual North American practice. Others accept a three limb core, especially with a buried delta tertiary winding or on reliably balanced loads. Check the specification for the project before ordering.
What goes wrong with a plain three limb core on unbalanced load?
Zero sequence flux has no return path in the steel, so it closes through the tank and oil. Under significant unbalance that causes tank heating, unstable zero sequence impedance and neutral voltage shift. On balanced loads the effect is small.
Do I need a five legged core on a 34.5 kV delta primary unit?
No. With a delta primary there is no zero sequence path required on the primary, so a three limb core is correct and normally cheaper.
How do I verify the core rather than take the factory's word?
Require the core type in writing before the order and on the nameplate, and require the zero sequence impedance in the routine or type test report. That measured figure is what proves the construction.
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