Transformer vector groups
The key idea
A code like Dyn11 is not a part number — it records how the transformer is wired inside. The capital letter is the high-voltage winding, the lowercase letter the low-voltage winding, an n means the star point is brought out to a terminal, and the number is the hour on a clock face where the low-voltage phasor lands. One hour is 30°.
The idea
Every power transformer nameplate carries a short code next to the ratings: Dyn11, YNd1, Yy0. It looks like a catalog number, but it is really a wiring diagram in four characters. It tells you how to protect the transformer, how to earth it, and what you may connect alongside it.
Read it in three pieces.
The capital letter is the high-voltage winding. D means the three coils join into a closed triangle, a delta. Y means each coil runs from a line to one shared center point, a star (also called a wye). These are the same two shapes as in wye and delta connections, except that here they describe the coils inside the transformer rather than a load.
The lowercase letter is the low-voltage winding, in the same alphabet: d for delta, y for star. High-voltage letter first, low-voltage letter second — always.
An n or N means the star point is brought out to a terminal. Without it, the star point stays sealed inside the tank: nothing to earth, no neutral to take. The case follows the side, N for the HV winding and n for the LV.
That leaves the number, and this is the part people misread. Picture a clock face. The HV phase-A voltage is a hand nailed to 12; the number says which hour the LV phase-a voltage points to. Twelve hours cover a full turn, so one hour is 30°.
Dyn11 puts the LV phasor at 11 o'clock: 30° counterclockwise, so the LV side leads by 30°. Dyn1 puts it at 1 o'clock, the same 30° in the other direction, so the LV side lags.
That 30° is not a design choice anyone made. It falls out of the winding shapes themselves. In a star winding each coil sits between a line and the neutral; in a delta winding each coil sits between two lines. Those two voltages differ by √3 in size and 30° in angle. Pair a star with a delta and 30° is left over at the terminals, which is why mixed pairs always land on an odd hour. Star with star, or delta with delta, leaves no shift, so those groups land on an even hour, in practice 0 or 6.
The letters carry one more consequence. An earth fault drives the same current in all three phases at once: the zero-sequence part of the symmetrical components. Three equal currents in the same direction need a fourth conductor to return through, and only an earthed star point provides one.
So an earthed star lets zero-sequence current pass between its system and the winding; an unearthed star blocks it completely. A delta has no neutral terminal, so it passes none to its own system. But its closed triangle still gives zero-sequence current a private loop to circulate in, and that circulating path is what lets the winding on the other side carry the current.
Try it
Step through the five common groups. Watch the LV phasor move around the clock, and watch the neutrals and zero-sequence paths change with the letters.
clock hour: 11 · 330° lag = 30° lead
- D
- HV: delta
- yn
- LV: star with the neutral brought out
- 11
- LV a at 11 o'clock
Displacement
330° lag = 30° lead
hour 11 × 30° = 330° of lag
Neutral available
- HVno
- LVyes — the star point comes out
Zero-sequence current
- HV: the delta winding has no terminal for zero-sequence current, so none reaches the HV system.
- LV: the earthed star point carries zero-sequence current between the LV system and the winding.
- A delta winding closes the loop, so zero-sequence current circulates inside the transformer and never leaves it.
The drawing shows symbolic windings only. It has no turns ratio and no impedance, and it uses the ABC phase sequence throughout. The clock hour decides whether two transformers can run in parallel. They can share a busbar only if their hours match. A Dyn11 and a Dyn1 sit 60° apart, and that 60° would push a heavy circulating current through both of them.
Why it matters
- Two transformers can share a busbar only if their clock numbers match. A Dyn11 and a Dyn1 sit 60° apart. Connect both to the same bus and that 60° appears across the pair of windings, driving a circulating current that only the winding impedances limit.
- A differential relay needs to know the vector group. A relay comparing HV and LV currents sees a standing 30° error unless something rotates one side back: modern relays do it with a setting, older schemes did it with the CT connections. Set it wrong and the relay trips a perfectly healthy transformer, which is why commissioning tests check the group. See protection.
- Positive and negative sequence shift in opposite directions. Through a Dyn11, positive sequence gains 30° and negative sequence loses 30°. An unbalanced fault study that carries sequence quantities across a transformer has to apply both, not one.
- The letters decide the earthing. Dyn11 is the standard distribution transformer for a reason: the LV star point can be earthed, giving both a neutral and a solid earth-fault path, while the HV delta keeps zero-sequence current out of the HV system entirely. Each side's earthing arrangement starts from what the code allows.
The math, if you want itOptional — the page reads completely without it
The clock hour is the whole displacement, 30° at a time:
clock hour to angle
displacement = hour × 30° · Dyn11 → 330° lag = 30° lead
The 30° itself comes from the star winding, where the line voltage leads the phase voltage:
star winding, line and phase
VLL = √3 · Vph ∠+30° · Vph = VLL√3 ∠−30°
A delta winding sits directly across the line voltage and adds no shift of its own. Pair the two windings and the star's 30° shows up at the terminals; the nameplate records it as an hour on the clock.
The displacement is not the same for every sequence, and protection settings depend on that:
through a Dyn11
I1 ∠+30° · I2 ∠−30° · I0 does not cross
Positive sequence gains the 30°, negative sequence loses it — equal and opposite, for any group. Zero sequence does not cross this transformer at all: it circulates inside the HV delta and never appears on the HV lines. Real current still flows on the HV side during an LV earth fault, but none of it is zero-sequence.
The model uses ideal windings throughout: no leakage impedance, no magnetizing current, no tap changer, and the standard ABC phase sequence. Real windings change the magnitudes; none of them changes the hour.
See it in Phasor
A transformer element in Phasor carries its vector group as a property: the connection of each winding, the earthing of each star point, and the clock number. The single-line diagram draws one symbol for all of it, but that property still decides the zero-sequence path an earth-fault study finds, and the phase reference of the results.