Transformer vector groups
The key idea
A code like Dyn11 is not a part number. It records the internal wiring of the transformer. The capital letter is the high-voltage winding. The lowercase letter is the low-voltage winding. An n means the star point comes out to a terminal. The number is the hour on a clock face where the low-voltage phasor lands, and one hour is 30°.
The idea
Every power transformer nameplate carries a short code next to the ratings: Dyn11, YNd1, Yy0. The code looks like a catalog number, but it is not one. It describes how the coils are joined inside the tank. It also tells you how to protect the transformer, how to earth it, and what you can connect alongside it.
Read the code 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. Here the shapes describe the coils inside the transformer, not a load.
The lowercase letter is the low-voltage winding, in the same alphabet: d for delta, y for star. The high-voltage letter always comes first. The low-voltage letter always comes second.
An n or N means the star point comes out to a terminal. Without that letter the star point stays inside the transformer. You cannot earth it, and you cannot take a neutral from it. The case follows the side. N belongs to the HV winding, and n belongs to the LV winding.
That leaves the number. Readers misread this part most often. Picture a clock face. The HV phase-A voltage is a hand fixed at 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 from the HV phasor. The LV side therefore leads by 30°. Dyn1 puts the LV phasor at 1 o'clock. That is the same 30° in the other direction, so the LV side lags.
That 30° is not a design choice. It comes from the shapes of the windings. 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 by 30° in angle.
So a star paired with a delta leaves 30° at the terminals, and mixed pairs always land on an odd hour. A star with a star gives no shift, and a delta with a delta gives no shift. 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. That current is the zero-sequence part of the symmetrical components. Three equal currents in the same direction need a fourth conductor for the return path. Only an earthed star point gives them one.
So an earthed star lets zero-sequence current pass between its system and the winding. An unearthed star blocks that current completely. A delta has no neutral terminal, so it passes none to its own system. The closed triangle of the delta still gives zero-sequence current a path to circulate in. 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. The neutrals and the 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. The 60° then drives a circulating current, and only the winding impedances limit it.
- A differential relay needs to know the vector group. A relay that compares HV and LV currents sees a standing 30° error, unless a setting rotates one side back. Modern relays make that correction with a setting. Older schemes made it with the CT connections. Set it wrong, and the relay trips a healthy transformer. For that reason the 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°. Any 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 good reason. You can earth its LV star point, and that gives both a neutral and a solid earth-fault path. Its HV delta keeps zero-sequence current out of the HV system completely. 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 it adds no shift of its own. Pair the two windings, and the 30° of the star shows at the terminals. The nameplate records that 30° as an hour on the clock.
The displacement is not the same for every sequence. Protection settings depend on that fact:
through a Dyn11
I1 ∠+30° · I2 ∠−30° · I0 does not cross
Positive sequence gains the 30°, and negative sequence loses it. The two shifts are 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. None of that current 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 property holds 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. That property still decides the zero-sequence path that an earth-fault study finds. It also decides the phase reference for the results.