The vector group tells you three things at once: how the high-voltage winding is connected, how the low-voltage winding is connected, and the phase angle between them. It appears on every rating plate as a short code — Dyn11, Yyn0, YNd1 — and it determines whether two transformers can operate in parallel, whether a neutral is available for single-phase loads, and how the unit behaves under unbalanced conditions.
Reading the Notation
Symbol
Meaning
D
HV winding connected in delta
Y
HV winding connected in star (wye)
Z
HV winding connected in zigzag (interconnected star)
d
LV winding connected in delta
y
LV winding connected in star
z
LV winding connected in zigzag
N
HV neutral point brought out to a terminal
n
LV neutral point brought out to a terminal
0–11
Clock number — phase displacement in 30° steps
Capital letters always describe the higher voltage winding, lower case the lower voltage winding — regardless of which side is the input.
So Dyn11 reads as: HV in delta, LV in star with the neutral brought out, LV phasor at the 11 o'clock position relative to HV.
The Clock Convention
Imagine the HV phase-to-neutral voltage phasor fixed at 12 o'clock. The clock number is the position of the corresponding LV phasor. Each hour represents 30° of lag.
Clock positions in 30° increments. Position 11 corresponds to a 30° lead, position 1 to a 30° lag.
Clock 11 is often described as "LV leads HV by 30°" and clock 1 as "LV lags HV by 30°". Both descriptions refer to the same convention viewed from opposite directions — the standard defines the clock number as the lag, measured clockwise.
Connection Diagrams
Dyn11 — Delta HV, Star LV with Neutral
The most widely used vector group for distribution transformers. The delta HV winding blocks zero-sequence currents from propagating upstream, while the star LV neutral provides a reference for single-phase loads and earthing.
Dyn5 — Delta HV, Star LV, 150° Displacement
Electrically identical to Dyn11 in every respect except the phase relationship. Common in networks where the utility standard specifies clock 5. Dyn5 and Dyn11 cannot be paralleled with each other.
Dyn1 — Delta HV, Star LV, 30° Lag
Found in networks following ANSI practice, where a 30° lag is the convention. The mirror image of Dyn11 in terms of phase relationship.
Yyn0 — Star HV, Star LV, No Displacement
Both windings in star, no phase shift. Simple and economical, but without a delta winding the transformer offers no path for zero-sequence current, which limits its tolerance to unbalanced loading. Often supplied with a delta-connected tertiary winding to address this.
YNd11 — Star HV with Neutral, Delta LV
Standard for step-up duty and for transmission-level power transformers. The HV neutral allows the winding to be earthed directly, while the delta LV stabilises the neutral and suppresses third-harmonic voltages.
YNd1 — Star HV with Neutral, Delta LV, 30° Lag
The clock 1 counterpart of YNd11, used where network convention or existing installed plant requires a 30° lag rather than a lead.
Dd0 — Delta HV, Delta LV
No neutral on either side, so unsuitable for four-wire distribution. Used in industrial applications feeding three-wire loads, and where continued operation with one phase out of service is required through open-delta connection.
Dzn0 — Delta HV, Zigzag LV with Neutral
Each LV phase is formed from two half-windings on different core limbs, connected at 60° to each other — shown here as the two segments of each arm. Their resultant places terminals a, b and c 120° apart and in phase with the HV delta. This construction gives excellent tolerance to unbalanced and single-phase loading, at the cost of roughly 15 % more winding material for the same rating. Common in rural distribution and in earthing transformer applications.
The Complete IEC 60076-1 List
IEC groups vector configurations by phase displacement. Every standard combination falls into one of six clock positions:
Clock
Displacement
Vector Groups
Typical Application
0
0°
Yy0, YNy0, Yyn0, YNyn0 Dd0 Dz0, Dzn0
Distribution where no phase shift is permitted; interconnection of networks already in phase
1
30° lag
Dy1, Dyn1 Yd1, YNd1 Yz1, YNz1, Yzn1, YNzn1
ANSI-convention networks; step-down where a 30° lag is standard
5
150° lag
Dy5, Dyn5 Yd5, YNd5 Yz5, YNz5, Yzn5, YNzn5
Utility standard in several European and Middle Eastern networks
6
180°
Yy6, YNy6, Yyn6, YNyn6 Dd6 Dz6, Dzn6
Phase reversal applications; specific rectifier and converter duties
7
210° lag
Dy7, Dyn7 Yd7, YNd7 Yz7, YNz7, Yzn7, YNzn7
Less common; used where a specific displacement is needed for parallel converter groups
The most widely used group internationally, both for distribution and step-up duty
Note: Adding N or n to any group indicates that the corresponding neutral point is brought out to a terminal. This does not change the clock number or the electrical relationship between windings.
Parallel Operation
Two transformers can only be operated in parallel if their vector groups are compatible. Connecting units with different phase displacement produces a circulating current driven by the vector difference, which can be large enough to damage both machines even at no load.
Combination
Parallel Operation
Same vector group (Dyn11 + Dyn11)
Permitted — the standard case
Same clock, different connection (Dyn11 + Yd11)
Permitted, provided voltage ratios and impedances match
Clock 1 with clock 11 (Dyn1 + Dyn11)
Possible only by reversing two phase connections on one unit
Clock 5 with clock 11 (Dyn5 + Dyn11)
Possible only by reversing two phase connections on one unit
Clock 0 with clock 11 (Yyn0 + Dyn11)
Not permitted — 30° difference cannot be corrected by reconnection
Clock 0 with clock 6 (Yyn0 + Yyn6)
Possible by reversing all three phase connections on one unit
Beyond vector group compatibility, parallel operation also requires matching voltage ratios, matching impedance voltages within a few percent, and the same tap position. Impedance mismatch causes uneven load sharing rather than circulating current — see Transformer Short-Circuit Current Calculation for how impedance affects fault contribution when units run in parallel.
Why the Delta Winding Matters
A delta winding on one side of the transformer serves purposes beyond phase shifting:
Zero-sequence isolation — earth fault currents on the LV side circulate within the delta rather than propagating to the HV network, simplifying protection coordination.
Third-harmonic suppression — triplen harmonic currents circulate in the closed delta loop, keeping the flux waveform sinusoidal and preventing voltage distortion.
Unbalanced load tolerance — the delta provides a path for the zero-sequence component of an unbalanced load, limiting neutral voltage displacement.
This is why Dyn11 dominates distribution: the delta HV gives network isolation and harmonic control, while the star LV neutral serves single-phase loads. A star-star transformer without a delta tertiary offers neither.
Practical Notes
The vector group is fixed at the design stage. It cannot be changed after manufacture without rewinding, although some transformers are built with external links allowing reconnection between delta and star on one side.
Where an existing installation is being extended, the vector group of the new unit must match what is already installed unless the units will be operated on separate busbars.
Utility specifications almost always state the required vector group. Where a project is exported to a different network, confirm the local convention — clock 11, 5 and 1 are all common in different regions.
The clock number describes the relationship between corresponding phases. Physically swapping two phase connections shifts the apparent group by 4 hours; swapping all three shifts it by 6.
Note: Diagrams in this article show the connection topology and phase relationship. Actual terminal marking and internal winding arrangement are stated on the transformer rating plate and in the test report.
Need a specific vector group for your network?
DATSAN manufactures oil-immersed transformers in Dyn11 as standard, with other vector groups available on request in accordance with IEC 60076-1. Browse the full product range or contact our engineering team to discuss your requirements.
DATSAN Transformer manufactures oil-immersed distribution and power transformers for industrial, utility, and infrastructure projects worldwide.
Established in 1992, DATSAN combines over 30 years of engineering experience with modern manufacturing and testing capabilities. Our products are designed in accordance with IEC 60076, ANSI C57 and relevant international standards, with Ecodesign-compliant options available upon request.
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