Why do so many new distribution transformers use Dyn11, while Yyn0 is becoming less common? The main reasons lie in their winding connections and operating characteristics.
Dyn11 has a delta-connected high-voltage winding and a star-connected low-voltage winding with a neutral. The LV voltage leads the HV voltage by 30°. Yyn0 has star-connected windings on both sides, with an LV neutral and no phase displacement.
1. Handling Triplen Harmonics
The delta winding in Dyn11 provides an internal circulation path for zero-sequence triplen harmonic currents, such as the third harmonic, limiting their transfer to the upstream line currents.
Yyn0 has no delta circulation path, so its harmonic performance depends more on core design and grounding. Dyn11 offers an advantage here, although it does not eliminate all harmonics.
2. Clearing LV Earth Faults
Zero-sequence impedance affects single-phase earth-fault current. With suitable LV neutral grounding, Dyn11 generally has lower zero-sequence impedance than Yyn0, allowing higher fault current and helping overcurrent protection detect and clear faults.
Yyn0 may produce lower earth-fault current, requiring closer attention to protection settings. Actual performance also depends on grounding and circuit impedance.
3. Supplying Unbalanced Single-Phase Loads
Lighting, household appliances and single-phase EV chargers can create uneven loading across the three phases.
For some Yyn0 designs, neutral current is limited to 25% of the rated LV winding current, restricting their ability to supply unbalanced single-phase loads. Some Dyn11 designs allow neutral current of 75% or more of the rated LV phase current, offering greater flexibility under uneven loading. (These percentages are design-dependent; the manufacturer’s specified ratings take precedence.)
As single-phase loads increase, Dyn11’s advantages become more noticeable.
Does Yyn0 Still Have a Place?
Yes. Yyn0 can remain an economical option where loads are reasonably balanced and the connection meets network requirements. Its cost advantage depends on the actual design.
Dyn11 is often preferred for mixed and unbalanced loads, but the right choice depends on the application.
