TRANSFORMER VECTOR GROUP SELECTION

Yyn0 vs Dyn11: Transformer Vector Groups and the Battle Against Triplen Harmonics

In Vietnam's distribution power systems, when opening the design drawings of a transformer substation (TBA), engineers always encounter a familiar parameter called the Vector Group, with Yyn0 and Dyn11 being the most common. Many engineers view this merely as a theoretical formality, simply copying and pasting what was used in previous projects into new ones.

This is an extremely dangerous mistake in the modern context. Today, our electrical rooms are no longer filled with purely resistive and inductive loads as they used to be; instead, they are flooded with non-linear loads (VFD inverters, UPS, computers, LED lights, solar power). These devices continuously pump an 'addictive poison' called harmonics into the system. Below is a breakdown of the nature of Dyn11 and Yyn0 vector groups and how they either 'control' or 'abet' harmonics.

1. Core Nature of Yyn0 and Dyn11
- Yyn0 (Star/Star-Neutral): Primary winding connected in Star (Y), secondary winding also connected in Star with a neutral wire (yn). The phase shift angle between the primary and secondary voltages is 0° (0 o'clock).
- Dyn11 (Delta/Star-Neutral): Primary winding connected in Delta (D), secondary winding connected in Star with a neutral wire (yn). The phase shift angle between the primary and secondary is 30° (secondary leads primary, pointing to the 11 o'clock position on a clock face).

2. The 'Achilles' Heel': Triplen Harmonics
Among all types of harmonics, the most terrifying for 3-phase 4-wire systems are triplen harmonics (3rd, 9th, 15th, etc.).
Fundamental (1st), 5th, and 7th harmonics in the 3 phases are 120° out of phase and cancel each other out at the neutral point if the load is balanced. However, triplen harmonics are completely in phase (0°).
Consequence: Instead of canceling out, the 3rd harmonic currents from all 3 phases algebraically add up on the neutral wire. The neutral wire current can soar to 1.73 times, or even twice, the phase current, causing the neutral wire to burn out even when the load is very balanced!

3. Impact of Vector Groups on Harmonics: Dyn11 vs Yyn0

A. Yyn0 Transformer (The 'Abettor')
When 3rd harmonic currents flow toward the secondary side (yn), they require a corresponding induced current on the primary side to balance the magnetic flux according to Lenz's law. However, the Y-connected primary side lacks a neutral wire (or the neutral is not grounded back to the source)—meaning 3rd harmonic currents (zero sequence) have no path to travel on the primary side.
Because there is no opposing induced current, the 3rd harmonic magnetic flux is not canceled out; instead, it escapes the steel core and closes its circuit through mechanical parts and the transformer tank body.
Consequence: The transformer suffers from magnetic saturation, maximum overheating, loud humming, rapid oil heating, and severe capacity reduction. The secondary voltage suffers from severe sinusoidal waveform distortion.

B. Dyn11 Transformer (The 'Harmonic Killer')
This is why EVN (Electricity of Vietnam) currently almost mandates the use of Dyn11 for modern distribution substations: when 3rd harmonic currents flow toward the secondary side (yn), they induce a current on the primary side (D). Because the primary side is connected in Delta—a closed-loop circuit—the 3rd harmonic currents will automatically circulate around inside the Delta winding and cancel each other out right there, absolutely preventing them from escaping into the primary grid.
Result: Harmonic magnetic flux is canceled, the steel core does not saturate, the secondary voltage maintains a smooth sinusoidal waveform, and the transformer runs much cooler and quieter.

4. Practical Field Analysis Example
Problem: Designing power for a Grade A office building. The loads primarily consist of computers, electronic circuit boards, LED lights, and a Chiller air conditioning system using VFD inverters. The expected total rated current is 1000A. The total harmonic distortion of current (THDi) measured is 30%, mainly consisting of 3rd harmonics.

Option 1 – Using a Yyn0 Transformer (Old mindset):
Estimated total 3rd harmonic current flowing into the neutral wire: $3 \times (1000\text{A} \times 30\%) = 900\text{A}$.
Field phenomena: The neutral wire becomes boiling hot, showing signs of melted PVC insulation even though phase currents have not reached rated load. The transformer emits a very loud 'humming' noise and heats up abnormally (oil temperature surges past $90^\circ\text{C}$ even at 70% load). Voltage at floor panels is distorted, causing sensitive computer power supplies and UPS units to continuously report errors and reboot.

Option 2 – Switching to a Dyn11 Transformer (Expert design):
The 900A of 3rd harmonic current flowing to the neutral still exists, but once it reaches the transformer, all of this harmonic energy is 'captured' and canceled within the closed-loop primary Delta (D) winding.
Field phenomena: The transformer runs smoothly and quietly, with oil temperature stabilizing at a safe level ($65–70^\circ\text{C}$). Voltage quality (THDv) at the main MSB switchboard busbar drops significantly (below 5% according to Circular 39/EVN standards), safely protecting all high-tech equipment in the building.

5. Essential Advice for Design and Operations Engineers

- Always prioritize Dyn11: For all civil projects (buildings, shopping malls) and industrial projects (factories with many inverters, robots), firmly lock in Dyn11 in the technical specifications.

- Phase Unbalance Handling Capacity: Dyn11 allows phase unbalance up to 100% of that phase's rated capacity, whereas Yyn0 only allows a maximum unbalance of 10–15%. With Vietnam's power grid where phase balancing is only relative, Dyn11 is the safest shield.

- K-Factor Rating: If a system has excessive harmonic levels (THDi > 35%), in addition to selecting the Dyn11 winding configuration, it is mandatory to choose a transformer with a K-Factor rating (e.g., K-4, K-13) so that it can withstand high-frequency currents without overheating.