Module 1 · Lesson 2
Transformers in Distribution
Power Distribution & Protection Design — Book 4 of the Electrical Series
Why this lesson matters: Every fault current calculation, every protection coordination study, and every
parallel-transformer decision later in this book depends on getting the connection type and vector group right.
A mismatched vector group between two transformers isn't a paperwork detail — it can prevent paralleling
entirely or cause severe circulating currents if forced.
1. Common Distribution Transformer Connections — Click to Compare
Select a connection type above.
Field mistake seen often: Two transformers are paralleled onto the same bus because both are "400V, same
kVA rating," without checking that their vector groups actually match. Even a 30° phase displacement mismatch
between them drives large circulating currents the moment both are connected live — sometimes large enough to
trip protection immediately, sometimes just quietly overheating both units.
2. Vector Groups — Interactive Clock Notation
Vector groups describe the phase displacement between primary and secondary windings using clock-face notation
(Dyn11 means Delta primary, Wye secondary, 11 o'clock = 330° lead, etc). Click a clock position to see the phase
relationship.
| Common Vector Group | Typical Application |
|---|
| Dyn11 | Very common distribution transformer group (Delta primary, Wye-grounded secondary) — the neutral point on the secondary supports single-phase loads |
| Dyn1 | Same connection family, opposite phase displacement direction — matters for paralleling with other Dyn1 units, not with Dyn11 |
| Yyn0 | Wye-Wye with grounded secondary neutral — less common for distribution due to third-harmonic and unbalanced-load limitations without a tertiary winding |
| Dyn5 / Dyn7 | Less common phase displacements sometimes seen on older or region-specific installations — always verify before assuming standard Dyn11 |
Field practice for paralleling transformers: Match vector group, turns ratio, and percent impedance
before ever attempting to parallel two transformers onto the same bus. Vector group and ratio mismatches cause
circulating current; impedance mismatches cause unequal load sharing even when everything else matches.
3. Transformer Sizing — Quick Field Calculator
Field mistake seen often: A transformer is sized exactly to today's connected load total with zero
diversity factor and zero growth allowance. The transformer then runs at or near its nameplate rating from day
one, offering no headroom for load growth and reducing insulation life due to running consistently hot.
4. Common Mistakes
Mistake 1 — Paralleling transformers with mismatched vector groups
Even transformers with identical voltage ratings and kVA can be electrically incompatible
for paralleling if their vector groups differ in phase displacement. Always confirm vector group compatibility
before closing a tie breaker between two sources.
Mistake 2 — Ignoring percent impedance mismatch when paralleling
Transformers with different percent impedance values sharing a bus will not share load
proportionally to their kVA ratings — the lower-impedance unit takes a disproportionate share of the load,
potentially overloading it while the other runs comfortably under capacity.
Mistake 3 — Sizing with no diversity factor or growth allowance
As shown in the sizing mistake box above, ignoring diversity factor (not all connected
load runs simultaneously at full demand) and future growth leads to a transformer that's either oversized and
inefficient or undersized with no headroom — both are avoidable with basic planning inputs.
Knowledge Check
1. Two transformers have identical voltage and kVA ratings but different vector groups. Can they be safely paralleled?
Yes, voltage and kVA matching is all that's required
Not safely — mismatched vector groups cause phase displacement that drives circulating currents
Only if they're both Delta-primary
2. Two correctly vector-matched transformers with different percent impedance are paralleled. What happens to load sharing?
Load always splits exactly 50/50 regardless of impedance
The lower-impedance unit takes a disproportionate share of the load
Load sharing is unaffected by impedance differences
3. Why include a diversity factor when sizing a distribution transformer for total connected load?
Diversity factor only matters for residential installations
Not all connected loads run simultaneously at full demand, so sizing to the raw total often overstates actual peak demand
Diversity factor increases the required transformer size, never decreases it
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