Three-phase transformer connections

A three-phase transformer (or bank of three single-phase units) can have its primary and secondary windings connected in star (Y) or delta (Δ), giving four common connections. Let $K$ = turns ratio per phase ($N_2/N_1$) and $V_L$, $I_L$ = primary line voltage and current. In the sketches, S is one phase winding.

1. Star-Star (Y-y)

 Primary (Y)        Secondary (y)
  A   B   C           a   b   c
  |   |   |           |   |   |
  S   S   S           S   S   S
   \  |  /             \  |  /
    \ | /               \ | /
      N                   n

• Phase voltage = $V_L/\sqrt3$, so less insulation is needed; a neutral is available on both sides.
• Secondary line voltage = $KV_L$, with no phase shift (0°).
• Problems: third-harmonic voltages in the phase voltages, and neutral shifting with unbalanced loads, unless the primary neutral is earthed or a tertiary delta winding is added.
• Use: small high-voltage transformers, and interconnecting systems where both neutrals are needed (with a tertiary winding).

2. Delta-Delta (D-d)

 Primary (D)          Secondary (d)
  A ----+              a ----+
       / \                  / \
      S   S                S   S
     /     \              /     \
  B +---S---+ C        b +---S---+ c

• Line voltage = phase voltage; winding current = $I_L/\sqrt3$.
• No third-harmonic problem (the third-harmonic current circulates inside the delta), and it handles unbalanced loads well.
• If one unit fails, the remaining two can work in open delta (V-V) at 57.7% of the bank rating.
• No neutral is available. Use: large low-voltage, high-current systems and industrial supply.

3. Star-Delta (Y-d)

 Primary (Y)          Secondary (d)
  A   B   C            a ----+
  |   |   |                 / \
  S   S   S                S   S
   \  |  /                /     \
    \ | /              b +---S---+ c
      N

• Primary star, secondary delta. Secondary line voltage = $\frac{KV_L}{\sqrt3}$, with a 30° phase shift.
• The delta secondary suppresses third harmonics; the primary neutral can be earthed.
• Use: step-down transformers at the receiving end of transmission lines (substations).

4. Delta-Star (D-y)

 Primary (D)          Secondary (y)
  A ----+               a   b   c
       / \              |   |   |
      S   S             S   S   S
     /     \             \  |  /
  B +---S---+ C           \ | /
                            n  (neutral brought out)

• Primary delta, secondary star. Secondary line voltage = $\sqrt3 KV_L$, with a 30° phase shift.
• The star secondary gives a neutral for a 3-phase, 4-wire supply.
• Use: distribution transformers (e.g. 11 kV/400 V in Nepal) and step-up transformers at the generating station.

Summary

Other connections: open delta (V-V) for emergencies or growing loads, and the Scott (T-T) connection for 3-phase to 2-phase conversion.