Different Types of Primary and Secondary Connections of Transformer

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Different Types of Primary and Secondary Connections of Transformer

Q. State the suitability of following type of 3-phase transformer: (i) Y-Y (ii) Y-∆  (iii) ∆– ∆ (iv) ∆ – Y

Q. Draw the connection diagram, symbol and phasor diagram of phasor group-1 (New phase displacement) of transformer connection (i) Y-Y (ii) ∆–∆ (iii) ∆-Z

Q. Explain Zig-Zag connection of transformer.

Types of Primary and Secondary Connections

       A) Y-Y Connection of Transformer

       B) Y-∆ Connection of Transformer

       C) ∆–∆ Connections of Transformer

       D) ∆-Y Connection of Transformer

       E) Zig-Zag Connection of Transformer


Y-Y Star Star Connection of Transformer

This is typically for 3-phase 4 wire system. Distribution transformer 11 kV / 400 V. On the secondary side line to line voltage is 400 V and line to neutral voltage is 230 V hence suitable for 3 ph 400 V power supply as well as 230 V single phase supply.

Y-Y Star Star Connection of Transformer

  • Noted from the phasor diagram that there is no phase displacement between primary and secondary (they are in phase).
  • This connection is not physible for 3 phase 3-wire system. Due to 3rd harmonic the flux is not purely sinusoidal therefore the secondary voltages are distorted.
  • The resultant line to neutral voltage may exceed the rated value. This may result into stresses in insulation.
  • If the loads are unbalance this also causes unbalance into line to neutral voltage in turn again stresses in insulation.
  • Problems of unbalance voltages and 3rd harmonic can be solved by solid grounding of neutrals or adding additional ∆-connected tertiary winding.

∆–∆ Delta Delta Connection of Transformer

As noted from the phasor diagram line voltage and phase voltages are same (both sides). As compared with Y-Y connection in each phase winding the number of turns are more in ∆ – ∆ connection but as (Ip = IL
/ √3) the K.S. area of conductor required is less than Y-Y connection.

∆-∆ Delta Delta Connection of Transformer

  • For lower voltage ratings but high capacity requirements ∆–∆ transformers prove to be economical.
  • Third harmonic effect is damped in closed ∆ connection.
  • Artificial neutral is to be provided.
  • There is a zero phase-displacement in primary and secondary voltages. (See the phasor diagram Figure).
  • Care is to be taken in paralleling the two ∆ – ∆ type transformers that there should be same phase displacement.
  • If one connection disconnects can operate open ∆ with 58% of normal rating.

Y – ∆ Star Delta Connections of Transformer

As seen in the connection diagram VL = √3 Vp in primary but VL = Vp in secondary IL = IP in primary but IL = √3 Ip in secondary.

Y - ∆ Star Delta Connections of Transformer

  • Third harmonic problem is suppressed. (This is because it is consumed in circulating current in ∆ winding).
  • The connection is suitable for balance or unbalance load conditions.
  • As seen in the phasor diagram, there is a phase displacement of – 30° between primary and secondary voltages.
  • For successful parallel operation care shall be taken to both transformer must be wound for the same phase displacement.
  • This connection is suitable for high voltage to low voltage stopping. In receiving station the transformer is Y-∆ type.

∆ -Y Delta Star Connection of Transformer

∆ -Y Delta Star Connection of Transformer

  • In primary side VP = VL same, but in secondary side VP = VL / √3
  • In primary side IP = IL / √3 where as in secondary side IP = IL same.
  • As noted in phasor diagram there is phase displacement of + 30° between primary and secondary voltages.
  • In this case also there is no problem of 3rd harmonic as ∆ winding helps in stabilizing the potential of star-point.
  • Useful in sending end and receiving end substations in power system.
  • Distribution substation receives ∆– 11 kV and steps down to 400 V, 3 phase 4 wire supply. d400 V, 3 phase as well as single phase 230 V supply.

Zig-Zag Connection of Transformer (Interconnected Y-Connection)

Zig-Zag Connection of Transformer (Interconnected Y-Connection)


phasor diagram of Zig Zag connection of Transformer

  • In this type of connection the secondary winding is Y connected but divided into two halves. Such 6 halves of 3 windings are connected as shown in the Figure (Primary is normal ∆ connected or may be Y normally connected).
  • If voltage of each halve is V/2 then.
  • Resultant of each phase = (√3 / 2)V and therefore the secondary line voltage.

       =   √3 x (√3/2) v   =   (3/2) v

  • Resultant of a ; b’ is displaced by 30° from a.
  • Similarly the other resultants are also displaced by 30° from b and c by 30° which is shown in the above phasor diagram.
  • Use of this type Zig-Zag-star connection is to remove the phase displacement of 30° between ∆-Y connection (see Figure).

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