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Open Delta or V-V Connection of Transformer

Open delta or V-V connection of 3 ϕ transformer: V-V connection has only two windings instead of three windings in both primary and secondary.
It is used when one winding of the transformer goes into maintenance.

Power transfer capacity reduces to 57.7%(or 1/√ 3) times of Δ-Δ connection or closed delta connection.

Open Delta or V-V Connection of Transformer

If one transformer is removed in the Δ-Δ connection of three single-phase transformers, the resulting connection becomes an open delta or V-V connection. In a complete Δ-Δ connection, the voltage of any one phase is equal and opposite to the sum of the voltages of the other two phases. Therefore, under no-load conditions, if one transformer is removed, the other two will maintain the same three-line voltages on the secondary side. Under load conditions, the secondary line voltages will be slightly unbalanced because of the unsymmetrical relation of the impedance drops in the transformers.

For a complete Δ-Δ connection

When all three windings of the transformer in the circuit
Let,
Vph – Phase Voltage of each secondary
Iph – Phase current of each secondary
In a closed delta connection, the line voltage is equal to the phase voltage.
VL = Vph
VA rating of the closed delta transformer
= √3 VL IL
= 1.732 Vph x √3 Iph
= 1.732 Vph x 1.732 Iph
= 3 Vph Iph      ………..(a)

Open Delta Connection

In a closed delta connection, the line current is 1.732 times the phase current, but in an open delta connection, the line current is equal to the phase current.
VL = √3 Vph

VA rating of the open delta transformer

VA rating of the open delta transformer can be calculated as follows.

The KW rating of the open delta transformer is;

=Vph Iph Cos(30-Φ) + Vph Iph Cos(30-Φ)
= Vph Iph [ Cos(30-Φ)+Cos(30-Φ)]

= Vph Iph [ 2Cos30 CosΦ]
=Vph Iph[ 2 x√3/2 CosΦ]
= Vph Iph[ √3 CosΦ]
= √3 Vph Iph CosΦ
= √3 Vph IphCosΦ   ——(1)

From equation(1), the VA rating of the open delta transformer is;

= √3 Vph Iph
=  1.732 Vph Iph
= 1.732 Vph Iph

The relationship between the VA rating of open delta and close delta connection is as follows.

VA rating of Open Delta / close Delta

VA rating of open delta transformer
From above it is clear that the VA delivering capacity of the open delta transformer is 57.7 % of the VA delivering capacity when all three transformers are connected in the delta.

Transformer Utility Factor(TUF)
The ratio of actual VA available to the total VA available in open delta connection is known as the transformer utility factor(TUF).

Open Delta

Applications of Open Delta or Open Delta (V-V) Connection

The open delta circuit has a number of features that are advantageous. A few applications are given below by way of illustration:

  1. The circuit can be employed in an emergency situation when one transformer in a complete Δ-Δ circuit must be removed for repair and continuity of service is required.
  2. Upon failure of the primary or secondary of one transformer of a complete Δ-Δ circuit, the system can be operated as an open delta (V-V) circuit and can deliver 3-phase power (with reduced capacity) to a 3-phase load.
  3. A circuit is sometimes installed as an open delta (V-V) circuit with the understanding that its capacity may be increased by adding one more transformer to form a complete Δ-Δ circuit. As shown earlier,
  • Pv/PΔ = 1/√3 = 0.577
  • PΔ = √3 Pv

Thus if a V-V circuit is changed to a complete Δ-Δ circuit, the capacity is increased by a factor of √3.

Check out the given below pdf for the details of the Open Delta or V-V Connection of the Transformer.

 


I hope the above Hindi video lecture and the handwritten note can help you understand the Open Delta or V-V Connections. Stay with us for more educational video lectures and handwritten notes with free job alerts as well.

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