Open Delta connections; V-V connections

Open Delta (V–V) Transformer Connection

In electrical systems, we generally use single-phase transformers (with one winding on each side) and three-phase transformers (with three windings on each side). These windings can be connected in different configurations such as star–delta, delta–delta, or star–star.

There is also a special type of connection known as the Open Delta Connection (also called V–V Connection). This method uses only two transformer windings instead of three.

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What is an Open Delta (V–V) Connection?

• In an open delta configuration, only two transformers are connected in a three-phase system, while the third transformer winding is absent (or removed due to a fault).

• This connection is often used as a temporary solution when one transformer in a delta–delta bank fails, allowing the system to continue supplying power but at reduced capacity.

• Open delta transformers are also used for supplying small three-phase loads where installing a full three-transformer bank is uneconomical.

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Circuit Representation

• Delta–Delta Transformer Connection (3 windings)

• Open Delta (V–V) Transformer Connection (2 windings)

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Power Delivered by Open Delta Connection

For a Delta Connection:

$$I_{line} = \sqrt{3} \, I_{phase}$$ $$P_{\Delta} = 3 \, V_{L} \, I_{phase}$$

For an Open Delta (V–V) Connection:

$$I_{line} = I_{phase}$$ $$P_{V-V} = \sqrt{3} \, V_{L} \, I_{phase}$$

Therefore,

$$\frac{P_{V-V}}{P_{\Delta}} = \frac{\sqrt{3}}{3} = 0.577 \, (57.7\%)$$

👉 Hence, an open delta connection delivers only 57.7% of the power that a closed delta connection would deliver.

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System Power Factor

• In open delta, when the load power factor = 1, the system power factor reduces to 0.866.

• In a closed delta, the system power factor remains equal to the load power factor.

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Advantages of Open Delta Connection

1. Cost-effective for small loads – Instead of using three transformers, only two are required.

2. Continuity of supply – If one transformer of a delta–delta bank fails, the system can continue operation in open delta mode (with reduced load capacity).

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Disadvantages of Open Delta Connection

1. Reduced capacity – Delivers only 57.7% of the original delta-connected transformer’s power.

2. Reduced power factor – System power factor falls to 0.866 even if load has unity power factor.

3. Not suitable for large loads – Due to limitations in power delivery and system efficiency.

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Comparison Table: Delta vs. Open Delta

Parameter	Delta Connection (Δ–Δ)	Open Delta (V–V)
Number of Transformers	3	2
Power Delivered	100%	57.7%
System Power Factor (pf=1)	1.0	0.866
Application	Large Loads	Small Loads / Emergency use

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Applications of Open Delta Connection

• Temporary supply during failure of one transformer in a delta–delta bank.

• Economical solution for supplying small three-phase loads.

• Rural and remote areas where power demand is limited