What is Open Delta (V-V) Transformer Connection? Complete Guide with Diagram

What is an Open Delta Connection?

Definition

Open Delta connection also known as V-V connection is a method of attaining three phase (3 phase) power transformation using only 2 single phase transformers instead of the conventional 3 transformers required in a delta-delta (D-D) connection.

Open Delta Configuration

The open delta configuration represents an innovative and practical method to 3 phase power supply providing a cost-effective solution for specific applications. 

This connection derives its alternative name V-V from the visual representation of two transformers connected in a V-shaped arrangement on secondary side. 

Unlike other delta-delta transformer banks that require three transformers operating in parallel the open delta configuration achieves complete three phase power transformation using just 2 single phase transformers making it an economical alternative in certain conditions.

How Open Delta Connection works?

In an open delta connection 3 phase supply is connected to the primary sides of two single phase transformers. 

The transformers are connected such that they form an incomplete (open) delta configuration. 

The secondary sides of these transformers are arranged to provide a three phase output with voltages that are equal in magnitude but differ in phase by 120 degrees.

When no load is connected to the secondary side three equal voltages appear across the secondary terminals. 

However the essential termination from a balanced delta-delta connection becomes evident when analyzing the power handling capacity and voltage characteristics under loaded conditions.

Why open Delta?

Open deltas only need the utility to install 2 transformers. 

Future Capacity can be increased by simply installing a 3^rd similar sized transformer vs installing 2 – 3 larger transformers.

How Open Delta connection configured?

2 single phase transformers are connected with their primary windings receiving three-phase supply (A, B, C phases). 

The secondary windings are interconnected to produce three phase output.

One transformer carries phases A-B while the second transformer carries phases B-C creating the open delta arrangement. 

The missing transformer (C-A phase) is what creates the “open” configuration.

Load Carrying Capacity – 57.7% Rule

One of the most significant characteristics of open delta connection is its reduced load carrying capacity compared to delta-delta connection. 

This reduction is expressed mathematically and represents one of the key tradeoff when using this configuration.

Capacity Calculation

Delta-Delta Connection

S_DD = √3 x V_L x I_L = √3 x V_L x I_ph

S_DD = 3 x V_L x I_ph

Where

S_DD – Delta-Delta Connection

V_L – Line Voltage

I_L – Line Current

I_ph – Phase Voltage

Open Delta Connection

S_OD = √3 x V_L x I_L = √3 x V_L x I_ph

Where

S_OD – Open Delta Connection

Capacity Ratio

S_OD / S_DD = √3 x V_L x I_ph / (3 x V_L x I_ph)

S_OD / S_DD = √3 / 3 = 1/√3 = 0.577

Therefore open delta carries only 57.7% of the load that a delta-delta bank can carry over.

The basis for establishing the efficacy of open delta connections is this 57.7% capacity rating.

While this reduction may seem significant it is often acceptable for applications where the initial load is small and there is potential for future expansion.

Transformer Overload Analysis

When transitioning from a three transformer delta-delta bank to an open delta configuration (whether intentional / due to failure) the remaining 2 transformers experience increased stress. Understanding this overload is important for ensuring safe operation.

Overload Factor Calculation

Total Load / Individual Transformer VA Rating = √3 x V_L x I_ph / (V_L x I_ph)

Overload Factor = √3 = 1.73

Each transformer in an open delta configuration experiences approximately 1.73 times the current it would manage in a balanced three transformer delta-delta bank.

This 1.73 x overload is only suitable for short-term and temporary operation.

Continuous operation at this overload level will cause excessive heating and potential breakdown of the transformers. 

Proper load management and cooling provisions are so essential.

Power Factor Considerations

The 2 transformers operate at separate power factors which distinguishes open delta connections.

This asymmetry creates imbalances in the system that should be understood and managed.

Transformer Power Factors

• Transformer 1

cos(30° – φ) with delivered power = kVA x cos(30° – φ)

• Transformer 2

cos(30° + φ) with delivered power = kVA x cos(30° + φ)

Where 

φ – Load Power Factor Angle

The 30-degree phase shift between the two transformers operating points results from the geometry of open delta configuration. 

This means that even with a balanced load the 2 transformers share the power unequally with one transformer typically carrying more of the reactive burden than the other.

Advantages of Open Delta Connection

• When three phase loads are too small to justify the cost of a full three transformer delta-delta bank.

• When one transformer in a D-D bank becomes damaged (or) fails the remaining 2 transformers can temporarily continue service at the reduced capacity.

• When anticipated load growth will eventually necessitate the installation of a full three transformer bank and an open delta setup can bridge the gap cost-effectively

• Significant savings on transformer procurement, installation and maintenance expenses.

• In installations where physical space is limited using two transformers instead of three transformers provides practical advantages.

Disadvantages of Open Delta Connection

• Secondary terminal voltages become an unbalanced under load conditions creating unequal voltage distribution to the connected equipment.

• The two transformers utilize different power factors (cos(30°-φ) and cos(30°+φ) resulting in unequal power distribution & reactive power circulation.

• Maximum power handling capacity is limited to the 57.7% of a delta-delta configuration.

• Each transformer experiences approximately 1.73 x overload limiting the operational duration.

• Under typical load levels power handling is reduced by approximately 15.5%.

• The asymmetrical loading can contribute to harmonic generation & distortion.

• Cannot be used as a permanent solution for the sustained three-phase power distribution.

Comparative Analysis

Parameter	Delta-Delta Connection	Open Delta Connection
Number of Transformers	3	2
Load Capacity	100%	57.7%
Initial Cost	Higher	Lower (33% savings)
Voltage Balance	Balanced	Unbalanced under load
Transformer Overload Factor	1.0	1.73
Suitable for Permanent Installation	Yes	No (Temporary only)
Power Factor	Equal for all units	Unequal (cos(30°±φ))

Applications of Open Delta Connection

When implementing open delta connections several practical considerations must be addressed to ensure safe and reliable operation that are:

• Load Monitoring,

• Cooling Requirements,

• Voltage Regulation,

• Phase Sequence,

• Future Planning &

• Documentation.

What is the difference between Open Delta and Closed Delta?

Features	Open Delta Connection (V-V)	Closed Delta Connection (Δ-Δ)
Number of transformers used	Uses 2 transformers	Uses 3 transformers
Circuit shape	Forms an incomplete delta (V-shape)	Forms a complete delta (triangle)
Power capacity	Used to deliver merely 57.7% of the closed delta’s rated capacity	Delivers 100% of its rated capacity
Cost	Lower cost (one transformer saved)	Higher cost (needs 3 transformers)
Efficiency	Lower efficiency	Higher efficiency
Voltage Regulation	Poorer regulation	Better voltage regulation
Power factor handling	Performs poorly with a low power factor (PF) loads	Handles low power factor (PF) loads better
Load balance	More sensitive to the unbalanced loads	Used to handle unbalanced loads better
Reliability	Used as emergency operation if one transformer fails	More reliable under normal operation
Losses	Higher losses per kVA delivered	Lower losses per kVA delivered
Typical applications	Temporary supply, emergency service, rural loads	Industrial plants, substations, continuous duty
Common name	Also called V–V connection	Also called Delta–Delta connection

Conclusion

Open delta (V-V) transformer connection represents a practical and economical solution for the specific three phase power distribution conditions. 

With its ability to deliver a complete three phase power using only two transformers it provides significant cost savings and flexibility. 

However, the inherent limitations including reduced load capacity (57.7%), voltage imbalance, unequal power factor operation and transformer overload (1.73 x) making it suitable only for temporary (or) short-term applications (or) when loads are genuinely small.