Transformer connections play a crucial role in the efficient and reliable operation of power systems. The choice of connection type directly impacts voltage regulation, phase shift, and fault current levels. In this blog post, we will explore the various types of transformer connections and their unique characteristics.

Winding Configurations

Star (Wye) Connection

In a star or wye (Y) connection, the three transformer windings are connected at a common point, forming a neutral connection. The other ends of the windings are brought out as line terminals. The star connection is denoted by the symbol “Y” and is commonly used in three-phase power systems. Some key characteristics of the star connection include:

• The phase voltage in a star-connected winding is equal to the line voltage divided by √3.
• The line current is equal to the phase current.
• The neutral point allows for the connection of a neutral conductor, which is essential for serving unbalanced loads and providing a return path for harmonic currents.
• Star-connected transformers have a higher voltage rating compared to delta-connected transformers of the same power rating.
• The star connection is suitable for applications requiring a neutral connection, such as in distribution transformers and four-wire three-phase systems.

Delta Connection

In a delta (Δ) connection, the three transformer windings are connected end-to-end, forming a closed loop or delta configuration. The delta connection is denoted by the symbol “Δ” and is widely used in three-phase power systems. Some key characteristics of the delta connection include:

• The line voltage is equal to the phase voltage in a delta-connected winding.
• The line current is equal to the phase current multiplied by √3.
• There is no neutral connection in a delta-connected transformer, making it unsuitable for serving unbalanced loads or providing a return path for harmonic currents.
• Delta-connected transformers have a higher current rating compared to star-connected transformers of the same power rating.
• The delta connection is suitable for applications where a neutral connection is not required, such as in transmission systems and three-wire three-phase systems.

Three-Phase Transformer Connections

Three-phase transformers are widely used in power distribution systems to step up or step down voltages and transfer electrical energy between circuits. The four most common types of three-phase transformer connections are:

1. Star-Star (Y-Y) Connection
2. Delta-Delta (Δ-Δ) Connection
3. Star-Delta (Y-Δ) Connection
4. Delta-Star (Δ-Y) Connection

Star-Star (Y-Y) Connection

In a star-star (Y-Y) connection, both the primary and secondary windings of the transformer are connected in a star or wye configuration. The neutral points of the primary and secondary windings are typically grounded or connected together. Some key characteristics of the Y-Y connection include:

• The primary and secondary line voltages are √3 times the respective phase voltages.
• The phase currents and line currents are equal in magnitude.
• The neutral connection allows for the supply of unbalanced loads without causing voltage imbalances.
• Ground faults on the secondary side do not induce high voltages on the primary side, making it safer.

Y-Y connected transformers are commonly used in high-voltage transmission systems and distribution networks where a neutral connection is required to serve unbalanced loads or provide a ground reference.

Delta-Delta (Δ-Δ) Connection

In a delta-delta (Δ-Δ) connection, both the primary and secondary windings of the transformer are connected in a delta or mesh configuration. The key characteristics of the Δ-Δ connection are:

• The primary and secondary line voltages are equal to the respective phase voltages.
• The line currents are √3 times the phase currents.
• No neutral connection is available, making it unsuitable for serving unbalanced loads.
• Third harmonic currents and their multiples circulate within the delta windings, providing a path for their elimination.

Delta-delta connected transformers are commonly used in power distribution systems where the loads are predominantly balanced and where isolation from ground faults is desired.

Star-Delta (Y-Δ) Connection

In a star-delta (Y-Δ) connection, the primary winding is connected in a star (wye) configuration, while the secondary winding is connected in a delta configuration. The main characteristics of the Y-Δ connection include:

• The secondary line voltage is √3 times the primary phase voltage, and the secondary phase voltage is equal to the primary phase voltage.
• The secondary line current is equal to the primary phase current, and the secondary phase current is √3 times the primary phase current.
• There is a 30° phase shift between the primary and secondary voltages, with the secondary leading the primary.
• The delta secondary provides a path for third harmonic currents and their multiples to circulate, preventing their flow into the primary side.

Star-delta connected transformers are often used in step-down applications, such as distribution substations, where a lower secondary voltage is required, and the neutrals on the primary and secondary sides do not need to be connected.

Delta-Star (Δ-Y) Connection

In a delta-star (Δ-Y) connection, the primary winding is connected in a delta configuration, while the secondary winding is connected in a star (wye) configuration. The main characteristics of the Δ-Y connection are:

• The secondary phase voltage is equal to the primary line voltage divided by √3, and the secondary line voltage is equal to the primary line voltage.
• The secondary phase current is equal to the primary line current, and the secondary line current is the primary line current divided by √3.
• There is a 30° phase shift between the primary and secondary voltages, with the primary leading the secondary.
• The star-connected secondary allows for the supply of unbalanced loads and provides a neutral point for grounding or earthing.

Delta-star connected transformers are commonly used in step-up applications, such as in power generation plants, where the generated voltage needs to be increased for transmission over long distances.

Special Connections and Applications

While the most common transformer connections are star-star (Y-Y), delta-delta (Δ-Δ), star-delta (Y-Δ), and delta-star (Δ-Y), there are special transformer connections designed for specific applications. These connections offer unique advantages in terms of load balancing, voltage regulation, and fault protection.

Open-Delta Connection

The open-delta connection, also known as the V-V connection, is a three-phase transformer connection that uses only two single-phase transformers. This configuration is often employed when a limited amount of three-phase power is required, and the load is not expected to be fully balanced.

In an open-delta connection, the primary windings of the two transformers are connected in series, forming a closed loop. The secondary windings are also connected in series, but with a phase shift of 180 degrees between them. This arrangement allows the transformers to supply three-phase power to the load, albeit with reduced capacity compared to a full three-phase system.

One key advantage of the open-delta connection is its ability to continue supplying power even if one of the transformers fails. However, this connection has some limitations, such as reduced load capacity, increased voltage imbalance, and potential overheating of the transformers under unbalanced load conditions.

Zig-Zag Connection

The zig-zag connection, also called the interconnected star connection, is a special transformer connection used primarily for grounding and providing a neutral connection in three-phase systems. This configuration is achieved by connecting the primary and secondary windings of three single-phase transformers in a specific pattern, creating a zig-zag arrangement.

In a zig-zag connection, the primary windings are connected in series, while the secondary windings are divided into two halves. These halves are connected in a zig-zag pattern, with each half of the secondary winding connected to a different phase. This unique arrangement allows for the cancellation of triplen harmonics (3rd, 9th, 15th, etc.) in the secondary voltages.

The zig-zag connection is particularly useful in providing a stable neutral point for unbalanced loads and minimizing the impact of harmonics on the power system. It also helps in reducing the fault current during ground faults, as the zero-sequence impedance of the zig-zag transformer is relatively high.

Scott-T Connection

The Scott-T connection, named after its inventor Charles F. Scott, is a special transformer connection used to convert three-phase power to two-phase power, or vice versa. This connection is particularly useful in applications where two-phase loads, such as older industrial motors or railway traction systems, need to be powered from a three-phase supply.

In a Scott-T connection, two single-phase transformers with specific winding ratios are used. The primary windings of both transformers are connected to the three-phase supply, with a phase shift of 90 degrees between them. The secondary windings of the main transformer (T-connected) provide the two-phase output, while the secondary winding of the teaser transformer (connected in series with one of the primary windings) helps to maintain the proper phase relationships.

The Scott-T connection offers several advantages, such as balanced loading of the three-phase supply, isolation between the primary and secondary circuits, and the ability to supply two-phase loads from a three-phase source without the need for a complex phase conversion process.

How a power transformer is connected

Step 1: Identify the transformer windings

The first step in connecting a power transformer is to identify the primary and secondary windings. The primary winding is connected to the power source, while the secondary winding provides the output voltage. In three-phase transformers, there are three sets of windings for each phase. The transformer nameplate should specify the voltage ratings and connection types for each winding.

Step 2: Determine the required connection type

Power transformers can be connected in various configurations, such as delta-delta, star-star, delta-star, or star-delta, depending on the application requirements. For example, delta-delta connections are commonly used in distribution transformers for their ability to handle unbalanced loads and provide a stable neutral connection.

Step 3: Connect the primary windings

Once the connection type is determined, the primary windings are connected according to the chosen configuration. In a delta connection, the windings are connected in a closed loop, with each phase connected to the next. In a star connection, one end of each winding is connected to a common neutral point, while the other ends form the three phases. The primary line voltages and phase voltages will depend on the connection type and the transformer’s voltage ratio.

Step 4: Connect the secondary windings

The secondary windings are connected in the same configuration as the primary windings, following the same principles for delta or star connections. The secondary voltage and current ratings will depend on the transformation ratio and the load requirements. In some cases, a tertiary winding may be present for additional functionality, such as supplying auxiliary loads or improving voltage regulation.

Step 5: Establish a neutral connection

In star-connected transformers, the neutral point of the star is often grounded to provide a reference point and to facilitate the detection and isolation of ground faults. The neutral connection also allows for the supply of single-phase loads in a three-phase system. In delta-connected transformers, a neutral connection is not necessary, as the closed loop provides a return path for currents.

Step 6: Verify phase relationships

Before energizing the transformer, it is crucial to verify the phase relationships between the primary and secondary windings. Incorrect phase connections can lead to phase shifts, circulating currents, and inefficient operation. Phasor diagrams and phase sequence tests can be used to confirm the correct phase relationships and to ensure that the transformer will operate as intended.

Step 7: Test and commission the transformer

After the connections are complete, the transformer should undergo a series of tests and commissioning procedures to ensure its proper functioning and safety. These tests may include insulation resistance tests, turns ratio tests, and load tests. Once the transformer passes all the necessary tests, it can be energized and integrated into the power system.

What Is the Most Common Transformer Connection

The most common transformer connection for three-phase power systems is the delta-wye (Δ-Y) configuration. This connection is widely used in both step-up and step-down transformers, particularly in distribution transformers that supply power to residential and commercial settings.

Which Type of Connection Is Used in a Power Transformer

Power transformers, which are crucial components in electrical power transmission and distribution systems, commonly employ the delta-wye (Δ-Y) connection. This connection type is suitable for both generator step-up (GSU) transformers at power generation facilities and step-down transformers at substations.

In some cases, power transformers may also utilize the wye-wye (Y-Y) connection, particularly when a neutral point is required on both the primary and secondary sides. However, the delta-wye connection remains the most prevalent choice for its versatility and ability to mitigate harmonic distortion.

How to Tell if a Transformer Is Delta or Wye

To determine whether a transformer is connected in a delta or wye configuration, you can follow these steps:

1. Consult the transformer’s nameplate: The nameplate often specifies the connection type, such as “Δ-Y” for delta-wye or “Y-Y” for wye-wye.
2. Count the number of bushings or terminals: A delta-connected winding typically has three bushings, while a wye-connected winding has four bushings (three phases and a neutral).
3. Measure the voltage between phases and between phase and neutral: In a delta connection, the phase-to-phase voltage is equal to the phase voltage. In a wye connection, the phase-to-neutral voltage is equal to the phase voltage divided by the square root of three (1.732).
4. Perform a phase sequence test: The phase sequence of the primary and secondary windings can help identify the connection type. In a delta-wye transformer, the secondary phase sequence is typically shifted by 30 degrees with respect to the primary sequence.