# Three-Phase Transformer Connections: Y–Y, Δ–Δ, Y–Δ, Δ–Y, V–V, T-T

- Link: https://studyelectrical.com/2025/07/three-phase-transformer-connections-y-y-%ce%b4-%ce%b4-y-%ce%b4-%ce%b4-y-v-v-t-t.html
- Published: 2025-07-05T18:30:41+00:00
- Author: Tesla

Three-phase transformers are essential in power transmission and distribution systems.
They are designed to step up or step down voltages in three-phase systems, and the
type of connection used determines their performance under various load and system
conditions. This article discusses various **three-phase transformer connections**
including their working, advantages, and applications.

## Types of Three-Phase Transformer Connections

 * [Star-Star (Y–Y) Connection](https://studyelectrical.com/2014/11/star-star-connection-of-three-phase-transformer.html)
 * [Delta-Delta (Δ–Δ) Connection](https://studyelectrical.com/2025/07/delta-delta-%ce%b4-%ce%b4-transformer-connection-advantages-working-and-application.html)
 * Star-Delta (Y–Δ) Connection
 * Delta-Star (Δ–Y) Connection
 * [Open Delta (V–V) Connection](https://studyelectrical.com/2019/11/open-delta-v-v-connection-of-transformers.html)
 * Scott or T-T Connection

## 1. Star-Star (Y–Y) Transformer Connection

This connection is commonly used in **small, high-voltage transformers**. The primary
and secondary windings are both connected in a star (Y) configuration. It is economical
because each phase winding experiences only 1/√3 of the line voltage, which reduces
the insulation requirement.

![Star Star Connection](https://studyelectrical.com/wp-content/uploads/2025/07/Star-
Star-Connection.avif)

[Star Star Connection](https://studyelectrical.com/2014/11/star-star-connection-of-three-phase-transformer.html)

**Key Points:**

 * Line-to-line voltage ratio is same as the transformation ratio of each transformer.
 * There is a 30° phase shift between line and phase voltages.
 * Performs well under balanced load conditions only

**Limitations:**

 * Under unbalanced loading, the neutral point shifts, leading to unequal phase 
   voltages.
 * Power supply to single-phase loads becomes inefficient due to high impedance 
   in the isolated primary coils.
 * If the primary neutral is isolated, triple-frequency components of magnetizing
   current (third harmonics) have no return path, leading to waveform distortion.

**Solutions:**

 * Ground the primary neutral to the generator neutral to stabilize the voltage 
   and neutral point.
 * Use a tertiary delta-connected winding to allow third harmonic components to 
   circulate and maintain sinusoidal flux and voltage waveforms.

## 2. Delta-Delta (Δ–Δ) Transformer Connection

This connection is suitable for **large, low-voltage transformers**. Both the primary
and secondary windings are delta connected. It is preferred in industrial applications
for its robustness and ease of parallel operation.

![Delta delta transformer connection](https://studyelectrical.com/wp-content/uploads/
2025/07/Delta-delta-transformer-connection.avif)

[Delta delta transformer connection](https://studyelectrical.com/2025/07/delta-delta-%ce%b4-%ce%b4-transformer-connection-advantages-working-and-application.html)

**Key Advantages:**

 * No phase displacement between primary and secondary sides.
 * Third harmonic components of magnetizing current circulate within the delta loop,
   maintaining sinusoidal voltages.
 * Performs well under unbalanced load conditions.
 * If one transformer fails, the system can continue in open-delta (V–V) configuration
   with 58% of the total capacity.

## 3. Star-Delta (Y–Δ) Transformer Connection

This connection is used at the **receiving end (substations)** of transmission lines
for stepping down voltage. The primary is Y-connected and the secondary is Δ-connected.

![Star Delta connnection of transformer](https://studyelectrical.com/wp-content/
uploads/2025/07/Star-Delta-connnection-of-transformer.avif)

Star Delta connnection of transformer

 * The line voltage ratio is (1/√3) × transformation ratio of each transformer.
 * A 30° phase shift exists between primary and secondary line voltages.
 * Cannot be paralleled with Y–Y or Δ–Δ systems due to phase displacement.
 * Delta side provides path for third harmonic currents.

## 4. Delta-Star (Δ–Y) Transformer Connection

This connection is primarily used for **stepping up voltage** at the beginning of
transmission lines. The primary winding is delta connected, and the secondary is
star connected with the neutral grounded.

![Delta Star Connection](https://studyelectrical.com/wp-content/uploads/2025/07/
Delta-Star-Connection.avif)

Delta Star Connection

 * Common for 3-phase 4-wire distribution networks.
 * Supports both three-phase loads and single-phase lighting loads.
 * Third harmonic components are handled by the delta side, preventing waveform 
   distortion.
 * Not parallel compatible with Δ–Δ or Y–Y due to 30° phase shift.
 * Voltage ratio is √3 × the transformation ratio of each transformer.

## 5. Open Delta (V–V) Transformer Connection

The open-delta or [V–V connection](https://studyelectrical.com/2019/11/open-delta-v-v-connection-of-transformers.html)
is used when one transformer from a Δ–Δ bank becomes faulty or is removed. It enables
continued operation at reduced capacity (58%) without full system shutdown.

## 6. Scott or T–T Transformer Connection

This special connection is used to convert **three-phase power to two-phase power**
or vice versa. It is commonly used in systems involving two-phase motor loads or
arc furnaces.

## Comparison Table

| **Connection Type** | **Phase Shift** | **Harmonic Suppression** | **Unbalanced Load Handling** | **Neutral Availability** | **Typical Use Case** | 
| **Star-Star (Y–Y)** | 0° (but 30° phase difference between phase & line voltages) | Poor (needs tertiary winding or grounding) | Poor (neutral shift issues) | Yes (requires grounding) | High-voltage, small-load systems | 
| **Delta-Delta (Δ–Δ)** | 0° | Good (third harmonics circulate in delta) | Excellent | No | Industrial loads, low-voltage applications | 
| **Star-Delta (Y–Δ)** | 30° lag of secondary line voltage | Good | Good | No | Substation end of transmission line (step-down) | 
| **Delta-Star (Δ–Y)** | 30° lead of secondary line voltage | Good | Good | Yes (star neutral grounded) | Generator end (step-up), power distribution | 
| **Open Delta (V–V)** | Depends on original configuration | Fair | Fair (derated capacity) | No | Backup when one transformer fails in Δ–Δ | 
| **Scott (T–T)** | None (converts 3-phase to 2-phase) | Not applicable | Good | Depends on configuration | Interfacing 3-phase and 2-phase systems |

Comparison of Different types of three phase transformer connections.

## Conclusion

The choice of transformer connection in a three-phase system depends on the application,
voltage level, load conditions, and system compatibility. **Y–Y and Δ–Δ** are used
in balanced systems, **Y–Δ and Δ–Y** in substations and transmission, and **open
delta and Scott connections** in special or fault-tolerant setups. Understanding
the advantages and limitations of each connection helps electrical engineers design
efficient and resilient power systems.

## FAQs

**Q1. Why is the Y–Y connection not ideal for unbalanced loads?**
 Because the neutral
shifts under unbalanced conditions, causing unequal phase voltages.

**Q2. Which transformer connection is suitable for stepping down voltage?**
 Y–Δ
connection is most commonly used at substations for stepping down voltage.

**Q3. What is the use of an open-delta connection?**
 It allows continued system
operation at 58% capacity if one transformer in a Δ–Δ set fails.

**Keywords:** three-phase transformer connections, Y–Y transformer, Delta-Delta 
transformer, star-delta transformer, delta-star transformer, open delta transformer,
Scott connection, transformer phase shift

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