Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE) PDF Download

The three-phase transformers are mainly of two types:

  1. Single unit three-phase transformer.
  2. Three unit three-phase transformer.

Figure: Three phase shell type transformer Figure: Three phase shell type transformer 

Advantages of Bank of three units:
In case of damage of one winding the power can be transmitted through the two units by using an open delta connection, so 50% power can be transferred.

Advantages of single unit transformer:

  1. These transformers use less space.
  2. These transformers are lighter, smaller, and cheaper.
  3. These transformers are slightly more efficient.

Disadvantages of single unit transformer:
If the single winding of the transformer gets damaged, then we have to change the complete unit.

Connections of three-phase transformer:
A three-phase transformer has three transformers connected in it, either separately or combined on one core. We can connect the primary and secondary winding of a 3-phase transformer in either a star (Y) or delta (∆). There are four ways to connect the 3-phase transformer bank:

  1. ∆ - ∆ (Delta primary - Delta secondary)
  2. Y - Y (Star primary - Star secondary)
  3. ∆ - Y (Delta primary - Star secondary)
  4. Y - ∆ (Star primary - Delta secondary)

For connecting the transformers into star or delta, we have to assume that the transformers we are connecting, are all of the same KVA ratings.

What are the factors that affect the choice of connections?
The factors that affect the choice are as follows:

  1. We have to check the availability of a neutral connection for grounding, protection or load current.
  2. Insulation to ground and voltage stress.
  3. We have to check that the path for the flow of third harmonic and zero sequence current is available or not.
  4. When one circuit is out of service, we need the partial capacity.
  5. Parallel operation with other transformers.
  6. We have to check the economic considerations.

1. Delta - Delta (∆ - ∆) Connection
In delta-delta connection the line voltage of the transformer is equal to the supply voltage of the transformer.
Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
The above diagram shows the delta-delta connection of three windings of single phase transformer. The secondary winding a1 a2 corresponds to the primary winding A1A2, b1b2 corresponds to B1B2, and c1c2 corresponds to C1C2, similarly 'a' corresponds to A, 'b' corresponds to B and 'c' corresponds to C. The terminals 'a1' and A1 have the same polarity. The Phasor diagram drawn above is for lagging power factor cos Φ. For balanced conditions, the line current is Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)three times the Phase current.
The turn ratio for 3 phase transformer is
Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
And the current ratio when the magnetizing current is neglected is
Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
We can see from the above-drawn diagram that both the primary and secondary line voltages are in phase. This connection is called 0° - connection.

If we reverse the connections of the phase winding, we obtain the phase difference of 180° between the primary and secondary systems. This connection is called 180° - connection.

The advantage of ∆ - ∆ Transformation:

  1. The delta-delta connection is good for balanced and unbalanced loading.
  2. If a third harmonic is present, it circulates in the closed path and therefore does not appear in the output voltage wave.
  3. The main advantage of ∆ - ∆ transformer is that if one transformer stops working, then the other two transformers will keep on working. This is called an open delta connection.

The disadvantage of ∆ - ∆ Transformation:
The disadvantage of ∆ - ∆ transformer is that it does not contain a neutral point and this can only be used when neither primary nor secondary requires neutral, and the required voltage is low and moderate.

2. Star-Star (Y - Y) Connection
Fig: Star-Star connection of transformer (0◦ Phase Shift) Fig: Star-Star connection of transformer (0◦ Phase Shift) 

The voltage ratios for ideal transformer are:
Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
And current ratios are:
Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
There are two serious problems in star-star connection:

  1. In the star-star connection when the load is unbalanced and neutral is not provided, then the phase voltage tends to become severely unbalanced. Therefore, the star-star connection is not suitable for unbalanced loading.
  2. The magnetizing current of any transformer is very non-sinusoidal and contains a very large third harmonic, which is necessary to overcome saturation to produce a sinusoidal flux.

3. Delta-Star (∆ - Y) Connection
In ∆ - Y connection of 3-phase transformers, the primary line voltage is equal to the primary phase voltage (VLP = VpP). The relationship between secondary voltages is Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE) therefore, the line-line voltage ratio of this connection is
Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
Figure: Delta - star connection of transformer (Phase shift 30◦ lead), (b) Phasor diagram Figure: Delta - star connection of transformer (Phase shift 30◦ lead), (b) Phasor diagram 

The phasor diagram drawn above shows the delta-star connection supplying a balanced load at power factor cos Φ lagging. It is seen from the phasor diagram that the secondary phase voltage Van leads primary phase voltage VAN by 30◦. Similarly, Vbn leads VBN by 30◦ and Vcn leads VCN by 30◦. This is also the phase relationship between the respective line-to-line voltages. This connection is called +30◦ connection.

4. Star-Delta (Y - ∆) Connection
The Y - ∆ connection of three-phase transformers is shown below. In this connection, the primary line voltage is equal toThree-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)times the primary phase voltageThree-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)The secondary line voltage is equal to the secondary phase voltage (VLS = VpS). The voltage ration of each phase is
Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
Therefore line-to-line voltage ratio of a Y - ∆ connection is
Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE)
Figure: Y - ∆ connection of transformer (Phase shift of 30◦ lead) Figure: Y - ∆ connection of transformer (Phase shift of 30◦ lead) 

When there is a phase shift of 30◦ lead between respective line-to-line voltages then this type of connection is called as +30◦ connection.
And when there is a phase shift of 30◦ lag between the line-to-line voltages then the connection is known as -30◦ connection.
The ∆ - Y connection or Y - ∆ connection has no problem with unbalanced loads and third harmonics.

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FAQs on Three-Phase Transformers - Electrical Machines for Electrical Engg. - Electrical Engineering (EE)

1. What is a three-phase transformer in electrical engineering?
Ans. A three-phase transformer is a type of transformer used in electrical power systems to transfer electrical energy between three-phase circuits. It consists of three separate primary and secondary windings, which are connected in various configurations to provide different voltage and current levels.
2. What are the advantages of using a three-phase transformer?
Ans. There are several advantages of using a three-phase transformer in electrical engineering. These include: - Improved efficiency: Three-phase transformers have a higher efficiency compared to single-phase transformers, resulting in lower energy losses. - Smaller size and weight: Three-phase transformers can handle higher power levels with smaller physical dimensions, making them more compact and lightweight. - Balanced power flow: Due to the nature of three-phase power, a three-phase transformer ensures balanced power flow across all three phases, reducing the risk of voltage and current imbalances. - Lower cost: Three-phase transformers are often more cost-effective compared to multiple single-phase transformers for the same power rating. - Greater reliability: Three-phase transformers provide a reliable and robust solution for power distribution in industrial and commercial applications.
3. What are the different types of connections used in three-phase transformers?
Ans. Three-phase transformers can be connected in different configurations, including: - Delta-Delta (Δ-Δ) connection: Both primary and secondary windings are connected in a delta configuration, suitable for applications where the load is balanced and requires a three-phase power supply. - Wye-Wye (Y-Y) connection: Both primary and secondary windings are connected in a wye configuration, commonly used for distributing power to residential and commercial areas. - Delta-Wye (Δ-Y) connection: The primary winding is connected in a delta configuration, while the secondary winding is connected in a wye configuration. This configuration is often used to step down a high voltage to a lower voltage for distribution purposes. - Wye-Delta (Y-Δ) connection: The primary winding is connected in a wye configuration, while the secondary winding is connected in a delta configuration. This configuration is typically used to step up a low voltage to a higher voltage for transmission purposes.
4. How does a three-phase transformer work?
Ans. A three-phase transformer works by transferring electrical energy between three-phase circuits. The primary windings are connected to the source of electrical power, while the secondary windings are connected to the load. When three-phase power is applied to the primary windings, a magnetic field is created, which induces a voltage in the secondary windings. This voltage is then delivered to the load, providing the required electrical power. The turns ratio of the windings determines the voltage transformation ratio, allowing the three-phase transformer to step up or step down the voltage levels as needed.
5. What are some common applications of three-phase transformers?
Ans. Three-phase transformers are widely used in various electrical engineering applications, including: - Power distribution: Three-phase transformers play a crucial role in transmitting electrical power from power plants to substations and then to residential, commercial, and industrial areas. - Industrial machinery: Many industrial machines and equipment, such as motors, pumps, compressors, and generators, require three-phase power supply, which is provided by three-phase transformers. - Renewable energy systems: Three-phase transformers are used in renewable energy systems, such as wind farms and solar power plants, to step up the generated electricity for efficient transmission and distribution. - Data centers: Data centers often require large amounts of power, and three-phase transformers are used to distribute and manage the electrical load efficiently. - Electric vehicles: Charging stations for electric vehicles often use three-phase transformers to provide the required power for charging the vehicles' batteries.
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