What is Three-Phase Transformer : Construction and Its Working

The three-phase system is used to generate, transmit, and distribute electrical power. It generates power on a large scale to meet the needs of industries and commercial establishments. Three identical single-phase transformers are connected suitably or combined on a single core to form a three-phase system. Based on various types of industrial needs, the step-up and step-down transformers are employed for generating, transmission, and distributing the electric power.  The building of a three-phase transformer unit is economical as it consumes less material compared to connecting three individual single-phase transformers. Additionally, the three-phase system transfers AC power instead of DC and is simple to construct.

What is a Three-Phase Transformer?

As known, a single-phase transformer is a device that is capable of transferring electrical energy from one circuit to one or more circuits based on the concept of mutual induction. It comprises two coils – a primary and a secondary coil, which helps to transform the energy. The primary coil is connected to a single-phase supply, while the secondary is connected to a load.

Similarly, a three-phase transformer consists of three primary coils and three secondary coils and is represented as 3-phase or 3ɸ. A three-phase system can be constructed using three individual identical single-phase transformers, and such a 3-phase transformer is known as the bank of three transformers. On the other hand, the three-phase transformer can be built on a single core. The windings of a transformer can be connected in either delta or wye configurations. The working of the 3-phase system is similar to a single-phase transformer, and they are normally employed in power generation plants.

Three-Phase Transformer Construction

The diagram of a three-phase transformer is shown in the figure below.

A three-phase transformer of a single unit is used widely because it is lighter, cheaper and occupies less space than the bank of three single-phase transformers. The three-phase transformer construction is of two types: Core type and Shell type.

Core Type Construction

In this type of construction, there are three cores and two yokes. Each core has both primary and secondary windings wounded spirally as shown in the figure. Each leg of the core carries high voltage as well as low voltage windings. The core is laminated to minimize eddy current losses on core and yoke. As it is easier to laminate low voltage (LV) winding than the high voltage (HV) winding. The LV windings are positioned near the core with appropriate insulation and oil ducts in between them whereas, the HV windings are placed above the LV windings with appropriate insulation and oil ducts between them.

Shell Type Transformer

The three-phase shell type transformer is generally constructed by stacking three individual single-phase transformers. Three phases of a shell-type transformer are independent than the core-type transformer, while each phase has an individual magnetic circuit. These magnetic circuits are parallel to each other and flux induced by each winding is in phase. Shell type transformer is highly preferred as the voltage waveforms are less distorted.

Working of Three-Phase Transformers

The figure below shows the three-phase transformer, wherein three cores are placed at 120˚ from each other. This figure is simplified to show only primary windings and their connection to the three-phase power supply. As soon as the three-phase supply is excited, the currents IR, IY, and IB are carried by the primary windings and thus inducing the fluxes ɸR, ɸY, and ɸB individually in each core. The center leg will carry the sum of all the fluxes, and the center leg combined all the legs of a core.

For instance, if the sum of the currents IR+IY+IB is zero in a three-phase system, then the sum of all the three fluxes also becomes zero, resulting in the center leg carrying no flux. Therefore, removing the center leg makes no difference for other transformer conditions.

Three-Phase Transformer Connections

Various three-phase transformer connection is described below.

Wye and Delta configurations are applied for three-phase transformers because Wye connections provide the options to have multiple voltages, whereas delta configurations offer high reliability. The phase diagram of Wye and Delta is given below. For Wye connection, either all the minus or all the plus points of windings shall be tied together. However, in delta connection, polarities of winding are connected in a converse way. The phase difference between any two phases is 120˚.

Wye-wye Connection

The diagram of Y-Y connected transformers is shown below. It can serve both single-phase and three-phase loads. In this connection, all the windings ending with dots are connected to phases A, B, and C, while non-dots endings are connected to become the centers of “Y” configuration.

Wye-Delta Connection

The Y-Delta connection shown in the figure below shows that the secondary windings (which are at the bottom in the figure) are connected to form a chain. The windings with dot connection on one side are connected with the non-dot connection of the other side to form the “Delta” loop.

Delta-Wye Connection

The connection of Delta-Y is shown in the figure below. This type of configuration allows wye-connected secondary to connect multiple voltages such as line-to-line or neutral. As the delta-wye configuration presents a 30˚ phase shift between primary and secondary, it cannot be used to connect in parallel with delta-delta and Y-Y configurations.

Delta-Delta Connection

The diagram of the delta-delta connection is shown below. These connections can be made either with three identical single-phase transformers or one three-phase transformer. The delta-delta configuration is preferred due to its inherent reliability.

Advantages/Disadvantages of a Three-Phase Transformer

The advantages and disadvantages of a three-phase transformer are discussed below.

Advantages of a three-phase transformer

• Needs less space to install and it is easier to install
• Less weight and reduced size
• Higher efficiency
• Low cost
• Transportation cost is low

Disadvantages of a three-phase transformer

• The entire unit shuts down in case of fault or loss occurs in any one unit of a transformer as a common core is shared by all three units.
• Repair costs are higher
• Cost of spare units are high

FAQs

1). Mention the applications of 3-phase transformer

Three-phase transformers are used in electrical grids, power transformer, and as distribution transformers

2). What are the types of 3-phase transformer?

The four types of 3-phase transformers include: Delta-Delta (Dd), Star-Star (Yy), Star-Delta (Yd), and Delta-Star (Dy)

3). What happens if a 3-phase motor loses a phase?

If a 3-phase motor loses a phase during operation, the motor continues to operate at less speed and experiences vibrations. The current also increases abruptly in other phases leading to internal heating of the components of a motor.

4). Under which condition delta/wye works satisfactorily?

The wye-delta connection works satisfactorily with large unbalanced and balanced loads. It can handle third harmonic components because of the circulating currents in the delta.

5). For the Wye-Wye connection, what is the phase shift?

The phase shift is 0 degrees.

Although a single-phase transformer is preferred by most industries, it is not suitable for large power distribution. Therefore, 3-phase systems are used by large industries to generate power on a large scale.

In this article, we discussed various benefits and a few disadvantages offered by a 3-phase transformer. We also focused on a three-phase transformer and its construction and various configurations. Here is a question for you, what is the function of the three-phase transformer?