Everything You Need to Know About Power Transformers

9 May 2023

A power transformer is a type of transformer used to transmit electrical energy at any component of an electronic or electrical circuit between distribution primary circuits and generators. Power transformers are used for voltage reduction, voltage boost, and for interfacing voltages in distribution networks. Their lifespan is typically around 30 years.

They are categorized into three types based on capacity: large power transformers (100 MVA and above), medium power transformers (7,500 kVA – 100 MVA), and small power transformers (500-750 kVA). These transformers are responsible for transmitting voltage. They consist of a low-voltage and high-current circuit on one side and a high-voltage and low-current circuit on the other side.

Power transformers operate based on Faraday’s law of electromagnetic induction. Each winding in the system is designed according to the ratios defined by the power transformer, which dictates how the power network operates. They are constructed using metal encased in laminated sheets and are classified as shell-type or core-type transformers. Transformer structures are connected and wound using conductors to produce three single-phase or one three-phase transformer.

Difference Between Power Transformers and Distribution Transformers

Power transformers are typically operated at full load since they are designed to have high efficiency at 100% load. On the other hand, distribution transformers offer higher efficiency when the load falls between 50% and 70%. Therefore, distribution transformers are not intended to operate continuously at 100% load.

Power transformers require significant insulation due to the high voltages involved in stepping up or stepping down voltages, compared to distribution transformers or instrument transformers. They are massive and heavy due to the high insulation applied. Since power transformers are generally not directly connected to households, they experience minimal load fluctuations. In contrast, distribution transformers deal with significant load variations.

Power transformers have a higher magnetic flux density compared to distribution transformers.

Types of Power Transformers

Transformers are classified based on their purpose, application, and manufacturing methods.

Step-Up and Step-Down Transformers:

Step-up transformers convert low-voltage, high-current input from the primary section of the transformer into high-voltage, low-current output on the secondary side. Step-down transformers, on the other hand, convert high-voltage, low-current input from the primary section into low-voltage, high-current output on the secondary side.

Three-Phase and Single-Phase Transformers:

Three-phase transformers are more efficient compared to single-phase types. They are typically used in three-phase power systems. However, for ease of transmission, it’s often more convenient to use three single-phase units than one three-phase transformer.

Power Transformers, Distribution Transformers, and Instrument Transformers:

Power transformers are typically used in transmission applications to either lower or raise voltage levels. They operate typically at full load and achieve maximum efficiency at or near 100% load. Distribution transformers are used for reducing voltage levels for commercial and local consumers. They provide proper voltage regulation and achieve their highest efficiency at around 50-70% load, making them suitable for 24/7 operation.

Instrument transformers include current and voltage transformers used to control high voltages and currents with conventional devices.

Two-Winding Transformers and Autotransformers:

Two-winding transformers are used when there’s a significant difference between low-voltage and high-voltage sides. Autotransformers are more efficient when the ratio between the different sides is less than 2.

Outdoor and Indoor Transformers:

Outdoor transformers are designed for outdoor installations, while indoor transformers are intended for indoor use.

Oil-Immersed and Dry-Type Transformers:

Transformers are categorized based on their cooling configuration. Oil-immersed transformers use transformer oil for cooling, while dry-type transformers utilize air for cooling.

Types of Windings in Power Transformers

There are two main types of windings in power transformers: shell-type and core-type. Additionally, berry-type transformers are produced for specific applications.

• Core-Type Transformer: A core-type transformer has two horizontal limbs called the yoke and two vertical legs. Cylindrical coils are mounted on both legs.
• Shell-Type Transformer: A shell-type transformer has two outer limbs and a central limb. The YG and AG coils are mounted on the central limb. Shell-type transformers have a dual magnetic circuit.
• Berry-Type Transformer: Metal sheet tanks are tightly fitted to encase the transformer with oil for insulation.

The core in a core-type transformer looks like the fingers of a wheel in a berry transformer. Metal sheet tanks are tightly fitted to encase the transformer with oil for insulation and protection.