Power Transformers: Structure and Types

9 May 2023

Importance of Transformers

One of the most crucial features of electrical energy is its ability to be easily transported from the generation source to distant regions. To efficiently achieve this, voltage must be sufficiently high.

As is known, electrical energy is produced as direct current (DC) or alternating current (AC). In recent times, high-voltage DC energy transmission has gained importance. However, achieving the desired level in this regard has proven challenging. In contrast, the voltage of alternating current electrical energy can be increased or decreased with the help of transformers, which is why transporting energy in AC remains significant. Transformers, serving as electric machines that raise or lower the voltage of AC without altering its power or frequency, play a crucial role in the transmission of electrical energy in AC.

Transformer Components

COOLING

Transformers are cooled using removable radiator panels. Radiators are mounted on the tank surface in various shapes depending on the design of the transformer, with common cooling methods being ONAN/ONAF, OFAF, ODAF, OFWF, and ODWF. The design is customized according to customer preferences.

WINDINGS

Windings are created using copper conductors. Paper-covered flat wires and CTC conductors are used as conductors. Windings are wound vertically and horizontally in accordance with the design, using both vertical and horizontal machines.

TANK

The tank design is created using Autodesk Invertor software with 2-D and 3-D CAD for assembly, transport, and commissioning convenience. Membranes that isolate the tank from the atmosphere are used in the expansion tanks. All junction points, welds, and connections are checked and tested for oil leakage. The design adheres to customer specifications and requirements.

CORE

Low-loss silicon steel particles with directed orientation are used to construct the transformer core using highly modern machinery in our own facilities.

OFF-LOAD TAP CHANGER

The off-load tap changer is an accessory used to manually adjust the tap position of the OG windings when the transformer is not loaded, ensuring that the transformer always provides the correct output voltage.

ON-LOAD TAP CHANGER

The on-load tap changer is an accessory used to automatically adjust the tap position of the OG windings when the transformer is loaded, ensuring that the transformer always provides the correct output voltage.

BUCHHOLZ RELAY

Used only in conservator type transformers, the Buchholz relay is a protective relay that signals any dangerous conditions, such as gas formation inside the transformer or the oil level dropping below a critical limit. It is installed between the oil expansion tank and the tank.

SILICA GEL CONTAINER

Used only in conservator type transformers, the silica gel container is placed to capture moisture from the air that enters or exits the oil reservoir during oil expansion or contraction.

MAGNETIC OIL LEVEL INDICATORS

Used in conservator type transformers, these accessories are used to monitor the oil level, and they come in contact and non-contact versions.

PRESSURE RELIEF VALVE

This accessory is used to relieve pressure in case the operating pressure of the transformer exceeds the set limit, thereby protecting the tank from excessive pressure.

OIL TEMPERATURE INDICATOR

Used to monitor the oil temperature in the transformer, this accessory sends signals to relays if the temperature exceeds the set threshold. It can be configured with up to six contacts based on customer requirements.

General Description and Structure

General Description: A transformer is a machine that changes voltage and current values within a certain ratio without changing the frequency through electromagnetic induction. Transformers are commonly referred to as “trafos.”

General Structure: Transformers are composed of two coils, primary and secondary, wound on insulated conductors and placed on a closed magnetic circuit called the iron core. Except for autotransformers, the two coils are electrically isolated from each other.

Core Structure and Types

In transformers, the magnetic core (body) is formed by stacking insulated silicon steel sheets with a thickness of 0.30 to 0.50 mm to prevent eddy and hysteresis losses. Carlite is used as the insulating material.

The magnetic core consists of two parts:

Leg or limb: The part of the magnetic core where the windings are wound.

Yoke: The part that connects the legs.

Transformer cores are made in three ways: core-type, shell-type, and distributed-type.

Core-Type Transformer Core-type transformers have more space for insulation. Therefore, they are used in high-power and high-voltage applications. One advantage of core-type transformers is that winding control is easier.

Shell-Type Transformer In shell-type transformers, the average magnetic path length is shorter compared to core-type transformers. This results in lower iron losses. Shell-type transformers are used in low-voltage and low-power applications.

Distributed-Type Transformer Distributed-type transformers have minimal leakage flux, which reduces no-load current. As a result, internal voltage drops are also reduced. Distributed-type transformers are mainly used in small, special-purpose applications.

Types of Windings

Primary Winding The winding to which voltage is applied in a transformer is called the primary winding or the first winding.

In step-down transformers, it is wound with thin cross-section conductors in a tightly coiled manner. In step-up transformers, it is wound with thick cross-section conductors in a loosely coiled manner. Secondary Winding The winding from which voltage is taken in a transformer is called the secondary winding or the second winding.

In step-down transformers, it is wound with thick cross-section conductors in a loosely coiled manner. In step-up transformers, it is wound with thin cross-section conductors in a tightly coiled manner.

Types of Transformers

Transformers play a pivotal role in the entire process of electrical energy, from generation to reaching consumers. They come in various types, each serving a specific purpose:

Power Transformers: These are large and used for increasing or decreasing voltage in high-voltage power transmission.

Substation Transformers: These are used to lower voltage levels at substations.

Distribution Transformers: These further reduce voltage levels for distribution to consumers.

Instrument Transformers: These are used for accurate measurements and are often found in electrical equipment.

Isolation Transformers: Used for isolating DC currents, ensuring electrical safety, and providing voltage transformation when both primary and secondary voltages contain AC components.

Auto Transformers: These transformers have windings that are electrically connected, often used for voltage adjustment.

Pulse Transformers: Specialized for transmitting pulses or brief electrical signals.

Types of Transformers by Operating Voltage:

• Low Voltage: 0-1 kV
• Medium Voltage: 1-3-5-10-20-25-30 kV
• High Voltage: 45-60-110 kV
• Extra High Voltage: 150-220-380-400 kV

Cooling Systems in Transformers

Transformers experience more heating issues compared to other machines due to the absence of rotating parts. In machines with rotating parts, there is at least some air circulation when they operate. Transformers are cooled in five ways:

• A – Air cooling
• N – Natural convection cooling
• O – Oil cooling
• F – Forced air cooling (fan cooling)
• W – Water cooling

Types of Transformers by Application:

• Feeding Transformers
• Isolation Transformers
• Fixed Frequency Transformers
• Line Transformers
• Impedance Transformers
• Auto Transformers
• Pulse Transformers