What is Transformer Winding?
Transformer winding consists of several turns of insulated copper or aluminium conductors wound around the transformer’s core. These windings are responsible for transferring electrical energy from one circuit to another through electromagnetic induction.
Transformer windings are designed according to the input and output requirements of the transformer, particularly the voltage and current ratings. There are two main types of transformer windings:
- Primary Winding: The primary winding receives electrical energy from the input power supply.
- Secondary Winding: The secondary winding delivers electrical energy to the connected load.
Materials Used in Transformer Windings
Copper and aluminium are the most commonly used materials for transformer windings. Both materials have different electrical, mechanical, and economic properties.
- Copper: Copper has high electrical conductivity and good mechanical strength. It is widely used in power and distribution transformers because of its excellent electrical properties.
- Aluminium: Aluminium is lighter and generally less expensive than copper. However, it has higher electrical resistivity, so a larger conductor cross-section is generally required to achieve comparable electrical performance.
Types of Transformer Windings
Transformer windings are classified into different types according to their construction, voltage rating, current capacity, and application.
The main types of transformer windings are:
- Multi-Layer Helical Windings
- Helical or Spiral Windings
- Disc Windings
- Foil Windings
- Cylindrical Windings
- Cross-Over Windings
- Disc and Continuous Disc Windings
- Aluminium Windings
1. Multi-Layer Helical Windings
Multi-layer helical windings are commonly used in high-voltage transformers, particularly those with voltage ratings of 110 kV and above. These windings consist of several cylindrical layers connected in sequence. The outer layers may be made shorter than the inner layers to obtain a suitable capacitance distribution. This arrangement helps improve the transformer’s behaviour under surge conditions.
2. Helical or Spiral Windings
Helical windings, also known as spiral windings, are generally used in low-voltage, high-capacity transformers. They are suitable for applications involving high current and a relatively small number of turns. These windings are commonly used in transformers with ratings ranging from approximately 160 to 1000 kVA and voltage ratings between 0.23 and 15 kV. The conductor is generally made using multiple parallel strips to carry high current. Helical windings are classified into three types:
- Single Helical Winding: The conductor is wound in a helical or screw-like path along the axial direction. Each winding consists of a single layer of turns.
- Double Helical Winding: This arrangement uses parallel conductors in a double-helical configuration. It helps reduce circulating and eddy-current losses within the conductors.
- Disc-Helical Winding: The conductor strips are arranged side by side in a radial pattern to form the winding structure.
3. Disc Windings
Disc windings are constructed by connecting several conductor discs in series. Each disc is made by winding multiple turns of insulated conductors and connecting them to form the required winding arrangement. Spacers are provided between the discs to maintain proper separation and allow cooling oil to circulate. Disc windings are mechanically strong and are commonly used in high-voltage transformers. Their construction also provides flexibility in arranging the conductors and insulation.
4. Foil Windings
Foil windings are made using thin sheets or strips of copper or aluminium. These conductors are wound in a spiral arrangement to form the transformer winding. Depending on the design, a single foil or multiple parallel foils may be used. Foil windings are suitable for high-current applications and large-capacity transformers. They provide a compact winding arrangement and help distribute current across the conductor width.
5. Cylindrical Windings
Cylindrical windings are generally used in low-voltage transformers. They are constructed by winding insulated conductors around the transformer core in cylindrical layers. These windings may use round or rectangular conductors. In some designs, vertical spacers are provided between winding layers to create oil ducts that improve cooling. Cylindrical windings are commonly used in distribution transformers and other applications where a simple and mechanically strong winding arrangement is required.
6. Cross-Over Windings
Cross-over windings are generally used in small transformers. They are divided into several coils to reduce the voltage difference between adjacent winding layers. The individual coils are separated by insulating materials to provide proper electrical insulation. This arrangement helps reduce insulation stress and improves the mechanical strength of the winding.
7. Disc and Continuous Disc Windings
Disc and continuous disc windings are commonly used in large-capacity transformers. They consist of several flat coils or discs connected in series or parallel according to the required electrical design. These windings are generally made using rectangular conductor strips wound in a radial arrangement. The discs are separated by pressboard spacers and insulating barriers. Oil ducts are provided between the winding sections to support cooling. One of the main advantages of this construction is its high mechanical strength, particularly against axial forces.
8. Aluminium Windings
Aluminium windings are widely used in dry-type and low-voltage transformers. Aluminium is preferred in some applications because it is lightweight and generally has a lower initial material cost than copper. However, aluminium has higher electrical resistivity than copper. Therefore, a larger conductor cross-section is generally required to achieve comparable electrical performance. Aluminium conductors are also relatively easy to shape and manufacture. The final performance of an aluminium-wound transformer depends on its design, conductor dimensions, insulation, and cooling arrangement.
Transformer Winding Resistance Meter
A transformer winding resistance meter is an important diagnostic instrument used to measure the electrical resistance of transformer windings. It helps evaluate the condition of winding conductors, joints, and connections.
Winding resistance measurements are useful for identifying problems such as loose connections, poor contacts, damaged connections, and shorted turns. The measurements are generally taken between the appropriate winding terminals or phases and compared with expected values or previous test results.
Transformer winding resistance meters are also used for testing:
- Transformer windings
- Generator windings
- Motor windings
- Inductors
- Tap changers
- Power cables
- Busbar connections
These instruments are designed to measure resistance in highly inductive electrical equipment, including high-voltage power transformers.
Transformer Winding Temperature Indicator
The winding temperature of a transformer is an important operating parameter. As the load increases, the temperature of the winding generally rises due to increased electrical losses.
To maintain safe operating conditions, transformer temperature is monitored using temperature indicators. Two commonly used instruments are:
- Winding Temperature Indicator (WTI): Used to indicate the estimated or measured winding temperature.
- Oil Temperature Indicator (OTI): Used to monitor the temperature of the transformer oil.
Functions of Winding Temperature Indicator
The main function of a Winding Temperature Indicator (WTI) is to monitor the temperature of the transformer’s high-voltage and low-voltage windings. It can be connected to alarm, trip, and cooling-control circuits. In traditional pointer-type indicators, different pointers may show the current winding temperature and the maximum temperature recorded during operation. Temperature settings can also be provided for alarm and trip operations. When the winding temperature reaches the preset limits, the indicator activates the corresponding control or protection functions.
Advantages of Transformer Windings
- Properly designed windings provide good electromagnetic coupling between the primary and secondary circuits.
- They can provide high mechanical strength when suitable conductor and insulation arrangements are used.
- They support efficient energy transfer through electromagnetic induction.
- Suitable winding designs can reduce electrical losses.
- They can be designed for different voltage, current, and power ratings.
- They provide reliable operation when properly manufactured and maintained.
Disadvantages of Transformer Windings
- Winding construction and manufacturing require skilled labour and specialised equipment.
- Improper winding placement can affect electrical performance and insulation characteristics.
- Manufacturing and insulation processes can increase production costs.
- Winding faults, such as shorted turns or loose connections, can affect transformer performance.
- High winding temperatures can reduce insulation life if proper cooling is not provided.
Applications of Transformer Windings
Transformer windings are essential components of transformers used in electrical power systems and electronic equipment. Their main applications include:
- Power Distribution: Used in distribution transformers to supply electrical energy at suitable voltage levels.
- Voltage Transformation: Used in step-up and step-down transformers to increase or decrease AC voltage.
- Electrical Isolation: Used in isolation transformers to provide electrical separation between two circuits.
- Power Transmission: Used in power transformers at generating stations and substations.
- Electronic Equipment: Used in power supplies, chargers, and other electrical and electronic devices.
- Industrial Applications: Used in transformers that supply power to industrial machines and equipment.
Conclusion
Transformer winding is an essential part of a transformer and plays an important role in transferring electrical energy through electromagnetic induction. The primary and secondary windings are made using conductors such as copper or aluminium. Different winding types are selected according to the transformer’s voltage, current, capacity, and operating requirements. Proper winding design, resistance testing, and temperature monitoring help ensure the safe and reliable operation of transformers.