What Is a Three-Phase Transformer?—A Complete Guide

In modern power systems, three-phase transformers play an important role in ensuring stable and efficient voltage conversion. This article helps electrical engineers and system integrators understand how three-phase transformers work, their configurations, and types, helping you make decisions suitable for your projects.

What Is a Three-phase Transformer?

A three-phase transformer is an electrical device used in a three-phase AC system to transmit electrical energy between two or more circuits. It is mainly based on the principle of electromagnetic induction. And energy can be transferred from the primary winding to the secondary winding through the magnetic field, thus stepping up or down the voltage.

It can handle three alternating currents at the same time, each 120° out of phase with the others. Three-phase transformers are widely used in power generation, transmission, distribution, and industrial power consumption.

Components of a Three-Phase Transformer

Isolation: It is mainly used to insulate the winding and core, preventing short-circuit. Insulating media mainly include oil, gas and paper.

Core: Cores are typically made of laminated silicon steel to reduce energy loss and provide a path for magnetic flux. It can be divided into core-type construction and shell-type construction.

Winding: The winding is usually made of copper or aluminum, used to input or output electrical energy. It helps you achieve voltage conversion through the turns ratio.

Tank:  It is used to store insulating oil and protect transformer windings and cores from external influences.

Breather: It allows air to move in and out to equalize pressure, while a silica gel breather prevents moisture from entering the transformer.

Circuit de refroidissement: The cooling system circulates coolant through water/oil tubes to dissipate heat from the oil, which then returns to the core and windings. The cooling methods are mainly ONAN, ONAF, AN, and AF.

Temperature Sensors: These sensors monitor winding and oil temperatures to guarantee safe operation.

Types of Three-phase Transformer

Three-phase transformers can be classified according to their construction, winding connection, cooling method and application. Understanding the different types of three-phase transformers is important to choose the proper transformer for your project.

Core-Type Three-Phase Transformer

The core has three limbs. The primary and secondary windings are evenly distributed on each limb. It allows the three-phase magnetic flux to share a return path through the outer limbs. This is also a simple design in high-voltage power distribution. Besides, it facilitates heat dissipation and maintenance.

Shell-Type Three-Phase Transformer

Shell-type three-phase transformers have a core that surrounds the windings. It often uses a five-limb construction to provide better mechanical strength and magnetic flux distribution.

Besides, its core is mostly exposed to the cooler, so it has better cooling characteristics. What’s more, it also has short circuit resistance and reduces the leakage flux. You can apply it in heavy-duty industrial systems or low-voltage applications.

Dry-Type Transformer

Dry-type three-phase transformers are defined by their cooling and insulation methods. It uses air or solids (resin or varnish) to insulate windings. Common types include cast resin transformers and vacuum pressure impregnated(VPI) transformers.  And, it can be cooled by natural convection or forced air.

Dry-type three-phase transformers have no oil leakage or fire risk. And it is eco-friendly and has low maintenance needs. So they are ideal for indoor or safety-critical environments.

Compared with oil-filled transformers, dry-type transformers have lower cooling efficiency and have limited capacity for very high power applications.

How Does a Three-phase Transformer Work?

The three-phase transformer works based on the principle of electromagnetic induction. When three-phase AC passes through the primary winding, each phase carries AC power. These currents are 120° out of phase with each other.

And, the current generates an alternating magnetic flux in the core of the transformer. The magnetic flux induces a voltage in the secondary winding, so that energy is transferred from the input to the output without direct electrical contact.

The voltage level of a three-phase transformer varies according to the turns ratio between the primary winding and the secondary winding. If the secondary winding has more turns, it is a step-up transformer. If the secondary winding has fewer turns, it is a step-down transformer.

A three-phase transformer generates a rotating magnetic field. Its total magnetic flux remains almost constant and differs by 120°. Therefore, a three-phase transformer can achieve smoother and more continuous power output. Besides, it reduces vibration and losses. Three-phase transformers are more efficient than single-phase transformers.

Advantages of Three-Phase Transformers

Higher Energy Efficiency

The three-phase transformer’s power transmission is more consistent and there are no noticeable fluctuations. This results in smoother energy transfer and improves the overall system efficiency. The three-phase transformer also has lower energy loss when operating. It is suitable for your large-scale and continuous operation.

Continuous and Stable Power Output

The 120° phase shift guarantees continuous power delivery with no zero points. Using a three-phase transformer can reduce your voltage fluctuations and improve the performance of sensitive and heavy devices. Besides, this reduces the risk of interruption of your operation.

Higher Power Handling Capacity

Three-phase transformers are designed for heavy-duty applications. It can handle large electrical loads efficiently. And it can achieve reliable operation under continuous load conditions. What’s more, it can support high-power industrial equipment. For the manufacturing and energy industries, three-phase transformers are very important equipment.

Flexibility in System Configuration

You can implement a variety of connection types(star, delta and combinations) on a three-phase transformer. Different connecting ways can match your different system needs. This flexible configuration allows you to integrate it into various power systems. Besides, the three-phase transformer can also adapt to different voltage and grounding requirements.

Improved Load Balance

The three-phase transformer evenly distributes the power to the three phases, thereby reducing the risk of single-phase overload. This will also increase your system stability and extend your device’s lifespan.

Lower Installation and Operation Cost

The three-phase transformer reduces the material use of the core, winding, and housing. Compared to transformer banks, three-phase transformers have lower installation complexity and cost, and require less maintenance.

Three-Phase Transformer Connections

Three-phase transformer connection mainly refers to the electrical connection between the primary and secondary windings. Different connection types will directly affect your voltage and current levels, system grounding protection, and related systems.

Star Connection

The star connection consists of four terminals and three windings. The three windings form a three-phase circuit. One end of each three-phase is connected to the fourth terminal. The fourth terminal is the common neutral point. This connection makes each winding under low voltage, so it has low insulation requirements.

This connection method is suitable for high-voltage applications and can supply single-phase and three-phase loads. However, it is more sensitive to load imbalance and requires you to ground it for stable operation.

Delta Connection

The delta connection is a closed circuit formed by connecting three windings end to end. This connection method does not provide a neutral point. It can effectively handle unbalanced loads.

Besides, if one phase fails, the circuit can still operate continuously. However, it also has higher insulation requirements. Most importantly, if you do not modify the circuit, you cannot directly supply single-phase loads.

Common Three-Phase Transformer Connection Types

Star-Star Connection

Both the primary and the secondary sides are star-connected methods. And each side has neutral points. This connection is commonly used in high-voltage transmission. But it requires careful neutral grounding to avoid instability. You can use this method in the power transmission system.

Delta-Delta Connection

This connection method is highly reliable but has no neutral point. And it has excellent performance under unbalanced loads. You can apply it to the heavy motor loads in industrial systems.

Star-Delta Connection

The primary is the star connection, and the secondary is the delta connection. This connection method is often used for voltage step-down. It can handle heavy industrial loads very well. You can apply it in industrial plants and motor loads.

Delta-Star Connection

The primary is the delta connection, and the secondary is the star connection. There is a usable neutral point on the secondary side. This connection method is the most commonly used configuration. It is usually used to step up the voltage and can provide you with a stable output voltage. You can apply it in power distribution networks or commercial/industrial power supply systems.

Three Phase Voltages and Currents

In a three-phase transformer, the voltage and current are mainly determined by the connection method of the windings. Understanding the relationship between phase voltage(VP)/current(IP) and line voltage(VL)/current(IL) is very important for you to design the system correctly.

Usually, in a star connection, the line voltage is √3 times the phase voltage, and the line current is equal to the phase current. In practical applications, this connection is more suitable for high-voltage applications. And its neutral point allows grounding or mixed-load configurations.

In a delta connection, the line voltage is equal to the phase voltage, but the line current is √3 times the phase current. This connection is more suitable for applications with high current and low voltage.

Applications of Three-Phase Transformers

Three-phase transformers can achieve efficient power transmission, distribution and utilization in modern power systems. It has become an indispensable device in many industries.

Power Generation and Transmission: At the power station, three-phase transformers are used for long-distance transmission to step up voltage. It is used in substations to safely distribute and step down voltage.

Industrial Manufacturing Facilities: A three-phase transformer can power your high-load equipment, including motors, compressors, and pumps.

Systèmes d'énergie renouvelable: The three-phase transformer can be applied in solar/electric power plants. It can match voltage to grid demand, improving power quality and stability.

FAQ

Can a Three-Phase Transformer Be Connected to Different Voltage Systems?

Yes. But it needs to be properly configured for different voltages. Three-phase transformers can use the turns ratio to step-up/down the voltage level. Different connections can be compatible with different system voltages and grounding requirements. Tap changers can also adjust voltage to match your system needs.

What Is the Difference Between Oil-filled and Dry-type Transformers?

The main differences between oil-filled transformers and dry-type transformers lie in the cooling method, safety, and application.

Oil-filled transformers mainly use insulating oil for insulation and cooling, but they have high maintenance requirements. Besides, they also have oil leakage and fire risks. They are suitable for outdoor and industrial applications.

Dry-type transformers use air or insulating solids. They provide higher security and require less maintenance. And they are usually used for indoor and safety-demanding environments.

How Do You Choose the Right Size Three-Phase Transformer for Your Project?

Choosing the right size for a three-phase transformer requires matching the transformer capacity to your load demand. First, you need to calculate the equipment power(kW), then convert the power into kVA. Then, you should add a 20-30% margin to avoid overloading in future expansions.

Next, you need to check that the primary and secondary voltages match your system. If you are using motors and drives, you might need a higher capacity. Finally, you also need to determine whether the size is proper based on your installation environment.

Is It Better to Use One Three-Phase Transformer or Three Single-Phase Units?

For most projects, a three-phase transformer is usually better. It is more compact, more cost-effective, more efficient, and has lower installation and operation costs.

If you are more concerned with reliability and flexibility, transformer banks are preferred. If a transformer fails, the system can continue to operate.

Final Thought

KDM is a manufacturer with many years of experience in electrical enclosure customization. Our products use high-quality raw materials and advanced production techniques to provide you with high-quality three-phase transformers and enceintes. What’s more, we have a professional team responsible for custom service. KDM is looking forward to serving you.