How Do Substation Transformers Work?

Substation transformers are a critical component in the electrical power distribution network. They play a vital role in stepping up or stepping down the voltage levels of electricity as it moves from power plants to consumers. Without transformers, efficient long-distance power transmission would be nearly impossible, and homes, industries, and businesses would not have reliable access to electricity. In this article, we will explore the functionality, working principles, and components of substation transformers in detail.

Introduction to Substation Transformers

A substation transformer is an electrical device that converts high-voltage electricity from the transmission network into lower voltage electricity suitable for distribution. This step-down process is necessary because electricity is generated at high voltages and needs to be transmitted over long distances efficiently. Once the power reaches its destination, such as a city or industrial area, transformers at substations reduce the voltage to levels that can be safely used by homes and businesses.

In contrast, some substations are equipped with step-up transformers, which increase the voltage levels for efficient long-distance transmission. Regardless of the type, transformers are essential in ensuring that electricity reaches consumers efficiently and reliably.

Basic Working Principle of Transformers

The working of a transformer is based on the principle of electromagnetic induction. This principle states that a change in the magnetic field within a conductor induces an electric current in another conductor placed within the same magnetic field.

A transformer consists of two coils of wire called the primary and secondary windings, wound around a common magnetic core. When an alternating current (AC) flows through the primary winding, it creates a varying magnetic field in the core. This varying magnetic field induces a voltage in the secondary winding, which can either be higher (step-up transformer) or lower (step-down transformer) than the voltage in the primary winding, depending on the number of turns in each winding.

The voltage transformation ratio between the primary and secondary windings is given by the following equation:

$$\frac{V_s}{V_p} = \frac{N_s}{N_p}$$

Where:

• $V_s$ = Secondary voltage
• $V_p$ = Primary voltage
• $N_s$ = Number of turns in the secondary winding
• $N_p$ = Number of turns in the primary winding

Thus, the voltage induced in the secondary coil is directly proportional to the number of turns in the coil. For instance, if the secondary winding has fewer turns than the primary winding, the transformer will step down the voltage.

Key Components of Substation Transformers

Substation transformers consist of several key components that ensure their proper operation:

1. Core

The magnetic core of the transformer is made from laminated steel to minimize energy losses due to eddy currents. The core serves as the pathway for the magnetic flux, linking the primary and secondary windings.

2. Windings

The primary and secondary windings are made of copper or aluminum conductors, with insulation to prevent electrical shorts. These windings are responsible for transferring energy between the different voltage levels. The ratio of turns between these windings determines the voltage transformation.

3. Bushings

Bushings provide electrical insulation where conductors pass through the transformer casing. They ensure that the high-voltage terminals do not come into contact with the outer case or other components.

4. Tap Changer

Some transformers are equipped with tap changers, which allow the adjustment of the number of active windings. This enables operators to regulate the output voltage without shutting down the transformer, ensuring that voltage levels remain within acceptable limits.

5. Cooling System

Large transformers generate significant heat due to electrical losses. To manage this heat, transformers are equipped with cooling systems, which may include fans, radiators, or even oil-based cooling. The oil not only cools the transformer but also provides insulation between live components.

6. Protection Relays

Transformers are fitted with protection relays to detect faults such as overloading, overheating, or short circuits. When a fault is detected, these relays trigger protective devices such as circuit breakers, preventing damage to the transformer and the network.

Types of Substation Transformers

There are two primary types of substation transformers based on their role in the power grid:

1. Step-Up Transformers

Step-up transformers are typically located near power generation stations. They increase the voltage from the generator's output to higher levels, which is crucial for efficient long-distance transmission. Higher voltage transmission minimizes losses due to resistance in transmission lines, making it more efficient.

2. Step-Down Transformers

Step-down transformers are found at distribution substations, where they reduce the voltage from high transmission levels to lower distribution levels. For instance, the transmission voltage may be stepped down from 132 kV to 11 kV for distribution to local substations, and then further reduced to 400 V for residential or commercial use.

Working of a Substation Transformer

A typical electrical substation contains several transformers, along with other electrical equipment like circuit breakers, switchgear, and protection systems. The process of transforming voltage at a substation follows these steps:

1. High Voltage Input: High-voltage electricity enters the substation transformer from the transmission lines. For example, the input may be 132 kV or 220 kV, depending on the region and the grid structure.
2. Primary Winding: The high-voltage electricity flows through the primary winding of the transformer. The current in this winding generates a magnetic field in the transformer's core.
3. Electromagnetic Induction: The alternating current in the primary winding causes the magnetic flux in the core to vary, which induces a voltage in the secondary winding through electromagnetic induction.
4. Voltage Transformation: The voltage induced in the secondary winding is either higher or lower than the primary voltage, depending on the number of turns in each winding. For step-down transformers, the voltage is reduced for safer and more efficient distribution.
5. Low Voltage Output: The output from the secondary winding is at a lower voltage suitable for distribution, typically 11 kV or 33 kV. This electricity is then fed into the local distribution network, where further transformations may take place before it reaches end users.

Transformer Losses and Efficiency

Substation transformers are designed to operate with high efficiency, often exceeding 98%. However, some energy is lost in the form of heat due to core losses (hysteresis and eddy currents) and copper losses (I²R losses) in the windings. Modern transformers are designed with advanced materials and cooling systems to minimize these losses and maximize efficiency.

Conclusion

Substation transformers are indispensable in the power grid, facilitating the safe and efficient transmission of electricity over long distances. By stepping up or stepping down voltage levels, they ensure that power generated at distant plants can be delivered to consumers with minimal losses. The basic operation of transformers, grounded in the principles of electromagnetic induction, makes them one of the most essential and enduring technologies in electrical engineering.

Understanding how substation transformers work helps us appreciate the complexity and importance of the electrical grid in delivering reliable power to homes and industries around the world.