Transformer

4.5.6 The transformer - Use in high-voltage transmission

Transformers are crucial components in the electrical grid, particularly for the efficient transmission of electricity over long distances. This section explains how transformers are used to step up voltage for transmission and step down voltage for safe use in homes and industries.

Why Use Transformers for High-Voltage Transmission?

Transmitting electricity at high voltages offers significant advantages:

• Reduced Current: For a given amount of power, increasing the voltage decreases the current.
• Reduced Power Loss: Power loss in transmission lines due to resistance (I²R) is proportional to the square of the current. Therefore, reducing the current significantly reduces power loss.
• Economic Efficiency: Lower transmission losses translate to a more economical and efficient electricity supply.

Step-Up Transformers

At power stations, transformers are used to step up the voltage of the electricity generated. A step-up transformer increases the voltage from the station's output to a high voltage suitable for transmission. This is achieved by having more turns in the secondary coil than in the primary coil.

The relationship between the number of turns (N) and the voltage (V) in a transformer is given by:

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

Where:

• $V_p$ is the voltage in the primary coil.
• $V_s$ is the voltage in the secondary coil.
• $N_p$ is the number of turns in the primary coil.
• $N_s$ is the number of turns in the secondary coil.

For a step-up transformer, $N_s > N_p$, so $V_s > V_p$

Step-Down Transformers

As electricity reaches homes and businesses, it needs to be reduced to a safe and usable voltage. Step-down transformers are used to step down the voltage from the high transmission voltage to lower voltages suitable for domestic and industrial use (e.g., 230V in the UK).

Again, the relationship between the number of turns and voltage applies:

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

Where:

• $V_p$ is the voltage in the primary coil.
• $V_s$ is the voltage in the secondary coil.
• $N_p$ is the number of turns in the primary coil.
• $N_s$ is the number of turns in the secondary coil.

For a step-down transformer, $N_s < N_p$, so $V_s < V_p$

Transformer Construction (Simplified)

A transformer typically consists of two coils of wire (primary and secondary) wound around a common iron core. The iron core concentrates the magnetic flux, improving efficiency. When an alternating current flows through the primary coil, it creates a changing magnetic field. This changing magnetic field induces a voltage in the secondary coil.

Component	Description
Iron Core	Provides a path of low reluctance for the magnetic flux.
Primary Coil	Receives the input AC voltage.
Secondary Coil	Provides the output AC voltage.

Efficiency of Transformers

Real transformers are not perfectly efficient. Some energy is lost due to:

• Hysteresis Loss: Energy lost due to the magnetization and demagnetization of the iron core.
• Eddy Current Loss: Circulating currents induced in the iron core by the changing magnetic field.
• Copper Loss: Energy lost due to the resistance of the wires in the coils.

Transformer efficiency is the ratio of output power to input power, expressed as a percentage.

Diagram