Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Explain why transformers require cooling.
• Describe four methods of cooling transformers.
• Identify transformer types for each cooling method.
• Analyze the pros and cons of each method.

Lesson summary

Transformers are the backbone of our national power grid, managed by bodies like GRIDCo and ECG/NEDCo. You see them every day on poles in your community or in large substations. These devices, however, are not 100% efficient; they lose energy in the form of heat. If this heat is not removed, the transformer can overheat, get damaged, and cause a power outage, what we locally call "dumsor". This lesson explores why transformers get hot and the clever engineering methods used to keep them cool and working reliably, ensuring we have stable power for our homes, schools, and businesses. Understanding this is crucial for anyone pursuing a career in electrical engineering or technology.

Lesson notes

This section covers the core content needed to understand transformer cooling. Part A: Why Do Transformers Need Cooling? The Problem of Heat

No machine is perfectly efficient, and transformers are no exception. The electrical energy that is lost during the process of stepping up or stepping down voltage is converted directly into heat. If this heat is not dissipated (removed), the temperature inside the transformer will rise continuously.

Consequences of Overheating: Insulation Damage: The windings (coils of wire) in a transformer are coated with insulation (like varnish or paper). High temperatures will cause this insulation to become brittle, crack, and fail, leading to short circuits. Oil Degradation: In oil-filled transformers, excessive heat breaks down the mineral oil, reducing its ability to insulate and cool. This process can also produce flammable gases. Reduced Lifespan & Failure: Continuous operation at high temperatures significantly shortens the transformer's life and can lead to catastrophic failure.

The heat is generated from two main sources, known as transformer losses: Copper Losses (I²R Losses): What it is: This is the heat generated due to the electrical resistance of the copper windings. Just like any wire, the copper coils in the primary and secondary windings resist the flow of current. Formula: Power Loss (Heat) = I²R, where 'I' is the current flowing and 'R' is the resistance of the winding. When it occurs: These losses vary with the load on the transformer. When you connect more appliances and draw more current (high load), the copper losses increase significantly. Iron Losses (Core Losses): What it is: This is the heat generated in the transformer's laminated iron core. It is present whenever the transformer is energized, even with no load connected. It has two components: Hysteresis Loss: Energy is lost as heat each time the magnetic domain in the core material changes direction. Since AC current changes direction 50 times per second (50 Hz) in Ghana, this process happens continuously, generating heat. Eddy Current Loss: The changing magnetic field induces small, circulating currents within the core material itself, called eddy currents. These currents flow against the resistance of the iron, generating heat. To minimise this, the core is built from thin, insulated sheets of iron called laminations. Part B: Methods of Cooling Transformers

Evaluation guide

• Describe the methods of cooling transformers
• Focal Area: Methods Of Cooling Transformers
• Air cooling
• Oil cooling