Lesson Notes By Weeks and Term v4 - SHS 3
ELECTRICAL SYSTEMS DESIGN
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ELECTRICAL SYSTEMS DESIGN
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Subject: Applied Technology
Class: SHS 3
Term: 2nd Term
Week: 9
Grade code: 2.4.1.LI.7
Strand code: 4
Sub-strand code: 1
Content standard code: 2.4.1.CS.1
Indicator code: 2.4.1.LI.7
Theme: ELECTRICAL AND ELECTRONIC TECHNOLOGY
Subtheme: ELECTRICAL SYSTEMS DESIGN
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This lesson focuses on the critical importance of cooling systems in electrical transformers. Transformers are everywhere in our Ghanaian communities, from the large ones at GRIDCo and ECG substations that manage power for our cities, to the smaller pole-mounted ones that bring electricity to our homes, schools, and businesses. When these transformers work, they generate a lot of heat. If this heat is not removed effectively, the transformer can get damaged, leading to power outages ("dumsor"), costly repairs, and even dangerous fires.
A. Why Do Transformers Need Cooling?
Transformers are highly efficient, but not 100% efficient. This means some of the electrical energy passing through them is lost and converted into heat. If this heat is not dissipated, the internal temperature will rise continuously. High temperatures can cause: Insulation Breakdown: The paper and oil insulation inside the transformer will degrade, leading to short circuits and complete failure. Reduced Lifespan: For every 6-8°C increase above its rated operating temperature, a transformer's life is halved. Reduced Efficiency: As the windings get hotter, their electrical resistance increases, leading to even more heat generation (a vicious cycle). Safety Hazards: Overheating can cause the oil to expand and potentially cause an explosion or fire.
There are two primary sources of heat generation (known as transformer losses): Copper Losses (or I²R Losses): This heat is generated in the primary and secondary windings. The copper wire used for the windings has some electrical resistance (R). As current (I) flows through it, heat is produced according to the formula P = I²R. These losses vary with the load on the transformer – the more current drawn, the more heat is produced. *Simple Analogy:* Think of how your phone charger or a pressing iron gets warm when you use it. It's the same principle. Iron Losses (or Core Losses): This heat is generated in the transformer's laminated steel core. It is constant as long as the transformer is connected to a power supply, regardless of the load. Iron losses have two components: Hysteresis Loss: This is like "magnetic friction." Energy is lost as the magnetic domains in the core are constantly and rapidly realigned by the alternating magnetic field. Eddy Current Loss: The alternating magnetic field also induces small, circular currents (eddy currents) within the core material itself. These currents flow through the resistance of the core and produce heat. This is why cores are made of thin, insulated layers (laminations) – to reduce these currents. B. Methods of Cooling Transformers
The method used to cool a transformer depends on its size, power rating (kVA or MVA), and location. We can classify them broadly into natural and forced (artificial) methods. "Artificial Cooling" in the curriculum refers to any method where we actively use fans or pumps to speed up the cooling process.