Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Explain the operation of a transformer and its role in power distribution.
• Calculate voltage, current, and turns ratios.
• Describe different types of transformers.
• Explain power losses in transformers and how to minimize them.
• Discuss the impact of transformer efficiency in South Africa.

Lesson summary

This week, we delve into the fascinating world of transformers and power distribution, crucial components of South Africa's electricity grid. Understanding how electricity is generated, stepped up for efficient transmission, and stepped down for safe use in our homes and industries is not just academic; it’s directly linked to our daily lives and the functioning of our economy. Load shedding, voltage fluctuations, and the cost of electricity are all impacted by the efficiency and reliability of our power distribution network. By understanding transformers, we gain insight into how power reaches every corner of South Africa, from the bustling streets of Johannesburg to rural communities.

Lesson notes

2.1 The Transformer: A Core Component of Power Distribution A transformer is a static device that transfers electrical energy from one circuit to another through electromagnetic induction. It essentially changes the voltage level of an alternating current (AC) without changing the frequency. This ability to “step up” or “step down” voltage is fundamental to efficient power transmission and distribution. 2.1.1 Principle of Operation: Transformers operate on the principle of mutual inductance. A transformer consists of two or more coils of wire electrically isolated but magnetically linked. These coils are wound around a common ferromagnetic core (usually laminated steel to reduce eddy current losses).

Primary Winding: Connected to the input voltage source.

Secondary Winding: Connected to the load. When an alternating current flows through the primary winding, it creates a time-varying magnetic flux in the core. This flux links with the secondary winding, inducing a voltage in it according to Faraday's Law of Electromagnetic Induction. The magnitude of the induced voltage depends on the number of turns in each winding. 2.1.2 Transformer Ratio (Turns Ratio): The turns ratio (N) is the ratio of the number of turns in the secondary winding (N s ) to the number of turns in the primary winding (N p ): N = N s / N p This ratio directly determines the voltage transformation.

Step-Up Transformer: N > 1 (Secondary voltage is higher than primary voltage)

Step-Down Transformer: N s / V p = N s / N p = N Where: V s is the secondary voltage V p is the primary voltage Ideally, the power entering the primary winding is equal to the power leaving the secondary winding (assuming no losses): P in = P out V p I p = V s I s Therefore, the current ratio is inversely proportional to the turns ratio: I s / I p = N p / N s = 1/N Where: I s is the secondary current I p is the primary current 2.2 Types of Transformers in Power Distribution: Step-Up Transformers: Used at power generation stations to increase the voltage from generators (typically around 11kV) to higher voltages (e.g., 132kV, 400kV) for long-distance transmission. Higher voltages reduce current, minimizing I 2 R losses in transmission lines.

Step-Down Transformers: Used in substations and distribution transformers to reduce the high transmission voltages to lower voltages (e.g., 11kV) suitable for distribution to industrial and residential areas.

Distribution Transformers: Small step-down transformers typically found on utility poles or in underground vaults, which further reduce the voltage to levels used in homes and businesses (e.g., 230V in South Africa).

Instrument Transformers: Used to measure high voltages and currents in power systems. They include Current Transformers (CTs) and Voltage Transformers (VTs). 2.3 Power Losses in Transformers: Transformers are not perfectly efficient; some power is lost during the energy transfer process.

These losses can be categorized as: Core Losses (Iron Losses): These are constant losses and occur in the transformer core due to: Hysteresis Loss: Energy lost due to the repeated magnetization and demagnetization of the core material. It depends on the frequency and the core material.

Eddy Current Loss: Circulating currents induced in the core material by the changing magnetic field. Lamination of the core reduces eddy current losses.

Copper Losses (I 2 R Losses): These are variable losses and occur in the windings due to the resistance of the copper wire. They are proportional to the square of the current flowing through the windings. 2.4 Importance of Transformer Efficiency in South Africa: Transformer efficiency directly impacts the cost of electricity and the environment. Lower efficiency means more power is lost as heat, requiring more fuel to be burned at power plants. This increases CO2 emissions and contributes to climate change. In South Africa, with our reliance on coal-fired power plants, improving transformer efficiency is crucial for reducing our carbon footprint and minimizing load shedding caused by insufficient power generation. Eskom is continuously investing in upgrading transformers with higher efficiency ratings.

Worked example

Example 1: A transformer has 500 turns on its primary winding and 100 turns on its secondary winding. If the primary voltage is 220V, calculate the secondary voltage.

Solution:

N = N s / N p = 100 / 500 = 0.2

V s / V p = N

V s = N V p = 0.2 220V = 44V

Therefore, the secondary voltage is 44V. This is a step-down transformer.