4.1 Name THREE losses that occur in transformers - NSC Electrical Technology Power Systems - Question 4 - 2018 - Paper 1

The delta-star transformer is commonly used for:

1. Power distribution in commercial settings: It allows for the efficient transfer of power across long distances.
2. Voltage step-down in industrial applications: It adapts high voltage from transmission lines to lower voltage suitable for use in factories.

The oil in transformers serves two major functions:

1. Insulation: It improves the insulation within the transformer, preventing electrical arcing and enhancing safety.
2. Cooling: The oil conducts heat away from the windings, preventing overheating during operation.

A transformer operates on the principle of electromagnetic induction. It consists of two coils of wire, known as the primary and secondary windings, wound around a magnetic core. When an alternating current flows through the primary winding, it creates a magnetic field that induces a voltage in the secondary winding. The voltage ratio between the primary and secondary is determined by the turns ratio of the coils: $\frac{V_P}{V_S} = \frac{N_P}{N_S}$ This allows the transformer to step-up or step-down voltages depending on the turn counts.

Transformers are more efficient than many other machines due to several reasons:

1. No moving parts: Transformers do not have mechanical components that can wear out, which minimizes mechanical losses.
2. Electromagnetic energy transfer: They operate on efficient electromagnetic induction, resulting in less energy lost as heat compared to devices that rely on mechanical motion.

The Buchholz relay is utilized in transformers to detect abnormal gas formation, which may indicate problems such as insulation failure. It provides an early warning by isolating the transformer from the supply if gas accumulation occurs, thus preventing extensive damage.

To calculate the secondary phase voltage, we use the formula: $V_{phase} = \frac{V_{L(S)}}{\sqrt{3}}$ Given that: $V_{L(S)} = 380 \text{ V}$ Thus, $V_{phase} = \frac{380}{\sqrt{3}} \approx 219.39 \text{ V}$

The transformation ratio (TR) is calculated using: $TR = \frac{V_{P}}{V_{S}}$ Substituting the values: $TR = \frac{2200}{219.39} \approx 10:1$

To find the number of secondary turns ($N_S$), we can relate it to the primary turns using the transformation ratio: $\frac{V_{P}}{V_{S}} = \frac{N_{P}}{N_{S}}$ Given:

• $N_{P} = 1500$ turns
• $V_{S} = 219.39$ V

Rearranging gives: $N_{S} = N_{P} \cdot \frac{V_{S}}{V_{P}}$ So, $N_{S} = 1500 \cdot \frac{219.39}{2200} \approx 149.58 \text{ turns}$

The transformer with a turns ratio of 30:1 is categorized as a step-down transformer, since the primary side has more turns than the secondary side ($N_P > N_S$).

Transformers allow for the efficient distribution of electrical power by stepping down high transmission voltages to lower, safer levels suitable for domestic and industrial use. The star-delta configuration enhances this capability by providing a neutral point for three-phase systems, ensuring balanced loads in various applications.