5.1 Name the process used by transformers to transfer energy from the primary winding to the secondary winding - NSC Electrical Technology Power Systems - Question 5 - 2023 - Paper 1

An alternating magnetic field is created in the primary winding of a transformer through the alternating current (AC) flowing in the primary coil. As the AC voltage is applied, the current increases and decreases, which causes the magnetic field around the coils to also expand and collapse alternately, resulting in a changing magnetic field.

• Size: The physical dimensions must be comparable to handle equivalent loads.
• Voltage Rating: All transformers must have the same voltage rating to ensure compatibility.
• Efficiency: Each transformer should have similar efficiency characteristics to ensure equal performance.

Delta-delta transformers are mainly used in industries where high power transfer is essential. This design allows for better phase balancing and reduced losses during high-load conditions, making it suitable for heavy industrial applications.

5.5.1 Is more expensive: The method of using single-phase transformers is typically more expensive due to the higher initial cost and additional complexity in installation.

5.5.2 Has a higher efficiency: Pre-manufactured three-phase transformers usually have higher efficiency because they are designed as a single unit, leading to lower losses compared to individual single-phase transformers.

5.5.3 Uses thicker-sized conductors: The method involving single-phase transformers often requires thicker conductors due to increased current handling and potential for greater heat generation.

• Dry type transformers
• Oil immersed transformers

• Air Natural Cooling: Utilizes natural airflow for cooling.
• Oil Natural, Air Forced (ONAF): Uses oil circulation and fans to enhance cooling.

Buchholz relay is the protective device that monitors gas formation in high-power transformers, providing an early warning of potential issues.

The secondary phase voltage can be calculated using the transformation ratio: V_{PH2} = rac{N_2}{N_1} V_{PH1} Substituting the given values, we have: V_{PH2} = rac{1}{25} imes 6000 = 240 ext{ V}

The secondary line voltage is obtained from the phase voltage as follows: V_{L2} = rac{ ext{Phase Voltage} imes ext{sqrt}(3)}{25} By substituting in the known values: $V_{L2} = ext{sqrt}(3) imes V_{PH2} = ext{sqrt}(3) imes 240 ext{ V} = 415.69 ext{ V}$

The apparent power can be calculated using: S = rac{P}{ ext{pf}} = rac{50000}{0.9} = 55,555.56 ext{ VA} ext{ or } 55.56 ext{ kVA}

The primary line current can be found using: I_{L1} = rac{S}{ ext{sqrt}(3) imes V_{L1} imes ext{pf}} = rac{50000}{ ext{sqrt}(3) imes 6000 imes 0.9} ext{ A} = 5.35 ext{ A}