A distribution transformer for a building is specified as 6.6 kV/415 V, 2 MVA, ONAN, DYN11. (a) Explain the term ONAN. (b) State two (2) physical differences between this transformer and an ANAF transformer. (c) Calculate the current flow in the load side of this transformer when the load of [4] 1500 kVA is connected to it? (d) What is the greatest risk associated with this transformer installing in a building? [1]

Let's address each part of your question regarding the distribution transformer specified as 6.6 kV/415 V, 2 MVA, ONAN, DYN11. ### (a) Explain the term ONAN. **ONAN** stands for **Oil Natural Air Natural**. It is a cooling method used in transformers. In an ONAN transformer, the heat generated by the transformer during operation is dissipated through natural convection of the oil and air. The oil circulates naturally due to the temperature difference (hot oil rises and cooler oil descends), and air is also allowed to circulate naturally around the transformer to aid in cooling. This method is commonly used for transformers that operate at lower power ratings and in environments where forced cooling is not necessary. ### (b) State two (2) physical differences between this transformer and an ANAF transformer. 1. **Cooling Method**: - An **ONAN** transformer relies on natural circulation of oil and air for cooling, while an **ANAF** (Air Natural, Air Forced) transformer uses natural air cooling combined with forced air cooling (usually with fans) to enhance heat dissipation. This means that ANAF transformers can handle higher loads or operate in higher ambient temperatures more effectively than ONAN transformers. 2. **Design and Size**: - Due to the forced cooling mechanism, ANAF transformers are typically larger and may have additional components such as fans or blowers to facilitate air movement. In contrast, ONAN transformers are generally more compact since they do not require additional cooling equipment. ### (c) Calculate the current flow in the load side of this transformer when the load of 1500 kVA is connected to it. To calculate the current on the load side (secondary side) of the transformer, we can use the formula: \[ I = \frac{S}{\sqrt{3} \times V} \] Where: - \( I \) = current (in Amperes) - \( S \) = apparent power (in kVA) - \( V \) = voltage (in Volts) For the secondary side of the transformer: - \( S = 1500 \, \text{kVA} \) - \( V = 415 \, \text{V} \) Now, substituting the values into the formula: \[ I = \frac{1500 \times 1000}{\sqrt{3} \times 415} \] Calculating: \[ I = \frac{1500000}{1.732 \times 415} \approx \frac{1500000}{718.78} \approx 2086.4 \, \text{A} \] So, the current flow in the load side of the transformer when a load of 1500 kVA is connected is approximately **2086.4 A**. ### (d) What is the greatest risk associated with this transformer installing in a building? The greatest risk associated with installing a transformer in a building is the potential for **fire hazards**. Transformers can overheat due to overload, insulation failure, or short circuits, which can lead to ignition of surrounding materials. Additionally, if the transformer is filled with oil (as in the case of ONAN transformers), there is a risk of oil leaks or spills, which can further increase the fire risk. Proper installation, maintenance, and fire safety measures are essential to mitigate these risks.