TION $$ 3 \quad $$ (11 marks) A distribution transformer for a polytechnic is specified as $$ 22 \mathrm{kV} / 415 \mathrm{~V}, 2 $$ MVA, ONAN, DYN11. (i) Explain the term DYN11. (ii) State two (2) physical differences between this transformer and an ANAF transformer.

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### (i) Explanation of the term DYN11

The term **DYN11** refers to the vector group of the transformer, which indicates the configuration of the windings and the phase displacement between the primary (high voltage) and secondary (low voltage) sides.

- **D**: This indicates that the primary winding is connected in a **Delta** configuration.
- **Y**: This indicates that the secondary winding is connected in a **Star** (or Wye) configuration.
- **N**: This indicates that the neutral point of the star-connected winding is brought out for grounding.
- **11**: This number indicates the phase displacement between the primary and secondary windings. In this case, the "11" means that the secondary voltage lags the primary voltage by 30 degrees (or 11 hours on a clock face).

This configuration is commonly used in distribution transformers as it allows for effective voltage transformation and provides a neutral point for grounding, which enhances safety and stability in the electrical system.

### (ii) Two physical differences between this transformer and an ANAF transformer

1. **Cooling Method**:
   - **DYN11 Transformer (ONAN)**: The cooling method is **Oil Natural Air Natural (ONAN)**, which means that the transformer is cooled by natural circulation of oil and air. The oil absorbs heat from the windings and rises, while cooler oil descends, creating a natural convection current.
   - **ANAF Transformer**: The cooling method is **Air Natural Air Forced (ANAF)**, which typically involves natural air cooling combined with forced air cooling. This means that while the transformer may rely on natural air circulation, it also uses fans or blowers to enhance cooling, which can lead to a more compact design and potentially higher power ratings.

2. **Winding Configuration**:
   - **DYN11 Transformer**: As mentioned, this transformer has a **Delta/Star** winding configuration, which allows for phase shifting and provides a neutral point for grounding.
   - **ANAF Transformer**: This transformer may have different winding configurations, such as **Star/Star** or **Delta/Delta**, which do not provide the same phase shift or neutral grounding capabilities. The specific configuration can affect the transformer's performance, including its ability to handle unbalanced loads and its response to fault conditions.

These differences can impact the transformer's performance, efficiency, and suitability for specific applications in electrical distribution systems.
### (i) Explanation of the term DYN11 DYN11 is a transformer vector group that indicates winding configuration and phase displacement. It means Delta primary, Star secondary with neutral, and a 30-degree phase shift. ### (ii) Two physical differences between this transformer and an ANAF transformer 1. **Cooling Method**: DYN11 (ONAN) uses natural oil and air cooling, while ANAF uses natural air cooling with forced air. 2. **Winding Configuration**: DYN11 has Delta/Star configuration for phase shifting and grounding, while ANAF may have different configurations without phase shifting.

TION (11 marks)
A distribution transformer for a polytechnic is specified as
MVA, ONAN, DYN11.
(i) Explain the term DYN11.
(ii) State two (2) physical differences between this transformer and an
ANAF transformer.

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(i) Explanation of the term DYN11

(ii) Two physical differences between this transformer and an ANAF transformer

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TION (11 marks) A distribution transformer for a polytechnic is specified as MVA, ONAN, DYN11. (i) Explain the term DYN11. (ii) State two (2) physical differences between this transformer and an ANAF transformer.