## 29/03/2020

### Design of Transformer Interview Question Answer - 8

 106 Which type of core construction is preferred in the large power transformer? Cruciform core 107 Why the core flux density in the distribution transformer is kept low? As the distribution transformer is designed for higher all day efficiency, the value of flux density is kept low in order to keep iron losses minimum in it. 108 Why LV winding is placed nearer to core of the transformer? LV winding nearer to core The LV winding is placed nearer to core of the transformer because the potential difference between LV winding and core ( at earth potential ) is very small. Therefore there is less chances of fault occurs between them. There is small thickness of insulation requires between LV winding and core. The major insulation requires only between core and LV winding. HV winding nearer to core However if the HV winding is placed nearer to the core, the thickness of insulation should be large and it will result in length of mean turn of winding increases. The cost of insulation increases because the major insulation requires on both side i.e. core side and LV side. 109 Which are the factors to be considered while designing insulation of the transformer? Points to be considered while designing insulation The winding voltage produces electro static field in the dielectric material and produces stress in it. The current in the winding produces electromagnetic field in the dielectric material and produces mechanical stress in it. The temperature rise due to losses produces thermal stress on the insulation. 110 What should be width of duct kept for small and large capacity power transformer? The width of oil duct for small capacity power transformer should be 6 mm and it is in the range of 7.0 – 12.0 mm for large capacity power transformer. 111 How insulation is classified in the transformer? Types of insulation Major insulation Minor insulation Insulation between two phases Insulation between relative to tank

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