The trend of design is now towards lowering the manufacturing costs per unit even if it is at the expense of quality. However, the tender appraisal is mostly confined to the initial cost, along with no consideration for maintenance of the transformer up to the end of its fair life period. Therefore the safety factor is affected adversely. The subsequent aspects of transformer design impact transformer failure:
1 Transformer tank size: Inadequate clearance for free circulation of oil could lead to abnormal temperature rise, causing great damage to the HV winding insulation and, consequently, premature failure of transformers.
2 Percentage impedance (mechanical strength of coil): Most of the distribution transformers are situated in remote areas and many a times it is not possible to provide special attention to the operating conditions. Harsh conditions could also lead to failure. The solution to this problem lies in designing transformers along with large impedance so as to increase their short circuit withstands capacity.
Percentage impedance depends upon the subsequent factors.
- Size of wire used in HV coils - Economical size of coil yields lower size gauge wire, other than this decrease the mechanical capability of coils. As a result, the coils might not be able to withstand higher current densities that occur during the short circuit conditions.
- Radial distance between HV and LV coils - Increasing the radial distance among HV and LV coils increases the percentage impedance. It also leads to better mechanical strength of the coil to withstand higher short circuit stresses developed during short circuit conditions. But this will lead to higher cost.
- Effect of impedance on the short circuit stresses -The short circuit stresses is proportional to the square of the short circuit current. If the impedance is increased from 4.5 % to 5 % - 5.5%, the effect on the short circuit stresses developed within the transformer is decreased considerably.
• Improper use of aluminium wires: Improper use of aluminium wires leads to HV coil failure. The use of aluminium conductors has been recommended for windings up to 200 kVA transformers. Therefore, the super enamel covering the aluminium wire tends to crack during asymmetrical conditions and leads to coil failure. Thus, the use of higher cross-section conductors along with double paper covering would be desirable.
• Improper use of interlayer papers: Coil failure is commonly seen as an electrical failure. This commonly occurs while interlayer insulation breaks down at the end of the turn and creeps to the further layer. This type of insulation failure could be prevented through using folding papers and reinforcing the end turn insulation along with proper sleevings. Uniform separation of HV coil along with the inner coil, using spacers helps to prevent pressing of end turns as well as any further shrinkage during service.
• Use of inferior quality materials: Use of inferior quality wires for coils, poor quality of oil and other insulation material and etc., to bring down the cost of the transformer also raises the probability of failure of the transformer before full life of the transformer. The design computation of the tenderers should conform to the quantity and grade of input materials of core and windings furnished in the tender. For ensuring this, the transformer should be subjected to strip test. This will make sure in which the losses and impedance furnished in the tender have been actually achieved through the transformer.
The cost of repairs during the fair life period plus the initial cost is known as the estimated cost of the transformer. It is at present a few times more than the initial cost at that the distribution transformer is procured.